A Programmer's Guide To Java SE 8 Oracle Certified Associate (OCA)

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AProgrammer’sGuidetoJava®SE8
OracleCertifiedAssociate(OCA)
AComprehensivePrimer
KhalidA.Mughal
RolfW.Rasmussen
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ISBN-13:978-0-13-293021-5
ISBN-10:0-13-293021-8
TextprintedintheUnitedStatesonrecycledpaperatRRDonnelleyinCrawfordsville,
Indiana.
Firstprinting,July2016
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Tothelovingmemoryofmymother,ZubaidaBegum,
andmyfather,MohammedAzim
—K.A.M.
ForOliviaE.Rasmussenand
LouiseJ.Dahlmo
—R.W.R.
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ContentsOverview
Figures
Tables
Examples
Foreword
Preface
1BasicsofJavaProgramming
2LanguageFundamentals
3Declarations
4AccessControl
5OperatorsandExpressions
6ControlFlow
7Object-OrientedProgramming
8FundamentalClasses
9ObjectLifetime
10TheArrayList<E>ClassandLambdaExpressions
11DateandTime
ATakingtheJavaSE8ProgrammerIExam
BExamTopics:JavaSE8ProgrammerI
CAnnotatedAnswerstoReviewQuestions
DSolutionstoProgrammingExercises
EMockExam:JavaSE8ProgrammerI
FAnnotatedAnswerstoMockExamI
Index
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Contents
Figures
Tables
Examples
Foreword
Preface
1BasicsofJavaProgramming
1.1Introduction
1.2Classes
DeclaringMembers:FieldsandMethods
1.3Objects
ClassInstantiation,ReferenceValues,andReferences
ObjectAliases
1.4InstanceMembers
InvokingMethods
1.5StaticMembers
1.6Inheritance
1.7Associations:AggregationandComposition
1.8TenetsofJava
ReviewQuestions
1.9JavaPrograms
1.10SampleJavaApplication
EssentialElementsofaJavaApplication
CompilingandRunninganApplication
1.11ProgramOutput
FormattedOutput
1.12TheJavaEcosystem
Object-OrientedParadigm
Interpreted:TheJVM
Architecture-NeutralandPortableBytecode
Simplicity
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DynamicandDistributed
RobustandSecure
HighPerformanceandMultithreaded
ReviewQuestions
ChapterSummary
ProgrammingExercise
2LanguageFundamentals
2.1BasicLanguageElements
LexicalTokens
Identifiers
Keywords
Separators
Literals
IntegerLiterals
Floating-PointLiterals
UnderscoresinNumericalLiterals
BooleanLiterals
CharacterLiterals
StringLiterals
Whitespace
Comments
ReviewQuestions
2.2PrimitiveDataTypes
TheIntegerTypes
ThecharType
TheFloating-PointTypes
ThebooleanType
ReviewQuestions
2.3VariableDeclarations
DeclaringandInitializingVariables
ReferenceVariables
2.4InitialValuesforVariables
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DefaultValuesforFields
InitializingLocalVariablesofPrimitiveDataTypes
InitializingLocalReferenceVariables
LifetimeofVariables
ReviewQuestions
ChapterSummary
ProgrammingExercise
3Declarations
3.1ClassDeclarations
3.2MethodDeclarations
Statements
InstanceMethodsandtheObjectReferencethis
MethodOverloading
3.3Constructors
TheDefaultConstructor
OverloadedConstructors
ReviewQuestions
3.4Arrays
DeclaringArrayVariables
ConstructinganArray
InitializinganArray
UsinganArray
AnonymousArrays
MultidimensionalArrays
SortingArrays
SearchingArrays
ReviewQuestions
3.5ParameterPassing
PassingPrimitiveDataValues
PassingReferenceValues
PassingArrays
ArrayElementsasActualParameters
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finalParameters
3.6VariableArityMethods
CallingaVariableArityMethod
VariableArityandFixedArityMethodCalls
3.7Themain()Method
ProgramArguments
3.8EnumeratedTypes
DeclaringType-safeEnums
UsingType-safeEnums
SelectedMethodsforEnumTypes
ReviewQuestions
ChapterSummary
ProgrammingExercise
4AccessControl
4.1JavaSourceFileStructure
4.2Packages
DefiningPackages
UsingPackages
CompilingCodeintoPackages
RunningCodefromPackages
4.3SearchingforClasses
ReviewQuestions
4.4ScopeRules
ClassScopeforMembers
BlockScopeforLocalVariables
4.5AccessibilityModifiersforTop-LevelTypeDeclarations
4.6Non-AccessibilityModifiersforClasses
abstractClasses
finalClasses
ReviewQuestions
4.7MemberAccessibilityModifiers
publicMembers
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protectedMembers
DefaultAccessibilityforMembers
privateMembers
ReviewQuestions
4.8Non-AccessibilityModifiersforMembers
staticMembers
finalMembers
abstractMethods
synchronizedMethods
nativeMethods
transientFields
volatileFields
ReviewQuestions
ChapterSummary
ProgrammingExercise
5OperatorsandExpressions
5.1Conversions
WideningandNarrowingPrimitiveConversions
WideningandNarrowingReferenceConversions
BoxingandUnboxingConversions
OtherConversions
5.2TypeConversionContexts
AssignmentContext
MethodInvocationContext
CastingContextoftheUnaryTypeCastOperator:(type)
NumericPromotionContext
5.3PrecedenceandAssociativityRulesforOperators
5.4EvaluationOrderofOperands
Left-HandOperandEvaluationFirst
OperandEvaluationbeforeOperationExecution
Left-to-RightEvaluationofArgumentLists
5.5RepresentingIntegers
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CalculatingTwo’sComplement
ConvertingBinaryNumberstoDecimals
ConvertingDecimalstoBinaryNumbers
RelationshipsamongBinary,Octal,andHexadecimalNumbers
5.6TheSimpleAssignmentOperator=
AssigningPrimitiveValues
AssigningReferences
MultipleAssignments
TypeConversionsinanAssignmentContext
ReviewQuestions
5.7ArithmeticOperators:*,/,%,+,-
ArithmeticOperatorPrecedenceandAssociativity
EvaluationOrderinArithmeticExpressions
RangeofNumericValues
UnaryArithmeticOperators:-,+
MultiplicativeBinaryOperators:*,/,%
AdditiveBinaryOperators:+,-
NumericPromotionsinArithmeticExpressions
ArithmeticCompoundAssignmentOperators:*=,/=,%=,+=,-=
ReviewQuestions
5.8TheBinaryStringConcatenationOperator+
5.9VariableIncrementandDecrementOperators:++,--
TheIncrementOperator++
TheDecrementOperator--
ReviewQuestions
5.10BooleanExpressions
5.11RelationalOperators:<,<=,>,>=
5.12Equality
PrimitiveDataValueEquality:==,!=
ObjectReferenceEquality:==,!=
ObjectValueEquality
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5.13BooleanLogicalOperators:!,^,&,|
OperandEvaluationforBooleanLogicalOperators
BooleanLogicalCompoundAssignmentOperators:&=,^=,|=
5.14ConditionalOperators:&&,||
Short-CircuitEvaluation
5.15IntegerBitwiseOperators:~,&,|,^
BitwiseCompoundAssignmentOperators:&=,^=,|=
ReviewQuestions
5.16TheConditionalOperator:?:
5.17OtherOperators:new,[],instanceof,->
ReviewQuestions
ChapterSummary
ProgrammingExercise
6ControlFlow
6.1OverviewofControlFlowStatements
6.2SelectionStatements
TheSimpleifStatement
Theif-elseStatement
TheswitchStatement
ReviewQuestions
6.3IterationStatements
ThewhileStatement
Thedo-whileStatement
Thefor(;;)Statement
Thefor(:)Statement
6.4TransferStatements
LabeledStatements
ThebreakStatement
ThecontinueStatement
ThereturnStatement
ReviewQuestions
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6.5Stack-BasedExecutionandExceptionPropagation
6.6ExceptionTypes
TheExceptionClass
TheRuntimeExceptionClass
TheErrorClass
CheckedandUncheckedExceptions
DefiningCustomizedExceptions
6.7ExceptionHandling:try,catch,andfinally
ThetryBlock
ThecatchClause
ThefinallyClause
6.8ThethrowStatement
6.9ThethrowsClause
OverridingthethrowsClause
6.10AdvantagesofExceptionHandling
ReviewQuestions
ChapterSummary
ProgrammingExercises
7Object-OrientedProgramming
7.1SingleImplementationInheritance
Relationships:is-aandhas-a
TheSupertype–SubtypeRelationship
7.2OverridingMethods
InstanceMethodOverriding
CovariantreturninOverridingMethods
OverridingversusOverloading
7.3HidingMembers
FieldHiding
StaticMethodHiding
7.4TheObjectReferencesuper
ReviewQuestions
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7.5ChainingConstructorsUsingthis()andsuper()
Thethis()ConstructorCall
Thesuper()ConstructorCall
ReviewQuestions
7.6Interfaces
DefiningInterfaces
AbstractMethodsinInterfaces
ImplementingInterfaces
ExtendingInterfaces
InterfaceReferences
DefaultMethodsinInterfaces
StaticMethodsinInterfaces
ConstantsinInterfaces
ReviewQuestions
7.7ArraysandSubtyping
ArraysandSubtypeCovariance
ArrayStoreCheck
7.8ReferenceValuesandConversions
7.9ReferenceValueAssignmentConversions
7.10MethodInvocationConversionsInvolvingReferences
OverloadedMethodResolution
7.11ReferenceCastingandtheinstanceofOperator
TheCastOperator
TheinstanceofOperator
ReviewQuestions
7.12PolymorphismandDynamicMethodLookup
7.13InheritanceversusAggregation
7.14BasicConceptsinObject-OrientedDesign
Encapsulation
Cohesion
Coupling
ReviewQuestions
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ChapterSummary
ProgrammingExercises
8FundamentalClasses
8.1Overviewofthejava.langPackage
8.2TheObjectClass
ReviewQuestions
8.3TheWrapperClasses
CommonWrapperClassConstructors
CommonWrapperClassUtilityMethods
NumericWrapperClasses
TheCharacterClass
TheBooleanClass
ReviewQuestions
8.4TheStringClass
Immutability
CreatingandInitializingStrings
TheCharSequenceInterface
ReadingCharactersfromaString
ComparingStrings
CharacterCaseinaString
ConcatenationofStrings
JoiningofCharSequenceObjects
SearchingforCharactersandSubstrings
ExtractingSubstrings
ConvertingPrimitiveValuesandObjectstoStrings
FormattedStrings
ReviewQuestions
8.5TheStringBuilderandStringBufferClasses
Thread-Safety
Mutability
ConstructingStringBuilders
ReadingandChangingCharactersinStringBuilders
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ConstructingStringsfromStringBuilders
Appending,Inserting,andDeletingCharactersinStringBuilders
ControllingStringBuilderCapacity
ReviewQuestions
ChapterSummary
ProgrammingExercises
9ObjectLifetime
9.1GarbageCollection
9.2ReachableObjects
9.3FacilitatingGarbageCollection
9.4ObjectFinalization
9.5FinalizerChaining
9.6InvokingGarbageCollectionProgrammatically
ReviewQuestions
9.7Initializers
9.8FieldInitializerExpressions
DeclarationOrderofInitializerExpressions
9.9StaticInitializerBlocks
DeclarationOrderofStaticInitializers
9.10InstanceInitializerBlocks
DeclarationOrderofInstanceInitializers
9.11ConstructingInitialObjectState
ReviewQuestions
ChapterSummary
10TheArrayList<E>ClassandLambdaExpressions
10.1TheArrayList<E>Class
Lists
DeclaringReferencesandConstructingArrayLists
ModifyinganArrayList
QueryinganArrayList
TraversinganArrayList
ConvertinganArrayListtoanArray
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SortinganArrayList
ArraysversusArrayList
ReviewQuestions
10.2LambdaExpressions
BehaviorParameterization
FunctionalInterfaces
DefiningLambdaExpressions
TypeCheckingandExecutionofLambdaExpressions
FilteringRevisited:ThePredicate<T>FunctionalInterface
ReviewQuestions
ChapterSummary
ProgrammingExercise
11DateandTime
11.1BasicDateandTimeConcepts
11.2WorkingwithTemporalClasses
CreatingTemporalObjects
QueryingTemporalObjects
ComparingTemporalObjects
CreatingModifiedCopiesofTemporalObjects
TemporalArithmetic
11.3WorkingwithPeriods
CreatingPeriods
QueryingPeriods
CreatingModifiedCopiesofPeriods
MoreTemporalArithmetic
ReviewQuestions
11.4FormattingandParsing
DefaultFormatters
PredefinedFormatters
LocalizedFormatters
CustomizedFormatters
ReviewQuestions
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ChapterSummary
ProgrammingExercise
ATakingtheJavaSE8ProgrammerIExam
A.1PreparingfortheExam
A.2RegisteringfortheExam
ContactInformation
ObtaininganExamVoucher
SigningUpfortheTest
AfterTakingtheExam
A.3HowtheExamIsConducted
TheTestingLocations
UtilizingtheAllottedTime
TheExamProgram
TheExamResult
A.4TheQuestions
AssumptionsabouttheExamQuestions
TypesofQuestionsAsked
TypesofAnswersExpected
TopicsCoveredbytheQuestions
BExamTopics:JavaSE8ProgrammerI
CAnnotatedAnswerstoReviewQuestions
DSolutionstoProgrammingExercises
EMockExam:JavaSE8ProgrammerI
FAnnotatedAnswerstoMockExamI
Index
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Figures
1.1UMLNotationforClasses
1.2UMLNotationforObjects
1.3Aliases
1.4ClassDiagramShowingStaticMembersofaClass
1.5MembersofaClass
1.6ClassDiagramDepictingInheritanceRelationship
1.7ClassDiagramDepictingAssociations
1.8ClassDiagramDepictingComposition
2.1PrimitiveDataTypesinJava
3.1ArrayofArrays
3.2ParameterPassing:PrimitiveDataValues
3.3ParameterPassing:ReferenceValues
3.4ParameterPassing:Arrays
4.1JavaSourceFileStructure
4.2PackageHierarchy
4.3FileHierarchy
4.4SearchingforClasses
4.5BlockScope
4.6PublicAccessibilityforMembers
4.7ProtectedAccessibilityforMembers
4.8DefaultAccessibilityforMembers
4.9PrivateAccessibilityforMembers
5.1WideningPrimitiveConversions
5.2ConvertingamongBinary,Octal,andHexadecimalNumbers
5.3OverflowandUnderflowinFloating-PointArithmetic
5.4NumericPromotioninArithmeticExpressions
6.1ActivityDiagramforifStatements
6.2ActivityDiagramforaswitchStatement
6.3ActivityDiagramforthewhileStatement
6.4ActivityDiagramforthedo-whileStatement
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6.5ActivityDiagramfortheforStatement
6.6EnhancedforStatement
6.7MethodExecution
6.8ExceptionPropagation
6.9PartialExceptionInheritanceHierarchy
6.10Thetry-catch-finallyConstruct
6.11ExceptionHandling(Scenario1)
6.12ExceptionHandling(Scenario2)
6.13ExceptionHandling(Scenario3)
7.1InheritanceHierarchy
7.2InheritanceHierarchyforExample7.2andExample7.3
7.3InheritanceHierarchies
7.4InheritanceRelationshipsforInterfaceConstants
7.5ReferenceTypeHierarchy:ArraysandSubtypeCovariance
7.6TypeHierarchyThatIllustratesPolymorphism
7.7ImplementingDataStructuresbyInheritanceandAggregation
8.1PartialInheritanceHierarchyinthejava.langPackage
8.2ConvertingValuesamongPrimitive,Wrapper,andStringTypes
9.1MemoryOrganizationatRuntime
10.1PartialArrayListInheritanceHierarchy
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Tables
1.1TerminologyforClassMembers
1.2FormatSpecifierExamples
2.1KeywordsinJava
2.2ReservedLiteralsinJava
2.3ReservedKeywordsNotCurrentlyinUse
2.4SeparatorsinJava
2.5ExamplesofLiterals
2.6ExamplesofDecimal,Binary,Octal,andHexadecimalLiterals
2.7ExamplesofCharacterLiterals
2.8EscapeSequences
2.9ExamplesofEscapeSequence\ddd
2.10RangeofIntegerValues
2.11RangeofCharacterValues
2.12RangeofFloating-PointValues
2.13BooleanValues
2.14SummaryofPrimitiveDataTypes
2.15DefaultValues
3.1ParameterPassingbyValue
4.1AccessingMemberswithinaClass
4.2SummaryofAccessibilityModifiersforTop-LevelTypes
4.3SummaryofNon-AccessibilityModifiersforClasses
4.4SummaryofAccessibilityModifiersforMembers
4.5SummaryofNon-AccessibilityModifiersforMembers
5.1SelectedConversionContextsandConversionCategories
5.2OperatorSummary
5.3RepresentingSignedbyteValuesUsingTwo’sComplement
5.4ExamplesofTruncatedValues
5.5ArithmeticOperators
5.6ExamplesofArithmeticExpressionEvaluation
5.7ArithmeticCompoundAssignmentOperators
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5.8RelationalOperators
5.9PrimitiveDataValueEqualityOperators
5.10ReferenceEqualityOperators
5.11TruthValuesforBooleanLogicalOperators
5.12BooleanLogicalCompoundAssignmentOperators
5.13ConditionalOperators
5.14TruthValuesforConditionalOperators
5.15IntegerBitwiseOperators
5.16ResultTableforBitwiseOperators
5.17ExamplesofBitwiseOperations
5.18BitwiseCompoundAssignmentOperators
6.1ThereturnStatement
7.1OverridingversusOverloading
7.2SameSignatureforSubclassandSuperclassMethod
7.3TypesandValues
10.1SummaryofArraysversusArrayLists
10.2SelectedFunctionalInterfacesfromthejava.util.functionPackage
11.1SelectedCommonMethodPrefixoftheTemporalClasses
11.2SelectedISO-BasedPredefinedFormattersforDateandTime
11.3FormatStylesforDateandTime
11.4CombinationofFormatStylesandLocalizedFormatters
11.5SelectedDate/TimePatternLetters
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Examples
1.1BasicElementsofaClassDeclaration
1.2StaticMembersinClassDeclaration
1.3DefiningaSubclass
1.4AnApplication
1.5FormattedOutput
2.1DefaultValuesforFields
2.2FlaggingUninitializedLocalVariablesofPrimitiveDataTypes
2.3FlaggingUninitializedLocalReferenceVariables
3.1UsingthethisReference
3.2Namespaces
3.3UsingArrays
3.4UsingAnonymousArrays
3.5UsingMultidimensionalArrays
3.6PassingPrimitiveValues
3.7PassingReferenceValues
3.8PassingArrays
3.9ArrayElementsasPrimitiveDataValues
3.10ArrayElementsasReferenceValues
3.11CallingaVariableArityMethod
3.12PassingProgramArguments
3.13UsingEnums
4.1DefiningPackagesandUsingTypeImport
4.2SingleStaticImport
4.3AvoidingtheInterfaceConstantAntipattern
4.4ImportingEnumConstants
4.5ShadowingStaticImport
4.6ConflictinImportingStaticMethodwiththeSameSignature
4.7ClassScope
4.8AccessibilityModifiersforClassesandInterfaces
4.9AbstractClasses
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4.10PublicAccessibilityofMembers
4.11AccessingStaticMembers
4.12UsingfinalModifier
4.13SynchronizedMethods
5.1EvaluationOrderofOperandsandArguments
5.2NumericPromotioninArithmeticExpressions
5.3Short-CircuitEvaluationInvolvingConditionalOperators
5.4BitwiseOperations
6.1Fall-ThroughinaswitchStatement
6.2UsingbreakinaswitchStatement
6.3NestedswitchStatement
6.4StringsinswitchStatement
6.5EnumsinswitchStatement
6.6ThebreakStatement
6.7LabeledbreakStatement
6.8continueStatement
6.9LabeledcontinueStatement
6.10ThereturnStatement
6.11MethodExecution
6.12Thetry-catchConstruct
6.13ExceptionPropagation
6.14Thetry-catch-finallyConstruct
6.15Thetry-finallyConstruct
6.16ThefinallyClauseandthereturnStatement
6.17ThrowingExceptions
6.18ThethrowsClause
7.1ExtendingClasses:InheritanceandAccessibility
7.2Overriding,Overloading,andHiding
7.3UsingthesuperKeyword
7.4ConstructorOverloading
7.5Thethis()ConstructorCall
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7.6Thesuper()ConstructorCall
7.7ImplementingInterfaces
7.8DefaultMethodsinInterfaces
7.9DefaultMethodsandMultipleInheritance
7.10StaticMethodsinInterfaces
7.11ConstantsinInterfaces
7.12InheritingConstantsinInterfaces
7.13AssigningandPassingReferenceValues
7.14ChoosingtheMostSpecificMethod(SimpleCase)
7.15OverloadedMethodResolution
7.16TheinstanceofandCastOperators
7.17UsingtheinstanceofOperator
7.18PolymorphismandDynamicMethodLookup
7.19ImplementingDataStructuresbyInheritanceandAggregation
8.1MethodsintheObjectClass
8.2StringRepresentationofIntegers
8.3StringConstructionandEquality
8.4ReadingCharactersfromaString
9.1GarbageCollectionEligibility
9.2UsingFinalizers
9.3InvokingGarbageCollection
9.4InitializerExpressionOrderandMethodCalls
9.5StaticInitializersandForwardReferences
9.6InstanceInitializersandForwardReferences
9.7ObjectStateConstruction
9.8InitializationAnomalyunderObjectStateConstruction
10.1UsinganArrayList
10.2ImplementingCustomizedMethodsforFilteringanArrayList
10.3ImplementinganInterfaceforFilteringanArrayList
10.4User-DefinedFunctionalInterfaceforFilteringanArrayList
10.5UsingthePredicate<T>FunctionalInterfaceforFilteringanArrayList
10.6AccessingMembersinanEnclosingObject
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10.7AccessingLocalVariablesinanEnclosingMethod
10.8FilteringanArrayList
11.1CreatingTemporalObjects
11.2UsingTemporalObjects
11.3TemporalArithmetic
11.4Period-BasedLoop
11.5MoreTemporalArithmetic
11.6UsingDefaultDateandTimeFormatters
11.7UsingPredefinedFormatStyleswithTime-BasedValues
11.8UsingPredefinedFormatStyleswithDate-BasedValues
11.9UsingPredefinedFormatStyleswithDateandTime-BasedValues
11.10FormattingandParsingwithLetterPatterns
11.11FormattingwithDate/TimeLetterPatterns
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Foreword
Javaisnowovertwentyyearsoldandthecurrentrelease,JDK8,despiteitsname,is
reallytheeleventhsignificantreleaseoftheplatform.Whilststayingtruetotheoriginal
ideasoftheplatform,therehavebeennumerousdevelopmentsaddingavarietyoffeatures
tothelanguagesyntaxaswellasahugenumberofAPIstothecoreclasslibraries.This
hasenableddeveloperstobecomesubstantiallymoreproductiveandhashelpedto
eliminateavarietyofcommonsituationsthatcaneasilyresultinbugs.
Javahascontinuedtogrowinpopularity,whichisinlargepartattributabletothe
continuedevolutionoftheplatform,whichkeepsitfreshandaddressesthingsthat
developerswant.Accordingtosomesources,therearemorethanninemillionJava
programmersacrosstheglobeandthisnumberlookssettocontinuetogrowasmost
universitiesuseJavaasaprimaryteachinglanguage.
WithsomanyJavaprogrammersavailabletoemployers,howdotheyensurethat
candidateshavethenecessaryskillstodevelophigh-quality,reliablecode?Theansweris
certification:astandardizedtestofadeveloper’sknowledgeaboutthewidevarietyof
featuresandtechniquesrequiredtouseJavaefficientlyandeffectively.Originally
introducedbySunMicrosystems,thecertificationprocessandexamhasbeenupdatedto
matchthefeaturesofeachreleaseofJava.OraclehascontinuedthissinceacquiringSun
in2010.
Takingandpassingtheexamsisnotasimpletask.Toensurethatdevelopersmeetahigh
standardofknowledgeaboutJava,thecandidatemustdemonstratetheabilityto
understandawidevarietyofprogrammingtechniques,acleargraspoftheJavasyntax,
andacomprehensiveknowledgeofthestandardclasslibraryAPIs.Withthereleaseof
JDK8,notonlydoJavadevelopersneedtounderstandthedetailsofimperativeand
object-orientedprogramming,theynowneedtohaveagraspoffunctionalprogramming
sotheycaneffectivelyusethekeynewfeatures:lambdaexpressionsandtheStreamsAPI.
Whichiswhy,ultimately,youneedthisbooktohelpyoupreparefortheexam.The
authorshavedoneagreatjobofpresentingthematerialyouneedtoknowtopassthe
examinanapproachableandeasy-to-graspway.Thebookstartswiththefundamental
conceptsandlanguagesyntaxandworksitswaythroughwhatyouneedtoknowabout
object-orientedprogrammingbeforeaddressingmorecomplextopicslikegenerictypes.
ThelatterpartofthebookaddressesthemostrecentchangesinJDK8,thatoflambda
expressions,theStreamsAPI,andthenewDateandTimeAPI.
HavingworkedwithJavaalmostsinceitwasfirstreleased,bothatSunMicrosystemsand
thenatOracleCorporation,Ithinkyouwillfindthisbookaninvaluableguidetohelpyou
passtheOracleCertifiedAssociateExamforJavaSE8.Iwishyouthebestofluck!
—SimonRitter
DeputyCTO,AzulSystems
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Preface
WritingThisBook
DearReader,whatyouholdinyourhandistheresultofameticuloushigh-techoperation
thattookmanymonthsandrequiredinspectingmanyparts,removingcertainparts,
retrofittingsomeoldparts,andaddingmanynewpartstoourpreviousbookonanearlier
Javaprogrammercertificationexam,untilwewerecompletelysatisfiedwiththeresult.
Afteryouhavereadthebookandpassedtheexam,wehopethatyouwillappreciatethe
TLC(tenderlovingcare)thathasgoneintothisoperation.Thisishowitallcameabout.
LearningthenamesofJavacertificationsandtherequiredexamsisthefirstitemonthe
agenda.ThisbookprovidescoveragefortheexamtoearnOracleCertifiedAssociate
(OCA),JavaSE8ProgrammerCertification(alsoknowasOCAJP8).Theexamrequired
forthiscertificationhasthenameJavaSE8ProgrammerIExam(Examnumber1Z0-
808).ItisthefirstoftwoexamsrequiredtoobtainOracleCertifiedProfessional(OCP),
JavaSE8ProgrammerCertification(alsoknownasOCPJP8).Thesecondexamrequired
forthisprofessionalcertificationhasthenameJavaSE8ProgrammerIIExam(Exam
number1Z0-809).Toreiterate,thisbookcoversonlythetopicsfortheJavaSE8
ProgrammerIExamthatisrequiredtoobtainOCAJP8certification.
AbookonthenewJavaSE8certificationwasalongtimecoming.ThemantleofJava
hadbeenpassedontoOracleandJava7hadhitthenewsstand.Westartedouttowritea
booktocoverthetopicsforthetwoexamsrequiredtoearntheOracleCertified
Professional,JavaSE7ProgrammerCertification.SoonafterthereleaseofJava8,
OracleannouncedthecertificationforJavaSE8.Wedecidedtoswitchtothenewversion.
Itwasnotadifficultdecisiontomake.Java8marksawatershedwhenthelanguagewent
frombeingapureobject-orientedlanguagetoonethatalsoincorporatesfeaturesof
functional-styleprogramming.Asthesayinggoes,Java8changedthewholeballgame.
Javapasseditstwentiethbirthdayin2015.Java8,releasedayearearlier,representeda
significantmilestoneinitshistory.Therewaslittlereasontodwellonearlierversions.
Thenextdecisionconcernedwhetheritwouldbebesttoprovidecoverageforthetwo
JavaSE8ProgrammerCertificationexamsinoneortwobooks.Pragmaticreasons
dictatedtwobooks.Itwouldtakefartoolongtocompleteabookthatcoveredboth
exams,mainlybecausethesecondexamwaslargelyrevampedandwouldrequirealotof
newmaterial.Wedecidedtocompletethebookforthefirstexam.Oncethatdecisionwas
made,ourdraftmanuscriptwentbackontheoperatingtable.
Ourapproachtowritingthisbookhasnotchangedfromtheoneweemployedforour
previousbooks,mainlybecauseithasprovedsuccessful.Nostoneswereleftunturnedto
createthisbook,asweexplainhere.
ThemostnoticeablechangesintheexamforOCAJP8aretheinclusionofthecoreclasses
inthenewDateandTimeAPIandthewritingofpredicatesusinglambdaexpressions.
Theemphasisremainsonanalyzingcodescenarios,ratherthanindividuallanguage
constructs.Theexamcontinuestorequireactualexperiencewiththelanguage,notjust
mererecitationoffacts.Westillclaimthatproficiencyinthelanguageisthekeyto
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success.
SincetheexamemphasizesthecorefeaturesofJava,thisbookprovidesin-depthcoverage
oftopicsrelatedtothosefeatures.Asinourearlierbooks,supplementarytopicsarealso
includedtoaidinmasteringtheexamtopics.
Thisbookisnodifferentfromourpreviousbooksinoneotherimportantaspect:Itisa
one-stopguide,providingamixtureoftheoryandpracticethatenablesreaderstoprepare
fortheexam.ItcanbeusedtolearnJavaandtopreparefortheexam.Aftertheexamis
passed,itcanalsocomeinhandyasalanguageguide.
Apartfromincludingcoverageofthenewtopics,ourdiscussionsofnumeroustopicsfrom
thepreviousexamwereextensivelyrevised.Allelementsfoundinourpreviousbooks
(e.g.,sections,examples,figures,tables,reviewquestions,mockexamquestions)were
closelyscrutinized.Newexamples,figures,tables,andreviewquestionswerespecifically
createdforthenewtopicsaswellasfortherevisedones.WecontinuetouseUML
(UnifiedModelingLanguage)extensivelytoillustrateconceptsandlanguageconstructs,
andallnumberedexamplescontinuetobecompleteJavaprogramsreadyfor
experimenting.
Feedbackfromreadersregardingourpreviousbookswasinvaluableinshapingthisbook.
Everyquestion,suggestion,andcommentreceivedwasdeliberatedupon.Wearegrateful
foreverysingleemailwehavereceivedovertheyears;thatinputprovedinvaluablein
improvingthisbook.
DearReader,wewishyouallthebestshouldyoudecidetogodownthepathofJava
certification.Mayyourloopsterminateandyourexceptionsgetcaught!
AboutThisBook
ThisbookprovidesextensivecoverageofthecorefeaturesoftheJavaprogramming
languageanditscoreapplicationprogramminginterface(API),withparticularemphasis
onitssyntaxandusage.Thebookisprimarilyintendedforprofessionalswhowantto
preparefortheJavaSE8ProgrammerIexam,butitisreadilyaccessibletoany
programmerwhowantstomasterthelanguage.Forbothpurposes,itprovidesin-depth
coverageofessentialfeaturesofthelanguageanditscoreAPI.
Thedemandforwell-trainedandhighlyskilledJavaprogrammersremainsunabated.
OracleoffersmanyJavacertificationsthatprofessionalscantaketovalidatetheirskills
(seehttp://education.oracle.com).Thecertificationprovidesmembersofthe
ITindustrywithastandardtousewhenhiringsuchprofessionals,anditallows
professionalstoturntheirJavaskillsintocredentialsthatareimportantforcareer
advancement.
ThebookprovidesextensivecoverageofalltheobjectivesdefinedbyOraclefortheJava
SE8ProgrammerIexam.Theexamobjectivesareselective,however,anddonotinclude
manyoftheessentialfeaturesofJava.Thisbookcoversmanyadditionaltopicsthatevery
Javaprogrammershouldmastertobetrulyproficient.Inthisregard,thebookisa
comprehensiveprimerforlearningtheJavaprogramminglanguage.Aftermasteringthe
languagebyworkingthroughthisbook,thereadercanconfidentlysitfortheexam.
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ThisbookisnotacompletereferenceforJava,asitdoesnotattempttolisteverymember
ofeveryclassfromtheJavaSEplatformAPIdocumentation.Thepurposeisnotto
documenttheJavaSEplatformAPI.TheemphasisismoreontheJavaprogramming
languagefeatures—theirsyntaxandcorrectusagethroughcodeexamples—andlesson
teachingprogrammingtechniques.
Thebookassumeslittlebackgroundinprogramming.Webelievetheexamisaccessibleto
anyprogrammerwhoworksthroughthebook.AJavaprogrammercaneasilyskipover
materialthatiswellunderstoodandconcentrateonpartsthatneedreinforcing,whereasa
programmernewtoJavawillfindtheconceptsexplainedfrombasicprinciples.
Eachtopicisexplainedanddiscussedthoroughlywithexamples,andbackedbyreview
questionsandexercisestoreinforcetheconcepts.Thebookisnotbiasedtowardany
particularplatform,butprovidesplatform-specificdetailswherenecessary.
UsingThisBook
Thereadercanchoosealinearoranonlinearroutethroughthebook,dependingonhisor
herprogrammingbackground.Non-JavaprogrammerswishingtomigratetoJavacanread
Chapter1,whichprovidesashortintroductiontoobject-orientedprogrammingconcepts,
andtheprocedureforcompilingandrunningJavaapplications.Forthosepreparingfor
JavaSE8ProgrammerIexam,thebookhasaseparateappendix(AppendixA)providing
allthepertinentinformationonpreparingforandtakingtheexam.
Cross-referencesareprovidedwherenecessarytoindicatetherelationshipsamongthe
variousconstructsofthelanguage.Tounderstandalanguageconstruct,allpertinent
detailsareprovidedwheretheconstructiscovered,butinaddition,cross-referencesare
providedtoindicateitsrelationshiptootherconstructs.Sometimesitisnecessaryto
postponediscussionofcertainaspectsofatopiciftheydependonconceptsthathavenot
yetbeencoveredinthebook.Atypicalexampleistheconsequencesofobject-oriented
programmingconcepts(forexample,inheritance)onthememberdeclarationsthatcan
occurinaclass.Thisapproachcanresultinforwardreferencesintheinitialchaptersof
thebook.
Thetableofcontents;listingsoftables,examples,andfigures;andacomprehensiveindex
facilitatelocatingtopicsdiscussedinthebook.
Inparticular,wedrawattentiontothefollowingfeaturesofthebook:
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ProgrammerIExamObjectives
0.1Examobjectivesarestatedclearlyatthebeginningofeverychapter.
0.2Thenumberinfrontoftheobjectiveidentifiestheexamobjective,asdefined
byOracle,andcanbefoundinAppendixB.
0.3Theobjectivesareorganizedintomajorsections,detailingthecurriculumfor
theexam.
0.4TheobjectivesfortheJavaSE8ProgrammerIexamarereproducedverbatim
inAppendixB,whereforeachsectionofthesyllabus,referencesareincludedto
pointthereadertorelevanttopicsinthebook.
SupplementaryObjectives
•Supplementaryobjectivescovertopicsthatarenotontheexam,butwhichwe
believeareimportantformasteringthetopicsthatareontheexam.
•Anysupplementaryobjectiveislistedasabulletatthebeginningofthechapter.
ReviewQuestions
Reviewquestionsareprovidedaftereverymajortopictotestandreinforcethematerial.
Thereviewquestionspredominantlyreflectthekindofmultiple-choicequestionsthatcan
beaskedontheactualexam.Ontheexam,theexactnumberofanswerstochooseforeach
questionisexplicitlystated.Thereviewquestionsinthisbookfollowthatpractice.
Manyquestionsontheactualexamcontaincodesnippetswithlinenumberstoindicate
thatcompleteimplementationisnotprovided,andthatthenecessarymissingcodeto
compileandrunthecodesnippetscanbeassumed.Thereviewquestionsinthisbook
providecompletecodeimplementationswherepossible,sothatthecodecanbereadily
compiledandrun.
AnnotatedanswerstothereviewquestionsareprovidedinAppendixC.
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Example0.1ExampleSourceCode
Weencouragereaderstoexperimentwiththecodeexamplestoreinforcethematerialfrom
thebook.Theseexamplescanbedownloadedfromthebookwebsite(seep.xxxiv).
Javacodeispresentedinamonospacedfont.Linesofcodeintheexamplesorin
codesnippetsarereferencedinthetextbyanumber,whichisspecifiedbyusingasingle-
linecommentinthecode.Forexample,inthefollowingcodesnippet,thecalltothe
methoddoSomethingInteresting()at(1)doessomethinginteresting:
Clickheretoviewcodeimage
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Namesofclassesandinterfacesstartwithanuppercaseletter.Namesofpackages,
variables,andmethodsstartwithalowercaseletter.Constantsareinalluppercaseletters.
InterfacenamesbeginwiththeprefixI,whenitmakessensetodistinguishthemfrom
classnames.Codingconventionsarefollowed,exceptwhenwehavehadtodeviatefrom
theseconventionsintheinterestofspaceorclarity.
ChapterSummary
Eachchapterconcludeswithasummaryofthetopicscoveredinthechapter,pointingout
themajorconceptsthatwereintroduced.
ProgrammingExercises
Programmingexercisesattheendofeachchapterprovidetheopportunitytoputconcepts
intopractice.SolutionstotheprogrammingexercisesareprovidedinAppendixD.
MockExam
ThemockexaminAppendixEshouldbeattemptedwhenthereaderfeelsconfidentabout
thetopicsontheexam.ItishighlyrecommendedtoreadAppendixAbeforeattempting
themockexam,asAppendixAcontainspertinentinformationaboutthequestionsto
expectontheactualexam.Eachmultiple-choicequestioninthemockexamexplicitly
stateshowmanyanswersareapplicableforagivenquestion,asisthecaseontheactual
exam.AnnotatedanswerstothequestionsinthemockexamareprovidedinAppendixF.
JavaSEPlatformAPIDocumentation
Averticalgraybarisusedtohighlightmethodsandfieldsfoundintheclassesof
theJavaSEPlatformAPI.
AnyexplanationfollowingtheAPIinformationisalsosimilarlyhighlighted.
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ToobtainthemaximumbenefitfromusingthisbookinpreparingfortheJavaSE8
ProgrammerIexam,westronglyrecommendinstallingthelatestversion(Release8or
newer)oftheJDKanditsaccompanyingAPIdocumentation.Thebookfocusessolelyon
Java8,anddoesnotacknowledgepreviousversions.
BookWebsite
Thisbookisbackedbyawebsiteprovidingauxiliarymaterial:
www.ii.uib.no/~khalid/ocajp8/
Thecontentsofthewebsiteincludethefollowing:
•Sourcecodeforalltheexamplesinthebook
•Solutionstotheprogrammingexercisesinthebook
•Annotatedanswerstothereviewsquestionsinthebook
•Annotatedanswerstothemockexaminthebook
•Tableofcontents,samplechapter,andindexfromthebook
•Errataforthebook
•LinkstomiscellaneousJavaresources(e.g.,certification,discussiongroups,tools)
InformationabouttheJavaStandardEdition(SE)anditsdocumentationcanbefoundat
thefollowingwebsite:
www.oracle.com/technetwork/java/javase/overview/index.html
ThecurrentauthoritativetechnicalreferencefortheJavaprogramminglanguage,The
Java®LanguageSpecification:JavaSE8Edition(alsopublishedbyAddison-Wesley),
canbefoundatthiswebsite:
http://docs.oracle.com/javase/specs/index.html
RequestforFeedback
Considerableefforthasbeenmadetoensuretheaccuracyofthecontentofthisbook.All
codeexamples(includingcodefragments)havebeencompiledandtestedonvarious
platforms.Inthefinalanalysis,anyerrorsremainingarethesoleresponsibilityofthe
authors.
Anyquestions,comments,suggestions,andcorrectionsarewelcome.Letusknow
whetherthebookwashelpful(ornot)foryourpurpose.Anyfeedbackisvaluable.The
principalauthorcanbereachedatthefollowingemailaddress:
khalid.mughal@uib.no
RegisteryourcopyofAProgrammer’sGuidetoJava®SE8OracleCertifiedAssociate
(OCA)atinformit.comforconvenientaccesstodownloads,updates,andcorrectionsas
theybecomeavailable.Tostarttheregistrationprocess,gotoinformit.com/registerand
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AbouttheAuthors
KhalidA.Mughal
KhalidA.MughalisanassociateprofessorattheDepartmentofInformaticsatthe
UniversityofBergen,Norway,wherehehasbeenresponsiblefordesigningand
implementingvariouscoursesininformatics.Overtheyears,hehastaughtprogramming
(primarilyJava),softwareengineering(object-orientedsystemdevelopment),databases
(datamodelinganddatabasemanagementsystems),compilertechniques,webapplication
development,andsoftwaresecuritycourses.For15years,hewasresponsiblefor
developingandrunningweb-basedprogrammingcoursesinJava,whichwereofferedto
off-campusstudents.Hehasalsogivennumerouscoursesandseminarsatvariouslevels
inobject-orientedprogrammingandsystemdevelopmentusingJavaandJava-related
technologies,bothattheUniversityofBergenandfortheITindustry.
Mughalistheprincipalauthorandsolelyresponsibleforthecontentsofthisbook.Heis
alsotheprincipalauthorofthreebooksonpreviousversionsoftheJavaprogrammer
certification—AProgrammer’sGuidetoJava™SCJPCertification:AComprehensive
Primer,ThirdEdition(0321556054);AProgrammer’sGuidetoJava™Certification:A
ComprehensivePrimer,SecondEdition(0201728281);andAProgrammer’sGuideto
Java™Certification(0201596148)—andthreeintroductorytextbooksonprogrammingin
Java:JavaActually:AFirstCourseinProgramming(1844804186);JavaActually:A
ComprehensivePrimerinJavaProgramming(1844809331);andJavasomførste
programmeringsspråk/JavaasFirstProgrammingLanguage,ThirdEdition
(8202245540).
Mughalcurrentlyworksonsecurityissuesrelatedtomobiledatacollectionsystemsfor
deliveringhealthservicesinlow-andmiddle-incomecountries.
RolfW.Rasmussen
RolfW.RasmussenisasystemdevelopmentmanageratVizrt,acompanythatdevelops
solutionsfortheTVbroadcastindustry,includingreal-time3Dgraphicrenderers,and
contentandcontrolsystems.Rasmussenworksmainlyoncontrolandautomationsystems,
videoprocessing,typography,andreal-timevisualization.Hehasworkedonclean-room
implementationsoftheJavaclasslibrariesinthepastandisacontributortotheFree
SoftwareFoundation.
Overtheyears,Rasmussenhasworkedbothacademicallyandprofessionallywith
numerousprogramminglanguages,includingJava.Hewasprimarilyresponsiblefor
developingthereviewquestionsandanswers,theprogrammingexercisesandtheir
solutions,themockexam,andallthepracticalaspectsrelatedtotakingtheexaminour
threepreviousbooksonJavaprogrammercertification.Selectedearliercontenthasbeen
utilizedinthisbook.TogetherwithMughal,heisalsoaco-authorofthreeintroductory
textbooksonprogramminginJava.
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Acknowledgments
AtAddison-Wesley,GregDoenchwasagainoureditor,whoeffectivelymanagedthe
processofpublishingthisbook.Regulardialogwithhiminrecentmonthshelpedtokeep
thisprojectontrack.JulieNahilwasthein-housecontactatAddison-Wesley,who
professionallymanagedtheproductionofthebook.AnnaPopickwastheprojecteditor,
whodiligentlyhandledtheday-to-dayprojectmanagementforthisbook.JillHobbsdida
trulymarvelousjobcopyeditingthebook.ThefolksatTheCIPGroupperformedthe
typesettingwizardrynecessarytomaterializethebook.Wewouldliketoextendour
sincerethankstoGreg,Julie,Anna,Jill,thefolksatTheCIPGroup,andallthosebehind
thescenesatAddison-Wesley,whohelpedtoputthispublicationonthebookshelf.
Forthetechnicalreviewofthebook,wewereluckythatRoelDeNijsagreedtotakeon
thetask.IfyoudropinonCodeRanch.com,youareboundtofindhimexecutinghis
dutiesasaSheriff,especiallyhelpinggreenhornsfindtheirbearingintheJava
certificationcorrals.HeisafreelanceJavadeveloperwithmanyITcompaniesasclients
andamultitudeofJavacertificationaccoladesunderhisbelt(SCJA,SCJP,SCJD,
OCAJP7).Andnotleast,heisaTechnicalReviewerParExcellence.Withoutdoubt,Roel
hasameticulouseyefordetail.Itisnoexaggerationtosaythathisexhaustivefeedback
hasbeeninvaluableinimprovingthequalityofthisbookatalllevels.Roel,youhaveour
mostsincerethanksforyourmanyexcellentcommentsandsuggestions,andaboveall,for
weedingoutnumerouspeskyerrorsinthemanuscript.
Overtheyears,wehavealsobeenluckytohaveourownpersonalmanuscriptquality
controller:MaritSeljeflotMughal.Asdiligentlyaswithourpreviousbooks,shetirelessly
proofreadseveralchapterdraftsforthisbook,andputherfingeronmanyunmentionable
mistakesanderrorsinthemanuscript.Hervaluablecommentsandsuggestionshavealso
beeninstrumentalinimprovingthequalityofthisbook.IfMarit,whohasnoIT
background,couldmakesenseoftheJavajargonwewrote,thenwewereconfidentour
readerswouldaswell.Ourmostsincerethanks.
Greatefforthasbeenmadetoeliminatemistakesanderrorsinthisbook.Weacceptfull
responsibilityforanyremainingoversights.WehopethatwhenourDearReadersfind
any,theywillbringthemtoourattention.
Manyfamilyoccasionshavebeenmissedwhileworkingonthisbook.Withoutfamily
support,thisbookwouldnothaveseenthelightofday.Khalidisevergratefultohis
familyfortheirlove,support,andunderstanding—butespeciallywhenheisworkingona
book.Nowthatthisbookisoutthedoor,heisofftoplaywithhisthreegrandchildren.
—KhalidA.Mughal
17May2016
Bergen,Norway
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1.BasicsofJavaProgramming
1.1Introduction
BeforeembarkingontheroadtoJavaprogrammercertification,itisimportantto
understandthebasicterminologyandconceptsinobject-orientedprogramming(OOP).In
thischapter,theemphasisisonprovidinganintroductiontoOOP,ratherthanexhaustive
coverage.In-depthcoverageoftheconceptsfollowsinsubsequentchaptersofthebook.
Javasupportsthewritingofmanydifferentkindsofexecutables:applications,applets,and
servlets.ThebasicelementsofaJavaapplicationareintroducedinthischapter.Theold
adagethatpracticemakesperfectiscertainlytruewhenlearningaprogramminglanguage.
Toencourageprogrammingonthecomputer,themechanicsofcompilingandrunninga
Javaapplicationareoutlined.
1.2Classes
Oneofthefundamentalwaysinwhichwehandlecomplexityisbyusingabstractions.An
abstractiondenotestheessentialpropertiesandbehaviorsofanobjectthatdifferentiateit
fromotherobjects.TheessenceofOOPismodelingabstractions,usingclassesand
objects.Thehardpartofthisendeavorisfindingtherightabstraction.
Aclassdenotesacategoryofobjects,andactsasablueprintforcreatingobjects.Aclass
modelsanabstractionbydefiningthepropertiesandbehaviorsfortheobjectsrepresenting
theabstraction.Anobjectexhibitsthepropertiesandbehaviorsdefinedbyitsclass.The
propertiesofanobjectofaclassarealsocalledattributes,andaredefinedbyfieldsin
Java.Afieldinaclassisavariablethatcanstoreavaluethatrepresentsaparticular
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propertyofanobject.Thebehaviorsofanobjectofaclassarealsoknownasoperations,
andaredefinedusingmethodsinJava.Fieldsandmethodsinaclassdeclarationare
collectivelycalledmembers.
Animportantdistinctionismadebetweenthecontractandtheimplementationthataclass
providesforitsobjects.Thecontractdefineswhichservicesareprovided,andthe
implementationdefineshowtheseservicesareprovidedbytheclass.Clients(i.e.,other
objects)needtoknowonlythecontractofanobject,andnotitsimplementation,toavail
themselvesoftheobject’sservices.
Asanexample,wewillimplementdifferentversionsofaclassthatmodelstheabstraction
ofastackthatcanpushandpopcharacters.Thestackwilluseanarrayofcharactersto
storethecharacters,andafieldtoindicatethetopelementinthestack.UsingUnified
ModelingLanguage(UML)notation,aclasscalledCharStackisgraphicallydepicted
inFigure1.1,whichmodelstheabstraction.Bothfieldsandmethodnamesareshownin
Figure1.1a.
Figure1.1UMLNotationforClasses
DeclaringMembers:FieldsandMethods
Example1.1showsthedeclarationoftheclassCharStackdepictedinFigure1.1.Its
intentionistoillustratethesalientfeaturesofaclassdeclarationinJava,ratherthanan
effectiveimplementationofstacks.Thecharactersequence//inthecodeindicatesthe
startofasingle-linecommentthatcanbeusedtodocumentthecode.Allcharactersafter
thissequenceandtotheendofthelineareignoredbythecompiler.
Aclassdeclarationcontainsmemberdeclarationsthatdefinethefieldsandthemethodsof
theobjectstheclassrepresents.InthecaseoftheclassCharStack,ithastwofields
declaredat(1):
•stackArray,whichisanarraytoholdtheelementsofthestack(inthiscase,
characters)
•topOfStack,whichdenotesthetopelementofthestack(i.e.,theindexofthelast
characterstoredinthearray)
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TheclassCharStackhasfivemethods,declaredat(3),thatimplementtheessential
operationsonastack:
•push()pushesacharacterontothestack.
•pop()removesandreturnsthetopelementofthestack.
•peek()returnsthetopelementofthestackforinspection.
•isEmpty()determineswhetherthestackisempty.
•isFull()determineswhetherthestackisfull.
Theclassdeclarationalsohasamethod-likedeclarationat(2)withthesamenameasthe
class.Suchdeclarationsarecalledconstructors.Asweshallsee,aconstructorisexecuted
whenanobjectiscreatedfromtheclass.However,theimplementationdetailsinthe
examplearenotimportantforthepresentdiscussion.
Example1.1BasicElementsofaClassDeclaration
Clickheretoviewcodeimage
//File:CharStack.java
publicclassCharStack{//Classname
//ClassDeclarations:
//Fields:(1)
privatechar[]stackArray;//Thearrayimplementingthestack
privateinttopOfStack;//Thetopofthestack
//Constructor:(2)
publicCharStack(intcapacity){
stackArray=newchar[capacity];
topOfStack=-1;
}
//Methods:(3)
publicvoidpush(charelement){stackArray[++topOfStack]=element;}
publiccharpop(){returnstackArray[topOfStack—];}
publiccharpeek(){returnstackArray[topOfStack];}
publicbooleanisEmpty(){returntopOfStack==-1;}
publicbooleanisFull(){returntopOfStack==stackArray.length-
1;}
}
1.3Objects
ClassInstantiation,ReferenceValues,andReferences
Theprocessofcreatingobjectsfromaclassiscalledinstantiation.Anobjectisan
instanceofaclass.Theobjectisconstructedusingtheclassasablueprintandisa
concreteinstanceoftheabstractionthattheclassrepresents.Anobjectmustbecreated
beforeitcanbeusedinaprogram.
Areferencevalueisreturnedwhenanobjectiscreated.Areferencevaluedenotesa
particularobject.Avariabledenotesalocationinmemorywhereavaluecanbestored.
Anobjectreference(orsimplyreference)isavariablethatcanstoreareferencevalue.
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Thusareferenceprovidesahandletoanobject,asitcanindirectlydenoteanobjectwhose
referencevalueitholds.InJava,anobjectcanbemanipulatedonlyviaitsreferencevalue,
orequivalentlybyareferencethatholdsitsreferencevalue.
Thissetupformanipulatingobjectsrequiresthatareferencebedeclared,aclassbe
instantiatedtocreateanobject,andthereferencevalueoftheobjectcreatedbestoredin
thereference.Thesestepsareaccomplishedbyadeclarationstatement.
Clickheretoviewcodeimage
CharStackstack1=newCharStack(10);//Stacklength:10chars
Intheprecedingdeclarationstatement,theleft-handsideofthe=operatordeclaresthat
stack1isareferenceofclassCharStack.Thereferencestack1,therefore,canrefer
toobjectsofclassCharStack.
Theright-handsideofthe=operatorcreatesanobjectofclassCharStack.Thisstep
involvesusingthenewoperatorinconjunctionwithacalltoaconstructoroftheclass
(newCharStack(10)).ThenewoperatorcreatesaninstanceoftheCharStack
classandreturnsthereferencevalueofthisinstance.The=operator(calledthe
assignmentoperator)storesthereferencevalueinthereferencestack1declaredonthe
left-handsideoftheassignmentoperator.Thereferencestack1cannowbeusedto
manipulatetheobjectwhosereferencevalueisstoredinit.
Analogously,thefollowingdeclarationstatementdeclaresthereferencestack2tobeof
classCharStack,createsanobjectofclassCharStack,andassignsitsreferencevalue
tothereferencestack2:
Clickheretoviewcodeimage
CharStackstack2=newCharStack(5);//Stacklength:5chars
Eachobjectthatiscreatedhasitsowncopyofthefieldsdeclaredintheclassdeclaration
inExample1.1.Thatis,thetwostackobjects,referencedbystack1andstack2,will
havetheirownstackArrayandtopOfStackfields.
Thepurposeoftheconstructorcallontheright-handsideofthenewoperatoristo
initializethenewlycreatedobject.Inthisparticularcase,foreachnewCharStack
objectcreatedusingthenewoperator,theconstructorat(2)inExample1.1createsan
arrayofcharacters.Thelengthofthisarrayisgivenbythevalueoftheargumenttothe
constructor.TheconstructoralsoinitializesthetopOfStackfield.
Figure1.2showstheUMLnotationforobjects.Thegraphicalrepresentationofanobject
isverysimilartothatofaclass.Figure1.2showsthecanonicalnotation,wherethename
ofthereferencedenotingtheobjectisprefixedtotheclassnamewithacolon(:).Ifthe
nameofthereferenceisomitted,asinFigure1.2b,thisdenotesananonymousobject.
SinceobjectsinJavadonothavenames,butratheraredenotedbyreferences,amore
elaboratenotationisshowninFigure1.2c,wherereferencesoftheCharStackclass
explicitlyrefertoCharStackobjects.Inmostcases,themorecompactnotationwill
suffice.
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Figure1.2UMLNotationforObjects
ObjectAliases
Severalreferencescanrefertothesameobject,meaningthattheystorethereferencevalue
ofthesameobject.Suchreferencesarecalledaliases.Theobjectcanbemanipulatedvia
anyoneofitsaliases,aseachonereferstothesameobject.
Clickheretoviewcodeimage
//Createtwodistinctstacksofchars.
CharStackstackA=newCharStack(12);//Stacklength:12chars
CharStackstackB=newCharStack(6);//Stacklength:6chars
stackB=stackA;//(1)aliasesafterassignment
//ThestackpreviouslyreferencedbystackBcannowbegarbagecollected.
Twostackobjectsarecreatedintheprecedingcode.Beforetheassignmentat(1),the
situationisasdepictedinFigure1.3a.Aftertheassignmentat(1),thereferencesstackA
andstackBwilldenotethesamestack,asdepictedinFigure1.3b.Thereferencevalue
instackAisassignedtostackB.ThereferencesstackAandstackBarealiases
aftertheassignment,astheyrefertothesameobject.Whathappenstothestackobject
thatwasdenotedbythereferencestackBbeforetheassignment?Whenobjectsareno
longerinuse,theirmemoryis,ifnecessary,reclaimedandreallocatedforotherobjects.
Thisprocessiscalledautomaticgarbagecollection.GarbagecollectioninJavaistaken
careofbytheruntimeenvironment.
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Figure1.3Aliases
1.4InstanceMembers
Eachobjectcreatedwillhaveitsowncopiesofthefieldsdefinedinitsclass.Thefieldsof
anobjectarecalledinstancevariables.Thevaluesoftheinstancevariablesinanobject
constituteitsstate.Twodistinctobjectscanhavethesamestateiftheirinstancevariables
havethesamevalues.Themethodsofanobjectdefineitsbehavior;suchmethodsare
calledinstancemethods.Itisimportanttonotethatthesemethodspertaintoeachobjectof
theclass.Incontrast,theimplementationofthemethodsissharedbyallinstancesofthe
class.Instancevariablesandinstancemethods,whichbelongtoobjects,arecollectively
calledinstancemembers,todistinguishthemfromstaticmembers,whichbelongtothe
classonly.Staticmembersarediscussedin§1.5.
InvokingMethods
Objectscommunicatebymessagepassing.Asaconsequence,anobjectcanbemadeto
exhibitaparticularbehaviorbysendingtheappropriatemessagetotheobject.InJava,this
isdonebycallingamethodontheobjectusingthebinarydot(.)operator.Amethodcall
spellsoutthecompletemessage:theobjectthatisthereceiverofthemessage,themethod
tobeinvoked,andtheargumentstobepassedtothemethod,ifany.Themethodinvoked
onthereceivercanalsosendinformationbacktothesender,viaasinglereturnvalue.The
methodcalledmustbeonethatisdefinedfortheobject;otherwise,thecompilerreports
anerror.
Clickheretoviewcodeimage
CharStackstack=newCharStack(5);//Createastack
stack.push(‘J’);//(1)Character‘J’pushed
charc=stack.pop();//(2)Onecharacterpoppedandreturned:‘J’
stack.printStackElements();//(3)Compile-timeerror:Nosuchmethodin
CharStack
Thesamplecodegivenhereinvokesmethodsontheobjectdenotedbythereference
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stack.Themethodcallat(1)pushesonecharacteronthestack,andthemethodcallat
(2)popsonecharacteroffthestack.Boththepush()andpop()methodsaredefinedin
theclassCharStack.Thepush()methoddoesnotreturnanyvalue,butthepop()
methodreturnsthecharacterpopped.Tryingtoinvokeamethodnamed
printStackElementsonthestackresultsinacompile-timeerror,asnosuchmethod
isdefinedintheclassCharStack.
Thedot(.)notationcanalsobeusedwithareferencetoaccessthefieldsofanobject.
Useofthedotnotationisgovernedbytheaccessibilityofthemember.Thefieldsinthe
classCharStackhaveprivateaccessibility,indicatingthattheyarenotaccessible
fromoutsidetheclass.ThusthefollowingcodeinaclientoftheCharStackclasswill
notcompile:
Clickheretoviewcodeimage
stack.topOfStack++;//Compile-timeerror:topOfStackisnotvisible.
1.5StaticMembers
Insomecases,certainmembersshouldbelongonlytotheclass;thatis,theyshouldnotbe
partofanyinstanceoftheclass.Asanexample,supposeaclasswantstokeeptrackof
howmanyobjectsoftheclasshavebeencreated.Definingacounterasaninstance
variableintheclassdeclarationfortrackingthenumberofobjectscreateddoesnotsolve
theproblem.Eachobjectcreatedwillhaveitsowncounterfield.Whichcountershould
thenbeupdated?Thesolutionistodeclarethecounterfieldasbeingstatic.Sucha
fieldiscalledastaticvariable.Itbelongstotheclass,ratherthantoanyspecificobjectof
theclass.Astaticvariableisinitializedwhentheclassisloadedatruntime.Similarly,a
classcanhavestaticmethodsthatbelongtotheclass,ratherthantoanyspecificobjectsof
theclass.Staticvariablesandstaticmethodsarecollectivelyknownasstaticmembers,
andaredeclaredwiththekeywordstatic.
Figure1.4showstheclassdiagramfortheclassCharStack.Ithasbeenaugmentedby
twostaticmembers,whosenamesareunderlined.Theaugmenteddefinitionofthe
CharStackclassisgiveninExample1.2.Thefieldcounterisastaticvariable
declaredat(1).Itwillbeallocatedandinitializedtothedefaultvalue0whentheclassis
loaded.EachtimeanobjectoftheCharStackclassiscreated,theconstructorat(2)is
executed.Theconstructorexplicitlyincrementsthecounterintheclass.Themethod
getInstanceCount()at(3)isastaticmethodbelongingtotheclass.Itreturnsthe
countervaluewhencalled.
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Figure1.4ClassDiagramShowingStaticMembersofaClass
Example1.2StaticMembersinClassDeclaration
Clickheretoviewcodeimage
//File:CharStack.java
publicclassCharStack{
//Instancevariables:
privatechar[]stackArray;//Thearrayimplementingthestack
privateinttopOfStack;//Thetopofthestack
//Staticvariable
privatestaticintcounter;//(1)
//Constructornowincrementsthecounterforeachobjectcreated.
publicCharStack(intcapacity){//(2)
stackArray=newchar[capacity];
topOfStack=-1;
counter++;
}
//Instancemethods:
publicvoidpush(charelement){stackArray[++topOfStack]=element;}
publiccharpop(){returnstackArray[topOfStack—];}
publiccharpeek(){returnstackArray[topOfStack];}
publicbooleanisEmpty(){returntopOfStack==-1;}
publicbooleanisFull(){returntopOfStack==stackArray.length-
1;}
//Staticmethod(3)
publicstaticintgetInstanceCount(){returncounter;}
}
Figure1.5showstheclassificationofthemembersintheclassCharStack,usingthe
terminologywehaveintroducedsofar.Table1.1providesasummaryoftheterminology
usedindefiningmembersofaclass.
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Figure1.5MembersofaClass
Table1.1TerminologyforClassMembers
Clientscanaccessstaticmembersintheclassbyusingtheclassname.Thefollowingcode
invokesthegetInstanceCount()methodintheclassCharStack:
Clickheretoviewcodeimage
intcount=CharStack.getInstanceCount();//Classnametoinvokestatic
method
Staticmemberscanalsobeaccessedviaobjectreferences,althoughdoingsoisconsidered
badstyle:
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Clickheretoviewcodeimage
CharStackmyStack=newCharStack(20);
intcount=myStack.getInstanceCount();//Referenceinvokesstaticmethod
Staticmembersinaclasscanbeaccessedbothbytheclassnameandviaobject
references,butinstancememberscanbeaccessedonlybyobjectreferences.
1.6Inheritance
Therearetwofundamentalmechanismsforbuildingnewclassesfromexistingones:
inheritanceandaggregation.ItmakessensetoinheritfromanexistingclassVehicleto
defineaclassCar,sinceacarisavehicle.TheclassVehiclehasseveralparts;
therefore,itmakessensetodefineacompositeobjectoftheclassVehiclethathas
constituentobjectsofsuchclassesasEngine,Axle,andGearBox,whichmakeupa
vehicle.
Inheritanceisillustratedherebyanexamplethatimplementsastackofcharactersthatcan
printitselementsontheterminal.Thisnewstackhasallthepropertiesandbehaviorsof
theCharStackclass,alongwiththeadditionalcapabilityofprintingitselements.Given
thatthisprintablestackisastackofcharacters,itcanbederivedfromtheCharStack
class.ThisrelationshipisshowninFigure1.6.TheclassPrintableCharStackis
calledthesubclass,andtheclassCharStackiscalledthesuperclass.TheCharStack
classisageneralizationforallstacksofcharacters,whereastheclass
PrintableCharStackisaspecializationofstacksofcharactersthatcanalsoprint
theirelements.
Figure1.6ClassDiagramDepictingInheritanceRelationship
InJava,derivinganewclassfromanexistingclassrequirestheuseoftheextends
clauseinthesubclassdeclaration.Asubclasscanextendonlyonesuperclass.Thesubclass
caninheritmembersofthesuperclass.Thefollowingcodefragmentimplementsthe
PrintableCharStackclass:
Clickheretoviewcodeimage
classPrintableCharStackextendsCharStack{//(1)
//Instancemethod
publicvoidprintStackElements(){//(2)
//…implementationofthemethod…
}
//Theconstructorcallstheconstructorofthesuperclassexplicitly.
publicPrintableCharStack(intcapacity){super(capacity);}//(3)
}
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ThePrintableCharStackclassextendstheCharStackclassat(1).Implementing
theprintStackElements()methodinthePrintableCharStackclassrequires
accesstothefieldstackArrayfromthesuperclassCharStack.However,thisfieldis
privateand,therefore,notaccessibleinthesubclass.Thesubclasscanaccessthesefields
iftheaccessibilityofthefieldsischangedtoprotectedintheCharStackclass.Example
1.3usesaversionoftheclassCharStack,whichhasbeenmodifiedtosupportthis
access.ImplementationoftheprintStackElements()methodisshownat(2).The
constructorofthePrintableCharStackclassat(3)callstheconstructorofthe
superclassCharStacktoinitializethestackproperly.
Example1.3DefiningaSubclass
Clickheretoviewcodeimage
//File:CharStack.java
publicclassCharStack{
//Instancevariables
protectedchar[]stackArray;//Thearraythatimplementsthestack
protectedinttopOfStack;//Thetopofthestack
//TherestofthedefinitionisthesameasinExample1.2.
}
//File:PrintableCharStack.java
publicclassPrintableCharStackextendsCharStack{//(1)
//Instancemethod
publicvoidprintStackElements(){//(2)
for(inti=0;i<=topOfStack;i++)
System.out.print(stackArray[i]);//Printeachcharonterminal
System.out.println();
}
//Constructorcallstheconstructorofthesuperclassexplicitly.
PrintableCharStack(intcapacity){super(capacity);}//(3)
}
ObjectsofthePrintableCharStackclasswillrespondjustliketheobjectsofthe
CharStackclass,buttheyalsohavetheadditionalfunctionalitydefinedinthesubclass:
Clickheretoviewcodeimage
PrintableCharStackpcStack=newPrintableCharStack(3);
pcStack.push(‘H’);
pcStack.push(‘i’);
pcStack.push(‘!’);
pcStack.printStackElements();//Prints“Hi!”ontheterminal
1.7Associations:AggregationandComposition
Anassociationdefinesastaticrelationshipbetweenobjectsoftwoclasses.Onesuch
association,calledaggregation,expresseshowanobjectusesotherobjects.Javasupports
aggregationofobjectsbyreference,sinceobjectscannotcontainotherobjectsexplicitly.
Theaggregateobjectusuallyhasfieldsthatdenoteitsconstituentobjects.Aconstituent
objectcanbesharedwithotheraggregateobjects.
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Forexample,anobjectofclassAirplanemighthaveafieldthatdenotesanobjectof
classPilot.ThisPilotobjectofanAirplaneobjectmightbesharedamongother
aggregateobjects(notnecessarilyAirplaneobjects)oncethepilothasfinisheddutyon
oneairplane.Infact,thePilotobjectcanstillbeusedevenwhenitsAirplaneobject
nolongerexists.ThisaggregationrelationshipisdepictedbytheUMLdiagraminFigure
1.7(emptydiamond),showingthateachobjectoftheAirplaneclasshaszeroorone
objectofclassPilotassociatedwithit.
Figure1.7ClassDiagramDepictingAssociations
Theaggregateassociationcanbemadestrongeriftheconstituentobjectscannotbeshared
withotheraggregateobjects—forexample,anAirplaneobjectwithtwoWingobjects.
TheWingobjectscannotbesharedandcanexistonlywiththeirAirplaneobject;that
is,theAirplaneobjecthasownershipofitsWingobjects.Conversely,theWing
objectsareapartoftheirAirplaneobject.Thisstrongeraggregationassociationis
calledcompositionandisdepictedbytheUMLdiagraminFigure1.7(filleddiamond),
showingthateachobjectoftheAirplaneclassownstwoobjectsofclassWing.
InthecaseoftheCharStackclassusedintheearlierexamples,eachobjectofthisclass
hasafieldtostorethereferencevalueofanarrayobjectthatholdsthecharacters.Itwould
notbeagoodideatosharethisarraywithotherstackobjects.Thestackownsthearrayof
characters.Therelationshipbetweenthestackobjectanditsconstituentarrayobjectcan
beexpressedbycomposition(Figure1.8),showingthateachobjectoftheCharStack
classwillownonearrayobjectoftypecharassociatedwithit.
Figure1.8ClassDiagramDepictingComposition
1.8TenetsofJava
•CodeinJavamustbeencapsulatedinclasses.
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•TherearetwokindsofvaluesinJava:objectsthatareinstancesofclassesorarrays,
andatomicvaluesofprimitivedatatypes.
•Referencesstorereferencevaluesthatdenoteobjects,andareusedtomanipulate
objects.
•ObjectsinJavacannotcontainotherobjects;theycanonlyhavereferencestoother
objects.
•Duringexecution,reclamationofobjectsthatarenolongerinuseismanagedbythe
runtimeenvironment.
ReviewQuestions
1.1Whichstatementistrueaboutmethods?
Selecttheonecorrectanswer.
(a)Amethodisanimplementationofanabstraction.
(b)Amethodisanattributedefiningthepropertyofaparticularabstraction.
(c)Amethodisacategoryofobjects.
(d)Amethodisanoperationdefiningthebehaviorforaparticularabstraction.
(e)Amethodisablueprintformakingoperations.
1.2Whichstatementistrueaboutobjects?
Selecttheonecorrectanswer.
(a)Anobjectiswhatclassesareinstantiatedfrom.
(b)Anobjectisaninstanceofaclass.
(c)Anobjectisablueprintforcreatingconcreterealizationofabstractions.
(d)Anobjectisareference.
(e)Anobjectisavariable.
1.3Whichisthefirstlineofaconstructordeclarationinthefollowingcode?
Clickheretoviewcodeimage
publicclassCounter{//(1)
intcurrent,step;
publicCounter(intstartValue,intstepValue){//(2)
setCurrent(startValue);//(3)
setStep(stepValue);
}
publicintgetCurrent(){returncurrent;}//(4)
publicvoidsetCurrent(intvalue){current=value;}//(5)
publicvoidsetStep(intstepValue){step=stepValue;}//(6)
}
Selecttheonecorrectanswer.
(a)(1)
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(b)(2)
(c)(3)
(d)(4)
(e)(5)
(f)(6)
1.4GiventhatThingisaclass,howmanyobjectsandhowmanyreferencesare
createdbythefollowingcode?
Thingitem,stuff;
item=newThing();
Thingentity=newThing();
Selectthetwocorrectanswers.
(a)Oneobjectiscreated.
(b)Twoobjectsarecreated.
(c)Threeobjectsarecreated.
(d)Onereferenceiscreated.
(e)Tworeferencesarecreated.
(f)Threereferencesarecreated.
1.5Whichstatementistrueaboutinstancemembers?
Selecttheonecorrectanswer.
(a)Aninstancememberisalsocalledastaticmember.
(b)Aninstancememberisalwaysafield.
(c)Aninstancememberisneveramethod.
(d)Aninstancememberbelongstoaninstance,nottotheclassasawhole.
(e)Aninstancememberalwaysrepresentsanoperation.
1.6HowdoobjectscommunicateinJava?
Selecttheonecorrectanswer.
(a)Theycommunicatebymodifyingeachother’sfields.
(b)Theycommunicatebymodifyingthestaticvariablesofeachother’sclasses.
(c)Theycommunicatebycallingeachother’sinstancemethods.
(d)Theycommunicatebycallingstaticmethodsofeachother’sclasses.
1.7Giventhefollowingcode,whichstatementsaretrue?
classA{
protectedintvalue1;
}
classBextendsA{
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intvalue2;
}
Selectthetwocorrectanswers.
(a)ClassAextendsclassB.
(b)ClassBisthesuperclassofclassA.
(c)ClassAinheritsfromclassB.
(d)ClassBisasubclassofclassA.
(e)ObjectsofclassAhaveafieldnamedvalue2.
(f)ObjectsofclassBhaveafieldnamedvalue1.
1.8Giventhefollowingcode,whichstatementsexpressthemostaccurateassociation?
Clickheretoviewcodeimage
classCarriage{}
classTrainDriver{}
classTrain{
privateCarriage[]carriages;
privateTrainDriverdriver;
Train(TrainDrivertrainDriver,intnoOfCarriages){
carriages=newCarriage[noOfCarriages];
driver=trainDriver;
}
voidinsertCarriage(CarriagenewCarriage){/*…*/}
}
Selectthethreecorrectanswers.
(a)ATrainobjecthasanarrayofCarriageobjects.
(b)ATrainobjectownsanarrayofCarriageobjects.
(c)ATrainobjectownsCarriageobjects.
(d)ATrainobjecthasaTrainDriverobject.
(e)ATrainobjectownsaTrainDriverobject.
(f)ATrainDriverobjectispartofaTrainobject.
(g)AnarrayofCarriageobjectsispartofaTrainobject.
(h)CarriageobjectsarepartofaTrainobject.
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1.9JavaPrograms
AJavasourcefilecancontainmorethanoneclassdeclaration.Eachsourcefilenamehas
theextension.java.TheJDK(JavaDevelopmentKit)enforcestherulethatanyclassin
thesourcefilethathaspublicaccessibilitymustbedeclaredinitsownfile,meaning
thatsuchapublicclassmustbedeclaredinasourcefilewhosefilenamecomprisesthe
nameofthispublicclasswith.javaasitsextension.Thisruleimpliesthatasourcefile
cancontainatmostonepublicclass.Ifthesourcefilecontainsapublicclass,thefile
namingruleisenforcedbytheJDK.
Eachclassdeclarationinasourcefileiscompiledintoaseparateclassfile,containing
Javabytecode.Thenameofthisfilecomprisesthenameoftheclasswith.classasits
extension.TheJDKprovidestoolsforcompilingandrunningprograms,asexplainedin
thenextsection.TheclassesintheJavaSEplatformAPIarealreadycompiled,andthe
JDKtoolsknowwheretofindthem.
1.10SampleJavaApplication
Thetermapplicationisjustasynonymforaprogram,referringtosourcecodethatis
compiledanddirectlyexecuted.TocreateanapplicationinJava,theprogrammusthavea
classthatdefinesamethodnamedmain,whichisthestartingpointfortheexecutionof
anyapplication.
EssentialElementsofaJavaApplication
Example1.4isanexampleofanapplicationinwhichaclientusestheCharStackclass
toreverseastringofcharacters.
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Example1.4AnApplication
Clickheretoviewcodeimage
//File:CharStack.java
publicclassCharStack{
//SameasinExample1.2.
}
//File:Client.java
publicclassClient{
publicstaticvoidmain(String[]args){
//Createastack.
CharStackstack=newCharStack(40);
//Createastringtopushonthestack:
Stringstr=“!notisotnuferaskcatS”;
System.out.println(“Originalstring:”+str);//(1)
intlength=str.length();
//Pushthestringcharbycharontothestack:
for(inti=0;i<length;i++){
stack.push(str.charAt(i));
}
System.out.print(“Reversedstring:“);//(2)
//Popandprinteachcharfromthestack:
while(!stack.isEmpty()){//Checkifthestackisnotempty.
System.out.print(stack.pop());
}
System.out.println();//(3)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Originalstring:!notisotnuferaskcatS
Reversedstring:Stacksarefuntositon!
ThepublicclassClientdefinesamethodwiththenamemain.Tostarttheapplication,
themain()methodinthispublicclassisinvokedbytheJavainterpreter,alsocalledthe
JavaVirtualMachine(JVM).Themethodheaderofthismain()methodmustbe
declaredasshowninthefollowingmethodstub:
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args)//Methodheader
{/*Implementation*/}
Themain()methodhaspublicaccessibility—thatis,itisaccessiblefromanyclass.
Thekeywordstaticmeansthemethodbelongstotheclass.Thekeywordvoid
indicatesthatthemethoddoesnotreturnanyvalue.Theparameterargsisanarrayof
stringsthatcanbeusedtopassinformationtothemain()methodwhenexecutionstarts.
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CompilingandRunninganApplication
JavasourcefilescanbecompiledusingtheJavacompilertooljavac,whichispartof
theJDK.
ThesourcefileClient.javacontainsthedeclarationoftheClientclass.Thissource
filecanbecompiledbygivingthefollowingcommandatthecommandline(thecharacter
>isthecommandprompt):
>javacClient.java
ThiscommandcreatestheclassfileClient.classcontainingtheJavabytecodefor
theClientclass.TheClientclassusestheCharStackclass,andifthefile
CharStack.classdoesnotalreadyexist,thecompilerwillalsocompilethesource
fileCharStack.java.
CompiledclassescanbeexecutedbytheJavainterpreterjava,whichisalsopartofthe
JDK.TorunExample1.4,givethefollowingcommandonthecommandline:
Clickheretoviewcodeimage
>javaClient
Originalstring:!notisotnuferaskcatS
Reversedstring:Stacksarefuntositon!
Notethatonlythenameoftheclassisspecified,resultingintheexecutionstartinginthe
main()methodofthespecifiedclass.TheapplicationinExample1.4terminateswhen
theexecutionofthemain()methodiscompleted.
1.11ProgramOutput
Dataproducedbyaprogramiscalledoutput.Thisoutputcanbesenttodifferentdevices.
Theexamplespresentedinthisbooksendtheiroutputtoaterminalwindow,wherethe
outputisprintedaslineofcharacterswithacursorthatadvancesascharactersareprinted.
AJavaprogramcansenditsoutputtotheterminalwindowusinganobjectcalledstandard
out.Thisobject,whichcanbeaccessedusingthepublicstaticfinalfieldoutinthe
Systemclass,isanobjectoftheclassjava.io.PrintStreamthatprovidesmethods
forprintingvalues.Thesemethodsconvertvaluestotheirstringrepresentationandprint
theresultingstring.
Example1.4illustratestheprocessofprintingvaluestotheterminalwindow.The
argumentinthecalltotheprintln()methodat(1)isfirstevaluated,andtheresulting
stringisprintedtotheterminalwindow.Thismethodalwaysterminatesthecurrentline,
whichresultsinthecursorbeingmovedtothebeginningofthenextline:
Clickheretoviewcodeimage
System.out.println(“Originalstring:”+str);//(1)
Theprint()methodat(2)printsitsargumenttotheterminalwindow,butitdoesnot
terminatethecurrentline:
Clickheretoviewcodeimage
System.out.print(“Reversedstring:“);//(2)
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Toterminatealine,withoutprintinganyvalues,wecanusetheno-argumentprintln()
method:
Clickheretoviewcodeimage
System.out.println();//(3)
FormattedOutput
Tohavemorecontroloverhowthevaluesareprinted,wecancreateformattedoutput.The
followingmethodofthejava.io.PrintStreamclasscanbeusedforthispurpose:
Clickheretoviewcodeimage
PrintStreamprintf(Stringformat,Object…args)
TheStringparameterformatspecifieshowformattingwillbedone.Itcontains
formatspecificationsthatdeterminehoweachsubsequentvalueintheparameter
argswillbeformattedandprinted.TheparameterdeclarationObject…args
representsanarrayofzeroormoreargumentstobeformattedandprinted.The
resultingstringfromtheformattingwillbeprintedtothedestinationstream.
(System.outwillprinttothestandardoutobject.)
Anyerrorintheformatstringwillresultinaruntimeexception.
Thefollowingcalltotheprintf()methodonthestandardoutobjectformatsandprints
threevalues:
Clickheretoviewcodeimage
System.out.printf(“Formattedvalues|%5d|%8.3f|%5s|%n”,//Formatstring
2016,Math.PI,“Hi”);//Valuestoformat
Atruntime,thefollowinglineisprintedintheterminalwindow:
Clickheretoviewcodeimage
Formattedvalues|2016|3.142|Hi|
Theformatstringisthefirstargumentinthemethodcall.Itcontainsfourformat
specifiers.Thefirstthreeare%5d,%8.3f,and%5s,whichspecifyhowthethree
argumentsshouldbeprocessed.Theletterintheformatspecifierindicatesthetypeof
valuetoformat.Theirlocationintheformatstringspecifieswherethetextual
representationoftheargumentsshouldbeinserted.Thefourthformatspecifier,%n,isa
platform-specificlineseparator.Itsoccurrencecausesthecurrentlinetobeterminated,
withthecursormovingtothestartofthenextline.Allothertextintheformatstringis
fixed,includinganyotherspacesorpunctuation,andisprintedverbatim.
Intheprecedingexample,thefirstvalueisformattedaccordingtothefirstformat
specifier,thesecondvalueisformattedaccordingtothesecondformatspecifier,andso
on.The|characterhasbeenusedintheformatstringtoshowhowmanycharacter
positionsaretakenupbythetextrepresentationofeachvalue.Theoutputshowsthatthe
intvaluewaswrittenright-justified,spanningfivecharacterpositionsusingtheformat
specifier%5d;thedoublevalueofMath.PItookupeightcharacterpositionsandwas
roundedtothreedecimalplacesusingtheformatspecifier%8.3f;andtheStringvalue
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waswrittenright-justified,spanningfivecharacterpositionsusingtheformatspecifier
%5s.Theformatspecifier%nterminatesthecurrentline.Allothercharactersinthe
formatstringareprintedverbatim.
Table1.2showsexamplesofsomeselectedformatspecifiersthatcanbeusedtoformat
values.TheirusageisillustratedinExample1.5,whichprintsasimpleinvoice.
Table1.2FormatSpecifierExamples
AtthetopoftheinvoiceprintedbyExample1.5,thecompanynameisprintedat(1)with
aformatstringthatcontainsonlyfixedtext.Thedateandtimeofdayareprintedonthe
sameline,withleadingzerosat(2).Aheaderisthenprintedat(3).Thecolumnnames
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Item,Price,Quantity,andAmountarepositionedappropriatelywiththeformat
specifications%-20s,%7s,%9s,and%8s,respectively.
Beneaththeheading,theitemspurchasedareprintedat(5),(6),and(7)usingthesame
fieldwidthsasthecolumnheadings.Theformatforeachitemisdefinedbytheformat
stringat(4).Theitemnameisprintedwiththeformatstring"%-20s",resultingina20-
character-widestring,left-justified.Theitempriceandthetotalamountforeachtypeof
itemareprintedasfloating-pointvaluesusingtheformatspecifications%7.2fand
%8.2f,respectively.Thequantityisprintedasanintegerusingtheformatspecification
%9d.Thestringsareleft-justified,whileallnumbersareright-justified.Thecharactersis
theconversioncodeforobjects,whilefloating-pointandintegervaluesareprintedusing
thecodesfandd,respectively.
At(8),thetotalcostofallitemsisprintedusingtheformatspecification%8.2f.To
positionthisvaluecorrectlyunderthecolumnAmount,weprintthestring"Total:"
usingtheformat%-36s.Thewidthof36charactersisfoundbyaddingthewidthofthe
firstthreecolumnsoftheinvoice.
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Example1.5FormattedOutput
Clickheretoviewcodeimage
//File:Invoice.java
publicclassInvoice{
publicstaticvoidmain(String[]args){
System.out.printf(“SecureDataInc.”);//(1)
System.out.printf(“%02d/%02d/%04d,%02d:%02d%n%n”,//(2)
2,13,2016,11,5);
System.out.printf(“%-20s%7s%9s%8s%n”,//(3)
“Item”,“Price”,“Quantity”,“Amount”);
intquantity=4;
doubleprice=120.25,amount=quantity*price,total=amount;
StringitemFormat=“%-20s%7.2f%9d%8.2f%n”;//(4)
System.out.printf(itemFormat,
“FlashDrive,250GB”,price,quantity,amount);//(5)
quantity=2;
price=455.0;amount=quantity*price;total=total+amount;
System.out.printf(itemFormat,
“UltraHD,4TB”,price,quantity,amount);//(6)
quantity=1;
price=8.50;amount=quantity*price;total=total+amount;
System.out.printf(itemFormat,
“USB3.0cable”,price,quantity,amount);//(7)
System.out.printf(“%-36s%8.2f%n”,“Total:”,total);//(8)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
SecureDataInc.02/13/2016,11:05
ItemPriceQuantityAmount
FlashDrive,250GB120.254481.00
UltraHD,4TB455.002910.00
USB3.0cable8.5018.50
Total:1399.50
1.12TheJavaEcosystem
SinceitsinitialreleaseasJavaDevelopmentKit1.0(JDK1.0)in1996,thenameJavahas
becomesynonymouswithathrivingecosystemthatprovidesthecomponentsandthetools
necessaryfordevelopingsystemsfortoday’smulticoreworld.Itsdiversecommunity,
comprisingamultitudeofvolunteers,organizations,andcorporations,continuestofuelits
evolutionandgrowwithitssuccess.Manyfreeopen-sourcetechnologiesnowexistthat
arewellproven,mature,andsupported,makingtheiradoptionlessdaunting.Thesetools
andframeworksprovidesupportforallphasesofthesoftwaredevelopmentlifecycleand
beyond.
TherearethreemajorJavaPlatformsfortheJavaprogramminglanguage:
•JavaSE(StandardEdition)
•JavaEE(EnterpriseEdition)
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•JavaME(MicroEdition)
Eachplatformprovidesahardware/operatingsystem–specificJVMandanAPI
(applicationprogramminginterface)todevelopapplicationsforthatplatform.TheJava
SEplatformprovidesthecorefunctionalityofthelanguage.TheJavaEEplatformisa
supersetoftheJavaSEplatformand,asthemostextensiveofthethreeplatforms,targets
enterpriseapplicationdevelopment.TheJavaMEplatformisasubsetoftheJavaSE
platform,havingthesmallestfootprint,andissuitablefordevelopingmobileand
embeddedapplications.TheupshotofthisclassificationisthataJavaprogramdeveloped
foroneJavaplatformwillnotnecessaryrunundertheJVMofanotherJavaplatform.The
JVMmustbecompatiblewiththeJavaplatformthatwasusedtodeveloptheprogram.
TheAPIandthetoolsfordevelopingandrunningJavaapplicationsarebundledtogether
asJDK.JusttheJVMandtheruntimelibrariesarealsobundledseparatelyasJRE(Java
RuntimeEnvironment).
ThesubjectofthisbookisJavaSE8.WerecommendinstallingtheappropriateJDKfor
JavaSE8(oranewerversion)dependingonthehardwareandoperatingsystem.
Therestofthissectionsummarizessomeofthefactorsthathavecontributedtothe
evolutionofJavafromanobject-orientedprogramminglanguagetoafull-fledged
ecosystemfordevelopingallsortsofsystems,includinglarge-scalebusinesssystemsand
embeddedsystemsforportablecomputingdevices.Alotofjargonisusedinthissection,
andmightbedifficulttounderstandatthefirstreading,butwerecommendcomingback
afterworkingthroughthebooktoappreciatethefactorsthathavecontributedtothe
successofJava.
Object-OrientedParadigm
TheJavaprogramminglanguagesupportstheobject-orientedparadigm,inwhichthe
propertiesofanobjectanditsbehaviorareencapsulatedintheobject.Thepropertiesand
thebehaviorarerepresentedbythefieldsandthemethodsoftheobject,respectively.The
objectscommunicatethroughmethodcallsinaproceduralmanner.Encapsulationensures
thatobjectsareimmunetotamperingexceptwhenmanipulatedthroughtheirpublic
interface.Encapsulationexposesonlywhatanobjectdoesandnothowitdoesit,sothat
itsimplementationcanbechangedwithminimumimpactonitsclients.Somebasic
conceptsofobject-orientedprogramming,suchasinheritanceandaggregation,were
introducedearlierinthischapter,andsubsequentchapterswillexpandonthistopic.
Aboveall,object-orientedsystemdevelopmentpromotescodereusewhereexisting
objectscanbereusedtoimplementnewobjects.Italsofacilitatesimplementationoflarge
systems,allowingtheirdecompositionintomanageablesubsystems.
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Interpreted:TheJVM
JavaprogramsarecompiledtobytecodethatisinterpretedbytheJVM.Various
optimizationtechnologies(e.g.,just-in-time[JIT]delivery)haveledtotheJVMbecoming
aleanandmeanvirtualmachinewithregardtoperformance,stability,andsecurity.Many
otherlanguages,suchasScala,Groovy,andClojure,nowcompiletobytecodeand
seamlesslyexecuteontheJVM.TheJVMhasthusevolvedintoanecosysteminitsown
right.
Architecture-NeutralandPortableBytecode
Theoften-citedslogan“Writeonce,runeverywhere”istrueonlyifacompatibleJVMis
availableforthehardwareandsoftwareplatform.Inotherwords,torunJavaSE
applicationsunderWindows10ona64-bithardwarearchitecture,therightJVMmustbe
installed.Fortunately,theJVMhasbeenportedtorunundermostplatformsandoperative
systemsthatexisttoday,includinghardwaredevicessuchassmartcards,mobiledevices,
andhomeappliances.
Thespecificationofthebytecodeisarchitectureneutral,meaningitisindependentofany
hardwarearchitecture.Itisexecutedbyareadilyavailablehardwareandoperating
system–specificJVM.TheportabilityoftheJavabytecodethuseasestheburdenofcross-
platformsystemdevelopment.
Simplicity
LanguagedesignofJavahasbeendrivenbyadesiretosimplifytheprogrammingprocess.
AlthoughJavaborrowsheavilyfromtheC++programminglanguage,certainfeaturesthat
weredeemedproblematicwerenotincorporatedintoitsdesign.Forexample,Javadoes
nothaveapreprocessor,anditdoesnotallowpointerhandling,user-definedoperator
overloading,ormultipleclassinheritance.
Javaoptedforautomaticgarbagecollection,whichfreestheprogrammerfromdealing
withmanyissuesrelatedtomemorymanagement,suchasmemoryleaks.
However,thejuryisstilloutonwhetherthesyntaxofnestedclassesorintroductionof
wildcardsforgenericscanbeconsideredsimple.
DynamicandDistributed
TheJVMcandynamicallyloadclasslibrariesfromthelocalfilesystemaswellasfrom
machinesonthenetwork,whenthoselibrariesareneededatruntime.Thisfeature
facilitateslinkingthecodeasandwhennecessaryduringtheexecutionofaprogram.Itis
alsopossibletoqueryprogrammaticallyaclassoranobjectatruntimeaboutitsmeta-
information,suchasitsmethodsandfields.
Javaprovidesextensivesupportfornetworkingtobuilddistributedsystems,whereobjects
areabletocommunicateacrossnetworksusingvariouscommunicationprotocolsand
technologies,suchasRemoteMethodInvocation(RMI)andsocketconnections.
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RobustandSecure
Javapromotesthedevelopmentofreliable,robust,andsecuresystems.Itisastrong
staticallytypedlanguage:Thecompilerguaranteesruntimeexecutionifthecodecompiles
withouterrors.Eliminationofpointers,runtimeindexchecksforarraysandstrings,and
automaticgarbagecollectionaresomeofthefeaturesofJavathatpromotereliability.The
exceptionhandlingfeatureofJavaiswithoutdoubtthemainfactorthatfacilitatesthe
developmentofrobustsystems.
Javaprovidesmultilevelprotectionfrommaliciouscode.Thelanguagedoesnotallow
directaccesstomemory.Abytecodeverifierdetermineswhetheranyuntrustedcode
loadedintheJVMissafe.Thesandboxmodelisusedtoconfineandexecuteany
untrustedcode,limitingthedamagethatsuchcodecancause.Thesefeatures,among
others,areprovidedbyacomprehensiveJavasecuritymodeltoensurethatapplication
codeexecutessecurelyintheJVM.
HighPerformanceandMultithreaded
TheperformanceofJavaprogramshasimprovedsignificantlywithvariousoptimizations
thatareappliedtothebytecodeatruntimebytheJVM.TheJITfeaturemonitorsthe
programatruntimetoidentifyperformance-criticalbytecode(calledhotspots)thatcanbe
optimized.Suchcodeisusuallytranslatedtomachinecodetoboostperformance.The
performanceachievedbytheJVMisabalancebetweennativecodeexecutionand
interpretationoffullyscriptedlanguages,whichfortunatelyisadequateformany
applications.
Javahasalwaysprovidedhigh-levelsupportformultithreading,allowingmultiplethreads
ofexecutiontoperformdifferenttasksconcurrentlyinanapplication.Ithasrisentothe
newchallengesthathaveemergedinrecentyearstoharnesstheincreasedcomputing
powermadeavailablebymulticorearchitectures.Functionalprogramming,inwhich
computationistreatedasside-effects–freeevaluationoffunctions,isseenasaboonto
meetthesechallenges.Java8bringselementsoffunctional-styleprogrammingintothe
language,providinglanguageconstructs(lambdaexpressionsandfunctionalinterfaces)
andAPIsupport(throughitsFork&JoinFrameworkandStreamAPI)toefficiently
utilizethemanycorestoprocesslargeamountsofdatainparallel.
ReviewQuestions
1.9WhichcommandfromtheJDKshouldbeusedtocompilethefollowingsource
codecontainedinafilenamedSmallProg.java?
Clickheretoviewcodeimage
publicclassSmallProg{
publicstaticvoidmain(String[]args){System.out.println(“Good
luck!”);}
}
Selecttheonecorrectanswer.
(a)javaSmallProg
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(b)javacSmallProg
(c)javaSmallProg.java
(d)javacSmallProg.java
(e)javaSmallProgmain
1.10WhichcommandfromtheJDKshouldbeusedtoexecutethemain()methodof
aclassnamedSmallProg?
Selecttheonecorrectanswer.
(a)javaSmallProg
(b)javacSmallProg
(c)javaSmallProg.java
(d)javaSmallProg.class
(e)javaSmallProg.main()
1.11WhichstatementistrueaboutJava?
Selecttheonecorrectanswer.
(a)AJavaprogramcanbeexecutedbyanyJVM.
(b)Javabytecodecannotbetranslatedtomachinecode.
(c)OnlyJavaprogramscanbeexecutedbyaJVM.
(d)AJavaprogramcancreateanddestroyobjects.
(e)Noneoftheabove
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•EssentialelementsofaJavaapplication
•Accessingobjectfieldsandcallingmethods
•CompilingandrunningJavaapplications
•Formattingandprintingvaluestotheterminalwindow
•BasicterminologyandconceptsinOOP,andhowtheseconceptsaresupportedin
Java
•FactorsandfeaturesoftheJavaecosystemthathavecontributedtoitsevolutionand
success
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ProgrammingExercise
1.1ModifytheClientclassfromExample1.4tousethePrintableCharStack
class,ratherthantheCharStackclassfromExample1.2.Utilizethe
printStackElements()methodfromthePrintableCharStackclass.Is
thenewprogrambehavior-wiseanydifferentfromExample1.4?
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2.LanguageFundamentals
2.1BasicLanguageElements
Likeanyotherprogramminglanguage,theJavaprogramminglanguageisdefinedby
grammarrulesthatspecifyhowsyntacticallylegalconstructscanbeformedusingthe
languageelements,andbyasemanticdefinitionthatspecifiesthemeaningofsyntactically
legalconstructs.
LexicalTokens
Thelow-levellanguageelementsarecalledlexicaltokens(orjusttokens)andarethe
buildingblocksformorecomplexconstructs.Identifiers,numbers,operators,andspecial
charactersareallexamplesoftokensthatcanbeusedtobuildhigh-levelconstructslike
expressions,statements,methods,andclasses.
Identifiers
Anameinaprogramiscalledanidentifier.Identifierscanbeusedtodenoteclasses,
methods,variables,andlabels.
InJava,anidentifieriscomposedofasequenceofcharacters,whereeachcharactercanbe
eitheraletteroradigit.However,thefirstcharacterinanidentifiermustalwaysbea
letter,asexplainedlater.
SinceJavaprogramsarewrittenintheUnicodecharacterset(p.32),charactersallowedin
identifiernamesareinterpretedaccordingtothischaracterset.UseoftheUnicode
charactersetopensupthepossibilityofwritingidentifiernamesinmanywritingscripts
usedaroundtheworld.Asonewouldexpect,thecharactersA-Zanda-zareletters,and
charactersfrom0-9aredigits.Aconnectingpunctuationcharacter(suchasunderscore
_)andanycurrencysymbol(suchas$,¢,¥,or£)arealsoallowedaslettersinidentifier
names,butthesecharactersshouldbeusedjudiciously.
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IdentifiersinJavaarecasesensitive.Forexample,priceandPricearetwodifferent
identifiers.
ExamplesofLegalIdentifiers
Clickheretoviewcodeimage
number,Number,sum_$,bingo,$$_100,_007,mål,grüß
ExamplesofIllegalIdentifiers
48chevy,all@hands,grand-sum
Thename48chevyisnotalegalidentifierbecauseitstartswithadigit.Thecharacter@
isnotalegalcharacterinanidentifier.Itisalsonotalegaloperator,sothatall@hands
cannotbeinterpretedasalegalexpressionwithtwooperands.Thecharacter-isnota
legalcharacterinanidentifier,butitisalegaloperator;thusgrand-sumcouldbe
interpretedasalegalexpressionwithtwooperands.
Keywords
Keywordsarereservedwordsthatarepredefinedinthelanguageandcannotbeusedto
denoteotherentities.AllJavakeywordsarelowercase,andincorrectusageresultsin
compile-timeerrors.
KeywordscurrentlydefinedinthelanguagearelistedinTable2.1.Inaddition,three
identifiersarereservedaspredefinedliteralsinthelanguage:thenullreference,andthe
booleanliteralstrueandfalse(Table2.2).Keywordscurrentlyreserved,butnotin
use,arelistedinTable2.3.Areservedwordcannotbeusedasanidentifier.Theindex
containsreferencestorelevantsectionswherecurrentlyusedkeywordsareexplained.
Table2.1KeywordsinJava
Table2.2ReservedLiteralsinJava
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Table2.3ReservedKeywordsNotCurrentlyinUse
Separators
Separators(alsoknownaspunctuators)aretokensthathavemeaningdependingonthe
contextinwhichtheyareused;theyaidthecompilerinperformingsyntaxandsemantic
analysisofaprogram(Table2.4).Dependingonthecontext,brackets([]),parentheses
(()),andthedotoperator(.)canalsobeinterpretedasoperators(§5.3,p.150).Seethe
indexentriesfortheseseparatorsformoredetails.
Table2.4SeparatorsinJava
Literals
Aliteraldenotesaconstantvalue;inotherwords,thevaluethataliteralrepresents
remainsunchangedintheprogram.Literalsrepresentnumerical(integerorfloating-
point),character,boolean,orstringvalues.Inaddition,theliteralnullrepresentsthenull
reference.Table2.5showsexamplesofliteralsinJava.
Table2.5ExamplesofLiterals
IntegerLiterals
Integerdatatypescomprisethefollowingprimitivedatatypes:int,long,byte,and
short(§2.2,p.37).
Thedefaultdatatypeofanintegerliteralisalwaysint,butitcanbespecifiedaslong
byappendingthesuffixL(orl)totheintegervalue.ThesuffixLisoftenpreferred
becausethesuffixlandthedigit1canbehardtodistinguish.Withoutthesuffix,the
longliterals2000Land0Lwillbeinterpretedasintliterals.Thereisnodirectwayto
specifyashortorabyteliteral.
Inadditiontothedecimalnumbersystem,integerliteralscanbespecifiedinthebinary
(base2,digits0-1),octal(base8,digits0-7),andhexadecimal(base16,digits0-9and
a-f)numbersystems.Thedigitsatofinthehexadecimalsystemcorrespondtodecimal
values10to15.Binary,octal,andhexadecimalnumbersarespecifiedwith0b(or0B),
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0,and0x(or0X)asthebaseorradixprefix,respectively.Examplesofdecimal,binary,
octal,andhexadecimalliteralsareshowninTable2.6.Notethattheleading0(zero)digit
isnottheuppercaseletterO.Thehexadecimaldigitsfromatofcanalsobespecified
withthecorrespondinguppercaseforms(AtoF).Negativeintegers(e.g.,-90)canbe
specifiedbyprefixingtheminussign(-)tothemagnitudeoftheintegerregardlessofthe
numbersystem(e.g.,-0b1011010,-0132,or-0X5A).Integerrepresentationis
discussedin§5.5,p.154.
Table2.6ExamplesofDecimal,Binary,Octal,andHexadecimalLiterals
Floating-PointLiterals
Floating-pointdatatypescomeintwoflavors:floatordouble.
Thedefaultdatatypeofafloating-pointliteralisdouble,butitcanbeexplicitly
designatedbyappendingthesuffixD(ord)tothevalue.Afloating-pointliteralcanalso
bespecifiedtobeafloatbyappendingthesuffixF(orf).
Floating-pointliteralscanalsobespecifiedinscientificnotation,whereE(ore)standsfor
exponent.Forexample,thedoubleliteral194.9E-2inscientificnotationisinterpreted
as194.9×10-2(i.e.,1.949).
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ExamplesofdoubleLiterals
Clickheretoviewcodeimage
0.00.0d0D
0.49.49.49D
49.049.49D
4.9E+14.9E+1D4.9e1d4900e-2.49E2
ExamplesoffloatLiterals
0.0F0f
0.49F.49F
49.0F49.F49F
4.9E+1F4900e-2f.49E2F
Notethatthedecimalpointandtheexponentareoptional,andthatatleastonedigitmust
bespecified.Also,fortheexamplesoffloatliteralspresentedhere,thesuffixFis
mandatory;ifitwasomitted,theywouldbeinterpretedasdoubleliterals.
UnderscoresinNumericalLiterals
Theunderscorecharacter(_)canbeusedtoimprovethereadabilityofnumericalliterals
inthesourcecode.Anynumberofunderscorescanbeinsertedbetweenthedigitsthat
makeupthenumericalliteral.Thisrulesoutunderscoresadjacenttothesign(+,-),the
radixprefix(0b,0B,0x,0X),thedecimalpoint(.),theexponent(e,E),andthedata
typesuffix(l,L,d,D,f,F),aswellasbeforethefirstdigitandafterthelastdigit.Note
thatoctalradixprefix0ispartofthedefinitionofanoctalliteralandistherefore
consideredthefirstdigitofanoctalliteral.
Underscoresinidentifiersaretreatedasletters.Forexample,thenames_XXLand_XXL_
aretwodistinctlegalidentifiers.Incontrast,underscoresareusedasanotational
conveniencefornumericalliterals,beingignoredbythecompilerwhenusedinsuch
literals.Inotherwords,anumericalliteralcanbespecifiedinthesourcecodeusing
underscoresbetweendigits,suchthat2_0_1_5and20__15representthesame
numericalliteral2015insourcecode.
ExamplesofLegalUseofUnderscoresinNumericalLiterals
Clickheretoviewcodeimage
0b0111_1111_1111_1111_1111_1111_1111_1111
0_377_777_7770xff_ff_ff_ff
-123_456.001_2.345_678e1_2
2009__08__1349_03_01d
ExamplesofIllegalUseofUnderscoresinNumericalLiterals
Clickheretoviewcodeimage
_0_b_01111111111111111111111111111111_
_0377777777__0_x_ffffffff_
+_123456_._00__12_._345678_e_12_
_20090813__490301_d_
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BooleanLiterals
Theprimitivedatatypebooleanrepresentsthetruthvaluestrueandfalsethatare
denotedbythereservedliteralstrueandfalse,respectively.
CharacterLiterals
Acharacterliteralisquotedinsinglequotes(').Allcharacterliteralshavetheprimitive
datatypechar.
Acharacterliteralisrepresentedaccordingtothe16-bitUnicodecharacterset,which
subsumesthe8-bitISO-Latin-1andthe7-bitASCIIcharacters.InTable2.7,notethat
digits(0to9),uppercaseletters(AtoZ),andlowercaseletters(atoz)havecontiguous
Unicodevalues.AUnicodecharactercanalwaysbespecifiedasafour-digithexadecimal
number(i.e.,16bits)withtheprefix\u.
Table2.7ExamplesofCharacterLiterals
EscapeSequences
Certainescapesequencesdefinespecialcharacters,asshowninTable2.8.Theseescape
sequencescanbesingle-quotedtodefinecharacterliterals.Forexample,thecharacter
literals\tand\u0009areequivalent.However,thecharacterliterals\u000aand
\u000dshouldnotbeusedtorepresentnewlineandcarriagereturninthesourcecode.
Thesevaluesareinterpretedasline-terminatorcharactersbythecompiler,andwillcause
compile-timeerrors.Youshouldusetheescapesequences\nand\r,respectively,for
correctinterpretationofthesecharactersinthesourcecode.
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Table2.8EscapeSequences
Wecanalsousetheescapesequence\dddtospecifyacharacterliteralasanoctalvalue,
whereeachdigitdcanbeanyoctaldigit(0–7),asshowninTable2.9.Thenumberof
digitsmustbethreeorfewer,andtheoctalvaluecannotexceed\377;inotherwords,
onlythefirst256characterscanbespecifiedwiththisnotation.
Table2.9ExamplesofEscapeSequence\ddd
StringLiterals
Astringliteralisasequenceofcharactersthatmustbeenclosedindoublequotesand
mustoccuronasingleline.AllstringliteralsareobjectsoftheclassString(§8.4,p.
357).
EscapesequencesaswellasUnicodevaluescanappearinstringliterals:
Clickheretoviewcodeimage
“Herecomesatab.\tAndherecomesanotherone\u0009!”(1)
“What’sonthemenu?”(2)
“"Stringliteralsaredouble-quoted."”(3)
“Left!\nRight!”(4)
“Don’tsplit(5)
meup!”
In(1),thetabcharacterisspecifiedusingtheescapesequenceandtheUnicodevalue,
respectively.In(2),thesingleapostropheneednotbeescapedinstrings,butitwouldbeif
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specifiedasacharacterliteral('\'').In(3),thedoublequotesinthestringmustbe
escaped.In(4),weusetheescapesequence\ntoinsertanewline.Theexpressionin(5)
generatesacompile-timeerror,asthestringliteralissplitoverseverallines.Printingthe
stringsfrom(1)to(4)willgivethefollowingresult:
Clickheretoviewcodeimage
Herecomesatab.Andherecomesanotherone!
What’sonthemenu?
“Stringliteralsaredouble-quoted.”
Left!
Right!
Oneshouldalsousetheescapesequences\nand\r,respectively,forcorrect
interpretationofthecharacters\u000a(newline)and\u000d(formfeed)instring
literals.
Whitespace
Awhitespaceisasequenceofspaces,tabs,formfeeds,andlineterminatorcharactersina
Javasourcefile.Lineterminatorsincludethenewline,carriagereturn,oracarriage
return–newlinesequence.
AJavaprogramisafree-formatsequenceofcharactersthatistokenizedbythecompiler—
thatis,brokenintoastreamoftokensforfurtheranalysis.Separatorsandoperatorshelpto
distinguishtokens,butsometimeswhitespacehastobeinsertedexplicitlyasaseparator.
Forexample,theidentifierclassRoomwillbeinterpretedasasingletoken,unless
whitespaceisinsertedtodistinguishthekeywordclassfromtheidentifierRoom.
Whitespaceaidsnotonlyinseparatingtokens,butalsoinformattingtheprogramsothatit
iseasytoread.Thecompilerignoresthewhitespaceoncethetokensareidentified.
Comments
Aprogramcanbedocumentedbyinsertingcommentsatrelevantplacesinthesource
code.Thesecommentsarefordocumentationpurposesonlyandareignoredbythe
compiler.
Javaprovidesthreetypesofcommentsthatcanbeusedtodocumentaprogram:
•Asingle-linecomment://...totheendoftheline
•Amultiple-linecomment:/*...*/
•Adocumentation(Javadoc)comment:/**...*/'
Single-LineComment
Allcharactersafterthecomment-startsequence//throughtotheendoftheline
constituteasingle-linecomment.
Clickheretoviewcodeimage
//Thiscommentendsattheendofthisline.
intage;//Fromcomment-startsequencetotheendofthelineisa
comment.
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Multiple-LineComment
Amultiple-linecomment,asthenamesuggests,canspanseverallines.Suchacomment
startswiththesequence/*andendswiththesequence*/.
/*Acomment
onseveral
lines.
*/
Thecomment-startsequences(//,/*,/**)arenottreateddifferentlyfromother
characterswhenoccurringwithincomments,sotheyareignored.Thismeansthattryingto
nestmultiple-linecommentswillresultinacompile-timeerror:
Clickheretoviewcodeimage
/*Formulaforalchemy.
gold=wizard.makeGold(stone);
/*ButitonlyworksonSundays.*/
*/
Thesecondoccurrenceofthecomment-startsequence/*isignored.Thelastoccurrence
ofthesequence*/inthecodeisnowunmatched,resultinginasyntaxerror.
DocumentationComment
Adocumentationcommentisaspecial-purposemultiple-linecommentthatisusedbythe
javadoctooltogenerateHTMLdocumentationfortheprogram.Documentation
commentsareusuallyplacedinfrontofclasses,interfaces,methods,andfielddefinitions.
Specialtagscanbeusedinsideadocumentationcommenttoprovidemorespecific
information.Suchacommentstartswiththesequence/**andendswiththesequence
*/:
Clickheretoviewcodeimage
/**
*Thisclassimplementsagizmo.
*@authorK.A.M.
*@version4.0
*/
Fordetailsonthejavadoctool,seethetoolsdocumentationprovidedbytheJDK.
ReviewQuestions
2.1Whichofthefollowingisnotalegalidentifier?
Selecttheonecorrectanswer.
(a)a2z
(b)ödipus
(c)52pickup
(d)_class
(e)ca$h
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(f)_8to5
2.2WhichofthefollowingarenotlegalliteralsinJava?
Selectthefourcorrectanswers.
(a)0Xbad
(b)0B_101_101
(c)09
(d)+_825
(e)1_2e4f
(f)'\x'
(g)"what\'syourfancy?"
2.3Whichstatementistrue?
Selecttheonecorrectanswer.
(a)newanddeletearekeywordsintheJavalanguage.
(b)try,catch,andthrownarekeywordsintheJavalanguage.
(c)static,unsigned,andlongarekeywordsintheJavalanguage.
(d)exit,class,andwhilearekeywordsintheJavalanguage.
(e)return,goto,anddefaultarekeywordsintheJavalanguage.
(f)for,while,andnextarekeywordsintheJavalanguage.
2.4WhichofthefollowingisnotalegalcommentinJava?
Selecttheonecorrectanswer.
(a)/*//*/
(b)/**///
(c)///**/
(d)/*/**/
(e)/*/**/*/
(f)////
2.2PrimitiveDataTypes
Figure2.1givesanoverviewoftheprimitivedatatypesinJava.
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Figure2.1PrimitiveDataTypesinJava
PrimitivedatatypesinJavacanbedividedintothreemaincategories:
•Integraltypes—representsignedintegers(byte,short,int,long)and
unsignedcharactervalues(char)
•Floating-pointtypes(float,double)—representfractionalsignednumbers
•Booleantype(boolean)—representslogicalvalues
Primitivedatavaluesarenotobjects.Eachprimitivedatatypedefinestherangeofvalues
inthedatatype,andoperationsonthesevaluesaredefinedbyspecialoperatorsinthe
language(Chapter5,p.143).
Eachprimitivedatatypealsohasacorrespondingwrapperclassthatcanbeusedto
representaprimitivevalueasanobject.Wrapperclassesarediscussedin§8.3,p.346.
TheIntegerTypes
Theintegerdatatypesarebyte,short,int,andlong(Table2.10).Theirvaluesare
signedintegersrepresentedbytwo’scomplement(§5.5,p.155).
Table2.10RangeofIntegerValues
The Type
Thedatatypecharrepresentscharacters(Table2.11).Theirvaluesareunsignedintegers
thatdenoteallofthe65536(216)charactersinthe16-bitUnicodecharacterset.Thisset
includesletters,digits,andspecialcharacters.
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Table2.11RangeofCharacterValues
Thefirst128charactersoftheUnicodesetarethesameasthe128charactersofthe7-bit
ASCIIcharacterset,andthefirst256charactersoftheUnicodesetcorrespondtothe256
charactersofthe8-bitISOLatin-1characterset.
Theintegertypesandthechartypearecollectivelycalledintegraltypes.
TheFloating-PointTypes
Floating-pointnumbersarerepresentedbythefloatanddoubledatatypes.
Floating-pointnumbersconformtotheIEEE754-1985binaryfloating-pointstandard.
Table2.12showstherangeofvaluesforpositivefloating-pointnumbers,buttheseapply
equallytonegativefloating-pointnumberswiththeminussign(-)asaprefix.Zerocanbe
either0.0or-0.0.
Table2.12RangeofFloating-PointValues
Sincethesizeforrepresentationisafinitenumberofbits,certainfloating-pointnumbers
canberepresentedonlyasapproximations.Forexample,thevalueoftheexpression
(1.0/3.0)isrepresentedasanapproximationduetothefinitenumberofbitsusedto
representfloating-pointnumbers.
The Type
Thedatatypebooleanrepresentsthetwologicalvaluesdenotedbytheliteralstrue
andfalse(Table2.13).
Table2.13BooleanValues
Booleanvaluesareproducedbyallrelational(§5.11,p.180),conditional(§5.14,p.186),
andbooleanlogicaloperators(§5.13,p.184),andareprimarilyusedtogoverntheflow
ofcontrolduringprogramexecution.
Table2.14summarizesthepertinentfactsabouttheprimitivedatatypes:theirwidthor
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size,whichindicatesthenumberofbitsrequiredtostoreaprimitivevalue;theirrangeof
legalvalues,whichisspecifiedbytheminimumandthemaximumvaluespermissible;
andthenameofthecorrespondingwrapperclass(§8.3,p.346).
Table2.14SummaryofPrimitiveDataTypes
ReviewQuestions
2.5WhichofthefollowingdonotdenoteaprimitivedatavalueinJava?
Selectthetwocorrectanswers.
(a)"t"
(b)'k'
(c)50.5F
(d)"hello"
(e)false
2.6Whichofthefollowingprimitivedatatypesarenotintegertypes?
Selectthethreecorrectanswers.
(a)boolean
(b)byte
(c)float
(d)short
(e)double
2.7WhichintegraltypeinJavahastheexactrangefrom-2147483648(i.e.,-231)to
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2147483647(i.e.,231-1),inclusive?
Selecttheonecorrectanswer.
(a)byte
(b)short
(c)int
(d)long
(e)char
2.3VariableDeclarations
Avariablestoresavalueofaparticulartype.Avariablehasaname,atype,andavalue
associatedwithit.InJava,variablescanstoreonlyvaluesofprimitivedatatypesand
referencevaluesofobjects.Variablesthatstorereferencevaluesofobjectsarecalled
referencevariables(orobjectreferencesorsimplyreferences).
DeclaringandInitializingVariables
Variabledeclarationsareusedtospecifythetypeandthenameofvariables.This
implicitlydeterminestheirmemoryallocationandthevaluesthatcanbestoredinthem.
Examplesofdeclaringvariablesthatcanstoreprimitivevaluesfollow:
Clickheretoviewcodeimage
chara,b,c;//a,bandcarecharactervariables.
doublearea;//areaisafloating-pointvariable.
booleanflag;//flagisabooleanvariable.
Thefirstdeclarationisequivalenttothefollowingthreedeclarations:
chara;
charb;
charc;
Adeclarationcanalsobecombinedwithaninitializationexpressiontospecifyan
appropriateinitialvalueforthevariable.Suchdeclarationsarecalleddeclaration
statements.
Clickheretoviewcodeimage
inti=10,//iisanintvariablewithinitialvalue10.
j=0b101;//jisanintvariablewithinitialvalue5.
longbig=2147483648L;//bigisalongvariablewithspecifiedinitial
value.
ReferenceVariables
Areferencevariablecanstorethereferencevalueofanobject,andcanbeusedto
manipulatetheobjectdenotedbythereferencevalue.
Avariabledeclarationthatspecifiesareferencetype(i.e.,aclass,anarray,aninterface
name,oranenumtype)declaresareferencevariable.Analogoustothedeclarationof
variablesofprimitivedatatypes,thesimplestformofreferencevariabledeclaration
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specifiesthenameandthereferencetypeonly.Thedeclarationdetermineswhichobjects
canbereferencedbyareferencevariable.Beforewecanuseareferencevariableto
manipulateanobject,itmustbedeclaredandinitializedwiththereferencevalueofthe
object.
Clickheretoviewcodeimage
PizzayummyPizza;//VariableyummyPizzacanreferenceobjectsofclass
Pizza.
HamburgerbigOne,//VariablebigOnecanreferenceobjectsofclass
Hamburger,
smallOne;//andsocanvariablesmallOne.
Itisimportanttonotethattheprecedingdeclarationsdonotcreateanyobjectsofclass
PizzaorHamburger.Rather,theysimplycreatevariablesthatcanstorereference
valuesofobjectsofthespecifiedclasses.
Adeclarationcanalsobecombinedwithaninitializerexpressiontocreateanobject
whosereferencevaluecanbeassignedtothereferencevariable:
Clickheretoviewcodeimage
PizzayummyPizza=newPizza(“Hot&Spicy”);//Declarationstatement
ThereferencevariableyummyPizzacanreferenceobjectsofclassPizza.Thekeyword
new,togetherwiththeconstructorcallPizza("Hot&Spicy"),createsanobjectof
theclassPizza.Thereferencevalueofthisobjectisassignedtothevariable
yummyPizza.ThenewlycreatedobjectofclassPizzacannowbemanipulated
throughthereferencevariableyummyPizza.
2.4InitialValuesforVariables
Thissectiondiscusseswhatvalue,ifany,isassignedtoavariablewhennoexplicitinitial
valueisprovidedinthedeclaration.
DefaultValuesforFields
DefaultvaluesforfieldsofprimitivedatatypesandreferencetypesarelistedinTable
2.15.Thevalueassigneddependsonthetypeofthefield.
Table2.15DefaultValues
Ifnoexplicitinitializationisprovidedforastaticvariable,itisinitializedwiththedefault
valueofitstypewhentheclassisloaded.Similarly,ifnoinitializationisprovidedforan
instancevariable,itisinitializedwiththedefaultvalueofitstypewhentheclassis
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instantiated.Thefieldsofreferencetypesarealwaysinitializedwiththenullreference
valueifnoinitializationisprovided.
Example2.1illustratesthedefaultinitializationoffields.Notethatstaticvariablesare
initializedwhentheclassisloadedthefirsttime,andinstancevariablesareinitialized
accordinglyineveryobjectcreatedfromtheclassLight.
Example2.1DefaultValuesforFields
Clickheretoviewcodeimage
publicclassLight{
//Staticvariable
staticintcounter;//Defaultvalue0whenclassisloaded
//Instancevariables:
intnoOfWatts=100;//Explicitlysetto100
booleanindicator;//Implicitlysettodefaultvaluefalse
Stringlocation;//Implicitlysettodefaultvaluenull
publicstaticvoidmain(String[]args){
Lightbulb=newLight();
System.out.println(“Staticvariablecounter:”+Light.counter);
System.out.println(“InstancevariablenoOfWatts:”+bulb.noOfWatts);
System.out.println(“Instancevariableindicator:”+bulb.indicator);
System.out.println(“Instancevariablelocation:”+bulb.location);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Staticvariablecounter:0
InstancevariablenoOfWatts:100
Instancevariableindicator:false
Instancevariablelocation:null
InitializingLocalVariablesofPrimitiveDataTypes
Localvariablesarevariablesthataredeclaredinmethods,constructors,andblocks
(Chapter3,p.47).Theyarenotinitializedimplicitlywhentheyareallocatedmemoryat
methodinvocation—thatis,whentheexecutionofamethodbegins.Thesameappliesto
localvariablesinconstructorsandblocks.Localvariablesmustbeexplicitlyinitialized
beforebeingused.Thecompilerwillreportanerroronlyifanattemptismadetousean
uninitializedlocalvariable.
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Example2.2FlaggingUninitializedLocalVariablesofPrimitiveDataTypes
Clickheretoviewcodeimage
publicclassTooSmartClass{
publicstaticvoidmain(String[]args){
intweight=10,thePrice;//(1)Localvariables
if(weight<10)thePrice=1000;
if(weight>50)thePrice=5000;
if(weight>=10)thePrice=weight*10;//(2)Alwaysexecuted
System.out.println(“Thepriceis:”+thePrice);//(3)Compile-time
error!
}
}
InExample2.2,thecompilercomplainsthatthelocalvariablethePriceusedinthe
printlnstatementat(3)maynotbeinitialized.However,atruntime,thelocalvariable
thePricewillgetthevalue100inthelastifstatementat(2),beforeitisusedinthe
printlnstatement.Thecompilerdoesnotperformarigorousanalysisoftheprogramin
thisregard.Itcompilesthebodyofaconditionalstatementonlyifitcandeducethatthe
conditionistrue.Theprogramwillcompilecorrectlyifthevariableisinitializedinthe
declaration,orifanunconditionalassignmentismadetothevariable.
Replacingthedeclarationofthelocalvariablesat(1)inExample2.2withthefollowing
declarationsolvestheproblem:
Clickheretoviewcodeimage
intweight=10,thePrice=0;//(1’)Bothlocalvariables
initialized
InitializingLocalReferenceVariables
Localreferencevariablesareboundbythesameinitializationrulesaslocalvariablesof
primitivedatatypes.
Example2.3FlaggingUninitializedLocalReferenceVariables
Clickheretoviewcodeimage
publicclassVerySmartClass{
publicstaticvoidmain(String[]args){
StringimportantMessage;//Localreferencevariable
System.out.println(“Themessagelengthis:”+
importantMessage.length());//Compile-timeerror!
}
}
InExample2.3,thecompilercomplainsthatthelocalvariableimportantMessage
usedintheprintlnstatementmaynotbeinitialized.Ifthevariableimportant-
Messageissettothevaluenull,theprogramwillcompile.However,aruntimeerror
(NullPointerException)willoccurwhenthecodeisexecuted,becausethevariable
importantMessagewillnotdenoteanyobject.Thegoldenruleistoensurethata
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referencevariable,whetherlocalornot,isassignedareferencevaluedenotinganobject
beforeitisused—thatis,toensurethatitdoesnothavethevaluenull.
Theprogramcompilesandrunsifwereplacethedeclarationwiththefollowing
declarationofthelocalvariable,whichcreatesastringliteralandassignsitsreference
valuetothelocalreferencevariableimportantMessage:
Clickheretoviewcodeimage
StringimportantMessage=“Initializebeforeuse!”;
Arraysandtheirdefaultvaluesarediscussedin§3.4,p.58.
LifetimeofVariables
Thelifetimeofavariable—thatis,thetimeavariableisaccessibleduringexecution—is
determinedbythecontextinwhichitisdeclared.Thelifetimeofavariable,whichisalso
calleditsscope,isdiscussedinmoredetailin§4.4,p.114.Wedistinguishamongthe
lifetimesofvariablesinthreecontexts:
•Instancevariables—membersofaclass,whicharecreatedforeachobjectofthe
class.Inotherwords,everyobjectoftheclasswillhaveitsowncopiesofthese
variables,whicharelocaltotheobject.Thevaluesofthesevariablesatanygiven
timeconstitutethestateoftheobject.Instancevariablesexistaslongastheobject
theybelongtoisinuseatruntime.
•Staticvariables—membersofaclass,butwhicharenotcreatedforanyspecific
objectoftheclassand,therefore,belongonlytotheclass(§4.4,p.114).Theyare
createdwhentheclassisloadedatruntime,andexistaslongastheclassisavailable
atruntime.
•Localvariables(alsocalledmethodautomaticvariables)—declaredinmethods,
constructors,andblocks;andcreatedforeachexecutionofthemethod,constructor,
orblock.Aftertheexecutionofthemethod,constructor,orblockcompletes,local
(non-final)variablesarenolongeraccessible.
ReviewQuestions
2.8Whichofthefollowingdeclarationsarevalid?
Selectthethreecorrectanswers.
(a)chara='\u0061';
(b)char'a'='a';
(c)char\u0061='a';
(d)ch\u0061ra='a';
(e)ch'a'ra='a';
2.9Giventhefollowingcodewithinamethod,whichstatementistrue?
inti,j;
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j=5;
Selecttheonecorrectanswer.
(a)Localvariableiisnotdeclared.
(b)Localvariablejisnotdeclared.
(c)Localvariableiisdeclaredbutnotinitialized.
(d)Localvariablejisdeclaredbutnotinitialized.
(e)Localvariablejisinitializedbutnotdeclared.
2.10Inwhichofthesevariabledeclarationswillthevariableremainuninitializedunless
itisexplicitlyinitialized?
Selecttheonecorrectanswer.
(a)Declarationofaninstancevariableoftypeint
(b)Declarationofastaticvariableoftypefloat
(c)Declarationofalocalvariableoftypefloat
(d)DeclarationofastaticvariableoftypeObject
(e)Declarationofaninstancevariableoftypeint[]
2.11Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassInit{
Stringtitle;
booleanpublished;
staticinttotal;
staticdoublemaxPrice;
publicstaticvoidmain(String[]args){
InitinitMe=newInit();
doubleprice;
if(true)
price=100.00;
System.out.println(“|”+initMe.title+“|”+initMe.published+“|”
+
Init.total+“|”+Init.maxPrice+“|”+price+
“|”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,andprint|null|false|0|0.0|0.0|at
runtime.
(c)Theprogramwillcompile,andprint|null|true|0|0.0|100.0|at
runtime.
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(d)Theprogramwillcompile,andprint||false|0|0.0|0.0|atruntime.
(e)Theprogramwillcompile,andprint|null|false|0|0.0|100.0|at
runtime.
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Basiclanguageelements:identifiers,keywords,separators,literals,whitespace,and
comments
•Primitivedatatypes:integral,floating-point,andboolean
•Notationalrepresentationofnumbersindecimal,binary,octal,andhexadecimal
systems
•Declarationandinitializationofvariables,includingreferencevariables
•Usageofdefaultvaluesforinstancevariablesandstaticvariables
•Lifetimeofinstancevariables,staticvariables,andlocalvariables
ProgrammingExercise
2.1Thefollowingprogramhasseveralerrors.Modifytheprogramsothatitwill
compileandrunwithouterrors.
Clickheretoviewcodeimage
//File:Temperature.java
PUBLICCLASStemperature{
PUBLICvoidmain(stringargs){
doublefahrenheit=62.5;
*/Convert/*
doublecelsius=f2c(fahrenheit);
System.out.println(fahrenheit+‘F’+”=”+Celsius+‘C’);
}
doublef2c(floatfahr){
RETURN(fahr-32.0)*5.0/9.0;
}
}
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3.Declarations
3.1ClassDeclarations
Aclassdeclarationintroducesanewreferencetype.Forthepurposeofthisbook,wewill
usethefollowingsimplifiedsyntaxofaclassdeclaration:
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class_modifiersclassclass_name
extends_clause
implements_clause//Classheader
{//Classbody
field_declarations
method_declarations
constructor_declarations
}
Intheclassheader,thenameoftheclassisprecededbythekeywordclass.Inaddition,
theclassheadercanspecifythefollowinginformation:
•Anaccessibilitymodifier(§4.5,p.118)
•Additionalclassmodifiers(§4.6,p.120)
•Anyclassitextends(§7.1,p.264)
•Anyinterfacesitimplements(§7.6,p.290)
Theclassbody,enclosedinbraces({}),cancontainmemberdeclarations.Inthisbook,
wediscussthefollowingtwokindsofmemberdeclarations:
•Fielddeclarations(§2.3,p.40)
•Methoddeclarations(§3.2,p.49)
Membersdeclaredstaticbelongtotheclassandarecalledstaticmembers.Non-static
membersbelongtotheobjectsoftheclassandarecalledinstancemembers.Inaddition,
thefollowingdeclarationscanbeincludedinaclassbody:
•Constructordeclarations(§3.3,p.53)
Thedeclarationscanappearinanyorderintheclassbody.Theonlymandatorypartsof
theclassdeclarationsyntaxarethekeywordclass,theclassname,andtheclassbody
braces({}),asexemplifiedbythefollowingclassdeclaration:
classX{}
Tounderstandwhichcodecanbelegallydeclaredinaclass,wedistinguishbetweenstatic
contextandnon-staticcontext.Astaticcontextisdefinedbystaticmethods,staticfield
initializers,andstaticinitializerblocks.Anon-staticcontextisdefinedbyinstance
methods,non-staticfieldinitializers,instanceinitializerblocks,andconstructors.Bystatic
code,wemeanexpressionsandstatementsinastaticcontext;bynon-staticcode,wemean
expressionsandstatementsinanon-staticcontext.Onecrucialdifferencebetweenthetwo
contextsisthatstaticcodecanreferonlytootherstaticmembers.
3.2MethodDeclarations
Forthepurposeofthisbook,wewillusethefollowingsimplifiedsyntaxofamethod
declaration:
Clickheretoviewcodeimage
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method_modifiersreturn_typemethod_name
(formal_parameter_list)throws_clause//Methodheader
{//Methodbody
local_variable_declarations
statements
}
Inadditiontothenameofthemethod,themethodheadercanspecifythefollowing
information:
•Scopeoraccessibilitymodifier(§4.7,p.123)
•Additionalmethodmodifiers(§4.8,p.131)
•Thetypeofthereturnvalue,orvoidifthemethoddoesnotreturnanyvalue(§6.4,
p.224)
•Aformalparameterlist
•Anyexceptionsthrownbythemethod,whicharespecifiedinathrowsclause
(§6.9,p.251)
Theformalparameterlistisacomma-separatedlistofparametersforpassinginformation
tothemethodwhenthemethodisinvokedbyamethodcall(§3.5,p.72).Anempty
parameterlistmustbespecifiedby().Eachparameterisasimplevariabledeclaration
consistingofitstypeandname:
Clickheretoviewcodeimage
optional_parameter_modifiertypeparameter_name
Theparameternamesarelocaltothemethod(§4.4,p.117).Theoptionalparameter
modifierfinalisdiscussedin§3.5,p.80.Itisrecommendedtousethe@paramtagina
Javadoccommenttodocumenttheformalparametersofamethod.
Thesignatureofamethodcomprisesthenameofthemethodandthetypesoftheformal
parametersonly.
Themethodbodyisablockcontainingthelocalvariabledeclarations(§2.3,p.40)and
thestatementsofthemethod.
Themandatorypartsofamethoddeclarationarethereturntype,themethodname,andthe
methodbodybraces({}),asexemplifiedbythefollowingmethoddeclaration:
voidnoAction(){}
Likemembervariables,membermethodscanbecharacterizedasoneoftwotypes:
•Instancemethods,whicharediscussedlaterinthissection
•Staticmethods,whicharediscussedin§4.8,p.132
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Statements
StatementsinJavacanbegroupedintovariouscategories.Variabledeclarationswith
explicitinitializationofthevariablesarecalleddeclarationstatements(§2.3,p.40,and
§3.4,p.60).Otherbasicformsofstatementsarecontrolflowstatements(§6.1,p.200)and
expressionstatements.
Anexpressionstatementisanexpressionterminatedbyasemicolon.Anyvaluereturned
bytheexpressionisdiscarded.Onlycertaintypesofexpressionshavemeaningas
statements:
•Assignments(§5.6,p.158)
•Incrementanddecrementoperators(§5.9,p.176)
•Methodcalls(§3.5,p.72)
•Objectcreationexpressionswiththenewoperator(§5.17,p.195)
Asolitarysemicolondenotestheemptystatement,whichdoesnothing.
Ablock,{},isacompoundstatementthatcanbeusedtogroupzeroormorelocal
declarationsandstatements(§4.4,p.117).Blockscanbenested,sinceablockisa
statementthatcancontainotherstatements.Ablockcanbeusedinanycontextwherea
simplestatementispermitted.Thecompoundstatementthatisembodiedinablockbegins
attheleftbrace,{,andendswithamatchingrightbrace,}.Suchablockmustnotbe
confusedwithanarrayinitializerindeclarationstatements(§3.4,p.60).
Labeledstatementsarediscussedin§6.4onpage220.
InstanceMethodsandtheObjectReference
Instancemethodsbelongtoeveryobjectoftheclassandcanbeinvokedonlyonobjects.
Allmembersdefinedintheclass,bothstaticandnon-static,areaccessibleinthecontext
ofaninstancemethod.Thereasonisthatallinstancemethodsarepassedanimplicit
referencetothecurrentobject—thatis,theobjectonwhichthemethodisbeinginvoked.
Thecurrentobjectcanbereferencedinthebodyoftheinstancemethodbythekeyword
this.Inthebodyofthemethod,thethisreferencecanbeusedlikeanyotherobject
referencetoaccessmembersoftheobject.Infact,thekeywordthiscanbeusedinany
non-staticcontext.Thethisreferencecanbeusedasanormalreferencetoreferencethe
currentobject,butthereferencecannotbemodified—itisafinalreference(§4.8,p.
133).
Thethisreferencetothecurrentobjectisusefulinsituationswherealocalvariable
hides,orshadows,afieldwiththesamename.InExample3.1,thetwoparameters
noOfWattsandindicatorintheconstructoroftheLightclasshavethesame
namesasthefieldsintheclass.Theexamplealsodeclaresalocalvariablelocation,
whichhasthesamenameasoneofthefields.Thereferencethiscanbeusedto
distinguishthefieldsfromthelocalvariables.At(1),thethisreferenceisusedto
identifythefieldnoOfWatts,whichisassignedthevalueoftheparameter
noOfWatts.Withoutthethisreferenceat(2),thevalueoftheparameterindicator
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isassignedbacktothisparameter,andnottothefieldbythesamename,resultingina
logicalerror.Similarlyat(3),withoutthethisreference,itisthelocalvariable
locationthatisassignedthevalueoftheparametersite,andnotthefieldwiththe
samename.
Example3.1UsingthethisReference
Clickheretoviewcodeimage
publicclassLight{
//Fields:
intnoOfWatts;//Wattage
booleanindicator;//Onoroff
Stringlocation;//Placement
//Constructor
publicLight(intnoOfWatts,booleanindicator,Stringsite){
Stringlocation;
this.noOfWatts=noOfWatts;//(1)Assignmenttofield
indicator=indicator;//(2)Assignmenttoparameter
location=site;//(3)Assignmenttolocalvariable
this.superfluous();//(4)
superfluous();//equivalenttocallat(4)
}
publicvoidsuperfluous(){
System.out.printf(“Currentobject:%s%n”,this);//(5)
}
publicstaticvoidmain(String[]args){
Lightlight=newLight(100,true,“loft”);
System.out.println(“No.ofwatts:”+light.noOfWatts);
System.out.println(“Indicator:”+light.indicator);
System.out.println(“Location:”+light.location);
}
}
Probableoutputfromtheprogram:
Currentobject:Light@1bc4459
Currentobject:Light@1bc4459
No.ofwatts:100
Indicator:false
Location:null
Ifamemberisnotshadowedbyalocaldeclaration,thesimplenamememberis
consideredashort-handnotationforthis.member.Inparticular,thethisreference
canbeusedexplicitlytoinvokeothermethodsintheclass.Thisusageisillustratedat(4)
inExample3.1,wherethemethodsuperfluous()iscalled.
If,forsomereason,amethodneedstopassthecurrentobjecttoanothermethod,itcando
sousingthethisreference.Thisapproachisillustratedat(5)inExample3.1,wherethe
currentobjectispassedtotheprintf()method.Theprintf()methodprintsthe
stringrepresentationofthecurrentobject(whichcomprisesthenameoftheclassofthe
currentobjectandthehexadecimalrepresentationofthecurrentobject’shashcode).(The
hashcodeofanobjectisanintvaluethatcanbeusedtostoreandretrievetheobject
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fromspecialdatastructurescalledhashtables.)
Notethatthethisreferencecannotbeusedinastaticcontext,asstaticcodeisnot
executedinthecontextofanyobject.
MethodOverloading
Eachmethodhasasignature,whichcomprisesthenameofthemethodplusthetypesand
orderoftheparametersintheformalparameterlist.Severalmethodimplementationsmay
havethesamename,aslongasthemethodsignaturesdiffer.Thispracticeiscalled
methodoverloading.Becauseoverloadedmethodshavethesamename,theirparameter
listsmustbedifferent.
Ratherthaninventingnewmethodnames,methodoverloadingcanbeusedwhenthesame
logicaloperationrequiresmultipleimplementations.TheJavaSEplatformAPImakes
heavyuseofmethodoverloading.Forexample,theclassjava.lang.Mathcontainsan
overloadedmethodmin(),whichreturnstheminimumoftwonumericvalues.
Clickheretoviewcodeimage
publicstaticdoublemin(doublea,doubleb)
publicstaticfloatmin(floata,floatb)
publicstaticintmin(inta,intb)
publicstaticlongmin(longa,longb)
Inthefollowingexamples,fiveimplementationsofthemethodmethodAareshown:
Clickheretoviewcodeimage
voidmethodA(inta,doubleb){/*…*/}//(1)
intmethodA(inta){returna;}//(2)
intmethodA(){return1;}//(3)
longmethodA(doublea,intb){returnb;}//(4)
longmethodA(intx,doubley){returnx;}//(5)NotOK.
Thecorrespondingsignaturesofthefivemethodsareasfollows:
Clickheretoviewcodeimage
methodA(int,double)1’
methodA(int)2’:Numberofparameters
methodA()3’:Numberofparameters
methodA(double,int)4’:Orderofparameters
methodA(int,double)5’:Sameas1’
ThefirstfourimplementationsofthemethodnamedmethodAareoverloadedcorrectly,
eachtimewithadifferentparameterlistand,therefore,differentsignatures.The
declarationat(5)hasthesamesignaturemethodA(int,double)asthedeclaration
at(1)and,therefore,isnotavalidoverloadingofthismethod.
Clickheretoviewcodeimage
voidbake(Cakek){/*…*/}//(1)
voidbake(Pizzap){/*…*/}//(2)
inthalfIt(inta){returna/2;}//(3)
doublehalfIt(inta){returna/2.0;}//(4)NotOK.Same
signature.
Themethodnamedbakeiscorrectlyoverloadedat(1)and(2),withtwodifferent
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parameterlists.Intheimplementation,changingjustthereturntype(asshownat(3)and
(4)intheprecedingexample),isnotenoughtooverloadamethod,andwillbeflaggedas
acompile-timeerror.Theparameterlistinthedeclarationsmustbedifferent.
Onlymethodsdeclaredinthesameclassandthosethatareinheritedbytheclasscanbe
overloaded.Overloadedmethodsshouldbeconsideredtobeindividualmethodsthatjust
happentohavethesamename.Methodswiththesamenameareallowed,sincemethods
areidentifiedbytheirsignature.Atcompiletime,therightimplementationofan
overloadedmethodischosen,basedonthesignatureofthemethodcall.Detailsofmethod
overloadingresolutioncanbefoundin§7.10onpage316.Methodoverloadingshouldnot
beconfusedwithmethodoverriding(§7.2,p.268).
3.3Constructors
Themainpurposeofconstructorsistosettheinitialstateofanobject,whentheobjectis
createdbyusingthenewoperator.
Forthepurposeofthisbook,wewillusethefollowingsimplifiedsyntaxofaconstructor:
Clickheretoviewcodeimage
accessibility_modifierclass_name(formal_parameter_list)
throws_clause//Constructorheader
{//Constructorbody
local_variable_declarations
statements
}
Constructordeclarationsareverymuchlikemethoddeclarations.However,thefollowing
restrictionsonconstructorsshouldbenoted:
•Modifiersotherthananaccessibilitymodifierarenotpermittedintheconstructor
header.Foraccessibilitymodifiersforconstructors,see§4.7,p.123.
•Constructorscannotreturnavalueand,therefore,donotspecifyareturntype,not
evenvoid,intheconstructorheader.Buttheirdeclarationcanusethereturn
statementthatdoesnotreturnavalueintheconstructorbody(§6.4,p.224).
•Theconstructornamemustbethesameastheclassname.
Classnamesandmethodnamesexistindifferentnamespaces.Thus,therearenoname
conflictsinExample3.2,whereamethoddeclaredat(2)hasthesamenameasthe
constructordeclaredat(1).Amethodmustalwaysspecifyareturntype,whereasa
constructordoesnot.However,usingsuchnamingschemesisstronglydiscouraged.
Aconstructorthathasnoparameters,liketheoneat(1)inExample3.2,iscalledano-
argumentconstructor.
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Example3.2Namespaces
Clickheretoviewcodeimage
publicclassName{
Name(){//(1)No-argumentconstructor
System.out.println(“Constructor”);
}
voidName(){//(2)Instancemethod
System.out.println(“Method”);
}
publicstaticvoidmain(String[]args){
newName().Name();//(3)Constructorcallfollowedbymethod
call
}
}
Outputfromtheprogram:
Constructor
Method
TheDefaultConstructor
Ifaclassdoesnotspecifyanyconstructors,thenadefaultconstructorisgeneratedforthe
classbythecompiler.Thedefaultconstructorisequivalenttothefollowing
implementation:
Clickheretoviewcodeimage
class_name(){super();}//Noparameters.Calls
superclassconstructor.
Adefaultconstructorisano-argumentconstructor.Theonlyactiontakenbythedefault
constructoristocallthesuperclassconstructor.Thisensuresthattheinheritedstateofthe
objectisinitializedproperly(§7.5,p.282).Inaddition,allinstancevariablesintheobject
aresettothedefaultvalueoftheirtype,barringthosethatareinitializedbyan
initializationexpressionintheirdeclaration.
Inthefollowingcode,theclassLightdoesnotspecifyanyconstructors:
Clickheretoviewcodeimage
classLight{
//Fields:
intnoOfWatts;//Wattage
booleanindicator;//Onoroff
Stringlocation;//Placement
//Noconstructors
//…
}
classGreenhouse{
//…
LightoneLight=newLight();//(1)Calltodefaultconstructor
}
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Inthiscode,thefollowingdefaultconstructoriscalledwhenaLightobjectiscreatedby
theobjectcreationexpressionat(1):
Light(){super();}
Creatinganobjectusingthenewoperatorwiththedefaultconstructor,asat(1),will
initializethefieldsoftheobjecttotheirdefaultvalues(thatis,thefieldsnoOfWatts,
indicator,andlocationinaLightobjectwillbeinitializedto0,false,and
null,respectively).
Aclasscanchoosetoprovideitsownconstructors,ratherthanrelyingonthedefault
constructor.Inthefollowingexample,theclassLightprovidesano-argument
constructorat(1).
Clickheretoviewcodeimage
classLight{
//…
Light(){//(1)No-argumentconstructor
noOfWatts=50;
indicator=true;
location=“X”;
}
//…
}
classGreenhouse{
//…
LightextraLight=newLight();//(2)Callofexplicitdefault
constructor
}
Theno-argumentconstructorensuresthatanyobjectcreatedwiththeobjectcreation
expressionnewLight(),asat(2),willhaveitsfieldsnoOfWatts,indicator,and
locationinitializedto50,true,and"X",respectively.
Ifaclassdefinesanyconstructor,itcannolongerrelyonthedefaultconstructortosetthe
stateofitsobjects.Ifsuchaclassrequiresano-argumentconstructor,itmustprovideits
ownimplementation,asintheprecedingexample.InthenextexampletheclassLight
doesnotprovideano-argumentconstructor,butratherincludesanon-zeroargument
constructorat(1).Itiscalledat(2)whenanobjectoftheclassLightiscreatedwiththe
newoperator.Anyattempttocallthedefaultconstructorwillbeflaggedasacompile-
timeerror,asshownat(3).
Clickheretoviewcodeimage
classLight{
//…
//Onlynon-zeroargumentconstructor:
Light(intnoOfWatts,booleanindicator,Stringlocation){//(1)
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
}
//…
}
classGreenhouse{
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//…
LightmoreLight=newLight(100,true,“Greenhouse”);//(2)OK
LightfirstLight=newLight();//(3)Compile-time
error
}
OverloadedConstructors
Likemethods,constructorscanbeoverloaded.Sincetheconstructorsinaclassallhave
thesamenameastheclass,theirsignaturesaredifferentiatedbytheirparameterlists.In
thefollowingexample,theclassLightnowprovidesexplicitimplementationoftheno-
argumentconstructorat(1)andthatofanon-zeroargumentconstructorat(2).The
constructorsareoverloaded,asisevidentbytheirsignatures.Thenon-zeroargument
constructorat(2)iscalledwhenanobjectoftheclassLightiscreatedat(3),andtheno-
argumentconstructorislikewisecalledat(4).Overloadingofconstructorsallows
appropriateinitializationofobjectsoncreation,dependingontheconstructorinvoked(see
chainingofconstructorsin§7.5,p.282).Itisrecommendedtousethe@paramtagina
Javadoccommenttodocumenttheformalparametersofaconstructor.
Clickheretoviewcodeimage
classLight{
//…
//No-argumentconstructor:
Light(){//(1)
noOfWatts=50;
indicator=true;
location=“X”;
}
//Non-zeroargumentconstructor:
Light(intnoOfWatts,booleanindicator,Stringlocation){//(2)
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
}
//…
}
classGreenhouse{
//…
LightmoreLight=newLight(100,true,“Greenhouse”);//(3)OK
LightfirstLight=newLight();//(4)OK
}
ReviewQuestions
3.1Whichoneofthesedeclarationsisavalidmethoddeclaration?
Selecttheonecorrectanswer.
(a)voidmethod1{/*...*/}
(b)voidmethod2(){/*...*/}
(c)voidmethod3(void){/*...*/}
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(d)method4(){/*...*/}
(e)method5(void){/*...*/}
3.2Whichstatements,wheninsertedat(1),willnotresultincompile-timeerrors?
Clickheretoviewcodeimage
publicclassThisUsage{
intplanets;
staticintsuns;
publicvoidgaze(){
inti;
//(1)INSERTSTATEMENTHERE
}
}
Selectthethreecorrectanswers.
(a)i=this.planets;
(b)i=this.suns;
(c)this=newThisUsage();
(d)this.i=4;
(e)this.suns=planets;
3.3Giventhefollowingpairsofmethoddeclarations,whichstatementsaretrue?
Clickheretoviewcodeimage
voidfly(intdistance){}
intfly(inttime,intspeed){returntime*speed;}
voidfall(inttime){}
intfall(intdistance){returndistance;}
voidglide(inttime){}
voidGlide(inttime){}
Selectthetwocorrectanswers.
(a)Thefirstpairofmethodswillcompile,andoverloadthemethodnamefly.
(b)Thesecondpairofmethodswillcompile,andoverloadthemethodname
fall.
(c)Thethirdpairofmethodswillcompile,andoverloadthemethodnameglide.
(d)Thefirstpairofmethodswillnotcompile.
(e)Thesecondpairofmethodswillnotcompile.
(f)Thethirdpairofmethodswillnotcompile.
3.4GivenaclassnamedBook,whichoneoftheseconstructordeclarationsisvalidfor
theclassBook?
Selecttheonecorrectanswer.
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(a)Book(Bookb){}
(b)BookBook(){}
(c)privatefinalBook(){}
(d)voidBook(){}
(e)publicstaticvoidBook(String[]args){}
(f)abstractBook(){}
3.5Whichstatementsaretrue?
Selectthetwocorrectanswers.
(a)Aclassmustdefineaconstructor.
(b)Aconstructorcanbedeclaredprivate.
(c)Aconstructorcanreturnavalue.
(d)Aconstructormustinitializeallfieldswhenaclassisinstantiated.
(e)Aconstructorcanaccessthenon-staticmembersofaclass.
3.6Whatwillbetheresultofcompilingthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
longvar;
publicvoidMyClass(longparam){var=param;}//(1)
publicstaticvoidmain(String[]args){
MyClassa,b;
a=newMyClass();//(2)
b=newMyClass(5);//(3)
}
}
Selecttheonecorrectanswer.
(a)Acompile-timeerrorwilloccurat(1).
(b)Acompile-timeerrorwilloccurat(2).
(c)Acompile-timeerrorwilloccurat(3).
(d)Theprogramwillcompilewithouterrors.
3.4Arrays
Anarrayisadatastructurethatdefinesanindexedcollectionofafixednumberof
homogeneousdataelements.Thismeansthatallelementsinthearrayhavethesamedata
type.Apositioninthearrayisindicatedbyanon-negativeintegervaluecalledtheindex.
Anelementatagivenpositioninthearrayisaccessedusingtheindex.Thesizeofan
arrayisfixedandcannotbechangedafterthearrayhasbeencreated.
InJava,arraysareobjects.Arrayscanbeofprimitivedatatypesorreferencetypes.Inthe
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formercase,allelementsinthearrayareofaspecificprimitivedatatype.Inthelatter
case,allelementsarereferencesofaspecificreferencetype.Referencesinthearraycan
thendenoteobjectsofthisreferencetypeoritssubtypes.Eacharrayobjecthasapublic
finalfieldcalledlength,whichspecifiesthearraysize(i.e.,thenumberofelements
thearraycanaccommodate).Thefirstelementisalwaysatindex0andthelastelementat
indexn–1,wherenisthevalueofthelengthfieldinthearray.
Simplearraysareone-dimensionalarrays—thatis,asimplelistofvalues.Sincearrays
canstorereferencevalues,theobjectsreferencedcanalsobearrayobjects.Thus,
multidimensionalarraysareimplementedasarrayofarrays.
Passingarrayreferencesasparametersisdiscussedin§3.5,p.72.Typeconversionsfor
arrayreferencesonassignmentandonmethodinvocationarediscussedin§7.7,p.309.
DeclaringArrayVariables
Aone-dimensionalarrayvariabledeclarationhaseitherofthefollowingsyntaxes:
element_type[]array_name;
or
element_typearray_name[];
whereelement_typecanbeaprimitivedatatypeorareferencetype.Thearrayvariable
array_namehasthetypeelement_type[].Notethatthearraysizeisnotspecified.Asa
consequence,thearrayvariablearray_namecanbeassignedthereferencevalueofan
arrayofanylength,aslongasitselementshaveelement_type.
Itisimportanttounderstandthatthedeclarationdoesnotactuallycreateanarray.Instead,
itsimplydeclaresareferencethatcanrefertoanarrayobject.The[]notationcanalsobe
specifiedafteravariablenametodeclareitasanarrayvariable,butthenitappliestojust
thatvariable.
Clickheretoviewcodeimage
intanIntArray[],oneInteger;
Pizza[]mediumPizzas,largePizzas;
ThesetwodeclarationsdeclareanIntArrayandmediumPizzastobereference
variablesthatcanrefertoarraysofintvaluesandarraysofPizzaobjects,respectively.
ThevariablelargePizzascandenoteanarrayofPizzaobjects,butthevariable
oneIntegercannotdenoteanarrayofintvalues—itisasimplevariableofthetype
int.
Anarrayvariablethatisdeclaredasafieldinaclass,butisnotexplicitlyinitializedtoany
array,willbeinitializedtothedefaultreferencevaluenull.Thisdefaultinitialization
doesnotapplytolocalreferencevariablesand,therefore,doesnotapplytolocalarray
variableseither(§2.4,p.42).Thisbehaviorshouldnotbeconfusedwithinitializationof
theelementsofanarrayduringarrayconstruction.
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ConstructinganArray
Anarraycanbeconstructedforafixednumberofelementsofaspecifictype,usingthe
newoperator.Thereferencevalueoftheresultingarraycanbeassignedtoanarray
variableofthecorrespondingtype.Thesyntaxofthearraycreationexpressionisshown
ontheright-handsideofthefollowingassignmentstatement:
Clickheretoviewcodeimage
array_name=newelement_type[array_size];
Theminimumvalueofarray_sizeis0;inotherwordszero-lengtharrayscanbe
constructedinJava.Ifthearraysizeisnegative,aNegativeArraySizeException
isthrownatruntime.
Giventhedeclarations
Clickheretoviewcodeimage
intanIntArray[],oneInteger;
Pizza[]mediumPizzas,largePizzas;
thethreearraysinthedeclarationscanbeconstructedasfollows:
Clickheretoviewcodeimage
anIntArray=newint[10];//arrayfor10integers
mediumPizzas=newPizza[5];//arrayof5pizzas
largePizzas=newPizza[3];//arrayof3pizzas
Thearraydeclarationandconstructioncanbecombined.
Clickheretoviewcodeimage
element_type1[]array_name=newelement_type2[array_size];
Intheprecedingsyntax,thearraytypeelement_type2[]mustbeassignabletothearray
typeelement_type1[](§7.7,p.309).Whenthearrayisconstructed,allofitselementsare
initializedtothedefaultvalueforelement_type2.Thisistrueforbothmemberandlocal
arrayswhentheyareconstructed.
Inthenextexamples,thecodeconstructsthearray,andthearrayelementsareimplicitly
initializedtotheirdefaultvalues.Forexample,allelementsofthearrayanIntArrayget
thevalue0,andallelementsofthearraymediumPizzasgetthevaluenullwhenthe
arraysareconstructed.
Clickheretoviewcodeimage
int[]anIntArray=newint[10];//Defaultelementvalue:0
Pizza[]mediumPizzas=newPizza[5];//Defaultelementvalue:
null
Thevalueofthefieldlengthineacharrayissettothenumberofelementsspecified
duringtheconstructionofthearray;forexample,mediumPizzas.lengthhasthe
value5.
Onceanarrayhasbeenconstructed,itselementscanalsobeexplicitlyinitialized
individually—forexample,inaloop.Theexamplesintherestofthissectionmakeuseof
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alooptotraversetheelementsofanarrayforvariouspurposes.
InitializinganArray
Javaprovidesthemeansofdeclaring,constructing,andexplicitlyinitializinganarrayin
onedeclarationstatement:
Clickheretoviewcodeimage
element_type[]array_name={array_initialize_list};
Thisformofinitializationappliestofieldsaswellastolocalarrays.The
array_initialize_listisacomma-separatedlistofzeroormoreexpressions.Suchanarray
initializerresultsintheconstructionandinitializationofthearray.
Clickheretoviewcodeimage
int[]anIntArray={13,49,267,15,215};
Inthisdeclarationstatement,thevariableanIntArrayisdeclaredasareferencetoan
arrayofints.Thearrayinitializerresultsintheconstructionofanarraytoholdfive
elements(equaltothelengthofthelistofexpressionsintheblock),wherethefirst
elementisinitializedtothevalueofthefirstexpression(13),thesecondelementtothe
valueofthesecondexpression(49),andsoon.
Clickheretoviewcodeimage
Pizza[]pizzaOrder={newPizza(),newPizza(),null};
Inthisdeclarationstatement,thevariablepizzaOrderisdeclaredasareferencetoan
arrayofPizzaobjects.Thearrayinitializerconstructsanarraytoholdthreeelements.
TheinitializationcodesetsthefirsttwoelementsofthearraytorefertotwoPizza
objects,whilethelastelementisinitializedtothenullreference.Thereferencevalueof
thearrayofPizzaobjectsisassignedtothereferencepizzaOrder.Notealsothatthis
declarationstatementactuallycreatesthreeobjects:thearrayobjectwiththreereferences
andthetwoPizzaobjects.
Theexpressionsinthearray_initialize_listareevaluatedfromlefttoright,andthearray
nameobviouslycannotoccurinanyoftheexpressionsinthelist.Inthepreceding
examples,thearray_initialize_lististerminatedbytherightbrace,},oftheblock.Thelist
canalsobelegallyterminatedbyacomma.Thefollowingarrayhaslength2,andnot3:
Clickheretoviewcodeimage
Topping[]pizzaToppings={newTopping(“cheese”),newTopping(“tomato”),};
Thedeclarationstatementat(1)inthefollowingcodedefinesanarrayoffourString
objects,whilethedeclarationstatementat(2)showsthataStringobjectisnotthesame
asanarrayofchar.
Clickheretoviewcodeimage
//Arraywith4Stringobjects:
String[]pets={“crocodiles”,“elephants”,“crocophants”,“elediles”};//
(1)
//Arrayof3characters:
char[]charArray={‘a’,‘h’,‘a’};//(2)Notthesameas“aha”
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UsinganArray
Thearrayobjectisreferencedbythearrayname,butindividualarrayelementsare
accessedbyspecifyinganindexwiththe[]operator.Thearrayelementaccess
expressionhasthefollowingsyntax:
array_name[index_expression]
Eachindividualelementistreatedasasimplevariableoftheelementtype.Theindexis
specifiedbytheindex_expression,whosevalueshouldbepromotabletoanintvalue;
otherwise,acompile-timeerrorisflagged.Sincethelowerboundofanarrayindexis
always0,theupperboundis1lessthanthearraysize—thatis,array_name.length-1.
Theithelementinthearrayhasindex(i-1).Atruntime,theindexvalueis
automaticallycheckedtoensurethatitiswithinthearrayindexbounds.Iftheindexvalue
islessthan0,orgreaterthanorequaltoarray_name.length,an
ArrayIndexOutOfBoundsExceptionisthrown.Aprogramcaneithercheckthe
indexexplicitlyorcatchtheruntimeexception(§6.5,p.230),butanillegalindexis
typicallyanindicationofaprogrammingerror.
Inthearrayelementaccessexpression,thearray_namecanbeanyexpressionthatreturns
areferencetoanarray.Forexample,theexpressionontheright-handsideofthefollowing
assignmentstatementreturnsthecharacter'H'atindex1inthecharacterarrayreturned
byacalltothetoCharArray()methodoftheStringclass:
Clickheretoviewcodeimage
charletter=“AHA”.toCharArray()[1];//‘H’
Thearrayoperator[]isusedtodeclarearraytypes(Topping[]),specifythearraysize
(newTopping[3]),andaccessarrayelements(toppings[1]).Thisoperatorisnot
usedwhenthearrayreferenceismanipulated,suchasinanarrayreferenceassignment
(§7.9,p.312),orwhenthearrayreferenceispassedasanactualparameterinamethod
call(§3.5,p.77).
Example3.3showstraversalofarraysusingforloops(§6.3,p.215andp.217).A
for(;;)loopat(3)inthemain()methodinitializesthelocalarraytrialArray
declaredat(2)fivetimeswithpseudo-randomnumbers(from0.0to100.0),bycalling
themethodrandomize()declaredat(5).Theminimumvalueinthearrayisfoundby
callingthemethodfindMinimum()declaredat(6),andisstoredinthearray
storeMinimumdeclaredat(1).Bothofthesemethodsalsouseafor(;;)loop.The
loopvariableisinitializedtoastartvalue—0in(3)and(5),and1in(6).Theloop
conditiontestswhethertheloopvariableislessthanthelengthofthearray;this
guaranteesthattheloopwillterminatewhenthelastelementhasbeenaccessed.Theloop
variableisincrementedaftereachiterationtoaccessthenextelement.
Afor(:)loopat(4)inthemain()methodisusedtoprinttheminimumvaluesfrom
thetrials,aselementsarereadconsecutivelyfromthearray,withoutkeepingtrackofan
indexvalue.
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Example3.3UsingArrays
Clickheretoviewcodeimage
publicclassTrials{
publicstaticvoidmain(String[]args){
//Declareandconstructthelocalarrays:
double[]storeMinimum=newdouble[5];//(1)
double[]trialArray=newdouble[15];//(2)
for(inti=0;i<storeMinimum.length;++i){//(3)
//Initializethearray.
randomize(trialArray);
//Findandstoretheminimumvalue.
storeMinimum[i]=findMinimum(trialArray);
}
//Printtheminimumvalues:(4)
for(doubleminValue:storeMinimum)
System.out.printf(“%.4f%n”,minValue);
}
publicstaticvoidrandomize(double[]valArray){//(5)
for(inti=0;i<valArray.length;++i)
valArray[i]=Math.random()*100.0;
}
publicstaticdoublefindMinimum(double[]valArray){//(6)
//Assumethearrayhasatleastoneelement.
doubleminValue=valArray[0];
for(inti=1;i<valArray.length;++i)
minValue=Math.min(minValue,valArray[i]);
returnminValue;
}
}
Probableoutputfromtheprogram:
6.9330
2.7819
6.7427
18.0849
26.2462
AnonymousArrays
Asshownearlierinthissection,thefollowingdeclarationstatementcanbeusedto
constructarraysusinganarraycreationexpression:
Clickheretoviewcodeimage
element_type1[]array_name=newelement_type2[array_size];//(1)
int[]intArray=newint[5];
Thesizeofthearrayisspecifiedinthearraycreationexpression,whichcreatesthearray
andinitializesthearrayelementstotheirdefaultvalues.Bycomparison,thefollowing
declarationstatementbothcreatesthearrayandinitializesthearrayelementstospecific
valuesgiveninthearrayinitializer:
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Clickheretoviewcodeimage
element_type[]array_name={array_initialize_list};//(2)
int[]intArray={3,5,2,8,6};
However,thearrayinitializerisnotanexpression.Javahasanotherarraycreation
expression,calledananonymousarray,whichallowstheconceptofthearraycreation
expressionfrom(1)tobecombinedwiththearrayinitializerfrom(2),soastocreateand
initializeanarrayobject:
Clickheretoviewcodeimage
newelement_type[]{array_initialize_list}
newint[]{3,5,2,8,6}
Thisconstructhasenoughinformationtocreateanamelessarrayofaspecifictype.
Neitherthenameofthearraynorthesizeofthearrayisspecified.Theconstructreturns
thereferencevalueofthenewlycreatedarray,whichcanbeassignedtoreferencesand
passedasargumentinmethodcalls.Inparticular,thefollowingdeclarationstatementsare
equivalent:
Clickheretoviewcodeimage
int[]intArray={3,5,2,8,6};//(1)
int[]intArray=newint[]{3,5,2,8,6};//(2)
In(1),anarrayinitializerisusedtocreateandinitializetheelements.In(2),an
anonymousarrayexpressionisused.Itistemptingtousethearrayinitializerasan
expression—forexample,inanassignmentstatement,asashortcutforassigningvaluesto
arrayelementsinonego.However,thisisillegal;instead,ananonymousarrayexpression
shouldbeused.Theconceptoftheanonymousarraycombinesthedefinitionandthe
creationofthearrayintooneoperation.
Clickheretoviewcodeimage
int[]daysInMonth;
daysInMonth={31,28,31,30,31,30,
31,31,30,31,30,31};//Compile-time
error
daysInMonth=newint[]{31,28,31,30,31,30,31,31,30,31,30,31};//
OK
InExample3.4,ananonymousarrayisconstructedat(1),andpassedasanactual
parametertothestaticmethodfindMinimum()definedat(2).Notethatnoarrayname
orarraysizeisspecifiedfortheanonymousarray.
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Example3.4UsingAnonymousArrays
Clickheretoviewcodeimage
publicclassAnonArray{
publicstaticvoidmain(String[]args){
System.out.println(“Minimumvalue:”+
findMinimum(newint[]{3,5,2,8,6}));//(1)
}
publicstaticintfindMinimum(int[]dataSeq){//(2)
//Assumethearrayhasatleastoneelement.
intmin=dataSeq[0];
for(intindex=1;index<dataSeq.length;++index)
if(dataSeq[index]<min)
min=dataSeq[index];
returnmin;
}
}
Outputfromtheprogram:
Minimumvalue:2
MultidimensionalArrays
Sinceanarrayelementcanbeanobjectreferenceandarraysareobjects,arrayelements
canthemselvesrefertootherarrays.InJava,anarrayofarrayscanbedefinedasfollows:
Clickheretoviewcodeimage
element_type[][]...[]array_name;
or
Clickheretoviewcodeimage
element_typearray_name[][]...[];
Infact,thesequenceofsquarebracketpairs,[],indicatingthenumberofdimensions,can
bedistributedasapostfixtoboththeelementtypeandthearrayname.Arraysofarrays
areoftencalledmultidimensionalarrays.
Thefollowingdeclarationsareallequivalent:
Clickheretoviewcodeimage
int[][]mXnArray;//2-dimensionalarray
int[]mXnArray[];//2-dimensionalarray
intmXnArray[][];//2-dimensionalarray
Itiscustomarytocombinethedeclarationwiththeconstructionofthemultidimensional
array.
Clickheretoviewcodeimage
int[][]mXnArray=newint[4][5];//4x5matrixofints
ThepreviousdeclarationconstructsanarraymXnArrayoffourelements,whereeach
elementisanarray(row)offiveintvalues.Theconceptofrowsandcolumnsisoften
usedtodescribethedimensionsofa2-dimensionalarray,whichisoftencalledamatrix.
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However,suchaninterpretationisnotdictatedbytheJavalanguage.
EachrowinthepreviousmatrixisdenotedbymXnArray[i],where0≤i<4.Each
elementintheithrow,mXnArray[i],isaccessedbymXnArray[i][j],where0≤
j<5.ThenumberofrowsisgivenbymXnArray.length,inthiscase4,andthe
numberofvaluesintheithrowisgivenbymXnArray[i].length,inthiscase5for
alltherows,where0≤i<4.
Multidimensionalarrayscanalsobeconstructedandexplicitlyinitializedusingthearray
initializersdiscussedforsimplearrays.Notethateachrowisanarraythatusesanarray
initializertospecifyitsvalues:
Clickheretoviewcodeimage
double[][]identityMatrix={
{1.0,0.0,0.0,0.0},//1.row
{0.0,1.0,0.0,0.0},//2.row
{0.0,0.0,1.0,0.0},//3.row
{0.0,0.0,0.0,1.0}//4.row
};//4x4floating-pointmatrix
Arraysinamultidimensionalarrayneednothavethesamelength;whentheydonot,they
arecalledraggedarrays.ThearrayofarrayspizzaGaloreinthefollowingcodehas
fiverows;thefirstfourrowshavedifferentlengthsbutthefifthrowisleftunconstructed:
Clickheretoviewcodeimage
Pizza[][]pizzaGalore={
{newPizza(),null,newPizza()},//1.rowisanarrayof3elements.
{null,newPizza()},//2.rowisanarrayof2elements.
newPizza[1],//3.rowisanarrayof1element.
{},//4.rowisanarrayof0elements.
null//5.rowisnotconstructed.
};
Whenconstructingmultidimensionalarrayswiththenewoperator,thelengthofthe
deeplynestedarraysmaybeomitted.Insuchacase,thesearraysareleftunconstructed.
Forexample,anarrayofarraystorepresentaroomonafloorinahotelonastreetina
citycanhavethetypeHotelRoom[][][][].Fromlefttoright,thesquarebrackets
representindicesforstreet,hotel,floor,androom,respectively.This4-dimensionalarray
ofarrayscanbeconstructedpiecemeal,startingwiththeleftmostdimensionand
proceedingtotherightmostsuccessively.
Clickheretoviewcodeimage
HotelRoom[][][][]rooms=newHotelRoom[10][5][][];//Juststreetsand
hotels.
Theprecedingdeclarationconstructsthearrayofarraysroomspartiallywithtenstreets,
whereeachstreethasfivehotels.Floorsandroomscanbeaddedtoaparticularhotelona
particularstreet:
Clickheretoviewcodeimage
rooms[0][0]=newHotelRoom[3][];//3floorsin1sthotelon1st
street.
rooms[0][0][0]=newHotelRoom[8];//8roomson1stfloorinthis
hotel.
rooms[0][0][0][0]=newHotelRoom();//Initializes1stroomonthis
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floor.
Thenextcodesnippetconstructsanarrayofarraysmatrix,wherethefirstrowhasone
element,thesecondrowhastwoelements,andthethirdrowhasthreeelements.Notethat
theouterarrayisconstructedfirst.Theseconddimensionisconstructedinaloopthat
constructsthearrayineachrow.Theelementsinthemultidimensionalarraywillbe
implicitlyinitializedtothedefaultdoublevalue(0.0D).InFigure3.1,thearrayof
arraysmatrixisdepictedaftertheelementshavebeenexplicitlyinitialized.
Clickheretoviewcodeimage
double[][]matrix=newdouble[3][];//Numberofrows.
for(inti=0;i<matrix.length;++i)
matrix[i]=newdouble[i+1];//Constructarow.
Figure3.1ArrayofArrays
Twootherwaysofinitializingsuchanarrayofarraysareshownnext.Thefirstapproach
usesarrayinitializers,andthesecondusesananonymousarrayofarrays.
Clickheretoviewcodeimage
double[][]matrix2={//Usingarrayinitializers.
{0.0},//1.row
{0.0,0.0},//2.row
{0.0,0.0,0.0}//3.row
};
double[][]matrix3=newdouble[][]{//Usingananonymousarrayofarrays.
{0.0},//1.row
{0.0,0.0},//2.row
{0.0,0.0,0.0}//3.row
};
Thetypeofthevariablematrixisdouble[][],atwo-dimensionalarrayofdouble
values.Thetypeofthevariablematrix[i](where0≤i<matrix.length)is
double[],aone-dimensionalarrayofdoublevalues.Thetypeofthevariable
matrix[i][j](where0≤i<matrix.lengthand0≤j<matrix[i].length)
isdouble,asimplevariableoftypedouble.
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Nestedloopsareanaturalmatchformanipulatingmultidimensionalarrays.InExample
3.5,arectangular4×3intmatrixisdeclaredandconstructedat(1).Theprogramfinds
theminimumvalueinthematrix.Theouterloopat(2)traversestherows
(mXnArray[i],where0≤i<mXnArray.length),andtheinnerloopat(3)
traversestheelementsineachrowinturn(mXnArray[i][j],where0≤j<
mXnArray[i].length).TheouterloopisexecutedmXnArray.lengthtimes,or4
times,andtheinnerloopisexecuted(mXnArray.length)×
(mXnArray[i].length),or12times,sinceallrowshavethesamelength3.
Thefor(:)loopalsoprovidesasafeandconvenientwayoftraversinganarray.Several
examplesofitsuseareprovidedin§6.3,p.217.
Example3.5UsingMultidimensionalArrays
Clickheretoviewcodeimage
publicclassMultiArrays{
publicstaticvoidmain(String[]args){
//DeclareandconstructtheMXNmatrix.
int[][]mXnArray={//(1)
{16,7,12},//1.row
{9,20,18},//2.row
{14,11,5},//3.row
{8,5,10}//4.row
};//4x3intmatrix
//FindtheminimumvalueinaMXNmatrix:
intmin=mXnArray[0][0];
for(inti=0;i<mXnArray.length;++i)//(2)
//FindmininmXnArray[i],intherowgivenbyindexi:
for(intj=0;j<mXnArray[i].length;++j)//(3)
min=Math.min(min,mXnArray[i][j]);
System.out.println(“Minimumvalue:”+min);
}
}
Outputfromtheprogram:
Minimumvalue:5
SortingArrays
Sortingimpliesorderingtheelementsaccordingtosomerankingcriteria,usuallybasedon
thevaluesoftheelements.Thevaluesofnumericdatatypescanbecomparedandranked
byusingtherelationaloperators.Forcomparingobjectsofaclass,theclasstypically
implementsthecompareTo()methodoftheComparableinterface.Theordering
definedbythismethodiscalledthenaturalorderingfortheobjectsoftheclass.The
wrapperclassesforprimitivevaluesandtheStringclassimplementthe
compareTo()method(§8.3,p.350,and§8.4,p.363,respectively).
Thejava.util.Arraysclassprovidesmanyoverloadedversionsofthesort()
methodtosortpracticallyanytypeofarray.
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voidsort(type[]array)
Permittedtypeforelementsincludesbyte,char,double,float,int,long,
short,andObject.Themethodsortstheelementsinthearrayaccordingtotheir
naturalordering.Inthecaseofanarrayofobjectsbeingpassedasargument,the
objectsmustbemutuallycomparable;thatis,itshouldbepossibletocompareany
twoobjectsinthearrayaccordingtothenaturalorderingdefinedbythe
compareTo()methodoftheComparableinterface.
Anappropriateimportstatementshouldbeincludedinthesourcecodetoaccessthe
java.util.Arraysclass.Inthenextcodesnippet,anarrayofstringsissorted
accordingtonaturalorderingforstrings—thatis,basedontheUnicodevaluesofthe
charactersinthestrings:
Clickheretoviewcodeimage
String[]strArray={“biggest”,“big”,“bigger”,“Bigfoot”};
Arrays.sort(strArray);//Naturalordering:[Bigfoot,big,bigger,
biggest]
Thenextexamplesillustratesortinganarrayofprimitivevalues(int)at(1),andanarray
oftypeObjectcontainingmutuallycomparableelements(String)at(2).In(3),the
numericalvaluesareautoboxedintotheircorrespondingwrapperclasses(§8.3,p.346),
buttheobjectsofdifferentwrapperclassesandtheStringclassarenotmutually
comparable.In(4),thenumericalvaluesarealsoautoboxedintotheircorresponding
wrapperclasses,butagaintheobjectsofdifferentwrapperclassesarenotmutually
comparable.AClassCastExceptionisthrownwhentheelementsarenotmutually
comparable.
Clickheretoviewcodeimage
int[]intArray={5,3,7,1};//int
Arrays.sort(intArray);//(1)Naturalordering:[1,3,
5,7]
Object[]objArray1={“I”,“am”,“OK”};//String
Arrays.sort(objArray1);//(2)Naturalordering:[I,OK,
am]
Object[]objArray2={23,“ten”,3.14};//Notmutuallycomparable
Arrays.sort(objArray2);//(3)ClassCastException
Number[]numbers={23,3.14,10L};//Notmutuallycomparable
Arrays.sort(numbers);//(4)ClassCastException
SearchingArrays
Acommonoperationonanarrayistosearchthearrayforagivenelement,calledthekey.
Thejava.util.Arraysclassprovidesoverloadedversionsofthe
binarySearch()methodtosearchinpracticallyanytypeofarraythatissorted.
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intbinarySearch(type[]array,typekey)
Permittedtypeforelementsincludebyte,char,double,float,int,long,
short,andObject.Thearraymustbesortedinascendingorderbeforecalling
thismethod,ortheresultsareunpredictable.Inthecasewhereanarrayofobjectsis
passedasargument,theobjectsmustbesortedinascendingorderaccordingtotheir
naturalordering,asdefinedbytheComparableinterface.
Themethodreturnstheindextothekeyinthesortedarray,ifthekeyexists.The
indexisthenguaranteedtobegreaterorequalto0.Ifthekeyisnotfound,a
negativeindexisreturned,correspondingto–(insertionpoint+1),whereinsertion
pointistheindexoftheelementwherethekeywouldhavebeenfound,ifithad
beeninthearray.Ifthereareduplicateelementsequaltothekey,thereisno
guaranteewhichduplicate’sindexwillbereturned.Theelementsandthekeymust
bemutuallycomparable.
Anappropriateimportstatementshouldbeincludedinthesourcecodetoaccessthe
java.util.Arraysclass.Inthecodethatfollows,thereturnvalue–3indicatesthat
thekeywouldhavebeenfoundatindex2haditbeeninthelist:
Clickheretoviewcodeimage
//SortedStringarray(naturalordering):[Bigfoot,big,bigger,biggest]
//Searchinnaturalordering:
intindex1=Arrays.binarySearch(strArray,“bigger”);//Successful:2
intindex2=Arrays.binarySearch(strArray,“bigfeet”);//Unsuccessful:-3
intindex3=Arrays.binarySearch(strArray,“bigmouth”);//Unsuccessful:-5
Resultsareunpredictableifthearrayisnotsorted,oriftheorderingusedinthesearchis
notthesameasthesortordering.SearchinginthestrArrayusingnaturalordering
whenthearrayissortedinreversenaturalorderinggivesthewrongresult:
Clickheretoviewcodeimage
//SortedStringarray(inversenaturalordering):[biggest,bigger,big,
Bigfoot]
//Searchinnaturalordering:
intindex4=Arrays.binarySearch(strArray,“big”);//-1(INCORRECT)
AClassCastExceptionisthrownifthekeyandtheelementsarenotmutually
comparable:
Clickheretoviewcodeimage
intindex5=Arrays.binarySearch(strArray,4);//Key:4=>
ClassCastException
However,thisincompatibilityiscaughtatcompiletimeinthecaseofarrayswith
primitivevalues:
Clickheretoviewcodeimage
//Sortedintarray(naturalordering):[1,3,5,7]
intindex6=Arrays.binarySearch(intArray,4.5);//Key:4.5=>compile-time
error!
ThemethodbinarySearch()derivesitsnamefromthedivide-and-conqueralgorithm
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thatitusestoperformthesearch.Itrepeatedlydividestheremainingelementstobe
searchedintotwohalvesandselectsthehalfcontainingthekeytocontinuethesearchin,
untileitherthekeyisfoundortherearenomoreelementslefttosearch.
ReviewQuestions
3.7Giventhefollowingdeclaration,whichexpressionreturnsthesizeofthearray,
assumingthatthearrayreferencehasbeenproperlyinitialized?
int[]array;
Selecttheonecorrectanswer.
(a)array[].length()
(b)array.length()
(c)array[].length
(d)array.length
(e)array[].size()
(f)array.size()
(g)array[].size
(h)array.size
3.8Isitpossibletocreatearraysoflengthzero?
Selecttheonecorrectanswer.
(a)Yes,youcancreatearraysofanytypewithlengthzero.
(b)Yes,butonlyforprimitivedatatypes.
(c)Yes,butonlyforarraysofreferencetypes.
(d)No,youcannotcreatezero-lengtharrays,butthemain()methodmaybe
passedazero-lengtharrayofStringswhennoprogramargumentsare
specified.
(e)No,itisnotpossibletocreatearraysoflengthzeroinJava.
3.9Whichoneofthefollowingarraydeclarationstatementsisnotlegal?
Selecttheonecorrectanswer.
(a)int[]a[]=newint[4][4];
(b)inta[][]=newint[4][4];
(c)inta[][]=newint[][4];
(d)int[]a[]=newint[4][];
(e)int[][]a=newint[4][4];
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3.10Whichofthesearraydeclarationstatementsarenotlegal?
Selectthetwocorrectanswers.
(a)int[]i[]={{1,2},{1},{},{1,2,3}};
(b)inti[]=newint[2]{1,2};
(c)inti[][]=newint[][]{{1,2,3},{4,5,6}};
(d)inti[][]={{1,2},newint[2]};
(e)inti[4]={1,2,3,4};
3.11Whatwouldbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
intsize=20;
int[]arr=newint[size];
for(inti=0;i<size;++i){
System.out.println(arr[i]);
}
}
}
Selecttheonecorrectanswer.
(a)Thecodewillnotcompile,becausethearraytypeint[]isincorrect.
(b)Theprogramwillcompile,butwillthrowan
ArrayIndexOutOfBoundsExceptionwhenrun.
(c)Theprogramwillcompileandrunwithouterror,butwillproducenooutput.
(d)Theprogramwillcompileandrunwithouterror,andwillprintthenumbers0
through19.
(e)Theprogramwillcompileandrunwithouterror,andwillprint0twentytimes.
(f)Theprogramwillcompileandrunwithouterror,andwillprintnulltwenty
times.
3.12Whatwouldbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassDefaultValuesTest{
int[]ia=newint[1];
booleanb;
inti;
Objecto;
publicstaticvoidmain(String[]args){
DefaultValuesTestinstance=newDefaultValuesTest();
instance.print();
}
publicvoidprint(){
System.out.println(ia[0]+””+b+””+i+””+o);
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}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompilebecauseofuninitializedvariables.
(b)Theprogramwillthrowajava.lang.NullPointerExceptionwhen
run.
(c)Theprogramwillprint0falseNaNnull.
(d)Theprogramwillprint0false0null.
(e)Theprogramwillprintnull00null.
(f)Theprogramwillprintnullfalse0null.
3.5ParameterPassing
Objectscommunicatebycallingmethodsoneachother.Amethodcallisusedtoinvokea
methodonanobject.Parametersinthemethodcallprovideonewayofexchanging
informationbetweenthecallerobjectandthecalleeobject(whichneednotbedifferent).
Declaringmethodsisdiscussedin§3.2,p.49.Invokingstaticmethodsonclassesis
discussedin§4.8,p.132.
Thesyntaxofamethodcallcanbeanyoneofthefollowing:
Clickheretoviewcodeimage
object_reference.method_name(actual_parameter_list)
class_name.static_method_name(actual_parameter_list)
method_name(actual_parameter_list)
Theobject_referencemustbeanexpressionthatevaluatestoareferencevaluedenoting
theobjectonwhichthemethodiscalled.Ifthecallerandthecalleearethesame,object
referencecanbeomitted(seethediscussionofthethisreferencein§3.2,p.50).The
class_namecanbethefullyqualifiedname(§4.2,p.97)oftheclass.The
actual_parameter_listiscommaseparatedifthereismorethanoneparameter.The
parenthesesaremandatoryeveniftheactualparameterlistisempty.Thisdistinguishesthe
methodcallfromfieldaccess.Onecanspecifyfullyqualifiednamesforclassesand
packagesusingthedotoperator(.).
Clickheretoviewcodeimage
objRef.doIt(time,place);//Explicitobjectreference
inti=java.lang.Math.abs(-1);//Fullyqualifiedclassname
intj=Math.abs(-1);//Simpleclassname
someMethod(ofValue);//Objectorclassimplicitlyimplied
someObjRef.make().make().make();//make()returnsareferencevalue
Thedotoperator(.)hasleftassociativity.Inthelastcodeline,thefirstcallofthe
make()methodreturnsareferencevaluethatdenotestheobjectonwhichtoexecutethe
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nextcall,andsoon.Thisisanexampleofcallchaining.
Eachactualparameter(alsocalledanargument)isanexpressionthatisevaluated,and
whosevalueispassedtothemethodwhenthemethodisinvoked.Itsvaluecanvaryfrom
invocationtoinvocation.Formalparametersareparametersdefinedinthemethod
declaration(§3.2,p.49)andarelocaltothemethod(§2.4,p.44).
InJava,allparametersarepassedbyvalue—thatis,anactualparameterisevaluatedand
itsvalueisassignedtothecorrespondingformalparameter.Table3.1summarizesthe
valuethatispasseddependingonthetypeoftheparameters.Inthecaseofprimitivedata
types,thedatavalueoftheactualparameterispassed.Iftheactualparameterisa
referencetoanobject,thereferencevalueofthedenotedobjectispassedandnotthe
objectitself.Analogously,iftheactualparameterisanarrayelementofaprimitivedata
type,itsdatavalueispassed,andifthearrayelementisareferencetoanobject,thenits
referencevalueispassed.
Table3.1ParameterPassingbyValue
Itshouldalsobestressedthateachinvocationofamethodhasitsowncopiesoftheformal
parameters,asisthecaseforanylocalvariablesinthemethod(§6.5,p.230).
Theorderofevaluationintheactualparameterlistisalwaysfromlefttoright.The
evaluationofanactualparametercanbeinfluencedbyanearlierevaluationofanactual
parameter.Giventhefollowingdeclaration:
inti=4;
themethodcall
leftRight(i++,i);
iseffectivelythesameas
leftRight(4,5);
andnotthesameas
leftRight(4,4);
Anoverviewoftheconversionsthatcantakeplaceinamethodinvocationcontextis
providedin§5.2,p.148.Methodinvocationconversionsforprimitivevaluesare
discussedinthenextsubsection(p.73),andthoseforreferencetypesarediscussedin
§7.10,p.315.Callingvariablearitymethodsisdiscussedin§3.6,p.81.
Forthesakeofsimplicity,theexamplesinsubsequentsectionsprimarilyshowmethod
invocationonthesameobjectorthesameclass.Theparameterpassingmechanismisno
differentwhendifferentobjectsorclassesareinvolved.
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PassingPrimitiveDataValues
Anactualparameterisanexpressionthatisevaluatedfirst,withtheresultingvaluethen
beingassignedtothecorrespondingformalparameteratmethodinvocation.Theuseof
thisvalueinthemethodhasnoinfluenceontheactualparameter.Inparticular,whenthe
actualparameterisavariableofaprimitivedatatype,thevalueofthevariableiscopiedto
theformalparameteratmethodinvocation.Sinceformalparametersarelocaltothe
method,anychangesmadetotheformalparameterwillnotbereflectedintheactual
parameterafterthecallcompletes.
Legaltypeconversionsbetweenactualparametersandformalparametersofprimitivedata
typesaresummarizedherefromTable5.1,p.147:
•Wideningprimitiveconversion
•Unboxingconversion,followedbyanoptionalwideningprimitiveconversion
Theseconversionsareillustratedbyinvokingthefollowingmethod
Clickheretoviewcodeimage
staticvoiddoIt(longi){/*…*/}
withthefollowingcode:
Clickheretoviewcodeimage
IntegerintRef=34;
LonglongRef=34L;
doIt(34);//(1)Primitivewideningconversion:long<—int
doIt(longRef);//(2)Unboxing:long<—Long
doIt(intRef);//(3)Unboxing,followedbyprimitivewideningconversion:
//long<—int<—Integer
However,forparameterpassing,therearenoimplicitnarrowingconversionsforinteger
constantexpressions(§5.2,p.148).
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Example3.6PassingPrimitiveValues
Clickheretoviewcodeimage
publicclassCustomerOne{
publicstaticvoidmain(String[]args){
PizzaFactorypizzaHouse=newPizzaFactory();
intpricePrPizza=15;
System.out.println(“ValueofpricePrPizzabeforecall:”+pricePrPizza);
doubletotPrice=pizzaHouse.calcPrice(4,pricePrPizza);//
(1)
System.out.println(“ValueofpricePrPizzaaftercall:”+pricePrPizza);
}
}
classPizzaFactory{
publicdoublecalcPrice(intnumberOfPizzas,doublepizzaPrice){//
(2)
pizzaPrice=pizzaPrice/2.0;//Changesprice.
System.out.println(“Changedpizzapriceinthemethod:”+pizzaPrice);
returnnumberOfPizzas*pizzaPrice;
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
ValueofpricePrPizzabeforecall:15
Changedpizzapriceinthemethod:7.5
ValueofpricePrPizzaaftercall:15
InExample3.6,themethodcalcPrice()isdefinedintheclassPizzaFactoryat
(2).ItiscalledfromtheCustomerOne.main()methodat(1).Thevalueofthefirst
actualparameter,4,iscopiedtotheintformalparameternumberOfPizzas.Notethat
thesecondactualparameterpricePrPizzaisofthetypeint,whilethecorresponding
formalparameterpizzaPriceisofthetypedouble.Beforethevalueoftheactual
parameterpricePrPizzaiscopiedtotheformalparameterpizzaPrice,itis
implicitlywidenedtoadouble.ThepassingofprimitivevaluesisillustratedinFigure
3.2.
Figure3.2ParameterPassing:PrimitiveDataValues
ThevalueoftheformalparameterpizzaPriceischangedinthecalcPrice()
method,butthisdoesnotaffectthevalueoftheactualparameterpricePrPizzaon
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return:Itstillhasthevalue15.Thebottomlineisthattheformalparameterisalocal
variable,andchangingitsvaluedoesnotaffectthevalueoftheactualparameter.
PassingReferenceValues
Iftheactualparameterexpressionevaluatestoareferencevalue,theresultingreference
valueisassignedtothecorrespondingformalparameterreferenceatmethodinvocation.
Inparticular,ifanactualparameterisareferencetoanobject,thereferencevaluestored
intheactualparameterispassed.Consequently,boththeactualparameterandtheformal
parameterarealiasestotheobjectdenotedbythisreferencevalueduringtheinvocationof
themethod.Inparticular,thisimpliesthatchangesmadetotheobjectviatheformal
parameterwillbeapparentafterthecallreturns.
Typeconversionsbetweenactualandformalparametersofreferencetypesarediscussed
in§7.10,p.315.
InExample3.7,aPizzaobjectiscreatedat(1).AnyobjectoftheclassPizzacreated
usingtheclassdeclarationat(5)alwaysresultsinabeefpizza.Inthecalltothebake()
methodat(2),thereferencevalueoftheobjectreferencedbytheactualparameter
favoritePizzaisassignedtotheformalparameterpizzaToBeBakedinthe
declarationofthebake()methodat(3).
Example3.7PassingReferenceValues
Clickheretoviewcodeimage
publicclassCustomerTwo{
publicstaticvoidmain(String[]args){
PizzafavoritePizza=newPizza();//(1)
System.out.println(“Meatonpizzabeforebaking:”+favoritePizza.meat);
bake(favoritePizza);//(2)
System.out.println(“Meatonpizzaafterbaking:”+favoritePizza.meat);
}
publicstaticvoidbake(PizzapizzaToBeBaked){//(3)
pizzaToBeBaked.meat=“chicken”;//Changethemeatonthepizza.
pizzaToBeBaked=null;//(4)
}
}
classPizza{//(5)
Stringmeat=“beef”;
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Meatonpizzabeforebaking:beef
Meatonpizzaafterbaking:chicken
Oneparticularconsequenceofpassingreferencevaluestoformalparametersisthatany
changesmadetotheobjectviaformalparameterswillbereflectedbackinthecalling
methodwhenthecallreturns.Inthiscase,thereferencefavoritePizzawillshowthat
chickenhasbeensubstitutedforbeefonthepizza.Settingtheformalparameter
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pizzaToBeBakedtonullat(4)doesnotchangethereferencevalueintheactual
parameterfavoritePizza.Thesituationatmethodinvocation,andjustbeforethe
returnfrommethodbake(),isillustratedinFigure3.3.
Figure3.3ParameterPassing:ReferenceValues
Insummary,theformalparametercanonlychangethestateoftheobjectwhosereference
valuewaspassedtothemethod.
TheparameterpassingstrategyinJavaiscallbyvalueandnotcallbyreference,
regardlessofthetypeoftheparameter.Callbyreferencewouldhaveallowedvaluesinthe
actualparameterstobechangedviaformalparameters;thatis,thevaluein
pricePrPizzawouldbehalvedinExample3.6andfavoritePizzawouldbeset
tonullinExample3.7.However,thiscannotbedirectlyimplementedinJava.
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PassingArrays
Thediscussionofpassingreferencevaluesintheprevioussectionisequallyvalidfor
arrays,asarraysareobjectsinJava.Methodinvocationconversionsforarraytypesare
discussedalongwiththoseforotherreferencetypesin§7.10,p.315.
InExample3.8,theideaistorepeatedlyswapneighboringelementsinanintegerarray
untilthelargestelementinthearraypercolatestothelastpositioninthearray.
Example3.8PassingArrays
Clickheretoviewcodeimage
publicclassPercolate{
publicstaticvoidmain(String[]args){
int[]dataSeq={8,4,6,2,1};//Createandinitializeanarray.
//Writearraybeforepercolation:
printIntArray(dataSeq);
//Percolate:
for(intindex=1;index<dataSeq.length;++index)
if(dataSeq[index-1]>dataSeq[index])
swap(dataSeq,index-1,index);//(1)
//Writearrayafterpercolation:
printIntArray(dataSeq);
}
publicstaticvoidswap(int[]intArray,inti,intj){//(2)
inttmp=intArray[i];intArray[i]=intArray[j];intArray[j]=tmp;
}
publicstaticvoidswap(intv1,intv2){//(3)Logical
error!
inttmp=v1;v1=v2;v2=tmp;
}
publicstaticvoidprintIntArray(int[]array){//(4)
for(intvalue:array)
System.out.print(””+value);
System.out.println();
}
}
Outputfromtheprogram:
84621
46218
Notethatinthedeclarationofthemethodswap()at(2),theformalparameter
intArrayisofthearraytypeint[].Theswap()methodiscalledinthemain()
methodat(1),whereoneoftheactualparametersisthearrayvariabledataSeq.The
referencevalueofthearrayvariabledataSeqisassignedtothearrayvariable
intArrayatmethodinvocation.Afterreturnfromthecalltotheswap()method,the
arrayvariabledataSeqwillreflectthechangesmadetothearrayviathecorresponding
formalparameter.ThissituationisdepictedinFigure3.4atthefirstcallandreturnfrom
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theswap()method,indicatinghowthevaluesoftheelementsatindices0and1inthe
arrayhavebeenswapped.
Figure3.4ParameterPassing:Arrays
However,thedeclarationoftheswap()methodat(3)willnotswaptwovalues.The
methodcall
Clickheretoviewcodeimage
swap(dataSeq[index-1],dataSeq[index]);
willhavenoeffectonthearrayelements,astheswappingisdoneonthevaluesofthe
formalparameters.
ThemethodprintIntArray()at(4)alsohasaformalparameterofarraytype
int[].Notethattheformalparameterisspecifiedasanarrayreferenceusingthe[]
notation,butthisnotationisnotusedwhenanarrayispassedasanactualparameter.
ArrayElementsasActualParameters
Arrayelements,likeothervariables,canstorevaluesofprimitivedatatypesorreference
valuesofobjects.Inthelattercase,theycanalsobearrays—thatis,arraysofarrays(§3.4,
p.63).Ifanarrayelementisofaprimitivedatatype,itsdatavalueispassed;ifitisa
referencetoanobject,thereferencevalueispassed.Themethodinvocationconversions
applytothevaluesofarrayelementsaswell.
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Example3.9ArrayElementsasPrimitiveDataValues
Clickheretoviewcodeimage
publicclassFindMinimum{
publicstaticvoidmain(String[]args){
int[]dataSeq={6,4,8,2,1};
intminValue=dataSeq[0];
for(intindex=1;index<dataSeq.length;++index)
minValue=minimum(minValue,dataSeq[index]);//(1)
System.out.println(“Minimumvalue:”+minValue);
}
publicstaticintminimum(inti,intj){//(2)
return(i<=j)?i:j;
}
}
Outputfromtheprogram:
Minimumvalue:1
InExample3.9,thevalueofallbutoneelementofthearraydataSeqisretrievedand
passedconsecutivelyat(1)totheformalparameterjoftheminimum()methoddefined
at(2).Thediscussionin§3.5,p.73,onpassingprimitivevaluesalsoappliestoarray
elementsthathaveprimitivevalues.
InExample3.10,theformalparameterseqofthefindMinimum()methoddefinedat
(4)isanarrayvariable.Thevariablematrixdenotesanarrayofarraysdeclaredat(1)
simulatingamultidimensionalarray,whichhasthreerows,whereeachrowisasimple
array.Thefirstrow,denotedbymatrix[0],ispassedtothefindMinimum()method
inthecallat(2).Eachremainingrowispassedbyitsreferencevalueinthecalltothe
findMinimum()methodat(3).
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Example3.10ArrayElementsasReferenceValues
Clickheretoviewcodeimage
publicclassFindMinimumMxN{
publicstaticvoidmain(String[]args){
int[][]matrix={{8,4},{6,3,2},{7}};//(1)
intmin=findMinimum(matrix[0]);//(2)
for(inti=1;i<matrix.length;++i){
intminInRow=findMinimum(matrix[i]);//(3)
min=Math.min(min,minInRow);
}
System.out.println(“Minimumvalueinmatrix:”+min);
}
publicstaticintfindMinimum(int[]seq){//(4)
intmin=seq[0];
for(inti=1;i<seq.length;++i)
min=Math.min(min,seq[i]);
returnmin;
}
}
Outputfromtheprogram:
Minimumvalueinmatrix:2
Parameters
Aformalparametercanbedeclaredwiththekeywordfinalprecedingtheparameter
declarationinthemethoddeclaration.Afinalparameterisalsoknownasablankfinal
variable;thatis,itisblank(uninitialized)untilavalueisassignedtoit,(e.g.,atmethod
invocation)andthenthevalueinthevariablecannotbechangedduringthelifetimeofthe
variable(seealsothediscussionin§4.8,p.133).Thecompilercantreatfinalvariables
asconstantsforcodeoptimizationpurposes.Declaringparametersasfinalprevents
theirvaluesfrombeingchangedinadvertently.Aformalparameter’sdeclarationas
finaldoesnotaffectthecaller’scode.
ThedeclarationofthemethodcalcPrice()fromExample3.6isshownnext,withthe
formalparameterpizzaPricedeclaredasfinal:
Clickheretoviewcodeimage
publicdoublecalcPrice(intnumberOfPizzas,finaldoublepizzaPrice){//
(2’)
pizzaPrice=pizzaPrice/2.0;//(3)Notallowed
returnnumberOfPizzas*pizzaPrice;
}
IfthisdeclarationofthecalcPrice()methodiscompiled,thecompilerwillnotallow
thevalueofthefinalparameterpizzaPricetobechangedat(3)inthebodyofthe
method.
Asanotherexample,thedeclarationofthemethodbake()fromExample3.7isshown
here,withtheformalparameterpizzaToBeBakeddeclaredasfinal:
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publicstaticvoidbake(finalPizzapizzaToBeBaked){//(3)
pizzaToBeBaked.meat=“chicken”;//(3a)Allowed
pizzaToBeBaked=null;//(4)Notallowed
}
Ifthisdeclarationofthebake()methodiscompiled,thecompilerwillnotallowthe
referencevalueofthefinalparameterpizzaToBeBakedtobechangedat(4)inthe
bodyofthemethod.Notethatthisappliestothereferencevalueinthefinalparameter,
butnottotheobjectdenotedbythisparameter.Thestateoftheobjectcanbechangedas
before,asshownat(3a).
3.6VariableArityMethods
Afixedaritymethodmustbecalledwiththesamenumberofactualparameters(also
calledarguments)asthenumberofformalparametersspecifiedinitsdeclaration.Ifthe
methoddeclarationspecifiestwoformalparameters,everycallofthismethodmust
specifyexactlytwoarguments.Wesaythatthearityofthismethodis2.Inotherwords,
thearityofsuchamethodisfixed,anditisequaltothenumberofformalparameters
specifiedinthemethoddeclaration.
Javaalsoallowsdeclarationofvariablearitymethods,meaningthatthenumberof
argumentsinitscallcanbevaried.Asweshallsee,invocationsofsuchamethodmay
containmoreactualparametersthanformalparameters.Variablearitymethodsareheavily
employedinformattingtextrepresentationofvalues,asdemonstratedbythevariablearity
methodSystem.out.printf()thatisusedinmanyexamplesforthispurpose.
Thelastformalparameterinavariablearitymethoddeclarationisdeclaredasfollows:
type...formal_parameter_name
Theellipsis(...)isspecifiedbetweenthetypeandtheformal_parameter_name.The
typecanbeaprimitivetype,areferencetype,oratypeparameter.Whitespacecanbe
specifiedonbothsidesoftheellipsis.Suchaparameterisusuallycalledavariablearity
parameter(alsoknownasvarargs).
Apartfromthevariablearityparameter,avariablearitymethodisidenticaltoafixedarity
method.Themethodpublish()isavariablearitymethod:
Clickheretoviewcodeimage
publicstaticvoidpublish(intn,String…data){//(int,String[])
System.out.println(“n:”+n+“,datasize:”+data.length);
}
Thevariablearityparameterinavariablearitymethodisalwaysinterpretedashavingan
arraytype:
type[]
Inthebodyofthepublish()method,thevariablearityparameterdatahasthetype
String[],soitisasimplearrayofStrings.
Onlyonevariablearityparameterispermittedintheformalparameterlist,anditisalways
thelastparameterintheformalparameterlist.Giventhatthemethoddeclarationhasn
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formalparameters,andthemethodcallhaskactualparameters,kmustbeequaltoor
greaterthann–1.Thelastk–n+1actualparametersareevaluatedandstoredinanarray
whosereferencevalueispassedasthevalueoftheactualparameter.Inthecaseofthe
publish()method,nisequalto2,sokcanbe1,2,3,andsoon.Thefollowing
invocationsofthepublish()methodshowwhichargumentsarepassedineachmethod
call:
Clickheretoviewcodeimage
publish(1);//(1,newString[]{})
publish(2,“two”);//(2,newString[]{“two”})
publish(3,“two”,“three”);//(3,newString[]{“two”,“three”})
Eachmethodcallresultsinanimplicitarraybeingcreatedandpassedasanargument.
Thisarraycancontainzeroormoreargumentvaluesthatdonotcorrespondtotheformal
parametersprecedingthevariablearityparameter.Thisarrayisreferencedbythevariable
arityparameterdatainthemethoddeclaration.Theprecedingcallswouldresultinthe
publish()methodprintingthefollowingoutput:
n:1,datasize:0
n:2,datasize:1
n:3,datasize:2
Tooverloadavariablearitymethod,itisnotenoughtochangethetypeofthevariable
arityparametertoanexplicitarraytype.Thecompilerwillcomplainifanattemptismade
tooverloadthemethodtransmit(),asshowninthefollowingcode:
Clickheretoviewcodeimage
publicstaticvoidtransmit(String…data){}//Compile-timeerror!
publicstaticvoidtransmit(String[]data){}//Compile-timeerror!
Thesedeclarationswouldresultintwomethodswithequivalentsignaturesinthesame
class,whichisnotpermitted.
Overloadingandoverridingofmethodswithvariablearityarediscussedin§7.10,p.316.
CallingaVariableArityMethod
Example3.11illustratesvariousaspectsofcallingavariablearitymethod.Themethod
flexiPrint()intheVarargsDemoclasshasavariablearityparameter:
Clickheretoviewcodeimage
publicstaticvoidflexiPrint(Object…data){//Object[]
//…
}
ThevariablearitymethodprintsthenameoftheClassobjectrepresentingtheactual
arraythatispassedatruntime.Itprintsthenumberofelementsinthisarrayaswellasthe
textrepresentationofeachelementinthearray.
ThemethodflexiPrint()iscalledinthemain()method.Firstwiththevaluesof
primitivetypesandStrings((1)to(8)),thenitiscalledwiththeprogramarguments(p.
85)suppliedinthecommandline,((9)to(11)).
Compilingtheprogramresultsinawarningat(9),whichweignoreforthetimebeing.
Theprogramcanstillberun,asshowninExample3.11.Thenumbersattheendofthe
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linesintheoutputrelatetonumbersinthecode,andarenotprintedbytheprogram.
Example3.11CallingaVariableArityMethod
Clickheretoviewcodeimage
publicclassVarargsDemo{
publicstaticvoidflexiPrint(Object…data){//Object[]
//PrintthenameoftheClassobjectforthevarargsparameter.
System.out.print(“Type:”+data.getClass().getName());
System.out.println(“No.ofelements:”+data.length);
System.out.print(“Elementvalues:“);
for(Objectelement:data)
System.out.print(element+”“);
System.out.println();
}
publicstaticvoidmain(String…args){
intday=13;
StringmonthName=“August”;
intyear=2009;
//Passingprimitivesandnon-arraytypes:
flexiPrint();//(1)newObject[]{}
flexiPrint(day);//(2)newObject[]
{Integer.valueOf(day)}
flexiPrint(day,monthName);//(3)newObject[]
{Integer.valueOf(day),
//monthName}
flexiPrint(day,monthName,year);//(4)newObject[]
{Integer.valueOf(day),
//monthName,
//Integer.valueOf(year)}
System.out.println();
//Passinganarraytype:
Object[]dateInfo={day,//(5)newObject[]
{Integer.valueOf(day),
monthName,//monthName,
year};//Integer.valueOf(year)}
flexiPrint(dateInfo);//(6)Non-varargscall
flexiPrint((Object)dateInfo);//(7)newObject[]{(Object)
dateInfo}
flexiPrint(newObject[]{dateInfo});//(8)Non-varargscall
System.out.println();
//Explicitvarargsornon-varargscall:
flexiPrint(args);//(9)Warning!
flexiPrint((Object)args);//(10)Explicitvarargscall
flexiPrint((Object[])args);//(11)Explicitnon-varargscall
}
}
Compilingtheprogram:
Clickheretoviewcodeimage
>javacVarargsDemo.java
VarargsDemo.java:41:warning:non-varargscallofvarargsmethodwithinexact
argumenttypeforlastparameter;
flexiPrint(args);//(9)Warning!
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^
casttoObjectforavarargscall
casttoObject[]foranon-varargscallandtosuppressthiswarning
1warning
Runningtheprogram:
Clickheretoviewcodeimage
>javaVarargsDemoToargornottoarg
Type:[Ljava.lang.Object;No.ofelements:0(1)
Elementvalues:
Type:[Ljava.lang.Object;No.ofelements:1(2)
Elementvalues:13
Type:[Ljava.lang.Object;No.ofelements:2(3)
Elementvalues:13August
Type:[Ljava.lang.Object;No.ofelements:3(4)
Elementvalues:13August2009
Type:[Ljava.lang.Object;No.ofelements:3(6)
Elementvalues:13August2009
Type:[Ljava.lang.Object;No.ofelements:1(7)
Elementvalues:[Ljava.lang.Object;@1eed786
Type:[Ljava.lang.Object;No.ofelements:1(8)
Elementvalues:[Ljava.lang.Object;@1eed786
Type:[Ljava.lang.String;No.ofelements:6(9)
Elementvalues:Toargornottoarg
Type:[Ljava.lang.Object;No.ofelements:1(10)
Elementvalues:[Ljava.lang.String;@187aeca
Type:[Ljava.lang.String;No.ofelements:6(11)
Elementvalues:Toargornottoarg
VariableArityandFixedArityMethodCalls
Thecallsin(1)to(4)inExample3.11areallvariablearitycalls,asanimplicitObject
arrayiscreated,inwhichthevaluesoftheactualparametersarestored.Thereference
valueofthisarrayispassedtothemethod.Theprintoutshowsthatthetypeofthe
parameterisactuallyanarrayofObjects([Ljava.lang.Object;).
Thecallat(6)differsfromthepreviouscalls,inthattheactualparameterisanarraythat
hasthesametype(Object[])asthevariablearityparameter,withouthavingtocreate
animplicitarray.Insuchacase,noimplicitarrayiscreated,andthereferencevalueofthe
arraydateInfoispassedtothemethod.Seealsotheresultfromthiscallat(6)inthe
output.Thecallat(6)isafixedaritycall(alsocalledanon-varargscall),whereno
implicitarrayiscreated:
Clickheretoviewcodeimage
flexiPrint(dateInfo);//(6)Non-varargscall
However,iftheactualparameteriscasttothetypeObjectasin(7),avariablearitycall
isexecuted:
Clickheretoviewcodeimage
flexiPrint((Object)dateInfo);//(7)newObject[]{(Object)dateInfo}
Thetypeoftheactualargumentisnownotthesameasthatofthevariablearityparameter,
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resultinginanarrayofthetypeObject[]beingcreated,inwhichthearraydateInfo
isstoredasanelement.Theprintoutat(7)showsthatonlythetextrepresentationofthe
dateInfoarrayisprinted,andnotitselements,asitisthesoleelementoftheimplicit
array.
Thecallat(8)isafixedaritycall,forthesamereasonasthecallin(6).Now,however,the
arraydateInfoisexplicitlystoredasanelementinanarrayofthetypeObject[]that
matchesthetypeofthevariablearityparameter:
Clickheretoviewcodeimage
flexiPrint(newObject[]{dateInfo});//(8)Non-varargscall
Theoutputfrom(8)isthesameastheoutputfrom(7),wherethearraydateInfowas
passedasanelementinanimplicitlycreatedarrayoftypeObject[].
Thecompilerissuesawarningforthecallat(9):
Clickheretoviewcodeimage
flexiPrint(args);//(9)Warning!
TheactualparameterargsisanarrayofthetypeString[],whichisasubtypeof
Object[]—thetypeofthevariablearityparameter.Thearrayargscanbepassedina
fixedaritycallasanarrayofthetypeString[],orinavariablearitycallasanelement
inanimplicitlycreatedarrayofthetypeObject[].Bothcallsarefeasibleandvalidin
thiscase.Notethatthecompilerchoosesafixedaritycallratherthanavariablearitycall,
butalsoissuesawarning.Theresultat(9)confirmsthiscourseofaction.
ThearrayargsofthetypeString[]isexplicitlypassedasanObjectinavariable
aritycallat(10),similartothecallat(7):
Clickheretoviewcodeimage
flexiPrint((Object)args);//(10)Explicitvarargscall
ThearrayargsoftypeString[]isexplicitlypassedasanarrayofthetype
Object[]inafixedaritycallat(11).Thiscallisequivalenttothecallat(9),wherethe
wideningreferenceconversionisimplicit,butnowwithoutawarningatcompiletime.
Thetwocallsprintthesameinformation,asisevidentfromtheoutputat(9)and(11):
Clickheretoviewcodeimage
flexiPrint((Object[])args);//(11)Explicitnon-varargscall
3.7The Method
ThemechanicsofcompilingandrunningJavaapplicationsusingtheJDKareoutlinedin
§1.10,p.16.Thejavacommandexecutesamethodcalledmainintheclassspecified
onthecommandline.Anyclasscanhaveamain()method,butonlythemain()
methodoftheclassspecifiedinthejavacommandstartstheexecutionofaJava
application.
Themain()methodmusthavepublicaccessibilitysothattheJVMcancallthis
method(§4.7,p.123).Itisastaticmethodbelongingtotheclass,sothatnoobjectof
theclassisrequiredtostarttheexecution(§4.8,p.132).Itdoesnotreturnavalue;thatis,
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itisdeclaredasvoid(§6.4,p.224).ItalwayshasanarrayofStringobjectsasitsonly
formalparameter.Thisarraycontainsanyargumentspassedtotheprogramonthe
commandline(seethenextsubsection).Thefollowingmethodheaderdeclarationsfitthe
bill,andanyoneofthemcanbeusedforthemain()method:
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args)//Methodheader
publicstaticvoidmain(String…args)//Methodheader
Thethreemodifierscanoccurinanyorderinthemethodheader.Therequirementsgiven
intheseexamplesdonotexcludespecificationofadditionalmodifiers(§4.8,p.131)or
anythrowsclause(§6.9,p.251).Themain()methodcanalsobeoverloadedlikeany
othermethod(§3.2,p.52).TheJVMensuresthatthemain()methodhavingthe
previouslymentionedmethodheaderisthestartingpointofprogramexecution.
ProgramArguments
Anyargumentspassedtotheprogramonthecommandlinecanbeaccessedinthe
main()methodoftheclassspecifiedonthecommandline:
>javaColorsredgreenblue
Theseargumentsarecalledprogramarguments.Notethatthecommandname,java,and
theclassnameColorsarenotpassedtothemain()methodoftheclassColors,nor
areanyotheroptionsthatarespecifiedonthecommandlinepassedtothismethod.
Sincetheformalparameterofthemain()methodisanarrayofStringobjects,
individualStringelementsinthearraycanbeaccessedbyusingthe[]operator.
InExample3.12,thethreeargumentsred,green,andbluecanbeaccessedinthe
main()methodoftheColorsclassasargs[0],args[1],andargs[2],
respectively.ThetotalnumberofargumentsisgivenbythefieldlengthoftheString
arrayargs.Notethatprogramargumentscanbepassedonlyasstrings,andmustbe
explicitlyconvertedtoothervaluesbytheprogram,ifnecessary.
Whennoargumentsarespecifiedonthecommandline,anarrayofzeroString
elementsiscreatedandpassedtothemain()method.Thusthereferencevalueofthe
formalparameterinthemain()methodisnevernull.
Programargumentssupplyinformationtotheapplication,whichcanbeusedtotailorthe
runtimebehavioroftheapplicationaccordingtouserrequirements.
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Example3.12PassingProgramArguments
Clickheretoviewcodeimage
publicclassColors{
publicstaticvoidmain(String[]args){
System.out.println(“No.ofprogramarguments:”+args.length);
for(inti=0;i<args.length;i++)
System.out.println(“Argumentno.”+i+”(”+args[i]+“)has”+
args[i].length()+”characters.”);
}
}
Runningtheprogram:
Clickheretoviewcodeimage
>javaColorsredgreenblue
No.ofprogramarguments:3
Argumentno.0(red)has3characters.
Argumentno.1(green)has5characters.
Argumentno.2(blue)has4characters.
3.8EnumeratedTypes
Inthissectionweprovideabasicintroductiontoenumeratedtypes.Anenumeratedtype
definesafinitesetofsymbolicnamesandtheirvalues.Thesesymbolicnamesareusually
calledenumconstantsornamedconstants.
Onewaytodefineconstantsistodeclarethemasfinal,staticvariablesinaclass(or
interface)declaration:
Clickheretoviewcodeimage
publicclassMachineState{
publicstaticfinalintBUSY=1;
publicstaticfinalintIDLE=0;
publicstaticfinalintBLOCKED=-1;
}
Suchconstantsarenottype-safe,asanyintvaluecanbeusedwhereweneedtousea
constantdeclaredintheMachineStateclass.Suchaconstantmustbequalifiedbythe
class(orinterface)name,unlesstheclassisextended(ortheinterfaceisimplemented).
Whensuchaconstantisprinted,onlyitsvalue(forexample,0),andnotitsname(for
example,IDLE),isprinted.Aconstantalsoneedsrecompilingifitsvalueischanged,as
thevaluesofsuchconstantsarecompiledintotheclientcode.
AnenumeratedtypeinJavaisaspecialkindofclasstypethatismuchmorepowerfulthan
theapproachoutlinedearlierfordefiningcollectionsofnamedconstants.
DeclaringType-safeEnums
Thecanonicalformofdeclaringanenumtypeisshownhere:
Clickheretoviewcodeimage
publicenumMachineState//Enumheader
{//Enumbody
BUSY,IDLE,BLOCKED//Enumconstants
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}
Thekeywordenumisusedtodeclareanenumtype,asopposedtothekeywordclass
foraclassdeclaration.Thebasicnotationrequirestheenumtypenameinenumheader,
andacomma-separatedlistofenumconstantscanbespecifiedintheenumbody.
Optionally,anaccessmodifiercanalsobespecifiedintheenumheader,asfora(top-
level)class.Intheexampleenumdeclaration,thenameoftheenumtypeis
MachineState.Itdefinesthreeenumconstantswithexplicitnames.Anenumconstant
canbeanylegalJavaidentifier,buttheconventionistouseuppercaselettersinthename.
Essentially,anenumdeclarationdefinesareferencetypethathasafinitenumberof
permissiblevaluesreferencedbytheenumconstants,andthecompilerensurestheyare
usedinatype-safemanner.
Othermemberdeclarationscanbespecifiedinthebodyofanenumtype,butthe
canonicalformsufficesforthepurposeofthisbook.Analogoustoaclassdeclaration,an
enumtypeiscompiledtoJavabytecodethatisplacedinaseparateclassfile.
Theenumtypesjava.time.Monthandjava.time.DayOfWeekaretwoexamples
ofenumtypesfromtheJavaSEplatformAPI.Aswewouldexpect,theMonthenum
typerepresentsthemonthsfromJANUARYtoDECEMBER,andtheDayOfWeekenum
typerepresentsthedaysoftheweekfromMONDAYtoSUNDAY.Examplesoftheirusage
canbefoundin§11.2,p.462.
Someadditionalexamplesofenumtypesfollow:
Clickheretoviewcodeimage
publicenumMarchingOrders{LEFT,RIGHT}
publicenumTrafficLightState{RED,YELLOW,GREEN}
enumMealType{BREAKFAST,LUNCH,DINNER}
UsingType-safeEnums
Example3.13illustratestheuseofenumconstants.Anenumtypeisessentiallyusedin
thesamewayasanyotherreferencetype.Enumconstantsareactuallypublic,
static,finalfieldsoftheenumtype,andtheyareimplicitlyinitializedwithinstances
oftheenumtypewhentheenumtypeisloadedatruntime.Sincetheenumconstantsare
staticmembers,theycanbeaccessedusingthenameoftheenumtype—analogousto
accessingstaticmembersinaclassoraninterface.
Example3.13showsamachineclientthatusesamachinewhosestateisanenum
constant.Inthisexample,weseethatanenumconstantcanbepassedasanargument,as
shownas(1),andwecandeclarereferenceswhosetypeisanenumtype,asshownas(3),
butwecannotcreatenewconstants(thatis,objects)oftheenumtypeMachineState.
Anattempttodoso,at(5),resultsinacompile-timeerror.
Thestringrepresentationofanenumconstantisitsname,asshownat(4).Notethatitis
notpossibletopassatypeofvalueotherthanaMachineStateenumconstantinthe
calltothemethodsetState()oftheMachineclass,asshownat(2).
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Example3.13UsingEnums
Clickheretoviewcodeimage
//File:MachineState.java
publicenumMachineState{BUSY,IDLE,BLOCKED}
//File:Machine.java
publicclassMachine{
privateMachineStatestate;
publicvoidsetState(MachineStatestate){this.state=state;}
publicMachineStategetState(){returnthis.state;}
}
//File:MachineClient.java
publicclassMachineClient{
publicstaticvoidmain(String[]args){
Machinemachine=newMachine();
machine.setState(MachineState.IDLE);//(1)Passedasavalue.
//machine.setState(1);//(2)Compile-time
error!
MachineStatestate=machine.getState();//(3)Declaringa
reference.
System.out.println(
“Currentmachinestate:”+state//(4)Printingtheenum
name.
);
//MachineStatenewState=newMachineState();//(5)Compile-time
error!
System.out.println(“Allmachinestates:”);
for(MachineStatems:MachineState.values()){//(6)Traversingover
enum
System.out.println(ms+“:”+ms.ordinal());//contants.
}
System.out.println(“Comparison:”);
MachineStatestate1=MachineState.BUSY;
MachineStatestate2=state1;
MachineStatestate3=MachineState.BLOCKED;
System.out.println(state1+”==”+state2+“:”+
(state1==state2));//(7)
System.out.println(state1+”isequalto”+state2+“:”+
(state1.equals(state2)));//(8)
System.out.println(state1+”islessthan”+state3+“:”+
(state1.compareTo(state3)<0));//(9)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Currentmachinestate:IDLE
Allmachinestates:
BUSY:0
IDLE:1
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BLOCKED:2
Comparison:
BUSY==BUSY:true
BUSYisequaltoBUSY:true
BUSYislessthanBLOCKED:true
SelectedMethodsforEnumTypes
Allenumtypesimplicitlyhavethefollowingusefulmethod:
Clickheretoviewcodeimage
staticEnumTypeName[]values()
Returnsanarraycontainingtheenumconstantsofthisenumtype,intheorderthey
arespecified.
Theloopat(6)inExample3.13illustratestraversingoveralltheMachineStateenum
constantsintheordertheyarespecified.AnarraycontainingalltheMachineState
constantsisobtainedbycallingthestaticmethodvalues()ontheenumtype.
Allenumtypesaresubtypesofthejava.lang.Enumclass,whichprovidesthedefault
behavior.Allenumtypesinheritthefollowingselectedmethodsfromthe
java.lang.Enumclass:
Clickheretoviewcodeimage
finalbooleanequals(Objectother)
Thismethodreturnstrueifthespecifiedobjectisequaltothisenumconstant.
finalintcompareTo(Eother)
Thenaturalorderoftheenumconstantsinanenumtypeisbasedontheirordinal
values(seetheordinal()methodnext).ThecompareTo()methodofthe
Comparableinterfacereturnsthevaluezeroifthisenumconstantisequaltothe
otherenumconstant,avaluelessthanzeroifthisenumconstantislessthanthe
otherenumconstant,oravaluegreaterthanzeroifthisenumconstantisgreater
thantheotherenumconstant.
finalintordinal()
Thismethodreturnstheordinalvalueofthisenumconstant(thatis,itspositionin
itsenumtypedeclaration).Thefirstenumconstantisassignedanordinalvalueof
zero.Iftheordinalvalueofanenumconstantislessthantheordinalvalueof
anotherenumconstantofthesameenumtype,theformeroccursbeforethelatterin
theenumtypedeclaration.
Notethattheequalitytestimplementedbytheequals()methodisbasedonreference
equality(==)oftheenumconstants,notonvalueequality.Anenumtypehasafinite
numberofdistinctobjects.Comparingtwoenumreferencesforequalitymeans
determiningwhethertheystorethereferencevalueofthesameenumconstant—inother
words,whetherthereferencesarealiases.Thus,foranytwoenumreferencesstate1
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andstate2,theexpressionsstate1.equals(state2)andstate1==state2
areequivalent,asshownat(7)and(8)inExample3.13.
Theordinalvalueoftheconstantsinanenumtypedeterminestheresultofcomparing
suchconstantswiththecompareTo()method,asshownat(9)inExample3.13.
ReviewQuestions
3.13Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassParameterPass{
publicstaticvoidmain(String[]args){
inti=0;
addTwo(i++);
System.out.println(i);
}
staticvoidaddTwo(inti){
i+=2;
}
}
Selecttheonecorrectanswer.
(a)0
(b)1
(c)2
(d)3
3.14Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassPassing{
publicstaticvoidmain(String[]args){
inta=0;intb=0;
int[]bArr=newint[1];bArr[0]=b;
inc1(a);inc2(bArr);
System.out.println(“a=”+a+”b=”+b+”bArr[0]=”+bArr[0]);
}
publicstaticvoidinc1(intx){x++;}
publicstaticvoidinc2(int[]x){x[0]++;}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompile,sincex[0]++;isnotalegalstatement.
(b)Thecodewillcompileandwillprinta=1b=1bArr[0]=1atruntime.
(c)Thecodewillcompileandwillprinta=0b=1bArr[0]=1atruntime.
(d)Thecodewillcompileandwillprinta=0b=0bArr[0]=1atruntime.
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(e)Thecodewillcompileandwillprinta=0b=0bArr[0]=0atruntime.
3.15Whichstatements,wheninsertedat(1),willresultinacompile-timeerror?
Clickheretoviewcodeimage
publicclassParameterUse{
staticvoidmain(String[]args){
inta=0;
finalintb=1;
int[]c={2};
finalint[]d={3};
useArgs(a,b,c,d);
}
staticvoiduseArgs(finalinta,intb,finalint[]c,int[]d){
//(1)INSERTSTATEMENTHERE.
}
}
Selectthetwocorrectanswers.
(a)a++;
(b)b++;
(c)b=a;
(d)c[0]++;
(e)d[0]++;
(f)c=d;
3.16Whichofthefollowingmethoddeclarationsarevaliddeclarations?
Selectthethreecorrectanswers.
(a)voidcompute(int…is){}
(b)voidcompute(intis…){}
(c)voidcompute(int…is,inti,String…ss){}
(d)voidcompute(String…ds){}
(e)voidcompute(String…ss,intlen){}
(f)voidcompute(char[]ca,int…is){}
3.17Giventhefollowingcode:
Clickheretoviewcodeimage
publicclassRQ810A40{
staticvoidprint(Object…obj){
System.out.println(“Object…:”+obj[0]);
}
publicstaticvoidmain(String[]args){
//(1)INSERTMETHODCALLHERE.
}
}
Whichmethodcall,wheninsertedat(1),willnotresultinthefollowingoutput
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fromtheprogram:
Object…:9
Selecttheonecorrectanswer.
(a)print("9","1","1");
(b)print(9,1,1);
(c)print(newint[]{9,1,1});
(d)print(newInteger[]{9,1,1});
(e)print(newString[]{"9","1","1"});
(f)print(newObject[]{"9","1","1"});
(g)Noneoftheabove.
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Anoverviewofdeclarationsthatcanbespecifiedinaclass
•Declarationofmethods,usageofthethisreferenceinaninstancemethod,and
methodoverloading
•Declarationofconstructors,usageofthedefaultconstructor,andoverloadingof
constructors
•Explanationofdeclaration,construction,initialization,andusageofbothone-
dimensionalandmultidimensionalarrays,includinganonymousarrays
•Sortingandsearchingarrays
•Parameterpassing,bothprimitivevaluesandobjectreferences,includingarraysand
arrayelements;anddeclaringfinalparameters
•Declaringandcallingmethodswithvariablearity
•Declarationofthemain()methodwhoseexecutionstartstheapplication
•Passingprogramargumentstothemain()method
•Declaringandusingsimpleenumtypes
ProgrammingExercise
3.1Writeaprogramtogradeashortmultiple-choicequiz.Thecorrectanswersforthe
quizare
1.C5.B
2.A6.C
3.B7.C
4.D8.A
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Assumethatthepassingmarksareatleast5outof8.Theprogramstoresthe
correctanswersinanarray.Thesubmittedanswersarespecifiedasprogram
arguments.LetXrepresentaquestionthatwasnotansweredonthequiz.Usean
enumtypetorepresenttheresultofansweringaquestion.
Exampleofrunningtheprogram:
Clickheretoviewcodeimage
>javaQuizGraderCBBDBCAX
QuestionSubmittedAns.CorrectAns.Result
1CCCORRECT
2BAWRONG
3BBCORRECT
4DDCORRECT
5BBCORRECT
6CCCORRECT
7ACWRONG
8XAUNANSWERED
No.ofcorrectanswers:5
No.ofwronganswers:2
No.ofquestionsunanswered:1
ThecandidatePASSED.
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4.AccessControl
4.1JavaSourceFileStructure
ThestructureofaskeletalJavasourcefileisdepictedinFigure4.1.AJavasourcefilecan
havethefollowingelementsthat,ifpresent,mustbespecifiedinthefollowingorder:
1.Anoptionalpackagedeclarationtospecifyapackagename.Packagesare
discussedin§4.2.
2.Zeroormoreimportdeclarations.Sinceimportdeclarationsintroducetypeor
staticmembernamesinthesourcecode,theymustbeplacedbeforeanytype
declarations.Bothtypeandstaticimportdeclarationsarediscussedin§4.2.
3.Anynumberoftop-leveltypedeclarations.Class,enum,andinterfacedeclarations
arecollectivelyknownastypedeclarations.Sincethesedeclarationsbelongtothe
samepackage,theyaresaidtobedefinedatthetoplevel,whichisthepackagelevel.
Thetypedeclarationscanbedefinedinanyorder.Technically,asourcefileneednot
haveanysuchdeclarations,butthatishardlyuseful.
TheJDKimposestherestrictionthatatmostonepublicclassdeclarationper
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sourcefilecanbedefined.Ifapublicclassisdefined,thefilenamemustmatch
thispublicclass.Forexample,ifthepublicclassnameisNewApp,thefile
namemustbeNewApp.java.
Classesarediscussedin§3.1,p.48;enumsarediscussedin§3.8,p.87;and
interfacesarediscussedin§7.6,p.290.
Figure4.1JavaSourceFileStructure
Notethatexceptforthepackageandtheimportstatements,allcodeisencapsulated
inclasses,interfaces,andenums.Nosuchrestrictionappliestocommentsandwhitespace.
4.2Packages
ApackageinJavaisanencapsulationmechanismthatcanbeusedtogrouprelated
classes,interfaces,enums,andsubpackages.
Figure4.2showsanexampleofapackagehierarchy,comprisingapackagecalled
wizardthatcontainstwootherpackages:pandorasboxandspells.Thepackage
pandorasboxhasaclasscalledClownthatimplementsaninterfacecalledMagic,
alsofoundinthesamepackage.Inaddition,thepackagepandorasboxhasaclass
calledLovePotionandasubpackagecalledartifactscontainingaclasscalled
Ailment.Thepackagespellshastwoclasses:BaldnessandLovePotion.The
classBaldnessisasubclassofclassAilmentfoundinthesubpackageartifacts
inthepackagepandorasbox.
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Figure4.2PackageHierarchy
Thedot(.)notationisusedtouniquelyidentifypackagemembersinthepackage
hierarchy.Theclasswizard.pandorasbox.LovePotion,forexample,isdifferent
fromtheclasswizard.spells.LovePotion.TheAilmentclasscanbeeasily
identifiedbythenamewizard.pandorasbox.artifacts.Ailment,whichis
knownasthefullyqualifiednameofthetype.Notethatthefullyqualifiednameofthe
typeinanamedpackagecomprisesthefullyqualifiednameofthepackageandthesimple
nameofthetype.ThesimpletypenameAilmentandthefullyqualifiedpackagename
wizard.pandorasbox.artifactstogetherdefinethefullyqualifiedtypename
wizard.pandorasbox.artifacts.Ailment.Analogously,thefullyqualified
nameofasubpackagecomprisesthefullyqualifiednameoftheparentpackageandthe
simplenameofthesubpackage.
Javaprogrammingenvironmentsusuallymapthefullyqualifiednameofpackagestothe
underlying(hierarchical)filesystem.Forexample,onaUnixsystem,theclassfile
LovePotion.classcorrespondingtothefullyqualifiedname
wizard.pandorasbox.LovePotionwouldbefoundunderthedirectory
wizard/pandorasbox.
Conventionally,aglobalnamingschemebasedontheInternetdomainnamesisusedto
uniquelyidentifypackages.Ifthepackagewizardwasimplementedbyacompany
calledSorcerersLimitedthatownsthedomainsorcerersltd.com,itsfullyqualified
namewouldbe
com.sorcerersltd.wizard
Becausedomainnamesareunique,packageswiththisnamingschemeareglobally
identifiable.Itisnotadvisabletousethetop-levelpackagenamesjavaandsun,as
thesearereservedfortheJavadesigners.
Thesubpackagewizard.pandorasbox.artifactscouldeasilyhavebeenplaced
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elsewhere,aslongasitwasuniquelyidentified.Subpackagesinapackagedonotaffect
theaccessibilityoftheotherpackagemembers.Forallintentsandpurposes,subpackages
aremoreanorganizationalfeatureratherthanalanguagefeature.Accessibilityof
membersinapackageisdiscussedin§4.4.Accessibilityofmembersdefinedintype
declarationsisdiscussedin§4.7.
DefiningPackages
ApackagehierarchyrepresentsanorganizationoftheJavaclassesandinterfaces.Itdoes
notrepresentthesourcecodeorganizationoftheclassesandinterfaces.Thesourcecodeis
ofnoconsequenceinthisregard.EachJavasourcefile(alsocalledcompilationunit)can
containzeroormoretypedeclarations,butthecompilerproducesaseparateclassfile
containingtheJavabytecodeforeachofthem.AtypedeclarationcanindicatethatitsJava
bytecodeshouldbeplacedinaparticularpackage,usingapackagedeclaration.
Thepackagestatementhasthefollowingsyntax:
Clickheretoviewcodeimage
packagefully_qualified_package_name;
Atmostonepackagedeclarationcanappearinasourcefile,anditmustbethefirst
statementinthesourcefile.ThepackagenameissavedintheJavabytecodeforthetypes
containedinthepackage.Javanamingconventionsrecommendwritingpackagenamesin
lowercaseletters.
Notethatthisschemehastwoconsequences.First,alltheclassesandinterfacesina
sourcefilewillbeplacedinthesamepackage.Second,severalsourcefilescanbeusedto
specifythecontentsofapackage.
Ifapackagedeclarationisomittedinacompilationunit,theJavabytecodeforthe
declarationsinthecompilationunitwillbelongtoanunnamedpackage(alsocalledthe
defaultpackage),whichistypicallysynonymouswiththecurrentworkingdirectoryonthe
hostsystem.
Example4.1illustrateshowthepackagesinFigure4.2canbedefinedusingthepackage
declaration.Therearefourcompilationunits.Eachcompilationunithasapackage
declaration,ensuringthatthetypedeclarationsarecompiledintothecorrectpackage.The
completecodecanbefoundinExample4.8onpage118.
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Example4.1DefiningPackagesandUsingTypeImport
Clickheretoviewcodeimage
//Filename:Clown.java
packagewizard.pandorasbox;//Packagedeclaration
importwizard.pandorasbox.artifacts.Ailment;//Importingspecificclass
publicclassClownimplementsMagic{/*…*/}
interfaceMagic{/*…*/}
//Filename:LovePotion.java
packagewizard.pandorasbox;//Packagedeclaration
publicclassLovePotion{/*…*/
//Filename:Ailment.java
packagewizard.pandorasbox.artifacts;//Packagedeclaration
publicclassAilment{/*…*/}
//Filename:Baldness.java
packagewizard.spells;//Packagedeclaration
importwizard.pandorasbox.*;//(1)Type-import-on-demand
importwizard.pandorasbox.artifacts.*;//(2)Importfromsubpackage
publicclassBaldnessextendsAilment{//SimplenameforAilment
wizard.pandorasbox.LovePotiontlcOne;//(3)Fullyqualifiedclass
name
LovePotiontlcTwo;//Classinsamepackage
//…
}
classLovePotion{/*…*/}
UsingPackages
TheimportfacilityinJavamakesiteasiertousethecontentsofpackages.Thissubsection
discussesimportingreferencetypesandstaticmembersofreferencetypesfrompackages.
ImportingReferenceTypes
Theaccessibilityoftypes(classes,interfaces,andenums)inapackagedeterminestheir
accessfromotherpackages.Givenareferencetypethatisaccessiblefromoutsidea
package,thereferencetypecanbeaccessedintwoways.Onewayistousethefully
qualifiednameofthetype.However,writinglongnamescanbecometedious.Thesecond
wayistousetheimportdeclarationthatprovidesashorthandnotationforspecifying
thenameofthetype,oftencalledtypeimport.
Theimportdeclarationsmustbethefirststatementafteranypackagedeclarationina
sourcefile.Thesimpleformoftheimportdeclarationhasthefollowingsyntax:
Clickheretoviewcodeimage
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importfully_qualified_type_name;
Thisiscalledsingle-type-import.Asthenameimplies,suchanimportdeclaration
providesashorthandnotationforasingletype.Thesimplenameofthetype(thatis,its
identifier)cannowbeusedtoaccessthisparticulartype.Giventheimportdeclaration
Clickheretoviewcodeimage
importwizard.pandorasbox.Clown;
thesimplenameClowncanbeusedinthesourcefiletorefertothisclass.
Alternatively,thefollowingformoftheimportdeclarationcanbeused:
Clickheretoviewcodeimage
importfully_qualified_package_name.*;
Thisiscalledtype-import-on-demand.Itallowsanytypefromthespecifiedpackagetobe
accessedbyitssimplename.
Animportdeclarationdoesnotrecursivelyimportsubpackages.Thedeclarationalso
doesnotresultininclusionofthesourcecodeofthetypes;rather,itsimplyimportstype
names(thatis,itmakestypenamesavailabletothecodeinacompilationunit).
Allcompilationunitsimplicitlyimportthejava.langpackage(§8.1,p.342).Thisis
thereasonwhywecanrefertotheclassStringbyitssimplename,andneednotuseits
fullyqualifiednamejava.lang.Stringallthetime.
Example4.1showsseveralusagesoftheimportstatement.Herewewilldrawattention
totheclassBaldnessinthefileBaldness.java.Thisclassreliesontwoclassesthat
havethesamesimplenameLovePotionbutareindifferentpackages:
wizard.pandorasboxandwizard.spells,respectively.Todistinguishbetween
thetwoclasses,wecanusetheirfullyqualifiednames.However,sinceoneofthemisin
thesamepackageastheclassBaldness,itisenoughtofullyqualifytheclassfromthe
otherpackage.ThissolutionisusedinExample4.1at(3).Notethattheimportofthe
wizard.pandorasboxpackageat(1)becomesredundant.Suchnameconflictscan
usuallyberesolvedbyusingvariationsoftheimportstatementtogetherwithfully
qualifiednames.
TheclassBaldnessextendstheclassAilment,whichisinthesubpackage
artifactsofthewizard.pandorasboxpackage.Theimportdeclarationat(2)
isusedtoimportthetypesfromthesubpackageartifacts.
Thefollowingexampleshowshowasingle-type-importdeclarationcanbeusedto
disambiguateatypenamewhenaccesstothetypeisambiguousbyitssimplename.The
followingimportstatementallowsthesimplenameListtobeusedasshorthandfor
thejava.awt.Listtypeasexpected:
Clickheretoviewcodeimage
importjava.awt.*;//importsallreferencetypesfromjava.awt
Giventhetwoimportdeclarations
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importjava.awt.*;//importsalltypenamesfromjava.awt
importjava.util.*;//importsalltypenamesfromjava.util
thesimplenameListisnowambiguous,becauseboththetypesjava.util.List
andjava.awt.Listmatch.
Addingasingle-type-importdeclarationforthejava.awt.Listtypeallowsthesimple
nameListtobeusedasashorthandnotationforthistype:
Clickheretoviewcodeimage
importjava.awt.*;//importsalltypenamesfromjava.awt
importjava.util.*;//importsalltypenamesfromjava.util
importjava.awt.List;//importsthetypeListfromjava.awt
explicitly
ImportingStaticMembersofReferenceTypes
Analogoustothetypeimportfacility,Javaalsoallowsimportofstaticmembersof
referencetypesfrompackages,oftencalledstaticimport.
Staticimportallowsaccessiblestaticmembersdeclaredinatypetobeimported,sothat
theycanbeusedbytheirsimplenames,andthereforeneednotbequalified.Theimport
appliestothewholecompilationunit,andimportingfromtheunnamedpackageisnot
permissible.
Thetwoformsofstaticimportareshownhere:
Clickheretoviewcodeimage
//Single-static-import:importsaspecificstaticmemberfromthedesignated
type
importstaticfully_qualified_type_name.static_member_name;
//Static-import-on-demand:importsallstaticmembersinthedesignatedtype
importstaticfully_qualified_type_name.*;
Bothformsrequiretheuseofthekeywordimportfollowedbythekeywordstatic,
althoughthefeatureiscalledstaticimport.Inbothcases,thefullyqualifiednameofthe
referencetypeweareimportingfromisrequired.
Thefirstformallowssinglestaticimportofindividualstaticmembers,andis
demonstratedinExample4.2.TheconstantPI,whichisastaticfieldinthe
classjava.lang.Math,isimportedat(1).Notetheuseofthefullyqualifiednameof
thetypeinthestaticimportstatement.Thestaticmethodnamedsqrtfromthe
classjava.lang.Mathisimportedat(2).Onlythenameofthestaticmethodis
specifiedinthestaticimportstatement;noparametersarelisted.Useofanyotherstatic
memberfromtheMathclassrequiresthatthefullyqualifiednameoftheclassbe
specified.Sincetypesfromthejava.langpackageareimportedimplicitly,thefully
qualifiednameoftheMathclassisnotnecessary,asshownat(3).
Staticimportondemandiseasilydemonstratedbyreplacingthetwoimportstatements
inExample4.2bythefollowingimportstatement:
Clickheretoviewcodeimage
importstaticjava.lang.Math.*;
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WecanalsodispensewiththeuseoftheclassnameMathin(3),asallstaticmembers
fromtheMathclassarenowimported:
Clickheretoviewcodeimage
doublehypotenuse=hypot(x,y);//(3’)Typenamecannowbeomitted.
Example4.2SingleStaticImport
Clickheretoviewcodeimage
importstaticjava.lang.Math.PI;//(1)Staticfield
importstaticjava.lang.Math.sqrt;//(2)Staticmethod
//Onlyspecifiedstaticmembersareimported.
publicclassCalculate3{
publicstaticvoidmain(String[]args){
doublex=3.0,y=4.0;
doublesquareroot=sqrt(y);//Simplenameofstaticmethod
doublehypotenuse=Math.hypot(x,y);//(3)Requirestypename
doublearea=PI*y*y;//Simplenameofstaticfield
System.out.printf(“Squareroot:%.2f,hypotenuse:%.2f,area:%.2f%n”,
squareroot,hypotenuse,area);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Squareroot:2.00,hypotenuse:5.00,area:50.27
Example4.3illustrateshowstaticimportcanbeusedtoaccessinterfaceconstants(§7.6,
p.302).Thestaticimportstatementat(1)allowstheinterfaceconstantsinthepackage
mypkgtobeaccessedbytheirsimplenames.Thestaticimportfacilityavoidsthe
MyFactoryclasshavingtoimplementtheinterfacesoastoaccesstheconstantsbytheir
simplename(oftenreferredtoastheinterfaceconstantantipattern):
Clickheretoviewcodeimage
publicclassMyFactoryimplementsmypkg.IMachineState{
//…
}
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Example4.3AvoidingtheInterfaceConstantAntipattern
Clickheretoviewcodeimage
packagemypkg;
publicinterfaceIMachineState{
//Fieldsarepublic,staticandfinal.
intBUSY=1;
intIDLE=0;
intBLOCKED=-1;
}
importstaticmypkg.IMachineState.*;//(1)Staticimportinterface
constants
publicclassMyFactory{
publicstaticvoidmain(String[]args){
int[]states={IDLE,BUSY,IDLE,BLOCKED};
for(ints:states)
System.out.print(s+”“);
}
}
Outputfromtheprogram:
010-1
Staticimportisidealforimportingenumconstantsfrompackages,assuchconstantsare
staticmembersofanenumtype.Example4.4combinestypeandstaticimports.Theenum
constantscanbeaccessedat(5)usingtheirsimplenamesbecauseofthestaticimport
statementat(2).Thetypeimportat(1)isrequiredtoaccesstheenumtypeStatebyits
simplenameat(4)and(6).
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Example4.4ImportingEnumConstants
Clickheretoviewcodeimage
packagemypkg;
publicenumState{BUSY,IDLE,BLOCKED}
//File:Factory.java(inunnamedpackage)
importmypkg.State;//(1)Singletypeimport
importstaticmypkg.State.*;//(2)Staticimportondemand
importstaticjava.lang.System.out;//(3)Singlestaticimport
publicclassFactory{
publicstaticvoidmain(String[]args){
State[]states={//(4)Usingtypeimportimpliedby(1)
IDLE,BUSY,IDLE,BLOCKED//(5)Usingstaticimportimpliedby
(2)
};
for(States:states)//(6)Usingtypeimportimpliedby(1)
out.print(s+”“);//(7)Usingstaticimportimpliedby
(3)
}
}
Outputfromtheprogram:
IDLEBUSYIDLEBLOCKED
Identifiersinaclasscanshadowstaticmembersthatareimported.Example4.5illustrates
thecasewheretheparameteroutofthemethodwriteInfo()hasthesamenameas
thestaticallyimportedfieldjava.lang.System.out.Thetypeoftheparameteris
ShadowImportandthatofthestaticallyimportedfieldisPrintStream.Bothclasses
PrintStreamandShadowImportdefinethemethodprintln()thatiscalledin
theprogram.Theonlywaytoaccesstheimportedfieldoutinthemethod
writeInfo()istouseitsfullyqualifiedname.
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Example4.5ShadowingStaticImport
Clickheretoviewcodeimage
importstaticjava.lang.System.out;//(1)Staticimport
publicclassShadowImport{
publicstaticvoidmain(String[]args){
out.println(“Callingprintln()injava.lang.System.out”);
ShadowImportsbi=newShadowImport();
writeInfo(sbi);
}
//Parametershadowsjava.lang.System.out:
publicstaticvoidwriteInfo(ShadowImportout){
out.println(“Callingprintln()intheparameterout”);
System.out.println(“Callingprintln()injava.lang.System.out”);//
Qualify
}
publicvoidprintln(Stringmsg){
out.println(msg+”oftypeShadowImport”);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Callingprintln()injava.lang.System.out
Callingprintln()intheparameteroutoftypeShadowImport
Callingprintln()injava.lang.System.out
Thenextcodesnippetillustratesacommonconflictthatoccurswhenastaticfieldwiththe
samenameisimportedbyseveralstaticimportstatements.Thisconflictisreadily
resolvedbyusingthefullyqualifiednameofthefield.Inthecaseshownhere,wecanuse
thesimplenameofclassinwhichthefieldisdeclared,asthejava.langpackageis
implicitlyimportedbyallcompilationunits.
Clickheretoviewcodeimage
importstaticjava.lang.Integer.MAX_VALUE;
importstaticjava.lang.Double.MAX_VALUE;
publicclassStaticFieldConflict{
publicstaticvoidmain(String[]args){
System.out.println(MAX_VALUE);//(1)Ambiguous!Compile-time
error!
System.out.println(Integer.MAX_VALUE);//OK
System.out.println(Double.MAX_VALUE);//OK
}
}
Conflictscanalsooccurwhenastaticmethodwiththesamesignatureisimportedby
severalstaticimportstatements.InExample4.6,amethodnamedbinarySearchis
imported21timesbythestaticimportstatements.Thismethodisoverloadedtwiceinthe
java.util.Collectionsclassand18timesinthejava.util.Arraysclass,in
additiontoonedeclarationinthemypkg.Auxiliaryclass.Theclasses
java.util.Arraysandmypkg.Auxiliaryhaveadeclarationofthismethodwith
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thesamesignaturethatmatchesthemethodcallat(2),resultinginasignatureconflictthat
isflaggedasacompile-timeerror.Theconflictcanagainberesolvedbyspecifyingthe
fullyqualifiednameofthemethod.
Ifthestaticimportstatementat(1)isremoved,thereisnoconflict,asonlytheclass
java.util.Arrayshasamethodthatmatchesthemethodcallat(2).Ifthe
declarationofthemethodbinarySearch()at(3)isallowed,thereisalsonoconflict,
asthismethoddeclarationwillshadowtheimportedmethodwhosesignatureitmatches.
Example4.6ConflictinImportingStaticMethodwiththeSameSignature
Clickheretoviewcodeimage
packagemypkg;
publicclassAuxiliary{
publicstaticintbinarySearch(int[]a,intkey){//Samein
java.util.Arrays
//Implementationisomitted.
return-1;
}
}
//File:MultipleStaticImport.java(inunnamedpackage)
importstaticjava.util.Collections.binarySearch;//2overloadedmethods
importstaticjava.util.Arrays.binarySearch;//+18overloadedmethods
importstaticmypkg.Auxiliary.binarySearch;//(1)Causessignatureconflict
publicclassMultipleStaticImport{
publicstaticvoidmain(String[]args){
intindex=binarySearch(newint[]{10,50,100},50);//(2)Ambiguous!
System.out.println(index);
}
//publicstaticintbinarySearch(int[]a,intkey){//(3)
//return-1;
//}
}
CompilingCodeintoPackages
Conventionsforspecifyingpathnamesvaryondifferentplatforms.Inthischapter,wewill
usepathnameconventionsusedonaUnixplatform.Whiletryingouttheexamplesinthis
section,attentionshouldbepaidtoplatformdependenciesinthisregard—especiallythe
factthattheseparatorcharactersinfilepathsfortheUnixandWindowsplatformsare/
and\,respectively.
Asmentionedearlier,apackagecanbemappedonahierarchicalfilesystem.Wecanthink
ofapackagenameasapathnameinthefilesystem.ReferringtoExample4.1,the
packagenamewizard.pandorasboxcorrespondstothepathname
wizard/pandorasbox.TheJavabytecodeforalltypesdeclaredinthesourcefiles
Clown.javaandLovePotion.javawillbeplacedinthepackagedirectorywiththe
pathnamewizard/pandorasbox,asthesesourcefileshavethefollowingpackage
declaration:
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packagewizard.pandorasbox;
Thelocationinthefilesystemwherethepackagedirectoryshouldbecreatedisspecified
usingthedoption(dfordestination)ofthejavaccommand.Thetermdestination
directoryisasynonymforthislocationinthefilesystem.Thecompilerwillcreatethe
packagedirectorywiththepathnamewizard/pandorasbox(includingany
subdirectoriesrequired)underthespecifiedlocation,andplacetheJavabytecodeforthe
typesdeclaredinthesourcefilesClown.javaandLovePotion.javainsidethe
packagedirectory.
Assumingthatthecurrentdirectory(.)isthedirectory/pgjc/work,andthefour
sourcefilesinExample4.1arefoundinthisdirectory,thefollowingcommandissuedin
thecurrentdirectorywillcreateafilehierarchyunderthisdirectory(Figure4.3)that
mirrorsthepackagehierarchyinFigure4.2:
Clickheretoviewcodeimage
>javac-d.Clown.javaLovePotion.javaAilment.javaBaldness.java
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Figure4.3FileHierarchy
Notethesubdirectoriesthatarecreatedforafullyqualifiedpackagename,andwherethe
classfilesarelocated.Inthiscommandline,thespacebetweenthe-doptionandits
argumentismandatory.
Wecanspecifyanyrelativepathnamethatdesignatesthedestinationdirectory,orits
absolutepathname:
Clickheretoviewcodeimage
>javac-d/pgjc/workClown.javaLovePotion.javaAilment.javaBaldness.java
Wecan,ofcourse,specifydestinationsotherthanthecurrentdirectorywheretheclass
fileswiththebytecodeshouldbestored.Thefollowingcommandinthecurrentdirectory
/pgjc/workwillcreatethenecessarypackageswiththeclassfilesunderthedestination
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directory/pgjc/myapp:
Clickheretoviewcodeimage
>javac-d../myappClown.javaLovePotion.javaAilment.javaBaldness.java
Withoutthe-doption,thedefaultbehaviorofthejavaccompileristoplaceallclass
filesdirectlyunderthecurrentdirectory(wherethesourcefilesarelocated),ratherthanin
theappropriatesubdirectoriescorrespondingtothepackages.
Thecompilerwillreportanerrorifthereisanyproblemwiththedestinationdirectory
specifiedwiththe-doption(e.g.,ifitdoesnotexistordoesnothavetherightfile
permissions).
RunningCodefromPackages
ReferringtoExample4.1,ifthecurrentdirectoryhastheabsolutepathname
/pgjc/workandwewanttorunClown.classinthedirectorywiththepathname
./wizard/pandorasbox,thefullyqualifiednameoftheClownclassmustbe
specifiedinthejavacommand:
Clickheretoviewcodeimage
>javawizard.pandorasbox.Clown
ThiswillloadthebytecodeoftheclassClownfromthefilewiththepathname
./wizard/pandorasbox/Clown.class,andstarttheexecutionofitsmain()
method.
4.3SearchingforClasses
ThedocumentationfortheJDKtoolsexplainshowtoorganizepackagesinmoreelaborate
schemes.Inparticular,theCLASSPATHenvironmentvariablecanbeusedtospecifythe
classsearchpath(usuallyabbreviatedtojustclasspath),whicharepathnamesor
locationsinthefilesystemwhereJDKtoolsshouldlookwhensearchingforclassesand
otherresourcefiles.Alternatively,the-classpathoption(oftenabbreviatedto-cp)of
theJDKtoolcommandscanbeusedforthesamepurpose.TheCLASSPATHenvironment
variableisnotrecommendedforthispurpose,asitsclasspathvalueaffectsallJava
applicationsonthehostplatform,andanyapplicationcanmodifyit.However,the-cp
optioncanbeusedtosettheclasspathforeachapplicationindividually.Thisway,an
applicationcannotmodifytheclasspathforotherapplications.Theclasspathspecifiedin
the-cpoptionsupersedesthepathorpathssetbytheCLASSPATHenvironmentvariable
whiletheJDKtoolcommandisrunning.WewillnotdiscusstheCLASSPATH
environmentvariablehere,andassumeittobeundefined.
Basically,theJDKtoolsfirstlookinthedirectorieswheretheJavastandardlibrariesare
installed.Iftheclassisnotfoundinthestandardlibraries,thetoolsearchesintheclass
path.Whennoclasspathisdefined,thedefaultvalueoftheclasspathisassumedtobe
thecurrentdirectory.Ifthe-cpoptionisusedandthecurrentdirectoryshouldbe
searchedbytheJDKtool,thecurrentdirectorymustbespecifiedasanentryintheclass
path,justlikeanyotherdirectorythatshouldbesearched.Thisismostconvenientlydone
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byincluding'.'asoneoftheentriesintheclasspath.
WewillusethefilehierarchiesshowninFigure4.4toillustratesomeoftheintricacies
involvedwhensearchingforclasses.Thecurrentdirectoryhastheabsolutepathname
/top/src,wherethesourcefilesarestored.Thepackagepkgwillbecreatedunderthe
directorywiththeabsolutepathname/top/bin.Thesourcecodeinthetwosourcefiles
A.javaandB.javaisalsoshowninFigure4.4.
Figure4.4SearchingforClasses
ThefilehierarchybeforeanyfilesarecompiledisshowninFigure4.4a.SincetheclassB
doesnotuseanyotherclasses,wecompileitfirstwiththefollowingcommand,resulting
inthefilehierarchyshowninFigure4.4b:
>javac-d../binB.java
Next,wetrytocompilethefileA.java,andgetthefollowingresults:
>javac-d../binA.java
A.java:3:cannotfindsymbol
symbol:classB
location:classpkg.A
publicclassA{Bb;}
^
1error
ThecompilercannotfindtheclassB—thatis,thefileB.classcontainingtheJava
bytecodefortheclassB.InFigure4.4b,wecanseethatitisinthepackagepkgunderthe
directorybin,butthecompilercannotfindit.Thisishardlysurprising,asthereisno
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bytecodefilefortheclassBinthecurrentdirectory,whichisthedefaultvalueoftheclass
path.Thefollowingcommandsetsthevalueoftheclasspathtobe/top/bin,and
compilationissuccessful(Figure4.4c):
Clickheretoviewcodeimage
>javac-cp/top/bin-d../binA.java
ItisveryimportanttounderstandthatwhenwewanttheJDKtooltosearchinanamed
package,itisthelocationofthepackagethatisspecified;inotherwords,theclasspath
indicatesthedirectorythatcontainsthefirstelementofthefullyqualifiedpackagename.
InFigure4.4c,thepackagepkgiscontainedunderthedirectorywhoseabsolutepathis
/top/bin.Thefollowingcommandwillnotwork,asthedirectory/top/bin/pkg
doesnotcontainapackagewiththenamepkgthathasaclassB:
Clickheretoviewcodeimage
>javac-cp/top/bin/pkg-d../binA.java
Also,thecompilerisnotusingtheclasspathtofindthesourcefile(s)thatarespecifiedin
thecommandline.Intheprecedingcommand,thesourcefilehastherelativepathname
./A.java.Consequently,thecompilerlooksforthesourcefileinthecurrentdirectory.
TheclasspathisusedtofindtheclassesusedbytheclassA.
GiventhefilehierarchyinFigure4.3,thefollowing-cpoptionsetstheclasspathsothat
allpackages(wizard.pandorasbox,wizard.pandorasbox.artifacts,
wizard.spells)inFigure4.3willbesearched,asallpackagesarelocatedunderthe
specifieddirectory:
-cp/pgjc/work
However,thefollowing-cpoptionwillnothelpinfindinganyofthepackagesinFigure
4.3,asnoneofthepackagesarelocatedunderthespecifieddirectory:
Clickheretoviewcodeimage
>java-cp/pgjc/work/wizardpandorasbox.Clown
Thiscommandalsoillustratesanimportantpointaboutpackagenames:Thefullyqualified
packagenameshouldnotbesplit.Thepackagenamefortheclass
wizard.pandorasbox.Clowniswizard.pandorasbox,andmustbespecified
fully.ThefollowingcommandwillsearchallpackagesinFigure4.3forclassesthatare
usedbytheclasswizard.pandorasbox.Clown:
Clickheretoviewcodeimage
>java-cp/pgjc/workwizard.pandorasbox.Clown
Theclasspathcanspecifyseveralentries(i.e.,severallocations),andtheJDKtool
searchesthemintheordertheyarespecified,fromlefttoright.
-cp/pgjc/work:/top/bin:.
Wehaveusedthepath-separatorcharacter':'forUnixplatformstoseparatetheentries,
andalsoincludedthecurrentdirectory(.)asanentry.Thereshouldbenowhitespaceon
eithersideofthepath-separatorcharacter.
Thesearchintheclasspathentriesstopsoncetherequiredclassfileisfound.Therefore,
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theorderinwhichentriesarespecifiedcanbesignificant.IfaclassBisfoundina
packagepkglocatedunderthedirectory/ext/lib1,andalsoinapackagepkglocated
underthedirectory/ext/lib2,theorderinwhichtheentriesarespecifiedinthetwo-
cpoptionsshownnextissignificant.Theywillresultintheclasspkg.Bbeingfound
under/ext/lib1and/ext/lib2,respectively.
-cp/ext/lib1:/ext/lib2
-cp/ext/lib2:/ext/lib1
Theexamplessofarhaveusedabsolutepathnamesforclasspathentries.Wecan,of
course,userelativepathnamesaswell.Ifthecurrentdirectoryhastheabsolutepathname
/pgjc/workinFigure4.3,thefollowingcommandwillsearchthepackagesunderthe
currentdirectory:
Clickheretoviewcodeimage
>java-cp.wizard.pandorasbox.Clown
Ifthecurrentdirectoryhastheabsolutepathname/top/srcinFigure4.4,thefollowing
commandwillcompilethefile./A.java:
Clickheretoviewcodeimage
>javac-cp../bin-d../binA.java
Ifthenameofanentryintheclasspathincludeswhitespace,thenameshouldbedouble
quotedsothatitwillbeinterpretedcorrectly:
-cp“../newbin”
ReviewQuestions
4.1GiventhesourcefileA.java:
//File:A.java
packagenet.alphabet;
importjava.util.ArrayList;
publicclassA{}
classB{}
Selectthetwocorrectanswers.
(a)BothclassAandclassBwillbeplacedinthepackagenet.alphabet.
(b)OnlyclassAwillbeplacedinthepackagenet.alphabet.ClassBwillbe
placedinthedefaultpackage.
(c)BothclassAandclassBcanaccesstheimportedclass
java.util.ArrayListbyitssimplename.
(d)OnlyclassAcanaccesstheimportedclassjava.util.ArrayListbyits
simplename.
4.2Whichimportstatement,wheninsertedindependentlyat(1),willmakethecode
compile?
Clickheretoviewcodeimage
//File:Window.java
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packageapp;
publicclassWindow{
finalstaticStringframe=“Top-frame”;
}
___________________________________________________________________________
//File:Canvas.java
packageapp;
//(1)INSERTIMPORTSTATEMENTHERE.
publicclassCanvas{
privateStringstr=frame;
}
Selecttheonecorrectanswer.
(a)importapp.*;
(b)importapp.Window;
(c)importjava.lang.*;
(d)importjava.lang.String;
(e)importstaticapp.Window.frame;
4.3Whichimportstatements,wheninsertedindependentlyat(1),willmakethecode
compile?
Clickheretoviewcodeimage
//File:Window.java
packagemainpkg.subpkg1;
publicclassWindow{}
___________________________________________________________________________
//File:Window.java
packagemainpkg.subpkg2;
publicclassWindow{}
___________________________________________________________________________
//File:Screen.java
packagemainpkg;
//(1)INSERTIMPORTSTATEMENTSHERE.
publicclassScreen{
privateWindowwin;
}
Selectthefourcorrectanswers.
(a)importmainpkg.*;
(b)importmainpkg.subpkg1.*;
(c)importmainpkg.subpkg2.*;
(d)
Clickheretoviewcodeimage
importmainpkg.subpkg1.*;
importmainpkg.subpkg2.Window;
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(e)
Clickheretoviewcodeimage
importmainpkg.subpkg1.Window;
importmainpkg.subpkg2.*;
(f)
importmainpkg.subpkg1.*;
importmainpkg.subpkg2.*;
(g)
Clickheretoviewcodeimage
importmainpkg.subpkg1.Window;
importmainpkg.subpkg2.Window;
4.4Giventhefollowingcode:
Clickheretoviewcodeimage
//(1)INSERTONEIMPORTSTATEMENTHERE
publicclassRQ700A20{
publicstaticvoidmain(String[]args){
System.out.println(sqrt(49));
}
}
Whichimportstatements,wheninsertedindependentlyat(1),willmakethe
programprint7,whentheprogramiscompiledandrun?
Selectthetwocorrectanswers.
(a)importstaticMath.*;
(b)importstaticMath.sqrt;
(c)importstaticjava.lang.Math.sqrt;
(d)importstaticjava.lang.Math.sqrt();
(e)importstaticjava.lang.Math.*;
4.5GiventhesourcefileA.java:
packagetop.sub;
publicclassA{}
andthefollowingdirectoryhierarchy:
Clickheretoviewcodeimage
/proj
|–src
||–top
||–sub
||–A.java
|–bin
Assumingthatthecurrentdirectoryis/proj/src,whichofthefollowing
statementsaretrue?
Selectthethreecorrectanswers.
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(a)Thefollowingcommandwillcompile,andplacethebytecodeoftheclass
top.sub.Aunder/proj/bin:
javac-d.top/sub/A.java
(b)Thefollowingcommandwillcompile,andplacethebytecodeoftheclass
top.sub.Aunder/proj/bin:
Clickheretoviewcodeimage
javac-d/proj/bintop/sub/A.java
(c)Thefollowingcommandwillcompile,andplacethebytecodeoftheclass
top.sub.Aunder/proj/bin:
Clickheretoviewcodeimage
javac-D/proj/bin./top/sub/A.java
(d)Thefollowingcommandwillcompile,andplacethebytecodeoftheclass
top.sub.Aunder/proj/bin:
Clickheretoviewcodeimage
javac-d../bintop/sub/A.java
(e)Aftersuccessfulcompilation,theabsolutepathnameofthefileA.classwill
be:
/proj/bin/A.class
(f)Aftersuccessfulcompilation,theabsolutepathnameofthefileA.classwill
be:
/proj/bin/top/sub/A.class
4.6Giventhefollowingdirectorystructure:
Clickheretoviewcodeimage
/top
|–wrk
|–pkg
|–A.java
|–B.java
AssumethatthetwofilesA.javaandB.javacontainthefollowingcode,
respectively:
Clickheretoviewcodeimage
//File:A.java
packagepkg;
classA{Bb;}
___________________________________________________________________________
//File:B.java
packagepkg;
classB{…}
Forwhichcombinationsofcurrentdirectoryandcommandisthecompilation
successful?
Selectthetwocorrectanswers.
(a)
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Clickheretoviewcodeimage
Currentdirectory:/top/wrk
Command:javac-cp.:pkgA.java
(b)
Clickheretoviewcodeimage
Currentdirectory:/top/wrk
Command:javac-cp.pkg/A.java
(c)
Currentdirectory:/top/wrk
Command:javac-cppkgA.java
(d)
Clickheretoviewcodeimage
Currentdirectory:/top/wrk
Command:javac-cp.:pkgpkg/A.java
(e)
Clickheretoviewcodeimage
Currentdirectory:/top/wrk/pkg
Command:javacA.java
(f)
Clickheretoviewcodeimage
Currentdirectory:/top/wrk/pkg
Command:javac-cp.A.java
4.7Giventhefollowingdirectorystructure:
Clickheretoviewcodeimage
/proj
|–src
||–A.java
|
|
|–bin
|–top
|–sub
|–A.class
Assumethatthecurrentdirectoryis/proj/src.Whichclasspathspecifications
willfindthefileA.classoftheclasstop.sub.Adeclaredinthefile
/proj/src/A.java?
Selectthetwocorrectanswers.
(a)-cp/proj/bin/top
(b)-cp/proj/bin/top/sub
(c)-cp/proj/bin/top/sub/A.class
(d)-cp.:../bin
(e)-cp/proj
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(f)-cp/proj/bin
4.4ScopeRules
Javaprovidesexplicitaccessibilitymodifierstocontroltheaccessibilityofmembersina
classbyexternalclients(§4.7,p.123),butintwoareasaccessisgovernedbyspecific
scoperules:
•Classscopeformembers:howmemberdeclarationsareaccessedwithintheclass.
•Blockscopeforlocalvariables:howlocalvariabledeclarationsareaccessedwithin
ablock.
ClassScopeforMembers
Classscopeconcernsaccessingmembers(includinginheritedones)fromcodewithina
class.Table4.1givesanoverviewofhowstaticandnon-staticcodeinaclasscanaccess
membersoftheclass,includingthosethatareinherited.Table4.1assumesthefollowing
declarations:
Clickheretoviewcodeimage
classSuperName{
intinstanceVarInSuper;
staticintstaticVarInSuper;
voidinstanceMethodInSuper(){/*…*/}
staticvoidstaticMethodInSuper(){/*…*/}
//…
}
classClassNameextendsSuperName{
intinstanceVar;
staticintstaticVar;
voidinstanceMethod(){/*…*/}
staticvoidstaticMethod(){/*…*/}
//…
}
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Table4.1AccessingMemberswithinaClass
Thegoldenruleisthatstaticcodecanonlyaccessotherstaticmembersbytheirsimple
names.Staticcodeisnotexecutedinthecontextofanobject,sothereferencesthisand
superarenotavailable.Anobjecthasknowledgeofitsclass,sostaticmembersare
alwaysaccessibleinanon-staticcontext.
Notethatusingtheclassnametoaccessstaticmemberswithintheclassisnodifferent
fromhowexternalclientsaccessthesestaticmembers.
Thefollowingfactorscanallinfluencethescopeofamemberdeclaration:
•Shadowingofafielddeclaration,eitherbylocalvariables(§4.4,p.117)orby
declarationsinthesubclass(§7.3,p.275)
•Overridinganinstancemethodfromasuperclass(§7.2,p.268)
•Hidingastaticmethoddeclaredinasuperclass(§7.3,p.275)
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Withinaclass,referencesoftheclasscanbedeclaredandusedtoaccessallmembersin
theclass,regardlessoftheiraccessibilitymodifiers.InExample4.7,themethod
duplicateLightat(1)intheclassLighthastheparameteroldLightandthe
localvariablenewLightthatarereferencesoftheclassLight.Eventhoughthefields
oftheclassareprivate,theyareaccessiblethroughthetworeferences(oldLight
andnewLight)inthemethodduplicateLight()asshownat(2),(3),and(4).
Example4.7ClassScope
Clickheretoviewcodeimage
classLight{
//Instancevariables:
privateintnoOfWatts;//Wattage
privatebooleanindicator;//Onoroff
privateStringlocation;//Placement
//Instancemethods:
publicvoidswitchOn(){indicator=true;}
publicvoidswitchOff(){indicator=false;}
publicbooleanisOn(){returnindicator;}
publicstaticLightduplicateLight(LightoldLight){//(1)
LightnewLight=newLight();
newLight.noOfWatts=oldLight.noOfWatts;//(2)
newLight.indicator=oldLight.indicator;//(3)
newLight.location=oldLight.location;//(4)
returnnewLight;
}
}
BlockScopeforLocalVariables
Declarationsandstatementscanbegroupedintoablockusingbraces,{}.Blockscanbe
nested,andscoperulesapplytolocalvariabledeclarationsinsuchblocks.Alocal
declarationcanappearanywhereinablock.Thegeneralruleisthatavariabledeclaredin
ablockisinscopeinsidetheblockinwhichitisdeclared,butitisnotaccessibleoutside
ofthisblock.Itisnotpossibletoredeclareavariableifalocalvariableofthesamename
isalreadydeclaredinthecurrentscope.
Localvariablesofamethodincludetheformalparametersofthemethodandvariables
thataredeclaredinthemethodbody.Thelocalvariablesinamethodarecreatedeachtime
themethodisinvoked,andarethereforedistinctfromlocalvariablesinotherinvocations
ofthesamemethodthatmightbeexecuting(§6.5,p.230).
Figure4.5illustratesblockscope(alsoknownaslexicalscope)forlocalvariables.A
methodbodyisablock.Parameterscannotberedeclaredinthemethodbody,asshownat
(1)inBlock1.
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Figure4.5BlockScope
Alocalvariable—alreadydeclaredinanenclosingblockand,therefore,visibleinanested
block—cannotberedeclaredinthenestedblock.Thesecasesareshownat(3),(5),and
(6).
Alocalvariableinablockcanberedeclaredinanotherblockiftheblocksaredisjoint—
thatis,theydonotoverlap.Thisisthecaseforvariableiat(2)inBlock3andat(4)in
Block4,asthesetwoblocksaredisjoint.
Thescopeofalocalvariabledeclarationbeginsfromwhereitisdeclaredintheblockand
endswherethisblockterminates.Thescopeoftheloopvariableindexistheentire
Block2.EventhoughBlock2isnestedinBlock1,thedeclarationofthevariableindex
at(7)inBlock1isvalid.Thescopeofthevariableindexat(7)spansfromits
declarationtotheendofBlock1,anditdoesnotoverlapwiththatoftheloopvariable
indexinBlock2.
4.5AccessibilityModifiersforTop-LevelTypeDeclarations
Theaccessibilitymodifierpubliccanbeusedtodeclaretop-leveltypes(thatis,classes,
enums,andinterfaces)inapackagetobeaccessiblefromeverywhere,bothinsidetheir
ownpackageandinsideotherpackages.Iftheaccessibilitymodifierisomitted,theywill
beaccessibleonlyintheirownpackageandnotinanyotherpackagesorsubpackages.
Thisiscalledpackageordefaultaccessibility.
Example4.8AccessibilityModifiersforClassesandInterfaces
Clickheretoviewcodeimage
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//File:Clown.java
packagewizard.pandorasbox;//Packagedeclaration
importwizard.pandorasbox.artifacts.Ailment;//ImportingclassAilment
publicclassClownimplementsMagic{//(1)
LovePotiontlc;//Classinsamepackage
Ailmentproblem;//Simpleclassname
Clown(){
tlc=newLovePotion(“passion”);
problem=newAilment(“flu”);//Simpleclassname
}
@Overridepublicvoidlevitate(){//(2)
System.out.println(“Levitating”);
}
publicvoidmixPotion(){System.out.println(“Mixing”+tlc);}
publicvoidhealAilment(){System.out.println(“Healing”+problem);}
publicstaticvoidmain(String[]args){
Clownjoker=newClown();
joker.levitate();
joker.mixPotion();
joker.healAilment();
}
}
interfaceMagic{voidlevitate();}//(3)
//File:LovePotion.java
packagewizard.pandorasbox;//Packagedeclaration
publicclassLovePotion{//(4)Accessibleoutside
package
StringpotionName;
publicLovePotion(Stringname){potionName=name;}
publicStringtoString(){returnpotionName;}
}
//File:Ailment.java
packagewizard.pandorasbox.artifacts;//Packagedeclaration
publicclassAilment{//Accessibleoutsidepackage
StringailmentName;
publicAilment(Stringname){ailmentName=name;}
publicStringtoString(){returnailmentName;}
}
//File:Baldness.java
packagewizard.spells;//Packagedeclaration
importwizard.pandorasbox.*;//Redundant
importwizard.pandorasbox.artifacts.*;//Importofsubpackage
publicclassBaldnessextendsAilment{//SimplenameforAilment
wizard.pandorasbox.LovePotiontlcOne;//Fullyqualifiedname
LovePotiontlcTwo;//Classinsamepackage
Baldness(Stringname){
super(name);
tlcOne=newwizard.pandorasbox.//Fullyqualifiedname
LovePotion(“romance”);
tlcTwo=newLovePotion();//Classinsamepackage
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}
}
classLovePotion/*implementsMagic*/{//(5)Magicisnotaccessible
//@Overridepublicvoidlevitate(){}//(6)Cannotoverridemethod
}
Compilingandrunningtheprogramfromthecurrentdirectorygivesthefollowingresults:
Clickheretoviewcodeimage
>javac-d.Clown.javaLovePotion.javaAilment.javaBaldness.java
>javawizard.pandorasbox.Clown
Levitating
Mixingpassion
Healingflu
InExample4.8,theclassClownat(1)andtheinterfaceMagicat(3)areplacedina
packagecalledwizard.pandorasbox.ThepublicclassClownisaccessiblefrom
everywhere.TheMagicinterfacehasdefaultaccessibility,andcanbeaccessedonly
withinthepackagewizard.pandorasbox.Itisnotaccessiblefromotherpackages,
notevenfromsubpackages.
TheclassLovePotionat(4)isalsoplacedinthepackagecalled
wizard.pandorasbox.Theclasshaspublicaccessibilityand,therefore,is
accessiblefromotherpackages.ThetwofilesClown.javaandLovePotion.java
demonstratehowseveralcompilationunitscanbeusedtogroupclassesinthesame
package.
InthefileClown.java,theclassClownat(1)implementstheinterfaceMagicat(3)
fromthesamepackage.Wehaveusedtheannotation@Overrideinfrontofthe
declarationofthelevitate()methodat(2)sothatthecompilercanaidinchecking
thatthismethodisdeclaredcorrectlyasrequiredbytheinterfaceMagic.
InthefileBaldness.java,theclassLovePotionat(5)wishestoimplementthe
interfaceMagicat(3)fromthepackagewizard.pandorasbox,butcannotdoso,
althoughthesourcefileimportsfromthispackage.ThereasonisthattheinterfaceMagic
hasdefaultaccessibilityandcan,therefore,beaccessedonlywithinthepackage
wizard.pandorasbox.Themethodlevitate()oftheMagicinterfacetherefore
cannotbeoverriddeninclassLovePotionat(6).
Justbecauseareferencetypeisaccessibledoesnotnecessarilymeanthatmembersofthe
typearealsoaccessible.Memberaccessibilityisgovernedseparatelyfromtype
accessibility,asexplainedin§4.7,p.123.Table4.2givesasummaryofaccessibility
modifiersfortop-leveltypes.
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Table4.2SummaryofAccessibilityModifiersforTop-LevelTypes
4.6Non-AccessibilityModifiersforClasses
Thenon-accessibilitymodifiersabstractandfinalcanbeappliedtotop-level
classes.
Classes
Aclasscanbedeclaredwiththekeywordabstracttoindicatethatitcannotbe
instantiated.Aclassmightchoosetodothisiftheabstractionitrepresentsissogeneral
thatitneedstobespecializedtobeofpracticaluse.TheclassVehiclemightbe
specifiedasabstracttorepresentthegeneralabstractionofavehicle,ascreating
instancesoftheclasswouldnotmakemuchsense.Creatinginstancesofnon-abstract
subclasses,likeCarandBus,wouldmakemoresense,asthiswouldmakethe
abstractionmoreconcrete.
Anynormalclass(thatis,aclassdeclaredwiththekeywordclass)canbedeclaredas
abstract.However,ifsuchaclasshasoneormoreabstractmethods(§4.8,p.136),
itmustbedeclaredasabstract.Obviously,suchclassescannotbeinstantiated,astheir
implementationmightbeonlypartial.Aclassmightchoosethisstrategytodictatecertain
behavior,butallowitssubclassesthefreedomtoprovidetherelevantimplementation.In
otherwords,subclassesoftheabstractclasshavetotakeastandandprovide
implementationsofanyinheritedabstractmethodsbeforeinstancescanbecreated.A
subclassthatdoesnotprovideanimplementationofitsinheritedabstractmethods
mustalsobedeclaredasabstractorthecodewillnotcompile.
InExample4.9,thedeclarationoftheabstractclassLighthasanabstract
methodnamedkwhPriceat(1).Thisforcesitsconcrete(i.e.,non-abstract)subclassesto
provideanimplementationforthismethod.Suchaclassprovidesimplementationsofall
itsmethods.TheconcretesubclassTubeLightprovidesanimplementationforthe
methodkwhPrice()at(2).TheclassFactorycreatesaninstanceoftheclass
TubeLightat(3).Referencesofanabstractclasscanbedeclared,asshownat(4),
butanabstractclasscannotbeinstantiated,asshownat(5).Referencesofan
abstractclasscanrefertoobjectsofthesubclasses,asshownat(6).
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Example4.9AbstractClasses
Clickheretoviewcodeimage
abstractclassLight{
//Fields:
intnoOfWatts;//Wattage
booleanindicator;//Onoroff
Stringlocation;//Placement
//Instancemethods:
publicvoidswitchOn(){indicator=true;}
publicvoidswitchOff(){indicator=false;}
publicbooleanisOn(){returnindicator;}
//Abstractinstancemethod
publicabstractdoublekwhPrice();//(1)Nomethodbody
}
//______________________________________________________________________________
classTubeLightextendsLight{
//Field
inttubeLength;
//Implementationofinheritedabstractmethod.
@OverridepublicdoublekwhPrice(){return2.75;}//(2)
}
//______________________________________________________________________________
publicclassFactory{
publicstaticvoidmain(String[]args){
TubeLightcellarLight=newTubeLight();//(3)OK
LightnightLight;//(4)OK
//LighttableLight=newLight();//(5)Compile-time
error
nightLight=newTubeLight();//(6)OK
System.out.println(“KWHprice:$”+nightLight.kwhPrice());
}
}
Outputfromtheprogram:
KWHprice:$2.75
Classes
Aclasscanbedeclaredasfinaltoindicatethatitcannotbeextended;thatis,one
cannotdeclaresubclassesofafinalclass.Thisimpliesthatonecannotoverrideany
methodsdeclaredinsuchaclass.Inotherwords,theclassbehaviorcannotbechangedby
extendingtheclass.Afinalclassmarksthelowerboundaryofitsimplementation
inheritancehier-archy(§7.1,p.264).Onlyaconcreteclasscanbedeclaredasfinal.
Afinalclassmustbecomplete,whereasanabstractclassisconsideredincomplete.
Classes,therefore,cannotbebothfinalandabstractatthesametime.Interfacesare
inherentlyabstract,astheycanspecifymethodsthatareabstract,andthereforecannot
bedeclaredasfinal.Afinalclassandaninterfacerepresenttwoextremeswhenit
comestoprovidinganimplementation.Anabstractclassrepresentsacompromise
betweenthesetwoextremes.Table4.3providesasummaryofnon-accessibilitymodifiers
forclasses.
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Table4.3SummaryofNon-AccessibilityModifiersforClasses
TheJavaSEplatformAPIincludesmanyfinalclasses—forexample,the
java.lang.Stringclassandthewrapperclassesforprimitivevalues.
IfitisdecidedthattheclassTubeLightinExample4.9maynotbeextended,itcanbe
declaredasfinal:
Clickheretoviewcodeimage
finalclassTubeLightextendsLight{
//…
}
Discussionoffinalmethods,fields,andlocalvariablescanbefoundin§4.8,p.133.
ReviewQuestions
4.8Giventhefollowingclass,whichofthesealternativesarevalidwaysofreferringto
theclassfromoutsideofthepackagenet.basemaster?
packagenet.basemaster;
publicclassBase{
//…
}
Selectthetwocorrectanswers.
(a)BysimplyreferringtotheclassasBase
(b)Bysimplyreferringtotheclassasbasemaster.Base
(c)Bysimplyreferringtotheclassasnet.basemaster.Base
(d)Byimportingwithnet.basemaster.*,andreferringtotheclassasBase
(e)Byimportingwithnet.*,andreferringtotheclassasbasemaster.Base
4.9Whichoneofthefollowingclassdeclarationsisavaliddeclarationofaclassthat
cannotbeinstantiated?
Selecttheonecorrectanswer.
(a)classGhost{abstractvoidhaunt();}
(b)abstractclassGhost{voidhaunt();}
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(c)abstractclassGhost{voidhaunt(){};}
(d)abstractGhost{abstractvoidhaunt();}
(e)abstractclassGhost{abstracthaunt();}
4.10Whichoneofthefollowingclassdeclarationsisavaliddeclarationofaclassthat
cannotbeextended?
Selecttheonecorrectanswer.
(a)classLink{}
(b)abstractclassLink{}
(c)nativeclassLink{}
(d)finalclassLink{}
(e)abstractfinalclassLink{}
4.7MemberAccessibilityModifiers
Byspecifyingmemberaccessibilitymodifiers,aclasscancontrolwhichinformationis
accessibletoclients(thatis,otherclasses).Thesemodifiershelpaclasstodefinea
contractsothatclientsknowexactlywhichservicesareofferedbytheclass.
Theaccessibilityofmemberscanbeoneofthefollowing:
public
protected
Defaultaccessibility(alsoknownaspackageaccessibility),meaningthatno
accessibilitymodifierisspecified
private
Inthefollowingdiscussionofaccessibilitymodifiersformembersofaclass,keepinmind
thatthememberaccessibilitymodifierhasmeaningonlyiftheclass(oroneofits
subclasses)isaccessibletotheclient.Also,notethatonlyoneaccessibilitymodifiercan
bespecifiedforamember.
Thediscussioninthissectionappliestobothinstanceandstaticmembersoftop-level
classes.Itappliesequallytoconstructorsaswell.
InUMLnotation,theprefixes+,#,and-,whenappliedtoamembername,indicate
public,protected,andprivatememberaccessibility,respectively.Noprefix
indicatesdefaultorpackageaccessibility.
Members
Publicaccessibilityistheleastrestrictiveofalltheaccessibilitymodifiers.Apublic
memberisaccessiblefromanywhere,bothinthepackagecontainingitsclassandinother
packageswherethisclassisvisible.
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Example4.10containstwosourcefiles,shownat(1)and(6).Thepackagehierarchy
definedbythesourcefilesisdepictedinFigure4.6,showingthetwopackages,
packageAandpackageB,containingtheirrespectiveclasses.Theclassesin
packageBuseclassesfrompackageA.TheclassSuperclassAinpackageAhas
twosubclasses:SubclassAinpackageAandSubclassBinpackageB.
Figure4.6PublicAccessibilityforMembers
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Example4.10PublicAccessibilityofMembers
Clickheretoviewcodeimage
//File:SuperclassA.java(1)
packagepackageA;
publicclassSuperclassA{
publicintsuperclassVarA;//(2)
publicvoidsuperclassMethodA(){/*…*/}//(3)
}
classSubclassAextendsSuperclassA{
voidsubclassMethodA(){superclassVarA=10;}//(4)OK
}
classAnyClassA{
SuperclassAobj=newSuperclassA();
voidanyClassMethodA(){
obj.superclassMethodA();//(5)OK
}
}
//File:SubclassB.java(6)
packagepackageB;
importpackageA.*;
publicclassSubclassBextendsSuperclassA{
voidsubclassMethodB(){superclassMethodA();}//(7)OK
}
classAnyClassB{
SuperclassAobj=newSuperclassA();
voidanyClassMethodB(){
obj.superclassVarA=20;//(8)OK
}
}
AccessibilityisillustratedinExample4.10bytheaccessibilitymodifiersforthefield
superclassVarAandthemethodsuperclassMethodA()at(2)and(3),
respectively,definedintheclassSuperclassA.Thesemembersareaccessedfromfour
differentclientsinExample4.10.
•Client1:Fromasubclassinthesamepackage,whichaccessesaninheritedfield
fromtheclassSuperclassA.SubclassAissuchaclient,anddoesthisat(4).
•Client2:Fromanon-subclassinthesamepackage,whichinvokesamethodonan
instanceoftheSuperclassAclass.AnyClassAissuchaclient,anddoesthisat
(5).
•Client3:Fromasubclassinanotherpackage,whichinvokesaninheritedmethod
fromtheclassSuperclassA.SubclassBissuchaclient,anddoesthisat(7).
•Client4:Fromanon-subclassinanotherpackage,whichaccessesafieldinan
instanceoftheSuperclassAclass.AnyClassBissuchaclient,anddoesthisat
(8).
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InExample4.10,thefieldsuperclassVarAandthemethod
superclassMethodA()havepublicaccessibilityintheSuperclassAclass,and
areaccessiblebyallfouroftheseclients.Subclassescanaccesstheirinheritedpublic
membersbytheirsimplenames,andallclientscanaccesspublicmembersinaninstance
oftheSuperclassAclass.PublicaccessibilityisdepictedinFigure4.6.
Members
Aprotectedmemberisaccessibleinallclassesinthesamepackage,andbyallsubclasses
ofitsclassinanypackagewherethisclassisvisible.Inotherwords,non-subclassesin
otherpackagescannotaccessprotectedmembersfromotherpackages.Thiskindof
accessibilityismorerestrictivethanpublicmemberaccessibility.
InExample4.10,ifthefieldsuperclassVarAandthemethod
superclassMethodA()oftheclassSuperclassAhaveprotectedaccessibility,
theyareaccessiblewithinpackageA,andonlyaccessiblebysubclassesinanyother
packages.
Clickheretoviewcodeimage
publicclassSuperclassA{
protectedintsuperclassVarA;//(2)Protectedmember
protectedvoidsuperclassMethodA(){/*…*/}//(3)Protectedmember
}
Client4inpackageBcannotaccessthesemembers,asshowninFigure4.7.
Figure4.7ProtectedAccessibilityforMembers
Animportantcaveatisthatasubclassinanotherpackagecanaccessonlyprotected
membersinthesuperclassviareferencesofitsowntypeoritssubtypes.Thefollowing
newdeclarationofSubclassBinpackageBfromExample4.10illustratesthepoint:
Clickheretoviewcodeimage
//File:SubclassB.java
packagepackageB;
importpackageA.*;
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publicclassSubclassBextendsSuperclassA{//InpackageB
SuperclassAobjRefA=newSuperclassA();//(1)
voidsubclassMethodB(SubclassBobjRefB){
objRefB.superclassMethodA();//(2)OK
objRefB.superclassVarA=5;//(3)OK
objRefA.superclassMethodA();//(4)NotOK
objRefA.superclassVarA=10;//(5)NotOK
}
}
TheclassSubclassBdeclaresthefieldobjRefAoftypeSuperclassAat(1).The
methodsubclassMethodB()hastheformalparameterobjRefBoftype
SubclassB.AccessispermittedtoaprotectedmemberofSuperclassAin
packageAbyareferenceofthesubclass,asshownat(2)and(3),butnotbyareference
ofitssuperclass,asshownat(4)and(5).AccesstothefieldsuperclassVarAandthe
calltothemethodsuperclassMethodA()occurinSubclassB.Thesemembersare
declaredinSuperclassA.SubclassBisnotinvolvedintheimplementationof
SuperclassA,whichisthetypeofthereferenceobjRefA.Hence,accesstoprotected
membersat(4)and(5)isnotpermittedasthesearenotmembersofanobjectthatcanbe
guaranteedtobeimplementedbythecodeaccessingthem.
Accessibilitytoprotectedmembersofthesuperclasswouldalsobepermittedviaany
referencewhosetypeisasubclassofSubclassB.Thepreviouslymentionedrestriction
helpstoensurethatsubclassesinpackagesdifferentfromtheirsuperclasscanaccess
protectedmembersofthesuperclassonlyintheirpartoftheimplementationinheritance
hierarchy.Inotherwords,aprotectedmemberofasuperclassisaccessibleinasubclass
thatisinanotherpackageonlyifthememberisinheritedbyanobjectofthesubclass(or
byanobjectofasubclassofthissubclass).
DefaultAccessibilityforMembers
Whennomemberaccessibilitymodifierisspecified,thememberisaccessibleonlyto
otherclassesinitsownclass’spackage.Evenifitsclassisvisibleinanother(possibly
nested)package,thememberisnotaccessibleelsewhere.Defaultmemberaccessibilityis
morerestrictivethanprotectedmemberaccessibility.
InExample4.10,ifthefieldsuperclassVarAandthemethod
superclassMethodA()aredefinedwithnoaccessibilitymodifier,theyareaccessible
withinpackageA,butnotinanyotherpackages.
Clickheretoviewcodeimage
publicclassSuperclassA{
intsuperclassVarA;//(2)Default
accessibility
voidsuperclassMethodA(){/*…*/}//(3)Defaultaccessibility
}
TheclientsinpackageB(thatis,Clients3and4)cannotaccessthesemembers.This
situationisdepictedinFigure4.8.
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Figure4.8DefaultAccessibilityforMembers
Members
Theprivatemodifieristhemostrestrictiveofalltheaccessibilitymodifiers.Private
membersarenotaccessiblefromanyotherclasses.Thisalsoappliestosubclasses,
whethertheyareinthesamepackageornot.Sincetheyarenotaccessiblebytheirsimple
namesinasubclass,theyarealsonotinheritedbythesubclass.Astandarddesignstrategy
foraclassistomakeallfieldsprivateandprovidepublicaccessormethodsforthem.
Auxiliarymethodsareoftendeclaredasprivate,astheydonotconcernanyclient.
InExample4.10,ifthefieldsuperclassVarAandthemethod
superclassMethodA()haveprivateaccessibility,theyarenotaccessiblebyany
otherclients.
Clickheretoviewcodeimage
publicclassSuperclassA{
privateintsuperclassVarA;//(2)Privatemember
privatevoidsuperclassMethodA(){/*…*/}//(3)Privatemember
}
NoneoftheclientsinFigure4.9canaccessthesemembers.Table4.4providesasummary
ofaccessibilitymodifiersformembers.
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Figure4.9PrivateAccessibilityforMembers
Table4.4SummaryofAccessibilityModifiersforMembers
ReviewQuestions
4.11Giventhefollowingdeclarationofaclass,whichfieldisaccessiblefromoutside
thepackagecom.corporation.project?
Clickheretoviewcodeimage
packagecom.corporation.project;
publicclassMyClass{
inti;
publicintj;
protectedintk;
privateintl;
}
Selecttheonecorrectanswer.
(a)Fieldiisaccessibleinallclassesinotherpackages.
(b)Fieldjisaccessibleinallclassesinotherpackages.
(c)Fieldkisaccessibleinallclassesinotherpackages.
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(d)Fieldkisaccessibleinsubclassesonlyinotherpackages.
(e)Fieldlisaccessibleinallclassesinotherpackages.
(f)Fieldlisaccessibleinsubclassesonlyinotherpackages.
4.12Howrestrictiveisthedefaultaccessibilitycomparedtopublic,protected,
andprivateaccessibility?
Selecttheonecorrectanswer
(a)Lessrestrictivethanpublic
(b)Morerestrictivethanpublic,butlessrestrictivethanprotected
(c)Morerestrictivethanprotected,butlessrestrictivethanprivate
(d)Morerestrictivethanprivate
(e)Lessrestrictivethanprotectedfromwithinapackage,andmorerestrictive
thanprotectedfromoutsideapackage
4.13Whichstatementistrueabouttheaccessibilityofmembers?
Selecttheonecorrectanswer.
(a)Aprivatememberisalwaysaccessiblewithinthesamepackage.
(b)Aprivatemembercanbeaccessedonlywithintheclassofthemember.
(c)Amemberwithdefaultaccessibilitycanbeaccessedbyanysubclassofthe
classinwhichitisdeclared.
(d)Aprivatemembercannotbeaccessedatall.
(e)Package/defaultaccessibilityforamembercanbedeclaredusingthekeyword
default.
4.14Whichlinesthataremarkedwillcompileinthefollowingcode?
Clickheretoviewcodeimage
//Filename:A.java
packagepackageA;
publicclassA{
protectedintpf;
}
//Filename:B.java
packagepackageB;
importpackageA.A;
publicclassBextendsA{
voidaction(Aobj1,Bobj2,Cobj3){
pf=10;//(1)
obj1.pf=10;//(2)
obj2.pf=10;//(3)
obj3.pf=10;//(4)
}
}
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classCextendsB{
voidaction(Aobj1,Bobj2,Cobj3){
pf=10;//(5)
obj1.pf=10;//(6)
obj2.pf=10;//(7)
obj3.pf=10;//(8)
}
}
classD{
voidaction(Aobj1,Bobj2,Cobj3){
pf=10;//(9)
obj1.pf=10;//(10)
obj2.pf=10;//(11)
obj3.pf=10;//(12)
}
}
Selectthefivecorrectanswers.
(a)(1)
(b)(2)
(c)(3)
(d)(4)
(e)(5)
(f)(6)
(g)(7)
(h)(8)
(i)(9)
(j)(10)
(k)(11)
(l)(12)
4.8Non-AccessibilityModifiersforMembers
Thefollowingkeywordscanbeusedtospecifycertainaspectsofmembersinatype
declaration:
static
final
abstract
synchronized
native
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transient
volatile
Members
Staticmembersbelongtotheclassinwhichtheyaredeclaredandarenotpartofany
instanceoftheclass.Thedeclarationofstaticmembersisprefixedbythekeyword
statictodistinguishthemfrominstancemembers.Dependingontheaccessibility
modifiersofthestaticmembersinaclass,clientscanaccessthesemembersbyusingthe
classnameorthroughobjectreferencesoftheclass.Theclassneednotbeinstantiatedto
accessitsstaticmembers.
Staticvariables(alsocalledclassvariables)existonlyintheclassinwhichtheyare
defined.Theyarenotinstantiatedwhenaninstanceoftheclassiscreated.Inotherwords,
thevaluesofthesevariablesarenotapartofthestateofanyobject.Whentheclassis
loaded,staticvariablesareinitializedtotheirdefaultvaluesifnoexplicitinitialization
expressionisspecified.
Staticmethodsarealsoknownasclassmethods.Astaticmethodinaclasscandirectly
accessotherstaticmembersintheclass.Itcannotaccessinstance(i.e.,non-static)
membersoftheclassdirectly,asthereisnonotionofanobjectassociatedwithastatic
method.
Atypicalstaticmethodmightperformsometaskonbehalfofthewholeclassorfor
objectsoftheclass.InExample4.11,thestaticvariablecounterkeepstrackofthe
numberofinstancesoftheLightclassthathavebeencreated.Theexampleshowsthat
thestaticmethodwriteCount()canaccessstaticmembersdirectly,asshownat(2),
butnotnon-staticmembers,asshownat(3).Thestaticvariablecounterat(1)willbe
initializedtothedefaultvalue0whentheclassisloadedatruntime.Themain()method
at(4)intheclassWarehouseshowshowstaticmembersoftheclassLightcanbe
accessedusingtheclassnameandviaobjectreferencesofthetypeLight.
Asummaryofhowstaticmembersareaccessedinstaticandnon-staticcodeisgivenin
Table4.1,p.115.
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Example4.11AccessingStaticMembers
Clickheretoviewcodeimage
classLight{
//Fields:
intnoOfWatts;//Wattage
booleanindicator;//Onoroff
Stringlocation;//Placement
//Staticvariable
staticintcounter;//NumberofLightobjectscreated(1)
//Non-zeroargumentconstructor
Light(intnoOfWatts,booleanindicator,Stringlocation){
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
++counter;//Incrementcounter.
}
//Staticmethod
publicstaticvoidwriteCount(){
System.out.println(“Numberoflights:”+counter);//(2)
//Compile-timeerror.FieldnoOfWattsisnotaccessible:
//System.out.println(“NumberofWatts:”+noOfWatts);//(3)
}
}
//______________________________________________________________________________
publicclassWarehouse{
publicstaticvoidmain(String[]args){//(4)
Light.writeCount();//Invokedusingclass
name
Lightlight1=newLight(100,true,“basement”);//Createanobject
System.out.println(
“Valueofcounter:”+Light.counter//Accessedviaclass
name
);
Lightlight2=newLight(200,false,“garage”);//Createanotherobject
light2.writeCount();//Invokedusing
reference
Lightlight3=newLight(300,true,“kitchen”);//Createanotherobject
System.out.println(
“Valueofcounter:”+light3.counter//Accessedvia
reference
);
}
}
Outputfromtheprogram:
Numberoflights:0
Valueofcounter:1
Numberoflights:2
Valueofcounter:3
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Members
Afinalvariableisaconstantdespitebeingcalledavariable.Itsvaluecannotbe
changedonceithasbeeninitialized.Instanceandstaticvariablescanbedeclaredas
final.Notethatthekeywordfinalcanalsobeappliedtolocalvariables,including
formalparametersofamethod.Declaringavariableasfinalhasthefollowing
implications:
•Afinalvariableofaprimitivedatatypecannotchangeitsvalueonceithasbeen
initialized.
•Afinalvariableofareferencetypecannotchangeitsreferencevalueonceithas
beeninitialized.Thiseffectivelymeansthatafinalreferencewillalwaysreferto
thesameobject.However,thekeywordfinalhasnobearingonwhetherthestate
oftheobjectdenotedbythereferencecanbechanged.
•Aftertheconstructorexits,thefinalfieldsofaobjectareallguaranteedtobe
initialized.Thecompilerensuresthattheclassprovidestheappropriatecodeto
initializethefinalfields.
Afinalvariablemustbeexplicitlyinitializedonlyoncewithaninitializerexpression,
eitherinitsdeclarationorinaninitializerblock(§9.7,p.399).Afinalinstancevariable
canalsobeinitializedinaconstructor.
Notethatafinallocalvariableneednotbeinitializedinitsdeclaration,butitmustbe
initializedinthecodeoncebeforeitisused.Thesevariablesarealsoknownasblankfinal
variables.Foradiscussionoffinalparameters,see§3.5,p.80.
Afinalmethodinaclassisaconcretemethod(thatis,hasanimplementation)and
cannotbeoverriddeninanysubclass(§7.2,p.268).
Variablesdeclaredasfinalensurethatvaluescannotbechangedandmethodsdeclared
asfinalensurethatbehaviorcannotbechanged.Classesdeclaredasfinalare
discussedin§4.6,p.122.
Thecompilermaybeabletoperformcodeoptimizationsforfinalmembers,because
certainassumptionscanbemadeaboutsuchmembers.
Staticfinalvariablesarecommonlyusedtodefinemanifestconstants(alsocallednamed
constants)—forexample,Integer.MAX_VALUE,whichisthemaximumintvalue.
Variablesdefinedinaninterfaceareimplicitlyfinal(§7.6,p.290).
InExample4.12,theclassLightdefinestwopublicstaticfinalvariablesat
(1)and(2).ThepublicstaticfinalvariableKWH_PRICEisinitializedinthe
declarationat(1),andthepublicstaticfinalvariableMANUFACTURERis
initializedinthestaticinitializerblockat(3).Anattempttochangethevalueofthe
publicstaticfinalvariableKWH_PRICEat(9)resultsinacompile-timeerror.
TheclassLightalsodefinestwofinalinstancevariablesat(4)and(5).Thefinal
instancevariablecolorisinitializedintheinstanceinitializerblockat(6),andthe
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finalinstancevariableenergyRatingisinitializedintheconstructorat(7).
TheclassLightalsodefinesafinalmethodat(8).ThesubclassTubeLight
attemptstooverridethefinalmethodsetWatts()fromthesuperclassLightat
(10),whichisnotpermitted.
TheclassWarehousealsodefinesafinallocalreferenceworkLightat(11).The
stateoftheobjectdenotedbythereferenceworkLightischangedat(12),butits
referencevaluecannotbechangedasattemptedat(13).Anotherfinallocalreference
alarmLightisdeclaredat(14),butitisnotinitialized.Thecompilerreportsanerror
whenanattemptismadetousethisreferenceat(15).
Example4.12UsingfinalModifier
Clickheretoviewcodeimage
classLight{
//Staticfinalvariables
publicstaticfinaldoubleKWH_PRICE=3.25;//(1)
publicstaticfinalStringMANUFACTURER;//(2)
static{//Staticinitializerblock
MANUFACTURER=“Ozam”;//(3)Initializes(2)
}
//Instancevariables
intnoOfWatts;
finalStringcolor;//(4)
finalStringenergyRating;//(5)
{//Instanceinitializer
block
color=“offwhite”;//(6)Initializes(4)
}
//Constructor
Light(){
energyRating=“A++”;//(7)Initializes(5)
}
//Finalinstancemethod(8)
finalpublicvoidsetWatts(intwatt){
noOfWatts=watt;
}
publicvoidsetKWH(){
//KWH_PRICE=4.10;//(9)NotOK.Cannotbechanged.
}
}
//______________________________________________________________________________
classTubeLightextendsLight{
//Finalmethodinsuperclasscannotbeoverridden.
//Thismethodwillnotcompile.
/*
@Override
publicvoidsetWatts(intwatt){//(10)Attempttooverride.
noOfWatts=2*watt;
}
*/
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}
//______________________________________________________________________________
publicclassWarehouse{
publicstaticvoidmain(String[]args){
finalLightworkLight=newLight();//(11)Finallocalvariable.
workLight.setWatts(100);//(12)OK.Changingobjectstate.
//workLight=newLight();//(13)NotOK.Changingfinal
reference.
finalLightalarmLight;//(14)Notinitialized.
//alarmLight.setWatts(200);//(15)NotOK.
System.out.println(“KWH_PRICE:”+Light.KWH_PRICE);
System.out.println(“MANUFACTURER:”+Light.MANUFACTURER);
System.out.println(“noOfWatts:”+workLight.noOfWatts);
System.out.println(“color:”+workLight.color);
System.out.println(“energyRating:”+workLight.energyRating);
}
}
Outputfromtheprogram:
KWH_PRICE:3.25
MANUFACTURER:Ozam
noOfWatts:100
color:offwhite
energyRating:A++
Methods
Anabstractmethodinanabstractclasshasthefollowingsyntax:
Clickheretoviewcodeimage
accessibility_modifierabstractreturn_typemethod_name
(formal_parameter_list)
throws_clause;
Anabstractmethoddoesnothaveanimplementation;thatis,nomethodbodyisdefined
foranabstractmethod,andonlythemethodheaderisprovidedintheclassdeclaration.
Thekeywordabstractismandatoryintheheaderofanabstractmethoddeclaredina
class.Itsclassisthenincompleteandmustbeexplicitlydeclaredasabstract(§4.6,p.
120).Subclassesofanabstractclassmustthenprovidethemethodimplementation;
otherwise,theymustalsobedeclaredasabstract.Theaccessibilityofanabstract
methoddeclaredinatop-levelclasscannotbeprivate,assubclasseswouldnotbeable
tooverridethemethodandprovideanimplementation.See§4.6,whereExample4.9also
illustratestheuseofabstractmethods.
Onlyaninstancemethodcanbedeclaredasabstract.Sincestaticmethodscannotbe
overridden,declaringanabstractstaticmethodmakesnosense,andthecompilerwill
reportanerror.Afinalmethodcannotbeabstract(i.e.,cannotbeincomplete),and
viceversa.Thekeywordabstractcanbespecifiedonlyincombinationwiththe
publicorprotectedaccessibilitymodifier.
Abstractmethodsspecifiedinatop-levelinterfaceareimplicitlyabstract,andthe
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keywordabstractisseldomspecifiedintheirmethodheaders.Thesemethodscan
haveonlypublicaccessibility.See§7.6,p.291,foradiscussionofabstractmethodsin
top-levelinterfaces.
Methods
Athreadisanindependentpathofexecutioninaprogram.Severalthreadscanbe
executinginaprogram.Theymighttrytoexecuteseveralmethodsonthesameobject
simultaneously.Methodscanbedeclaredassynchronizedifitisdesiredthatonlyone
threadatatimebeabletoexecuteamethodoftheobject.Theirexecutionisthenmutually
exclusiveamongallthreads.Atanygiventime,atmostonethreadcanbeexecutinga
synchronizedmethodonanobject.Thisdiscussionalsoappliestostatic
synchronizedmethodsofaclass.
InExample4.13,boththepush()method,declaredat(1),andthepop()method,
declaredat(2),aresynchronizedintheclassStackImpl.Onlyonethreadatatime
canexecuteasynchronizedmethodinanobjectoftheclassStackImpl.
Consequently,itisnotpossibleforthestateofanobjectoftheclassStackImpltobe
corrupted,forexample,whileonethreadispushinganelementandanotherisattempting
topopthestack.
Example4.13SynchronizedMethods
Clickheretoviewcodeimage
classStackImpl{//Non-genericpartialimplementation
privateObject[]stackArray;
privateinttopOfStack;
//…
synchronizedpublicvoidpush(Objectelem){//(1)
stackArray[++topOfStack]=elem;
}
synchronizedpublicObjectpop(){//(2)
Objectobj=stackArray[topOfStack];
stackArray[topOfStack]=null;
topOfStack—;
returnobj;
}
//Othermethods,etc.
publicObjectpeek(){returnstackArray[topOfStack];}
}
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Methods
NativemethodsaremethodswhoseimplementationisnotdefinedinJavabutratherin
anotherprogramminglanguage,suchasCorC++.Suchamethodcanbedeclaredasa
memberinaJavaclassdeclaration.Sinceitsimplementationappearselsewhere,onlythe
methodheaderisspecifiedintheclassdeclaration.Thekeywordnativeismandatoryin
themethodheader.Anativemethodcanalsospecifycheckedexceptionsinathrows
clause(§6.9,p.251),butthecompilercannotcheckthem,sincethemethodisnot
implementedinJava.
TheJavaNativeInterface(JNI)isaspecialAPIthatallowsJavamethodstoinvokenative
functionsimplementedinC.
Inthefollowingexample,anativemethodintheclassNativeisdeclaredat(2).The
classalsousesastaticinitializerblocktoloadthenativelibrarywhentheclassisloaded.
ClientsoftheNativeclasscancallthenativemethodlikeanyanothermethod,asat
(3).
Clickheretoviewcodeimage
classNative{
/*
*Thestaticblockensuresthatthenativemethodlibrary
*isloadedbeforethenativemethodiscalled.
*/
static{
System.loadLibrary(“NativeMethodLib”);//(1)Loadnativelibrary.
}
nativevoidnativeMethod();//(2)Nativemethodheader.
//…
}
classClient{
//…
publicstaticvoidmain(String[]args){
NativetrueNative=newNative();
trueNative.nativeMethod();//(3)Nativemethodcall.
}
//…
}
Fields
Oftenitisdesirabletosavethestateofanobject—forexample,onafile.Suchobjectsare
saidtobepersistent.InJava,thestateofanobjectcanbestoredusingserialization.
Serializationtransformsobjectsintoanoutputformatthatisconduciveforstoringobjects.
Objectscanlaterberetrievedinthesamestateaswhentheywereserialized,meaningthat
allfieldsincludedintheserializationwillhavethesamevaluesasatthetimeof
serialization.
Sometimesthevalueofafieldinanobjectshouldnotbesaved,inwhichcasethefield
canbespecifiedastransientintheclassdeclaration.Thisdesignationimpliesthatits
valueshouldnotbesavedwhenobjectsoftheclassarewrittentopersistentstorage.Inthe
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followingexample,thefieldcurrentTemperatureisdeclaredastransientat(1),
becausethecurrenttemperatureismostlikelytohavechangedwhentheobjectisrestored
atalaterdate.However,thevalueofthefieldmass,declaredat(2),islikelytoremain
unchanged.WhenobjectsoftheclassExperimentareserialized,thevalueofthefield
currentTemperaturewillnotbesaved,butthatofthefieldmasswillbe,aspartof
thestateoftheserializedobject.
Clickheretoviewcodeimage
classExperimentimplementsSerializable{
//…
//ThevalueofcurrentTemperaturewillnotpersist.
transientintcurrentTemperature;//(1)Transientvalue.
doublemass;//(2)Persistentvalue.
}
Specifyingthetransientmodifierforstaticvariablesisredundantand,therefore,
discouraged.Staticvariablesarenotpartofthepersistentstateofaserializedobject.
Fields
Duringexecution,compiledcodemightcachethevaluesoffieldsforefficiencyreasons.
Sincemultiplethreadscanaccessthesamefield,itisvitalthatcachingisnotallowedto
causeinconsistencieswhenreadingandwritingthevalueinthefield.Thevolatile
modifiercanbeusedtoinformthecompilerthatitshouldnotattempttoperform
optimizationsonthefield,whichcouldcauseunpredictableresultswhenthefieldis
accessedbymultiplethreads.
Inthesimpleexamplethatfollows,thevalueofthefieldclockReadingmightbe
changedunexpectedlybyanotherthreadwhileonethreadisperformingataskthat
involvesalwaysusingthecurrentvalueofthefieldclockReading.Declaringthefield
asvolatileensuresthatawriteoperationwillalwaysbeperformedonthemasterfield
variable,andareadoperationwillalwaysreturnthecorrectcurrentvalue.
Clickheretoviewcodeimage
classVitalControl{
//…
volatilelongclockReading;
//Twosuccessivereadsmightgivedifferentresults.
}
Table4.5providesasummaryofnon-accessibilitymodifiersformembers.
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Table4.5SummaryofNon-AccessibilityModifiersforMembers
ReviewQuestions
4.15Whichstatementsabouttheuseofmodifiersaretrue?
Selectthetwocorrectanswers.
(a)Ifnoaccessibilitymodifier(public,protected,orprivate)isspecified
foramemberdeclaration,thememberisaccessibleonlybyclassesinthe
packageofitsclassandbysubclassesofitsclassinanypackage.
(b)Youcannotspecifyaccessibilityoflocalvariables.Theyareaccessibleonly
withintheblockinwhichtheyaredeclared.
(c)Subclassesofaclassmustresideinthesamepackageastheclasstheyextend.
(d)Localvariablescanbedeclaredasstatic.
(e)Theobjectsthemselvesdonothaveanyaccessibilitymodifiers;onlyfield
referencesdo.
4.16Giventhefollowingsourcecode,whichcommentlinecanbeuncommented
withoutintroducingerrors?
Clickheretoviewcodeimage
abstractclassMyClass{
abstractvoidf();
finalvoidg(){}
//finalvoidh(){}//(1)
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protectedstaticinti;
privateintj;
}
finalclassMyOtherClassextendsMyClass{
//MyOtherClass(intn){m=n;}//(2)
publicstaticvoidmain(String[]args){
MyClassmc=newMyOtherClass();
}
voidf(){}
voidh(){}
//voidk(){i++;}//(3)
//voidl(){j++;}//(4)
intm;
}
Selecttheonecorrectanswer.
(a)(1)
(b)(2)
(c)(3)
(d)(4)
4.17Whichstatementistrue?
Selecttheonecorrectanswer.
(a)Astaticmethodcancallothernon-staticmethodsinthesameclassbyusingthe
thiskeyword.
(b)Aclassmaycontainbothstaticandnon-staticvariables,andbothstaticand
non-staticmethods.
(c)Eachobjectofaclasshasitsowninstanceofthestaticvariablesdeclaredinthe
class.
(d)Instancemethodsmayaccesslocalvariablesofstaticmethods.
(e)Allmethodsinaclassareimplicitlypassedthethisreferenceasanargument,
wheninvoked.
4.18Whichoneoftheseisnotalegalmemberdeclarationwithinaclass?
Selecttheonecorrectanswer.
(a)staticinta;
(b)finalObject[]fudge={null};
(c)abstractintt;
(d)nativevoidsneeze();
(e)staticfinalprivatedoublePI=
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3.14159265358979323846;
4.19Whichstatementsaboutmodifiersaretrue?
Selectthetwocorrectanswers.
(a)Abstractclassescandeclarefinalmethods.
(b)Fieldscanbedeclaredasnative.
(c)Non-abstractmethodscanbedeclaredinabstractclasses.
(d)Classescanbedeclaredasnative.
(e)Abstractclassescanbedeclaredasfinal.
4.20Whichstatementistrue?
Selecttheonecorrectanswer.
(a)Thevaluesoftransientfieldswillnotbesavedduringserialization.
(b)Constructorscanbedeclaredasabstract.
(c)Theinitialstateofanarrayobjectconstructedwiththestatementint[]a=
newint[10]willdependonwhetherthearrayvariableaisalocalvariable
orafield.
(d)Asubclassofaclasswithanabstractmethodmustprovidean
implementationfortheabstractmethod.
(e)Onlystaticmethodscanaccessstaticmembers.
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•ThestructureofaJavasourcefile
•Defining,using,anddeployingpackages
•Classscopeformembers,andblockscopeforlocalvariables
•Accessibility(default,public)andothermodifiers(abstract,final)for
referencetypes
•Applicabilityofmemberaccessibility(default,public,protected,private)
andothermembermodifiers(static,final,abstract,synchronized,
native,transient,volatile)
ProgrammingExercise
4.1Designaclassforabankdatabase.Thedatabaseshouldsupportthefollowing
operations:
Depositacertainamountintoanaccount
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Withdrawacertainamountfromanaccount
Getthebalance(i.e.,thecurrentamount)inanaccount
Transferanamountfromoneaccounttoanother
Theamountinthetransactionsisavalueoftypedouble.Theaccountsare
identifiedbyinstancesoftheclassAccountthatisinthe
packagecom.megabankcorp.records.Thedatabaseclassshouldbeplaced
inapackagecalledcom.megabankcorp.system.
Thedeposit,withdraw,andbalanceoperationsshouldnothaveanyimplementation,
butshouldallowsubclassestoprovidetheimplementation.Thetransferoperation
shouldusethedepositandwithdrawoperationstoimplementthetransfer.Itshould
notbepossibletoalterthisoperationinanysubclass,andonlyclasseswithinthe
packagecom.megabankcorp.systemshouldbeallowedtousethisoperation.
Thedepositandwithdrawoperationsshouldbeaccessibleinallpackages.The
balanceoperationshouldbeaccessibleonlyinsubclassesandclasseswithinthe
packagecom.megabankcorp.system.
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5.OperatorsandExpressions
5.1Conversions
Inthissectionwefirstdiscussthedifferentkindsoftypeconversionsthatcanbeapplied
tovalues;inthenextsectionwediscussthecontextsinwhichtheseconversionsare
permitted.Sometypeconversionsmustbeexplicitlystatedintheprogram,whileothers
areperformedimplicitly.Sometypeconversionscanbecheckedatcompiletimeto
guaranteetheirvalidityatruntime,whileotherswillrequireanextracheckatruntime.
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WideningandNarrowingPrimitiveConversions
Fortheprimitivedatatypes,thevalueofanarrowerdatatypecanbeconvertedtoavalue
ofawiderdatatype.Thisiscalledawideningprimitiveconversion.Wideningconversions
fromoneprimitivetypetothenextwiderprimitivetypearesummarizedinFigure5.1.
Theconversionsshownaretransitive.Forexample,anintcanbedirectlyconvertedtoa
doublewithoutfirsthavingtoconvertittoalongandafloat.
Figure5.1WideningPrimitiveConversions
Notethatthetargettypeofawideningprimitiveconversionhasawiderrangeofvalues
thanthesourcetype—forexample,therangeofthelongtypesubsumestherangeofthe
inttype.Inwideningconversionsbetweenintegraltypes,thesourcevalueremains
intact,withnolossofmagnitudeinformation.However,awideningconversionfroman
intoralongvaluetoafloatvalue,orfromalongvaluetoadoublevalue,may
resultinalossofprecision.Thefloating-pointvalueinthetargettypeisthenacorrectly
roundedapproximationoftheintegervalue.Notethatprecisionrelatestothenumberof
significantbitsinthevalue,andmustnotbeconfusedwithmagnitude,whichrelateshow
bigavaluecanberepresented.
Convertingfromawiderprimitivetypetoanarrowerprimitivetypeiscalledanarrowing
primitiveconversion;itcanresultinlossofmagnitudeinformation,andpossiblyinaloss
ofprecisionaswell.Anyconversionthatisnotawideningprimitiveconversionaccording
toFigure5.1isanarrowingprimitiveconversion.Thetargettypeofanarrowingprimitive
conversionhasanarrowerrangeofvaluesthanthesourcetype—forexample,therange
oftheinttypedoesnotincludeallthevaluesintherangeofthelongtype.
Notethatallconversionsbetweencharandthetwointegertypesbyteandshortare
considerednarrowingprimitiveconversions.Thereasonisthattheconversionsbetween
theunsignedtypecharandthesignedtypesbyteorshortcanresultinlossof
information.Thesenarrowingconversionsaredoneintwosteps:firstconvertingthe
sourcevaluetotheinttype,andthenconvertingtheintvaluetothetargettype.
Wideningprimitiveconversionsareusuallydoneimplicitly,whereasnarrowingprimitive
conversionsusuallyrequireacast(§5.2,p.148).Itisnotillegaltouseacastfora
wideningconversion.However,thecompilerwillflaganyconversionthatrequiresacast
ifnonehasbeenspecified.Regardlessofanylossofmagnitudeorprecision,wideningand
narrowingprimitiveconversionsneverresultinaruntimeexception.
Ampleexamplesofwideningandnarrowingprimitiveconversionscanbefoundinthis
chapter.
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WideningandNarrowingReferenceConversions
Thesubtype–supertyperelationshipbetweenreferencetypesdetermineswhich
conversionsarepermissiblebetweenthem.Conversionsupthetypehierarchyarecalled
wideningreferenceconversions(alsocalledupcasting).Suchaconversionconvertsfroma
subtypetoasupertype:
Clickheretoviewcodeimage
Objectobj=“Upcastme”;//Widening:Object<–—String
Conversionsdownthetypehierarchyrepresentnarrowingreferenceconversions(also
calleddowncasting):
Clickheretoviewcodeimage
Stringstr=(String)obj;//Narrowingrequirescast:String<–—Object
Asubtypeisanarrowertypethanitssupertypeinthesensethatitisaspecializationofits
supertype.Contextsunderwhichreferenceconversionscanoccurarediscussedin§7.8,p.
311.
Wideningreferenceconversionsareusuallydoneimplicitly,whereasnarrowingreference
conversionsusuallyrequireacast,asillustratedintheseconddeclarationstatementinthis
subsection.Thecompilerwillrejectcaststhatarenotlegalorissueanuncheckedwarning
undercertaincircumstancesiftypesafetycannotbeguaranteed.
Wideningreferenceconversionsdonotrequireanyruntimechecksandneverresultinan
exceptionduringexecution.Thisisnotthecasefornarrowingreferenceconversions,
whichrequirearuntimecheckandcanthrowaClassCastExceptionifthe
conversionisnotlegal.
BoxingandUnboxingConversions
Boxingandunboxingconversionsallowinteroperabilitybetweenprimitivevaluesand
theirrepresentationasobjectsofthewrappertypes(§8.3,p.346).
Aboxingconversionconvertsthevalueofaprimitivetypetoacorrespondingvalueofits
wrappertype.IfpisavalueofaprimitiveType,boxingconversionconvertspintoa
referencerofthecorrespondingWrapperType,suchthatr.primitiveTypeValue()==
p.Inthecodethatfollows,theintvalue10resultsinanobjectofthetypeInteger
implicitlybeingcreated;thisobjectcontainstheintvalue10.Wesaythattheintvalue
10hasbeenboxedinanobjectofthewrappertypeInteger.Theterminology
autoboxedisalsousedforthisconversion.
Clickheretoviewcodeimage
IntegeriRef=10;//Boxing:Integer<–—int
System.out.println(iRef.intValue()==10);//true
Anunboxingconversionconvertsthevalueofawrappertypetoavalueofits
correspondingprimitivetype.IfrisareferenceofaWrapperType,anunboxing
conversionconvertsthereferencerintor.primitiveTypeValue(),whereprimitiveType
istheprimitivetypecorrespondingtotheWrapperType.Inthenextcodesnippet,thevalue
intheIntegerobjectreferencedbyiRefisimplicitlyconvertedtotheinttype.We
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saythatthewrapperobjecthasbeenunboxedtoitscorrespondingprimitivetype.
Clickheretoviewcodeimage
inti=iRef;//Unboxing:int<–—Integer
System.out.println(iRef.intValue()==i);//true
Notethatbothboxingandunboxingaredoneimplicitlyintherightcontext.Boxing
allowsprimitivevaluestobeusedwhereanobjectoftheirwrappertypeisexpected,and
unboxingallowstheconverse.UnboxingmakesitpossibletouseaBooleanwrapper
objectasabooleanvalueinabooleanexpression,andtouseanintegralwrapperobject
asanintegralprimitivevalueinanarithmeticexpression.Unboxingawrapperreference
thathasthenullvalueresultsinaNullPointerException.Ampleexamplesof
boxingandunboxingcanbefoundinthischapterandin§7.8,p.311.
OtherConversions
Webrieflymentionsomeotherconversions,andidentifywheretheyarecoveredinthis
book.
•Identityconversionsarealwayspermitted,astheyallowconversionsfromatypeto
thatsametype.Anidentityconversionisalwayspermitted.
Clickheretoviewcodeimage
inti=(int)10;//int<–-int
Stringstr=(String)“Hi”;//String<–-String
•StringconversionsallowavalueofanyothertypetobeconvertedtoaString
typeinthecontextofthestringconcatenationoperator+(§5.8,p.174).
•Uncheckedconversionsarepermittedtofacilitateoperabilitybetweenlegacyand
genericcode(§10.1,p.416).
5.2TypeConversionContexts
Selectedconversioncontextsandtheconversionsthatareapplicableinthesecontextsare
summarizedinTable5.1.Theconversionsshownineachcontextoccurimplicitly,without
theprogramhavingtotakeanyspecialaction.Forotherconversioncontexts,see§5.1,p.
146.
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Table5.1SelectedConversionContextsandConversionCategories
AssignmentContext
Assignmentconversionsthatcanoccurinanassignmentcontextareshowninthesecond
columnofTable5.1.Anassignmentconversionconvertsthetypeofanexpressiontothe
typeofatargetvariable.
Anexpression(oritsvalue)isassignabletothetargetvariable,ifthetypeofthe
expressioncanbeconvertedtothetypeofthetargetvariablebyanassignment
conversion.Equivalently,thetypeoftheexpressionisassignmentcompatiblewiththe
typeofthetargetvariable.
Forassignmentconversioninvolvingprimitivedatatypes,see§5.6,p.158.Notethe
specialcasewhereanarrowingconversionoccurswhenassigninganon-longinteger
constantexpression:
Clickheretoviewcodeimage
byteb=10;//Narrowingconversion:byte<–int
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Forassignmentconversionsinvolvingreferencetypes,see§7.8,p.311.
MethodInvocationContext
Methodinvocationconversionsthatcanoccurinamethodinvocationcontextareshown
inthethirdcolumnofTable5.1.Notethatmethodinvocationandassignmentconversions
differinonerespect:Methodinvocationconversionsdonotincludetheimplicitnarrowing
conversionperformedfornon-longintegralconstantexpressions.
Clickheretoviewcodeimage
//Assignment:(1)Implicitnarrowingfollowedby(2)boxing.
Characterspace1=32;//Character<-(2)—char<-(1)—int
//Invocationofmethodwithsignature:valueOf(char)
Characterspace2=Character.valueOf(32);//Compile-timeerror!
//Callsignature:
valueOf(int)
Characterspace3=Character.valueOf((char)32);//OK!
//Callsignature:
valueOf(char)
Amethodinvocationconversioninvolvesconvertingeachargumentvalueinamethodor
constructorcalltothetypeofthecorrespondingformalparameterinthemethodor
constructordeclaration.
Methodinvocationconversionsinvolvingparametersofprimitivedatatypesarediscussed
in§3.5,p.73,andthoseinvolvingreferencetypesarediscussedin§7.8,p.311.
CastingContextoftheUnaryTypeCastOperator:(type)
Java,beingastronglytypedlanguage,checksfortypecompatibility(i.e.,itchecksifa
typecansubstituteforanothertypeinagivencontext)atcompiletime.However,some
checksarepossibleonlyatruntime(e.g.,whichtypeofobjectareferenceactuallydenotes
duringexecution).Incaseswhereanoperatorwouldhaveincompatibleoperands(e.g.,
assigningadoubletoanint),Javademandsthatatypecastbeusedtoexplicitly
indicatethetypeconversion.Thetypecastconstructhasthefollowingsyntax:
(type)expression
Thecastoperator(type)isappliedtothevalueoftheexpression.Atruntime,acast
resultsinanewvalueoftype,whichbestrepresentsthevalueoftheexpressionintheold
type.Weusethetermcastingtomeanapplyingthecastoperatorforexplicittype
conversion.
However,inthecontextofcasting,implicitcastingconversionscantakeplace.These
castingconversionsareshowninthefourthcolumnofTable5.1.Castingconversions
includemoreconversioncategoriesthantheassignmentorthemethodinvocation
conversions.Inthecodethatfollows,thecommentsindicatethecategoryofthe
conversionthattakesplacebecauseofthecastoperatorontheright-handsideofeach
assignment—althoughcastsareonlynecessaryforthesakeoftheassignmentat(1)and
(2).
Clickheretoviewcodeimage
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longl=(long)10;//Wideningprimitiveconversion:long<–int
inti=(int)l;//(1)Narrowingprimitiveconversion:int<–long
Objectobj=(Object)“7Up”;//Wideningrefconversion:Object<–String
Stringstr=(String)obj;//(2)Narrowingrefconversion:String<–
Object
IntegeriRef=(Integer)i;//Boxing:Integer<–int
i=(int)iRef;//Unboxing:int<–Integer
Acastingconversionisappliedtothevalueoftheoperandexpressionofacastoperator.
Castingcanbeappliedtoprimitivevaluesaswellasreferences.Castingbetween
primitivedatatypesandreferencetypesisnotpermitted,exceptwhereboxingand
unboxingisapplicable.Booleanvaluescannotbecasttootherdatavalues,andviceversa.
Thereferenceliteralnullcanbecasttoanyreferencetype.
Examplesofcastingbetweenprimitivedatatypesareprovidedinthischapter.Casting
referencevaluesisdiscussedin§7.11,p.320.
NumericPromotionContext
Numericoperatorsallowonlyoperandsofcertaintypes.Numericpromotionresultsin
conversionsbeingappliedtotheoperandstoconvertthemtopermissibletypes.Numeric
promotionconversionsthatcanoccurinanumericpromotioncontextareshowninthe
fifthcolumnofTable5.1.Permissibleconversioncategoriesarewideningprimitive
conversionsandunboxingconversions.Adistinctionismadebetweenunaryandbinary
numericpromotion.
UnaryNumericPromotion
Unarynumericpromotionproceedsasfollows:
•IfthesingleoperandisoftypeByte,Short,Character,orInteger,itis
unboxed.Iftheresultingvalueisnarrowerthanint,itispromotedtoavalueof
typeintbyawideningconversion.
•Otherwise,ifthesingleoperandisoftypeLong,Float,orDouble,itisunboxed.
•Otherwise,ifthesingleoperandisofatypenarrowerthanint,itsvalueis
promotedtoavalueoftypeintbyawideningconversion.
•Otherwise,theoperandremainsunchanged.
Inotherwords,unarynumericpromotionresultsinanoperandvaluethatiseitherintor
wider.
Unarynumericpromotionisappliedinthefollowingexpressions:
•Operandoftheunaryarithmeticoperators+and-(§5.7,p.163)
•Arraycreationexpression;forexample,newint[20],wherethedimension
expression(inthiscase20)mustevaluatetoanintvalue(§3.4,p.59)
•Indexingarrayelements;forexample,objArray['a'],wheretheindex
expression(inthiscase'a')mustevaluatetoanintvalue(§3.4,p.61)
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BinaryNumericPromotion
Binarynumericpromotionimplicitlyappliesappropriatewideningprimitiveconversions
sothatapairofoperandshavethewidestnumerictypeofthetwo,whichisalwaysatleast
int.IfTisthewidestnumerictypeofthetwooperandsafteranyunboxingconversions
havebeenperformed,theoperandsarepromotedasfollowsduringbinarynumeric
promotion:
IfTiswiderthanint,bothoperandsareconvertedtoT;otherwise,bothoperandsare
convertedtoint.
Thismeansthattheresultingtypeoftheoperandsisatleastint.
Binarynumericpromotionisappliedinthefollowingexpressions:
•Operandsofthearithmeticoperators*,/,%,+,and-(§5.7,p.163)
•Operandsoftherelationaloperators<,<=,>,and>=(§5.11,p.180)
•Operandsofthenumericalequalityoperators==and!=(§5.12,p.181)
•Operandsoftheconditionaloperator?:,undercertaincircumstances(§5.16,p.
194)
5.3PrecedenceandAssociativityRulesforOperators
Precedenceandassociativityrulesarenecessaryfordeterministicevaluationof
expressions.TheoperatorsaresummarizedinTable5.2.Themajorityofthemare
discussedinsubsequentsectionsinthischapter.Seealsotheindexentriesforthese
operators.
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Table5.2OperatorSummary
ThefollowingremarksapplytoTable5.2:
•Theoperatorsareshownwithdecreasingprecedencefromthetopofthetable.
•Operatorswithinthesamerowhavethesameprecedence.
•Parentheses,(),canbeusedtooverrideprecedenceandassociativity.
•Theunaryoperators,whichrequireoneoperand,includethefollowing:thepostfix
increment(++)anddecrement(--)operatorsfromthefirstrow,alltheprefix
operators(+,-,++,--,~,!)inthesecondrow,andtheprefixoperators(object
creationoperatornew,castoperator(type))inthethirdrow.
•Theconditionaloperator(?:)isternary—thatis,itrequiresthreeoperands.
•Alloperatorsnotidentifiedpreviouslyasunaryorternaryarebinary—thatis,they
requiretwooperands.
•Allbinaryoperators,exceptfortherelationalandassignmentoperators,associate
fromlefttoright.Therelationaloperatorsarenonassociative.
•Exceptforunarypostfixincrementanddecrementoperators,allunaryoperators,all
assignmentoperators,andtheternaryconditionaloperatorassociatefromrightto
left.
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Dependingonthecontext,brackets([]),parentheses(()),colon(:)andthedotoperator
(.)canalsobeinterpretedasseparators(§2.1,p.29).Seetheindexentriesforthese
separatorsformoredetails.
Precedencerulesareusedtodeterminewhichoperatorshouldbeappliedfirstifthereare
twooperatorswithdifferentprecedence,andtheseoperatorsfolloweachotherinthe
expression.Insuchacase,theoperatorwiththehighestprecedenceisappliedfirst.
Theexpression2+3*4isevaluatedas2+(3*4)(withtheresult14)since*
hashigherprecedencethan+.
Associativityrulesareusedtodeterminewhichoperatorshouldbeappliedfirstifthereare
twooperatorswiththesameprecedence,andtheseoperatorsfolloweachotherinthe
expression.
Leftassociativityimpliesgroupingfromlefttoright:Theexpression7-4+2is
interpretedas((7-4)+2),sincethebinaryoperators+and-bothhavesame
precedenceandleftassociativity.
Rightassociativityimpliesgroupingfromrighttoleft:Theexpression--4is
interpretedas(-(-4))(withtheresult4),sincetheunaryoperator-hasright
associativity.
Theprecedenceandassociativityrulestogetherdeterminetheevaluationorderofthe
operators.
5.4EvaluationOrderofOperands
Tounderstandtheresultreturnedbyanoperator,itisimportanttounderstandthe
evaluationorderofitsoperands.Ingeneral,theoperandsofoperatorsareevaluatedfrom
lefttoright.Theevaluationorderalsorespectsanyparentheses,andtheprecedenceand
associativityrulesofoperators.
Examplesillustratinghowtheoperandevaluationorderinfluencestheresultreturnedby
anoperator,canbefoundin§5.6and§5.9.
Left-HandOperandEvaluationFirst
Theleft-handoperandofabinaryoperatorisfullyevaluatedbeforetheright-handoperand
isevaluated.
Theevaluationoftheleft-handoperandcanhavesideeffectsthatcaninfluencethevalue
oftheright-handoperand.Forexample,inthecode
Clickheretoviewcodeimage
intb=10;
System.out.println((b=3)+b);
thevalueprintedwillbe6andnot13.Theevaluationproceedsasfollows:
Clickheretoviewcodeimage
(b=3)+b
3+bbisassignedthevalue3
3+3
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6
Ifevaluationoftheleft-handoperandofabinaryoperatorthrowsanexception(§6.5,p.
230),wecannotrelyonthepresumptionthattheright-handoperandhasbeenevaluated.
OperandEvaluationbeforeOperationExecution
Javaguaranteesthatalloperandsofanoperatorarefullyevaluatedbeforetheactual
operationisperformed.Thisruledoesnotapplytotheshort-circuitconditionaloperators
&&,||,and?:.
Thisrulealsoappliestooperatorsthatthrowanexception(theintegerdivisionoperator/
andtheintegerremainderoperator%).Theoperationisperformedonlyiftheoperands
evaluatenormally.Anysideeffectsoftheright-handoperandwillhavebeeneffectuated
beforetheoperatorthrowsanexception.
Example5.1illustratestheevaluationorderoftheoperandsandprecedencerulesfor
arithmeticexpressions.Weusetheeval()methodat(3)inExample5.1todemonstrate
integerexpressionevaluation.Thefirstargumenttothismethodistheoperandvaluethat
isreturnedbythemethod,andthesecondargumentisastringtoidentifytheevaluation
order.
Theargumenttotheprintln()methodinthestatementat(1)isanintegerexpression
toevaluate2+3*4.Theevaluationofeachoperandintheexpressionat(1)resultsina
calloftheeval()methoddeclaredat(3).
Clickheretoviewcodeimage
out.println(eval(j++,”+“)+eval(j++,”*“)*eval(j,“\n”));//(1)
TheoutputfromExample5.1showsthattheoperandswereevaluatedfirst,fromleftto
right,beforeoperatorexecution,andthattheexpressionwasevaluatedas(2+(3*
4)),respectingtheprecedencerulesforarithmeticexpressionevaluation.Notehowthe
valueofvariablejchangessuccessivelyfromlefttorightasthefirsttwooperandsare
evaluated.
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Example5.1EvaluationOrderofOperandsandArguments
Clickheretoviewcodeimage
importstaticjava.lang.System.out;
publicclassEvalOrder{
publicstaticvoidmain(String[]args){
intj=2;
out.println(“Evaluationorderofoperands:”);
out.println(eval(j++,”+“)+eval(j++,”*“)*eval(j,“\n”));//
(1)
inti=1;
out.println(“Evaluationorderofarguments:”);
add3(eval(i++,“,“),eval(i++,“,“),eval(i,“\n”));//(2)Three
arguments.
}
publicstaticinteval(intoperand,Stringstr){//(3)
out.print(operand+str);//PrintintoperandandStringstr.
returnoperand;//Returnintoperand.
}
publicstaticvoidadd3(intoperand1,intoperand2,intoperand3){//
(4)
out.print(operand1+operand2+operand3);
}
}
Outputfromtheprogram:
Evaluationorderofoperands:
2+3*4
14
Evaluationorderofarguments:
1,2,3
6
Left-to-RightEvaluationofArgumentLists
Inamethodorconstructorinvocation,eachargumentexpressionintheargumentlistis
fullyevaluatedbeforeanyargumentexpressiontoitsright.
Ifevaluationofanargumentexpressiondoesnotcompletenormally,wecannotpresume
thatanyargumentexpressiontoitsrighthasbeenevaluated.
Wecanusetheadd3()methodat(4)inExample5.1,whichtakesthreearguments,to
demonstratetheorderinwhichtheargumentsinamethodcallareevaluated.Themethod
callat(2)
Clickheretoviewcodeimage
add3(eval(i++,“,“),eval(i++,“,“),eval(i,“\n”));//(2)Three
arguments.
resultsinthefollowingoutput,clearlyindicatingthattheargumentswereevaluatedfrom
lefttoright,beforebeingpassedtothemethod:
1,2,3
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Notehowthevalueofvariableichangessuccessivelyfromlefttorightasthefirsttwo
argumentsareevaluated.
5.5RepresentingIntegers
IntegerdatatypesinJavarepresentsignedintegervalues,meaningbothpositiveand
negativeintegervalues.Thevaluesofchartypecaneffectivelyberegardedasunsigned
16-bitintegers.
ValuesoftypebytearerepresentedasshowninTable5.3.Avalueoftypebyte
requires8bits.With8bits,wecanrepresent28or256values.Javausestwo’s
complement(explainedlater)tostoresignedvaluesofintegerdatatypes.Forthebyte
datatype,thismeansvaluesareintherange–128(i.e.,–27)to+127(i.e.,27–1),inclusive.
Table5.3RepresentingSignedbyteValuesUsingTwo’sComplement
Bitsinanintegralvalueareusuallynumberedfromrighttoleft,startingwiththeleast
significantbit0(alsocalledtherightmostbit).Therepresentationofthesignedtypessets
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themostsignificantbitto1,indicatingnegativevalues.Adding1tothemaximumint
value2147483647resultsintheminimumvalue-2147483648,suchthatthevalues
wraparoundforintegersandnooverfloworunderflowisindicated.
CalculatingTwo’sComplement
Beforewelookatthetwo’scomplement,weneedtounderstandtheone’scomplement.
Theone’scomplementofabinaryintegeriscomputedbyinvertingthebitsinthenumber.
Thus,theone’scomplementofthebinarynumber00101001is11010110.Theone’s
complementofabinarynumberN2isdenotedas~N2.Thefollowingrelationshold
betweenabinaryintegerN2,itsone’scomplement~N2,anditstwo’scomplement–N2:
–N2=~N2+1
0=–N2+N2
IfN2isapositivebinaryinteger,then–N2denotesitsnegativebinaryvalue,andvice
versa.ThesecondrelationstatesthataddingabinaryintegerN2toitstwo’scomplement–
N2equals0.
Givenapositivebytevalue,say41,thebinaryrepresentationof-41canbefoundas
follows:
AddinganumberN2toitstwo’scomplement–N2gives0,andthecarrybitfromthe
additionofthemostsignificantbits(afteranynecessaryextensionoftheoperands)is
ignored:
Subtractionbetweentwointegersisalsocomputedasadditionwithtwo’scomplement:
N2–M2=N2+(–M2)
Forexample,theexpression4110–310(withthecorrectresult3810)iscomputedas
follows:
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Thepreviousdiscussionofbytevaluesappliesequallytovaluesofotherintegertypes:
short,int,andlong.Thesetypeshavetheirvaluesrepresentedbytwo’scomplement
in16,32,and64bits,respectively.
ConvertingBinaryNumberstoDecimals
Abinarynumbercanbeconvertedtoitsequivalentdecimalvaluebycomputingthe
positionalvaluesofitsdigits.Eachdigitinthebinarynumbercontributestothefinal
decimalvaluebyvirtueofitsposition,startingwithposition0(units)fortherightmost
digitinthenumber.Thepositionalvalueofeachdigitisgivenby
digit×baseposition
Thenumber1010012correspondsto4110inthedecimalnumbersystem:
Thesametechniquecanbeusedtoconvertanumberfromanybase,forexample,octal
(base8)orhexadecimal(base16),toitsequivalentrepresentationinthedecimalnumber
system.
ConvertingDecimalstoBinaryNumbers
Toconvertdecimalstobinaries,wereversetheprocessoutlinedpreviouslyforconverting
abinarytoadecimal.
Thedivisorusedinthesestepsisthebaseofthetargetnumbersystem(binary,base2).
Thebinaryvalue,1010012,isrepresentedbytheremainders,withthelastremainderas
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theleftmostbit.
Analogously,wecanapplythisprocedureforconvertinganoctal(base8)orhexadecimal
(base16)numbertoitsbinaryequivalent.
RelationshipsamongBinary,Octal,andHexadecimalNumbers
Weneed3bitstorepresentalltheoctaldigits(8=23)and4bitstorepresentallthe
hexadecimaldigits(16=24).Wecanusethisfacttoconvertamongthebinary,octal,and
hexadecimalsystems,asshowninFigure5.2.
Figure5.2ConvertingamongBinary,Octal,andHexadecimalNumbers
Theprocedureforconvertinganoctaltoabinaryisshownbythearrowmarked(a).We
canconvertanoctalnumbertoitsequivalentbinarynumberbyreplacingeachdigitinthe
octalnumberbyits3-bitequivalentbinaryvalue.
Analogously,wecanconvertahexadecimalnumbertoitsequivalentbinarynumberby
replacingeachdigitinthehexadecimalnumberbyits4-bitequivalentbinaryvalue,as
shownbythearrowmarked(b).
Toconvertabinarytoitsoctalequivalent,wereversetheprocedureoutlinedearlier
(arrowmarked(c)inFigure5.2).Thebitsinthebinarynumberaregroupedinto3-bit
groupsfromrighttoleft.Eachsuchgroupisreplacedbyitsequivalentoctaldigit.
Analogously,wecanconvertabinarytoahexadecimalnumberbyreplacingeach4-bit
groupbyitsequivalenthexadecimaldigit(arrowmarked(d)inFigure5.2).
5.6TheSimpleAssignmentOperator
Theassignmentstatementhasthefollowingsyntax:
variable=expression
whichcanbereadas“thetarget,variable,getsthevalueofthesource,expression.”The
previousvalueofthetargetvariableisoverwrittenbytheassignmentoperator=.
Thetargetvariableandthesourceexpressionmustbeassignmentcompatible.Thetarget
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variablemustalsohavebeendeclared.Sincevariablescanstoreeitherprimitivevaluesor
referencevalues,expressionevaluatestoeitheraprimitivevalueorareferencevalue.
AssigningPrimitiveValues
Thefollowingexamplesillustrateassignmentofprimitivevalues:
Clickheretoviewcodeimage
intj,k;
j=0b10;//jgetsthevalue2.
j=5;//jgetsthevalue5.Previousvalueisoverwritten.
k=j;//kgetsthevalue5.
Theassignmentoperatorhasthelowestprecedence,sothattheexpressionontheright-
handsideisevaluatedbeforetheassignmentisdone.
Clickheretoviewcodeimage
inti;
i=5;//igetsthevalue5.
i=i+1;//igetsthevalue6.+hashigherprecedencethan=.
i=20-i*2;//igetsthevalue8:(20-(i*2))
AssigningReferences
Copyingreferencevaluesbyassignmentcreatesaliases,whicharediscussedin§1.3,p.6.
Thefollowingexamplerecapitulatesthatdiscussion:
Clickheretoviewcodeimage
Pizzapizza1=newPizza(“Hot&Spicy”);
Pizzapizza2=newPizza(“Sweet&Sour”);
pizza2=pizza1;
Thevariablepizza1isareferencetoapizzathatishotandspicy,andpizza2isa
referencetoapizzathatissweetandsour.Assigningpizza1topizza2meansthat
pizza2nowreferstothesamepizzaaspizza1,thehotandspicyone.Afterthe
assignment,thesevariablesarealiasesandeitheronecanbeusedtomanipulatethehot
andspicyPizzaobject.
Assigningareferencevaluedoesnotcreateacopyofthesourceobjectdenotedbythe
referencevariableontheright-handside.Itmerelyassignsthereferencevalueofthe
variableontheright-handsidetothevariableontheleft-handside,sothattheydenotethe
sameobject.Referenceassignmentalsodoesnotcopythestateofthesourceobjecttoany
objectdenotedbythereferencevariableontheleft-handside.
Amoredetaileddiscussionofreferenceassignmentcanbefoundin§7.8,p.311.
MultipleAssignments
Theassignmentstatementisanexpressionstatement,whichmeansthatapplicationofthe
binaryassignmentoperatorreturnsthevalueoftheexpressionontheright-handside.
Clickheretoviewcodeimage
intj,k;
j=10;//jgetsthevalue10,whichisreturned
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k=j;//kgetsthevalueofj,whichis10,andthisvalueis
returned
Thelasttwoassignmentscanbewrittenasmultipleassignments,illustratingtheright
associativityoftheassignmentoperator:
Clickheretoviewcodeimage
k=j=10;//(k=(j=10))
Multipleassignmentsareequallyvalidwithreferences:
Clickheretoviewcodeimage
PizzapizzaOne,pizzaTwo;
pizzaOne=pizzaTwo=newPizza(“Supreme”);//Aliases
Thefollowingexampleshowstheeffectofoperandevaluationorder:
Clickheretoviewcodeimage
int[]a={10,20,30,40,50};//Anarrayofint
intindex=4;
a[index]=index=2;//(1)
Whatisthevalueofindex,andwhicharrayelementa[index]isassignedavaluein
themultipleassignmentstatementat(1)?Theevaluationproceedsasfollows:
Clickheretoviewcodeimage
a[index]=index=2;
a[4]=index=2;
a[4]=(index=2);//indexgetsthevalue2.=isright
associative.
a[4]=2;//Thevalueofa[4]ischangedfrom50to2.
Thefollowingdeclarationstatementwillnotcompile,asthevariablev2hasnotbeen
declared:
Clickheretoviewcodeimage
intv1=v2=2016;//Onlyv1isdeclared.Compile-timeerror!
TypeConversionsinanAssignmentContext
Ifthetargetandthesourcehavethesametypeinanassignment,then,obviously,the
sourceandthetargetareassignmentcompatibleandthesourcevalueneednotbe
converted.Otherwise,ifawideningprimitiveconversionispermissible,thenthewidening
conversionisappliedimplicitly;thatis,thesourcetypeisconvertedtothetargettypein
anassignmentcontext.
Clickheretoviewcodeimage
//WideningPrimitiveConversions
intsmallOne=1234;//Nowideningnecessary.
longbigOne=2000;//Widening:inttolong.
doublelargeOne=bigOne;//Widening:longtodouble.
doublehugeOne=(double)bigOne;//Castredundantbutallowed.
Awideningprimitiveconversioncanresultinlossofprecision.Inthenextexample,the
precisionoftheleastsignificantbitsofthelongvaluemaybelostwhenitisconverting
toafloatvalue:
Clickheretoviewcodeimage
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longbigInteger=98765432112345678L;
floatfpNum=bigInteger;//Wideningbutlossofprecision:9.8765436E16
Additionally,implicitnarrowingprimitiveconversionsonassignmentcanoccurincases
whereallofthefollowingconditionsarefulfilled:
•Thesourceisaconstantexpressionofeitherbyte,short,char,orinttype.
•Thetargettypeiseitherbyte,short,orchartype.
•Thevalueofthesourceisdeterminedtobeintherangeofthetargettypeatcompile
time.
AconstantexpressionisanexpressionthatdenoteseitheraprimitiveoraStringliteral,
andiscomposedofoperandsthatcanbeonlyliteralsorconstantvariables,andoperators
thatcanbeevaluatedonlyatcompiletime(forexample,arithmeticandnumerical
comparisonoperators,butnotincrement/decrementoperatorsandmethodcalls).A
constantvariableisafinalvariableofeitheraprimitivetypeorStringtypethatis
initializedwithaconstantexpression.
Clickheretoviewcodeimage
intresult=100;//Notaconstantvariable.Notdeclared
final.
finalcharfinalGrade=‘A’;//Constantvariable.
System.out.printf(“%d%n%s%n%d%n%.2f%n%b%n%d%n%d%n”,
2106,//Constantexpression.
“Trust”+“me!”,//Constantexpression.
2+3*4,//Constantexpression.
Math.PI*Math.PI*10.0,//Constantexpression.
finalGrade==‘A’,//Constantexpression.
Math.min(2015,2016),//Notconstantexpression.Methodcall.
++result//Notconstantexpression.Increment
operator.
);
Herearesomeexamplesthatillustratehowtheconditionsmentionedpreviouslyaffect
narrowingprimitiveconversions:
Clickheretoviewcodeimage
//Conditionsfulfilledforimplicitnarrowingprimitiveconversions.
shorts1=10;//intvalueinrange.
shorts2=‘a’;//charvalueinrange.
charc1=32;//intvalueinrange.
charc2=(byte)35;//bytevalueinrange.(intvalueinrange,without
cast.)
byteb1=40;//intvalueinrange.
byteb2=(short)40;//shortvalueinrange.(intvalueinrange,without
cast.)
finalinti1=20;//Constantvariable
byteb3=i1;//finalvalueofi1inrange.
Allothernarrowingprimitiveconversionswillproduceacompile-timeerroron
assignmentandwillexplicitlyrequireacast.Herearesomeexamples:
Clickheretoviewcodeimage
//Conditionsnotfulfilledforimplicitnarrowingprimitiveconversions.
//Acastisrequired.
inti2=-20;//i2isnotaconstantvariable.i2isnotfinal.
finalinti3=i2;//i3isnotaconstantvariable,sincei2isnot.
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finalinti4=200;//i4isaconstantvariable.
finalinti5;//i5isnotaconstantvariable.
shorts3=(short)i2;//Notconstantexpression.
charc3=(char)i3;//Finalvalueofi3notdeterminableatcompile
time.
charc4=(char)i2;//Notconstantexpression.
byteb4=(byte)128;//intvaluenotinrange.
byteb5=(byte)i4;//Valueofconstantvariablei4isnotinrange.
i5=100;//Initializedatruntime.
shorts4=(short)i5;//Finalvalueofi5notdeterminableatcompile
time.
Floating-pointvaluesaretruncatedwhencasttointegralvalues.
Clickheretoviewcodeimage
//Thevalueistruncatedtofitthesizeofthetargettype.
floathuge=(float)1.7976931348623157d;//doubletofloat.
longgiant=(long)4415961481999.03D;//(1)doubletolong.
intbig=(int)giant;//(2)longtoint.
shortsmall=(short)big;//(3)inttoshort.
bytetiny=(byte)small;//(4)shorttobyte.
charsymbol=(char)112.5F;//(5)floattochar.
Table5.4showshowthevaluesaretruncatedforassignmentsfrom(1)to(5).
Table5.4ExamplesofTruncatedValues
Thediscussionofnumericassignmentconversionsalsoappliestonumericparameter
valuesatmethodinvocation(§3.5,p.73),exceptforthenarrowingconversions,which
alwaysrequireacast.
Thefollowingexamplesillustrateboxingandunboxinginanassignmentcontext:
Clickheretoviewcodeimage
BooleanboolRef=true;//Boxing.
BytebRef=2;//Constantinrange:narrowing,thenboxing.
//BytebRef2=257;//Constantnotinrange.Compile-timeerror!
shorts=10;//Narrowingfrominttoshort.
//IntegeriRef1=s;//shortnotassignabletoInteger.
IntegeriRef3=(int)s;//Explicitwideningwithcasttointandboxing
booleanbv1=boolRef;//Unboxing.
byteb1=bRef;//Unboxing.
intiVal=bRef;//Unboxingandwidening.
IntegeriRefVal=null;//Alwaysallowed.
//intj=iRefVal;//NullPointerExceptionatruntime.
if(iRef3!=null)iVal=iRef3;//Avoidexceptionatruntime.
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ReviewQuestions
5.1Giventhefollowingdeclaration:
charc=‘A’;
Whatisthesimplestwaytoconvertthecharactervalueinctoanint?
Selecttheonecorrectanswer.
(a)inti=c;
(b)inti=(int)c;
(c)inti=Character.getNumericValue(c);
5.2Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassAssignment{
publicstaticvoidmain(String[]args){
inta,b,c;
b=10;
a=b=c=20;
System.out.println(a);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile,sincethecompilerwillreportthatthevariable
cinthemultipleassignmentstatementa=b=c=20;hasnotbeen
initialized.
(b)Theprogramwillfailtocompile,becausethemultipleassignmentstatementa
=b=c=20;isillegal.
(c)Thecodewillcompile,andprint10atruntime.
(d)Thecodewillcompile,andprint20atruntime.
5.3Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
Stringa,b,c;
c=newString(“mouse”);
a=newString(“cat”);
b=a;
a=newString(“dog”);
c=b;
System.out.println(c);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
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(b)Theprogramwillprintmouseatruntime.
(c)Theprogramwillprintcatatruntime.
(d)Theprogramwillprintdogatruntime.
(e)Theprogramwillrandomlyprinteithercatordogatruntime.
5.7ArithmeticOperators: , , , ,
Arithmeticoperatorsareusedtoconstructmathematicalexpressionsasinalgebra.Their
operandsareofnumerictype(whichincludesthechartype).
ArithmeticOperatorPrecedenceandAssociativity
InTable5.5,theprecedenceoftheoperatorsappearsindecreasingorder,startingfromthe
toprow,whichhasthehighestprecedence.Unarysubtractionhashigherprecedencethan
multiplication.Theoperatorsinthesamerowhavethesameprecedence.Binary
multiplication,division,andremainderoperatorshavethesameprecedence.Theunary
operatorshaverightassociativity,andthebinaryoperatorshaveleftassociativity.
Table5.5ArithmeticOperators
EvaluationOrderinArithmeticExpressions
Javaguaranteesthattheoperandsarefullyevaluatedfromlefttorightbeforeanarithmetic
binaryoperatorisapplied.Ifevaluationofanoperandresultsinanerror,thesubsequent
operandswillnotbeevaluated.
Intheexpressiona+b*c,theoperandawillalwaysbefullyevaluatedbeforethe
operandb,whichwillalwaysbefullyevaluatedbeforetheoperandc.However,the
multiplicationoperator*willbeappliedbeforetheadditionoperator+,respectingthe
precedencerules.Notethata,b,andcarearbitraryarithmeticexpressionsthathavebeen
determinedtobetheoperandsoftheoperators.
Example5.1,p.153,illustratestheevaluationorderandprecedencerulesforarithmetic
expressions.
RangeofNumericValues
Aswehaveseen,allnumerictypeshavearangeofvalidvalues(§2.2,p.37).Thisrangeis
givenbytheconstantsnamedMAX_VALUEandMIN_VALUE,whicharedefinedineach
numericwrappertype.
Thearithmeticoperatorsareoverloaded,meaningthattheoperationofanoperatorvaries
dependingonthetypeofitsoperands.Floating-pointarithmeticisperformedifany
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operandofanoperatorisoffloating-pointtype;otherwise,integerarithmeticis
performed.
Valuesthatareoutofrangeoraretheresultsofinvalidexpressionsarehandleddifferently
dependingonwhetherintegerorfloating-pointarithmeticisperformed.
IntegerArithmetic
Integerarithmeticalwaysreturnsavaluethatisinrange,exceptinthecaseofinteger
divisionbyzeroandremainderbyzero,whichcauseanArithmeticException(see
thelaterdiscussionofthedivisionoperator/andtheremainderoperator%).Avalidvalue
doesnotnecessarilymeanthattheresultiscorrect,asdemonstratedbythefollowing
examples:
Clickheretoviewcodeimage
inttooBig=Integer.MAX_VALUE+1;//-2147483648whichis
Integer.MIN_VALUE.
inttooSmall=Integer.MIN_VALUE-1;//2147483647whichis
Integer.MAX_VALUE.
Theseresultsshouldbevaluesthatareoutofrange.However,integerarithmeticwraps
roundiftheresultisoutofrange;thatis,theresultisreducedmodulointherangeofthe
resulttype.Toavoidwrappingroundofout-of-rangevalues,programsshouldeitheruse
explicitchecksorawidertype.Ifthetypelongisusedintheearlierexamples,the
resultswouldbecorrectinthelongrange:
Clickheretoviewcodeimage
longnotTooBig=Integer.MAX_VALUE+1L;//2147483648Linrange.
longnotTooSmall=Integer.MIN_VALUE-1L;//-2147483649Linrange.
Floating-PointArithmetic
Certainfloating-pointoperationsresultinvaluesthatareoutofrange.Typically,addingor
multiplyingtwoverylargefloating-pointnumberscanresultinanoutof-rangevaluethat
isrepresentedbyinfinity(Figure5.3).Attemptingfloating-pointdivisionbyzeroalso
returnsinfinity.Thefollowingexamplesshowhowthisvalueisprintedassignedinfinity:
Clickheretoviewcodeimage
System.out.println(4.0/0.0);//Prints:Infinity
System.out.println(-4.0/0.0);//Prints:-Infinity
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Figure5.3OverflowandUnderflowinFloating-PointArithmetic
Bothpositiveandnegativeinfinityrepresentoverflowtoinfinity;thatis,thevalueistoo
largetoberepresentedasadoubleorfloat(Figure5.3).Signedinfinityisrepresented
bythenamedconstantsPOSITIVE_INFINITYandNEGATIVE_INFINITYinthe
wrapperclassesjava.lang.Floatandjava.lang.Double.Avaluecanbe
comparedwiththeseconstantstodetectoverflow.
Floating-pointarithmeticcanalsoresultinunderflowtozero,whenthevalueistoosmall
toberepresentedasadoubleorfloat(Figure5.3).Underflowoccursinthefollowing
situations:
•TheresultisbetweenDouble.MIN_VALUE(orFloat.MIN_VALUE)andzero,
aswiththeresultof(5.1E-324-4.9E-324).Underflowthenreturnspositive
zero0.0(or0.0F).
•Theresultisbetween-Double.MIN_VALUE(or-Float.MIN_VALUE)and
zero,aswiththeresultof(-Double.MIN_VALUE*1E-1).Underflowthen
returnsnegativezero-0.0(or-0.0F).
Negativezerocomparesequaltopositivezero;inotherwords,(-0.0==0.0)is
true.
Certainoperationshavenomathematicalresult,andarerepresentedbyNaN(Nota
Number).Forexample,calculatingthesquarerootof-1resultsinNaN.Anotherexample
is(floating-point)dividingzerobyzero:
Clickheretoviewcodeimage
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System.out.println(0.0/0.0);//Prints:NaN
NaNisrepresentedbytheconstantnamedNaNinthewrapperclasses
java.lang.Floatandjava.lang.Double.AnyoperationinvolvingNaN
producesNaN.Anycomparison(exceptinequality!=)involvingNaNandanyother
value(includingNaN)returnsfalse.AninequalitycomparisonofNaNwithanother
value(includingNaN)alwaysreturnstrue.However,therecommendedwayofchecking
avalueforNaNistousethestaticmethodisNaN()definedinbothwrapperclasses,
java.lang.Floatandjava.lang.Double.
StrictFloating-PointArithmetic:strictfp
Althoughfloating-pointarithmeticinJavaisdefinedinaccordancewiththeIEEE-75432-
bit(float)and64-bit(double)standardformats,thelanguagedoesallowJVM
implementationstouseotherextendedformatsforintermediateresults.Thismeansthat
floating-pointarithmeticcangivedifferentresultsonsuchJVMs,withpossiblelossof
precision.Suchabehavioristermednon-strict,incontrasttobeingstrictandadheringto
thestandardformats.
ToensurethatidenticalresultsareproducedonallJVMs,thekeywordstrictfpcanbe
usedtoenforcestrictbehaviorforfloating-pointarithmetic.Themodifierstrictfpcan
beappliedtoclasses,interfaces,andmethods.Astrictfpmethodensuresthatallcode
inthemethodisexecutedstrictly.Ifaclassorinterfaceisdeclaredtobestrictfp,then
allcode(inmethods,initializers,andnestedclassesandinterfaces)isexecutedstrictly.If
theexpressionisdeterminedtobeinastrictfpconstruct,itisexecutedstrictly.
Strictness,however,isnotinheritedbythesubclassesorsubinterfaces.Constant
expressionsarealwaysevaluatedstrictlyatcompiletime.
UnaryArithmeticOperators: ,
Theunaryoperatorshavethehighestprecedenceofallthearithmeticoperators.Theunary
operator-negatesthenumericvalueofitsoperand.Thefollowingexampleillustratesthe
rightassociativityoftheunaryoperators:
Clickheretoviewcodeimage
intvalue=--10;//(-(-10))is10
Noticetheblankneededtoseparatetheunaryoperators;otherwise,thesewouldbe
interpretedasthedecrementoperator--(§5.9,p.176),whichwouldresultinacompile-
timeerrorbecausealiteralcannotbedecremented.Theunaryoperator+hasnoeffecton
theevaluationoftheoperandvalue.
MultiplicativeBinaryOperators: , ,
MultiplicationOperator:*
Themultiplicationoperator*multipliestwonumbers.
Clickheretoviewcodeimage
intsameSigns=-4*-8;//result:32
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doubleoppositeSigns=4*-8.0;//Wideningofint4todouble.result:
-32.0
intzero=0*-0;//result:0
DivisionOperator:/
Thedivisionoperator/isoverloaded.Ifitsoperandsareintegral,theoperationresultsin
integerdivision.
Clickheretoviewcodeimage
inti1=4/5;//result:0
inti2=8/8;//result:1
doubled1=12/8;//result:1.0;integerdivision,thenwidening
conversion
Integerdivisionalwaysreturnsthequotientasanintegervalue;thatis,theresultis
truncatedtowardzero.Notethatthedivisionperformedisintegerdivisioniftheoperands
haveintegralvalues,eveniftheresultwillbestoredinafloating-pointtype.Theinteger
valueissubjectedtoawideningconversionintheassignmentcontext.
AnArithmeticExceptionisthrownwhenintegerdivisionwithzeroisattempted,
meaningthatintegerdivisionbyzeroisanillegaloperation.
Ifanyoftheoperandsisafloating-pointtype,theoperationperformsfloating-point
division,whererelevantoperandvaluesundergobinarynumericpromotion:
Clickheretoviewcodeimage
doubled2=4.0/8;//result:0.5
doubled3=8/8.0;//result:1.0
floatd4=12.0F/8;//result:1.5F
doubleresult1=12.0/4.0*3.0;//((12.0/4.0)*3.0)whichis9.0
doubleresult2=12.0*3.0/4.0;//((12.0*3.0)/4.0)whichis9.0
RemainderOperator:%
Inmathematics,whenwedivideanumber(thedividend)byanothernumber(thedivisor),
theresultcanbeexpressedintermsofaquotientandaremainder.Forexample,when7is
dividedby5,thequotientis1andtheremainderis2.Theremainderoperator%returnsthe
remainderofthedivisionperformedontheoperands.
Clickheretoviewcodeimage
intquotient=7/5;//Integerdivisionoperation:1
intremainder=7%5;//Integerremainderoperation:2
Forintegerremainderoperation,whereonlyintegeroperandsareinvolved,evaluationof
theexpression(x%y)alwayssatisfiesthefollowingrelation:
x==(x/y)*y+(x%y)
Inotherwords,theright-handsideyieldsavaluethatisalwaysequaltothevalueofthe
dividend.Thefollowingexamplesshowhowwecancalculatetheremaindersothatthis
relationissatisfied:
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Theremaindercanbenegativeonlyifthedividendisnegative,andthesignofthedivisor
isirrelevant.Ashortcuttoevaluatingtheremainderinvolvingnegativeoperandsisthe
following:ignorethesignsoftheoperands,calculatetheremainder,andnegatethe
remainderifthedividendisnegative.
Clickheretoviewcodeimage
intr0=7%7;//0
intr1=7%5;//2
longr2=7L%-5L;//2L
intr3=-7%5;//-2
longr4=-7L%-5L;//-2L
booleanrelation=-7L==(-7L/-5L)*-5L+r4;//true
AnArithmeticExceptionisthrownifthedivisorevaluatestozero.
Notethattheremainderoperatoracceptsnotonlyintegraloperands,butalsofloating-point
operands.Thefloating-pointremainderrisdefinedbytherelation
r==a-(b*q)
whereaandbarethedividendandthedivisor,respectively,andqistheintegerquotient
of(a/b).Thefollowingexamplesillustrateafloating-pointremainderoperation:
Clickheretoviewcodeimage
doubledr0=7.0%7.0;//0.0
floatfr1=7.0F%5.0F;//2.0F
doubledr1=7.0%-5.0;//2.0
floatfr2=-7.0F%5.0F;//-2.0F
doubledr2=-7.0%-5.0;//-2.0
booleanfpRelation=dr2==(-7.0)-(-5.0)*(long)(-7.0/-5.0);//true
floatfr3=-7.0F%0.0F;//NaN
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AdditiveBinaryOperators: ,
Theadditionoperator+andthesubtractionoperator-behaveastheirnamesimply:They
addandsubtractvalues,respectively.Thebinaryoperator+alsoactsasstring
concatenationifanyofitsoperandsisastring(§5.8,p.174).
Additiveoperatorshavelowerprecedencethanalltheotherarithmeticoperators.Table5.6
includesexamplesthatshowhowprecedenceandassociativityareusedinarithmetic
expressionevaluation.
Table5.6ExamplesofArithmeticExpressionEvaluation
NumericPromotionsinArithmeticExpressions
Unarynumericpromotionisappliedtothesingleoperandoftheunaryarithmetic
operators-and+.Whenaunaryarithmeticoperatorisappliedtoanoperandwhosetype
isnarrowerthanint,theoperandispromotedtoavalueoftypeint,withtheoperation
resultinginanintvalue.Iftheconditionsforimplicitnarrowingconversionarenot
fulfilled(p.160),assigningtheintresulttoavariableofanarrowertypewillrequirea
cast.Thisisdemonstratedbythefollowingexample,wherethebyteoperandbis
promotedtoanintintheexpression(-b):
Clickheretoviewcodeimage
byteb=3;//intliteralinrange.Narrowingconversion.
b=(byte)-b;//Castrequiredonassignment.
Binarynumericpromotionisappliedtooperandsofbinaryarithmeticoperators.Its
applicationleadstotypepromotionfortheoperands,asexplainedin§5.2,p.149.The
resultisofthepromotedtype,whichisalwaystypeintorwider.Fortheexpressionat
(1)inExample5.2,numericpromotionsproceedasshowninFigure5.4.Notetheinteger
divisionperformedinevaluatingthesubexpression(c/s).
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Figure5.4NumericPromotioninArithmeticExpressions
Example5.2NumericPromotioninArithmeticExpressions
Clickheretoviewcodeimage
publicclassNumPromotion{
publicstaticvoidmain(String[]args){
byteb=32;
charc=‘z’;//Unicodevalue122(\u007a)
shorts=256;
inti=10000;
floatf=3.5F;
doubled=0.5;
doublev=(d*i)+(f*-b)-(c/s);//(1)4888.0D
System.out.println(“Valueofv:”+v);
}
}
Outputfromtheprogram:
Valueofv:4888.0
Inadditiontothebinarynumericpromotionsinarithmeticexpressionevaluation,the
resultingvaluecanundergoanimplicitwideningconversionifassignedtoavariable.In
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thefirsttwodeclarationstatementsthatfollow,onlyassignmentconversionstakeplace.
Numericpromotionstakeplaceintheevaluationoftheright-handexpressionintheother
declarationstatements.
Clickheretoviewcodeimage
Byteb=10;//Constantinrange:narrowingandboxingon
assignment.
Shorts=20;//Constantinrange:narrowingandboxingon
assignment.
charc=‘z’;//122(\u007a)
inti=s*b;//Valuesinsandbpromotedtoint:unboxing,
widening.
longn=20L+s;//Valueinspromotedtolong:unboxing,widening.
floatr=s+c;//Valueinsisunboxed.Thisshortvalueandthechar
//valueincarepromotedtoint,followedbyimplicit
//wideningconversionofinttofloatonassignment.
doubled=r+i;//Valueinipromotedtofloat,followedbyimplicit
//wideningconversionoffloattodoubleonassignment.
Binarynumericpromotionforoperandsofbinaryoperatorsimpliesthateachoperandofa
binaryoperatorispromotedtotypeintorabroadernumerictype,ifnecessary.Aswith
unaryoperators,caremustbeexercisedinassigningthevalueresultingfromapplyinga
binaryoperatortooperandsofthesetypes.
Clickheretoviewcodeimage
shorth=40;//OK:intconvertedtoshort.Implicitnarrowing.
h=h+2;//Error:cannotassignaninttoshort.
Thevalueoftheexpressionh+2isoftypeint.Althoughtheresultoftheexpression
isintherangeofshort,thiscannotbedeterminedatcompiletime.Theassignment
requiresacast.
h=(short)(h+2);//OK
Noticethatapplyingthecastoperator(short)totheindividualoperandsdoesnotwork:
Clickheretoviewcodeimage
h=(short)h+(short)2;//Theresultingvalueshouldbecast.
Neitherdoesthefollowingapproach,whichresultsinacompile-timeerror:
Clickheretoviewcodeimage
h=(short)h+2;//Theresultingvalueshouldbecast.
Inthiscase,binarynumericpromotionleadstoanintvalueastheresultofevaluating
theexpressionontheright-handsideand,therefore,requiresanadditionalcasttonarrow
ittoashortvalue.
ArithmeticCompoundAssignmentOperators: , , , ,
Acompoundassignmentoperatorhasthefollowingsyntax:
variableop=expression
andthefollowingsemantics:
Clickheretoviewcodeimage
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variable=(type)((variable)op(expression))
Thetypeofthevariableistypeandthevariableisevaluatedonlyonce.Notethecastand
theparenthesesimpliedinthesemantics.Hereop=canbeanyofthecompound
assignmentoperatorsspecifiedinTable5.2.Thecompoundassignmentoperatorshavethe
lowestprecedenceofalltheoperatorsinJava,allowingtheexpressionontheright-hand
sidetobeevaluatedbeforetheassignment.Table5.7definesthearithmeticcompound
assignmentoperators.
Table5.7ArithmeticCompoundAssignmentOperators
Theimpliedcastoperator,(T),inthecompoundassignmentsbecomesnecessarywhen
theresultmustbenarrowedtothetargettype.Thisisillustratedbythefollowing
examples:
Clickheretoviewcodeimage
inti=2;
i*=i+4;//(1)Evaluatedasi=(int)((i)*(i+4)).
IntegeriRef=2;
iRef*=iRef+4;//(2)EvaluatedasiRef=(Integer)((iRef)*(iRef
+4)).
byteb=2;
b+=10;//(3)Evaluatedasb=(byte)(b+10).
b=b+10;//(4)Willnotcompile.Castisrequired.
At(1)thesourceintvalueisassignedtothetargetintvariable,andthecastoperator
(int)inthiscaseisanidentityconversion(i.e.,conversionfromatypetothesame
type).Suchcastsarepermitted.Theassignmentat(2)entailsunboxingtoevaluatethe
expressionontheright-handside,followedbyboxingtoassigntheintvalue.However,
at(3),asthesourcevalueisanintvaluebecausethebytevalueinbispromotedto
inttocarryouttheaddition,assigningittoatargetbytevariablerequiresanimplicit
narrowingconversion.Thesituationat(4)withsimpleassignmentwillnotcompile,
becauseimplicitnarrowingconversionisnotapplicable.
Thevariableisevaluatedonlyonceintheexpression,nottwice,asonemightinferfrom
thedefinitionofthecompoundassignmentoperator.Inthefollowingassignment,a[i]is
evaluatedjustonce:
Clickheretoviewcodeimage
int[]a=newint[]{2015,2016,2017};
inti=2;
a[i]+=1;//Evaluatesasa[2]=a[2]+1,anda[2]getsthevalue
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2018.
Implicitnarrowingconversionsarealsoappliedtoincrementanddecrementoperators
(§5.9,p.176).
Booleanlogicalcompoundassignmentoperatorsarecoveredin§5.13,p.184.
ReviewQuestions
5.4Whichofthefollowingexpressionswillbeevaluatedusingfloating-point
arithmetic?
Selectthethreecorrectanswers.
(a)2.0*3.0
(b)2*3
(c)2/3+5/7
(d)2.4+1.6
(e)0x10*1L*300.0
5.5Whatisthevalueoftheexpression(1/2+3/2+0.1)?
Selecttheonecorrectanswer.
(a)1
(b)1.1
(c)1.6
(d)2
(e)2.1
5.6Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassIntegers{
publicstaticvoidmain(String[]args){
System.out.println(0x10+10+010+0b10);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Whenrun,theprogramwillprint28.
(c)Whenrun,theprogramwillprint30.
(d)Whenrun,theprogramwillprint34.
(e)Whenrun,theprogramwillprint36.
(f)Whenrun,theprogramwillprint10101010.
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5.7Whichofthefollowingexpressionsarevalid?
Selectthethreecorrectanswers.
(a)(-1-)
(b)(++1)
(c)(+-+-+-1)
(d)(—1)
(e)(1**1)
(f)(--1)
5.8Whatisthevalueofevaluatingthefollowingexpression:(--1-3*10/5-
1)?
Selecttheonecorrectanswer.
(a)–8
(b)–6
(c)7
(d)8
(e)10
(f)Noneoftheabove
5.9Whichoftheseassignmentsarevalid?
Selectthefourcorrectanswers.
(a)shorts=12;
(b)longl=012;
(c)intother=(int)true;
(d)floatf=-123;
(e)doubled=0x12345678;
5.8TheBinaryStringConcatenationOperator
Thebinaryoperator+isoverloadedinthesensethattheoperationperformedis
determinedbythetypeoftheoperands.WhenoneoftheoperandsisaStringobject,a
stringconcatenationisperformedratherthannumericaddition.Stringconcatenation
resultsinanewlycreatedStringobjectinwhichthecharactersinthestring
representationoftheleft-handoperandprecedethecharactersinthestringrepresentation
oftheright-handoperand.Itmightbenecessarytoperformastringconversiononthe
non-Stringoperandbeforethestringconcatenationcanbeperformed.TheString
classisdiscussedin§8.4,p.357.
Astringconversionisperformedonthenon-Stringoperandasfollows:
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•Foranoperandofaprimitivedatatype,itsvalueisconvertedtoastring
representation.
•Forallreferencevalueoperands,astringrepresentationisconstructedbycallingthe
no-argumenttoString()methodonthereferredobject.Mostclassesoverride
thismethodfromtheObjectclasssoastoprovideamoremeaningfulstring
representationoftheirobjects.DiscussionofthetoString()methodcanbe
foundin§8.2,p.342.
•Valuesliketrue,false,andnullhavestringrepresentationsthatcorrespondto
theirnames.Areferencevariablewiththevaluenullalsohasthestring
representation"null"inthiscontext.
Theoperator+isleftassociativeandhasthesameprecedencelevelastheadditive
operators,whetheritisperformedasastringconcatenationorasanumericaddition.
Clickheretoviewcodeimage
StringstrVal=””+2016;//(1)“2016”
StringtheName=”Uranium”;
theName=”Pure”+theName;//(2)”PureUranium”
Stringtrademark1=100+“%”+theName;//(3)“100%PureUranium”
Sincethe+operatorisleft-associative,theevaluationin(3)proceedsasfollows:Theint
value100isconcatenatedwiththestringliteral"%",followedbyconcatenationwiththe
contentsoftheStringobjectreferredtobytheNamereference.
Notethatusingthecharacterliteral'%',insteadofthestringliteral"%"inline(2),does
notgivethesameresult:
Clickheretoviewcodeimage
Stringtrademark2=100+‘%’+theName;//(4)“137PureUranium”
Integeradditionisperformedbythefirst+operator:100+'%';thatis,(100+37).
Cautionshouldbeexercisedbecausethe+operatormightnotbeappliedasintended,as
shownbythefollowingexample:
Clickheretoviewcodeimage
System.out.println(“Wecanputtwoandtwotogetherandget”+2+2);//
(5)
Thisstatementprints"Wecanputtwoandtwotogetherandget22".
Stringconcatenationproceedsfromlefttoright:TheStringliteralisconcatenatedwith
thefirstintliteral2,followedbyconcatenationwiththesecondintliteral2.Both
occurrencesofthe+operatoraretreatedasstringconcatenation.Toconveytheintended
meaningofthesentence,parenthesesarenecessary:
Clickheretoviewcodeimage
System.out.println(“Wecanputtwoandtwotogetherandget”+(2+2));//
(6)
Thisstatementprints"Wecanputtwoandtwotogetherandget4",
sincetheparenthesesenforceintegeradditionintheexpression(2+2)beforestring
concatenationisperformedwiththecontentsoftheStringoperand.
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Thefollowingstatementwillprintthecorrectresult,evenwithouttheparentheses,
becausethe*operatorhashigherprecedencethanthe+operator:
Clickheretoviewcodeimage
System.out.println(“2*2=”+2*2);//(7)2*2=4
CreationoftemporaryStringobjectsmightbenecessarytostoretheresultsof
performingsuccessivestringconcatenationsinaString-valuedexpression.Fora
String-valuedconstantexpression((1),(5),(6)and(7)intheprecedingexamples),the
compilercomputessuchanexpressionatcompiletime,andtheresultistreatedasastring
literalintheprogram.Thecompilerusesastringbuildertoavoidtheoverheadof
temporaryStringobjectswhenapplyingthestringconcatenationoperator(+)in
String-valuednon-constantexpressions((2),(3)and(4)intheprecedingexamples),as
explainedin§8.5,p.378.
5.9VariableIncrementandDecrementOperators: ,
Variableincrement(++)anddecrement(--)operatorscomeintwoflavors:prefixand
postfix.Theseunaryoperatorshavethesideeffectofchangingthevalueofthearithmetic
operand,whichmustevaluatetoavariable.Dependingontheoperatorused,thevariable
iseitherincrementedordecrementedby1.
Theseoperatorscannotbeappliedtoavariablethatisdeclaredfinalandthathasbeen
initialized,asthesideeffectwouldchangethevalueinsuchavariable.
Theseoperatorsareveryusefulforupdatingvariablesinloopswhereonlythesideeffect
oftheoperatorisofinterest.
TheIncrementOperator
Theprefixincrementoperatorhasthefollowingsemantics:++iadds1tothevalueini,
andstoresthenewvalueini.Itreturnsthenewvalueasthevalueoftheexpression.Itis
equivalenttothefollowingstatements:
i+=1;
result=i;
returnresult;
Thepostfixincrementoperatorhasthefollowingsemantics:j++adds1tothevalueinj,
andstoresthenewvalueinj.Itreturnstheoriginalvaluethatwasinjasthevalueofthe
expression.Itisequivalenttothefollowingstatements:
result=j;
j+=1;
returnresult;
TheDecrementOperator
Theprefixdecrementoperatorhasthefollowingsemantics:--isubtracts1fromthe
valueofi,andstoresthenewvalueini.Itreturnsthenewvalueasthevalueofthe
expression.Itisequivalenttothefollowingstatements:
i-=1;
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result=i;
returnresult;
Thepostfixdecrementoperatorhasthefollowingsemantics:j--subtracts1fromthe
valueofj,andstoresthenewvalueinj.Itreturnstheoriginalvaluethatwasinjasthe
valueoftheexpression.Itisequivalenttothefollowingstatements:
result=j;
j-=1;
returnresult;
Thisbehaviorofdecrementandincrementoperatorsappliestoanyvariablewhosetypeis
anumericprimitivetypeoritscorrespondingnumericwrappertype.Necessarynumeric
promotionsareperformedonthevalue1andthevalueofthevariable.Beforethenew
valueisassignedtothevariable,itissubjectedtoanynarrowingprimitiveconversion
and/orboxingthatmightbenecessary.
Herearesomeexamplesthatillustratethebehaviorofincrementanddecrementoperators:
Clickheretoviewcodeimage
//(1)Prefixorder:increment/decrementoperandbeforeuse.
inti=10;
intk=++i+—i;//((++i)+(—i)).kgetsthevalue21andibecomes10.
—i;//Onlysideeffectutilized.iis9.(expression
statement)
IntegeriRef=11;//Boxingonassignment
—iRef;//Onlysideeffectutilized.iRefreferstoanInteger
//objectwiththevalue10.(expressionstatement)
k=++iRef+—iRef;//((++iRef)+(—iRef)).kgetsthevalue21and
//iRefreferstoanIntegerobjectwiththevalue10.
//(2)Postfixorder:increment/decrementoperandafteruse.
longj=10;
longn=j+++j—;//((j++)+(j—)).ngetsthevalue21Landjbecomes10L.
j++;//Onlysideeffectutilized.jis11L.(expression
statement)
Anincrementordecrementoperator,togetherwithitsoperand,canbeusedasan
expressionstatement(§3.2,p.50).
Executionoftheassignmentintheseconddeclarationstatementunder(1)proceedsas
follows:
Executionoftheexpressionstatement--iRef;under(1)proceedsasfollows:
•ThevalueintheIntegerobjectreferredtobythereferenceiRefisunboxed,
resultingintheintvalue11.
•Thevalue11isdecremented,resultinginthevalue10.
•Thevalue10isboxedinanIntegerobject,andthisobject’sreferencevalueis
assignedtothereferenceiRef.
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•Theintvalue10oftheexpressionstatementisdiscarded.
Expressionswherevariablesaremodifiedmultipletimesduringtheevaluationshouldbe
avoided,becausetheorderofevaluationisnotalwaysimmediatelyapparent.
Wecannotassociateincrementanddecrementoperators.Giventhataisavariable,we
cannotwrite(++(++a)).Thereasonisthatanyoperandto++mustevaluatetoa
variable,buttheevaluationof(++a)resultsinavalue.
Inthenextexample,bothbinarynumericpromotionandanimplicitnarrowingconversion
areperformedtoachievethesideeffectofmodifyingthevalueoftheoperand.Theint
valueoftheexpression(++b)(thatis,11),isassignedtotheintvariablei.Theside
effectofincrementingthevalueofthebytevariablebrequiresbinarynumeric
promotiontoperformintaddition,followedbyanimplicitnarrowingconversionofthe
intvaluetobytetoperformtheassignment.
Clickheretoviewcodeimage
byteb=10;
inti=++b;//iis11,andsoisb.
Thefollowingexampleillustratesapplyingtheincrementoperatortoafloating-point
operand.Thesideeffectofthe++operatorisoverwrittenbytheassignment.
Clickheretoviewcodeimage
doublex=4.5;
x=x+++x;//xgetsthevalue10.0.
ReviewQuestions
5.10Whichstatementsaretrue?
Selectthethreecorrectanswers.
(a)Theexpression(1+2+"3")evaluatestothestring"33".
(b)Theexpression("1"+2+3)evaluatestothestring"15".
(c)Theexpression(4+1.0f)evaluatestothefloatvalue5.0f.
(d)Theexpression(10/9)evaluatestotheintvalue1.
(e)Theexpression('a'+1)evaluatestothecharvalue'b'.
5.11Whathappenswhenyoutrytocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassProg1{
publicstaticvoidmain(String[]args){
intk=1;
inti=++k+k++++k;//(1)
System.out.println(i);
}
}
Selecttheonecorrectanswer.
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(a)Theprogramwillnotcompile,becauseoferrorsintheexpressionat(1).
(b)Theprogramwillcompileandprintthevalue3atruntime.
(c)Theprogramwillcompileandprintthevalue4atruntime.
(d)Theprogramwillcompileandprintthevalue7atruntime.
(e)Theprogramwillcompileandprintthevalue8atruntime.
5.12Whichisthefirstlinethatwillcauseacompile-timeerrorinthefollowing
program?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
charc;
inti;
c=‘a’;//(1)
i=c;//(2)
i++;//(3)
c=i;//(4)
c++;//(5)
}
}
Selecttheonecorrectanswer.
(a)(1)
(b)(2)
(c)(3)
(d)(4)
(e)(5)
(f)Noneoftheabove.Thecompilerwillnotreportanyerrors.
5.13Whatistheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassCast{
publicstaticvoidmain(String[]args){
byteb=128;
inti=b;
System.out.println(i);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile,becauseabytevaluecannotbeassignedtoan
intvariablewithoutusingacast.
(b)Theprogramwillcompile,andprint128atruntime.
(c)Theprogramwillnotcompile,becausethevalue128isnotintherangeof
valuesforthebytetype.
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(d)Theprogramwillcompile,butwillthrowaClassCastExceptionat
runtime.
(e)Theprogramwillcompile,andprint255atruntime.
5.14Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassEvaluationOrder{
publicstaticvoidmain(String[]args){
int[]array={4,8,16};
inti=1;
array[++i]=—i;
System.out.println(array[0]+array[1]+array[2]);
}
}
Selecttheonecorrectanswer.
(a)13
(b)14
(c)20
(d)21
(e)24
5.10BooleanExpressions
Asthenameimplies,abooleanexpressionhasthebooleandatatypeandcanevaluate
toonlythevaluestrueorfalse.Booleanexpressions,whenusedasconditionalsin
controlstatements,allowtheprogramflowtobecontrolledduringexecution.
Booleanexpressionscanbeformedusingrelationaloperators(§5.11,p.180),equality
operators(§5.12,p.181),booleanlogicaloperators(§5.13,p.184),conditionaloperators
(§5.14,p.186),theassignmentoperator(§5.6,p.158),andtheinstanceofoperator
(§7.11,p.321).
5.11RelationalOperators: , , ,
Giventhataandbrepresentnumericexpressions,therelational(alsocalledcomparison)
operatorsaredefinedasshowninTable5.8.
Table5.8RelationalOperators
Allrelationaloperatorsarebinaryoperatorsandtheiroperandsarenumericexpressions.
Binarynumericpromotionisappliedtotheoperandsoftheseoperators.Theevaluation
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resultsinabooleanvalue.Relationaloperatorshaveprecedencelowerthanarithmetic
operators,buthigherthanthatoftheassignmentoperators.
Clickheretoviewcodeimage
doublehours=45.5;
Doubletime=18.0;//Boxingofdoublevalue.
booleanovertime=hours>=35;//true.Binarynumericpromotion:double<—
int.
booleanbeforeMidnight=time<24.0;//true.Unboxingofvalueintime
reference.
charletterA=‘A’;
booleanorder=letterA<‘a’;//true.Binarynumericpromotion:int<—
char.
Relationaloperatorsarenonassociative.Mathematicalexpressionslikea≤b≤cmustbe
writtenusingrelationalandbooleanlogical/conditionaloperators.
Clickheretoviewcodeimage
inta=1,b=7,c=10;
booleanillegal=a<=b<=c;//(1)Illegal.
booleanvalid2=a<=b&&b<=c;//(2)OK.
Sincerelationaloperatorshaveleftassociativity,theevaluationoftheexpressiona<=b
<=cat(1)intheseexampleswouldproceedasfollows:((a<=b)<=c).
Evaluationof(a<=b)wouldyieldabooleanvaluethatisnotpermittedasan
operandofarelationaloperator;thatis,(booleanvalue<=c)wouldbeillegal.
5.12Equality
Wedistinguishbetweenprimitivedataequality,objectreferenceequality,andobjectvalue
equality.
Theequalityoperatorshavelowerprecedencethantherelationaloperators,buthigher
precedencethantheassignmentoperators.
PrimitiveDataValueEquality: ,
Giventhataandbrepresentoperandsofprimitivedatatypes,theprimitivedatavalue
equalityoperatorsaredefinedasshowninTable5.9.
Table5.9PrimitiveDataValueEqualityOperators
Theequalityoperator==andtheinequalityoperator!=canbeusedtocompareprimitive
datavalues,includingbooleanvalues.Binarynumericpromotionisappliedtothenon-
booleanoperandsoftheseequalityoperators.
Clickheretoviewcodeimage
intyear=2002;
booleanisEven=year%2==0;//true.
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booleancompare=‘1’==1;//false.Binarynumericpromotion
applied.
booleantest=compare==false;//true.
Caremustbeexercisedwhencomparingfloating-pointnumbersforequality,asaninfinite
numberoffloating-pointvaluescanbestoredonlyasapproximationsinafinitenumberof
bits.Forexample,theexpression(1.0-2.0/3.0==1.0/3.0)returnsfalse,
althoughmathematicallytheresultshouldbetrue.
Analogoustothediscussionforrelationaloperators,mathematicalexpressionslikea=b
=cmustbewrittenusingrelationalandlogical/conditionaloperators.Sinceequality
operatorshaveleftassociativity,theevaluationoftheexpressiona==b==cwould
proceedasfollows:((a==b)==c).Evaluationof(a==b)wouldyielda
booleanvaluethatispermittedasanoperandofadatavalueequalityoperator,but
(booleanvalue==c)wouldbeillegalifchadanumerictype.Thisproblemis
illustratedinthefollowingexamples.Theexpressionat(1)isillegal,butthoseat(2)and
(3)arelegal.
Clickheretoviewcodeimage
inta,b,c;
a=b=c=5;
booleanillegal=a==b==c;//(1)Illegal.
booleanvalid2=a==b&&b==c;//(2)Legal.
booleanvalid3=a==b==true;//(3)Legal.
ObjectReferenceEquality: ,
Theequalityoperator==andtheinequalityoperator!=canbeappliedtoreference
variablestotestwhethertheyrefertothesameobject.Giventhatrandsarereference
variables,thereferenceequalityoperatorsaredefinedasshowninTable5.10.
Table5.10ReferenceEqualityOperators
Theoperandsmustbecastcompatible:Itmustbepossibletocastthereferencevalueof
theoneintotheother’stype;otherwise,itisacompile-timeerror.Castingofreferencesis
discussedin§7.8,p.311.
Clickheretoviewcodeimage
PizzapizzaA=newPizza(“Sweet&Sour”);//newobject
PizzapizzaB=newPizza(“Sweet&Sour”);//newobject
PizzapizzaC=newPizza(“Hot&Spicy”);//newobject
Stringbanner=“Comeandgetit!”;//newobject
booleantest=banner==pizzaA;//(1)Compile-timeerror
booleantest1=pizzaA==pizzaB;//false
booleantest2=pizzaA==pizzaC;//false
pizzaA=pizzaB;//Denotethesameobject;are
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aliases
booleantest3=pizzaA==pizzaB;//true
Thecomparisonbanner==pizzaAin(1)isillegal,becausetheStringand
Pizzatypesarenotrelatedandthereforethereferencevalueofonetypecannotbecast
totheothertype.Thevaluesoftest1andtest2arefalsebecausethethree
referencesdenotedifferentobjects,regardlessofthefactthatpizzaAandpizzaBare
bothsweetandsourpizzas.Thevalueoftest3istruebecausenowbothpizzaAand
pizzaBdenotethesameobject.
Theequalityandinequalityoperatorsareappliedtoobjectreferencestocheckwhether
tworeferencesdenotethesameobject.Thestateoftheobjectsthatthereferencesdenote
isnotcompared.Thisisthesameastestingwhetherthereferencesarealiases,meaning
thattheydenotethesameobject.
Thenullliteralcanbeassignedtoanyreferencevariable,andthereferencevalueina
referencevariablecanbecomparedforequalitywiththenullliteral.Thecomparison
canbeusedtoavoidinadvertentuseofareferencevariablethatdoesnotdenoteany
object.
if(objRef!=null){
//…useobjRef…
}
Notethatonlywhenthetypeofbothoperandsiseitherareferencetypeorthenulltype,
dotheseoperatorstestforobjectreferenceequality.Otherwise,theytestforprimitivedata
equality(seealso§8.3,p.350).Inthefollowingcodesnippet,binarynumericpromotion
involvingunboxingisperformedat(2):
Clickheretoviewcodeimage
IntegeriRef=10;
booleanb1=iRef==null;//(1)Objectreferenceequality
booleanb2=iRef==10;//(2)Primitivedataequality
booleanb3=null==10;//Compile-timeerror!
ObjectValueEquality
TheObjectclassprovidesthemethodpublicbooleanequals(Objectobj),
whichcanbeoverridden(§7.2,p.268)togivetherightsemanticsofobjectvalueequality.
ThedefaultimplementationofthismethodintheObjectclassreturnstrueonlyifthe
objectiscomparedwithitself,asiftheequalityoperator==hadbeenusedtocompare
aliasesofanobject.Consequently,ifaclassdoesnotoverridethesemanticsofthe
equals()methodfromtheObjectclass,objectvalueequalityisthesameasobject
referenceequality.
CertainclassesinthestandardAPIoverridetheequals()method,suchas
java.lang.Stringandthewrapperclassesfortheprimitivedatatypes.Fortwo
Stringobjects,valueequalitymeanstheycontainidenticalcharactersequences.Forthe
wrapperclasses,valueequalitymeansthatthewrapperobjectshavethesameprimitive
valueandareofthesamewrappertype(seealso§8.3,p.350).
Clickheretoviewcodeimage
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//EqualityforStringobjectsmeansidenticalcharactersequences.
Stringmovie1=newString(“TheRevengeoftheExceptionHandler”);
Stringmovie2=newString(“HighNoonattheJavaCorral”);
Stringmovie3=newString(“TheRevengeoftheExceptionHandler”);
booleantest0=movie1.equals(movie2);//false.
booleantest1=movie1.equals(movie3);//true.
//Equalityforwrapperclassesmeanssametypeandsameprimitivevalue.
Booleanflag1=true;//Boxing.
Booleanflag2=false;//Boxing.
booleantest2=flag1.equals(“true”);//false.Notsametype.
booleantest3=flag1.equals(!flag2);//true.Sametypeand
value.
IntegeriRef=100;//Boxing.
ShortsRef=100;//Boxing.
booleantest4=iRef.equals(100);//true.Sametypeand
value.
booleantest5=iRef.equals(sRef);//false.Notsametype.
booleantest6=iRef.equals(3.14);//false.Notsametype.
//ThePizzaclassdoesnotoverridetheequals()method,sowecanuse
either
//equals()methodinheritedfromtheObjectclassorequalityoperator==.
Pizzapizza1=newPizza(“VeggiesDelight”);
Pizzapizza2=newPizza(“VeggiesDelight”);
Pizzapizza3=newPizza(“CheeseDelight”);
booleantest7=pizza1.equals(pizza2);//false.
booleantest8=pizza1.equals(pizza3);//false.
booleantest9=pizza1==pizza2;//false.
pizza1=pizza2;//Createsaliases.
booleantest10=pizza1.equals(pizza2);//true.
booleantest11=pizza1==pizza2;//true.
5.13BooleanLogicalOperators: , , ,
Booleanlogicaloperatorsincludetheunaryoperator!(logicalcomplement)andthe
binaryoperators&(logicalAND),|(logicalinclusiveOR),and^(logicalexclusiveOR,
alsocalledlogicalXOR).TheseoperatorscanbeappliedtobooleanorBoolean
operands,returningabooleanvalue.Theoperators&,|,and^canalsobeappliedto
integraloperandstoperformbitwiselogicaloperations,butarenotinthescopeofthis
book.
Giventhatxandyrepresentbooleanexpressions,thebooleanlogicaloperatorsaredefined
inTable5.11.Theprecedenceoftheoperatorsdecreasesfromlefttorightinthetable.
Table5.11TruthValuesforBooleanLogicalOperators
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Theseoperatorsalwaysevaluateboththeoperands,unliketheircounterpartconditional
operators&&and||(§5.14,p.186).Unboxingisappliedtotheoperandvalues,if
necessary.TruthvaluesforbooleanlogicaloperatorsareshowninTable5.11.
OperandEvaluationforBooleanLogicalOperators
IntheevaluationofbooleanexpressionsinvolvingbooleanlogicalAND,XOR,andOR
operators,boththeoperandsareevaluated.Theorderofoperandevaluationisalways
fromlefttoright.
Clickheretoviewcodeimage
if(i>0&i++<10){/*…*/}//iwillbeincremented,regardlessofvalue
ini.
Thebinarybooleanlogicaloperatorshaveprecedencelowerthanthearithmeticand
relationaloperators,buthigherprecedencethantheassignment,conditionalAND,andOR
operators(§5.14,p.186).Thisisillustratedinthefollowingexamples:
Clickheretoviewcodeimage
booleanb1,b2,b3=false,b4=false;
Booleanb5=true;
b1=4==2&1<4;//false,evaluatedas(b1=((4==2)&(1<
4)))
b2=b1|!(2.5>=8);//true
b3=b3^b5;//true,unboxingconversiononb5
b4=b4|b1&b2;//false
Here,theorderofevaluationisillustratedforthelastexpressionstatement:
Clickheretoviewcodeimage
(b4=(b4|(b1&b2)))
(b4=(false|(b1&b2)))
(b4=(false|(false&b2)))
(b4=(false|(false&true)))
(b4=(false|false))
(b4=false)
false
Notethatb2wasevaluatedalthough,strictlyspeaking,itwasnotnecessary.Thisbehavior
isguaranteedforbooleanlogicaloperators.
BooleanLogicalCompoundAssignmentOperators: , ,|
CompoundassignmentoperatorsforthebooleanlogicaloperatorsaredefinedinTable
5.12.Theleft-handoperandmustbeabooleanvariable,andtheright-handoperandmust
beabooleanexpression.Anidentityconversionisappliedimplicitlyonassignment.
Theseoperatorscanalsobeappliedtointegraloperandstoperformbitwisecompound
assignments,butarenotcoveredinthisbook.Seealsothediscussiononarithmetic
compoundassignmentoperatorsin§5.7,p.172.
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Table5.12BooleanLogicalCompoundAssignmentOperators
Herearesomeexamplestoillustratethebehaviorofbooleanlogicalcompound
assignmentoperators:
Clickheretoviewcodeimage
booleanb1=false,b2=true,b3=false;
Booleanb4=false;
b1|=true;//true
b4^=b1;//(1)true,unboxingin(b4^(b1)),boxingon
assignment
b3&=b1|b2;//(2)false,b3=(b3&(b1|b2))
b3=b3&b1|b2;//(3)true,b3=((b3&b1)|b2)
Theassignmentat(1)entailsunboxingtoevaluatetheexpressionontheright-handside,
followedbyboxingtoassignthebooleanresult.Itisalsoinstructivetocomparehow
theassignmentsat(2)and(3)areperformed,astheyleadtodifferentresultswiththesame
valuesoftheoperands,showinghowtheprecedenceaffectstheevaluation.
5.14ConditionalOperators: ,
Theconditionaloperators&&and||aresimilartotheircounterpartlogicaloperators&
and|,exceptthattheirevaluationisshort-circuited.Giventhatxandyrepresentvalues
ofbooleanorBooleanexpressions,theconditionaloperatorsaredefinedinTable
5.13.Inthetable,theoperatorsarelistedindecreasingprecedenceorder.
Table5.13ConditionalOperators
Unliketheirlogicalcounterparts&and|,whichcanalsobeappliedtointegraloperands
forbitwiseoperations,theconditionaloperators&&and||canbeappliedonlyto
booleanoperands.Theirevaluationresultsinabooleanvalue.Truthvaluesfor
conditionaloperatorsareshowninTable5.14.Notsurprisingly,theconditionaloperators
havethesametruthvaluesastheircounterpartlogicaloperators.However,unlikewith
theirlogicalcounterparts,therearenocompoundassignmentoperatorsfortheconditional
operators.
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Table5.14TruthValuesforConditionalOperators
Short-CircuitEvaluation
InevaluationofbooleanexpressionsinvolvingconditionalANDandOR,theleft-hand
operandisevaluatedbeforetheright-handoperand,andtheevaluationisshort-circuited
(i.e.,iftheresultofthebooleanexpressioncanbedeterminedfromtheleft-handoperand,
theright-handoperandisnotevaluated).Inotherwords,theright-handoperandis
evaluatedconditionally.
Thebinaryconditionaloperatorshavelowerprecedencethanthearithmetic,relational,
andlogicaloperators,buthigherprecedencethantheassignmentoperators.Unboxingof
theoperandvaluetakesplacewhennecessary,beforetheoperationisperformed.The
followingexamplesillustrateusageofconditionaloperators:
Clickheretoviewcodeimage
Booleanb1=4==2&&1<4;//false,short-circuitevaluatedas
//(b1=((4==2)&&(1<4)))
booleanb2=!b1||2.5>8;//true,short-circuitevaluatedas
//(b2=((!b1)||(2.5>8)))
Booleanb3=!(b1&&b2);//true
booleanb4=b1||!b3&&b2;//false,short-circuitevaluatedas
//(b4=(b1||((!b3)&&b2)))
Theorderofevaluationforcomputingthevaluestoredinthebooleanvariableb4
proceedsasfollows:
Clickheretoviewcodeimage
(b4=(b1||((!b3)&&b2)))
(b4=(false||((!b3)&&b2)))
(b4=(false||((!true)&&b2)))
(b4=(false||((false)&&b2)))
(b4=(false||false))
(b4=false)
Notethatb2isnotevaluated,short-circuitingtheevaluation.Example5.3illustratesthe
short-circuitevaluationoftheinitializationexpressionsinthedeclarationstatementsgiven
intheearliercodesnippet.Inaddition,itshowsanevaluation(seethedeclarationofb5)
involvingbooleanlogicaloperatorsthatalwaysevaluatebothoperands.SeealsoExample
5.1,p.153,whichusesasimilarapproachtoillustratetheorderofoperandevaluationin
arithmeticexpressions.
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Example5.3Short-CircuitEvaluationInvolvingConditionalOperators
Clickheretoviewcodeimage
publicclassShortCircuit{
publicstaticvoidmain(String[]args){
//Booleanb1=4==2&&1<4;
Booleanb1=operandEval(1,4==2)&&operandEval(2,1<4);
System.out.println();
System.out.println(“Valueofb1:”+b1);
//booleanb2=!b1||2.5>8;
booleanb2=!operandEval(1,b1)||operandEval(2,2.5>8);
System.out.println();
System.out.println(“Valueofb2:”+b2);
//Booleanb3=!(b1&&b2);
Booleanb3=!(operandEval(1,b1)&&operandEval(2,b2));
System.out.println();
System.out.println(“Valueofb3:”+b3);
//booleanb4=b1||!b3&&b2;
booleanb4=operandEval(1,b1)||!operandEval(2,b3)&&operandEval(3,
b2);
System.out.println();
System.out.println(“Valueofb4:”+b4);
//booleanb5=b1|!b3&b2;//Usingbooleanlogicaloperators
booleanb5=operandEval(1,b1)|!operandEval(2,b3)&operandEval(3,
b2);
System.out.println();
System.out.println(“Valueofb5:”+b5);
}
staticbooleanoperandEval(intopNum,booleanoperand){//
(1)
System.out.print(opNum);
returnoperand;
}
}
Outputfromtheprogram:
1
Valueofb1:false
1
Valueofb2:true
1
Valueofb3:true
12
Valueofb4:false
123
Valueofb5:false
Short-circuitevaluationcanbeusedtoensurethatareferencevariabledenotesanobject
beforeitisused.
Clickheretoviewcodeimage
if(objRef!=null&&objRef.equals(other)){/*…*/}
Themethodcallisnowconditionallydependentontheleft-handoperandandwillnotbe
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executedifthevariableobjRefhasthenullreference.Ifweusethelogical&operator
andthevariableobjRefhasthenullreference,evaluationoftheright-handoperand
willresultinaNullPointerException.
Insummary,weemploytheconditionaloperators&&and||iftheevaluationoftheright-
handoperandisconditionallydependentontheleft-handoperand.Weusetheboolean
logicaloperators&and|ifbothoperandsmustbeevaluated.Thesubtletyofconditional
operatorsisillustratedbythefollowingexamples:
Clickheretoviewcodeimage
if(i>0&&i++<10){/*…*/}//iisnotincrementedifi>0isfalse.
if(i>0||i++<10){/*…*/}//iisnotincrementedifi>0istrue.
5.15IntegerBitwiseOperators: , , ,
Areviewofintegerrepresentation(§5.5,p.154)isrecommendedbeforecontinuingwith
thissectiononhowintegerbitwiseoperatorscanbeappliedtovaluesofintegraldata
types.
Integerbitwiseoperatorsincludetheunaryoperator~(bitwisecomplement)andthe
binaryoperators&(bitwiseAND),|(bitwiseinclusiveOR),and^(bitwiseexclusiveOR,
alsoknownasbitwiseXOR).Theoperators&,|,and^areoverloaded,astheycanbe
appliedtobooleanorBooleanoperandstoperformbooleanlogicaloperations(§5.13,
p.184).AlthoughtheintegerbitwiseoperatorsarenotontheOCAJP8exam,theyare
includedheretocontrasttheirevaluationwiththatoftheirbooleancounterparts.
Thebinarybitwiseoperatorsperformbitwiseoperationsbetweencorrespondingindividual
bitvaluesintheoperands.Unarynumericpromotionisappliedtotheoperandoftheunary
bitwisecomplementoperator~,andbinarynumericpromotionisappliedtotheoperands
ofthebinarybitwiseoperators.Theresultisanewintegervalueofthepromotedtype,
whichcanbeeitherintorlong.
GiventhatAandBarecorrespondingbitvalues(either0or1)intheleft-handandright-
handoperands,respectively,thesebitwiseoperatorsaredefinedasshowninTable5.15.
Theoperatorsarelistedindecreasingprecedenceorder.
Table5.15IntegerBitwiseOperators
Theresultofapplyingbitwiseoperatorsbetweentwocorrespondingbitsintheoperandsis
showninTable5.16,whereAandBarecorrespondingbitvaluesintheleft-handright-
handoperands,respectively.Table5.16isanalogoustoTable5.11forbooleanlogical
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operators,ifweconsiderbitvalue1torepresenttrueandbitvalue0torepresent
false.
Table5.16ResultTableforBitwiseOperators
ExamplesofBitwiseOperatorApplication
Clickheretoviewcodeimage
charv1=‘)’;//Unicodevalue41
bytev2=13;
intresult1=~v1;//-42
intresult2=v1&v2;//9
intresult3=v1|v2;//45
intresult4=v1^v2;//36
Table5.17showshowtheresultiscalculated.Unaryandbinarynumericpromotionsare
appliedfirst,convertingtheoperandstointinthesecases.Notethattheoperator
semanticsisappliedtocorrespondingindividualbits—thatis,firstbitofleft-handoperand
andfirstbitofright-handoperand,secondbitofleft-handoperandandsecondbitofright-
handoperand,andsoon.
Table5.17ExamplesofBitwiseOperations
Itisinstructivetorunexamplesandprinttheresultofabitwiseoperationindifferent
notations,asshowninExample5.4.Theintegerbitwiseoperatorssupportaprogramming
techniquecalledbitmasking.Thevaluev2isusuallycalledabitmask.Dependingonthe
bitwiseoperationperformedonthevaluev1andthemaskv2,weseehowtheresulting
valuereflectsthebitwiseoperationperformedbetweentheindividualcorrespondingbits
ofthevaluev1andthemaskv2.Bychoosingappropriatevaluesforthebitsinthemask
v2andtherightbitwiseoperation,itispossibletoextract,set,andtogglespecificbitsin
thevaluev1.
Methodsforconvertingintegerstostringsindifferentnotationscanbefoundinthe
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Integerclass(§8.3,p.353).Convertingintegerstodifferentnumbersystemsis
discussedin§5.5,p.154.
Example5.4BitwiseOperations
Clickheretoviewcodeimage
publicclassBitOperations{
publicstaticvoidmain(String[]args){
charv1=‘)’;//Unicodevalue41
bytev2=13;
printIntToStr(“v1:”,v1);//41
printIntToStr(“v2:”,v2);//13
printIntToStr(“~v1:”,~v1);//-42
printIntToStr(“v1&v2:”,v1&v2);//9
printIntToStr(“v1|v2:”,v1|v2);//45
printIntToStr(“v1^v2:”,v1^v2);//36
}
publicstaticvoidprintIntToStr(Stringlabel,intresult){
System.out.println(label);
System.out.println(“Binary:”+Integer.toBinaryString(result));
System.out.println(“Hex:”+Integer.toHexString(result));
System.out.println(“Decimal:”+result);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
v1:
Binary:101001
Hex:29
Decimal:41
v2:
Binary:1101
Hex:d
Decimal:13
~v1:
Binary:11111111111111111111111111010110
Hex:ffffffd6
Decimal:-42
v1&v2:
Binary:1001
Hex:9
Decimal:9
v1|v2:
Binary:101101
Hex:2d
Decimal:45
v1^v2:
Binary:100100
Hex:24
Decimal:36
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BitwiseCompoundAssignmentOperators: , ,
BitwisecompoundassignmentoperatorsforthebitwiseoperatorsaredefinedinTable
5.18.Typeconversionsfortheseoperators,whenappliedtointegraloperands,arethe
sameasforothercompoundassignmentoperators:animplicitnarrowingconversionis
performedonassignmentwhenthedestinationdatatypeiseitherbyte,short,or
char.Theseoperatorscanalsobeappliedtobooleanoperandstoperformlogical
compoundassignments(§5.13,p.185).
Table5.18BitwiseCompoundAssignmentOperators
ExamplesofBitwiseCompoundAssignment
Clickheretoviewcodeimage
intv0=-42;
charv1=‘)’;//41
bytev2=13;
v0&=15;//1…11010110&0…00001111=>0…00000110(=6)
v1|=v2;//(1)0…00101001|0…00001101=>0…00101101(=45,‘-‘)
At(1)intheseexamples,boththecharvalueinv1andthebytevalueinv2arefirst
promotedtoint.Theresultisimplicitlynarrowedtothedestinationtypecharon
assignment.
ReviewQuestions
5.15Whichofthefollowingexpressionsevaluatetotrue?
Selectthetwocorrectanswers.
(a)(false|true)
(b)(null!=null)
(c)(4<=4)
(d)(!true)
(e)(true&false)
5.16Whichofthefollowingstatementsaretrue?
Selectthetwocorrectanswers.
(a)Theremainderoperator%canbeusedonlywithintegraloperands.
(b)Short-circuitevaluationoccurswithbooleanlogicaloperators.
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(c)Thearithmeticoperators*,/,and%havethesamelevelofprecedence.
(d)Ashortvaluerangesfrom-128to+127,inclusive.
(e)(+15)isalegalexpression.
5.17Whichstatementsaretrueaboutthelinesofoutputprintedbythefollowing
program?
Clickheretoviewcodeimage
publicclassBoolOp{
staticvoidop(booleana,booleanb){
booleanc=a!=b;
booleand=a^b;
booleane=c==d;
System.out.println(e);
}
publicstaticvoidmain(String[]args){
op(false,false);
op(true,false);
op(false,true);
op(true,true);
}
}
Selectthethreecorrectanswers.
(a)Alllinesprintedarethesame.
(b)Atleastonelinecontainsfalse.
(c)Atleastonelinecontainstrue.
(d)Thefirstlinecontainsfalse.
(e)Thelastlinecontainstrue.
5.18Whatistheresultofrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassOperandOrder{
publicstaticvoidmain(String[]args){
inti=0;
int[]a={3,6};
a[i]=i=9;
System.out.println(i+””+a[0]+””+a[1]);
}
}
Selecttheonecorrectanswer.
(a)Whenrun,theprogramthrowsanArrayIndexOutOfBoundsException.
(b)Whenrun,theprogramwillprint996.
(c)Whenrun,theprogramwillprint906.
(d)Whenrun,theprogramwillprint936.
(e)Whenrun,theprogramwillprint939.
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5.19Whichstatementsaretrueabouttheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
publicclassLogic{
publicstaticvoidmain(String[]args){
inti=0;
intj=0;
booleant=true;
booleanr;
r=(t&0<(i+=1));
r=(t&&0<(i+=2));
r=(t|0<(j+=1));
r=(t||0<(j+=2));
System.out.println(i+””+j);
}
}
Selectthetwocorrectanswers.
(a)Thefirstdigitprintedis1.
(b)Thefirstdigitprintedis2.
(c)Thefirstdigitprintedis3.
(d)Theseconddigitprintedis1.
(e)Theseconddigitprintedis2.
(f)Theseconddigitprintedis3.
5.16TheConditionalOperator:
Theternaryconditionaloperator?:allowsconditionalexpressionstobedefined.The
conditionalexpressionhasthefollowingsyntax:
Clickheretoviewcodeimage
condition?expression1:expression2
Itiscalledternarybecauseithasthreeoperands.Ifthebooleanexpressionconditionis
true,thenexpression1isevaluated;otherwise,expression2isevaluated.Both
expression1andexpression2mustevaluatetovaluesthatcanbeconvertedtothetypeof
theconditionalexpression.Thistypeisdeterminedfromthetypesofthetwoexpressions.
Thevalueoftheexpressionevaluatedisconvertedtothetypeoftheconditional
expression,andmayinvolveautoboxingandunboxing.
Evaluationofaconditionalexpressionisanexampleofshort-circuitevaluation.Asonly
oneofthetwoexpressionsisevaluated,oneshouldbewaryofanysideeffectsina
conditionalexpression.
Inthefollowingcodesnippetat(1),bothexpressionsintheconditionalexpressionareof
typebyte.Thetypeoftheconditionalexpressionisthereforebyte.Thatavalueoftype
bytecanbeconvertedtoanintbyanimplicitwideningnumericconversiontobe
assignmentcompatiblewiththeintvariabledaysInFebruaryissecondaryin
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determiningthetypeoftheconditionalexpression.Notethattheconditionaloperatorat
(1)hashigherprecedencethantheassignmentoperator=,makingitunnecessaryto
enclosetheconditionalexpressioninparentheses.
Clickheretoviewcodeimage
booleanleapYear=false;
bytev29=29;
bytev28=28;
intdaysInFebruary=leapYear?v29:v28;//(1)
Thefollowingexamplesillustratetheuseofconditionalexpressions.Thetypeofthe
conditionalexpressionat(2)isint,andnoconversionofanyexpressionvalueis
necessary.Thetypeoftheconditionalexpressionat(3)isdouble,duetobinarynumeric
promotion:Theintvalueofthefirstexpressionsispromotedtoadouble.The
compilerreportsanerrorbecauseadoublecannotbeassignedtoanintvariable.The
typeoftheconditionalexpressionat(4)isalsodoubleasin(3),butnowthedouble
valueisassignmentcompatiblewiththedoublevariableminDoubleValue.
Clickheretoviewcodeimage
inti=3;
intj=4;
intminValue1=i<j?i:j;//(2)int
intminValue2=i<j?i:Double.MIN_VALUE;//(3)double.NotOK.
doubleminDoubleValue=i<j?i:Double.MIN_VALUE;//(4)double
Inthefollowingcodesnippetin(5),theprimitivevaluesoftheexpressionscanbeboxed
andassignedtoanObjectreference.In(6),theintvalueofthefirstexpressioncanbe
boxedinanInteger.Theprintln()methodcreatesandprintsastring
representationofanyobjectwhosereferencevalueispassedasparameter.
Clickheretoviewcodeimage
//Assumeiandjareoftypeintandinitializedcorrectly.
Objectobj=i<j?i:true;//(5)valueofiboxedinIntegeror
//literaltrueboxedinBoolean
System.out.println(i<j?i:“Hi”);//(6)valueofiboxedinIntegeror
//Stringobject“Hi”
Theconditionalexpressionisnotanexpressionstatement.Thefollowingcodewillnot
compile:
Clickheretoviewcodeimage
(i<j)?i:j;//Compile-timeerror!
Theconditionalexpressioncanbenested,andtheconditionaloperatorassociatesfrom
righttoleft.
Clickheretoviewcodeimage
a?b:c?d:e?f:gevaluatesas(a?b:(c?d:(e?f:g)))
Thevalueofthisconditionalexpressionisgif,andonlyif,a,c,andearefalse.A
nestedconditionalexpressionisusedinthenextexample.Asaconvention,thecondition
inaconditionalexpressionisenclosedinparenthesestoaidreadingthecode.Typically,a
conditionalexpressionisusedwhenitmakesthecodeeasiertoread,especiallywhenthe
expressionsareshortandwithoutsideeffects.
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intn=3;
Stringmsg=(n==0)?“nocookies.”:(n==1)?“onecookie.”:“many
cookies.”;
System.out.println(“Youget”+msg);//Yougetmanycookies.
Theconditionaloperatoristheexpressionequivalentoftheif-elsestatement(§6.2,p.
201).
5.17OtherOperators: , , ,->
Thenewoperatorisusedtocreateobjects,suchasinstancesofclassesandarrays.Itis
usedwithaconstructorcalltoinstantiateclasses(§3.3,p.53)andwiththe[]notationto
createarrays(§3.4,p.59).Itisalsousedtoinstantiateanonymousarrays(§3.4,p.63).
Clickheretoviewcodeimage
PizzaonePizza=newPizza();//CreateaninstanceofthePizzaclass.
The[]notationisusedtodeclareandconstructarrays,andisalsotoaccessarray
elements(§3.4,p.58).
Clickheretoviewcodeimage
int[]anArray=newint[5];//Declareandconstructanintarrayof5
elements.
anArray[4]=anArray[3];//Elementatindex4getsvalueofelementat
index3.
Theboolean,binary,andinfixoperatorinstanceofisusedtotestthetypeofanobject
(§7.11,p.320).
Clickheretoviewcodeimage
PizzamyPizza=newPizza();
booleantest1=myPizzainstanceofPizza;//true.
booleantest2=“Pizza”instanceofPizza;//Compileerror.Stringisnot
Pizza.
booleantest3=nullinstanceofPizza;//Alwaysfalse.nullisnotan
instance.
Thearrowoperator->isusedinthedefinitionofalambdaexpression(§10.2,p.444).
Clickheretoviewcodeimage
java.util.function.Predicate<String>predicate=str->str.length()%2==
0;
booleantest4=predicate.test(“Thelambdastrikesback!”);//true.
ReviewQuestions
5.20WhichofthefollowingarenotoperatorsinJava?
Selectthetwocorrectanswers.
(a)%
(b)&&
(c)%=
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(d)&&=
(e)<=
(f)%%
(g)->
5.21Whichstatementswheninsertedat(1)willnotresultinacompile-timeerror?
Clickheretoviewcodeimage
publicclassRQ05A200{
publicstaticvoidmain(String[]args){
inti=20;
intj=30;
//(1)INSERTSTATEMENTHERE.
}
}
Selectthethreecorrectanswers.
(a)intresult1=i<j?i:j*10D;
(b)intresult2=i<j?{++i}:{++j};
(c)Numbernumber=i<j?i:j*10D;
(d)System.out.println(i<j?i);
(e)System.out.println(i<j?++i:++j);
(f)System.out.println(i==j?i==j:"inotequalto
j");
5.22Whichstatementsaretrueaboutthefollowingcode?
Clickheretoviewcodeimage
publicclassRQ05A100{
publicstaticvoidmain(String[]args){
intn1=10,n2=10;
intm1=20,m2=30;
intresult=n1!=n2?n1:m1!=m2?m1:m2;
System.out.println(result);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Whenrun,theprogramthrowsanArithmeticExceptionatruntime.
(c)Whenrun,theprogramwillprint10.
(d)Whenrun,theprogramwillprint20.
(e)Whenrun,theprogramwillprint30.
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ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Typeconversioncategoriesandconversioncontexts,andwhichconversionsare
permissibleineachconversioncontext.
•Definingandevaluatingarithmeticandbooleanexpressions,andtheorderinwhich
operandsandoperatorsareevaluated.
•Representingintegersindifferentnumbersystemsandinmemory.
•OperatorsinJava,includingprecedenceandassociativityrules.
ProgrammingExercise
5.1Thefollowingprogramissupposedtocalculateandprintthetimeittakesforlight
totravelfromthesuntotheearth.Itcontainssomelogicalerrors.Fixtheprogram
sothatitwillcompile,compute,andprintthecorrectresultwhenrun.
Clickheretoviewcodeimage
//File:Sunlight.java
publicclassSunlight{
publicstaticvoidmain(String[]args){
//Distancefromsun(150millionkilometers)
intkmFromSun=150_000_000;
intlightSpeed=299_792_458;//meterspersecond
//Convertdistancetometers.
intmFromSun=kmFromSun*1000;
intseconds=mFromSun/lightSpeed;
System.out.print(“Lightwilluse“);
printTime(seconds);
System.out.println(”totravelfromthesuntotheearth.”);
}
publicstaticvoidprintTime(intsec){
intmin=sec/60;
sec=sec-(min*60);
System.out.print(min+”minute(s)and”+sec+”second(s)”);
}
}
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6.ControlFlow
6.1OverviewofControlFlowStatements
Controlflowstatementsgoverntheflowofcontrolinaprogramduringexecution,
meaningtheorderinwhichstatementsareexecutedinarunningprogram.Therearethree
maincategoriesofcontrolflowstatements:
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•Selectionstatements:if,if-else,andswitch.
•Iterationstatements:while,do-while,basicfor,andenhancedfor.
•Transferstatements:break,continue,return,try-catch-finally,
throw,andassert.
Onlythebasicformofthetry-catch-finallyconstructiscoveredhere,andthe
assertfacilityisnotinthescopeofthisbook.
6.2SelectionStatements
Javaprovidesselectionstatementsthatallowtheprogramtochoosebetweenalternative
actionsduringexecution.Thechoiceisbasedoncriteriaspecifiedintheselection
statement.Theseselectionstatementsare
•Simpleifstatement
•if-elsestatement
•switchstatement
TheSimple Statement
Thesimpleifstatementhasthefollowingsyntax:
if(condition)
statement
Itisusedtodecidewhetheranactionistobeperformedornot,basedonacondition.The
actiontobeperformedisspecifiedbystatement,whichcanbeasinglestatementoracode
block.TheconditionmustevaluatetoabooleanorBooleanvalue.Inthelattercase,
theBooleanvalueisunboxedtothecorrespondingbooleanvalue.
Thesemanticsofthesimpleifstatementarestraightforward.Theconditionisevaluated
first.Ifitsvalueistrue,statement(calledtheifblock)isexecutedandthenexecution
continueswiththerestoftheprogram.Ifthevalueisfalse,theifblockisskippedand
executioncontinueswiththerestoftheprogram.Thesemanticsareillustratedbythe
activitydiagraminFigure6.1a.
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Figure6.1ActivityDiagramforifStatements
Inthefollowingexamplesoftheifstatement,itisassumedthatthevariablesandthe
methodshavebeenappropriatelydefined:
Clickheretoviewcodeimage
if(emergency)//emergencyisabooleanvariable
operate();
if(temperature>critical)
soundAlarm();
if(isLeapYear()&&endOfCentury())
celebrate();
if(catIsAway()){//Block
getFishingRod();
goFishing();
}
Notethatstatementcanbeablock,andtheblocknotationisnecessaryifmorethanone
statementistobeexecutedwhentheconditionistrue.
Sincetheconditionevaluatestoabooleanvalue,itavoidsacommonprogramming
error:usinganexpressionoftheform(a=b)asthecondition,whereinadvertentlyan
assignmentoperatorisusedinsteadofarelationaloperator.Thecompilerwillflagthisas
anerror,unlessbothaandbareboolean.
Notethattheifblockcanbeanyvalidstatement.Inparticular,itcanbetheempty
statement(;)ortheemptyblock({}).Acommonprogrammingerrorisinadvertentuse
oftheemptystatement.
Clickheretoviewcodeimage
if(emergency);//Emptyifblock
operate();//Executedregardlessofwhetheritwasanemergency
The Statement
Theif-elsestatementisusedtodecidebetweentwoactions,basedonacondition.It
hasthefollowingsyntax:
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if(condition)
statement1
else
statement2
Theconditionisevaluatedfirst.Ifitsvalueistrue(orunboxedtotrue),statement1
(theifblock)isexecutedandthenexecutioncontinueswiththerestoftheprogram.If
thevalueisfalse(orunboxedtofalse),statement2(theelseblock)isexecutedand
thenexecutioncontinueswiththerestoftheprogram.Inotherwords,oneoftwomutually
exclusiveactionsisperformed.Theelseclauseisoptional;ifomitted,theconstructis
equivalenttothesimpleifstatement.Thesemanticsareillustratedbytheactivity
diagraminFigure6.1b.
Inthefollowingexamplesoftheif-elsestatement,itisassumedthatallvariablesand
methodshavebeenappropriatelydefined:
if(emergency)
operate();
else
joinQueue();
if(temperature>critical)
soundAlarm();
else
businessAsUsual();
if(catIsAway()){
getFishingRod();
goFishing();
}else
playWithCat();
Sinceactionscanbearbitrarystatements,theifstatementscanbenested.
Clickheretoviewcodeimage
if(temperature>=upperLimit){//(1)
if(danger)//(2)Simpleif.
soundAlarm();
if(critical)//(3)
evacuate();
else//Goeswithifat(3).
turnHeaterOff();
}else//Goeswithifat(1).
turnHeaterOn();
Theuseoftheblocknotation,{},canbecriticaltotheexecutionofifstatements.The
ifstatements(A)and(B)inthefollowingexamplesdonothavethesamemeaning.The
ifstatements(B)and(C)arethesame,withextraindentationusedin(C)tomakethe
meaningevident.Leavingouttheblocknotationinthiscasecouldhavecatastrophic
consequences:Theheatercouldbeturnedonwhenthetemperatureisabovetheupper
limit.
Clickheretoviewcodeimage
//(A):
if(temperature>upperLimit){//(1)Blocknotation.
if(danger)soundAlarm();//(2)
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}else//Goeswithifat(1).
turnHeaterOn();
//(B):
if(temperature>upperLimit)//(1)Withoutblocknotation.
if(danger)soundAlarm();//(2)
elseturnHeaterOn();//Goeswithifat(2).
//(C):
if(temperature>upperLimit)//(1)
if(danger)//(2)
soundAlarm();
else//Goeswithifat(2).
turnHeaterOn();
Theruleformatchinganelseclauseisthatanelseclausealwaysreferstothenearest
ifthatisnotalreadyassociatedwithanotherelseclause.Blocknotationandproper
indentationcanbeusedtomakethemeaningobvious.
Cascadingif-elsestatementscompriseasequenceofnestedif-elsestatements
wheretheifblockofthenextif-elsestatementisjoinedtotheelseclauseofthe
previousone.Thedecisiontoexecuteablockisthenbasedonalltheconditionsevaluated
sofar.
Clickheretoviewcodeimage
if(temperature>=upperLimit){//(1)
soundAlarm();
turnHeaterOff();
}elseif(temperature<lowerLimit){//(2)
soundAlarm();
turnHeaterOn();
}elseif(temperature==(upperLimit-lowerLimit)/2){//(3)
doingFine();
}else//(4)
noCauseToWorry();
Theblockcorrespondingtothefirstifconditionthatevaluatestotrueisexecuted,and
theremainingifstatementsareskipped.Intheprecedingexample,theblockat(3)will
executeonlyiftheconditionsat(1)and(2)arefalseandtheconditionat(3)istrue.
Ifnoneoftheconditionsistrue,theblockassociatedwiththelastelseclauseis
executed.Ifthereisnolastelseclause,noactionsareperformed.
The Statement
Conceptually,theswitchstatementcanbeusedtochooseoneamongmanyalternative
actions,basedonthevalueofanexpression.Itsgeneralformisasfollows:
switch(switch_expression){
caselabel1:statement1
caselabel2:statement2
…
caselabeln:statementn
default:statement
}//endswitch
Thesyntaxoftheswitchstatementcomprisesaswitchexpressionfollowedbythe
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switchbody,whichisablockofstatements.Theswitchexpressionmustevaluatetoa
valueofthefollowingtypes:
•Oneofthefollowingprimitivedatatypes:char,byte,short,orint
•Oneofthefollowingwrappertypes:Character,Byte,Short,orInteger
•Stringtype
•Anenumeratedtype
Notethatthetypeoftheswitchexpressioncannotbeboolean,long,orfloating-
point.Thestatementsintheswitchbodycanbelabeled,therebydefiningentrypointsin
theswitchbodywherecontrolcanbetransferreddependingonthevalueofthe
switchexpression.Theexecutionoftheswitchstatementisasfollows:
•Theswitchexpressionisevaluatedfirst.Ifthevalueisawrappertype,an
unboxingconversionisperformed.
•Thevalueoftheswitchexpressioniscomparedwiththecaselabels.Controlis
transferredtothestatementassociatedwiththecaselabelthatisequaltothevalue
oftheswitchexpression.Afterexecutionoftheassociatedstatement,controlfalls
throughtothenextstatementunlessthiswasthelaststatementdeclaredorcontrol
wastransferredoutoftheswitchstatement.
•Ifnocaselabelisequaltothevalueoftheswitchexpression,thestatement
associatedwiththedefaultlabelisexecuted.Afterexecutionoftheassociated
statement,controlfallsthroughtothenextstatementunlessthiswasthelast
statementdeclaredorcontrolwastransferredoutoftheswitchstatement.
Figure6.2illustratestheflowofcontrolthroughaswitchstatementwherethe
defaultlabelisdeclaredlast.
Figure6.2ActivityDiagramforaswitchStatement
Alllabels(includingthedefaultlabel)areoptional,andcanbedefinedinanyorderin
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theswitchbody.Atmostonedefaultlabelcanbepresentinaswitchstatement.If
novalidcaselabelsarefoundandthedefaultlabelisomitted,thewholeswitch
statementisskipped.
Thecaselabelsareconstantexpressionswhosevaluesmustbeunique,meaningno
duplicatevaluesareallowed.Infact,acaselabelmustbeacompile-timeconstant
expressionwhosevalueisassignabletothetypeoftheswitchexpression(§5.2,p.147).
Inparticular,allcaselabelvaluesmustbeintherangeofthetypeoftheswitch
expression.Thetypeofthecaselabelcannotbeboolean,long,orfloating-point.
Thecompilerisabletogenerateefficientcodeforaswitchstatement,asthisstatement
onlytestsforequalitybetweentheswitchexpressionandtheconstantvalueofthe
caselabels,soastodeterminewhichcodetoexecuteatruntime.Incontrast,asequence
ofifstatementsdeterminestheflowofcontrolatruntime,basedonarbitraryconditions
whosevaluemightbepossibletodetermineonlyatruntime.
InExample6.1,dependingonthevalueofthehowMuchAdviceparameter,different
adviceisprintedintheswitchstatementat(1)inthemethoddispenseAdvice().
TheexampleshowstheoutputwhenthevalueofthehowMuchAdviceparameteris
LOTS_OF_ADVICE.Intheswitchstatement,theassociatedstatementat(2)is
executed,givingonepieceofadvice.Controlthenfallsthroughtothestatementat(3),
givingthesecondadvice.Controlnextfallsthroughto(4),dispensingthethirdpieceof
advice,andfinallyexecutionofthebreakstatementat(5)causescontroltoexitthe
switchstatement.Withoutthebreakstatementat(5),controlwouldcontinuetofall
throughtheremainingstatements—inthiscase,tothestatementat(6)beingexecuted.
Executionofthebreakstatementinaswitchbodytransferscontroloutofthe
switchstatement(§6.4,p.221).IftheparameterhowMuchAdvicehasthevalue
MORE_ADVICE,thentheadviceatboth(3)and(4)isgiven.Thevalue
LITTLE_ADVICEresultsinonlyonepieceofadviceat(4)beinggiven.Anyothervalue
resultsinthedefaultaction,whichannouncesthatthereisnoadvice.
Theassociatedstatementofacaselabelcanbealistofstatements(whichneednotbea
statementblock).Thecaselabelisprefixedtothefirststatementineachcase.Thisis
illustratedbytheassociatedstatementforthecaselabelLITTLE_ADVICEinExample
6.1,whichcomprisesstatements(4)and(5).
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Example6.1Fall-ThroughinaswitchStatement
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publicclassAdvice{
privatestaticfinalintLITTLE_ADVICE=0;
privatestaticfinalintMORE_ADVICE=1;
privatestaticfinalintLOTS_OF_ADVICE=2;
publicstaticvoidmain(String[]args){
dispenseAdvice(LOTS_OF_ADVICE);
}
publicstaticvoiddispenseAdvice(inthowMuchAdvice){
switch(howMuchAdvice){//(1)
caseLOTS_OF_ADVICE:
System.out.println(“Seenoevil.”);//(2)
caseMORE_ADVICE:
System.out.println(“Speaknoevil.”);//(3)
caseLITTLE_ADVICE:
System.out.println(“Hearnoevil.”);//(4)
break;//(5)
default:
System.out.println(“Noadvice.”);//(6)
}
}
}
Outputfromtheprogram:
Seenoevil.
Speaknoevil.
Hearnoevil.
Example6.2makesuseofabreakstatementinsideaswitchstatementtoconverta
charvaluerepresentingadigittothecorrespondingwordinEnglish.Notethatthe
breakstatementisthelaststatementinthelistofstatementsassociatedwitheachcase
label.Itiseasytothinkthatthebreakstatementisapartoftheswitchstatement
syntax,buttechnicallyitisnot.
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Example6.2UsingbreakinaswitchStatement
Clickheretoviewcodeimage
publicclassDigits{
publicstaticvoidmain(String[]args){
System.out.println(digitToString(‘7’)+””+digitToString(‘8’)+””+
digitToString(‘6’));
System.out.println(digitToString(‘2’)+””+digitToString(‘a’)+””+
digitToString(‘5’));
}
publicstaticStringdigitToString(chardigit){
Stringstr=””;
switch(digit){
case‘1’:str=“one”;break;
case‘2’:str=“two”;break;
case‘3’:str=“three”;break;
case‘4’:str=“four”;break;
case‘5’:str=“five”;break;
case‘6’:str=“six”;break;
case‘7’:str=“seven”;break;
case‘8’:str=“eight”;break;
case‘9’:str=“nine”;break;
case‘0’:str=“zero”;break;
default:System.out.println(digit+”isnotadigit!”);
}
returnstr;
}
}
Outputfromtheprogram:
seveneightsix
aisnotadigit!
twofive
Severalcaselabelscanprefixthesamestatement.Theywillallresultintheassociated
statementbeingexecuted.ThisbehaviorisillustratedinExample6.3fortheswitch
statementat(1).
Thefirststatementintheswitchbodymusthaveacaseordefaultlabel,or
otherwiseitwillbeunreachable.Thisstatementwillneverbeexecuted,becausecontrol
canneverbetransferredtoit.Thecompilerwillflagthisasanerror.Anemptyswitch
blockisperfectlylegal,butnotofmuchuse.
Sinceeachactionassociatedwithacaselabelcanbeanarbitrarystatement,itcanbe
anotherswitchstatement.Inotherwords,switchstatementscanbenested.Sincea
switchstatementdefinesitsownlocalblock,thecaselabelsinaninnerblockdonot
conflictwithanycaselabelsinanouterblock.Labelscanberedefinedinnestedblocks;
incontrast,variablescannotberedeclaredinnestedblocks(§4.4,p.117).InExample6.3,
aninnerswitchstatementisdefinedat(2),whichallowsfurtherrefinementoftheaction
totakeonthevalueoftheswitchexpressionincaseswheremultiplelabelsareusedin
theouterswitchstatement.Abreakstatementterminatestheinnermostswitch
statementinwhichitisexecuted.
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Example6.3NestedswitchStatement
Clickheretoviewcodeimage
publicclassSeasons{
publicstaticvoidmain(String[]args){
intmonthNumber=11;
switch(monthNumber){//(1)Outer
case12:case1:case2:
System.out.println(“Snowinthewinter.”);
break;
case3:case4:case5:
System.out.println(“Greengrassinthespring.”);
break;
case6:case7:case8:
System.out.println(“Sunshineinthesummer.”);
break;
case9:case10:case11://(2)
switch(monthNumber){//Nestedswitch(3)Inner
case10:
System.out.println(“Halloween.”);
break;
case11:
System.out.println(“Thanksgiving.”);
break;
}//Endnestedswitch
//Alwaysprintedforcaselabels9,10,11
System.out.println(“Yellowleavesinthefall.”);//(4)
break;
default:
System.out.println(monthNumber+”isnotavalidmonth.”);
}
}
}
Outputfromtheprogram:
Thanksgiving.
Yellowleavesinthefall.
Example6.4illustratesusingstringsinaswitchstatement.Thethingtonoteiswhat
constitutesaconstantstringexpressionthatcanbeusedasacaselabel.Thecaselabels
in(3),(4),(5),and(6)areallvalidconstantstringexpressions,asthecompilercanfigure
outtheirvaluesatcompiletime.Stringliterals,usedin(3)and(6),andconstantfield
values,declaredin(1)and(2a),andusedin(4)and(5),areallvalidcaselabels.In
contrast,theHOTreferencefromdeclarations(2b)and(2c)cannotbeusedasacase
label.Fromthedeclarationin(2a),thecompilercannotguaranteethatthevalueofthe
referencewillnotchangeatruntime.Fromthedeclarationin(2c),itcannotdeducethe
valueatcompiletime,astheconstructormustberuntoconstructthevalue.
Switchingonstringsisessentiallybasedonequalitycomparisonofintegervaluesthatare
hashvaluesofstrings,followedbyanobjectequalitytesttoruleoutthepossibilityof
collisionbetweentwodifferentstringshavingthesamehashvalue.Switchingonstrings
shouldbeusedjudiciously,asitislessefficientthanswitchingonintegers.Switchingon
stringsisnotadvisableifthevaluesbeingswitchedonarenotalreadystrings.
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Example6.4StringsinswitchStatement
Clickheretoviewcodeimage
publicclassSwitchingOnAString{
publicstaticfinalStringMEDIUM=“Medium”;//(1)
publicstaticfinalStringHOT=“Hot”;//(2a)
//publicstaticStringHOT=“Hot”;//(2b)NotOKascase
lablel
//publicstaticfinalStringHOT=newString(“Hot”);//(2c)NotOKascase
lablel
publicstaticvoidmain(String[]args){
StringspiceLevel=“Medium_Hot”;
switch(spiceLevel){
case“Mild”://(3)
caseMEDIUM+“_”+HOT://(4)
System.out.println(“Enjoyyourmeal!”);
break;
caseHOT://(5)
System.out.println(“Havefun!”);
break;
case“Suicide”://(6)
System.out.println(“Goodluck!”);
break;
default:
System.out.println(“Youbeingfunny?”);
}
}
}
Outputfromtheprogram:
Enjoyyourmeal!
Example6.5illustratestheuseofenumtypesinaswitchstatement.Theenumtype
SpiceGradeisdefinedat(1).ThetypeoftheswitchexpressionisSpiceGrade.
Notethattheenumconstantsarenotspecifiedwiththeirfullyqualifiedname(see(2a)).
Usingthefullyqualifiednameresultsinacompile-timeerror,asshownat(2b).Only
enumconstantsthathavethesameenumtypeastheswitchexpressioncanbespecified
ascaselabelvalues.
Thesemanticsoftheswitchstatementarethesameasdescribedearlier.However,ifa
switchexpressionevaluatestothenullreference,aNullPointerExceptionwill
bethrown.Switchingonenumeratedvaluesisessentiallybasedonequalitycomparisonof
uniqueintegervaluesthatareordinalvaluesassignedbythecompilertotheconstantsof
anenumtype.
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Example6.5EnumsinswitchStatement
Clickheretoviewcodeimage
enumSpiceGrade{//(1)
MILD,MEDIUM_HOT,HOT,SUICIDE;
}
publicclassSwitchingFun{
publicstaticvoidmain(String[]args){
SpiceGradespicing=SpiceGrade.HOT;
switch(spicing){
caseHOT://(2a)OK!
//caseSpiceGrade.HOT://(2b)Compile-timeerror!
System.out.println(“Havefun!”);
break;
caseSUICIDE:
System.out.println(“Goodluck!”);
break;
default://CanonlybeMILDor
MEDIUM_HOT.
System.out.println(“Enjoyyoumeal!”);
}
}
}
Outputfromtheprogram:
Havefun!
ReviewQuestions
6.1Whatwillbetheresultofattemptingtocompileandrunthefollowingclass?
Clickheretoviewcodeimage
publicclassIfTest{
publicstaticvoidmain(String[]args){
if(true)
if(false)
System.out.println(“a”);
else
System.out.println(“b”);
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecausethesyntaxoftheifstatementis
incorrect.
(b)Thecodewillfailtocompilebecausethecompilerwillnotbeabletodetermine
whichifstatementtheelseclausebelongsto.
(c)Thecodewillcompilecorrectly,anddisplaytheletteraatruntime.
(d)Thecodewillcompilecorrectly,anddisplaytheletterbatruntime.
(e)Thecodewillcompilecorrectly,butwillnotdisplayanyoutput.
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6.2Whichofthefollowingstatementsaretrue?
Selectthethreecorrectanswers.
(a)Theconditioninanifstatementcanhavemethodcalls.
(b)IfaandbareoftypebooleanorBoolean,theexpression(a=b)can
betheconditionofanifstatement.
(c)Anifstatementcanhaveeitheranifclauseoranelseclause.
(d)Thestatementif(false);else;isillegal.
(e)Onlyexpressionsthatevaluateorcanbeunboxedtoabooleanvaluecanbe
usedastheconditioninanifstatement.
6.3What,ifanything,iswrongwiththefollowingcode?
voidtest(intx){
switch(x){
case1:
case2:
case0:
default:
case4:
}
}
Selecttheonecorrectanswer.
(a)Thevariablexdoesnothavetherighttypeforaswitchexpression.
(b)Thecaselabel0mustprecedethecaselabel1.
(c)Eachcasesectionmustendwithabreakstatement.
(d)Thedefaultlabelmustbethelastlabelintheswitchstatement.
(e)Thebodyoftheswitchstatementmustcontainatleastonestatement.
(f)Thereisnothingwrongwiththecode.
6.4Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassSwitching{
publicstaticvoidmain(String[]args){
finalintiLoc=3;
switch(6){
case1:
caseiLoc:
case2*iLoc:
System.out.println(“IamnotOK.”);
default:
System.out.println(“YouareOK.”);
case4:
System.out.println(“It’sOK.”);
}
}
}
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Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecauseofthecaselabelvalue2*iLoc.
(b)Thecodewillfailtocompilebecausethedefaultlabelisnotspecifiedlast
intheswitchstatement.
(c)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
IamnotOK.
YouareOK.
It’sOK.
(d)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
YouareOK.
It’sOK.
(e)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
It’sOK.
6.5Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassMoreSwitching{
publicstaticvoidmain(String[]args){
finalintiLoc=3;
IntegeriRef=5;
switch(iRef){
default:
System.out.println(“YouareOK.”);
case1:
caseiLoc:
case2*iLoc:
System.out.println(“IamnotOK.”);
break;
case4:
System.out.println(“It’sOK.”);
}
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecausethetypeoftheswitchexpressionisnot
valid.
(b)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
YouareOK.
IamnotOK.
(c)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
YouareOK.
IamnotOK.
It’sOK.
(d)Thecodewillcompilecorrectlyandwillprintthefollowingatruntime:
It’sOK.
6.6Whichcaselabeldeclarationcanbeinsertedat(1)sothatthefollowingprogram
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willcompile,run,andprintHi,TomTom!?
Clickheretoviewcodeimage
publicclassSwitcheroo{
publicstaticvoidmain(String[]args){
finalStringTOM1=“Tom”;
StringTOM2=“Tom”;
finalStringTOM3=newString(“Tom”);
switch(“TomTom”){
default:
System.out.println(“Whatever!”);
break;
//(1)INSERTcaseLABELDECLARATIONHERE.
System.out.println(“Hi,TomTom!”);
}
}
}
Selectthefourcorrectanswers.
(a)case"TomTom":
(b)caseTOM1+TOM1:
(c)caseTOM1+TOM2:
(d)caseTOM1+TOM3:
(e)caseTOM2+TOM3:
(f)case"Tom"+TOM1:
(g)case"Tom"+TOM2:
(h)case"Tom"+TOM3:
(i)case'T'+'o'+'m'+TOM1:
(j)case"T"+'o'+'m'+TOM1:
6.3IterationStatements
Loopsallowablockofstatementstobeexecutedrepeatedly(thatis,iterated).Aboolean
condition(calledtheloopcondition)iscommonlyusedtodeterminewhentoterminatethe
loop.Thestatementsexecutedintheloopconstitutetheloopbody.Theloopbodycanbea
singlestatementorablock.
Javaprovidesthreelanguageconstructsforloopconstruction:
•Thewhilestatement
•Thedo-whilestatement
•Thebasicforstatement
Theseloopsdifferintheorderinwhichtheyexecutetheloopbodyandtesttheloop
condition.Thewhileloopandthebasicforlooptesttheloopconditionbefore
executingtheloopbody,whereasthedo-whileloopteststheloopconditionafter
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executionoftheloopbody.
Inadditiontothebasicforloop,aspecializedloopcalledtheenhancedforloop(also
calledthefor-eachloop)simplifiesiteratingoverarraysandcollections.Wewillusethe
notationsfor(;;)andfor(:)todesignatethebasicforloopandtheenhancedfor
loop,respectively.
The Statement
Thesyntaxofthewhileloopis
while(loop_condition)
loop_body
Theloopconditionisevaluatedbeforeexecutingtheloopbody.Thewhilestatement
executestheloopbodyaslongastheloopconditionistrue.Whentheloopcondition
becomesfalse,theloopisterminatedandexecutioncontinueswiththestatement
immediatelyfollowingtheloop.Iftheloopconditionisfalsetobeginwith,theloop
bodyisnotexecutedatall.Inotherwords,awhileloopcanexecutezeroormoretimes.
TheloopconditionmustevaluatetoabooleanoraBooleanvalue.Inthelattercase,
thereferencevalueisunboxedtoabooleanvalue.Theflowofcontrolinawhile
statementisshowninFigure6.3.
Figure6.3ActivityDiagramforthewhileStatement
Thewhilestatementisnormallyusedwhenthenumberofiterationsisnotknown.
while(noSignOfLife())
keepLooking();
Sincetheloopbodycanbeanyvalidstatement,inadvertentlyterminatingeachlinewith
theemptystatement(;)cangiveunintendedresults.Alwaysusingablockstatementas
theloopbodyhelpstoavoidsuchproblems.
Clickheretoviewcodeimage
while(noSignOfLife());//Emptystatementasloopbody!
keepLooking();//Statementnotintheloopbody.
The Statement
Thesyntaxofthedo-whileloopis
do
loop_body
while(loop_condition);
Inado-whilestatement,theloopconditionisevaluatedafterexecutingtheloopbody.
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TheloopconditionmustevaluatetoabooleanorBooleanvalue.Thevalueofthe
loopconditionissubjectedtounboxingifitisofthetypeBoolean.Thedo-while
statementexecutestheloopbodyuntiltheloopconditionbecomesfalse.Whentheloop
conditionbecomesfalse,theloopisterminatedandexecutioncontinueswiththe
statementimmediatelyfollowingtheloop.Notethattheloopbodyisexecutedatleast
once.Figure6.4illustratestheflowofcontrolinado-whilestatement.
Figure6.4ActivityDiagramforthedo-whileStatement
Theloopbodyinado-whileloopisinvariablyastatementblock.Itisinstructiveto
comparethewhileanddo-whileloops.Intheexamplesthatfollow,themicemight
nevergettoplayifthecatisnotaway,asintheloopat(1).Themicedogettoplayat
leastonce(attheperiloflosingtheirlife)intheloopat(2).
Clickheretoviewcodeimage
while(cat.isAway()){//(1)
mice.play();
}
do{//(2)
mice.play();
}while(cat.isAway());
The Statement
Thefor(;;)loopisthemostgeneralofalltheloops.Itismostlyusedforcounter-
controlledloops,inwhichthenumberofiterationsisknownbeforehand.
Thesyntaxoftheloopisasfollows:
Clickheretoviewcodeimage
for(initialization;loop_condition;update_expression)
loop_body
Theinitializationusuallydeclaresandinitializesaloopvariablethatcontrolsthe
executionoftheloopbody.TheloopconditionmustevaluatetoabooleanorBoolean
value.Inthelattercase,thereferencevalueisconvertedtoabooleanvalueby
unboxing.Theloopconditionusuallyinvolvestheloopvariable,andiftheloopcondition
istrue,theloopbodyisexecuted;otherwise,executioncontinueswiththestatement
followingthefor(;;)loop.Aftereachiteration(thatis,executionoftheloopbody),
theupdateexpressionisexecuted.Thisusuallymodifiesthevalueoftheloopvariableto
ensureeventuallooptermination.Theloopconditionisthentestedtodeterminewhether
theloopbodyshouldbeexecutedagain.Notethattheinitializationisexecutedonlyonce,
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onentryintotheloop.Thesemanticsofthefor(;;)loopareillustratedinFigure6.5,
andaresummarizedbythefollowingequivalentwhileloopcodetemplate:
initialization
while(loop_condition){
loop_body
update_expression
}
Figure6.5ActivityDiagramfortheforStatement
Thefollowingcodecreatesanintarrayandsumsthevaluesinthearray:
Clickheretoviewcodeimage
intsum=0;
int[]array={12,23,5,7,19};
for(intindex=0;index<array.length;index++)//(1)
sum+=array[index];
Theloopvariableindexisdeclaredandinitializedintheinitializationsectionofthe
loop.Itisincrementedintheupdateexpressionsection.Thisloopisanexampleofa
forwardfor(;;)loop,wheretheloopvariableisincremented.
Thenextcodesnippetisanexampleofabackwardfor(;;)loop,wheretheloop
variableisdecrementedtosumthevaluesinthearray:
Clickheretoviewcodeimage
intsum=0;
int[]array={12,23,5,7,19};
for(intindex=array.length-1;index>=0;index—)
sum+=array[index];
Itisinstructivetocomparethespecificationoftheloopheaderintheforward
andbackwardfor(;;)loopsintheseexamples.
Thefor(;;)loopdefinesalocalblocksothatthescopeofthisdeclarationisthe
for(;;)block,whichcomprisestheinitialization,theloopcondition,theloopbody,
andtheupdateexpressionsections.Anyvariabledeclaredinthefor(;;)block,
therefore,isnotaccessibleafterthefor(;;)loopterminates.Theloopat(1)earlier
showedhowadeclarationstatementcanbespecifiedintheinitializationsection.Sucha
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declarationstatementcanalsospecifyacomma-separatedlistofvariables:
Clickheretoviewcodeimage
for(inti=0,j=1,k=2;…;…)…;//(2)
Thevariablesi,j,andkinthedeclarationstatementallhavetypeint.Allvariables
declaredintheinitializationsectionarelocalvariablesinthefor(;;)blockandobey
thescoperulesforlocalblocks.Thefollowingcodewillnotcompile,however,asvariable
declarationsofdifferenttypes(inthiscase,intandString)requiredeclaration
statementsthatareterminatedbysemicolons:
Clickheretoviewcodeimage
for(inti=0,Stringstr=“@”;…;…)…;//(3)Compile-timeerror
Theinitializationsectioncanalsobeacomma-separatedlistofexpressionstatements
(§3.2,p.50).Anyvaluereturnedbyanexpressionstatementisdiscarded.Forexample,
theloopat(2)canberewrittenbyfactoringoutthevariabledeclaration:
Clickheretoviewcodeimage
inti,j,k;//Variabledeclaration
for(i=0,j=1,k=2;…;…)…;//(4)Onlyinitialization
Theinitializationsectionisnowacomma-separatedlistofthreeexpressions.The
expressionsinsuchalistarealwaysevaluatedfromlefttoright,andtheirvaluesare
discarded.Notethatthevariablesi,j,andkat(4)arenotlocaltotheloop.
Declarationstatementscannotbemixedwithexpressionstatementsintheinitialization
section,asisthecaseat(5)inthefollowingexample.Factoringoutthevariable
declaration,asat(6),leavesalegalcomma-separatedlistofexpressionstatements.
Clickheretoviewcodeimage
//(5)Notlegalandugly:
for(inti=0,System.out.println(“Thiswon’tdo!”);flag;i++){//Error!
//loopbody
}
//(6)Legal,butstillugly:
inti;//Declarationfactoredout.
for(i=0,System.out.println(“Thisislegal!”);flag;i++){//OK.
//loopbody
}
Theupdateexpressioncanalsobeacomma-separatedlistofexpressionstatements.The
followingcodespecifiesafor(;;)loopthathasacomma-separatedlistofthree
variablesintheinitializationsection,andacomma-separatedlistoftwoexpressionsinthe
updateexpressionsection:
Clickheretoviewcodeimage
//Legalusagebutnotrecommended.
int[][]sqMatrix={{3,4,6},{5,7,4},{5,8,9}};
for(inti=0,j=sqMatrix[0].length-1,asymDiagonal=0;//
initialization
i<sqMatrix.length;//loop
condition
i++,j—)//updateexpression
asymDiagonal+=sqMatrix[i][j];//loopbody
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Allsectionsinthefor(;;)headerareoptional.Anyorallofthemcanbeleftempty,
butthetwosemicolonsaremandatory.Inparticular,leavingouttheloopcondition
signifiesthattheloopconditionistrue.The“crab”,(;;),canbeusedtoconstructan
infiniteloop,whereterminationispresumablyachievedthroughcodeintheloopbody
(seethenextsectionontransferstatements):
Clickheretoviewcodeimage
for(;;)doProgramming();//Infiniteloop
The Statement
Theenhancedforloopisconvenientwhenweneedtoiterateoveranarrayora
collection,especiallywhensomeoperationneedstobeperformedoneachelementofthe
arrayorcollection.Inthissectionwediscussiteratingoverarrays;in§10.1,p.423,we
takealookatthefor(:)loopforiteratingoverArrayLists.
Earlierinthischapterweusedafor(;;)looptosumthevaluesofelementsinanint
array:
Clickheretoviewcodeimage
intsum=0;
int[]intArray={12,23,5,7,19};
for(intindex=0;index<intArray.length;index++){//(1)usingfor(;;)
loop
sum+=intArray[index];
}
Thefor(;;)loopat(1)isrewrittenusingthefor(:)loopinFigure6.6.
Figure6.6EnhancedforStatement
Thebodyoftheloopisexecutedforeachelementinthearray,wherethevariable
elementsuccessivelydenotesthecurrentelementinthearrayintArray.Whenthe
loopterminates,thevariablesumwillcontainthesumofallelementsinthearray.Wedo
notcareaboutthepositionoftheelementsinthearray,justthattheloopiteratesoverall
elementsofthearray.
FromFigure6.6weseethatthefor(:)loopheaderhastwoparts.Theexpressionmust
evaluatetoareferencevaluethatreferstoanarray—thatis,thearraywewanttoiterate
over.Thearraycanbeanarrayofprimitivevaluesorobjects,orevenanarrayofarrays.
Theexpressionisevaluatedonlyonce.Theelementdeclarationspecifiesalocalvariable
thatcanbeassignedavalueoftheelementtypeofthearray.Thetypeofthearray
intArrayinFigure6.6isint[],andtheelementtypeisint.Theelementvariableof
typeintcanbeassignedintvaluesfromthearrayofint.However,thisassignment
mightrequireeitheraboxingoranunboxingconversion,withoptionalwidening
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conversion.
Theelementvariableislocaltotheloopblockandisnotaccessibleaftertheloop
terminates.Also,changingthevalueofthecurrentvariabledoesnotchangeanyvaluein
thearray.Theloopbody,whichcanbeasimplestatementorastatementblock,is
executedforeachelementinthearrayandthereisnodangerofanyout-of-boundserrors.
Thefor(:)loophasitslimitations.Specifically,wecannotchangeelementvalues,and
thiskindofloopdoesnotprovideanyprovisionforpositionalaccessusinganindex.The
for(:)looponlyincrementsbyoneandalwaysinaforwarddirection.Itdoesnotallow
iterationsoverseveralarrayssimultaneously.Undersuchcircumstances,thefor(;;)
loopcanbemoreconvenient.
Herearesomecodeexamplesoflegalfor(:)loops:
Clickheretoviewcodeimage
//Some1-dimarrays:
int[]intArray={10,20,30};
Integer[]intObjArray={10,20,30};
String[]strArray={“one”,“two”};
//Some2-dimarrays:
Object[][]objArrayOfArrays={intObjArray,strArray};
Number[][]numArrayOfArrays={{1.5,2.5},intObjArray,{100L,200L}};
int[][]intArrayOfArrays={{20},intArray,{40}};
//IterateoveranarrayofStrings.
//ExpressiontypeisString[],andelementtypeisString.
//StringisassignabletoObject(wideningconversion).
for(Objectobj:strArray){}
//Iterateoveranarrayofints.
//Expressiontypeisint[],andelementtypeisint.
//intisassignabletoInteger(boxingconversion)
for(IntegeriRef:intArrayOfArrays[0]){}
//IterateoveranarrayofIntegers.
//ExpressiontypeisInteger[],andelementtypeisInteger.
//Integerisassignabletoint(unboxingconversion)
for(inti:intObjArray){}
//Iterateovera2-dimarrayofints.
//Outerloop:expressiontypeisint[][],andelementtypeisint[].
//Innerloop:expressiontypeisint[],andelementtypeisint.
for(int[]row:intArrayOfArrays)
for(intval:row){}
//Iterateovera2-dimarrayofNumbers.
//Outerloop:expressiontypeisNumber[][],andelementtypeisNumber[].
//Outerloop:Number[]isassignabletoObject[](wideningconversion).
//Innerloop:expressiontypeisObject[],andelementtypeisObject.
for(Object[]row:numArrayOfArrays)
for(Objectobj:row){}
//Outerloop:expressiontypeisInteger[][],andelementtypeisInteger[].
//Outerloop:Integer[]isassignabletoNumber[].
//Innerloop:expressiontypeisint[],andelementtypeisint.
//Innerloop:intisassignabletodouble.
for(Number[]row:newInteger[][]{intObjArray,intObjArray,intObjArray})
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for(doublenum:newint[]{}){}
Herearesomecodeexamplesoffor(:)loopsthatarenotlegal:
Clickheretoviewcodeimage
//ExpressiontypeisNumber[][],andelementtypeisNumber[].
//Number[]isnotassignabletoNumber.
for(Numbernum:numArrayOfArrays){}//Compile-timeerror.
//ExpressiontypeisNumber[],andelementtypeisNumber.
//Numberisnotassignabletoint.
for(introw:numArrayOfArrays[0]){}//Compile-timeerror.
//Outerloop:expressiontypeisint[][],andelementtypeisint[].
//int[]isnotassignabletoInteger[].
for(Integer[]row:intArrayOfArrays)//Compile-timeerror.
for(intval:row){}
//ExpressiontypeisObject[][],andelementtypeisObject[].
//Object[]isnotassignabletoInteger[].
for(Integer[]row:objArrayOfArrays){}//Compile-timeerror.
//Outerloop:expressiontypeisString[],andelementtypeisString.
//Innerloop:expressiontypeisString,whichisnotlegalhere.Notan
array.
for(Stringstr:strArray)
for(charval:str){}//Compile-timeerror.
Whenusingthefor(:)looptoiterateoveranarray,thetwomaincausesoferrorsarean
expressionintheloopheaderthatdoesnotrepresentanarrayand/oranelementtypeof
thearraythatisnotassignabletothelocalvariabledeclaredintheloopheader.
6.4TransferStatements
Javaprovidessixlanguageconstructsfortransferringcontrolinaprogram:
•break
•continue
•return
•try-catch-finally
•throw
•assert
Thissectiondiscussesthefirstthreestatements.Exceptfortheassertstatement(noton
theOCAJP8exam),theremainingstatementsarediscussedinsubsequentsections.
NotethatJavadoesnothaveagotostatement,althoughgotoisareservedword.
LabeledStatements
Astatementmayhavealabel:
label:statement
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Alabelisanyvalididentifier;italwaysimmediatelyprecedesthestatement.Labelnames
existintheirownnamespace,sothattheydonotconflictwithnamesofpackages,classes,
interfaces,methods,fields,andlocalvariables.Thescopeofalabelisthestatement
prefixedbythelabel,meaningthatitcannotberedeclaredasalabelinsidethelabeled
statement—analogoustothescopeoflocalvariables.
Clickheretoviewcodeimage
L1:if(i>0){
L1:System.out.println(i);//(1)NotOK.Labelredeclared.
}
L1:while(i<0){//(2)OK.
L2:System.out.println(i);
}
L1:{//(3)OK.Labeledblock.
intj=10;
System.out.println(j);
}
L1:try{//(4)OK.Labeledtry-catch-finallyblock.
intj=10,k=0;
L2:System.out.println(j/k);
}catch(ArithmeticExceptionae){
L3:ae.printStackTrace();
}finally{
L4:System.out.println(“Finallydone.”);
}
Astatementcanhavemultiplelabels:
Clickheretoviewcodeimage
LabelA:LabelB:System.out.println(“Mutliplelabels.Usejudiciously.”);
Adeclarationstatementcannothavealabel:
Clickheretoviewcodeimage
L0:inti=0;//Compile-timeerror!
Alabeledstatementisexecutedasifitwasunlabeled,unlessitisthebreakor
continuestatement.Thisbehaviorisdiscussedinthenexttwosubsections.
The Statement
Thebreakstatementcomesintwoforms:unlabeledandlabeled.
Clickheretoviewcodeimage
break;//theunlabeledform
breaklabel;//thelabeledform
Theunlabeledbreakstatementterminatesloops(for(;;),for(:),while,do-
while)andswitchstatements,andtransferscontroloutofthecurrentcontext(i.e.,the
closestenclosingblock).Therestofthestatementbodyisskipped,andexecution
continuesaftertheenclosingstatement.
InExample6.6,thebreakstatementat(1)isusedtoterminateafor(;;)loop.Control
istransferredto(2)whenthevalueofiisequalto4at(1),skippingtherestoftheloop
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bodyandterminatingtheloop.
Example6.6alsoshowsthattheunlabeledbreakstatementterminatesonlythe
innermostlooporswitchstatementthatcontainsthebreakstatement.Thebreak
statementat(3)terminatestheinnerfor(;;)loopwhenjisequalto2,andexecution
continuesintheouterswitchstatementat(4)afterthefor(;;)loop.
Example6.6ThebreakStatement
Clickheretoviewcodeimage
classBreakOut{
publicstaticvoidmain(String[]args){
System.out.println(“isqrt(i)”);
for(inti=1;i<=5;++i){
if(i==4)
break;//(1)Terminateloop.Controlto
(2).
//Restofloopbodyskippedwhenigetsthevalue4.
System.out.printf(“%d%.2f%n”,i,Math.sqrt(i));
}//endfor
//(2)Continuehere.
intn=2;
switch(n){
case1:
System.out.println(n);
break;
case2:
System.out.println(“Innerfor(;;)loop:“);
for(intj=0;j<=n;j++){
if(j==2)
break;//(3)Terminateloop.Controlto
(4).
System.out.println(“j=”+j);
}
default:
System.out.println(“default:n=”+n);//(4)Continuehere.
}
}
}
Outputfromtheprogram:
isqrt(i)
11.00
21.41
31.73
Innerfor(;;)loop:
j=0
j=1
default:n=2
Alabeledbreakstatementcanbeusedtoterminateanylabeledstatementthatcontains
thebreakstatement.Controlisthentransferredtothestatementfollowingtheenclosing
labeledstatement.Inthecaseofalabeledblock,therestoftheblockisskippedand
executioncontinueswiththestatementfollowingtheblock:
Clickheretoviewcodeimage
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out://Label.
{//(1)Labeledblock.
//…
if(j==10)breakout;//(2)Terminateblock.Controlto(3).
System.out.println(j);//Restoftheblocknotexecutedifj==10.
//…
}
//(3)Continuehere.
InExample6.7,theprogramcontinuestoaddtheelementsbelowthediagonalofasquare
matrixuntilthesumisgreaterthan10.Twonestedforloopsaredefinedat(1)and(2).
Theouterloopislabeledouterat(1).Theunlabeledbreakstatementat(3)transfers
controlto(5)whenitisexecuted;thatis,itterminatestheinnerloopandcontrolis
transferredtothestatementaftertheinnerloop.Thelabeledbreakstatementat(4)
transferscontrolto(6)whenitisexecuted;thatis,itterminatesboththeinnerandthe
outerloop,transferringcontroltothestatementafterthelooplabeledouter.
Example6.7LabeledbreakStatement
Clickheretoviewcodeimage
classLabeledBreakOut{
publicstaticvoidmain(String[]args){
int[][]squareMatrix={{4,3,5},{2,1,6},{9,7,8}};
intsum=0;
outer:for(inti=0;i<squareMatrix.length;++i){//(1)label
for(intj=0;j<squareMatrix[i].length;++j){//(2)
if(j==i)break;//(3)Terminateinnerloop.Controlto
(5).
System.out.println(“Element[”+i+“,”+j+“]:”+
squareMatrix[i][j]);
sum+=squareMatrix[i][j];
if(sum>10)breakouter;//(4)Terminatebothloops.Controlto
(6).
}//endinnerloop
//(5)Continuewithupdateexpressionintheouterloopheader.
}//endouterloop
//(6)Continuehere.
System.out.println(“sum:”+sum);
}
}
Outputfromtheprogram:
Element[1,0]:2
Element[2,0]:9
sum:11
The Statement
Likethebreakstatement,thecontinuestatementcomesintwoforms:unlabeledand
labeled.
Clickheretoviewcodeimage
continue;//theunlabeledform
continuelabel;//thelabeledform
Thecontinuestatementcanbeusedonlyinafor(;;),for(:),while,ordo-
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whilelooptoprematurelystopthecurrentiterationoftheloopbodyandproceedwith
thenextiteration,ifpossible.Inthecaseofthewhileanddo-whileloops,therestof
theloopbodyisskipped—thatis,thecurrentiterationisstopped,withexecution
continuingwiththeloopcondition.Inthecaseofthefor(;;)loop,therestoftheloop
bodyisskipped,withexecutioncontinuingwiththeupdateexpression.
InExample6.8,anunlabeledcontinuestatementisusedtoskipaniterationina
for(;;)loop.Controlistransferredto(2)whenthevalueofiisequalto4at(1),
skippingtherestoftheloopbodyandcontinuingwiththeupdateexpressioninthe
for(;;)statement.
Example6.8continueStatement
Clickheretoviewcodeimage
classSkip{
publicstaticvoidmain(String[]args){
System.out.println(“isqrt(i)”);
for(inti=1;i<=5;++i){
if(i==4)continue;//(1)Controlto(2).
//Restofloopbodyskippedwhenihasthevalue4.
System.out.printf(“%d%.2f%n”,i,Math.sqrt(i));
//(2)Continuewithupdateexpressionintheloopheader.
}//endfor
}
}
Outputfromtheprogram:
isqrt(i)
11.00
21.41
31.73
52.24
Alabeledcontinuestatementmustoccurwithinalabeledloopthathasthesamelabel.
Executionofthelabeledcontinuestatementthentransferscontroltotheendofthat
enclosinglabeledloop.InExample6.9,theunlabeledcontinuestatementat(3)
transferscontrolto(5)whenitisexecuted;thatis,therestoftheloopbodyisskippedand
executioncontinueswiththeupdateexpressionintheinnerloop.Thelabeledcontinue
statementat(4)transferscontrolto(6)whenitisexecuted;thatis,itterminatestheinner
loopbutexecutioncontinueswiththeupdateexpressioninthelooplabeledouter.Itis
instructivetocomparetheoutputfromExample6.7(labeledbreak)andthatfrom
Example6.9(labeledcontinue).
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Example6.9LabeledcontinueStatement
Clickheretoviewcodeimage
classLabeledSkip{
publicstaticvoidmain(String[]args){
int[][]squareMatrix={{4,3,5},{2,1,6},{9,7,8}};
intsum=0;
outer:for(inti=0;i<squareMatrix.length;++i){//(1)label
for(intj=0;j<squareMatrix[i].length;++j){//(2)
if(j==i)continue;//(3)Controlto
(5).
System.out.println(“Element[”+i+“,”+j+“]:”+
squareMatrix[i][j]);
sum+=squareMatrix[i][j];
if(sum>10)continueouter;//(4)Controlto
(6).
//(5)Continuewithupdateexpressionintheinnerloopheader.
}//endinnerloop
//(6)Continuewithupdateexpressionintheouterloopheader.
}//endouterloop
System.out.println(“sum:”+sum);
}
}
Outputfromtheprogram:
Element[0,1]:3
Element[0,2]:5
Element[1,0]:2
Element[1,2]:6
Element[2,0]:9
sum:25
The Statement
Thereturnstatementisusedtostopexecutionofamethod(oraconstructor)and
transfercontrolbacktothecallingcode(alsocalledthecallerorinvoker).Theusageof
thetwoformsofthereturnstatementisdictatedbywhetherthatstatementisusedina
voidoranon-voidmethod(Table6.1).Thefirstformdoesnotreturnanyvaluetothe
callingcode,butthesecondformdoes.Notethatthekeywordvoiddoesnotrepresent
anytype.
Table6.1ThereturnStatement
InTable6.1,theexpressionmustevaluatetoaprimitivevalueorareferencevalue,andits
typemustbeassignabletothereturntypespecifiedinthemethodheader(§5.6,p.158,
and§7.9,p.312).Seealsothediscussiononcovariantreturninconnectionwithmethod
overridingin§7.2,p.268.
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AscanbeseenfromTable6.1,avoidmethodneednothaveareturnstatement—in
whichcasethecontroltypicallyreturnstothecallerafterthelaststatementinthe
method’sbodyhasbeenexecuted.However,avoidmethodcanspecifyonlythefirst
formofthereturnstatement.Thisformofthereturnstatementcanalsobeusedin
constructors,astheylikewisedonotreturnavalue.
Table6.1alsoshowsthatthefirstformofthereturnstatementisnotallowedinanon-
voidmethod.Thesecondformofthereturnstatementismandatoryinanon-void
method,ifthemethodexecutionisnotterminatedprogrammatically—forexample,by
throwinganexception.Example6.10illustratestheuseofthereturnstatement
summarizedinTable6.1.Arecommendedbestpracticeistodocumentthevaluereturned
byamethodinaJavadoccommentusingthe@returntag.
Example6.10ThereturnStatement
Clickheretoviewcodeimage
publicclassReturnDemo{
publicstaticvoidmain(String[]args){//(1)voidmethodcanuse
return.
if(args.length==0)return;
output(checkValue(args.length));
}
staticvoidoutput(intvalue){//(2)voidmethodneednotusereturn.
System.out.println(value);
return‘a’;//NotOK.Cannotreturnavalue.
}
staticintcheckValue(inti){//(3)Non-voidmethod:Anyreturn
statement
//mustreturnavalue.
if(i>3)
returni;//OK.
else
return2.0;//NotOK.doublenotassignabletoint.
}
staticintabsentMinded(){//(4)Non-voidmethod.
thrownewRuntimeException();//OK:Noreturnstatementprovided,but
//methodterminatesbythrowingan
exception.
}
}
ReviewQuestions
6.7Whatwillbetheresultofattemptingtocompileandrunthefollowingcode?
Clickheretoviewcodeimage
classMyClass{
publicstaticvoidmain(String[]args){
booleanb=false;
inti=1;
do{
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i++;
b=!b;
}while(b);
System.out.println(i);
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecausebisaninvalidconditionforthedo-
whilestatement.
(b)Thecodewillfailtocompilebecausetheassignmentb=!bisnotallowed.
(c)Thecodewillcompilewithouterror,andwillprint1atruntime.
(d)Thecodewillcompilewithouterror,andwillprint2atruntime.
(e)Thecodewillcompilewithouterror,andwillprint3atruntime.
6.8Whatwillbetheoutputwhenrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassStillMyClass{
publicstaticvoidmain(String[]args){
inti=0;
intj;
for(j=0;j<10;++j){i++;}
System.out.println(i+””+j);
}
}
Selectthetwocorrectanswers.
(a)Thefirstnumberprintedwillbe9.
(b)Thefirstnumberprintedwillbe10.
(c)Thefirstnumberprintedwillbe11.
(d)Thesecondnumberprintedwillbe9.
(e)Thesecondnumberprintedwillbe10.
(f)Thesecondnumberprintedwillbe11.
6.9Whichofthefollowingforstatementsisvalid?
Selecttheonecorrectanswer.
(a)intj=10;for(inti=0,j+=90;i<j;i++){j-
-;}
(b)for(inti=10;i=0;i--){}
(c)for(inti=0,j=100;i<j;i++,--j){;}
(d)inti,j;for(j=100;i<j;j--){i+=2;}
(e)inti=100;for((i>0);i--){}
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6.10Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
classAnotherClass{
publicstaticvoidmain(String[]args){
inti=0;
for(;i<10;i++);//(1)
for(i=0;;i++)break;//(2)
for(i=0;i<10;)i++;//(3)
for(;;);//(4)
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecauseoferrorsintheforloopat(1).
(b)Thecodewillfailtocompilebecauseoferrorsintheforloopat(2).
(c)Thecodewillfailtocompilebecauseoferrorsintheforloopat(3).
(d)Thecodewillfailtocompilebecauseoferrorsintheforloopat(4).
(e)Thecodewillcompilewithouterror,andtheprogramwillrunandterminate
withoutanyoutput.
(f)Thecodewillcompilewithouterror,butwillneverterminatewhenrun.
6.11Whichofthefollowingstatementsarevalidwhenoccurringontheirown?
Selectthethreecorrectanswers.
(a)while()break;
(b)do{break;}while(true);
(c)if(true){break;}
(d)switch(1){default:break;}
(e)for(;true;)break;
6.12Giventhefollowingcodefragment,whichofthefollowinglineswillbeapartof
theoutput?
Clickheretoviewcodeimage
outer:
for(inti=0;i<3;i++){
for(intj=0;j<2;j++){
if(i==j){
continueouter;
}
System.out.println(“i=”+i+“,j=”+j);
}
}
Selectthetwocorrectanswers.
(a)i=1,j=0
(b)i=0,j=1
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(c)i=1,j=2
(d)i=2,j=1
(e)i=2,j=2
(f)i=3,j=3
(g)i=3,j=2
6.13Whatwillbetheresultofattemptingtocompileandrunthefollowingcode?
Clickheretoviewcodeimage
classMyClass{
publicstaticvoidmain(String[]args){
for(inti=0;i<10;i++){
switch(i){
case0:
System.out.println(i);
}
if(i){
System.out.println(i);
}
}
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecauseofanillegalswitchexpressioninthe
switchstatement.
(b)Thecodewillfailtocompilebecauseofanillegalconditionintheif
statement.
(c)Thecodewillcompilewithouterror,andwillprintthenumbers0through10
atruntime.
(d)Thecodewillcompilewithouterror,andwillprintthenumber0atruntime.
(e)Thecodewillcompilewithouterror,andwillprintthenumber0twiceat
runtime.
(f)Thecodewillcompilewithouterror,andwillprintthenumbers1through10at
runtime.
6.14Whichdeclarations,wheninsertedat(1),willresultintheprogramcompilingand
printing90atruntime?
Clickheretoviewcodeimage
publicclassRQ400A10{
publicstaticvoidmain(String[]args){
//(1)INSERTDECLARATIONHERE
intsum=0;
for(inti:nums)
sum+=i;
System.out.println(sum);
}
}
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Selectthetwocorrectanswers.
(a)Object[]nums={20,30,40};
(b)Number[]nums={20,30,40};
(c)Integer[]nums={20,30,40};
(d)int[]nums={20,30,40};
(e)Noneoftheabove
6.15Whichmethoddeclarations,wheninsertedat(1),willresultintheprogram
compilingandprinting90whenrun?
Clickheretoviewcodeimage
publicclassRQ400A30{
publicstaticvoidmain(String[]args){
doIt();
}
//(1)INSERTMETHODDECLARATIONHERE.
}
Selectthetwocorrectanswers.
(a)
publicstaticvoiddoIt(){
int[]nums={20,30,40};
for(intsum=0,i:nums)
sum+=i;
System.out.println(sum);
}
(b)
Clickheretoviewcodeimage
publicstaticvoiddoIt(){
for(intsum=0,i:{20,30,40})
sum+=i;
System.out.println(sum);
}
(c)
publicstaticvoiddoIt(){
intsum=0;
for(inti:{20,30,40})
sum+=i;
System.out.println(sum);
}
(d)
Clickheretoviewcodeimage
publicstaticvoiddoIt(){
intsum=0;
for(inti:newint[]{20,30,40})
sum+=i;
System.out.println(sum);
}
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(e)
publicstaticvoiddoIt(){
int[]nums={20,30,40};
intsum=0;
for(inti:nums)
sum+=i;
System.out.println(sum);
}
6.5Stack-BasedExecutionandExceptionPropagation
AnexceptioninJavasignalstheoccurrenceofanerrorsituationduetotheviolationof
somesemanticconstraintoftheJavaprogramminglanguageduringexecution—for
example,arequestedfilecannotbefound,anarrayindexisoutofbounds,oranetwork
linkfailed.Explicitchecksinthecodeforsuchsituationscaneasilyresultin
incomprehensiblecode.Javaprovidesanexceptionhandlingmechanismfor
systematicallydealingwithsucherrorsituations.
Theexceptionmechanismisbuiltaroundthethrow-and-catchparadigm.Tothrowan
exceptionistosignalthatanunexpectedconditionhasoccurred.Tocatchanexceptionis
totakeappropriateactiontodealwiththeexception.Anexceptioniscaughtbyan
exceptionhandler,andtheexceptionneednotbecaughtinthesamecontextinwhichit
wasthrown.Theruntimebehavioroftheprogramdetermineswhichexceptionsare
thrownandhowtheyarecaught.Thethrow-and-catchprincipleisembeddedinthetry-
catch-finallyconstruct.
SeveralthreadscanbeexecutingatthesametimeintheJVM.Eachthreadhasitsown
JVMstack(alsocalledaruntimestack,callstack,andinvocationstackintheliterature)
thatisusedtohandleexecutionofmethods.Eachelementonthestack(calledan
activationframeorastackframe)correspondstoamethodcall.Eachnewcallresultsina
newactivationframebeingpushedonthestack,whichstoresallthepertinentinformation
suchasthelocalvariables.Themethodwiththeactivationframeonthetopofthestackis
theonecurrentlyexecuting.Whenthismethodfinishesexecuting,itsactivationframeis
poppedfromthetopofthestack.Executionthencontinuesinthemethodcorrespondingto
theactivationframethatisnowuncoveredonthetopofthestack.Themethodsonthe
stackaresaidtobeactive,astheirexecutionhasnotcompleted.Atanygiventime,the
activemethodsonaJVMstackmakeupwhatiscalledthestacktraceofathread’s
execution.
Example6.11isasimpleprogramtoillustratemethodexecution.Itcalculatestheaverage
foralistofintegers,giventhesumofalltheintegersandthenumberofintegers.Ituses
threemethods:
•Themethodmain()callsthemethodprintAverage()withparametersgiving
thetotalsumoftheintegersandthetotalnumberofintegers,(1).
•ThemethodprintAverage()inturncallsthemethodcomputeAverage(),
(3).
•ThemethodcomputeAverage()usesintegerdivisiontocalculatetheaverage
andreturnstheresult,(7).
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Example6.11MethodExecution
Clickheretoviewcodeimage
publicclassAverage1{
publicstaticvoidmain(String[]args){
printAverage(100,20);//(1)
System.out.println(“Exitmain().”);//(2)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
intaverage=computeAverage(totalSum,totalNumber);//(3)
System.out.println(“Average=”+//(4)
totalSum+”/”+totalNumber+”=”+average);
System.out.println(“ExitprintAverage().”);//(5)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(6)
returnsum/number;//(7)
}
}
Outputofprogramexecution:
Computingaverage.
Average=100/20=5
ExitprintAverage().
Exitmain().
ExecutionofExample6.11isillustratedinFigure6.7.Eachmethodexecutionisshownas
aboxwiththelocalvariablesdeclaredinthemethod.Theheightoftheboxindicateshow
longamethodisactive.BeforethecalltothemethodSystem.out.println()at(6)
inFigure6.7,thestacktracecomprisesthethreeactivemethods:main(),
printAverage(),andcomputeAverage().Theresult5fromthemethod
computeAverage()isreturnedat(7)inFigure6.7.Theoutputfromtheprogram
correspondswiththesequenceofmethodcallsinFigure6.7.Astheprogramterminates
normally,thisprogrambehavioriscallednormalexecution.
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Figure6.7MethodExecution
Ifthemethodcallat(1)inExample6.11
Clickheretoviewcodeimage
printAverage(100,20);//(1)
isreplacedwith
Clickheretoviewcodeimage
printAverage(100,0);//(1)
andtheprogramisrunagain,theoutputisasfollows:
Clickheretoviewcodeimage
Computingaverage.
Exceptioninthread“main”java.lang.ArithmeticException:/byzero
atAverage1.computeAverage(Average1.java:18)
atAverage1.printAverage(Average1.java:10)
atAverage1.main(Average1.java:5)
Figure6.8illustratestheprogramexecutionwhenthemethodprintAverage()is
calledwiththearguments100and0at(1).Allgoeswelluntilthereturnstatementat
(7)inthemethodcomputeAverage()isexecuted.Anerrorconditionoccursin
calculatingtheexpressionsum/number,becauseintegerdivisionby0isanillegal
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operation.ThiserrorconditionissignaledbytheJVMbythrowinganArithmetic-
Exception(§6.6,p.233).ThisexceptionispropagatedbytheJVMthroughtheJVM
stackasexplainednext.
Figure6.8ExceptionPropagation
Figure6.8illustratesthecasewhereanexceptionisthrownandtheprogramdoesnottake
anyexplicitactiontodealwiththeexception.InFigure6.8,executionofthe
computeAverage()methodissuspendedatthepointwheretheexceptionisthrown.
Theexecutionofthereturnstatementat(7)nevergetscompleted.Sincethismethod
doesnothaveanycodetodealwiththeexception,itsexecutionislikewiseterminated
abruptlyanditsactivationframepopped.Wesaythatthemethodcompletesabruptly.The
exceptionisthenofferedtothemethodwhoseactivationisnowonthetopofthestack
(methodprintAverage()).Thismethoddoesnothaveanycodetodealwiththe
exceptioneither,soitsexecutioncompletesabruptly.Thestatementsat(4)and(5)inthe
methodprintAverage()nevergetexecuted.Theexceptionnowpropagatestothelast
activemethod(methodmain()).Thisdoesnotdealwiththeexceptioneither.The
main()methodalsocompletesabruptly.Thestatementat(2)inthemain()method
nevergetsexecuted.Sincetheexceptionisnotcaughtbyanyoftheactivemethods,itis
dealtwithbythemainthread’sdefaultexceptionhandler.Thedefaultexceptionhandler
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usuallyprintsthenameoftheexception,withanexplanatorymessage,followedbya
printoutofthestacktraceatthetimetheexceptionwasthrown.Anuncaughtexception,as
inthiscase,resultsinthedeathofthethreadinwhichtheexceptionoccurred.
Ifanexceptionisthrownduringtheevaluationoftheleft-handoperandofabinary
expression,thentheright-handoperandisnotevaluated.Similarly,ifanexceptionis
thrownduringtheevaluationofalistofexpressions(e.g.,alistofactualparametersina
methodcall),evaluationoftherestofthelistisskipped.
Ifthelinenumbersinthestacktracearenotprintedintheoutputasshownpreviously,use
thefollowingcommandtoruntheprogram:
Clickheretoviewcodeimage
>java-Djava.compiler=NONEAverage1
6.6ExceptionTypes
ExceptionsinJavaareobjects.Allexceptionsarederivedfromthe
java.lang.Throwableclass.Figure6.9showsapartialhierarchyofclassesderived
fromtheThrowableclass.ThetwomainsubclassesExceptionandError
constitutethemaincategoriesofthrowables,thetermusedtorefertobothexceptionsand
errors.Figure6.9alsoshowsthatnotallexceptionclassesarefoundinthejava.lang
package.
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Figure6.9PartialExceptionInheritanceHierarchy
TheThrowableclassprovidesaStringvariablethatcanbesettoprovideadetail
messagewhenanexceptionisconstructed.Thepurposeofthedetailmessageistoprovide
moreinformationabouttheactualexception.Allclassesofthrowablesdefineaone-
parameterconstructorthattakesastringasthedetailmessage.
TheclassThrowableprovidesthefollowingcommonmethodstoqueryanexception:
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StringgetMessage()
Returnsthedetailmessage.
voidprintStackTrace()
Printsthestacktraceonthestandarderrorstream.Thestacktracecomprisesthe
methodinvocationsequenceontheJVMstackwhentheexceptionwasthrown.The
stacktracecanalsobewrittentoaPrintStreamoraPrintWriterby
supplyingsuchadestinationasanargumenttooneofthetwooverloaded
printStackTrace()methods.Anysuppressedexceptionsassociatedwithan
exceptiononthestacktracearealsoprinted.Itwillalsoprintthecauseofan
exception(whichisalsoanexception)ifoneisavailable.
StringtoString()
Returnsashortdescriptionoftheexception,whichtypicallycomprisestheclass
nameoftheexceptiontogetherwiththestringreturnedbythegetMessage()
method.
Indealingwiththrowables,itisimportanttorecognizesituationsinwhichparticular
throwablescanoccur,andthesourcethatisresponsibleforthrowingthem.Bysourcewe
mean:
•TheJVMthatisresponsibleforthrowingthethrowable,or
•Thethrowablethatisexplicitlythrownprogrammaticallybythecodeinthe
applicationorbyanyAPIusedbytheapplication.
Infurtherdiscussionofexceptiontypes,weprovideanoverviewofsituationsinwhich
selectedthrowablescanoccurandthesourceresponsibleforthrowingthem.
The Class
TheclassExceptionrepresentsexceptionsthataprogramwouldnormallywantto
catch.ItssubclassRuntimeExceptionrepresentsmanycommonprogrammingerrors
thatcanmanifestatruntime(seethenextsubsection).Othersubclassesofthe
Exceptionclassdefineothercategoriesofexceptions,suchasI/O-relatedexceptionsin
thejava.iopackage(IOException,FileNotFoundException,
EOFException)anddatabase-relatedexceptionsinthejava.sqlpackage
(SQLException).
ClassNotFoundException
ThesubclassClassNotFoundExceptionsignalsthattheJVMtriedtoloadaclassby
itsstringname,buttheclasscouldnotbefound.Atypicalexampleofthissituationis
whentheclassnameismisspelledwhilestartingprogramexecutionwiththejava
command.ThesourceinthiscaseistheJVMthrowingtheexceptiontosignalthatthe
classcannotbefoundandthereforeexecutioncannotbestarted.
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The Class
Runtimeexceptionsareallsubclassesofthejava.lang.RuntimeExceptionclass,
whichisasubclassoftheExceptionclass.Astheseruntimeexceptionsareusually
causedbyprogrambugsthatshouldnotoccurinthefirstplace,itisusuallymore
appropriatetotreatthemasfaultsintheprogramdesignandletthembehandledbythe
defaultexceptionhandler.
ArithmeticException
Thisexceptionrepresentssituationswhereanillegalarithmeticoperationisattempted,
suchasintegerdivisionby0.ItistypicallythrownbytheJVM.SeeChapter5fordetails
onillegalarithmeticoperations.
ArrayIndexOutOfBoundsException
Javaprovidesruntimecheckingofthearrayindexvalue,meaningout-of-boundsarray
indices.ThesubclassArrayIndexOutOfBoundsExceptionrepresentsexceptions
thrownbytheJVMthatsignalout-of-bounderrorsspecificallyforarrays—thatis,anerror
inwhichaninvalidindexisusedtoaccessanelementinthearray.Theindexvaluemust
satisfytherelation0≤indexvalue<lengthofthearray.See§3.4,p.58,coveringarrays.
ClassCastException
ThisexceptionisthrownbytheJVMtosignalthatanattemptwasmadetocasta
referencevaluetoatypethatwasnotlegal,suchascastingthereferencevalueofan
IntegerobjecttotheLongtype.Castingreferencevaluesisdiscussedin§7.11,p.320.
IllegalArgumentExceptionandNumberFormatException
TheIllegalArgumentExceptionisthrownprogrammaticallytoindicatethata
methodwascalledwithanillegalorinappropriateargument.Forexample,the
ofPattern(Stringpattern)methodinthe
java.time.format.DateTimeFormatterclassthrowsan
IllegalArgumentExceptionwhentheletterpatternpassedasanargumentis
invalid(§11.4,p.495).
TheclassNumberFormatExceptionisasubclassofthe
IllegalArgumentExceptionclass,andisspecializedtosignalproblemswhen
convertingastringtoanumericvalueiftheformatofthecharactersinthestringisnot
appropriatefortheconversion.Thisexceptionisalsothrownprogrammatically.The
numericwrapperclassesallhavemethodsthatthrowthisexceptionwhenconversionfrom
astringtoanumericvalueisnotpossible(§8.3,p.346).
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NullPointerException
ThisexceptionistypicallythrownbytheJVMwhenanattemptismadetousethenull
valueasareferencevaluetorefertoanobject.Thismightinvolvecallinganinstance
methodusingareferencethathasthenullvalue,oraccessingafieldusingareference
thathasthenullvalue.Thisprogrammingerrorhasmadethisexceptiononeofthe
exceptionsmostoftenthrownbytheJVM.
The Class
TheclassErroranditssubclassesdefineerrorsthatareinvariablyneverexplicitly
caughtandareusuallyirrecoverable.Notsurprisingly,mostsucherrorsaresignaledbythe
JVM.Apartfromthesubclassesmentionedinthefollowingsubsections,othersubclasses
ofthejava.lang.Errorclassdefineerrorsthatindicateclasslinkage
(LinkageError),thread(ThreadDeath),andvirtualmachine
(VirtualMachineError)problems.
AssertionError
ThesubclassAssertionErrorofthejava.lang.ErrorclassisusedbytheJava
assertionfacility.ThiserroristhrownbytheJVMinresponsetotheconditioninthe
assertstatementevaluatingtofalse.Theassertionfacilityisnotdiscussedinthis
book.
StackOverflowError
ThiserroroccurswhentheJVMstackhasnomoreroomfornewmethodactivation
frames.Insuchacase,wesaythatthestackhasoverflowed.Thissituationcanoccurwhen
methodexecutioninanapplicationrecursestoodeeply.Hereisarecursivemethodto
illustratestackoverflow:
Clickheretoviewcodeimage
publicvoidcallMe(){
System.out.println(“Don’tdothisathome!”);
callMe();
}
Oncethismethodiscalled,itwillkeeponcallingitselfuntiltheJVMstackisfull,
resultingintheStackOverflowErrorbeingthrownbytheJVM.
CheckedandUncheckedExceptions
ExceptforRuntimeException,Error,andtheirsubclasses,allexceptionsare
checkedexceptions.Thatis,thecompilerensuresthatifamethodcanthrowachecked
exception,directlyorindirectly,themethodmustexplicitlydealwithit.Themethodmust
eithercatchtheexceptionandtaketheappropriateaction,orpassontheexceptiontoits
caller(§6.9,p.251).
ExceptionsdefinedbytheErrorandRuntimeExceptionclassesandtheir
subclassesareknownasuncheckedexceptions,meaningthatamethodisnotobligedto
dealwiththesekindsofexceptions(shownwithgraycolorinFigure6.9).Theyareeither
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irrecoverable(exemplifiedbytheErrorclass),inwhichcasetheprogramshouldnot
attempttodealwiththem,ortheyareprogrammingerrors(exemplifiedbythe
RuntimeExceptionclass)andshouldusuallybedealtwithassuch,andnotas
exceptions.
DefiningCustomizedExceptions
Customizedexceptionsareusuallydefinedtoprovidefine-grainedcategorizationoferror
situations,insteadofusingexistingexceptionclasseswithdescriptivedetailmessagesto
differentiateamongthevarioussituations.Newcustomizedexceptionsareusuallydefined
byeitherextendingtheExceptionclassoroneofitscheckedsubclasses,thereby
makingthenewexceptionschecked,orextendingtheRuntimeExceptionsubclassto
createnewuncheckedexceptions.
Asexceptionsaredefinedbyclasses,theycandeclarefieldsandmethods,thereby
providingmoreinformationastotheircauseandremedywhentheyarethrownand
caught.Thesuper()callcanbeusedtosetthedetailmessagefortheexception.Note
thattheexceptionclassmustbeinstantiatedtocreateanexceptionobjectthatcanbe
thrownandsubsequentlycaughtanddealtwith.Thefollowingcodesketchesaclass
declarationforanexceptionthatcanincludeallpertinentinformationabouttheexception.
Ataminimum,thenewexceptionclassshouldprovideaconstructortosetthedetail
message.
Clickheretoviewcodeimage
publicclassEvacuateExceptionextendsException{
//Data
Datedate;
Zonezone;
TransportModetransport;
//Constructor
publicEvacuateException(Dated,Zonez,TransportModet){
//Calltheconstructorofthesuperclass
super(“Evacuationofzone”+z);
//…
}
//Methods
//…
}
Severalexamplesinsubsequentsectionsillustrateexceptionhandling.
6.7ExceptionHandling: , ,and
Themechanismforhandlingexceptionsisembeddedinthetry-catch-finally
construct,whichhasthefollowingbasicform:
Clickheretoviewcodeimage
try{//tryblock
statements
}catch(exception_type1parameter1){//uni-catchclause
statements
}
…
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catch(exception_typenparametern){//uni-catchclause
statements
}finally{//finallyclause
statements
}
Afewaspectsaboutthesyntaxofthisconstructshouldbenoted.Foreachtryblock,
therecanbezeroormorecatchclauses(i.e.,itcanhavemultiplecatchclauses),but
onlyonefinallyclause.Thecatchclausesandthefinallyclausemustalways
appearinconjunctionwithatryblock,andintherightorder.Atryblockmustbe
followedbyatleastonecatchclause,orafinallyclausemustbespecified.In
additiontothetryblock,eachcatchclauseandthefinallyclausespecifyablock,
{}.Theblocknotationismandatory.
Exceptionsthrownduringexecutionofthetryblockcanbecaughtandhandledina
catchclause.Eachcatchclausedefinesanexceptionhandler.Theheaderofthe
catchclausespecifiesexactlyoneexceptionparameter.Theexceptiontypemustbeof
theThrowableclassoroneofitssubclasses.Thetypeoftheexceptionparameterofa
catchclauseisspecifiedbyasingleexceptiontypeinthesyntaxgivenearlier,andsuch
acatchclauseiscalledauni-catchclause.
Afinallyclauseisguaranteedtobeexecuted,regardlessofthecauseofexitfromthe
tryblock,orwhetheranycatchclausewasexecuted.Figure6.10showsthreetypical
scenariosofcontrolflowthroughthetry-catch-finallyconstruct.
Figure6.10Thetry-catch-finallyConstruct
Thetryblock,thecatchclause,andthefinallyclauseofatry-catch-
finallyconstructcancontainarbitrarycode,whichmeansthatatry-catch-
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finallyconstructcanbenestedinanyblockofthetry-catch-finallyconstruct.
However,suchnestingcaneasilymakethecodedifficulttoreadandisbestavoided,if
possible.
The Block
Thetryblockestablishesacontextforexceptionhandling.Terminationofatryblock
occursasaresultofencounteringanexception,orfromsuccessfulexecutionofthecode
inthetryblock.
Thecatchclausesareskippedforallnormalexitsfromthetryblockwhenno
exceptionsarethrown,andcontrolistransferredtothefinallyclauseifoneis
specified((1)inFigure6.10).
Forallexitsfromthetryblockresultingfromexceptions,controlistransferredtothe
catchclauses—ifanysuchclausesarespecified—tofindamatchingcatchclause((2)
inFigure6.10).Ifnocatchclausematchesthethrownexception,controlistransferred
tothefinallyclauseifoneisspecified((3)inFigure6.10).
The Clause
Onlyanexitfromatryblockresultingfromanexceptioncantransfercontroltoa
catchclause.Acatchclausecancatchthethrownexceptiononlyiftheexceptionis
assignabletotheparameterinthecatchclause(§7.8,p.311).Thecodeofthefirstsuch
catchclauseisexecuted,andallothercatchclausesareignored.
Onexitfromacatchclause,normalexecutioncontinuesunlessthereisanyuncaught
exceptionthathasbeenthrownandnothandled.Ifthisisthecase,themethodisaborted
andtheexceptionispropagateduptheJVMstackasexplainedin§6.5,p.230.
Afteracatchclausehasbeenexecuted,controlisalwaystransferredtothefinally
clauseifoneisspecified.Thisisalwaystrueaslongasthereisafinallyclause,
regardlessofwhetherthecatchclauseitselfthrowsanexception.
InExample6.12,themethodprintAverage()callsthemethod
computeAverage()inatry-catchconstructat(4).Thecatchclauseisdeclared
tocatchexceptionsoftypeArithmeticException.Thecatchclausehandlesthe
exceptionbyprintingthestacktraceandsomeadditionalinformationat(7)and(8),
respectively.NormalexecutionoftheprogramisillustratedinFigure6.11,whichshows
thatthetryblockisexecutedbutnoexceptionsarethrown,withnormalexecution
continuingafterthetry-catchconstruct.ThiscorrespondstoScenario1inFigure6.10.
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Figure6.11ExceptionHandling(Scenario1)
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Example6.12Thetry-catchConstruct
Clickheretoviewcodeimage
publicclassAverage2{
publicstaticvoidmain(String[]args){
printAverage(100,20);//(1)
System.out.println(“Exitmain().”);//(2)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
try{//(3)
intaverage=computeAverage(totalSum,totalNumber);//(4)
System.out.println(“Average=”+//(5)
totalSum+”/”+totalNumber+”=”+average);
}catch(ArithmeticExceptionae){//(6)
ae.printStackTrace();//(7)
System.out.println(“ExceptionhandledinprintAverage().”);//(8)
}
System.out.println(“ExitprintAverage().”);//(9)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(10)
returnsum/number;//(11)
}
}
Outputfromtheprogram,withcallprintAverage(100,20)at(1):
Computingaverage.
Average=100/20=5
ExitprintAverage().
Exitmain().
Outputfromtheprogram,withcallprintAverage(100,0)at(1):
Clickheretoviewcodeimage
Computingaverage.
java.lang.ArithmeticException:/byzero
atAverage2.computeAverage(Average2.java:23)
atAverage2.printAverage(Average2.java:11)
atAverage2.main(Average2.java:5)
ExceptionhandledinprintAverage().
ExitprintAverage().
Exitmain().
However,ifweruntheprograminExample6.12withthefollowingcallin(1):
printAverage(100,0)
anArithmeticExceptionisthrownbytheintegerdivisionoperatorinthemethod
computeAverage().InFigure6.12weseethattheexecutionofthemethod
compute-Average()isstoppedandtheexceptionpropagatedtomethod
printAverage(),whereitishandledbythecatchclauseat(6).Normalexecution
ofthemethodcontinuesat(9)afterthetry-catchconstruct,aswitnessedbytheoutput
fromthestatementsat(9)and(2).ThiscorrespondstoScenario2inFigure6.10.
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Figure6.12ExceptionHandling(Scenario2)
InExample6.13,themain()methodcallstheprintAverage()methodinatry-
catchconstructat(1).Thecatchclauseat(3)isdeclaredtocatchexceptionsoftype
ArithmeticException.TheprintAverage()methodcallsthe
computeAverage()methodinatry-catchconstructat(7),butherethecatch
clauseisdeclaredtocatchexceptionsoftypeIllegalArgumentException.
ExecutionoftheprogramisillustratedinFigure6.13,whichshowsthatthe
ArithmeticExceptionisfirstpropagatedtothecatchclauseinthe
printAverage()method.Becausethiscatchclausecannothandlethisexception,it
ispropagatedfurthertothecatchclauseinthemain()method,whereitiscaughtand
handled.Normalexecutioncontinuesat(6)aftertheexceptionishandled.
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Figure6.13ExceptionHandling(Scenario3)
Notethattheexecutionofthetryblockat(7)intheprintAverage()methodis
nevercompleted:Thestatementat(9)isneverexecuted.Thecatchclauseat(10)is
skipped.TheexecutionoftheprintAverage()methodisaborted:Thestatementat
(13)isneverexecuted,andtheexceptionispropagated.ThiscorrespondstoScenario3in
Figure6.10.
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Example6.13ExceptionPropagation
Clickheretoviewcodeimage
publicclassAverage3{
publicstaticvoidmain(String[]args){
try{//(1)
printAverage(100,0);//(2)
}catch(ArithmeticExceptionae){//(3)
ae.printStackTrace();//(4)
System.out.println(“Exceptionhandledinmain().”);//(5)
}
System.out.println(“Exitmain().”);//(6)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
try{//(7)
intaverage=computeAverage(totalSum,totalNumber);//(8)
System.out.println(“Average=”+//(9)
totalSum+”/”+totalNumber+”=”+average);
}catch(IllegalArgumentExceptioniae){//(10)
iae.printStackTrace();//(11)
System.out.println(“ExceptionhandledinprintAverage().”);//(12)
}
System.out.println(“ExitprintAverage().”);//(13)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(14)
returnsum/number;//(15)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Computingaverage.
java.lang.ArithmeticException:/byzero
atAverage3.computeAverage(Average3.java:28)
atAverage3.printAverage(Average3.java:16)
atAverage3.main(Average3.java:6)
Exceptionhandledinmain().
Exitmain().
Thescopeoftheparameternameinthecatchclauseistheclauseitself.Asmentioned
earlier,thetypeoftheexceptionobjectmustbeassignabletothetypeoftheargumentin
thecatchclause(§7.8,p.311).Inthebodyofthecatchclause,theexceptionobject
canbequeriedlikeanyotherobjectbyusingtheparametername.Thejavaccompiler
alsocomplainsifacatchclauseforasuperclassexceptionshadowsthecatchclause
forasubclassexception,asthecatchclauseofthesubclassexceptionwillneverbe
executed(asituationknownasunreachablecode).Thefollowingexampleshows
incorrectorderofthecatchclausesat(1)and(2),whichwillresultinacompile-time
error:ThesuperclassExceptionwillshadowthesubclassArithmeticException.
Clickheretoviewcodeimage
…
//Compiler-timeerrorat(1).
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catch(Exceptione){//(1)superclass
System.out.println(e);
}catch(ArithmeticExceptione){//(2)subclass
System.out.println(e);
}
…
The Clause
Ifthetryblockisexecuted,thenthefinallyclauseisguaranteedtobeexecuted,
regardlessofwhetheranycatchclausewasexecuted.Sincethefinallyclauseis
alwaysexecutedbeforecontroltransferstoitsfinaldestination,thefinallyclausecan
beusedtospecifyanyclean-upcode(e.g.,tofreeresourcessuchasfilesandnet
connections).However,thetry-with-resourcesstatementprovidesabettersolutionfor
handlingresources,andeliminatesuseofthefinallyclauseinmanycases.Butthatisa
storyforanotherday,asthistopicisnotontheOCAJP8exam.
Atry-finallyconstructcanbeusedtocontroltheinterplaybetweentwoactionsthat
mustbeexecutedinthecorrectorder,possiblywithotherinterveningactions.Inthe
followingcode,theoperationinthecalculateAverage()methodisdependenton
thesuccessofthesumNumbers()method,whichischeckedbythevalueofthesum
variablebeforecallingthecalculateAverage()method:
Clickheretoviewcodeimage
intsum=0;
try{
sum=sumNumbers();
//otheractions
}finally{
if(sum>0)calculateAverage();
}
Thiscodeguaranteesthatifthetryblockisentered,thesumNumbers()methodwill
beexecutedfirst,andlaterthecalculateAverage()methodwillbeexecutedinthe
finallyclause,regardlessofhowexecutionproceedsinthetryblock.Wecan,if
desired,includeanycatchclausestohandleanyexceptions.
Ifthefinallyclauseneitherthrowsanexceptionnorexecutesacontroltransfer
statementlikeareturnoralabeledbreak,theexecutionofthetryblockorany
catchclausedetermineshowexecutionproceedsafterthefinallyclause(Figure
6.10,p.239).
•Ifnoexceptionisthrownduringexecutionofthetryblockortheexceptionhas
beenhandledinacatchclause,normalexecutioncontinuesafterthefinally
clause.
•Ifthereisanyuncaughtexception(eitherbecausenocatchclausewasfoundor
becausethecatchclausethrewanexception),themethodisabortedandthe
exceptionispropagatedaftertheexecutionofthefinallyclause.
TheoutputofExample6.14showsthatthefinallyclauseat(9)isexecuted,regardless
ofwhetheranexceptionisthrowninthetryblockat(3).Ifanexceptionisthrown,itis
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caughtandhandledbythecatchclauseat(6).Aftertheexecutionofthefinally
clauseat(9),normalexecutioncontinuesat(10).
Example6.14Thetry-catch-finallyConstruct
Clickheretoviewcodeimage
publicclassAverage4{
publicstaticvoidmain(String[]args){
printAverage(100,20);//(1)
System.out.println(“Exitmain().”);//(2)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
try{//(3)
intaverage=computeAverage(totalSum,totalNumber);//(4)
System.out.println(“Average=”+//(5)
totalSum+”/”+totalNumber+”=”+average);
}catch(ArithmeticExceptionae){//(6)
ae.printStackTrace();//(7)
System.out.println(“ExceptionhandledinprintAverage().”);//(8)
}finally{//(9)
System.out.println(“Finallydone.”);
}
System.out.println(“ExitprintAverage().”);//(10)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(11)
returnsum/number;//(12)
}
}
Outputfromtheprogram,withthecallprintAverage(100,20)at(1):
Computingaverage.
Average=100/20=5
Finallydone.
ExitprintAverage().
Exitmain().
Outputfromtheprogram,withthecallprintAverage(100,0)at(1):
Clickheretoviewcodeimage
Computingaverage.
java.lang.ArithmeticException:/byzero
atAverage4.computeAverage(Average4.java:25)
atAverage4.printAverage(Average4.java:11)
atAverage4.main(Average4.java:5)
ExceptionhandledinprintAverage().
Finallydone.
ExitprintAverage().
Exitmain().
Onexitingfromthefinallyclause,ifthereisanyuncaughtexception,themethodis
abortedandtheexceptionpropagatedasexplainedearlier.ThisisillustratedinExample
6.15.ThemethodprintAverage()isabortedafterthefinallyclauseat(6)has
beenexecuted,astheArithmeticExceptionthrownat(9)isnothandledby
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anymethod.Inthiscase,theexceptionishandledbythedefaultexceptionhandler.
NoticethedifferenceintheoutputfromExample6.14andExample6.15.
Example6.15Thetry-finallyConstruct
Clickheretoviewcodeimage
publicclassAverage5{
publicstaticvoidmain(String[]args){
printAverage(100,0);//(1)
System.out.println(“Exitmain().”);//(2)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
try{//(3)
intaverage=computeAverage(totalSum,totalNumber);//(4)
System.out.println(“Average=”+//(5)
totalSum+”/”+totalNumber+”=”+average);
}finally{//(6)
System.out.println(“Finallydone.”);
}
System.out.println(“ExitprintAverage().”);//(7)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(8)
returnsum/number;//(9)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Computingaverage.
Finallydone.
Exceptioninthread“main”java.lang.ArithmeticException:/byzero
atAverage5.computeAverage(Average5.java:22)
atAverage5.printAverage(Average5.java:11)
atAverage5.main(Average5.java:4)
Ifthefinallyclauseexecutesacontroltransferstatement,suchasareturnora
labeledbreak,thiscontroltransferstatementdetermineshowtheexecutionwillproceed
—regardlessofhowthetryblockoranycatchclausewasexecuted.Inparticular,a
valuereturnedbyareturnstatementinthefinallyclausewillsupercedeanyvalue
returnedbyareturnstatementinthetryblockoracatchclause.
Example6.16showshowtheexecutionofacontroltransferstatementsuchasareturn
inthefinallyclauseaffectstheprogramexecution.Thefirstoutputfromtheprogram
showsthattheaverageiscomputedbutthevaluereturnedisfromthereturnstatement
at(8)inthefinallyclause,notfromthereturnstatementat(6)inthetryblock.
ThesecondoutputshowsthattheArithmeticExceptionthrowninthe
computeAverage()methodandpropagatedtotheprintAverage()methodis
nullifiedbythereturnstatementinthefinallyclause.Normalexecutioncontinues
afterthereturnstatementat(8),withthevalue0beingreturnedfromthe
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printAverage()method.
Ifthefinallyclausethrowsanexception,thisexceptionispropagatedwithallits
ramifications—regardlessofhowthetryblockoranycatchclausewasexecuted.In
particular,thenewexceptionoverrulesanypreviouslyuncaughtexception.
Example6.16ThefinallyClauseandthereturnStatement
Clickheretoviewcodeimage
publicclassAverage6{
publicstaticvoidmain(String[]args){
System.out.println(“Average:”+printAverage(100,20));//(1)
System.out.println(“Exitmain().”);//(2)
}
publicstaticintprintAverage(inttotalSum,inttotalNumber){
intaverage=0;
try{//(3)
average=computeAverage(totalSum,totalNumber);//(4)
System.out.println(“Average=”+//(5)
totalSum+”/”+totalNumber+”=”+average);
returnaverage;//(6)
}finally{//(7)
System.out.println(“Finallydone.”);
returnaverage*2;//(8)
}
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);//(9)
returnsum/number;//(10)
}
}
Outputfromtheprogram,withcallprintAverage(100,20)in(1):
Computingaverage.
Average=100/20=5
Finallydone.
Average:10
Exitmain().
Outputfromtheprogram,withcallprintAverage(100,0)in(1):
Computingaverage.
Finallydone.
Average:0
Exitmain().
6.8The Statement
Earlierexamplesinthischapterhaveshownhowanexceptioncanbethrownimplicitlyby
theJVMduringexecution.Nowwelookathowanapplicationcanprogrammatically
throwanexceptionusingthethrowstatement.Thegeneralformatofthethrow
statementisasfollows:
Clickheretoviewcodeimage
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throwobject_reference_expression;
ThecompilerensuresthattheobjectreferenceexpressionisofthetypeThrowableclass
oroneofitssubclasses.AtruntimeaNullPointerExceptionisthrownbytheJVM
iftheobjectreferenceexpressionisnull.ThisensuresthataThrowablewillalways
bepropagated.Adetailmessageisoftenpassedtotheconstructorwhentheexception
objectiscreated.
Clickheretoviewcodeimage
thrownewArithmeticException(“Integerdivisionby0”);
Whenanexceptionisthrown,normalexecutionissuspended.Theruntimesystem
proceedstofindacatchclausethatcanhandletheexception.Thesearchstartsinthe
contextofthecurrenttryblock,propagatingtoanyenclosingtryblocksandthrough
theJVMstacktofindahandlerfortheexception.Anyassociatedfinallyclauseofa
tryblockencounteredalongthesearchpathisexecuted.Ifnohandlerisfound,thenthe
exceptionisdealtwithbythedefaultexceptionhandleratthetoplevel.Ifahandleris
found,normalexecutionresumesafterthecodeinitscatchclausehasbeenexecuted,
barringanyrethrowingofanexception.
InExample6.17,anexceptionisthrownusingathrowstatementat(17).Thisexception
ispropagatedtothemain()method,whereitiscaughtandhandledbythecatch
clauseat(3).Notethatthefinallyclausesat(6)and(14)areexecuted.Execution
continuesnormallyat(7).
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Example6.17ThrowingExceptions
Clickheretoviewcodeimage
publicclassAverage7{
publicstaticvoidmain(String[]args){
try{//(1)
printAverage(100,0);//(2)
}catch(ArithmeticExceptionae){//(3)
ae.printStackTrace();//(4)
System.out.println(“Exceptionhandledinmain().”);//(5)
}finally{
System.out.println(“Finallyinmain().”);//(6)
}
System.out.println(“Exitmain().”);//(7)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber){
try{//(8)
intaverage=computeAverage(totalSum,totalNumber);//(9)
System.out.println(“Average=”+//(10)
totalSum+”/”+totalNumber+”=”+average);
}catch(IllegalArgumentExceptioniae){//(11)
iae.printStackTrace();//(12)
System.out.println(“ExceptionhandledinprintAverage().”);//(13)
}finally{
System.out.println(“FinallyinprintAverage().”);//(14)
}
System.out.println(“ExitprintAverage().”);//(15)
}
publicstaticintcomputeAverage(intsum,intnumber){
System.out.println(“Computingaverage.”);
if(number==0)//(16)
thrownewArithmeticException(“Integerdivisionby0”);//(17)
returnsum/number;//(18)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Computingaverage.
FinallyinprintAverage().
java.lang.ArithmeticException:Integerdivisionby0
atAverage7.computeAverage(Average7.java:33)
atAverage7.printAverage(Average7.java:18)
atAverage7.main(Average7.java:6)
Exceptionhandledinmain().
Finallyinmain().
Exitmain().
6.9The Clause
Athrowsclausecanbespecifiedinamethodoraconstructorheadertodeclareany
checkedexceptionsthatcanbethrownbyastatementinthebodyofamethodora
constructor.Itisdeclaredimmediatelyprecedingthebodyofthemethodorthe
constructor.
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…throwsExceptionType1,ExceptionType2,…,ExceptionTypen{…}
EachExceptionTypeiisanexceptiontype,althoughusuallyonlycheckedexceptionsare
specified.Thecompilerenforcesthatifacheckedexceptioncanbetheresultofexecuting
amethodoraconstructor,theneithertheexceptiontypeofthisexceptionorasupertype
ofitsexceptiontypeisspecifiedinthethrowsclauseofthemethodortheconstructor.
Thethrowsclausecanspecifyuncheckedexceptions,butthisisseldomdoneandthe
compilerdoesnotenforceanyrestrictionsontheirusage.
Thethrowsclauseispartofthecontractthatamethodoraconstructorofferstoits
clients.Thethrowsclausecanspecifyanynumberofexceptiontypes,eventhosethat
arenotthrownbythemethodortheconstructor.Thecompilersimplyensuresthatany
checkedexceptionthatcanactuallybethrowninthemethodorconstructorbodyis
coveredbythethrowsclause.Ofcourse,theclientscannotignorethechecked
exceptionsinthethrowsclause.
Inamethodoraconstructor,acheckedexceptioncanbethrowndirectlybyathrow
statement,orindirectlybycallingothermethodsorconstructorsthatcanthrowachecked
exception.Ifacheckedexceptionisthrown,itmustbehandledinoneofthreeways:
•Byusingatryblockandcatchingtheexceptioninahandleranddealingwithit
•Byusingatryblockandcatchingtheexceptioninahandler,butthrowinganother
exceptionthatiseitheruncheckedordeclaredinitsthrowsclause
•Byexplicitlyallowingpropagationoftheexceptiontoitscallerbydeclaringitinthe
throwsclauseofitsheader
Thismechanism(alsoknownascatch-or-declare)ensuresthatacheckedexceptionwill
bedealtwith,regardlessofthepathofexecution.Thisaidsdevelopmentofrobust
programs,asallowancecanbemadeformanycontingencies.Nativemethodscanalso
declarecheckedexceptionsintheirthrowsclause,butthecompilerisnotabletocheck
themforconsistency.
InExample6.18,anewcheckedexceptionisdefined,wherethecheckedexceptionclass
IntegerDivisionByZeroextendstheExceptionclass.Themethodmain()
callsthemethodprintAverage()inatryblockat(1).Intheifstatementat(9),the
methodcomputeAverage()throwsthecheckedexception
IntegerDivisionByZero.NeitherthecomputeAverage()methodnorthe
printAverage()methodcatchestheexception,butinsteadthrowsittothecaller,as
declaredinthethrowsclausesintheirheadersat(6)and(8).Theexceptionpropagates
tothemain()method.SincetheprintAverage()methodwascalledfromthe
contextofthetryblockat(1)inthemain()method,theexceptionissuccessfully
caughtbyitscatchclauseat(3).Theexceptionishandledandthefinallyclauseat
(4)executed,withnormalexecutionresumingfrom(5).Ifthemethodmain()didnot
catchtheexception,itwouldhavetodeclarethisexceptioninathrowsclause.Inthat
case,theexceptionwouldendupbeinghandledbythedefaultexceptionhandler.
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Example6.18ThethrowsClause
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//File:IntegerDivisionByZero.java
publicclassIntegerDivisionByZeroextendsException{
IntegerDivisionByZero(Stringstr){super(str);}
}
//File:Average8.java
publicclassAverage8{
publicstaticvoidmain(String[]args){
try{//(1)
printAverage(100,0);//(2)
}catch(IntegerDivisionByZeroidbz){//(3)
idbz.printStackTrace();
System.out.println(“Exceptionhandledinmain().”);
}finally{//(4)
System.out.println(“Finallydoneinmain().”);
}
System.out.println(“Exitmain().”);//(5)
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber)
throwsIntegerDivisionByZero{//(6)
intaverage=computeAverage(totalSum,totalNumber);
System.out.println(“Average=”+
totalSum+”/”+totalNumber+”=”+average);
System.out.println(“ExitprintAverage().”);//(7)
}
publicstaticintcomputeAverage(intsum,intnumber)
throwsIntegerDivisionByZero{//(8)
System.out.println(“Computingaverage.”);
if(number==0)//(9)
thrownewIntegerDivisionByZero(“IntegerDivisionByZero”);
returnsum/number;//(10)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Computingaverage.
IntegerDivisionByZero:IntegerDivisionByZero
atAverage8.computeAverage(Average8.java:27)
atAverage8.printAverage(Average8.java:17)
atAverage8.main(Average8.java:5)
Exceptionhandledinmain().
Finallydoneinmain().
Exitmain().
Asmentionedearlier,theexceptiontypespecifiedinthethrowsclausecanbea
superclassoftheactualexceptionsthrown;thatis,theexceptionsthrownmustbe
assignabletothetypeoftheexceptionsspecifiedinthethrowsclause.Ifamethodora
constructorcanthrowacheckedexception,thenthethrowsclausemustdeclareits
exceptiontypeorasupertypeofitsexceptiontype;otherwise,acompile-timeerrorwill
occur.IntheprintAverage()method,themethodheadercouldspecifythesuperclass
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ExceptionofthesubclassIntegerDivisionByZerointhethrowsclause.This
wouldalsoentailthatthemain()methodeithercatchanExceptionordeclareitina
throwsclause.
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args)throwsException{
/*…*/
}
publicstaticvoidprintAverage(inttotalSum,inttotalNumber)throws
Exception{
/*…*/
}
Itisgenerallyconsideredbadprogrammingstyletospecifyexceptionsuperclassesinthe
throwsclauseoftheheaderwhentheactualexceptionsthrownareinstancesoftheir
subclasses.Itisalsorecommendedtousethe@throwstaginaJavadoccommentto
documentthecheckedexceptionsthatamethodoraconstructorcanthrow,togetherwith
anyuncheckedexceptionsthatmightalsoberelevanttocatch.
Overridingthe Clause
Asubclasscanoverrideamethoddefinedinitssuperclassbyprovidinganew
implementation(§7.2,p.268).Whathappenswhenasuperclassmethodwithalistof
exceptionsinitsthrowsclauseisoverriddeninasubclass?Themethoddeclarationin
thesubclassneednotspecifyathrowsclauseifitdoesnotthrowanychecked
exceptions,andifitdoes,itcanspecifyonlycheckedexceptionclassesthatarealreadyin
thethrowsclauseofthesuperclassmethod,orthataresubclassesofthechecked
exceptionsinthethrowsclauseofthesuperclassmethod.Asaconsequence,an
overridingmethodcannotallowmorecheckedexceptionsinitsthrowsclausethanthe
superclassmethoddoes.Allowingmorecheckedexceptionsintheoverridingmethod
wouldcreateproblemsforclientswhoalreadydealwiththeexceptionsspecifiedinthe
superclassmethod.Suchclientswouldbeillpreparedifanobjectofthesubclassthrewa
checkedexceptiontheywerenotpreparedfor.However,therearenorestrictionson
specifyinguncheckedexceptionsinthethrowsclauseoftheoverridingmethod.The
precedingdiscussionalsoappliestomethodsfromaninterfacethataclassimplements,as
thesemethodsareoverriddenbyanyclassimplementingtheinterface.
Inthefollowingcode,themethodsuperclassMethodXinsuperclassAisoverridden
insubclassB.ThethrowsclauseofthemethodinsubclassBat(2)specifiesasubsetof
thecheckedexceptionsspecifiedinthethrowsclauseat(1)andaddsthemorespecific
subclassexception,SubFirstException,ofthesuperclassexception,First-
Exception,specifiedinthethrowsclauseat(1).
Clickheretoviewcodeimage
//Newexceptionclasses:
classFirstExceptionextendsException{}
classSecondExceptionextendsException{}
classThirdExceptionextendsException{}
classSubFirstExceptionextendsFirstException{}
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//Superclass
classA{
//…
protectedvoidsuperclassMethodX()
throwsFirstException,SecondException,ThirdException{/*…*/}//
(1)
//…
}
//Subclass
classBextendsA{
//…
@OverrideprotectedvoidsuperclassMethodX()
throwsFirstException,ThirdException,SubFirstException{/*…*/}//
(2)
//…
}
6.10AdvantagesofExceptionHandling
Robustnessreferstotheabilityofasoftwaresystemtorespondtoerrorsduringexecution.
Asystemshouldrespondtounexpectedsituationsatruntimeinaresponsibleway.
Applicationsthatprovidetheuserwithfrequentcrypticmessageswitherrorcodesorthat
repeatedlygivetheuserthesilenttreatmentwhensomethinggoeswrongcanhardlybe
consideredrobust.
TheexceptionhandlingmechanisminJavaoffersthefollowingadvantagesthatfacilitate
developingrobustapplicationsinJava:
•SeparationofExceptionHandlingCode
Thecodeforhandlingerrorsituationscanbeseparatedfromthecodeforthe
programlogicbyusingtheexceptionhandlingconstructsprovidedbythelanguage.
Codethatcanresultinerrorsituationsisconfinedinthetryblock,andtheir
handlinginthecatchclause.
•TransparentExceptionPropagation
PropagationofacheckedexceptionintheJVMstackcannotbeignoredbyanactive
method.Themethodmustcomplywiththecatch-or-declarerequirement:either
catchandhandletheexception,orpropagateitbydeclaringitinthemethod’s
throwsclause.Errorsituationscausingexceptionpropagationarethusalways
detected,andcanbecaughtandremedied.
•ExceptionCategorizationandSpecialization
TheexceptionanderrorclassesintheJavaSEplatformAPIareorganizedinan
inheritancehierarchy(Figure6.9,p.234).Classeshigherupinthishierarchy
representcategoriesofexceptionsanderrors(Exception,
RuntimeException,IO-Exception,Error),whereasclasseslowerinthis
hierarchyrepresentmorespecificexceptionsanderrors
(NullPointerException,FileNotFoundException,
AssertionError).Thetry-catchconstructallowsflexibilityincatchingand
handlingexceptions.Acatchclausecanspecifyanexceptioncategoryforcoarse-
grainedexceptionhandling,astheexceptioncategoryclasswillsubsumeitsmore
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specificexceptionsubclasses,oritcanspecifyamorespecificexceptionclassfor
fine-grainedexceptionhandling.Bestpracticedictatesthatfine-grainedexception
handlingbeused.
ReviewQuestions
6.16Whichdigits,andinwhichorder,willbeprintedwhenthefollowingprogramis
run?
Clickheretoviewcodeimage
publicclassDemoClass{
publicstaticvoidmain(String[]args){
intk=0;
try{
inti=5/k;
}catch(ArithmeticExceptione){
System.out.println(“1”);
}catch(RuntimeExceptione){
System.out.println(“2”);
return;
}catch(Exceptione){
System.out.println(“3”);
}finally{
System.out.println(“4”);
}
System.out.println(“5”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillonlyprint5.
(b)Theprogramwillonlyprint1and4,inthatorder.
(c)Theprogramwillonlyprint1,2,and4,inthatorder.
(d)Theprogramwillonlyprint1,4,and5,inthatorder.
(e)Theprogramwillonlyprint1,2,4,and5,inthatorder.
(f)Theprogramwillonlyprint3and5,inthatorder.
6.17Giventhefollowingprogram,whichstatementsaretrue?
Clickheretoviewcodeimage
publicclassExceptions{
publicstaticvoidmain(String[]args){
try{
if(args.length==0)return;
System.out.println(args[0]);
}finally{
System.out.println(“Theend”);
}
}
}
Selectthetwocorrectanswers.
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(a)Ifrunwithnoarguments,theprogramwillproducenooutput.
(b)Ifrunwithnoarguments,theprogramwillprintTheend.
(c)TheprogramwillthrowanArrayIndexOutOfBoundsException.
(d)Ifrunwithoneargument,theprogramwillsimplyprintthegivenargument.
(e)Ifrunwithoneargument,theprogramwillprintthegivenargumentfollowed
by"Theend".
6.18Whichofthefollowingstatementsaretrue?
Selectthetwocorrectanswers.
(a)Ifanexceptionisnotcaughtinamethod,themethodwillterminateandnormal
executionwillresume.
(b)Anoverridingmethodmustdeclarethatitthrowsthesameexceptionclassesas
themethoditoverrides.
(c)Themain()methodofaprogramcandeclarethatitthrowschecked
exceptions.
(d)Amethoddeclaringthatitthrowsanexceptionofacertainclassmaythrow
instancesofanysubclassofthatexceptionclass.
(e)finallyclausesareexecutedif,andonlyif,anexceptiongetsthrownwhile
insidethecorrespondingtryblock.
6.19Whichdigits,andinwhichorder,willbeprintedwhenthefollowingprogramis
run?
Clickheretoviewcodeimage
publicclassRQ6A19{
publicstaticvoidmain(String[]args)throwsInterruptedException{
try{
throwIt();
System.out.println(“1”);
}finally{
System.out.println(“2”);
}
System.out.println(“3”);
}
//InterruptedExceptionisadirectsubclassofException.
staticvoidthrowIt()throwsInterruptedException{
thrownewInterruptedException(“Timetogohome.”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillprint2andthrowInterruptedException.
(b)Theprogramwillprint1and2,inthatorder.
(c)Theprogramwillprint1,2,and3,inthatorder.
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(d)Theprogramwillprint2and3,inthatorder.
(e)Theprogramwillprint3and2,inthatorder.
(f)Theprogramwillprint1and3,inthatorder.
6.20Whatiswrongwiththefollowingcode?
Clickheretoviewcodeimage
publicclassRQ6A20{
publicstaticvoidmain(String[]args)throwsA{
try{
action();
}finally{
System.out.println(“Done.”);
}catch(Ae){
throwe;
}
}
publicstaticvoidaction()throwsB{
thrownewB();
}
}
classAextendsThrowable{}
classBextendsA{}
Selecttheonecorrectanswer.
(a)Themain()methodmustdeclarethatitthrowsB.
(b)Thefinallyclausemustfollowthecatchclauseinthemain()method.
(c)Thecatchclauseinthemain()methodmustdeclarethatitcatchesBrather
thanA.
(d)Asingletryblockcannotbefollowedbybothcatchandfinallyclauses.
(e)ThedeclarationofclassAisillegal.
6.21Whichthrowsclauseshouldbeinsertedat(1)fortheoverridingmethod
compute()inthefollowingcodesothatthecodewillcompilewithouterrors?
Clickheretoviewcodeimage
classA{
//InterruptedExceptionisadirectsubclassofException.
voidcompute()throwsArithmeticException,InterruptedException{
div(5,5);
}
intdiv(inti,intj)throwsArithmeticException{
returni/j;
}
}
publicclassClientextendsA{
voidcompute()/*(1)INSERTthrowsCLAUSEHERE.*/{
try{
div(5,0);
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}catch(ArithmeticExceptione){
return;
}
thrownewRuntimeException(“ArithmeticExceptionwasexpected.”);
}
}
Selecttheonecorrectanswer.
(a)Nothrowsclauseisnecessary.
(b)throwsArithmeticException
(c)throwsInterruptedException
(d)throwsRuntimeException
(e)throwsArithmeticException,InterruptedException
ChapterSummary
Thefollowinginformationwascoveredinthischapter:
•Theselectionstatements:if,if-else,switch
•Theiterationstatements:for(;;),for(:),while,do-while
•Thetransferstatements:break,continue,return
•ExceptionhandlingandexceptionclassesinthecoreAPI
•Definingcustomizedexceptiontypes
•Thetry-catch-finallyconstructandcontrolflowpathsthroughtheconstruct
•Usingmultiplecatchclauseswiththetrystatement
•Throwingexceptionsprogrammaticallywiththethrowstatement
•Usingthethrowsclausetospecifycheckedexceptions
ProgrammingExercises
6.1Createdifferentversionsofaprogramthatfindsalltheprimessmallerthan100.
Createoneversionthatusesonlythefor(;;)loop(i.e.,nowhileordo-
while).Createanotherversionthatusesonlythewhileloop.
6.2Hereisaskeletonofasystemforsimulatinganuclearpowerplant.Implementthe
methodsintheclassnamedControl.Modifythemethoddeclarationsif
necessary.TheJavadoccommentsforeachmethodgiveadescriptionofwhatthe
implementationshoulddo.Someofthemethodsintheotherclasseshave
unspecifiedimplementations.Assumethatthesemethodshavebeenproperly
implementedandprovidehookstotherestofthesystem.
Clickheretoviewcodeimage
packageenergy;
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/**APowerPlantwithareactorcore.*/
publicclassPowerPlant{
/**Eachpowerplanthasareactorcore.
ThisfieldhaspackageaccessibilitysothattheControlclass,
definedinthesamepackage,canaccessit.*/
finalReactorcore;
/**Initializesthepowerplant,createsareactorcore.*/
publicPowerPlant(){
core=newReactor();
}
/**Soundsthealarmtoevacuatethepowerplant.*/
publicvoidsoundEvacuateAlarm(){
//…implementationunspecified…
}
/**@returnthelevelofreactoroutputthatismostdesirableatthis
time.
(Unitsareunspecified.)*/
publicintgetOptimalThroughput(){
//…implementationunspecified…
return0;
}
/**Themainentrypointoftheprogram:setsupaPowerPlantobject
andaControlobjectandletstheControlobjectrunthepowerplant.
*/
publicstaticvoidmain(String[]args){
PowerPlantplant=newPowerPlant();
Controlctrl=newControl(plant);
ctrl.runSystem();
}
}
//______________________________________________________________________________
/**Areactorcorethathasathroughputthatcanbeeitherdecreasedor
increased.*/
classReactor{
/**@returnthecurrentthroughputofthereactor.(Unitsareunspecified.)
*/
publicintgetThroughput(){
//…implementationunspecified…
return0;
}
/**@returntrueifthereactorstatusiscritical,falseotherwise.*/
publicbooleanisCritical(){
//…implementationunspecified…
returnfalse;
}
/**Asksthereactortoincreasethroughput.*/
voidincreaseThroughput()throwsReactorCritical{
//…implementationunspecified…
}
/**Asksthereactortodecreasethroughput.*/
voiddecreaseThroughput(){
//…implementationunspecified…
}
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}
//______________________________________________________________________________
/**Thisexceptionclassshouldbeusedtoreportthatthereactorstatusis
critical.*/
classReactorCriticalextendsException{}
//______________________________________________________________________________
/**Acontrollerthatwillmanagethepowerplanttomakesurethatthe
reactorrunswithoptimalthroughput.*/
classControl{
privatefinalPowerPlantthePlant;
staticfinalintTOLERANCE=10;
/**@parampthepowerplanttocontrol*/
publicControl(PowerPlantp){
thePlant=p;
}
/**Runsthepowerplantbycontinuouslymonitoringthe
optimalthroughputandtheactualthroughputofthereactor.
Ifthethroughputsdifferbymorethan10units(i.e.tolerance),
adjustthereactorthroughput.
Ifthereactorgoescritical,theevacuatealarmis
soundedandthereactorisshutdown.
TherunSystem()methodcallsthemethodsneedAdjustment(),
adjustThroughput(),andshutdown().*/
publicvoidrunSystem(){
//…provideimplementationhere…
}
/**Reportswhetherthethroughputofthereactorneedsadjusting.
Thismethodshouldalsomonitorandreportifthereactorgoes
critical.
@paramtargetthedesiredthroughput.
@returntrueiftheoptimalandactualthroughputvaluesdifferby
morethan10units.*/
publicbooleanneedAdjustment(inttarget){
//…provideimplementationhere…
returntrue;
}
/**AdjuststhethroughputofthereactorbycallingincreaseThroughput()
anddecreaseThroughput()methodsuntiltheactualthroughputiswithin
10unitsofthetargetthroughput.
@paramtargetthedesiredthroughput.*/
publicvoidadjustThroughput(inttarget){
//…provideimplementationhere…
}
/**Shutsdownthereactorbyloweringthethroughputto0.*/
publicvoidshutdown(){
//…provideimplementationhere…
}
}
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7.Object-OrientedProgramming
7.1SingleImplementationInheritance
Inheritanceisoneofthefundamentalmechanismsforcodereuseinobject-oriented
programming(OOP).Itallowsnewclassestobederivedfromexistingones.Thenew
class(alsocalledasubclass,subtype,derivedclass,orchildclass)caninheritmembers
fromtheoldclass(alsocalledasuperclass,supertype,baseclass,orparentclass).The
subclasscanaddnewbehaviorandpropertiesand,undercertaincircumstances,modifyits
inheritedbehavior.
InJava,implementationinheritance(alsoknownasclassinheritance)isachievedby
extendingclasses(i.e.,addingnewfieldsandmethods)andmodifyinginheritedmembers
(§7.2,p.268).Inheritanceofmembersiscloselytiedtotheirdeclaredaccessibility.Ifa
superclassmemberisaccessiblebyitssimplenameinthesubclass(withouttheuseofany
extrasyntaxlikesuper),thatmemberisconsideredinherited.Conversely,private,
overridden,andhiddenmembersofthesuperclassarenotinherited.Inheritanceshouldnot
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beconfusedwiththeexistenceofsuchmembersinthestateofasubclassobject(Example
7.1).
Asubclassspecifiesthenameofitssuperclassinthesubclassheaderusingtheextends
clause.
Clickheretoviewcodeimage
classTubeLightextendsLight{…}//TubeLightisasubclassofLight.
Thesubclassspecifiesonlytheadditionalnewandmodifiedmembersinitsclassbody.
Therestofitsdeclarationismadeupofitsinheritedmembers.Ifnoextendsclauseis
specifiedintheheaderofaclassdeclaration,theclassimplicitlyinheritsfromthe
java.lang.Objectclass(§8.2,p.342).Thisimplicitinheritanceisassumedinthe
declarationoftheLightclassat(1)inExample7.1.AlsoinExample7.1,thesubclass
TubeLightat(2)explicitlyusestheextendsclauseandspecifiesonlymembersother
thanthosethatitalreadyinheritsfromthesuperclassLight(which,inturn,inheritsfrom
theObjectclass).MembersofthesuperclassLight,whichareaccessiblebytheir
simplenamesinthesubclassTubeLight,areinheritedbythesubclass,asevidentfrom
theoutputinExample7.1.
Privatemembersofthesuperclassarenotinheritedbythesubclassandcanbeaccessed
onlyindirectly.TheprivatefieldindicatorofthesuperclassLightisnotinherited,
butexistsinthesubclassobjectandisindirectlyaccessiblethroughpublicmethods.
Usingappropriateaccessibilitymodifiers,thesuperclasscanlimitwhichmemberscanbe
accesseddirectlyand,therefore,inheritedbyitssubclasses(§4.7,p.123).Asshownin
Example7.1,thesubclasscanusetheinheritedmembersasiftheyweredeclaredinits
ownclassbody.Thisisnotthecaseformembersthataredeclaredasprivateinthe
superclass.Membersthathavepackageaccessibilityinthesuperclassarealsonot
inheritedbysubclassesinotherpackages,asthesemembersareaccessiblebytheirsimple
namesonlyinsubclasseswithinthesamepackageasthesuperclass.
Sinceconstructors(§7.5,p.282)arenotmembersofaclass,theyarenotinheritedbya
subclass.
Example7.1ExtendingClasses:InheritanceandAccessibility
Clickheretoviewcodeimage
//File:Utility.java
classLight{//(1)
//Instancefields:
intnoOfWatts;//Wattage
privatebooleanindicator;//Onoroff
protectedStringlocation;//Placement
//Staticfield:
privatestaticintcounter;//NumberofLightobjectscreated
//No-argumentconstructor:
Light(){
noOfWatts=50;
indicator=true;
location=“X”;
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counter++;
}
//Instancemethods:
publicvoidswitchOn(){indicator=true;}
publicvoidswitchOff(){indicator=false;}
publicbooleanisOn(){returnindicator;}
privatevoidprintLocation(){
System.out.println(“Location:”+location);
}
//Staticmethods:
publicstaticvoidwriteCount(){
System.out.println(“Numberoflights:”+counter);
}
//…
}
//______________________________________________________________________________
classTubeLightextendsLight{//(2)Subclassusestheextends
clause.
//Instancefields:
privateinttubeLength=54;
privateintcolorNo=10;
//Instancemethods:
publicintgetTubeLength(){returntubeLength;}
publicvoidprintInfo(){
System.out.println(“Fromthesubclass:”);
System.out.println(“Tubelength:“+tubeLength);
System.out.println(“Colornumber:”+colorNo);
System.out.println(“Tubelength:“+getTubeLength());
System.out.println();
System.out.println(“Fromthesuperclass:”);
System.out.println(“Wattage:“+noOfWatts);//Inherited.
//System.out.println(“Indicator:“+indicator);//Notinherited.
System.out.println(“Location:“+location);//Inherited.
//System.out.println(“Counter:“+counter);//Notinherited.
switchOn();//Inherited
switchOff();//Inherited
System.out.println(“Indicator:“+isOn());//Inherited.
//printLocation();//Notinherited.
writeCount();//Inherited.
}
//…
}
//______________________________________________________________________________
publicclassUtility{//(3)
publicstaticvoidmain(String[]args){
newTubeLight().printInfo();
}
}
Outputfromtheprogram:
Fromthesubclass:
Tubelength:54
Colornumber:10
Tubelength:54
Fromthesuperclass:
Wattage:50
Location:X
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Indicator:false
Numberoflights:1
InJava,aclasscanextendonlyoneclass;thatis,itcanhaveonlyoneimmediate
superclass.Thiskindofinheritanceissometimescalledsingleorlinearimplementation
inheritance.Thenameisappropriate,asthesubclassinheritstheimplementationsofits
superclassmembers.Theinheritancerelationshipcanbedepictedasaninheritance
hierarchy(alsocalledaclasshierarchy).Classeshigherupinthehierarchyaremore
generalized(oftencalledbroader),astheyabstracttheclassbehavior.Classeslowerdown
inthehierarchyaremorespecialized(oftencallednarrower),astheycustomizethe
inheritedbehaviorbyadditionalpropertiesandbehavior.Figure7.1illustratesthe
inheritancerelationshipbetweentheclassLight,whichrepresentsthemoregeneral
abstraction,anditsmorespecializedsubclasses.Thejava.lang.Objectclassis
alwaysatthetop(theroot)ofanyJavainheritancehierarchy,asallclasses,withthe
exceptionoftheObjectclassitself,inherit(eitherdirectlyorindirectly)fromthisclass.
Figure7.1InheritanceHierarchy
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Relationships:is-aandhas-a
Inheritancedefinestherelationshipis-a(alsocalledthesuperclass–subclassrelationship)
betweenasuperclassanditssubclasses.Thus,anobjectofasubclassis-asuperclass
object,andcanbeusedwhereveranobjectofthesuperclasscanbeused.Thiscriterionis
oftenemployedasalitmustestforchoosinginheritanceinobject-orienteddesign.Ithas
particularconsequencesforhowobjectscanbeused.AnobjectoftheTubeLightclass
is-anobjectofthesuperclassLight.ReferringtoFigure7.1,anobjectofthe
TubeLightclasscanbeusedwhereveranobjectofthesuperclassLightcanbeused.
Theinheritancerelationshipistransitive:IfclassBextendsclassAandclassCextends
classB,thenclassCwillalsoinheritfromclassAviaclassB.Anobjectofthe
SpotLightBulbclassis-anobjectoftheclassLight.Theis-arelationshipdoesnot
holdbetweenpeerclasses:AnobjectoftheLightBulbclassisnotanobjectoftheclass
TubeLight,andviceversa.
Whereasinheritancedefinestherelationshipis-abetweenasuperclassanditssubclasses,
aggregationdefinestherelationshiphas-a(alsocalledthewhole–partrelationship)
betweenaninstanceofaclassanditsconstituents(alsocalledparts).Aggregation
comprisestheusageofobjects.AninstanceofclassLighthas(oruses)thefollowing
parts:afieldtostoreitswattage(noOfWatts),afieldtostorewhetheritisonoroff
(indicator),andaStringobjecttostoreitslocation(denotedbythefieldreference
location).InJava,acompositeobjectcannotcontainotherobjects.Itcanstoreonly
referencevaluesofitsconstituentobjectsinitsfields.Thisrelationshipdefinesan
aggregationhierarchy(alsocalledobjecthierarchy)thatembodiesthehasarelationship.
Asexplainedin§1.7,p.12,constituentobjectscanbesharedbetweenobjects.Iftheir
lifetimesaredependentonthelifetimeofthecompositeobject,thenthisrelationshipis
calledcomposition,andimpliesstrongownershipofthepartsbythecompositeobject.
Inheritanceandaggregationarecomparedin§7.13,p.331.
TheSupertype–SubtypeRelationship
Aclassdefinesareferencetype,adatatypewhoseobjectscanbeaccessedonlyby
references.Thereforetheinheritancehierarchycanberegardedasatypehierarchy,
embodyingthesupertype–subtyperelationshipbetweenreferencetypes.Inthecontextof
Java,thesupertype–subtyperelationshipimpliesthatthereferencevalueofasubtype
objectcanbeassignedtoasupertypereference,becauseasubtypeobjectcanbe
substitutedforasupertypeobject.Thisassignmentinvolvesawideningreference
conversion(§5.1,p.145),asreferencesareassigneduptheinheritancehierarchy.Using
thereferencetypesinExample7.1,thefollowingcodeassignsthereferencevalueofan
objectofthesubtypeTubeLighttothereferencelightofthesupertypeLight:
Clickheretoviewcodeimage
Lightlight=newTubeLight();//(1)wideningreference
conversion
Animplicitwideningconversiontakesplaceunderassignment,asthereferencevalueofa
narrowertype(subtypeTubeLight)objectisbeingassignedtoareferenceofbroader
type(supertypeLight).Wecannowusethereferencelighttoinvokethosemethods
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onthesubtypeobjectthatareinheritedfromthesupertypeLight:
Clickheretoviewcodeimage
light.switchOn();//(2)
Notethatthecompilerknowsaboutonlythedeclaredtype(statictype)ofthereference
light,whichisLight,andensuresthatonlymethodsfromthistypecanbecalled
usingthereferencelight.However,atruntime,thereferencelightwillrefertoan
objectofthesubtypeTubeLightwhenthecalltothemethodswitchOn()is
executed.Itisthetypeoftheobjectthatthereferencereferstoatruntimethatdetermines
whichmethodisexecuted.ThesubtypeobjectinheritstheswitchOn()methodfromits
supertypeLight,sothismethodisexecuted.Thetypeoftheobjectthatthereference
referstoatruntimeisoftencalledthedynamictypeofthereference.
OnemightbetemptedtoinvokemethodsexclusivetotheTubeLightsubtypeviathe
supertypereferencelight:
Clickheretoviewcodeimage
light.getTubeLength();//(3)NotOK.
Thiscodewillnotwork,asthecompilerdoesnotknowwhichobjectthereferencelight
willdenoteatruntime;itmerelyknowsthedeclaredtypeofthereference.Asthe
declarationoftheclassLightdoesnothaveamethodcalledgetTubeLength(),this
methodcallat(3)resultsinacompile-timeerror.Asweshallseelaterinthischapter,
elicitingsubtype-specificbehaviorusingasupertypereferencerequiresanarrowing
referenceconversionwithanexplicitcast(§7.11,p.320).
TherestofthischapterelaboratesonvariousaspectsofOOP.Understandingthemis
fundamentalinunderstandingtheconsequencesofthesubtype–supertyperelationship.
7.2OverridingMethods
InstanceMethodOverriding
Undercertaincircumstances,asubclasscanoverrideinstancemethodsthatitinheritsfrom
itssuperclass.Overridingsuchamethodallowsthesubclasstoprovideitsown
implementationofthemethod.Theoverriddenmethodinthesuperclassisnotinheritedby
thesubclass.Whenthemethodisinvokedonanobjectofthesubclass,itisthemethod
implementationinthesubclassthatisexecuted.Thenewmethodinthesubclassmust
abidebythefollowingrulesofmethodoverriding:
•Thenewmethoddefinitioninthesubclassmusthavethesamemethodsignature.In
otherwords,themethodname,andthetypesandthenumberofparameters,
includingtheirorder,mustbethesameasintheoverriddenmethodofthe
superclass.
Whetherparametersintheoverridingmethodshouldbefinalisatthediscretion
ofthesubclass(§3.7,p.86).Amethod’ssignaturedoesnotcomprisethefinal
modifierofparameters,onlytheirtypesandorder.
•Thereturntypeoftheoverridingmethodcanbeasubtypeofthereturntypeofthe
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overriddenmethod(calledcovariantreturn,p.273).
•Thenewmethoddefinitioncannotnarrowtheaccessibilityofthemethod,butitcan
widenit(§4.7,p.123).
•Thenewmethoddefinitioncanthroweitherallornone,orasubsetofthechecked
exceptions(includingtheirsubclasses)thatarespecifiedinthethrowsclauseof
theoverriddenmethodinthesuperclass(§6.9,p.253).
Theserequirementsalsoapplytointerfaces,whereasubinterfacecanoverrideabstract
anddefaultmethoddeclarationsfromitssuperinterfaces(§7.6,p.290).
Example7.2illustratesoverriding,overloading,andhidingofmembersinaclass.Figure
7.2givesanoverviewofthetwomainclassesinExample7.2.Thenewdefinitionofthe
energyCost()methodat(7)inthesubclassTubeLighthasthesamesignatureand
thesamereturntypeasthemethodat(2)inthesuperclassLight.Thenewdefinition
specifiesasubsetoftheexceptions(ZeroHoursException)thrownbytheoverridden
method(theexceptionclassInvalidHoursExceptionisasuperclassof
NegativeHoursExceptionandZeroHoursException).Thenewdefinitionalso
widenstheaccessibility(public)fromwhatitwasintheoverriddendefinition
(protected).Theoverridingmethoddeclarestheparametertobefinal,butthishas
nobearinginoverridingthemethod.
Figure7.2InheritanceHierarchyforExample7.2andExample7.3
Theastutereaderwillhavenoticedthe@Overrideannotationprecedingthemethod
definitionat(7).Thecompilerwillnowreportanerrorifthemethoddefinitionat(7)does
notoverrideaninheritedmethod.Theannotationhelpstoensurethatthemethod
definitionoverridestheinheritedmethod,ratherthanoverloadinganothermethodsilently.
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InvocationofthemethodenergyCost()onanobjectofsubclassTubeLightusing
referencesofthesubclassandthesuperclassat(15)and(16)resultsinthenewdefinition
at(7)beingexecuted,sincebothreferencesarealiasesoftheTubeLightobjectcreated
at(12).
Clickheretoviewcodeimage
tubeLight.energyCost(50);//(15)Invokesmethodat(7).
light1.energyCost(50);//(16)Invokesmethodat(7).
Notsurprisingly,theinvocationofthemethodenergyCost()onanobjectof
superclassLight,usingareferenceofthesuperclassat(17),resultsintheoverridden
definitionat(2)beingexecuted:
Clickheretoviewcodeimage
light2.energyCost(50);//(17)Invokesmethodat(2).
Example7.2Overriding,Overloading,andHiding
Clickheretoviewcodeimage
//File:Client2.java
//Exceptions
classInvalidHoursExceptionextendsException{}
classNegativeHoursExceptionextendsInvalidHoursException{}
classZeroHoursExceptionextendsInvalidHoursException{}
classLight{
protectedStringlightType=“GenericLight”;//(1)Instancefield
protecteddoubleenergyCost(intnoOfHours)//(2)Instancemethod
throwsInvalidHoursException{
System.out.print(“>>Light.energyCost(int):“);
if(noOfHours<0)
thrownewNegativeHoursException();
doublecost=00.20*noOfHours;
System.out.println(“Energycostfor”+lightType+“:”+cost);
returncost;
}
publicLightmakeInstance(){//(3)Instancemethod
System.out.print(“>>Light.makeInstance():“);
returnnewLight();
}
publicvoidshowSign(){//(4)Instancemethod
System.out.print(“>>Light.showSign():“);
System.out.println(“Lettherebelight!”);
}
publicstaticvoidprintLightType(){//(5)Staticmethod
System.out.print(“>>StaticLight.printLightType():“);
System.out.println(“GenericLight”);
}
}
//______________________________________________________________________________
classTubeLightextendsLight{
publicstaticStringlightType=“TubeLight”;//(6)Hidingfieldat(1).
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@Override
publicdoubleenergyCost(finalintnoOfHours)//(7)Overridinginstance
throwsZeroHoursException{//methodat(2).
System.out.print(“>>TubeLight.energyCost(int):“);
if(noOfHours==0)
thrownewZeroHoursException();
doublecost=00.10*noOfHours;
System.out.println(“Energycostfor”+lightType+“:”+cost);
returncost;
}
publicdoubleenergyCost(){//(8)Overloadingmethodat(7).
System.out.print(“>>TubeLight.energyCost():“);
doubleflatrate=20.00;
System.out.println(“Energycostfor”+lightType+“:”+flatrate);
returnflatrate;
}
@Override
publicTubeLightmakeInstance(){//(9)Overridinginstancemethodat
(3).
System.out.print(“>>TubeLight.makeInstance():“);
returnnewTubeLight();
}
publicstaticvoidprintLightType(){//(10)Hidingstaticmethodat(5).
System.out.print(“>>StaticTubeLight.printLightType():“);
System.out.println(lightType);
}
}
//______________________________________________________________________________
publicclassClient2{
publicstaticvoidmain(String[]args)//(11)
throwsInvalidHoursException{
TubeLighttubeLight=newTubeLight();//(12)
Lightlight1=tubeLight;//(13)Aliases.
Lightlight2=newLight();//(14)
System.out.println(“Invokeoverriddeninstancemethod:”);
tubeLight.energyCost(50);//(15)Invokesmethodat(7).
light1.energyCost(50);//(16)Invokesmethodat(7).
light2.energyCost(50);//(17)Invokesmethodat(2).
System.out.println(
”\nInvokeoverriddeninstancemethodwithcovariantreturn:”);
System.out.println(
light2.makeInstance().getClass());//(18)Invokesmethodat(3).
System.out.println(
tubeLight.makeInstance().getClass());//(19)Invokesmethodat(9).
System.out.println(“\nAccesshiddenfield:”);
System.out.println(tubeLight.lightType);//(20)Accessesfieldat(6).
System.out.println(light1.lightType);//(21)Accessesfieldat(1).
System.out.println(light2.lightType);//(22)Accessesfieldat(1).
System.out.println(“\nInvokehiddenstaticmethod:”);
tubeLight.printLightType();//(23)Invokesmethodat
(10).
light1.printLightType();//(24)Invokesmethodat(5).
light2.printLightType();//(25)Invokesmethodat(5).
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System.out.println(“\nInvokeoverloadedmethod:”);
tubeLight.energyCost();//(26)Invokesmethodat(8).
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Invokeoverriddeninstancemethod:
>>TubeLight.energyCost(int):EnergycostforTubeLight:5.0
>>TubeLight.energyCost(int):EnergycostforTubeLight:5.0
>>Light.energyCost(int):EnergycostforGenericLight:10.0
Invokeoverriddeninstancemethodwithcovariantreturn:
>>Light.makeInstance():classLight
>>TubeLight.makeInstance():classTubeLight
Accesshiddenfield:
TubeLight
GenericLight
GenericLight
Invokehiddenstaticmethod:
>>StaticTubeLight.printLightType():TubeLight
>>StaticLight.printLightType():GenericLight
>>StaticLight.printLightType():GenericLight
Invokeoverloadedmethod:
>>TubeLight.energyCost():EnergycostforTubeLight:20.0
Hereareafewmorefactstonoteaboutoverriding.First,asubclassmustusethekeyword
supertoinvokeanoverriddenmethodinthesuperclass(p.276).
Second,afinalmethodcannotbeoverridden,becausethemodifierfinalprevents
methodoverriding.Anattempttooverrideafinalmethodwillresultinacompile-time
error.Anabstractmethod,incontrast,requiresthenon-abstractsubclassestooverride
themethod,soastoprovideanimplementation.
Third,theaccessibilitymodifierprivateforamethodmeansthatthemethodisnot
accessibleoutsidetheclassinwhichitisdefined;therefore,asubclasscannotoverrideit.
However,asubclasscangiveitsowndefinitionofsuchamethod,whichmayhavethe
samesignatureasthemethodinitssuperclass.
Fourth,asubclasswithinthesamepackageasthesuperclasscanoverrideanynon-final
andnon-privatemethodsdeclaredinthesuperclass.However,asubclassinadifferent
packagecanoverrideonlythenon-finalmethodsthataredeclaredaseitherpublicor
protectedinthesuperclass.
Fifth,aninstancemethodinasubclasscannotoverrideastaticmethodinthe
superclass.Thecompilerwillflagsuchanattemptasanerror.Astaticmethodisclass-
specificandnotpartofanyobject,whileoverridingmethodsareinvokedonbehalfof
objectsofthesubclass.However,astaticmethodinasubclasscanhideastaticmethod
inthesuperclass,asweshallsee(p.275).Constructors,sincetheyarenotmethods,
cannotbeoverridden.
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Covariant inOverridingMethods
InExample7.2,thedefinitionofthemethodmakeInstance()at(9)overridesthe
methoddefinitionat(3).Notethatthemethodsignaturesarethesame,butthereturntype
at(9)isasubtypeofthereturntypeat(3).Themethodat(9)returnsanobjectofthe
subtypeTubeLight,whereasthemethodat(3)returnsanobjectofthesupertype
Light.Thisisanexampleofcovariantreturn.
DependingonwhetherwecallthemethodmakeInstance()onanobjectofthe
subtypeTubeLightoranobjectofthesupertypeLight,therespectivemethod
definitionwillbeexecuted.Thecodeat(18)and(19)illustrateswhichobjectisreturned
bythemethod,dependingonwhichmethoddefinitionisexecuted.
Notethatcovariantreturnappliesonlytoreferencetypes,nottoprimitivetypes.For
example,changingthereturntypeoftheenergyCost()methodat(7)tofloatwill
resultinacompile-timeerror.Thereisnosupertype–subtyperelationshipbetween
primitivetypes.
OverridingversusOverloading
Methodoverridingshouldnotbeconfusedwithmethodoverloading(§3.2,p.52).
Methodoverridingalwaysrequiresthesamemethodsignature(nameandparametertypes)
andthesameorcovariantreturntypes.Overloadingoccurswhenthemethodnamesare
thesame,buttheparameterlistsdiffer.Therefore,tooverloadmethods,theparameters
mustdifferineithertype,order,ornumber.Asthereturntypeisnotapartofthemethod
signature,justhavingdifferentreturntypesisnotenoughtooverloadmethods.
Onlynon-finalinstancemethodsinthesuperclassthataredirectlyaccessiblefromthe
subclassusingtheirsimplenamecanbeoverridden.Bothinstanceandstaticmethodscan
beoverloadedintheclasstheyaredefinedinorinasubclassoftheirclass.
Invokinganoverriddenmethodinthesuperclassfromasubclassrequiresaspecialsyntax
(e.g.,thekeywordsuper).Thisisnotnecessaryforinvokinganoverloadedmethodin
thesuperclassfromasubclass.Iftherightkindsofargumentsarepassedinthemethod
calloccurringinthesubclass,theoverloadedmethodinthesuperclasswillbeinvoked.In
Example7.2,themethodenergyCost()at(2)inclassLightisoverriddeninclass
TubeLightat(7)andoverloadedat(8).Wheninvokedat(26),theoverloaded
definitionat(8)isexecuted.
Foroverloadedmethods,whichmethodimplementationwillbeexecutedatruntimeis
determinedatcompiletime(§7.10,p.316).Incontrast,foroverriddenmethods,the
methodimplementationtobeexecutedisdeterminedatruntime(§7.12,p.329).Table7.1
providesacomparisonbetweenoverridingandoverloading.
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Table7.1OverridingversusOverloading
7.3HidingMembers
FieldHiding
Asubclasscannotoverrideinheritedfieldsofthesuperclass,butitcanhidethem.The
subclasscandefinefieldswiththesamenameasinthesuperclass.Ifthisisthecase,the
fieldsinthesuperclasscannotbeaccessedinthesubclassbytheirsimplenames;
therefore,theyarenotinheritedbythesubclass.Ahiddenstaticfieldcanalwaysbe
invokedbyusingthesuperclassnameinthesubclassdeclaration.Additionally,the
keywordsupercanbeusedinnon-staticcodeinthesubclassdeclarationtoaccess
hiddenstaticfields(§7.4,p.276).
Thefollowingdistinctionbetweeninvokinginstancemethodsonanobjectandaccessing
fieldsofanobjectmustbenoted.Whenaninstancemethodisinvokedonanobjectusing
areference,itisthedynamictypeofthereference(i.e.,thetypeofthecurrentobject
denotedbythereferenceatruntime),notthedeclaredtypeofthereference,that
determineswhichmethodimplementationwillbeexecuted.InExample7.2at(15),(16),
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and(17),thisisevidentfrominvokingtheoverriddenmethodenergyCost():The
methodfromtheclasscorrespondingtothecurrentobjectisexecuted,regardlessofthe
declaredreferencetype.Whenafieldofanobjectisaccessedusingareference,itisthe
declaredtypeofthereference,notthetypeofthecurrentobjectdenotedbythereference,
thatdetermineswhichfieldwillactuallybeaccessed.InExample7.2at(20),(21),and
(22),thisisevidentfromaccessingthehiddenfieldlightType:Thefieldaccessedis
theonedeclaredintheclasscorrespondingtothedeclaredreferencetype,regardlessof
theobjectdenotedbythereferenceatruntime.
Incontrasttomethodoverriding,whereaninstancemethodcannotoverrideastatic
method,therearenosuchrestrictionsonthehidingoffields.ThefieldlightTypeis
staticinthesubclass,butnotinthesuperclass.Thedeclaredtypeofthefieldsneednot
bethesameeither—onlythefieldnamemattersinthehidingoffields.
StaticMethodHiding
Astaticmethodinasubclasscannotoverrideaninstancemethodfromthesuperclass,but
itcanhideastaticmethodfromthesuperclassiftheexactrequirementsforoverriding
instancemethodsarefulfilled(§7.2,p.268).Ahiddensuperclassstaticmethodisnot
inherited.Thecompilerwillflagthecodeascontaininganerrorifthesignaturesarethe
same,buttheotherrequirementsregardingreturntype,throwsclause,andaccessibility
arenotmet.Ifthesignaturesaredifferent,themethodnameisoverloaded,nothidden.
Acalltoastaticorfinalmethodisboundtoamethodimplementationatcompile
time(privatemethodsareimplicitlyfinal).Example7.2illustratesinvocationof
staticmethods.Analogoustoaccessingfields,thestaticmethodinvokedin(23),(24),and
(25)isdeterminedbythedeclaredtypeofthereference.In(23),thedeclaredreference
typeisTubeLight;therefore,thestaticmethodprintLightType()at(10)inthis
classisinvoked.In(24)and(25),thedeclaredreferencetypeisLight,andthehidden
staticmethodprintLightType()at(5)inthatclassisinvoked.Thisisborneoutby
theoutputfromtheprogram.
Analogoustohiddenfields,ahiddenstaticmethodcanalwaysbeinvokedbyusingthe
superclassnameorbyusingthekeywordsuperinnon-staticcodeinthesubclass
declaration(§7.4,p.276).
Table7.2summarizestheconsequenceswhenasubclassmethodhasthesamesignatureas
amethodinthesuperclass.
Table7.2SameSignatureforSubclassandSuperclassMethod
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7.4TheObjectReference
Thethisreferencecanbeusedinnon-staticcodetorefertothecurrentobject(§3.2,p.
50).Thekeywordsuper,incontrast,canbeusedinnon-staticcodetoaccessfieldsand
invokemethodsfromthesuperclass(Table4.1,p.115).Thekeywordsuperprovidesa
referencetothecurrentobjectasaninstanceofitssuperclass.Inmethodinvocationswith
super,themethodfromthesuperclassisinvokedregardlessofwhattheactualtypeof
thecurrentobjectisorwhetherthecurrentclassoverridesthemethod.Thisapproachis
typicallyusedtoinvokemethodsthatareoverridden,andtoaccessmembersthatare
hiddeninthesubclass.Unlikethethiskeyword,thesuperkeywordcannotbeusedas
anordinaryreference.Forexample,itcannotbeassignedtootherreferencesorcastto
otherreferencetypes.
Example7.3usestheclassesLightandTubeLightfromExample7.2,whicharealso
showninFigure7.2.InExample7.3,theclassNeonLightextendstheclass
TubeLight.Thedeclarationofthemethoddemonstrate()at(11)intheclass
NeonLightmakesuseofthesuperkeywordtoaccessmembershigherupinits
inheritancehierarchy.ThisisthecasewhentheshowSign()methodisinvokedat(12).
Thismethodisdefinedat(4)intheclassLight,ratherthanintheimmediatesuperclass
TubeLightofthesubclassNeonLight.TheoverriddenmethodenergyCost()at
(7)anditsoverloadedversionat(8)intheclassTubeLightareinvoked,usingthe
objectreferencesuperat(13)and(14),respectively.
ThesuperclassLighthasafieldnamedlightTypeandamethodnamed
energyCostdefinedat(1)and(2),respectively.Onemightbetemptedtousethe
syntaxsuper.super.energyCost(20)inthesubclassNeonLighttoinvokethis
method,butthisisnotavalidconstruct.Onemightalsobetemptedtocastthethis
referencetotheclassLightandtryagain,asshownat(15).Theoutputshowsthatthe
methodenergyCost()at(7)intheclassTubeLightwasexecuted,nottheonefrom
theclassLight.Thereasonisthatacastsimplychangesthetypeofthereference(inthis
casetoLight),nottheclassoftheobject(whichisstillNeonLight).Method
invocationisdeterminedbytheclassofthecurrentobject,resultingintheinherited
methodenergyCost()intheclassTubeLightbeingexecuted.Thereisnowayto
invokethemethodenergyCost()intheclassLightfromthesubclassNeonLight,
withoutdeclaringareferenceofthetypeLight.
At(16),thekeywordsuperisusedtoaccessthefieldlightTypeat(6)intheclass
TubeLight,butisredundantinthiscase.At(17),thefieldlightTypefromtheclass
Lightisaccessedsuccessfullybycastingthethisreference,becauseitisthetypeof
thereferencethatdetermineswhichfieldisaccessed.Fromnon-staticcodeinasubclass,
itispossibletodirectlyaccessfieldsinaclasshigherupintheinheritancehierarchyby
castingthethisreference.However,itisfutiletocastthethisreferencetoinvoke
instancemethodsinaclasshigherupintheinheritancehierarchy,asillustratedearlierfor
theoverriddenmethodenergyCost().
Finally,thecallstothestaticmethodsat(18)and(19)usingthesuperandthis
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references,respectively,exhibitruntimebehavioranalogoustoaccessingfields,as
discussedpreviously.
Example7.3UsingthesuperKeyword
Clickheretoviewcodeimage
//File:Client3.java
//Exceptions
classInvalidHoursExceptionextendsException{}
classNegativeHoursExceptionextendsInvalidHoursException{}
classZeroHoursExceptionextendsInvalidHoursException{}
classLight{
protectedStringlightType=“GenericLight”;//(1)Instancefield
protecteddoubleenergyCost(intnoOfHours)//(2)Instancemethod
throwsInvalidHoursException{
System.out.print(“>>Light.energyCost(int):“);
if(noOfHours<0)
thrownewNegativeHoursException();
doublecost=00.20*noOfHours;
System.out.println(“Energycostfor”+lightType+“:”+cost);
returncost;
}
publicLightmakeInstance(){//(3)Instancemethod
System.out.print(“>>Light.makeInstance():“);
returnnewLight();
}
publicvoidshowSign(){//(4)Instancemethod
System.out.print(“>>Light.showSign():“);
System.out.println(“Lettherebelight!”);
}
publicstaticvoidprintLightType(){//(5)Staticmethod
System.out.print(“>>StaticLight.printLightType():“);
System.out.println(“GenericLight”);
}
}
//______________________________________________________________________________
classTubeLightextendsLight{
publicstaticStringlightType=“TubeLight”;//(6)Hidingfieldat(1).
@Override
publicdoubleenergyCost(finalintnoOfHours)//(7)Overridinginstance
throwsZeroHoursException{//methodat(2).
System.out.print(“>>TubeLight.energyCost(int):“);
if(noOfHours==0)
thrownewZeroHoursException();
doublecost=00.10*noOfHours;
System.out.println(“Energycostfor”+lightType+“:”+cost);
returncost;
}
publicdoubleenergyCost(){//(8)Overloadingmethodat(7).
System.out.print(“>>TubeLight.energyCost():“);
doubleflatrate=20.00;
System.out.println(“Energycostfor”+lightType+“:”+flatrate);
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returnflatrate;
}
@Override
publicTubeLightmakeInstance(){//(9)Overridinginstancemethodat
(3).
System.out.print(“>>TubeLight.makeInstance():“);
returnnewTubeLight();
}
publicstaticvoidprintLightType(){//(10)Hidingstaticmethodat(5).
System.out.print(“>>StaticTubeLight.printLightType():“);
System.out.println(lightType);
}
}
//______________________________________________________________________________
classNeonLightextendsTubeLight{
//…
publicvoiddemonstrate()//(11)
throwsInvalidHoursException{
super.showSign();//(12)Invokesmethodat
(4)
super.energyCost(50);//(13)Invokesmethodat
(7)
super.energyCost();//(14)Invokesmethodat
(8)
((Light)this).energyCost(50);//(15)Invokesmethodat
(7)
System.out.println(super.lightType);//(16)Accessesfieldat
(6)
System.out.println(((Light)this).lightType);//(17)Accessesfieldat
(1)
super.printLightType();//(18)Invokesmethodat
(10)
((Light)this).printLightType();//(19)Invokesmethodat
(5)
}
}
//______________________________________________________________________________
publicclassClient3{
publicstaticvoidmain(String[]args)
throwsInvalidHoursException{
NeonLightneonRef=newNeonLight();
neonRef.demonstrate();
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
>>Light.showSign():Lettherebelight!
>>TubeLight.energyCost(int):EnergycostforTubeLight:5.0
>>TubeLight.energyCost():EnergycostforTubeLight:20.0
>>TubeLight.energyCost(int):EnergycostforTubeLight:5.0
TubeLight
GenericLight
>>StaticTubeLight.printLightType():TubeLight
>>StaticLight.printLightType():GenericLight
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ReviewQuestions
7.1Whichofthefollowingstatementsaretrue?
Selectthetwocorrectanswers.
(a)InJava,theextendsclauseisusedtospecifytheinheritancerelationship.
(b)Thesubclassofanon-abstractclasscanbedeclaredasabstract.
(c)Allmembersofthesuperclassareinheritedbythesubclass.
(d)Afinalclasscanbeabstract.
(e)Aclassinwhichallthemembersaredeclaredprivatecannotbedeclaredas
public.
7.2Whichofthefollowingstatementsaretrue?
Selectthetwocorrectanswers.
(a)Aclasscanbeextendedbyonlyoneclass.
(b)EveryJavaobjecthasapublicmethodnamedequals.
(c)EveryJavaobjecthasapublicmethodnamedlength.
(d)Aclasscanextendanynumberofclasses.
(e)Anon-finalclasscanbeextendedbyanynumberofclasses.
7.3Giventhefollowingclassesanddeclarations,whichstatementsaretrue?
Clickheretoviewcodeimage
//Classes
classFoo{
privateinti;
publicvoidf(){/*…*/}
publicvoidg(){/*…*/}
}
classBarextendsFoo{
publicintj;
publicvoidg(){/*…*/}
}
//Declarations:
Fooa=newBar();
Barb=newBar();
Selectthethreecorrectanswers.
(a)TheBarclassisasubclassofFoo.
(b)Thestatementb.f();islegal.
(c)Thestatementa.j=5;islegal.
(d)Thestatementa.g();islegal.
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(e)Thestatementb.i=3;islegal.
7.4GivenclassesA,B,andC,whereBextendsA,andCextendsB,andwhereall
classesimplementtheinstancemethodvoiddoIt(),howcanthedoIt()
methodinAbecalledfromaninstancemethodinC?
Selecttheonecorrectanswer.
(a)doIt();
(b)super.doIt();
(c)super.super.doIt();
(d)this.super.doIt();
(e)A.this.doIt();
(f)((A)this).doIt();
(g)Itisnotpossible.
7.5Whatwouldbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassUserClass{
publicstaticvoidmain(String[]args){
Bb=newC();
System.out.println(b.max(13,29));
}
}
classA{
intmax(intx,inty){if(x>y)returnx;elsereturny;}
}
classBextendsA{
intmax(intx,inty){returnsuper.max(y,x)-10;}
}
classCextendsB{
intmax(intx,inty){returnsuper.max(x+10,y+10);}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompile.
(b)Thecodewillcompile,butthrowanexceptionatruntime.
(c)Thecodewillcompile,andprint13atruntime.
(d)Thecodewillcompile,andprint23atruntime.
(e)Thecodewillcompile,andprint29atruntime.
(f)Thecodewillcompile,andprint39atruntime.
7.6Whichisthesimplestexpressionthatcanbeinsertedat(1),sothattheprogram
printsthevalueofthetextfieldfromtheMessageclass?
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Clickheretoviewcodeimage
//File:MyClass.java
classMessage{
//Themessagethatshouldbeprinted:
Stringtext=“Hello,world!”;
}
classMySuperclass{
Messagemsg=newMessage();
}
publicclassMyClassextendsMySuperclass{
publicstaticvoidmain(String[]args){
MyClassobject=newMyClass();
object.print();
}
publicvoidprint(){
System.out.println(/*(1)INSERTTHESIMPLESTEXPRESSIONHERE*/);
}
}
Selecttheonecorrectanswer.
(a)text
(b)Message.text
(c)msg.text
(d)this.msg.text
(e)super.msg.text
(f)this.super.msg.text
7.7Whatwouldbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
classVehicle{
staticpublicStringgetModelName(){return“Volvo”;}
publiclonggetRegNo(){return12345;}
}
classCarextendsVehicle{
staticpublicStringgetModelName(){return“Toyota”;}
publiclonggetRegNo(){return54321;}
}
publicclassTakeARide{
publicstaticvoidmain(String[]args){
Carc=newCar();
Vehiclev=c;
System.out.println(“|”+v.getModelName()+“|”+c.getModelName()+
”|”+v.getRegNo()+“|”+c.getRegNo()+
“|”);
}
}
Selecttheonecorrectanswer.
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(a)Thecodewillfailtocompile.
(b)Thecodewillcompile,andprint|Toyota|Volvo|12345|54321|at
runtime.
(c)Thecodewillcompile,andprint|Volvo|Toyota|12345|54321|at
runtime.
(d)Thecodewillcompile,andprint|Toyota|Toyota|12345|12345|at
runtime.
(e)Thecodewillcompile,andprint|Volvo|Volvo|12345|54321|at
runtime.
(f)Thecodewillcompile,andprint|Toyota|Toyota|12345|12345|at
runtime.
(g)Thecodewillcompile,andprint|Volvo|Toyota|54321|54321|at
runtime.
7.5ChainingConstructorsUsing and
Constructorsarediscussedin§3.3,p.53.Otherusesofthekeywordsthisandsuper
canbefoundin§7.2,p.268.
The ConstructorCall
Constructorscannotbeinheritedoroverridden.Theycanbeoverloaded,butonlyinthe
sameclass.Sinceaconstructoralwayshasthesamenameastheclass,eachparameterlist
mustbedifferentwhendefiningmorethanoneconstructorforaclass.InExample7.4,the
classLighthasthreeoverloadedconstructors.Intheconstructorat(3),thethis
referenceisusedtoaccessthefieldsshadowedbytheparameters.Inthemain()method
at(4),theappropriateconstructorisinvokeddependingontheargumentsinthe
constructorcall,asillustratedbytheprogramoutput.
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Example7.4ConstructorOverloading
Clickheretoviewcodeimage
//File:DemoConstructorCall.java
classLight{
//Fields:
privateintnoOfWatts;//wattage
privatebooleanindicator;//onoroff
privateStringlocation;//placement
//Constructors:
Light(){//(1)No-argumentconstructor
noOfWatts=0;
indicator=false;
location=“X”;
System.out.println(“Returningfromno-argumentconstructorno.1.”);
}
Light(intwatts,booleanonOffState){//(2)
noOfWatts=watts;
indicator=onOffState;
location=“X”;
System.out.println(“Returningfromconstructorno.2.”);
}
Light(intnoOfWatts,booleanindicator,Stringlocation){//(3)
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
System.out.println(“Returningfromconstructorno.3.”);
}
}
//______________________________________________________________________________
publicclassDemoConstructorCall{
publicstaticvoidmain(String[]args){//(4)
System.out.println(“CreatingLightobjectno.1.”);
Lightlight1=newLight();
System.out.println(“CreatingLightobjectno.2.”);
Lightlight2=newLight(250,true);
System.out.println(“CreatingLightobjectno.3.”);
Lightlight3=newLight(250,true,“attic”);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
CreatingLightobjectno.1.
Returningfromno-argumentconstructorno.1.
CreatingLightobjectno.2.
Returningfromconstructorno.2.
CreatingLightobjectno.3.
Returningfromconstructorno.3.
Example7.5illustratestheuseofthethis()construct,whichisusedtoimplementlocal
chainingofconstructorsintheclasswhenaninstanceoftheclassiscreated.Thefirsttwo
constructorsat(1)and(2)fromExample7.4havebeenrewrittenusingthethis()
constructinExample7.5at(1)and(2),respectively.Thethis()constructcanbe
regardedasbeinglocallyoverloaded,sinceitsparameters(andhenceitssignature)can
vary,asshowninthebodyoftheconstructorsat(1)and(2).Thethis()callinvokesthe
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localconstructorwiththecorrespondingparameterlist.Inthemain()methodat(4),the
appropriateconstructorisinvokeddependingontheargumentsintheconstructorcall
wheneachofthethreeLightobjectsarecreated.Callingtheno-argumentconstructorat
(1)tocreateaLightobjectresultsintheconstructorsat(2)and(3)beingexecutedas
well.Thisisconfirmedbytheoutputfromtheprogram.Inthiscase,theoutputshowsthat
theconstructorat(3)completedfirst,followedbytheconstructorat(2),andfinallybythe
no-argumentconstructorat(1)thatwascalledfirst.Bearinginmindthedefinitionofthe
constructors,theconstructorsareinvokedinthereverseorder;thatis,invocationofthe
no-argumentconstructorimmediatelyleadstoinvocationoftheconstructorat(2)bythe
callthis(0,false),anditsinvocationleadstotheconstructorat(3)beingcalled
immediatelybythecallthis(watt,ind,"X"),withthecompletionofthe
executioninthereverseorderoftheirinvocation.Similarly,callingtheconstructorat(2)
tocreateaninstanceoftheLightclassresultsintheconstructorat(3)beingexecutedas
well.
Javarequiresthatanythis()callmustoccurasthefirststatementinaconstructor.The
this()callcanbefollowedbyanyotherrelevantcode.ThisrestrictionisduetoJava’s
handlingofconstructorinvocationinthesuperclasswhenanobjectofthesubclassis
created.Thismechanismisexplainedinthenextsubsection.
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Example7.5Thethis()ConstructorCall
Clickheretoviewcodeimage
//File:DemoThisCall.java
classLight{
//Fields:
privateintnoOfWatts;
privatebooleanindicator;
privateStringlocation;
//Constructors:
Light(){//(1)No-argumentconstructor
this(0,false);
System.out.println(“Returningfromno-argumentconstructorno.1.”);
}
Light(intwatt,booleanind){//(2)
this(watt,ind,“X”);
System.out.println(“Returningfromconstructorno.2.”);
}
Light(intnoOfWatts,booleanindicator,Stringlocation){//(3)
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
System.out.println(“Returningfromconstructorno.3.”);
}
}
//______________________________________________________________________________
publicclassDemoThisCall{
publicstaticvoidmain(String[]args){//(4)
System.out.println(“CreatingLightobjectno.1.”);
Lightlight1=newLight();//(5)
System.out.println(“CreatingLightobjectno.2.”);
Lightlight2=newLight(250,true);//(6)
System.out.println(“CreatingLightobjectno.3.”);
Lightlight3=newLight(250,true,“attic”);//(7)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
CreatingLightobjectno.1.
Returningfromconstructorno.3.
Returningfromconstructorno.2.
Returningfromno-argumentconstructorno.1.
CreatingLightobjectno.2.
Returningfromconstructorno.3.
Returningfromconstructorno.2.
CreatingLightobjectno.3.
Returningfromconstructorno.3.
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The ConstructorCall
Thesuper()constructisusedinasubclassconstructortoinvokeaconstructorinthe
immediatesuperclass.Thisallowsthesubclasstoinfluencetheinitializationofits
inheritedstatewhenanobjectofthesubclassiscreated.Asuper()callinthe
constructorofasubclasswillresultintheexecutionoftherelevantconstructorfromthe
superclass,basedonthesignatureofthecall.Sincethesuperclassnameisknowninthe
subclassdeclaration,thecompilercandeterminethesuperclassconstructorinvokedfrom
thesignatureoftheparameterlist.
Aconstructorinasubclasscanaccesstheclass’sinheritedmembersbytheirsimple
names.Thekeywordsupercanalsobeusedinasubclassconstructortoaccessinherited
membersviaitssuperclass.Onemightbetemptedtousethesuperkeywordina
constructortospecifyinitialvaluesforinheritedfields.However,thesuper()construct
providesabettersolutiontoinitializetheinheritedstate.
InExample7.6,theconstructorat(3)oftheclassLighthasasuper()call(withno
arguments)at(4).Althoughtheconstructorisnotstrictlynecessary,asthecompilerwill
insertone—asexplainedlater—itisincludedhereforexpositionalpurposes.The
constructorat(6)oftheclassTubeLighthasasuper()call(withthreearguments)at
(7).Thissuper()callwillmatchtheconstructorat(3)ofthesuperclassLight.Thisis
evidentfromtheprogramoutput.
Example7.6Thesuper()ConstructorCall
Clickheretoviewcodeimage
//File:Chaining.java
classLight{
//Fields:
privateintnoOfWatts;
privatebooleanindicator;
privateStringlocation;
//Constructors:
Light(){//(1)No-argumentconstructor
this(0,false);
System.out.println(
“Returningfromno-argumentconstructorno.1inclassLight”);
}
Light(intwatt,booleanind){//(2)
this(watt,ind,“X”);
System.out.println(
“Returningfromconstructorno.2inclassLight”);
}
Light(intnoOfWatts,booleanindicator,Stringlocation){//(3)
super();//(4)
this.noOfWatts=noOfWatts;
this.indicator=indicator;
this.location=location;
System.out.println(
“Returningfromconstructorno.3inclassLight”);
}
}
//______________________________________________________________________________
classTubeLightextendsLight{
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//Instancevariables:
privateinttubeLength;
privateintcolorNo;
//Constructors:
TubeLight(inttubeLength,intcolorNo){//(5)
this(tubeLength,colorNo,100,true,“Unknown”);
System.out.println(
“Returningfromconstructorno.1inclassTubeLight”);
}
TubeLight(inttubeLength,intcolorNo,intnoOfWatts,
booleanindicator,Stringlocation){//(6)
super(noOfWatts,indicator,location);//(7)
this.tubeLength=tubeLength;
this.colorNo=colorNo;
System.out.println(
“Returningfromconstructorno.2inclassTubeLight”);
}
}
//______________________________________________________________________________
publicclassChaining{
publicstaticvoidmain(String[]args){
System.out.println(“CreatingaTubeLightobject.”);
TubeLighttubeLightRef=newTubeLight(20,5);//(8)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
CreatingaTubeLightobject.
Returningfromconstructorno.3inclassLight
Returningfromconstructorno.2inclassTubeLight
Returningfromconstructorno.1inclassTubeLight
Thesuper()constructhasthesamerestrictionsasthethis()construct:Ifused,the
super()callmustoccurasthefirststatementinaconstructor,anditcanonlybeusedin
aconstructordeclaration.Thisimpliesthatthis()andsuper()callscannotboth
occurinthesameconstructor.Thethis()constructisusedtochainconstructorsinthe
sameclass.Theconstructorattheendofsuchachaincaninvokeasuperclassconstructor
usingthesuper()construct.Justasthethis()constructleadstochainingof
constructorsinthesameclass,sothesuper()constructleadstochainingofsubclass
constructorstosuperclassconstructors.Thischainingbehaviorguaranteesthatall
superclassconstructorsarecalled,startingwiththeconstructoroftheclassbeing
instantiated,allthewaytothetopoftheinheritancehierarchy,whichisalwaysthe
Objectclass.Notethatthebodyoftheconstructorisexecutedinthereverseordertothe
callorder,asthesuper()callcanoccuronlyasthefirststatementinaconstructor.This
orderofexecutionensuresthattheconstructorfromtheObjectclassiscompletedfirst,
followedbytheconstructorsintheotherclassesdowntotheclassbeinginstantiatedinthe
inheritancehierarchy.Itiscalled(subclass–superclass)constructorchaining.Theoutput
fromExample7.6clearlyillustratesthischainofeventswhenanobjectoftheclass
TubeLightiscreated.
Ifaconstructorattheendofathis()chain(whichmaynotbeachainatallifno
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this()callisinvoked)doesnothaveanexplicitcalltosuper(),thecallsuper()
(withouttheparameters)isimplicitlyinsertedbythecompilertoinvoketheno-argument
constructorofthesuperclass.Inotherwords,ifaconstructorhasneitherathis()call
norasuper()callasitsfirststatement,thecompilerinsertsasuper()calltotheno-
argumentconstructorinthesuperclass.Thecode
Clickheretoviewcodeimage
classA{
A(){}//No-argumentconstructor.
//…
}
classBextendsA{//Noconstructors.
//…
}
isequivalentto
Clickheretoviewcodeimage
classA{
A(){super();}//(1)Calltono-argumentsuperclassconstructor
inserted.
//…
}
classBextendsA{
B(){super();}//(2)Defaultconstructorinserted.
//…
}
wherethecompilerinsertsasuper()callintheno-argumentconstructorforclassAat
(1)andinsertsthedefaultconstructorforclassBat(2).Thesuper()callat(2)will
resultinacalltotheno-argumentconstructorinBat(1),andthesuper()callat(1)will
resultinacalltotheno-argumentconstructorinthesuperclassofA—thatis,theObject
class.
Ifasuperclassdefinesjustnon-zeroargumentconstructors(i.e.,onlyconstructorswith
parameters),itssubclassescannotrelyontheimplicitsuper()callbeinginserted.This
willbeflaggedasacompile-timeerror.Thesubclassesmustthenexplicitlycalla
superclassconstructor,usingthesuper()constructwiththerightarguments.
Clickheretoviewcodeimage
classNeonLightextendsTubeLight{
//Field
Stringsign;
NeonLight(){//(1)
super(10,2,100,true,“Roof-top”);//(2)Cannotbecommentedout.
sign=“Allwillberevealed!”;
}
//…
}
TheprecedingdeclarationofthesubclassNeonLightprovidesano-argument
constructorat(1).Thecalloftheconstructorat(2)inthesuperclassTubeLightcannot
beomitted.Ifitisomitted,anyinsertionofasuper()call(withnoarguments)inthis
constructorwilltrytomatchano-argumentconstructorinthesuperclassTubeLight,
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whichprovidesonlynon-zeroargumentconstructors.TheclassNeonLightwillnot
compileunlessanexplicitvalidsuper()callisinsertedat(2).
Ifthesuperclassprovidesjustnon-zeroargumentconstructors(i.e.,itdoesnothaveano-
argumentconstructor),thishasimplicationsforitssubclasses.Asubclassthatreliesonits
defaultconstructorwillfailtocompile,becausethedefaultconstructorofthesubclasswill
attempttocallthe(nonexistent)no-argumentconstructorinthesuperclass.Aconstructor
inasubclassmustexplicitlyusethesuper()call,withtheappropriatearguments,to
invokeanon-zeroargumentconstructorinthesuperclass.Thiscallisnecessarybecause
theconstructorinthesubclasscannotrelyonanimplicitsuper()calltotheno-
argumentconstructorinthesuperclass.
ReviewQuestions
7.8Whichconstructorscanbeinsertedat(1)inMySubwithoutcausingacompile-
timeerror?
Clickheretoviewcodeimage
classMySuper{
intnumber;
MySuper(inti){number=i;}
}
classMySubextendsMySuper{
intcount;
MySub(intcount,intnum){
super(num);
this.count=count;
}
//(1)INSERTCONSTRUCTORHERE
}
Selecttheonecorrectanswer.
(a)MySub(){}
(b)MySub(intcount){this.count=count;}
(c)MySub(intcount){super();this.count=count;}
(d)MySub(intcount){this.count=count;super(count);
}
(e)MySub(intcount){this(count,count);}
(f)MySub(intcount){super(count);this(count,0);}
7.9Whichofthefollowingstatementsistrue?
Selecttheonecorrectanswer.
(a)Asuper()orthis()callmustalwaysbeprovidedexplicitlyasthefirst
statementinthebodyofaconstructor.
(b)Ifbothasubclassanditssuperclassdonothaveanydeclaredconstructors,the
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implicitdefaultconstructorofthesubclasswillcallsuper()whenrun.
(c)Ifneithersuper()northis()isspecifiedasthefirststatementinthebody
ofaconstructor,this()willimplicitlybeinsertedasthefirststatement.
(d)Ifsuper()isthefirststatementinthebodyofaconstructor,this()canbe
declaredasthesecondstatement.
(e)Callingsuper()asthefirststatementinthebodyofaconstructorofa
subclasswillalwayswork,sinceallsuperclasseshaveadefaultconstructor.
7.10Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
Bb=newB(“Test”);
}
}
classA{
A(){this(“1”,“2”);}
A(Strings,Stringt){this(s+t);}
A(Strings){System.out.println(s);}
}
classBextendsA{
B(Strings){System.out.println(s);}
B(Strings,Stringt){this(t+s+“3”);}
B(){super(“4”);};
}
Selecttheonecorrectanswer.
(a)ItwilljustprintTest.
(b)ItwillprintTestfollowedbyTest.
(c)Itwillprint123followedbyTest.
(d)Itwillprint12followedbyTest.
(e)Itwillprint4followedbyTest.
7.6Interfaces
Extendingclassesusingsingleimplementationinheritancecreatesnewclasstypes.A
superclassreferencecanrefertoobjectsofitsowntypeanditssubclassesstrictly
accordingtotheinheritancehierarchy.Becausethisrelationshipislinear,itrulesout
multipleimplementationinheritance,inwhichasubclassinheritsfrommorethanone
superclass.InsteadJavaprovidesinterfaces,whichnotonlyallownewnamedreference
typestobeintroduced,butalsopermitmultipleinterfaceinheritance.
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DefiningInterfaces
Atop-levelinterfacehasthefollowingsimplifiedsyntax,whichwillsufficeforthe
purposesofthisbook:
Clickheretoviewcodeimage
accessibility_modifierinterfaceinterface_name
extends_interface_clause//Interface
header
{//Interfacebody
abstract_method_declarations
default_method_declarations
static_method_declarations
constant_declarations
}
Intheinterfaceheader,thenameoftheinterfaceisprecededbythekeyword
interface.Theinterfacenamecanalsoincludealistofformaltypeparametersfor
declaringagenericinterface.Inaddition,theinterfaceheadercanspecifythefollowing
information:
•Theaccessibilitymodifiermustbepublic,andthelackofanaccessibility
modifierimpliespackageaccessibility,asonewouldexpect(§4.5,p.118).
•Theextendsinterfaceclausespecifiesacomma-separatedlistofanysuperinterfaces
thattheinterfaceextends(p.294).
Theinterfacebodycancontainmemberdeclarationsthatincludeanyofthefollowing:
•Abstractmethoddeclarations(p.291)
•Defaultmethoddeclarations(p.297)
•Staticmethoddeclarations(p.300)
•Constantdeclarations(p.302)
Aninterfaceisabstractbydefinition,whichmeansthatitcannotbeinstantiated.
Declaringaninterfaceasabstractissuperfluousandseldomdoneinpractice.Itisthe
onlynon-accessibilitymodifierthatcanbespecifiedforatop-levelinterface(apartfrom
thekeywordstrictfp).
Thememberdeclarationscanappearinanyorderintheinterfacebody,whichcanbe
empty.Sinceinterfacesaremeanttobeimplementedbyclasses,interfacemembers
implicitlyhavepublicaccessibilityandthepublicmodifiercanbeomitted.The
followingdeclarationisanexampleofabare-bonesinterfacethathasanemptybody:
interfacePlayable{}
Interfaceswithemptybodiescanbeusedasmarkerstotagclassesashavingacertain
propertyorbehavior.Suchinterfacesarealsocalledabilityinterfaces.TheJavaSE
platformAPIprovidesseveralexamplesofsuchmarkerinterfaces—namely,
java.lang.Cloneable,java.io.Serializable,and
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java.util.EventListener.
AbstractMethodsinInterfaces
Aninterfacedefinesacontractbyspecifyingasetofabstractanddefaultmethod
declarations,butprovidesimplementationsonlyforthedefaultmethods—notforthe
abstractmethods.Theabstractmethodsinaninterfaceareallimplicitlyabstractand
publicbyvirtueoftheirdefinitions.Onlythemodifiersabstractandpublicare
allowed,buttheseareinvariablyomitted.Anabstractmethoddeclarationhasthe
followingsimpleforminatop-levelinterface:
Clickheretoviewcodeimage
return_typemethod_name(formal_parameter_list)throws_clause;
Anabstractmethoddeclarationisessentiallyamethodheaderterminatedbyasemicolon
(;).Notethatanabstractmethodisaninstancemethodwhoseimplementationwillbe
providedbyaclassthatimplementstheinterfaceinwhichtheabstractmethodisdeclared.
Thethrowsclauseisdiscussedin§6.9,p.251.
TheinterfacePlayableshownnextdeclaresanabstractmethodstartPlaying().
Thatitispublicandabstractisimplicitlyimplied.
Clickheretoviewcodeimage
interfacePlayable{
voidstartPlaying();//Abstractmethod:noimplementation
}
Aninterfacethathasnodirectsuperinterfacesimplicitlydeclaresapublicabstractmethod
foreachpublicinstancemethodinthejava.lang.Objectclass.
Incontrasttothesyntaxofabstractmethodsintop-levelinterfaces,abstractmethodsin
top-levelclassesmustbeexplicitlyspecifiedwiththekeywordabstract,andcanhave
public,protected,andpackageaccessibility(§4.8,p.136).
Functionalinterfaces,meaninginterfaceswithasingleabstractmethod,arediscussed
togetherwithlambdaexpressionsin§10.2,p.442.
Therestofthischapterprovidesnumerousexamplesofusinginterfaces.
ImplementingInterfaces
Aclasscanimplement,whollyorpartially,zeroormoreinterfaces.Aclassspecifiesthe
interfacesitimplementsasacomma-separatedlistofuniqueinterfacenamesinan
implementsclauseintheclassheader.Theinterfacemethodsmustallhavepublic
accessibilitywhenimplementedintheclass(oritssubclasses).Aclasscanneithernarrow
theaccessibilityofaninterfacemethodnorspecifynewexceptionsinthemethod’s
throwsclause,asattemptingtodosowouldamounttoalteringtheinterface’scontract,
whichisillegal.Thecriteriaforoverridingmethodsalsoapplywhenimplementing
abstractmethods(§7.2,p.268).
Aclasscanprovideimplementationsofmethodsdeclaredinaninterface.Toreapthe
benefitsofinterfaces,however,theclassmustalsospecifytheinterfacenameinits
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implementsclause.
InExample7.7,theclassStackImplimplementstheinterfaceIStack.Itboth
specifiestheinterfacenameusingtheimplementsclauseinitsclassheaderat(2)and
providestheimplementationfortheabstractmethodsintheinterfaceat(3)and(4).
Changingthepublicaccessibilityofthesemethodsintheclasswillresultinacompile-
timeerror,asthiswouldnarrowtheiraccessibility.
Example7.7ImplementingInterfaces
Clickheretoviewcodeimage
//File:RetailSeller.java
interfaceIStack{//(1)
voidpush(Objectitem);
Objectpop();
}
//______________________________________________________________________________
classStackImplimplementsIStack{//(2)
protectedObject[]stackArray;
protectedinttos;//topofstack
publicStackImpl(intcapacity){
stackArray=newObject[capacity];
tos=-1;
}
@Override
publicvoidpush(Objectitem){stackArray[++tos]=item;}//(3)
@Override
publicObjectpop(){//(4)
ObjectobjRef=stackArray[tos];
stackArray[tos]=null;
tos—;
returnobjRef;
}
publicObjectpeek(){returnstackArray[tos];}
}
//______________________________________________________________________________
interfaceISafeStackextendsIStack{//(5)
booleanisEmpty();
booleanisFull();
}
//______________________________________________________________________________
classSafeStackImplextendsStackImplimplementsISafeStack{//(6)
publicSafeStackImpl(intcapacity){super(capacity);}
@OverridepublicbooleanisEmpty(){returntos<0;}//
(7)
@OverridepublicbooleanisFull(){returntos>=stackArray.length-1;}//
(8)
}
//______________________________________________________________________________
publicclassStackUser{
publicstaticvoidmain(String[]args){//(9)
SafeStackImplsafeStackRef=newSafeStackImpl(10);
StackImplstackRef=safeStackRef;
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ISafeStackisafeStackRef=safeStackRef;
IStackistackRef=safeStackRef;
ObjectobjRef=safeStackRef;
safeStackRef.push(“Dollars”);//(10)
stackRef.push(“Kroner”);
System.out.println(isafeStackRef.pop());
System.out.println(istackRef.pop());
System.out.println(objRef.getClass());
}
}
Outputfromtheprogram:
Kroner
Dollars
classSafeStackImpl
Aclasscanchoosetoimplementonlysomeoftheabstractmethodsofitsinterfaces(i.e.,
giveapartialimplementationofitsinterfaces).Theclassmustthenbedeclaredas
abstract(§4.6,p.120).Notethatabstractmethodscannotbedeclaredasstatic,
becausetheycomprisethecontractfulfilledbytheobjectsoftheclassimplementingthe
interface.Abstractmethodsarealwaysimplementedasinstancemethods.
Theinterfacesthataclassimplementsandtheclassesthatitextends(directlyor
indirectly)arecalledsupertypesoftheclass.Conversely,theclassisasubtypeofits
supertypes.Classesimplementinginterfacesintroducemultipleinterfaceinheritanceinto
theirimplementationinheritancehierarchy.Evenso,regardlessofhowmanyinterfacesa
classimplementsdirectlyorindirectly,itprovidesjustasingleimplementationofany
abstractmethoddeclaredinmultipleinterfaces.
Singleimplementationofanabstractmethodisillustratedbythefollowingcode,where
theWorkerclassat(5)providesonlyoneimplementationofthedoIt()methodthatis
declaredinbothinterfaces,at(1)and(2).TheclassWorkerfulfillsthecontractforboth
interfaces,asthedoIt()methoddeclarationsat(1)and(2)havethesamemethod
signatureandreturntype.However,theclassCombinedat(3)declaresthatit
implementsthetwointerfaces,butdoesnotprovideanyimplementationofthedoIt()
method;consequently,itmustbedeclaredasabstract.
Clickheretoviewcodeimage
interfaceIA{intdoIt();}//(1)
interfaceIB{intdoIt();}//(2)
abstractclassCombinedimplementsIA,IB{}//(3)
publicclassWorkerimplementsIA,IB{//(4)
@Override
publicintdoIt(){return0;}//(5)
}
IfthedoIt()methodsinthetwointerfacesat(1)and(2)hadthesamesignaturesbut
differentreturntypes,theWorkerclasswouldnotbeabletoimplementbothinterfaces.
Thisisillustratedbythenextcodesnippet.ThedoIt()methodsat(1)and(2)havethe
samesignature,butdifferentreturntypes.TheWorkerclassprovidestwo
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implementationsofthedoIt()methodat(5)and(6),whichresultsincompile-time
errors,becauseaclasscannothavetwomethodswiththesamesignaturebutdifferent
returntypes.RemovingeitherimplementationfromtheWorkerclasswillbeflaggedasa
compile-timeerror,becausetheWorkerclasswillnotbeimplementingbothinterfaces.
ThereisnowaytheWorkerclasscanimplementbothinterfaces,giventhedeclarations
showninthecode.Inaddition,theabstractclassCombinedat(3)willnotcompile,
becauseitwillbeinheritingtwomethodswithconflictingabstractmethoddeclarations.In
fact,thecompilercomplainsofduplicatemethods.
Clickheretoviewcodeimage
interfaceIA{intdoIt();}//(1)
interfaceIB{doubledoIt();}//(2)
abstractclassCombinedimplementsIA,IB{}//(3)Compile-time
error.
publicclassLameWorkerimplementsIA,IB{//(4)
@Override
publicintdoIt(){return0;}//(5)Compile-time
error.
@Override
publicdoubledoIt(){//(6)Compile-time
error.
System.out.println(“Sorry!”);
return=0.0;
}
}
ExtendingInterfaces
Aninterfacecanextendotherinterfaces,usingtheextendsclause.Unlikewhen
extendingclasses,aninterfacecanextendseveralinterfaces.Theinterfacesextendedby
aninterface(directlyorindirectly)arecalledsuperinterfaces.Conversely,theinterfaceis
asubinterfaceofitssuperinterfaces.Sinceinterfacesdefinenewreferencetypes,
superinterfacesandsubinterfacesarealsosupertypesandsubtypes,respectively.
Asubinterfaceinheritsfromitssuperinterfaces,allmembersofthosesuperinterfaces,
exceptforthefollowing:
•Abstractordefaultmethodsthatitoverrides(p.297)
•Anystaticmethodsdeclaredinitssuperinterfaces(p.300)
•Anyconstantsthatithides(p.302)
Barringanyconflicts,asubinterfaceinheritsabstractanddefaultmethoddeclarationsthat
arenotoverridden,aswellasconstantsthatitdoesnothideinitssuperinterfaces.
Abstract,static,anddefaultmethoddeclarationscanalsobeoverloaded,analogousto
methodoverloadinginclasses.
Example7.7illustratestherelationshipsbetweenclassesandinterfaces.InExample7.7,
theinterfaceISafeStackextendstheinterfaceIStackat(5).Theclass
SafeStackImplbothextendstheStackImplclassandimplementsthe
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ISafeStackinterfaceat(6).Boththeimplementationandtheinterfaceinheritance
hierarchiesforclassesandinterfacesdefinedinExample7.7areshowninFigure7.3.
Figure7.3InheritanceHierarchies
InUML,aninterfaceresemblesaclass.Onewaytodifferentiatebetweenthemistouse
an«interface»stereotype,asinFigure7.3.Interfaceinheritanceisdepictedinasimilar
mannertoimplementationinheritance,butisindicatedbyanunbrokeninheritancearrow.
Thinkingintermsoftypes,everyreferencetypeinJavaisasubtypeoftheObjectclass.
Inturn,anyinterfacetypeisalsoasubtypeoftheObjectclass,butitdoesnotinherit
fromtheObjectclass.Aninterfacethathasnodirectsuperinterfacesimplicitlydeclares
apublicabstractmethodforeachpublicinstancemethodintheObjectclass.
Theseabstractmethoddeclarationsareinheritedbyallsubinterfacesofsuchaninterface.
Notethatthisdoesnotmeantheimplementationisinherited.Theimplicitpublic
abstractmethoddeclarationsinaninterfaceallowpublicinstancemethodsinthe
Objectclasstobeinvokedonobjectsreferredtobyaninterfacereference.Allclasses
implementthesemethods,whethertheyareinheritedoroverriddenfromtheObject
class.Anyinterfacecanalsoprovideexplicitpublicabstractmethoddeclarations
fornon-finalpublicinstancemethodsintheObjectclass.
Clickheretoviewcodeimage
interfaceIStack{//(1)
voidpush(Objectitem);
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Objectpop();
@Overridebooleanequals(Objectother);//publicmethodinObject
class.
@OverrideStringtoString();//publicmethodinObject
class.
//@OverrideClassgetClass();//Compile-timeerror!finalmethodinObject
class.
}
ItisinstructivetoconsiderhowtheclassSafeStackImplimplementstheIStack
interface:Itinheritstheimplementationsofthepush()andpop()methodsfromits
superclassStackImpl,whichitselfimplementstheIStackinterfaceinwhichthese
twomethodsaredeclared.TheclassSafeStackImplalsoimplementstheIStack
interfaceviatheISafeStackinterface.TheclassSafeStackImplprovidesitsown
implementationoftheisFull()andisEmpty()methodsdeclaredinthe
ISafeStackinterface,andhasinheritedimplementationsofthepush()andpop()
methodswhosedeclarationstheISafeStackinterfaceinheritsfromitssuperinterface
IStack.Thisisreadilyevidentfromthediamondshapeoftheinheritancehierarchyin
Figure7.3.Notethatthereisonlyonesingleimplementationinheritanceintotheclass
SafeStackImpl—fromitssuperclassStackImpl.Javadoesnotsupportmultiple
implementationinheritance.
Theassociationbetweenaclassandanyinterfaceitimplementsiscalledarealizationin
UML.InFigure7.3,therearethreerealizations:TheclassSafeStackImplimplements
theISafeStackinterfaceandalsoimplicitlyimplementstheIStackinterface,andthe
classStackImplimplementstheIStackinterface.
Thus,threedifferentinheritancerelationsareatworkwhendefininginheritanceamong
classesandinterfaces:
1.Singleimplementationinheritancehierarchybetweenclasses:aclassextends
anotherclass(subclasses–superclasses).
2.Multipleinheritancehierarchybetweeninterfaces:aninterfaceextendsother
interfaces(subinterfaces–superinterfaces).
3.Multipleinterfaceinheritancehierarchybetweenclassesandinterfaces:aclass
implementsinterfaces(realization).
InterfaceReferences
Althoughinterfacescannotbeinstantiated,referencesofaninterfacetypecanbedeclared.
Thereferencevalueofanobjectcanbeassignedtoreferencesoftheobject’ssupertypes.
InExample7.7,anobjectoftheclassSafeStackImpliscreatedinthemain()
methodoftheclassStackUserat(9).Thereferencevalueoftheobjectisassignedto
referencesofalltheobject’ssupertypes,whichareusedtomanipulatetheobject.The
referencesarealiasestothesameSafeStackImplobject,buttheycanonlybeusedto
manipulatethisobjectasanobjectofthereferencetype.Forexample,callingthemethod
isFull()onthisobjectusingthestackRefreferencewillbeflaggedasacompile-
timeerror,astheclassStackImpldoesnotprovidesuchamethod.Polymorphic
behaviorofsupertypereferencesisdiscussedin§7.12,p.329.
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DefaultMethodsinInterfaces
Onlyinterfacescandefinedefaultmethods.Adefaultmethodisaninstancemethod
declaredwiththekeyworddefaultandwhoseimplementationisprovidedbythe
interface.However,adefaultmethodinatop-levelinterfacealwayshaspublic
accessibility,whetherthekeywordpublicisspecifiedornot.
Clickheretoviewcodeimage
defaultreturn_typemethod_name(formal_parameter_list)throws_clause
{implementaion_of_method_body}
Aclassimplementinganinterfacecanoptionallydecidetooverrideanydefaultmethodin
theinterface.Iftheclassdoesnotoverrideadefaultmethodtoprovideanew
implementation,thedefaultimplementationprovidedbytheinterfaceisinheritedbythe
class.
Noothernon-accessibilitymodifiers,suchasabstract,final,orstatic,are
allowedinadefaultmethoddeclaration,exceptthekeywordstrictfp.Adefault
methodisnotabstractbecauseitprovidesanimplementation;isnotfinalbecauseit
canbeoverridden;andisnotstaticbecauseitcanbeinvokedonlyoninstancesofa
classthatimplementstheinterfaceinwhichthedefaultmethodisdeclared.
Example7.8illustratestheuseofdefaultmethods.Thedefaultmethod
printSlogan()at(1)intheinterfaceISloganisoverriddenat(2)intheclass
JavaGuru,andinheritedbytheclassJavaGeekat(3).Theoutputfromtheprogram
showsthatthisisthecase.
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Example7.8DefaultMethodsinInterfaces
Clickheretoviewcodeimage
//File:JavaParty.java
interfaceISlogan{
defaultvoidprintSlogan(){//(1)
System.out.println(“Happinessisgettingcertified!”);
}
}
//_______________________________________________________________________________
classJavaGuruimplementsISlogan{
@Override
publicvoidprintSlogan(){//(2)overrides(1)
System.out.println(“Happinessiscatchingalltheexceptions!”);
}
}
//_______________________________________________________________________________
classJavaGeekimplementsISlogan{}//(3)inherits(1)
//_______________________________________________________________________________
publicclassJavaParty{
publicstaticvoidmain(String[]args){
JavaGuruguru=newJavaGuru();
guru.printSlogan();//(4)
JavaGeekgeek=newJavaGeek();
geek.printSlogan();//(5)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Happinessiscatchingalltheexceptions!
Happinessisgettingcertified!
Thekeyworddefaultinthecontextofadefaultmethodshouldnotbeconfusedwith
defaultorpackageaccessibilityofamethodinaclass,whichisimpliedintheabsenceof
anyaccessibilitymodifier.Thekeyworddefaultisalsousedonlyfordefaultmethod
declarationsininterfacesthatprovideanimplementationforsuchmethods,andnotby
classesthatoverridethem.
Overridingadefaultmethodfromaninterfacedoesnotnecessarilyimplythatanew
implementationisbeingprovided.Thedefaultmethodcanalsobeoverriddenby
providinganabstractmethoddeclaration,asillustratedbythenextcodesnippet.The
defaultmethodprintSlogan()at(1)intheinterfaceISloganisoverriddenbyan
abstractmethoddeclarationat(2)and(3)intheinterfaceINewSloganandtheabstract
classJavaMaster,respectively.Thisstrategyeffectivelyforcesthesubtypesofthe
interfaceINewSloganandoftheabstractclassJavaMastertoprovideanew
concreteimplementationforthemethod,asonewouldexpectforanabstractmethod.
Clickheretoviewcodeimage
interfaceISlogan{
defaultvoidprintSlogan(){//(1)Defaultmethod.
System.out.println(“Happinessisgettingcertified!”);
}
}
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interfaceINewSloganextendsISlogan{
@Override
abstractvoidprintSlogan();//(2)overrides(1)withabstract
method.
}
abstractclassJavaMasterimplementsISlogan{
@Override
publicabstractvoidprintSlogan();//(3)overrides(1)withabstract
method.
}
Problemswithmultipleinheritancecanarisewhendefaultmethodsareinheritedfrom
multipleinterfaces.Example7.9illustratesonesuchcase.Thedefaultmethod
printSlogan()isdeclaredat(1)and(2)intheinterfacesICheapSloganand
IFunny-Slogan,respectively.Thetwomethoddeclarationshavethesamesignature.
TheinterfaceIAvailableSloganat(3)triestoextendthetwointerfaces
ICheapSloganandIFunnySlogan.Ifthiswasallowed,theinterface
IAvailableSloganwouldinherittwoimplementationsofmethodsthathavethesame
signature,whichofcourseisnotallowed—sothecompilerflagsitasanerror.Bythe
sametoken,thecompilerflagsanerrorat(4),indicatingthattheabstractclass
Wholesalercannotinherittwomethodswiththesamesignature.
Thewayoutofthisdilemmaistooverridetheconflictingmethods.Theabstractclass
RetailSellerthatimplementstheinterfacesICheapSloganandIFunnySlogan
overridestheconflictingmethodsbyprovidinganabstractmethoddeclarationofthe
defaultmethodprintSlogan()at(5).Similarly,theclassNetSellerthat
implementstheinterfacesICheapSloganandIFunnySloganoverridesthe
conflictingmethodsbyprovidinganimplementationofthedefaultmethod
printSlogan()at(6).
TheupshotofthissolutionisthatclientsoftheclassesRetailSellerand
NetSellernowhavetodealwiththenewdeclarationsoftheprintSlogan()
methodprovidedbytheseclasses.OnesuchclientistheclassMutlipleInheritance
at(10),whichcallsthemethodprintSlogan()onaninstanceofclassNetSellerat
(11).Notsurprisingly,theprogramoutputshowsthatthemethodintheNetSellerclass
wasexecuted.
WhatiftheclassNetSellerwantedtoinvokethedefaultmethodprintSlogan()in
theinterfacesitimplements?Theoverriddendefaultmethodcanbecalledbythe
overridingsubtype(inthiscase,NetSeller)usingthekeywordsuperinconjunction
withthefullyqualifiednameoftheinterfaceandthenameofthemethod,asshownat(8)
and(9).Thissyntaxworksforcallingoverriddendefaultmethodsinthedirect
superinterface,butnotatanyhigherlevelintheinheritancehierarchy.Theclass
NetSellercancallonlydefaultmethodsinitsdirectsuperinterfacesICheapSlogan
andIFunnySlogan.ItwouldnotbepossiblefortheclassNetSellertocallany
defaultmethodsinheritedbythesesuperinterfaces,eveniftheyhadany.
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Example7.9DefaultMethodsandMultipleInheritance
Clickheretoviewcodeimage
//File:MultipleInheritance.java
interfaceICheapSlogan{
defaultvoidprintSlogan(){//(1)
System.out.println(“Override,don’toverload.”);
}
}
//_______________________________________________________________________________
interfaceIFunnySlogan{
defaultvoidprintSlogan(){//(2)
System.out.println(“Catchexceptions,notbugs.”);
}
}
//_______________________________________________________________________________
interfaceIAvailableSlogan//(3)Compile-timeerror.
extendsICheapSlogan,IFunnySlogan{}
//_______________________________________________________________________________
abstractclassWholesaler//(4)Compile-timeerror.
implementsICheapSlogan,IFunnySlogan{}
//_______________________________________________________________________________
abstractclassRetailSellerimplementsICheapSlogan,IFunnySlogan{
@Override//Abstractmethod.
publicabstractvoidprintSlogan();//(5)overrides(1)and(2).
}
//_______________________________________________________________________________
classNetSellerimplementsICheapSlogan,IFunnySlogan{
@Override//Concretemethod.
publicvoidprintSlogan(){//(6)overrides(1)and(2).
System.out.println(“Thinkoutsideoftheclass.”);
}
publicvoidinvokeDirect(){//(7)
ICheapSlogan.super.printSlogan();//(8)calls
ICheapSlogan.printSlogan()
IFunnySlogan.super.printSlogan();//(9)calls
IFunnySlogan.printSlogan()
}
}
//_______________________________________________________________________________
publicclassMultipleInheritance{//(10)
publicstaticvoidmain(String[]args){
NetSellerseller=newNetSeller();
seller.printSlogan();//(11)
seller.invokeDirect();
}
}
Outputfromtheprogram:
Thinkoutsideoftheclass.
Override,don’toverload.
Catchexceptions,notbugs.
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StaticMethodsinInterfaces
Aninterfacecanalsodeclarestaticmethods.Staticmethoddeclarationsinatop-level
interfaceareanalogoustostaticmethoddeclarationsinaclass(§4.8,p.132).However,a
staticmethodinatop-levelinterfacealwayshaspublicaccessibility,whetherthe
keywordpublicisspecifiedornot.Aswithstaticmethodsinaclass,thekeyword
staticismandatory;otherwise,thecodewillnotcompile.Withoutthekeyword
static,themethoddeclarationisidenticaltothatofaninstancemethod,butsuch
instancemethodscannotbedeclaredinaninterface.
Clickheretoviewcodeimage
staticreturn_typemethod_name(formal_parameter_list)throws_clause
{implementaion_of_method_body}
Staticmethodsinaninterfacedifferfromthoseinaclassinoneimportantrespect:Static
methodsinaninterfacecannotbeinherited,unlikestaticmethodsinclasses.This
essentiallymeansthatsuchmethodscannotbeinvokeddirectlybycallingthemethodin
subinterfacesorinclassesthatextendorimplementinterfacescontainingsuchmethods,
respectively.Astaticmethodcanbeinvokedonlybyusingitsqualifiedname—thatis,the
nameoftheinterfaceinwhichitisdeclared—togetherwithitssimplename,usingthedot
notation(.).
Example7.10illustratestheuseofstaticmethodsininterfaces.Thestaticmethod
getNumOfCylinders()at(1)isdeclaredintheIMaxEngineSizeinterface.There
aretwoimplementationsofthemethodgetEngineSize(),at(2)and(3),inthe
interfaceIMaxEngineSizeanditssubinterfaceINewEngineSize,respectively.The
classCarRaceimplementsthesubinterfaceINewEngineSize.
ItisnotpossibletoinvokethemethodgetNumOfCylinders()directly,asshownat
(4).ItisalsonotpossibletoinvokedirectlythemethodgetEngineSize()fromeither
interface,asshownat(6).Therespectiveimplementationsofthestaticmethodscanbe
invokedonlybyusingtheirqualifiednames,asshownat(5),(7)and(8).Itdoesnot
matterthatastaticmethodisredeclaredinasubinterface;thestaticmethodisnot
inherited.EachstaticmethoddeclarationinExample7.10isanewmethod.
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Example7.10StaticMethodsinInterfaces
Clickheretoviewcodeimage
//File:CarRace.java
importstaticjava.lang.System.out;
interfaceIMaxEngineSize{
staticintgetNumOfCylinders(){return6;}//(1)Staticmethod
staticdoublegetEngineSize(){return1.6;}//(2)Staticmethod
}
//_______________________________________________________________________________
interfaceINewEngineSizeextendsIMaxEngineSize{
staticdoublegetEngineSize(){return2.4;}//(3)Staticmethod
}
//_______________________________________________________________________________
publicclassCarRaceimplementsINewEngineSize{
publicstaticvoidmain(String[]args){
//out.println(“No.ofcylinders:”+
//getNumOfCylinders());//(4)Compile-time
error.
out.println(“No.ofcylinders:”+
IMaxEngineSize.getNumOfCylinders());//(5)
//out.println(“Enginesize:”+getEngineSize());//(6)Compile-time
error.
out.println(“Maxenginesize:”+IMaxEngineSize.getEngineSize());//(7)
out.println(“Newenginesize:”+INewEngineSize.getEngineSize());//(8)
}
}
Outputfromtheprogram:
No.ofcylinders:6
Maxenginesize:1.6
Newenginesize:2.4
ConstantsinInterfaces
Aninterfacecanalsodefinenamedconstants.Namingconventionsrecommendusing
uppercaselettersfortheirnames,withmultiplewordsinthenamebeingseparatedby
underscores.Suchconstantsaredefinedbyfielddeclarationsandareconsideredtobe
public,static,andfinal.Thesemodifierscanbeomittedfromthedeclaration.
Suchaconstantmustbeinitializedwithaninitializerexpression.
Aninterfaceconstantcanbeaccessedbyanyclient(aclassorinterface)usingitsqualified
name,regardlessofwhethertheclientextendsorimplementsitsinterface.However,ifthe
clientisaclassthatimplementsthisinterfaceorisaninterfacethatextendsthisinterface,
thentheclientcanalsoaccesssuchconstantsdirectlybytheirsimplenames.Suchaclient
inheritstheinterfaceconstants.Typicalusageofconstantsininterfacesisillustratedin
Example7.11,showingaccessbothbytheconstant’ssimplenameanditsqualifiedname
intheprintstatementsat(1)and(2),respectively.
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Example7.11ConstantsinInterfaces
Clickheretoviewcodeimage
//File:Client.java
interfaceConstants{
doublePI_APPROXIMATION=3.14;
StringAREA_UNITS=“sq.cm.”;
StringLENGTH_UNITS=“cm.”;
}
//______________________________________________________________________________
publicclassClientimplementsConstants{
publicstaticvoidmain(String[]args){
doubleradius=1.5;
//(1)Usingsimplename:
System.out.printf(“Areaofcircleis%.2f%s%n”,
PI_APPROXIMATION*radius*radius,AREA_UNITS);
//(2)Usingqualifiedname:
System.out.printf(“Circumferenceofcircleis%.2f%s%n”,
2.0*Constants.PI_APPROXIMATION*radius,
Constants.LENGTH_UNITS);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Areaofcircleis7.06sq.cm.
Circumferenceofcircleis9.42cm.
Extendinganinterfacethathasconstantsisanalogoustoextendingaclasshavingstatic
variables.ThisisillustratedbyFigure7.4andExample7.12.Notethediamondshapeof
theinheritancehierarchy,indicatingthepresenceofmultipleinheritancepathsthrough
whichconstantscanbeinherited.TheconstantsIDLEandBUSYat(1)and(2)inthe
interfaceIBaseStatesareinheritedbythesubinterfaceIAllStatesviaboththe
interfaceIExtStatesAandtheinterfaceIExtStatesB.Insuchcases,theconstantis
consideredtobeinheritedonlyonce,andcanbeaccessedbyitssimplename,asshownat
(12)inExample7.12.
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Figure7.4InheritanceRelationshipsforInterfaceConstants
Constantscanbehiddenbythesubinterfaces.ThedeclarationoftheconstantBLOCKEDat
(6)intheinterfaceIAllStateshidesthedeclarationoftheconstantat(2)inthe
interfaceIBaseStates.Thenewdeclarationcanbeaccessedbyitssimplenameina
classimplementingtheinterfaceIAllStates,asshownat(10)inExample7.12.The
hiddenconstantdeclarationcanalwaysbeaccessedbyusingitsqualifiednameasshown
at(11)inExample7.12.
Inthecaseofmultipleinheritanceofinterfaceconstants,anynameconflictscanbe
resolvedbyusingthequalifiednametoaccesstheconstants.Thisisillustratedbythe
constantDISMANTLED,whichisdeclaredinboththeIExtStatesAand
IExtStatesBinterfaces.Bothdeclarationsareinheritedbythesubinterface
IAllStates.Suchdeclarationsaresaidtobeambiguous.Thecompilerwillreportan
erroronlyifsuchconstantsareaccessedbytheirsimplenames,asshownat(7)and(8)for
theconstantDISMANTLE.Onlythequalifiednamecanbeusedtodisambiguatesuch
constantsandresolvetheconflict,asshownat(7a)and(8a)fortheconstant
DISMANTLE.
Whendefiningasetofrelatedconstants,therecommendedpracticeistousean
enumeratedtype,ratherthannamedconstantsinaninterface.
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Example7.12InheritingConstantsinInterfaces
Clickheretoviewcodeimage
//File:Factory.java
interfaceIBaseStates{
StringIDLE=“idle”;//(1)
StringBUSY=“busy”;//(2)
StringBLOCKED=“blocked”;//(3)
}
//_______________________________________________________________________________
interfaceIExtStatesAextendsIBaseStates{
StringDISMANTLED=“dismantled”;//(4)
}
//_______________________________________________________________________________
interfaceIExtStatesBextendsIBaseStates{
StringDISMANTLED=“kaput”;//(5)
}
//_______________________________________________________________________________
interfaceIAllStatesextendsIExtStatesB,IExtStatesA{
StringBLOCKED=“outoforder”;//(6)hides(3)
//StringABSOLETE=BLOCKED+“,”+
//DISMANTLED+”andscrapped.”;//(7)Ambiguous
StringABSOLETE=BLOCKED+“,”+
IExtStatesB.DISMANTLED+”andscrapped”;//(7a)
}
//_______________________________________________________________________________
publicclassFactoryimplementsIAllStates{
publicstaticvoidmain(String[]args){
//System.out.println(“MachineAis”+DISMANTLED);//(8)
Ambiguous.
System.out.println(“MachineAis”+IExtStatesB.DISMANTLED);//(8a)
System.out.println(“MachineBis”+ABSOLETE);//
(9)IAllStates.ABSOLETE
System.out.println(“MachineCis”+BLOCKED);//(10)
IAllStates.BLOCKED
System.out.println(“MachineDis”+IBaseStates.BLOCKED);//(11)
System.out.println(“MachineEis”+BUSY);//(12)Simplename
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
MachineAiskaput
MachineBisoutoforder,kaputandscrapped
MachineCisoutoforder
MachineDisblocked
MachineEisbusy
ReviewQuestions
7.11Whichofthefollowingstatementsaboutinterfacesaretrue?
Selectthetwocorrectanswers.
(a)Interfacesallowmultipleimplementationinheritance.
(b)Interfacescanbeextendedbyanynumberofinterfaces.
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(c)Interfacescanextendanynumberofinterfaces.
(d)Membersofaninterfaceareneverstatic.
(e)Membersofaninterfacecanalwaysbedeclaredstatic.
7.12Whichmodifierscanmethodsdeclaredinatop-levelinterfacespecify?
Selectthefourcorrectanswers.
(a)public
(b)protected
(c)private
(d)default
(e)abstract
(f)static
(g)final
7.13Whichmodifiersareimplicitlyimpliedforinterfacevariables?
Selectthethreecorrectanswers.
(a)public
(b)protected
(c)private
(d)default
(e)abstract
(f)static
(g)final
7.14Howmanyerrorswillthecompilerreportforthefollowingcode?
Clickheretoviewcodeimage
publicinterfaceVehicle{
finalstaticintNUMBER_OF_HEADLIGHTS;//(1)
voidincreaseSpeed(intincrement){//(2)
System.out.println(“Increasingspeedby”+increment);
}
staticvoidreduceSpeed(intdecrement);//(3)
finaldefaultvoidstop(){//(4)
System.out.println(“Slammingthebrakes!”);
}
}
Selecttheonecorrectanswer.
(a)Noerrors
(b)1error
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(c)2errors
(d)3errors
(e)Morethan3errors
7.15Whichmethodcallscanbeinsertedatboth(1)and(2),sothatthefollowingcode
willstillcompile?
Clickheretoviewcodeimage
//File:Company.java
interfaceISlogan{
StringSLOGAN=“Happinesssharedishappinessdoubled!”;
defaultvoidprintSlogan(){System.out.println(SLOGAN);}
}
//______________________________________________________________________________
publicclassCompanyimplementsISlogan{
publicstaticvoidmain(String[]args){
Companyco=newCompany();
ISlogansl=co;
//(1)INSERTTHEMETHODCALLHERE.
}
publicvoidtestSlogan(){
Companyco=newCompany();
ISlogansl=co;
//(2)INSERTTHEMETHODCALLHERE.
}
}
Selectthetwocorrectanswers.
(a)printSlogan();
(b)co.printSlogan();
(c)sl.printSlogan();
(d)Company.printSlogan();
(e)ISlogan.printSlogan();
7.16Whichmethodcallcanbeinsertedatboth(1)and(2),sothatthefollowingcode
willstillcompile?
Clickheretoviewcodeimage
//File:Firm.java
interfaceINewSlogan{
StringSLOGAN=“Troublesharedistroublehalved!”;
staticvoidprintSlogan(){System.out.println(SLOGAN);}
}
//______________________________________________________________________________
publicclassFirmimplementsINewSlogan{
publicstaticvoidmain(String[]args){
Firmco=newFirm();
INewSlogansl=co;
//(1)INSERTTHESTATEMENTEXPRESSIONHERE.
}
voidtestSlogan(){
Firmco=newFirm();
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INewSlogansl=co;
//(2)INSERTTHESTATEMENTEXPRESSIONHERE.
}
}
Selecttheonecorrectanswer.
(a)printSlogan();
(b)co.printSlogan();
(c)sl.printSlogan();
(d)Firm.printSlogan();
(e)INewSlogan.printSlogan();
7.17Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
//File:RaceA.java
interfaceIJogger{
defaultbooleanjustDoIt(Stringmsg){returnfalse;}//(1)
staticbooleanjustDoIt(inti){returntrue;}//(2)
}
classAthleteimplementsIJogger{
publicbooleanjustDoIt(Stringmsg){returntrue;}//(3)
publicbooleanjustDoIt(inti){returnfalse;}//(4)
}
publicclassRaceA{
publicstaticvoidmain(String[]args){
Athleteathlete=newAthlete();
IJoggerjogger=athlete;
System.out.print(jogger.justDoIt(“Run”));//(5)
System.out.println(“|”+athlete.justDoIt(10));//(6)
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)true|true
(c)true|false
(d)false|true
(e)false|false
7.18Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
//File:HouseC
interfaceISwitch{
defaultbooleanisOn(){returnfalse;}//(1)
}
classLightimplementsISwitch{
booleanisOn(){returntrue;}//(2)
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}
publicclassHouseC{
publicstaticvoidmain(String[]args){
ISwitchlightswitch=newLight();
System.out.println(lightswitch.isOn());
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillcompile,andprinttruewhenrun.
(c)Theprogramwillcompile,andprintfalsewhenrun.
(d)Theprogramwillcompile,andthrowanexceptionwhenrun.
7.19Whichofthesefielddeclarationsarelegalwithinthebodyofaninterface?
Selectthethreecorrectanswers.
(a)publicstaticintANSWER=42;
(b)intANSWER;
(c)staticfinalintANSWER=42;
(d)publicintANSWER=42;
(e)privatestaticfinalintANSWER=42;
7.20Whichstatementsaboutthekeywordsextendsandimplementsaretrue?
Selectthetwocorrectanswers.
(a)Thekeywordextendsisusedtospecifythataninterfaceinheritsfrom
anotherinterface.
(b)Thekeywordextendsisusedtospecifythataclassimplementsaninterface.
(c)Thekeywordimplementsisusedtospecifythataninterfaceinheritsfrom
anotherinterface.
(d)Thekeywordimplementsisusedtospecifythataclassinheritsfroman
interface.
(e)Thekeywordimplementsisusedtospecifythataclassinheritsfromanother
class.
7.21Whichstatementistrueaboutthefollowingcode?
Clickheretoviewcodeimage
//File:MyClass.java
abstractclassMyClassimplementsInterface1,Interface2{
publicvoidf(){}
publicvoidg(){}
}
interfaceInterface1{
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intVAL_A=1;
intVAL_B=2;
voidf();
voidg();
}
interfaceInterface2{
intVAL_B=3;
intVAL_C=4;
voidg();
voidh();
}
Selecttheonecorrectanswer.
(a)MyClassimplementsonlyInterface1;theimplementationforvoid
h()fromInterface2ismissing.
(b)Thedeclarationsofvoidg()inthetwointerfacesareinconflict,sothecode
willnotcompile.
(c)ThedeclarationsofintVAL_Binthetwointerfacesareinconflict,sothe
codewillnotcompile.
(d)Nothingiswrongwiththecode;itwillcompilewithouterrors.
7.22Whichdeclarationcanbeinsertedat(1)withoutresultinginacompile-timeerror?
interfaceMyConstants{
intR=42;
intS=69;
//(1)INSERTCODEHERE
}
Selectthetwocorrectanswers.
(a)finaldoubleCIRCUMFERENCE=2*Math.PI*R;
(b)intTOTAL=TOTAL+R+S;
(c)intAREA=R*S;
(d)publicstaticMAIN=15;
(e)protectedintCODE=13082009;
7.7ArraysandSubtyping
Table7.3summarizesthetypesfoundinJava.Onlyprimitivedataandreferencevalues
canbestoredinvariables.Onlyclassandarraytypescanbeexplicitlyinstantiatedto
createobjects.
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Table7.3TypesandValues
ArraysandSubtypeCovariance
ArraysareobjectsinJava.Arraytypes(boolean[],Object[],StackImpl[])
implicitlyaugmenttheinheritancehierarchy.TheinheritancehierarchydepictedinFigure
7.3,forexample,canbeaugmentedbythecorrespondingarraytypestoproducethetype
hierarchyshowninFigure7.5.Anarraytypeisshownasa“class”withthe[]notation
appendedtothenameoftheelementtype.TheclassSafeStackImplisasubclassof
theclassStackImpl.Thecorrespondingarraytypes,SafeStackImpl[]and
StackImpl[],areshownasthesubtypeandthesupertype,respectively,inthetype
hierarchy.Figure7.5alsoshowsarraytypescorrespondingtosomeoftheprimitivedata
types.
Figure7.5ReferenceTypeHierarchy:ArraysandSubtypeCovariance
FromthetypehierarchyinFigure7.5,thefollowingfactsareapparent:
•AllreferencetypesaresubtypesoftheObjecttype.Thisappliestoclasses,
interfaces,enums,andarraytypes,astheseareallreferencetypes.
•AllarraysofreferencetypesarealsosubtypesofthearraytypeObject[],but
arraysofprimitivedatatypesarenot.NotethatthearraytypeObject[]isalsoa
subtypeoftheObjecttype.
•Ifanon-genericreferencetypeisasubtypeofanothernon-genericreferencetype,
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thecorrespondingarraytypesalsohaveananalogoussubtype–supertype
relationship.Thisiscalledthesubtypecovariancerelationship.
•Thereisnosubtype–supertyperelationshipbetweenatypeanditscorresponding
arraytype.
Wecancreateanarrayofaninterfacetype,butwecannotinstantiateaninterface(asisthe
casewithabstractclasses).Inthefollowingdeclarationstatement,thereference
iSafeStackArrayhastypeISafeStack[](i.e.,anarrayoftheinterfacetype
ISafeStack):
Clickheretoviewcodeimage
ISafeStack[]iSafeStackArray=newISafeStack[5];
ThearraycreationexpressioncreatesanarraywhoseelementtypeisISafeStack.The
arrayobjectcanaccommodatefivereferencesofthetypeISafeStack.Thedeclaration
statementdoesnotinitializethesereferencestorefertoanyobjects;instead,theyare
initializedtothedefaultvaluenull.
ArrayStoreCheck
Anarrayreferenceexhibitspolymorphicbehaviorlikeanyotherreference,subjecttoits
locationinthetypehierarchy(§7.12,p.329).However,aruntimecheckisnecessary
whenobjectsareinsertedinanarray,asthenextexampleillustrates.
Thefollowingassignmentisvalid,asasupertypereference(StackImpl[])canreferto
objectsofitssubtype(SafeStackImpl[]):
Clickheretoviewcodeimage
StackImpl[]stackImplArray=newSafeStackImpl[2];//(1)
SinceStackImplisasupertypeofSafeStackImpl,thefollowingassignmentisalso
valid:
Clickheretoviewcodeimage
stackImplArray[0]=newSafeStackImpl(10);//(2)
Theassignmentat(2)assignsthereferencevalueofanewSafeStackImplobjectto
thereferenceatindex0intheSafeStackImpl[]object(i.e.,thearrayof
SafeStackImpl)createdat(1).
SincethetypeofstackImplArray[i],(0≤i<2),isStackImpl,itshouldbe
possibletomakethefollowingassignmentaswell:
Clickheretoviewcodeimage
stackImplArray[1]=newStackImpl(20);//(3)
ArrayStoreException
Atcompiletimetherearenoproblems,asthecompilercannotdeducethatthearray
variablestackImplArraywillactuallydenoteaSafeStackImpl[]objectat
runtime.However,theassignmentat(3)resultsinanArrayStoreExceptionbeing
thrownatruntime,becauseanarrayofSafeStackImplobjectscannotpossiblycontain
objectsofitssupertypeStackImpl.
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Thearraystorecheckatruntimeensuresthatanobjectbeingstoredinthearrayis
assignmentcompatible(p.314)withtheelementtypeofthearray.Tomakethearraystore
checkfeasibleatruntime,thearrayretainsinformationaboutitsdeclaredelementtypeat
runtime.
7.8ReferenceValuesandConversions
Areviewofconversions(§5.1,p.144)isrecommendedbeforeproceedingwiththis
section.
Referencevalues,likeprimitivevalues,canbeassigned,cast,andpassedasarguments.
Conversionscanoccurinthefollowingcontexts:
•Assignment
•Methodinvocation
•Casting
Theruleofthumbfortheprimitivedatatypesisthatwideningconversionsarepermitted,
butnarrowingconversionsrequireanexplicitcast.Theruleofthumbforreferencevalues
isthatwideningconversionsupthetypehierarchyarepermitted,butnarrowing
conversionsdownthehierarchyrequireanexplicitcast.Inotherwords,conversionsthat
arefromasubtypetoitssupertypesareallowed,butotherconversionsrequireanexplicit
castorareotherwiseillegal.Thereisnonotionofpromotionforreferencevalues.
7.9ReferenceValueAssignmentConversions
Inthecontextofassignments,thefollowingconversionsarepermitted(Table5.1,p.147):
•Wideningprimitiveandreferenceconversions(long←int,Object←
String)
•Boxingconversionofprimitivevalues,followedbyoptionalwideningreference
conversion(Integer←int,Number←Integer←int)
•Unboxingconversionofaprimitivevaluewrapperobject,followedbyoptional
wideningprimitiveconversion(long←int←Integer)
Forassignmentconversionsonly,thefollowingconversionsarealsopossible:
•Narrowingconversionforconstantexpressionsofnon-longintegertypes,with
optionalboxing(Byte←byte←int)
Notethattheserulesimplythatawideningconversioncannotbefollowedbyanyboxing
conversion,buttheconverseispermitted.
Wideningreferenceconversionstypicallyoccurduringassignmentupthetypehierarchy,
withimplicitconversionofthesourcereferencevaluetothatofthedestinationreference
type:
Clickheretoviewcodeimage
Objectobj=“Upthetree”;//Wideningreferenceconversion:Object<—
String
Stringstr1=obj;//NotOK.Narrowingreferenceconversionrequiresa
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cast.
Stringstr2=newInteger(10);//Illegal.NorelationbetweenStringand
Integer.
Thesourcevaluecanbeaprimitivevalue,inwhichcasethevalueisboxedinawrapper
objectcorrespondingtotheprimitivetype.Ifthedestinationreferencetypeisasupertype
ofthewrappertype,awideningreferenceconversioncanoccur:
Clickheretoviewcodeimage
IntegeriRef=10;//Onlyboxing
Numbernum=10L;//Boxing,followedbywidening:Number<–Long<–long
Objectobj=100;//Boxing,followedbywidening:Object<–Integer<–int
Moreexamplesofboxingduringassignmentcanbefoundin§5.1,p.145.
Example7.13AssigningandPassingReferenceValues
Clickheretoviewcodeimage
interfaceIStack{/*FromExample7.7*/}
interfaceISafeStackextendsIStack{/*FromExample7.7*/}
classStackImplimplementsIStack{/*FromExample7.7*/}
classSafeStackImplextendsStackImpl
implementsISafeStack{/*FromExample7.7*/}
publicclassReferenceConversion{
publicstaticvoidmain(String[]args){
//Referencedeclarations:
ObjectobjRef;
StackImplstackRef;
SafeStackImplsafeStackRef;
IStackiStackRef;
ISafeStackiSafeStackRef;
//SourceTypeisaclasstype:
safeStackRef=newSafeStackImpl(10);
objRef=safeStackRef;//(1)Alwayspossible
stackRef=safeStackRef;//(2)Subclasstosuperclassassignment
iStackRef=stackRef;//(3)StackImplimplementsIStack
iSafeStackRef=safeStackRef;//(4)SafeStackImplimplements
ISafeStack
//SourceTypeisaninterfacetype:
objRef=iStackRef;//(5)Alwayspossible
iStackRef=iSafeStackRef;//(6)Sub-tosuper-interface
assignment
//SourceTypeisanarraytype:
Object[]objArray=newObject[3];
StackImpl[]stackArray=newStackImpl[3];
SafeStackImpl[]safeStackArray=newSafeStackImpl[5];
ISafeStack[]iSafeStackArray=newISafeStack[5];
int[]intArray=newint[10];
//Referencevalueassignments:
objRef=objArray;//(7)Alwayspossible
objRef=stackArray;//(8)Alwayspossible
objArray=stackArray;//(9)Alwayspossible
objArray=iSafeStackArray;//(10)Alwayspossible
objRef=intArray;//(11)Alwayspossible
//objArray=intArray;//(12)Compile-timeerror:
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//int[]notsubtypeofObject[]
stackArray=safeStackArray;//(13)Subclassarraytosuperclass
array
iSafeStackArray=safeStackArray;//(14)SafeStackImplimplements
ISafeStack
//Methodinvocationconversions:
System.out.println(“Firstcall:”);
sendParams(stackRef,safeStackRef,iStackRef,
safeStackArray,iSafeStackArray);//(15)
//CallSignature:sendParams(StackImpl,SafeStackImpl,IStack,
//SafeStackImpl[],ISafeStack[]);
System.out.println(“Secondcall:”);
sendParams(iSafeStackArray,stackRef,iSafeStackRef,
stackArray,safeStackArray);//(16)
//CallSignature:sendParams(ISafeStack[],StackImpl,ISafeStack,
//StackImpl[],SafeStackImpl[]);
}
publicstaticvoidsendParams(ObjectobjRefParam,StackImplstackRefParam,
IStackiStackRefParam,StackImpl[]stackArrayParam,
IStack[]iStackArrayParam){//(17)
//Signature:sendParams(Object,StackImpl,IStack,StackImpl[],
IStack[])
//Printclassnameofobjectdenotedbythereferenceatruntime.
System.out.println(objRefParam.getClass());
System.out.println(stackRefParam.getClass());
System.out.println(iStackRefParam.getClass());
System.out.println(stackArrayParam.getClass());
System.out.println(iStackArrayParam.getClass());
}
}
Outputfromtheprogram:
Firstcall:
classSafeStackImpl
classSafeStackImpl
classSafeStackImpl
class[LSafeStackImpl;
class[LSafeStackImpl;
Secondcall:
class[LSafeStackImpl;
classSafeStackImpl
classSafeStackImpl
class[LSafeStackImpl;
class[LSafeStackImpl;
Therulesforreferencevalueassignmentarestatedinthissection,basedonthefollowing
code:
Clickheretoviewcodeimage
SourceTypesrcRef;
//srcRefisappropriatelyinitialized.
DestinationTypedestRef=srcRef;
Ifanassignmentislegal,thereferencevalueofsrcRefissaidtobeassignable(or
assignmentcompatible)tothereferenceofDestinationType.Therulesareillustrated
byconcretecasesfromExample7.13.NotethatthecodeinExample7.13usesreference
typesfromExample7.7,p.292.
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•IftheSourceTypeisaclasstype,thereferencevalueinsrcRefmaybeassigned
tothedestRefreference,providedtheDestinationTypeisoneofthe
following:
DestinationTypeisasuperclassofthesubclassSourceType.
DestinationTypeisaninterfacetypethatisimplementedbytheclass
SourceType.
Clickheretoviewcodeimage
objRef=safeStackRef;//(1)Alwayspossible
stackRef=safeStackRef;//(2)Subclasstosuperclassassignment
iStackRef=stackRef;//(3)StackImplimplementsIStack
iSafeStackRef=safeStackRef;//(4)SafeStackImplimplements
ISafeStack
•IftheSourceTypeisaninterfacetype,thereferencevalueinsrcRefmaybe
assignedtothedestRefreference,providedtheDestinationTypeisoneof
thefollowing:
DestinationTypeistheObjectclass.
DestinationTypeisasuperinterfaceofthesubinterfaceSourceType.
Clickheretoviewcodeimage
objRef=iStackRef;//(5)Alwayspossible
iStackRef=iSafeStackRef;//(6)Subinterfacetosuperinterface
assignment
•IftheSourceTypeisanarraytype,thereferencevalueinsrcRefmaybe
assignedtothedestRefreference,providedtheDestinationTypeisoneof
thefollowing:
DestinationTypeistheObjectclass.
DestinationTypeisanarraytype,wheretheelementtypeofthe
SourceTypeisassignabletotheelementtypeoftheDestinationType.
Clickheretoviewcodeimage
objRef=objArray;//(7)Alwayspossible
objRef=stackArray;//(8)Alwayspossible
objArray=stackArray;//(9)Alwayspossible
objArray=iSafeStackArray;//(10)Alwayspossible
objRef=intArray;//(11)Alwayspossible
//objArray=intArray;//(12)Compile-timeerror:
//int[]notsubtypeofObject[]
stackArray=safeStackArray;//(13)Subclassarraytosuperclass
array
iSafeStackArray=safeStackArray;//(14)SafeStackImplimplements
ISafeStack
Therulesforassignmentareenforcedatcompiletime,guaranteeingthatnotype
conversionerrorwilloccurduringassignmentatruntime.Suchconversionsaretype-safe.
Thereasontherulescanbeenforcedatcompiletimeisthattheyconcernthedeclaredtype
ofthereference(whichisalwaysknownatcompiletime)ratherthantheactualtypeofthe
objectbeingreferenced(whichisknownatruntime).
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7.10MethodInvocationConversionsInvolvingReferences
Theconversionsforreferencevalueassignmentarealsoapplicabletomethodinvocation
conversions,exceptforthenarrowingconversionforconstantexpressionsofnon-long
integertype(Table5.1,p.147).Thisisreasonable,asparametersinJavaarepassedby
value(§3.5,p.72),requiringthatvaluesoftheactualparametersmustbeassignableto
formalparametersofthecompatibletypes.
InExample7.13,themethodsendParams()at(17)hasthefollowingsignature,
showingthetypesoftheformalparameters:
Clickheretoviewcodeimage
sendParams(Object,StackImpl,IStack,StackImpl[],IStack[])
Themethodcallat(15)hasthefollowingsignature,showingthetypesoftheactual
parameters:
Clickheretoviewcodeimage
sendParams(StackImpl,SafeStackImpl,IStack,SafeStackImpl[],ISafeStack[]);
Notethattheassignmentofthevaluesoftheactualparameterstothecorresponding
formalparametersislegal,accordingtotherulesforassignmentdiscussedearlier.The
methodcallat(16)providesanotherexampleoftheparameterpassingconversion.Ithas
thefollowingsignature:
Clickheretoviewcodeimage
sendParams(ISafeStack[],StackImpl,ISafeStack,StackImpl[],
SafeStackImpl[]);
Analogoustoassignment,therulesforparameterpassingconversionsarebasedonthe
referencetypeoftheparametersandareenforcedatcompiletime.TheoutputinExample
7.13showstheclassoftheactualobjectsreferencedbytheformalparametersatruntime,
whichinthiscaseturnsouttobeeitherSafeStackImplorSafeStackImpl[].The
characters[Lintheoutputindicateaone-dimensionalarrayofaclassorinterfacetype
(seetheClass.getName()methodintheJavaSEplatformAPIdocumentation).
OverloadedMethodResolution
Inthissubsection,wetakealookatsomeaspectsregardingoverloadedmethodresolution
—namely,howthecompilerdetermineswhichoverloadedmethodwillbeinvokedbya
givenmethodcallatruntime.
Resolutionofoverloadedmethodsselectsthemostspecificmethodforexecution.One
methodisconsideredmorespecificthananothermethodifallactualparametersthatcan
beacceptedbytheonemethodcanbeacceptedbytheothermethod.Ifmorethanone
suchmethodispresent,thecallisdescribedasambiguous.Thefollowingoverloaded
methodsillustratethissituation:
Clickheretoviewcodeimage
privatestaticvoidflipFlop(Stringstr,inti,IntegeriRef){//(1)
out.println(str+”==>(String,int,Integer)”);
}
privatestaticvoidflipFlop(Stringstr,inti,intj){//(2)
out.println(str+”==>(String,int,int)”);
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}
Theirmethodsignaturesfollow:
Clickheretoviewcodeimage
flipFlop(String,int,Integer)//See(1)
flipFlop(String,int,int)//See(2)
Thefollowingmethodcallisambiguous:
Clickheretoviewcodeimage
flipFlop(“(String,Integer,int)”,newInteger(4),2004);//(3)Ambiguous
call
Ithasthecallsignature:
Clickheretoviewcodeimage
flipFlop(String,Integer,int)//See(3)
Themethodat(1)canbecalledwiththesecondargumentunboxedandthethirdargument
boxed,ascanthemethodat(2)withonlythesecondargumentunboxed.Inotherwords,
forthecallat(3),noneofthemethodsismorespecificthantheothers.
Example7.14illustratesasimplecaseofhowmethodresolutionisdonetochoosethe
mostspecificoverloadedmethod.ThemethodtestIfOn()isoverloadedat(1)and(2)
intheclassOverload.Thecallclient.testIfOn(tubeLight)at(3)satisfies
theparameterlistsinbothimplementationsgivenat(1)and(2),asthereference
tubeLightcanalsobeassignedtoareferenceofitssuperclassLight.Themost
specificmethod,(2),ischosen,resultinginfalsebeingwrittenontheterminal.Thecall
client.testIfOn(light)at(4)satisfiesonlytheparameterlistinthe
implementationgivenat(1),resultingintruebeingwrittenontheterminal.Thisisalso
thecaseat(5).Theobjectreferredtobytheargumentinthecallisirrelevant;rather,itis
thetypeoftheargumentthatisimportantforoverloadedmethodresolution.
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Example7.14ChoosingtheMostSpecificMethod(SimpleCase)
Clickheretoviewcodeimage
classLight{/*…*/}
classTubeLightextendsLight{/*…*/}
publicclassOverload{
booleantestIfOn(LightaLight){returntrue;}//(1)
booleantestIfOn(TubeLightaTubeLight){returnfalse;}//(2)
publicstaticvoidmain(String[]args){
TubeLighttubeLight=newTubeLight();
Lightlight=newLight();
Lightlight2=newTubeLight();
Overloadclient=newOverload();
System.out.println(client.testIfOn(tubeLight));//(3)==>methodat(2)
System.out.println(client.testIfOn(light));//(4)==>methodat(1)
System.out.println(client.testIfOn(light2));//(5)==>methodat(2)
}
}
Outputfromtheprogram:
false
true
true
Thealgorithmusedbythecompilerfortheresolutionofoverloadedmethodsincorporates
thefollowingphases:
1.Thecompilerperformsoverloadresolutionwithoutpermittingboxing,unboxing,or
theuseofavariablearitycall.
2.Ifphase(1)fails,thecompilerperformsoverloadresolutionallowingboxingand
unboxing,butexcludingtheuseofavariablearitycall.
3.Ifphase(2)fails,thecompilerperformsoverloadresolutioncombiningavariable
aritycall,boxing,andunboxing.
Example7.15providessomeinsightintohowthecompilerdeterminesthemostspecific
overloadedmethodusingthesethreephases.Theexamplehassixoverloadeddeclarations
ofthemethodaction().Thesignatureofeachmethodisgivenbythelocalvariable
signatureineachmethod.Thefirstformalparameterofeachmethodisthesignature
ofthecallthatinvokedthemethod.Theprintoutfromeachmethodallowsustoseewhich
methodcallresolvedtowhichmethod.Themain()methodcontains10calls,(8)to(17),
oftheaction()method.Ineachcall,thefirstargumentisthesignatureofthatmethod
call.
Animportantpointtonoteisthatthecompilerchoosesafixedaritycalloveravariable
aritycall,asseeninthecallsfrom(8)to(12):
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Anunboxingconversion(Integertoint)takesplaceforthecallat(10).Awidening
primitiveconversion(bytetoint)takesplaceforthecallat(11).
Variablearitycallsarechosenfrom(13)to(17):
Whenavariablearitycallischosen,themethoddeterminedhasthemostspecificvariable
arityparameterthatisapplicablefortheactualargument.Forexample,inthemethodcall
at(14),thetypeInteger[]ismorespecificthaneitherNumber[]orObject[].
Notealsotheboxingoftheelementsoftheimplicitlycreatedarrayinthecallsfrom(13)
to(17).
Example7.15OverloadedMethodResolution
Clickheretoviewcodeimage
importstaticjava.lang.System.out;
classOverloadResolution{
publicvoidaction(Stringstr){//(1)
Stringsignature=“(String)”;
out.println(str+”=>”+signature);
}
publicvoidaction(Stringstr,intm){//(2)
Stringsignature=“(String,int)”;
out.println(str+”=>”+signature);
}
publicvoidaction(Stringstr,intm,intn){//(3)
Stringsignature=“(String,int,int)”;
out.println(str+”=>”+signature);
}
publicvoidaction(Stringstr,Integer…data){//(4)
Stringsignature=“(String,Integer[])”;
out.println(str+”=>”+signature);
}
publicvoidaction(Stringstr,Number…data){//(5)
Stringsignature=“(String,Number[])”;
out.println(str+”=>”+signature);
}
publicvoidaction(Stringstr,Object…data){//(6)
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Stringsignature=“(String,Object[])”;
out.println(str+”=>”+signature);
}
publicstaticvoidmain(String[]args){
OverloadResolutionref=newOverloadResolution();
ref.action(“(String)”);//(8)calls
(1)
ref.action(“(String,int)”,10);//(9)calls
(2)
ref.action(“(String,Integer)”,newInteger(10));//(10)calls
(2)
ref.action(“(String,int,byte)”,10,(byte)20);//(11)calls
(3)
ref.action(“(String,int,int)”,10,20);//(12)calls
(3)
ref.action(“(String,int,long)”,10,20L);//(13)calls
(5)
ref.action(“(String,int,int,int)”,10,20,30);//(14)calls
(4)
ref.action(“(String,int,double)”,10,20.0);//(15)calls
(5)
ref.action(“(String,int,String)”,10,“what?”);//(16)calls
(6)
ref.action(“(String,boolean)”,false);//(17)calls
(6)
}
}
Outputfromtheprogram(withremarkstotheoutputontheright):
Clickheretoviewcodeimage
(String)=>(String)(8)calls(1)
(String,int)=>(String,int)(9)calls(2)
(String,Integer)=>(String,int)(10)calls(2)
(String,int,byte)=>(String,int,int)(11)calls(3)
(String,int,int)=>(String,int,int)(12)calls(3)
(String,int,long)=>(String,Number[])(13)calls(5)
(String,int,int,int)=>(String,Integer[])(14)calls(4)
(String,int,double)=>(String,Number[])(15)calls(5)
(String,int,String)=>(String,Object[])(16)calls(6)
(String,boolean)=>(String,Object[])(17)calls(6)
7.11ReferenceCastingandthe Operator
TheCastOperator
Thetypecastexpressionforreferencetypeshasthefollowingsyntax:
Clickheretoviewcodeimage
(destination_type)reference_expression
wherethereferenceexpressionevaluatestoareferencevalueofanobjectofsome
referencetype.Atypecastexpressionchecksthatthereferencevaluereferstoanobject
whosetypeiscompatiblewiththedestinationtype,meaningthatitstypeisasubtypeof
thedestinationtype.Theconstruct(destination_type)isusuallycalledthecastoperator.
Theresultofatypecastexpressionforreferencesisalwaysareferencevalueofanobject.
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Theliteralnullcanbecasttoanyreferencetype.
Thenextcodesnippetillustratesthevariousscenariosthatarisewhenusingthecast
operator.Inthisdiscussion,itisthetypecastexpressionthatisimportant,notthe
evaluationoftheassignmentoperatorinthedeclarationstatements.In(1),thecastisfrom
thesuperclassObjecttothesubclassString;thecodecompilesandatruntimethis
castispermitted,asthereferenceobjwilldenoteanobjectofclassString.In(2),the
castisfromthesuperclassObjecttothesubclassInteger;thecodecompiles,butat
runtimethiscastresultsinaClassCastException,sincethereferenceobjwill
denoteanobjectofclassString,whichcannotbeconvertedtoanInteger.In(3),the
castisfromtheclassStringtotheclassInteger.Asthesetwoclassesareunrelated,
thecompilerflagsanerrorforthecast.
Clickheretoviewcodeimage
Objectobj=newString(“Castme!”);
Stringstr=(String)obj;//(1)CastfromObjecttoString.
IntegeriRef1=(Integer)obj;//(2)CastfromObjecttoInteger,but
//ClassCastExceptionatruntime.
IntegeriRef2=(Integer)str;//(3)Compile-timeerror!
//Castbetweenunrelatedtypes.
Thefollowingconversionscanbeappliedtotheoperandofacastoperator:
•Bothwideningandnarrowingreferenceconversions,followedoptionallybyan
uncheckedconversion
•Bothboxingandunboxingconversions
Boxingandunboxingconversionsthatcanoccurduringcastingareillustratedbythe
followingcode.Again,itisthetypecastexpressionthatisimportantinthisdiscussion,
ratherthanwhethertheassignmentoperatorrequiresoneinthedeclarationstatements.
Clickheretoviewcodeimage
//(1)Boxingandcasting:Number<—Integer<—int:
Numbernum=(Number)100;
//(2)Casting,boxing,casting:Object<—Integer<—int<—double:
Objectobj=(Object)(int)10.5;
//(3)Casting,unboxing,casting:double<–int<—Integer<—Object:
doubled=(double)(Integer)obj;
Notethattheresultingobjectin(1)and(2)isanInteger,buttheresultingvaluein(3)
isadouble.TheboxingconversionsfrominttoIntegerin(1)and(2)areimplicit,
andtheunboxingconversionfromIntegertointin(3)isalsoimplicit.
The Operator
Thebinaryinstanceofoperatorcanbeusedforcomparingtypes.Ithasthefollowing
syntax(notethatthekeywordiscomposedoflowercaselettersonly):
Clickheretoviewcodeimage
reference_expressioninstanceofdestination_type
Theinstanceofoperatorreturnstrueiftheleft-handoperand(i.e.,thereference
valuethatresultsfromtheevaluationofreferenceexpression)canbeasubtypeofthe
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right-handoperand(destination_type).Italwaysreturnsfalseiftheleft-handoperandis
null.Iftheinstanceofoperatorreturnstrue,thecorrespondingtypecast
expressionwillalwaysbevalid.Boththetypecastexpressionandtheinstanceof
operatorsrequireacompile-timecheckandaruntimecheck,asexplainedlaterinthis
section.
Thecompile-timecheckdetermineswhetherthereisasubtype–supertyperelationship
betweenthesourceanddestinationtypes.Giventhatthetypeofthereferenceexpression
issourcetype,thecompilerdetermineswhetherareferenceofsourcetypeandareference
ofdestinationtypecanrefertoobjectsofareferencetypethatareacommonsubtypeof
bothsourcetypeanddestinationtypeinthetypehierarchy.Ifthisisnotthecase,then
obviouslythereisnorelationshipbetweenthetypes,andneitherthecastnorthe
instanceofoperatorapplicationwouldbevalid.Atruntime,thereferenceexpression
evaluatestoareferencevalueofanobject.Itisthetypeoftheactualobjectthat
determinestheoutcomeoftheoperation,asexplainedearlier.
WiththeclassesLightandStringassourcetypeanddestinationtype,respectively,
thereisnosubtype–supertyperelationshipbetweensourcetypeanddestinationtype.The
compilerwouldrejectcastingareferenceoftypeLighttotypeStringorapplyingthe
instanceofoperator,asshownat(2)and(3)inExample7.16.Referencesofthe
classesLightandTubeLightcanrefertoobjectsoftheclassTubeLight(orits
subclasses)intheinheritancehierarchydepictedinFigure7.1.Therefore,itmakessense
toapplytheinstanceofoperatororcastareferenceofthetypeLighttothetype
TubeLightasshownat(4)and(5),respectively,inExample7.16.
Atruntime,theresultofapplyingtheinstanceofoperatorat(4)isfalse,because
thereferencelight1oftheclassLightwillactuallydenoteanobjectofthesubclass
LightBulb,andthisobjectcannotbedenotedbyareferenceofthepeerclass
TubeLight.Applyingthecastat(5)resultsinaClassCastExceptionforthesame
reason.Thisisthereasonwhycastconversionsaresaidtobeunsafe,astheymaythrowa
ClassCastExceptionatruntime.Notethatiftheresultoftheinstanceof
operatorisfalse,thecastinvolvingtheoperandswillthrowa
ClassCastException.
InExample7.16,theresultofapplyingtheinstanceofoperatorat(6)isalsofalse,
becausethereferencelight1willstilldenoteanobjectoftheclassLightBulb,whose
objectscannotbedenotedbyareferenceofitssubclassSpotLightBulb.Thus
applyingthecastat(7)causesaClassCastExceptiontobethrownatruntime.
Thesituationshownat(8),(9),and(10)illustratestypicalusageoftheinstanceof
operatortodeterminewhichobjectareferenceisdenoting,sothatitcanbecastforthe
purposeofcarryingoutsomespecificaction.Thereferencelight1oftheclassLight
isinitializedtoanobjectofthesubclassNeonLightat(8).Theresultofthe
instanceofoperatorat(9)istrue,becausethereferencelight1willdenotean
objectofthesubclassNeonLight,whoseobjectscanalsobedenotedbyareferenceof
itssuperclassTubeLight.Bythesametoken,thecastat(10)isvalid.Iftheresultofthe
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instanceofoperatoristrue,thecastinvolvingtheoperandswillbevalidaswell.
Example7.16TheinstanceofandCastOperators
Clickheretoviewcodeimage
//SeeFigure7.1,p.267,forinheritancehierarchy.
classLight{/*…*/}
classLightBulbextendsLight{/*…*/}
classSpotLightBulbextendsLightBulb{/*…*/}
classTubeLightextendsLight{/*…*/}
classNeonLightextendsTubeLight{/*…*/}
publicclassWhoAmI{
publicstaticvoidmain(String[]args){
booleanresult1,result2,result3;
Lightlight1=newLightBulb();//(1)
//Stringstr=(String)light1;//(2)Compile-time
error!
//result1=light1instanceofString;//(3)Compile-time
error!
result2=light1instanceofTubeLight;//(4)false:peer
class.
//TubeLighttubeLight1=(TubeLight)light1;//(5)
ClassCastException!
result3=light1instanceofSpotLightBulb;//(6)false:
superclass.
//SpotLightBulbspotRef=(SpotLightBulb)light1;//(7)
ClassCastException!
light1=newNeonLight();//(8)
if(light1instanceofTubeLight){//(9)true.
TubeLighttubeLight2=(TubeLight)light1;//(10)OK.
//CannowusetubeLight2toaccessanobjectoftheclassNeonLight,
//butonlythosemembersthattheobjectinheritsoroverrides
//fromtheclassTubeLight.
}
}
}
Aswehaveseen,theinstanceofoperatoreffectivelydetermineswhetherthereference
valueinthereferenceontheleft-handsidereferstoanobjectwhoseclassisasubtypeof
thetypeofthereferencespecifiedontheright-handside.Atruntime,itisthetypeofthe
actualobjectdenotedbythereferenceontheleft-handsidethatiscomparedwiththetype
specifiedontheright-handside.Inotherwords,whatmattersatruntimeisthetypeofthe
actualobjectdenotedbythereference,notthedeclaredtypeofthereference.
Example7.17providesmoreexamplesoftheinstanceofoperator.Itisinstructiveto
gothroughtheprintstatementsandunderstandwhythoseresultsprintedout.Theliteral
nullisnotaninstanceofanyreferencetype,asshownintheprintstatementsat(1),(2),
and(16).Aninstanceofasuperclassisnotaninstanceofitssubclass,asshowninthe
printstatementat(4).Aninstanceofaclassisnotaninstanceofatotallyunrelatedclass,
asshownintheprintstatementat(10).Aninstanceofaclassisnotaninstanceofan
interfacetypethattheclassdoesnotimplement,asshownintheprintstatementat(6).
Anyarrayofnon-primitivetypeisaninstanceofbothObjectandObject[]types,as
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shownintheprintstatementsat(14)and(15),respectively.
Example7.17UsingtheinstanceofOperator
Clickheretoviewcodeimage
//SeeFigure7.3,p.295,forinheritancehierarchy.
interfaceIStack{/*FromExample7.7*/}
interfaceISafeStackextendsIStack{/*FromExample7.7*/}
classStackImplimplementsIStack{/*FromExample7.7*/}
classSafeStackImplextendsStackImpl
implementsISafeStack{/*FromExample7.7*/}
publicclassIdentification{
publicstaticvoidmain(String[]args){
Objectobj=newObject();
StackImplstack=newStackImpl(10);
SafeStackImplsafeStack=newSafeStackImpl(5);
IStackiStack;
StringstrFormat=“(%d)%-25sinstanceof%-25s:%s%n”;
System.out.printf(strFormat,1,
null,Object.class,
nullinstanceofObject);//Alwaysfalse.
System.out.printf(strFormat,2,
null,IStack.class,
nullinstanceofIStack);//Alwaysfalse.
System.out.printf(strFormat,3,
stack.getClass(),Object.class,
stackinstanceofObject);//true:instanceofsubclassof
Object.
System.out.printf(strFormat,4,
obj.getClass(),StackImpl.class,
objinstanceofStackImpl);//false:Objectnotsubtypeof
StackImpl.
System.out.printf(strFormat,5,
stack.getClass(),StackImpl.class,
stackinstanceofStackImpl);//true:instanceofStackImpl.
System.out.printf(strFormat,6,
obj.getClass(),IStack.class,
objinstanceofIStack);//false:Objectdoesnotimplement
IStack.
System.out.printf(strFormat,7,
safeStack.getClass(),IStack.class,
safeStackinstanceofIStack);//true:SafeStackImplimplements
IStack.
obj=stack;//Nocastrequired:assigningsubclassto
superclass.
System.out.printf(strFormat,8,
obj.getClass(),StackImpl.class,
objinstanceofStackImpl);//true:instanceofStackImpl.
System.out.printf(strFormat,9,
obj.getClass(),IStack.class,
objinstanceofIStack);//true:StackImplimplementsIStack.
System.out.printf(strFormat,10,
obj.getClass(),String.class,
objinstanceofString);//false:norelationship.
iStack=(IStack)obj;//Castrequired:assigningsuperclassto
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subclass.
System.out.printf(strFormat,11,
iStack.getClass(),Object.class,
iStackinstanceofObject);//true:instanceofsubclassof
Object.
System.out.printf(strFormat,12,
iStack.getClass(),StackImpl.class,
iStackinstanceofStackImpl);//true:instanceofStackImpl.
String[]strArray=newString[10];
//System.out.printf(strFormat,13,
//strArray.getClass(),String.class,
//strArrayinstanceofString);//Compile-timeerror:no
relationship.
System.out.printf(strFormat,14,
strArray.getClass(),Object.class,
strArrayinstanceofObject);//true:arraysubclassofObject.
System.out.printf(strFormat,15,
strArray.getClass(),Object[].class,
strArrayinstanceofObject[]);//true:arraysubclassofObject[].
System.out.printf(strFormat,16,
strArray[0],Object.class,
strArray[0]instanceofObject);//false:strArray[0]isnull.
System.out.printf(strFormat,17,
strArray.getClass(),String[].class,
strArrayinstanceofString[]);//true:arrayofString.
strArray[0]=“Amoebastrip”;
System.out.printf(strFormat,18,
strArray[0].getClass(),String.class,
strArray[0]instanceofString);//true:strArray[0]instanceof
String.
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
(1)nullinstanceofclassjava.lang.Object:false
(2)nullinstanceofinterfaceIStack:false
(3)classStackImplinstanceofclassjava.lang.Object:true
(4)classjava.lang.ObjectinstanceofclassStackImpl:false
(5)classStackImplinstanceofclassStackImpl:true
(6)classjava.lang.ObjectinstanceofinterfaceIStack:false
(7)classSafeStackImplinstanceofinterfaceIStack:true
(8)classStackImplinstanceofclassStackImpl:true
(9)classStackImplinstanceofinterfaceIStack:true
(10)classStackImplinstanceofclassjava.lang.String:false
(11)classStackImplinstanceofclassjava.lang.Object:true
(12)classStackImplinstanceofclassStackImpl:true
(14)class[Ljava.lang.String;instanceofclassjava.lang.Object:true
(15)class[Ljava.lang.String;instanceofclass[Ljava.lang.Object;:true
(16)nullinstanceofclassjava.lang.Object:false
(17)class[Ljava.lang.String;instanceofclass[Ljava.lang.String;:true
(18)classjava.lang.Stringinstanceofclassjava.lang.String:true
ReviewQuestions
7.23Whichstatementaboutthefollowingprogramistrue?
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publicclassMyClass{
publicstaticvoidmain(String[]args){
A[]arrA;
B[]arrB;
arrA=newA[10];
arrB=newB[20];
arrA=arrB;//(1)
arrB=(B[])arrA;//(2)
arrA=newA[10];
arrB=(B[])arrA;//(3)
}
}
classA{}
classBextendsA{}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompilebecauseoftheassignmentat(1).
(b)Whenrun,theprogramwillthrowajava.lang.ClassCastException
intheassignmentat(2).
(c)Whenrun,theprogramwillthrowajava.lang.ClassCastException
intheassignmentat(3).
(d)Theprogramwillcompileandrunwithouterrors,evenifthecastoperator
(B[])inthestatementsat(2)and(3)isremoved.
(e)Theprogramwillcompileandrunwithouterrors,butwillnotdosoifthecast
operator(B[])instatementsat(2)and(3)isremoved.
7.24Whichstatementswillcauseacompile-timeerrorinthefollowingcode?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
MyClassa;
MySubclassb;
a=newMyClass();//(1)
b=newMySubclass();//(2)
a=b;//(3)
b=a;//(4)
a=newMySubclass();//(5)
b=newMyClass();//(6)
}
}
classMySubclassextendsMyClass{}
Selectthetwocorrectanswers.
(a)(1)
(b)(2)
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(c)(3)
(d)(4)
(e)(5)
(f)(6)
7.25Giventhefollowingtypeandreferencedeclarations,whichassignmentislegal?
Clickheretoviewcodeimage
//Typedeclarations:
interfaceI1{}
interfaceI2{}
classC1implementsI1{}
classC2implementsI2{}
classC3extendsC1implementsI2{}
//Referencedeclarations:
C1obj1=null;
C2obj2=null;
C3obj3=null;
Selecttheonecorrectanswer.
(a)obj2=obj1;
(b)obj3=obj1;
(c)obj3=obj2;
(d)I1a=obj2;
(e)I1b=obj3;
(f)I2c=obj1;
7.26Giventhefollowingclassandreferencedeclarations,whatcanbesaidaboutthe
statementy=(Sub)x?
//Classdeclarations:
classSuper{}
classSubextendsSuper{}
//Referencedeclarations:
Superx=null;
Suby=null;
Selecttheonecorrectanswer.
(a)Itisillegalatcompiletime.
(b)Itislegalatcompiletime,butmightbeillegalatruntime.
(c)Itisdefinitelylegalatruntime,butthecastoperator(Sub)isnotstrictly
needed.
(d)Itisdefinitelylegalatruntime,andthecastoperator(Sub)isneeded.
7.27GiventhreeclassesA,B,andC,whereBisasubclassofA,andCisasubclassof
B,whichoneofthesebooleanexpressionsistrueonlywhenthereferenceo
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referstoanobjectofclassB,andnottoanobjectofclassAorclassC?
Selecttheonecorrectanswer.
(a)(oinstanceofB)&&(!(oinstanceofA))
(b)(oinstanceofB)&&(!(oinstanceofC))
(c)!((oinstanceofA)||(oinstanceofB))
(d)(oinstanceofB)
(e)(oinstanceofB)&&!((oinstanceofA)||(o
instanceofC))
7.28Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassRQ07A100{
publicstaticvoidmain(String[]args){
Ix=newD();
if(xinstanceofI)System.out.print(“I”);
if(xinstanceofJ)System.out.print(“J”);
if(xinstanceofC)System.out.print(“C”);
if(xinstanceofD)System.out.print(“D”);
System.out.println();
}
}
interfaceI{}
interfaceJ{}
classCimplementsI{}
classDextendsCimplementsJ{}
Selecttheonecorrectanswer.
(a)Theprogramwillnotprintanyletters.
(b)ICD
(c)IJD
(d)IJCD
(e)ID
7.29Whatistheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
classYingYang{
voidyingyang(Integeri){
System.out.println(“Integer:”+i);
}
voidyingyang(Integer[]ints){
System.out.println(“Integer[]:”+ints[0]);
}
voidyingyang(Integer…ints){
System.out.println(“Integer…:”+ints[0]);
}
}
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publicclassRQ800A50{
publicstaticvoidmain(String[]args){
YingYangyy=newYingYang();
yy.yingyang(10);
yy.yingyang(10,12);
yy.yingyang(newInteger[]{10,20});
yy.yingyang(newInteger(10),newInteger(20));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompilebecauseoferrors.
(b)Theprogramwillcompile,butthrowanexceptionatruntime.
(c)Theprogramwillcompileandprint:
Integer:10
Integer…:10
Integer…:10
Integer…:10
(d)Theprogramwillcompileandprint:
Integer:10
Integer…:10
Integer[]:10
Integer…:10
7.30Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassRQ800A20{
staticvoidcompute(int…ia){//(1)
System.out.print(“|”);
for(inti:ia){
System.out.print(i+“|”);
}
System.out.println();
}
staticvoidcompute(int[]ia1,int…ia2){//(2)
compute(ia1);
compute(ia2);
}
staticvoidcompute(int[]ia1,int[]…ia2d){//(3)
for(int[]ia:ia2d){
compute(ia);
}
}
publicstaticvoidmain(String[]args){
compute(newint[]{10,11},newint[]{12,13,14});//(4)
compute(15,16);//(5)
compute(newint[]{17,18},newint[][]{{19},{20}});//(6)
compute(null,newint[][]{{21},{22}});//(7)
}
}
Selecttheonecorrectanswer.
(a)Theprogramdoesnotcompilebecauseoferrorsinoneormorecallstothe
compute()method.
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(b)Theprogramcompiles,butthrowsaNullPointerExceptionwhenrun.
(c)Theprogramcompilesandprints:
|10|11|
|12|13|14|
|15|16|
|19|
|20|
|21|
|22|
(d)Theprogramcompilesandprints:
|12|13|14|
|15|16|
|10|11|
|19|
|20|
|21|
|22|
7.12PolymorphismandDynamicMethodLookup
Whichobjectareferencewillactuallydenoteduringruntimecannotalwaysbedetermined
atcompiletime.Polymorphismallowsareferencetodenoteobjectsofdifferenttypesat
differenttimesduringexecution.Asupertypereferenceexhibitspolymorphicbehavior
sinceitcandenoteobjectsofitssubtypes.
Whenanon-privateinstancemethodisinvokedonanobject,themethoddefinition
actuallyexecutedisdeterminedbothbythetypeoftheobjectatruntimeandbythe
methodsignature.Dynamicmethodlookup(alsoknownaslatebinding,dynamicbinding,
andvirtualmethodinvocation)istheprocessofdeterminingwhichmethoddefinitiona
methodsignaturedenotesduringruntime,basedonthetypeoftheobject.However,acall
toaprivateinstancemethodisnotpolymorphic.Suchacallcanoccuronlywithinthe
classandgetsboundtotheprivatemethodimplementationatcompiletime.
TheinheritancehierarchydepictedinFigure7.6isimplementedinExample7.18.The
implementationofthemethoddraw()isoverriddeninallsubclassesoftheclass
Shape.Theinvocationofthedraw()methodinthetwoloopsat(3)and(4)inExample
7.18reliesonthepolymorphicbehaviorofreferencesanddynamicmethodlookup.The
arrayshapesholdsShapereferencesdenotingaCircle,aRectangle,anda
Square,asshownat(1).Atruntime,dynamiclookupdeterminesthedraw()
implementationthatwillexecute,basedonthetypeoftheobjectdenotedbyeachelement
inthearray.Thisisalsothecasefortheelementsofthearraydrawablesat(2),which
holdsIDrawablereferencesthatcanbeassignedthereferencevalueofanyobjectofa
classthatimplementstheIDrawableinterface.Thefirstloopwillstillworkwithoutany
changeifobjectsofnewsubclassesoftheclassShapeareaddedtothearrayshapes.If
theydidnotoverridethedraw()method,aninheritedversionofthemethodwouldbe
executed.Thispolymorphicbehaviorappliestothearraydrawables,wherethesubtype
objectsareguaranteedtohaveimplementedtheIDrawableinterface.
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Figure7.6TypeHierarchyThatIllustratesPolymorphism
Polymorphismanddynamicmethodlookupformapowerfulprogrammingparadigmthat
simplifiesclientdefinitions,encouragesobjectdecoupling,andsupportsdynamically
changingrelationshipsbetweenobjectsatruntime.
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Example7.18PolymorphismandDynamicMethodLookup
Clickheretoviewcodeimage
//File:PolymorphRefs.java
interfaceIDrawable{
voiddraw();
}
//_______________________________________________________________________________
classShapeimplementsIDrawable{
@Overridepublicvoiddraw(){System.out.println(“DrawingaShape.”);}
}
//_______________________________________________________________________________
classCircleextendsShape{
@Overridepublicvoiddraw(){System.out.println(“DrawingaCircle.”);}
}
//_______________________________________________________________________________
classRectangleextendsShape{
@Overridepublicvoiddraw(){System.out.println(“DrawingaRectangle.”);
}
}
//_______________________________________________________________________________
classSquareextendsRectangle{
@Overridepublicvoiddraw(){System.out.println(“DrawingaSquare.”);}
}
//_______________________________________________________________________________
classGraphimplementsIDrawable{
@Overridepublicvoiddraw(){System.out.println(“DrawingaGraph.”);}
}
//_______________________________________________________________________________
publicclassPolymorphRefs{
publicstaticvoidmain(String[]args){
Shape[]shapes={newCircle(),newRectangle(),newSquare()};//
(1)
IDrawable[]drawables={newShape(),newRectangle(),newGraph()};//
(2)
System.out.println(“Drawshapes:”);
for(Shapeshape:shapes)//
(3)
shape.draw();
System.out.println(“Drawdrawables:”);
for(IDrawabledrawable:drawables)//
(4)
drawable.draw();
}
}
Outputfromtheprogram:
Drawshapes:
DrawingaCircle.
DrawingaRectangle.
DrawingaSquare.
Drawdrawables:
DrawingaShape.
DrawingaRectangle.
DrawingaGraph.
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7.13InheritanceversusAggregation
Figure7.7isaUMLclassdiagramshowingseveralaggregationrelationshipsandone
inheritancerelationship.Thisclassdiagramshowsaqueuedefinedbyaggregationanda
stackdefinedbyinheritance,bothofwhicharebasedonlinkedlists.Alinkedlist,inturn,
isdefinedbyaggregation.Example7.19showsanon-genericimplementationofthese
datastructures.Thepurposeoftheexampleistoillustrateinheritanceandaggregation,not
industrial-strengthimplementationofqueuesandstacks.TheclassNodeat(1)is
straightforward,definingtwofields:onedenotingthedataandtheotherdenotingthenext
nodeinthelist.TheclassLinkedListat(2)keepstrackofthelistbymanaginghead
andtailreferences.Nodescanbeinsertedinthefrontorback,butdeletedonlyfromthe
frontofthelist.
Figure7.7ImplementingDataStructuresbyInheritanceandAggregation
Example7.19ImplementingDataStructuresbyInheritanceandAggregation
Clickheretoviewcodeimage
classNode{//(1)
privateObjectdata;//Data
privateNodenext;//Nextnode
//Constructorforinitializingdataandreferencetothenextnode.
Node(Objectdata,Nodenext){
this.data=data;
this.next=next;
}
//Methods:
publicvoidsetData(Objectobj){data=obj;}
publicObjectgetData(){returndata;}
publicvoidsetNext(Nodenode){next=node;}
publicNodegetNext(){returnnext;}
}
//______________________________________________________________________________
classLinkedList{//(2)
protectedNodehead=null;
protectedNodetail=null;
//Methods:
publicbooleanisEmpty(){returnhead==null;}
publicvoidinsertInFront(ObjectdataObj){
if(isEmpty())head=tail=newNode(dataObj,null);
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elsehead=newNode(dataObj,head);
}
publicvoidinsertAtBack(ObjectdataObj){
if(isEmpty())
head=tail=newNode(dataObj,null);
else{
tail.setNext(newNode(dataObj,null));
tail=tail.getNext();
}
}
publicObjectdeleteFromFront(){
if(isEmpty())returnnull;
Noderemoved=head;
if(head==tail)head=tail=null;
elsehead=head.getNext();
returnremoved.getData();
}
}
//______________________________________________________________________________
classQueueByAggregation{//(3)
privateLinkedListqList;
//Constructor
QueueByAggregation(){
qList=newLinkedList();
}
//Methods:
publicbooleanisEmpty(){returnqList.isEmpty();}
publicvoidenqueue(Objectitem){qList.insertAtBack(item);}
publicObjectdequeue(){
if(qList.isEmpty())returnnull;
returnqList.deleteFromFront();
}
publicObjectpeek(){
return(qList.isEmpty()?null:qList.head.getData());
}
}
//______________________________________________________________________________
classStackByInheritanceextendsLinkedList{//(4)
publicvoidpush(Objectitem){insertInFront(item);}
publicObjectpop(){
if(isEmpty())returnnull;
returndeleteFromFront();
}
publicObjectpeek(){
return(isEmpty()?null:head.getData());
}
}
//______________________________________________________________________________
publicclassClient{//(5)
publicstaticvoidmain(String[]args){
Stringstring1=“Queuesareboringtostandin!”;
intlength1=string1.length();
QueueByAggregationqueue=newQueueByAggregation();
for(inti=0;i<length1;i++)
queue.enqueue(newCharacter(string1.charAt(i)));
while(!queue.isEmpty())
System.out.print(queue.dequeue());
System.out.println();
Stringstring2=“!notisotnuferaskcatS”;
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intlength2=string2.length();
StackByInheritancestack=newStackByInheritance();
for(inti=0;i<length2;i++)
stack.push(newCharacter(string2.charAt(i)));
stack.insertAtBack(newCharacter(‘!’));//(6)
while(!stack.isEmpty())
System.out.print(stack.pop());
System.out.println();
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Queuesareboringtostandin!
Stacksarefuntositon!!
Choosingbetweeninheritanceandaggregationtomodelrelationshipscanbeacrucial
designdecision.Agooddesignstrategyadvocatesthatinheritanceshouldbeusedonlyif
therelationshipis-aisunequivocallymaintainedthroughoutthelifetimeoftheobjects
involved;otherwise,aggregationisthebestchoice.Aroleisoftenconfusedwithanis-a
relationship.Forexample,giventheclassEmployee,itwouldnotbeagoodideato
modeltherolesthatanemployeecanplay(suchasmanagerorcashier)byinheritance,if
theseroleschangeintermittently.Changingroleswouldinvolveanewobjecttorepresent
thenewroleeverytimethishappens.
Codereuseisalsobestachievedbyaggregationwhenthereisnois-arelationship.
Enforcinganartificialis-arelationshipthatisnotnaturallypresentisusuallynotagood
idea.SincetheclassStackByInheritanceat(4)inExample7.19isasubclassofthe
classLinkedListat(2),anyinheritedmethodfromthesuperclasscanbeinvokedonan
instanceofthesubclass.Also,methodsthatcontradicttheabstractionrepresentedbythe
subclasscanbeinvoked,asshownat(6).Usingaggregationinsuchacaseresultsina
bettersolution,asdemonstratedbytheclassQueueByAggregationat(3).Theclass
definestheoperationsofaqueuebydelegatingsuchrequeststotheunderlyingclass
LinkedList.Clientsimplementingaqueueinthismannerdonothaveaccesstothe
underlyingclassand,therefore,cannotbreaktheabstraction.
Bothinheritanceandaggregationpromoteencapsulationofimplementation,aschangesto
theimplementationarelocalizedtotheclass.Changingthecontractofasuperclasscan
haveconsequencesforthesubclasses(calledtherippleeffect)aswellasforclientsthat
aredependentonaparticularbehaviorofthesubclasses.
Polymorphismisachievedthroughinheritanceandinterfaceimplementation.Coderelying
onpolymorphicbehaviorwillstillworkwithoutanychangeifnewsubclassesornew
classesimplementingtheinterfaceareadded.Ifnoobviousis-arelationshipispresent,
polymorphismisbestachievedbyusingaggregationwithinterfaceimplementation.
7.14BasicConceptsinObject-OrientedDesign
Inthissection,weprovideabriefexplanationofsomebasicconceptsinobject-oriented
(OO)design.Formoredetails,thereaderisencouragedtoconsultthevastbodyof
literaturethatisreadilyavailableonthissubject.
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Encapsulation
Anobjecthaspropertiesandbehaviorsthatareencapsulatedinsidetheobject.The
servicesthattheobjectofferstoitsclientsmakeupitscontract,orpublicinterface.Only
thecontractdefinedbytheobjectisavailabletotheclients.Theimplementationofits
propertiesandbehaviorisnotaconcernoftheclients.Encapsulationhelpstomakeclear
thedistinctionbetweenanobject’scontractandimplementation.Thisdemarcationhas
majorconsequencesforprogramdevelopment,astheimplementationofanobjectcan
changewithoutaffectingtheclients.Encapsulationalsoreducescomplexity,asthe
internalsofanobjectarehiddenfromtheclients,whichcannotalteritsimplementation.
Encapsulationisachievedthroughinformationhiding,bymakingjudicioususeof
languagefeaturesprovidedforthispurpose.InformationhidinginJavacanbeachievedat
differentlevelsofgranularity:
•Methodorblocklevel
Localizinginformationinamethodhidesitfromtheoutside.
•Classlevel
Theaccessibilityofmembersdeclaredinaclasscanbecontrolledthroughmember
accessibilitymodifiers.Onemuch-advocatedinformation-hidingtechniqueisto
preventdirectaccessbyclientstodatamaintainedbyanobject.Thefieldsofthe
objectareprivate,anditscontractdefinespublicmethodsfortheservicesprovided
bytheobject.Suchtightencapsulationhelpstoseparatetheusefromthe
implementationofaclass.
•Packagelevel
Classesthatbelongtogethercanbegroupedintorelevantpackagesbyusingthe
packagestatement.Interpackageaccessibilityofclassescanbecontrolledbyclass
accessibilitymodifiers.
Cohesion
Cohesionisaninterclassmeasureofhowwellstructuredandcloselyrelatedthe
functionalityisinaclass.Theobjectiveistodesignclasseswithhighcohesion,that
performwell-definedandrelatedtasks(alsocalledfunctionalcohesion).Thepublic
methodsofahighlycohesiveclasstypicallyimplementasinglespecifictaskthatis
relatedtothepurposeoftheclass.Forexample,inanMVC-basedapplication,the
respectiveclassesfortheModel,theView,andtheControllershouldfocusonproviding
functionalitythatrelatestotheirindividualpurpose.Inotherwords,amethodinoneclass
shouldnotperformataskthatshouldactuallybeimplementedbyoneoftheothertwo
classes.
Lackofcohesioninaclassmeansthatthepurposeoftheclassisnotfocused,and
unrelatedfunctionalityisendingupintheclass(alsocalledcoincidentalcohesion)—
whichwilleventuallyimpactthemaintainabilityoftheapplication.
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Coupling
Couplingisameasureofintraclassdependencies.Becauseobjectsneedtointeractwith
oneanother,dependenciesbetweenclassesareinherentinOOdesign.However,these
dependenciesshouldbeminimizedtoachieveloosecoupling,whichfacilitatesthe
creationofextensibleapplications.
Onemajorsourceofintraclassdependenciesistheexposureofimplementationdetailsof
anobject.Suchdetailscanbeutilizedbyotherobjects,andthisdependencycanimpede
changesintheimplementation,resultinginlessextensibleapplications.
HighcohesionandloosecouplinghelptoachievethemaingoalsofOOdesign:
maintainability,reusability,extensibility,andreliability.
ReviewQuestions
7.31Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassPolymorphism{
publicstaticvoidmain(String[]args){
Aref1=newC();
Bref2=(B)ref1;
System.out.println(ref2.f());
}
}
classA{intf(){return0;}}
classBextendsA{intf(){return1;}}
classCextendsB{intf(){return2;}}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butwillthrowaClassCastExceptionat
runtime.
(c)Theprogramwillcompile,andprint0whenrun.
(d)Theprogramwillcompile,andprint1whenrun.
(e)Theprogramwillcompile,andprint2whenrun.
7.32Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassPolymorphism2{
publicstaticvoidmain(String[]args){
Aref1=newC();
Bref2=(B)ref1;
System.out.println(ref2.g());
}
}
classA{
privateintf(){return0;}
publicintg(){return3;}
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}
classBextendsA{
privateintf(){return1;}
publicintg(){returnf();}
}
classCextendsB{
publicintf(){return2;}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,andprint0whenrun.
(c)Theprogramwillcompile,andprint1whenrun.
(d)Theprogramwillcompile,andprint2whenrun.
(e)Theprogramwillcompile,andprint3whenrun.
7.33Whichstatementsaboutthefollowingprogramaretrue?
Clickheretoviewcodeimage
publicinterfaceHeavenlyBody{Stringdescribe();}
classStar{
StringstarName;
publicStringdescribe(){return“star”+starName;}
}
classPlanetextendsStar{
Stringname;
publicStringdescribe(){
return“planet”+name+”orbitingstar”+starName;
}
}
Selectthethreecorrectanswers:
(a)Thecodewillfailtocompile.
(b)ThecodedefinesaPlanetis-aStarrelationship.
(c)ThecodewillfailtocompileifthenamestarNameisreplacedwiththename
bodyNamethroughoutthedeclarationoftheStarclass.
(d)ThecodewillfailtocompileifthenamestarNameisreplacedwiththename
namethroughoutthedeclarationoftheStarclass.
(e)AninstanceofPlanetisavalidinstanceofHeavenlyBody.
(f)ThecodedefinesaPlanethas-aStarrelationship.
7.34Giventhefollowingcode,whichstatementistrue?
Clickheretoviewcodeimage
publicinterfaceHeavenlyBody{Stringdescribe();}
classStarimplementsHeavenlyBody{
StringstarName;
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publicStringdescribe(){return“star”+starName;}
}
classPlanet{
Stringname;
Starorbiting;
publicStringdescribe(){
return“planet”+name+”orbiting”+orbiting.describe();
}
}
Selecttheonecorrectanswer:
(a)Thecodewillfailtocompile.
(b)ThecodedefinesaPlanethas-aStarrelationship.
(c)ThecodewillfailtocompileifthenamestarNameisreplacedwiththename
bodyNamethroughoutthedeclarationoftheStarclass.
(d)ThecodewillfailtocompileifthenamestarNameisreplacedwiththename
namethroughoutthedeclarationoftheStarclass.
(e)AninstanceofPlanetisavalidinstanceofaHeavenlyBody.
(f)ThecodedefinesaPlanetis-aStarrelationship.
7.35Whichofthefollowingstatementsisnottrue?
Selecttheonecorrectanswer.
(a)Maximizingcohesionandminimizingcouplingarethehallmarksofawell-
designedapplication.
(b)CouplingisaninherentpropertyofanynontrivialOOdesign.
(c)Dependenciesbetweenclassescanbeminimizedbyhidingimplementation
details.
(d)Eachmethodimplementingasingletaskthatisrelatedtothepurposeofthe
classwillresultinaclassthathashighcohesion.
(e)Noneoftheabove.
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Inheritanceanditsimplicationsinobject-orientedprogramming
•Overridingandhidingofsuperclassmembers
•Methodoverridingversusmethodoverloading
•Useofthesuperreferencetoaccesssuperclassmembers
•Useofthis()andsuper()calls,includingconstructorchaining
•Interfacesandmultipleinterfaceinheritance
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•Subtype–supertyperelationships
•Conversionswhenassigning,casting,andpassingreferencevalues
•Resolutionofoverloadedmethods
•Identifyingthetypeofobjectsusingtheinstanceofoperator
•Polymorphismanddynamicmethodlookup
•Inheritance(is-a)versusaggregation(has-a)
•Bestpracticesforobject-orienteddesign:tightencapsulation,loosecoupling,and
highcohesioninclasses
ProgrammingExercises
7.1DeclareaninterfacecalledFunctionthathasamethodnamedevaluatethat
takesanintparameterandreturnsanintvalue.
CreateaclasscalledHalfthatimplementstheFunctioninterface.The
implementationofthemethodevaluate()shouldreturnthevalueobtainedby
dividingtheintargumentby2.
Inaclient,createamethodthattakesanarbitrarynumberofintvaluesasa
varargsparameter,andreturnsanarraythathaslengthequaltothenumberof
valuespassedinthevarargsparameter,butwiththevalueofanelementinthenew
arraybeinghalfthatofthecorrespondingvalueinthevarargsparameter.Letthe
implementationofthismethodcreateaninstanceofHalf,andusethisinstanceto
calculatevaluesforthearraythatisreturned.
7.2Rewritethemethodthatoperatedonintvaluesfromthepreviousexercise:The
methodshouldnowalsoacceptaFunctionreferenceasanargument,anduse
thisargumentinsteadofaninstanceoftheHalfclass.
CreateaclasscalledPrintthatimplementsthemethodevaluate()inthe
Functioninterface.Thismethodsimplyprintstheintvaluepassedasan
argument,andreturnsthisvalue.
Nowwriteaprogramthatdoesthefollowing:
PrintsanarbitrarynumberofintvaluesusinganinstanceofthePrintclassand
themethoddescribedearlier.
Halvesthevaluesinthearrayandprintsthevaluesagain,usingtheHalfand
Printclasses,andthemethoddescribedearlier.
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8.FundamentalClasses
8.1Overviewofthe Package
Thejava.langpackageisindispensablewhenprogramminginJava.Itisautomatically
importedintoeverysourcefileatcompiletime.ThepackagecontainstheObjectclass
thatisthesuperclassofallclasses,andthewrapperclasses(Boolean,Character,
Byte,Short,Integer,Long,Float,Double)thatareusedtohandleprimitive
valuesasobjects.ItprovidesclassesessentialforinteractingwiththeJVM(Runtime),
forsecurity(SecurityManager),forloadingclasses(ClassLoader),fordealing
withthreads(Thread),andforexceptions(Throwable,Error,Exception,
RuntimeException).Thejava.langpackagealsocontainsclassesthatprovidethe
standardinput,output,anderrorstreams(System),stringhandling(String,
StringBuilder,StringBuffer),andmathematicalfunctions(Math).
Figure8.1showstheimportantclassesthatarediscussedindetailinthischapter.
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Figure8.1PartialInheritanceHierarchyinthejava.langPackage
8.2The Class
AllclassesextendtheObjectclass,eitherdirectlyorindirectly.Aclassdeclaration,
withouttheextendsclause,implicitlyextendstheObjectclass(§7.1,p.264).Thus,
theObjectclassisalwaysattherootofanyinheritancehierarchy.TheObjectclass
definesthebasicfunctionalitythatallobjectsexhibitandallclassesinherit.This
relationshipalsoappliestoarrays,sincethesearegenuineobjectsinJava.
TheObjectclassprovidesthefollowinggeneralutilitymethods(seeExample8.1for
usageofsomeofthesemethods):
booleanequals(Objectobj)
Objectreferenceandvalueequalityarediscussedtogetherwiththe==and!=
operators(§5.12,p.181).Theequals()methodintheObjectclassreturns
trueonlyifthetworeferencescompareddenotethesameobject.Theequals()
methodisusuallyoverriddentoprovidethesemanticsofobjectvalueequality,asis
thecaseforthewrapperclasses(§8.3,p.350)andtheStringclass(§8.4,p.363).
inthashCode()
Whenstoringobjectsinhashtables,thismethodcanbeusedtogetahashvaluefor
anobject.Thisvalueisguaranteedtobeconsistentduringtheexecutionofthe
program,providedtheinformationusedintheequals()comparisonsonthe
objectdoesnotchange.Thismethodtriestoreturndistinctintegersfordistinct
objectsastheirdefaulthashvalue.ThehashCode()methodisusually
overriddenbyaclass,asisthecaseforthewrapperclassesandtheStringclass.
finalClass<?>getClass()
Returnstheruntimeclassoftheobject,whichisrepresentedbyanobjectofthe
classjava.lang.Classatruntime.
Clickheretoviewcodeimage
protectedObjectclone()throwsCloneNotSupportedException
Newobjectsthatareexactlythesame(i.e.,haveidenticalstates)asthecurrent
objectcanbecreatedbyusingtheclone()method;thatis,primitivevaluesand
referencevaluesarecopied.Thisiscalledshallowcopying.Aclasscanoverride
thismethodtoprovideitsownnotionofcloning.Forexample,cloningacomposite
objectbyrecursivelycloningtheconstituentobjectsiscalleddeepcopying.
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Whenoverridden,themethodinthesubclassisusuallydeclaredaspublicto
allowanyclienttocloneobjectsoftheclass.Iftheoverridingclone()methodin
thesubclassreliesontheclone()methodintheObjectclass(i.e.,ashallow
copy),thesubclassmustimplementtheCloneablemarkerinterfacetoindicate
thatitsobjectscanbesafelycloned.Otherwise,theclone()methodinthe
ObjectclasswillthrowacheckedCloneNotSupportedException.
StringtoString()
Ifasubclassdoesnotoverridethismethod,itreturnsatextualrepresentationofthe
object,whichhasthefollowingformat:
Clickheretoviewcodeimage
“<nameoftheclass>@<hashcodevalueofobject>”
Sincethedefaulthashvalueofanobjectisanintvalue,thisvalueisprintedasa
hexadecimalnumber(e.g.,3e25a5).Thismethodisusuallyoverridden.The
methodcallSystem.out.println(objRef)willimplicitlyconvertits
argumenttoatextualrepresentationbycallingthetoString()methodonthe
argument.Seealsothebinarystringconcatenationoperator+,discussedin§5.7on
page169.
Clickheretoviewcodeimage
protectedvoidfinalize()throwsThrowable
Thismethodisdiscussedinconnectionwithgarbagecollection(§9.4,p.390).Itis
calledonanobjectjustbeforeitisgarbagecollected,sothatanycleaningupcanbe
done.However,thedefaultfinalize()methodintheObjectclassdoesnot
doanythinguseful.
Inaddition,theObjectclassprovidessupportforthreadcommunicationinsynchronized
code,throughthefollowingmethods.Thisimportanttopicisbeyondthescopeofthis
book.
Clickheretoviewcodeimage
finalvoidwait(longtimeout)throwsInterruptedException
finalvoidwait(longtimeout,intnanos)throwsInterruptedException
finalvoidwait()throwsInterruptedException
finalvoidnotify()
finalvoidnotifyAll()
Athreadinvokesthesemethodsontheobjectwhoselockitholds.Athreadwaits
fornotificationbyanotherthread.
Example8.1MethodsintheObjectClass
Clickheretoviewcodeimage
//File:ObjectMethods.java
classMyClassimplementsCloneable{
@Override
publicMyClassclone(){
MyClassobj=null;
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try{obj=(MyClass)super.clone();}//Callsoverriddenmethod.
catch(CloneNotSupportedExceptione){System.out.println(e);}
returnobj;
}
}
//______________________________________________________________________________
publicclassObjectMethods{
publicstaticvoidmain(String[]args){
//TwoobjectsofMyClass.
MyClassobj1=newMyClass();
MyClassobj2=newMyClass();
//Twostrings.
Stringstr1=newString(“WhoAmI”);
Stringstr2=newString(“WhoAmI”);
//MethodhashCode()overriddeninStringclass.
//Stringsthatareequalhavethesamehashcode.
System.out.println(“hashcodeforstr1:”+str1.hashCode());
System.out.println(“hashcodeforstr2:”+str2.hashCode()+“\n”);
//HashcodesaredifferentfordifferentMyClassobjects.
System.out.println(“hashcodeforMyClassobj1:”+obj1.hashCode());
System.out.println(“hashcodeforMyClassobj2:”+
obj2.hashCode()+”\n”);
//Methodequals()overriddenintheStringclass.
System.out.println(“str1.equals(str2):”+str1.equals(str2));
System.out.println(“str1==str2:”+(str1==str2)+“\n”);
//Methodequals()fromtheObjectclasscalled.
System.out.println(“obj1.equals(obj2):”+obj1.equals(obj2));
System.out.println(“obj1==obj2:”+(obj1==obj2)+“\n”);
//Theruntimeobjectthatrepresentstheclassofanobject.
ClassrtStringClass=str1.getClass();
ClassrtMyClassClass=obj1.getClass();
//Thenameoftheclassrepresentedbytheruntimeobject.
System.out.println(“Classforstr1:”+rtStringClass);
System.out.println(“Classforobj1:”+rtMyClassClass+“\n”);
//ThetoString()methodisoverriddenintheStringclass.
StringtextRepStr=str1.toString();
StringtextRepObj=obj1.toString();
System.out.println(“Textrepresentationofstr1:”+textRepStr);
System.out.println(“Textrepresentationofobj1:”+textRepObj+“\n”);
//Shallowcopyingofarrays.
MyClass[]array1={newMyClass(),newMyClass(),newMyClass()};
MyClass[]array2=array1.clone();
//Arrayobjectsaredifferent,butsharetheelementobjects.
System.out.println(“array1==array2:”+(array1==array2));
for(inti=0;i<array1.length;i++){
System.out.println(“array1[”+i+“]==array2[”+i+“]:”+
(array1[i]==array2[i]));
}
System.out.println();
//CloneanobjectofMyClass.
MyClassobj3=obj1.clone();
System.out.println(“hashcodeforMyClassobj3:”+obj3.hashCode());
System.out.println(“obj1==obj3:”+(obj1==obj3));
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}
}
Probableoutputfromtheprogram:
Clickheretoviewcodeimage
hashcodeforstr1:-1704812257
hashcodeforstr2:-1704812257
hashcodeforMyClassobj1:25669322
hashcodeforMyClassobj2:14978587
str1.equals(str2):true
str1==str2:false
obj1.equals(obj2):false
obj1==obj2:false
Classforstr1:classjava.lang.String
Classforobj1:classMyClass
Textrepresentationofstr1:WhoAmI
Textrepresentationofobj1:MyClass@187aeca
array1==array2:false
array1[0]==array2[0]:true
array1[1]==array2[1]:true
array1[2]==array2[2]:true
hashcodeforMyClassobj3:19770577
obj1==obj3:false
ReviewQuestions
8.1WhatisthereturntypeofthehashCode()methodintheObjectclass?
Selecttheonecorrectanswer.
(a)String
(b)int
(c)long
(d)Object
(e)Class
8.2Whichofthefollowingstatementsistrue?
Selecttheonecorrectanswer.
(a)Ifthereferencesxandydenotetwodifferentobjects,theexpression
x.equals(y)isalwaysfalse.
(b)Ifthereferencesxandydenotetwodifferentobjects,theexpression
(x.hashCode()==y.hashCode())isalwaysfalse.
(c)ThehashCode()methodintheObjectclassisdeclaredasfinal.
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(d)Theequals()methodintheObjectclassisdeclaredasfinal.
(e)Allarrayshaveamethodnamedclone.
8.3Whichexceptioncantheclone()methodoftheObjectclassthrow?
Selecttheonecorrectanswer.
(a)CloneNotSupportedException
(b)NotCloneableException
(c)IllegalCloneException
(d)NoClonesAllowedException
8.3TheWrapperClasses
Wrapperclasseswereintroducedwiththediscussionoftheprimitivedatatypes(§2.2,p.
37),andalsoinconnectionwithboxingandunboxingofprimitivevalues(§5.1,p.145).
PrimitivevaluesinJavaarenotobjects.Tomanipulatethesevaluesasobjects,the
java.langpackageprovidesawrapperclassforeachoftheprimitivedatatypes
(showninthebottomleftofFigure8.2).Thenameofthewrapperclassisthenameofthe
primitivedatatypewithauppercaseletter,exceptforint(Integer)andchar
(Character).Allwrapperclassesarefinal.Theobjectsofallwrapperclassesthat
canbeinstantiatedareimmutable;inotherwords,thevalueinthewrapperobjectcannot
bechanged.
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Figure8.2ConvertingValuesamongPrimitive,Wrapper,andStringTypes
AlthoughtheVoidclassisconsideredawrapperclass,itdoesnotwrapanyprimitive
valueandisnotinstantiable(i.e.,hasnopublicconstructors).ItjustdenotestheClass
objectrepresentingthekeywordvoid.TheVoidclasswillnotbediscussedfurtherin
thissection.
Inadditiontothemethodsdefinedforconstructingandmanipulatingobjectsofprimitive
values,thewrapperclassesdefineusefulconstants,fields,andconversionmethods.
CommonWrapperClassConstructors
TheCharacterclasshasonlyonepublicconstructor,takingacharvalueasits
parameter.Theotherwrapperclassesallhavetwopublicone-argumentconstructors:
onetakesaprimitivevalueandtheothertakesastring.
WrapperType(typev)
WrapperType(Stringstr)
Thetypeisaprimitivedatatype.Thestringargumentisconvertedtoaprimitive
valuethatcorrespondstotheWrapperType.Anunchecked
NumberFormatExceptionisthrownifthestringcannotbeconvertedtoa
primitivevaluethatcorrespondstoanumericWrapperType.
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WrappingPrimitiveValuesinObjects
Boxingisaconvenientwaytowrapaprimitivevalueinanobject((1a)inFigure8.2and
§5.1,p.145).
CharactercharObj1=‘\n’;
BooleanboolObj1=true;
IntegerintObj1=2014;
DoubledoubleObj1=3.14;
Aconstructorthattakesaprimitivevaluecanbeusedtocreatewrapperobjects((1b)in
Figure8.2).
Clickheretoviewcodeimage
CharactercharObj1=newCharacter(‘\n’);
BooleanboolObj1=newBoolean(true);
IntegerintObj1=newInteger(2014);
DoubledoubleObj1=newDouble(3.14);
WecanalsousethevalueOf()methodthattakestheprimitivevaluetowrapasan
argument((1c)inFigure8.2).
Clickheretoviewcodeimage
CharactercharObj1=Character.valueOf(‘\n’);
BooleanboolObj1=Boolean.valueOf(true);
IntegerintObj1=Integer.valueOf(2014);
DoubledoubleObj1=Double.valueOf(3.14);
ConvertingStringstoWrapperObjects
AconstructorthattakesaStringobjectrepresentingtheprimitivevaluecanalsobe
usedtocreatewrapperobjects.Theconstructorsforthenumericwrappertypesthrowan
uncheckedNumberFormatExceptioniftheStringparameterdoesnotparsetoa
validnumber((2a)inFigure8.2).
Clickheretoviewcodeimage
BooleanboolObj2=newBoolean(“TrUe”);//caseignored:true
BooleanboolObj3=newBoolean(“XX”);//false
IntegerintObj2=newInteger(“2014”);
DoubledoubleObj2=newDouble(“3.14”);
LonglongObj1=newLong(“3.14”);//NumberFormatException
CommonWrapperClassUtilityMethods
ConvertingStringstoWrapperObjects
Eachwrapperclass(exceptCharacter)definesthestaticmethodvalueOf(String
str)thatreturnsthewrapperobjectcorrespondingtotheprimitivevaluerepresentedby
theStringobjectpassedasanargument((6b)inFigure8.2).Thismethodforthe
numericwrappertypesalsothrowsaNumberFormatExceptioniftheString
parameterisnotavalidnumber.
Clickheretoviewcodeimage
staticWrapperTypevalueOf(Stringstr)
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Clickheretoviewcodeimage
BooleanboolObj4=Boolean.valueOf(“false”);
IntegerintObj3=Integer.valueOf(“1949”);
DoubledoubleObj3=Double.valueOf(“-3.0”);
Inadditiontotheone-argumentvalueOf()method,theintegerwrapperclassesdefine
anoverloadedstaticvalueOf()methodthatcantakeasecondargument.Thisargument
specifiesthebase(orradix)inwhichtointerpretthestringrepresentingthesignedinteger
inthefirstargument.
Clickheretoviewcodeimage
staticIntegerWrapperTypevalueOf(Stringstr,intbase)
throwsNumberFormatException
Clickheretoviewcodeimage
BytebyteObj1=Byte.valueOf(“1010”,2);//Decimalvalue10
ShortshortObj2=Short.valueOf(“12”,8);//Not“\012”.Decimalvalue
10.
IntegerintObj4=Integer.valueOf(“-a”,16);//Not“-0xa”.Decimalvalue
-10.
LonglongObj2=Long.valueOf(“-a”,16);//Not“-0xa”.Decimalvalue
-10L.
ConvertingWrapperObjectstoStrings
EachwrapperclassoverridesthetoString()methodfromtheObjectclass.The
overridingmethodreturnsaStringobjectcontainingthestringrepresentationofthe
primitivevalueinthewrapperobject((3)inFigure8.2).
StringtoString()
Clickheretoviewcodeimage
StringcharStr=charObj1.toString();//“\n”
StringboolStr=boolObj2.toString();//“true”
StringintStr=intObj1.toString();//“2014”
StringdoubleStr=doubleObj1.toString();//“3.14”
ConvertingPrimitiveValuestoStrings
EachwrapperclassdefinesastaticmethodtoString(typev)thatreturnsthestring
correspondingtotheprimitivevalueoftype,whichispassedasanargument((6a)in
Figure8.2).
Clickheretoviewcodeimage
staticStringtoString(typev)
Clickheretoviewcodeimage
StringcharStr2=Character.toString(‘\n’);//“\n”
StringboolStr2=Boolean.toString(true);//“true”
StringintStr2=Integer.toString(2014);//Base10.“2014”
StringdoubleStr2=Double.toString(3.14);//“3.14”
Forintegerprimitivetypes,thebaseisassumedtobe10.Forfloating-pointnumbers,the
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textualrepresentation(decimalformorscientificnotation)dependsonthesignandthe
magnitude(absolutevalue)ofthenumber.TheNaNvalue,positiveinfinity,andnegative
infinitywillresultinthestrings"NaN","Infinity",and"-Infinity",
respectively.
Inaddition,thewrapperclassesIntegerandLongdefinemethodsforconverting
integerstostringrepresentationsindecimal,binary,octal,andhexadecimalnotation(p.
353).
ConvertingWrapperObjectstoPrimitiveValues
Unboxingisaconvenientwaytounwraptheprimitivevalueinawrapperobject((4a)in
Figure8.2and§5.1,p.145).
Clickheretoviewcodeimage
charc=charObj1;//‘\n’
booleanb=boolObj2;//true
inti=intObj1;//2014
doubled=doubleObj1;//3.14
EachwrapperclassdefinesatypeValue()methodthatreturnstheprimitivevalueinthe
wrapperobject((4b)inFigure8.2).
typetypeValue()
Clickheretoviewcodeimage
charc=charObj1.charValue();//‘\n’
booleanb=boolObj2.booleanValue();//true
inti=intObj1.intValue();//2014
doubled=doubleObj1.doubleValue();//3.14
Inaddition,eachnumericwrapperclassdefinestypeValue()methodsforconvertingthe
primitivevalueinthewrapperobjecttoavalueofanynumericprimitivedatatype.These
methodsarediscussedlater.
WrapperComparison,Equality,andHashCode
EachwrapperclassimplementstheComparable<Type>interface,whichdefinesthe
followingmethod:
intcompareTo(Typeobj2)
Thismethodreturnsavaluethatislessthan,equalto,orgreaterthanzero,dependingon
whethertheprimitivevalueinthecurrentwrapperTypeobjectislessthan,equalto,or
greaterthantheprimitivevalueinthewrapperTypeobjectdenotedbyargumentobj2.
Clickheretoviewcodeimage
//Comparisonsbasedonobjectscreatedearlier
CharactercharObj2=‘a’;
intresult1=charObj1.compareTo(charObj2);//result1<0
intresult2=intObj1.compareTo(intObj3);//result2>0
intresult3=doubleObj1.compareTo(doubleObj2);//result==0
intresult4=doubleObj1.compareTo(intObj1);//Compile-timeerror!
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Eachwrapperclassoverridestheequals()methodfromtheObjectclass.The
overridingmethodcomparestwowrapperobjectsforobjectvalueequality.
booleanequals(Objectobj2)
Clickheretoviewcodeimage
//Comparisonsbasedonobjectscreatedearlier
booleancharTest=charObj1.equals(charObj2);//false
booleanboolTest=boolObj2.equals(Boolean.FALSE);//false
booleanintTest=intObj1.equals(intObj2);//true
booleandoubleTest=doubleObj1.equals(doubleObj2);//true
booleantest=intObj1.equals(newLong(2014));//false.Notsametype.
Thefollowingvaluesareinternedwhentheyarewrappedduringboxing.Thatis,onlyone
wrapperobjectexistsintheprogramfortheseprimitivevalueswhenboxingisapplied:
•Thebooleanvaluestrueorfalse
•Abyte
•AcharwithaUnicodevalueintheinterval[\u0000,\u007f](i.e.,decimal
interval[0,127])
•Anintorshortvalueintheinterval[-128,127]
Ifreferencesw1andw2refertotwowrapperobjectsthatboxthesamevalue,whichis
amongtheonesmentionedhere,thenw1==w2isalwaystrue.Inotherwords,forthe
valueslistedpreviously,objectequalityandreferenceequalitygivethesameresult.
Clickheretoviewcodeimage
//Referenceandobjectequality
BytebRef1=10;
BytebRef2=10;
System.out.println(bRef1==bRef2);//true
System.out.println(bRef1.equals(bRef2));//true
IntegeriRef1=1000;
IntegeriRef2=1000;
System.out.println(iRef1==iRef2);//false
System.out.println(iRef1.equals(iRef2));//true
EachwrapperclassalsooverridesthehashCode()methodintheObjectclass.The
overridingmethodreturnsahashvaluebasedontheprimitivevalueinthewrapperobject.
inthashCode()
Clickheretoviewcodeimage
intindex=charObj1.hashCode();//10(‘\n’)
NumericWrapperClasses
ThenumericwrapperclassesByte,Short,Integer,Long,Float,andDoubleare
allsubclassesoftheabstractclassNumber(Figure8.1).
Eachnumericwrapperclassdefinesanassortmentofconstants,includingtheminimum
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andmaximumvaluesofthecorrespondingprimitivedatatype:
NumericWrapperType.MIN_VALUE
NumericWrapperType.MAX_VALUE
Thefollowingcoderetrievestheminimumandmaximumvaluesofvariousnumeric
types:
Clickheretoviewcodeimage
byteminByte=Byte.MIN_VALUE;//-128
intmaxInt=Integer.MAX_VALUE;//2147483647
doublemaxDouble=Double.MAX_VALUE;//1.7976931348623157e+308
ConvertingNumericWrapperObjectstoNumericPrimitiveTypes
EachnumericwrapperclassdefinesthefollowingsetoftypeValue()methodsfor
convertingtheprimitivevalueinthewrapperobjecttoavalueofanynumericprimitive
type:
bytebyteValue()
shortshortValue()
intintValue()
longlongValue()
floatfloatValue()
doubledoubleValue()
Seealso(4b)inFigure8.2.
Thefollowingcodeshowsconversionofvaluesinnumericwrapperobjectstoany
numericprimitivetype:
Clickheretoviewcodeimage
BytebyteObj2=newByte((byte)16);//Castmandatory
IntegerintObj5=newInteger(42030);
DoubledoubleObj4=newDouble(Math.PI);
shortshortVal=intObj5.shortValue();//(1)
longlongVal=byteObj2.longValue();
intintVal=doubleObj4.intValue();//(2)Truncation
doubledoubleVal=intObj5.doubleValue();
Noticethepotentialforlossofinformationat(1)and(2),whentheprimitivevalueina
wrapperobjectisconvertedtoanarrowerprimitivedatatype.
ConvertingStringstoNumericValues
EachnumericwrapperclassdefinesastaticmethodparseType(Stringstr),which
returnstheprimitivenumericvaluerepresentedbytheStringobjectpassedasan
argument.TheTypeinthemethodnameparseTypestandsforthenameofanumeric
wrapperclass,exceptforthenameoftheIntegerclass,whichisabbreviatedtoInt.
ThesemethodsthrowaNumberFormatExceptioniftheStringparameterisnota
validargument((5)inFigure8.2.)
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Clickheretoviewcodeimage
statictypeparseType(Stringstr)throwsNumberFormatException
Clickheretoviewcodeimage
bytevalue1=Byte.parseByte(“16”);
intvalue2=Integer.parseInt(“2010”);//parseInt,notparseInteger
intvalue3=Integer.parseInt(“7UP”);//NumberFormatException
doublevalue4=Double.parseDouble(“3.14”);
Fortheintegerwrappertypes,theoverloadedstaticmethodparseType()can
additionallytakeasecondargument,whichcanspecifythebaseinwhichtointerpretthe
stringrepresentingthesignedintegerinthefirstargument:
Clickheretoviewcodeimage
typeparseType(Stringstr,intbase)throwsNumberFormatException
Clickheretoviewcodeimage
bytevalue6=Byte.parseByte(“1010”,2);//Decimalvalue10.
shortvalue7=Short.parseShort(“12”,8);//“012”,not“\012”.Decimalvalue
10.
intvalue8=Integer.parseInt(“-a”,16);//Not“-0xa”.Decimalvalue-10.
longvalue9=Long.parseLong(“-a”,16);//Not“-0xa”.Decimalvalue-10L.
ConvertingIntegerValuestoStringsinDifferentNotations
ThewrapperclassesIntegerandLongprovidestaticmethodsforconvertingintegers
tostringrepresentationsindecimal,binary,octal,andhexadecimalnotation.Someof
thesemethodsfromtheIntegerclassarelistedhere,butanalogousmethodsarealso
definedintheLongclass.Example8.2demonstratestheuseofthesemethods.
Clickheretoviewcodeimage
staticStringtoBinaryString(inti)
staticStringtoHexString(inti)
staticStringtoOctalString(inti)
Thesethreemethodsreturnastringrepresentationoftheintegerargumentasan
unsignedintegerinbase2,16,and8,respectively,withnoextraleadingzeroes.
Clickheretoviewcodeimage
staticStringtoString(inti,intbase)
staticStringtoString(inti)
Thefirstmethodreturnstheminussign'-'asthefirstcharacteriftheintegeriis
negative.Inallcases,itreturnsthestringrepresentationofthemagnitudeofthe
integeriinthespecifiedbase.
ThelastmethodisequivalenttothemethodtoString(inti,intbase),
wherethebasehasthevalue10,andwhichreturnsthestringrepresentationasa
signeddecimal((6a)inFigure8.2).
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Example8.2StringRepresentationofIntegers
Clickheretoviewcodeimage
publicclassIntegerRepresentation{
publicstaticvoidmain(String[]args){
intpositiveInt=+41;//0b101001,051,0x29
intnegativeInt=-41;//0b11111111111111111111111111010111,
-0b101001,
//037777777727,-051,0xffffffd7,-0x29
System.out.println(“Stringrepresentationfordecimalvalue:”+
positiveInt);
integerStringRepresentation(positiveInt);
System.out.println(“Stringrepresentationfordecimalvalue:”+
negativeInt);
integerStringRepresentation(negativeInt);
}
publicstaticvoidintegerStringRepresentation(inti){
System.out.println(“Binary:”+Integer.toBinaryString(i));
System.out.println(“Octal:”+Integer.toOctalString(i));
System.out.println(“Hex:”+Integer.toHexString(i));
System.out.println(“Decimal:”+Integer.toString(i));
System.out.println(“UsingtoString(inti,intbase)method:”);
System.out.println(“Base2:”+Integer.toString(i,2));
System.out.println(“Base8:”+Integer.toString(i,8));
System.out.println(“Base16:”+Integer.toString(i,16));
System.out.println(“Base10:”+Integer.toString(i,10));
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Stringrepresentationfordecimalvalue:41
Binary:101001
Octal:51
Hex:29
Decimal:41
UsingtoString(inti,intbase)method:
Base2:101001
Base8:51
Base16:29
Base10:41
Stringrepresentationfordecimalvalue:-41
Binary:11111111111111111111111111010111
Octal:37777777727
Hex:ffffffd7
Decimal:-41
UsingtoString(inti,intbase)method:
Base2:-101001
Base8:-51
Base16:-29
Base10:-41
The Class
TheCharacterclassdefinesamyriadofconstants,includingthefollowing,which
representtheminimumandthemaximumvaluesofthechartype(§2.2,p.38):
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Character.MIN_VALUE
Character.MAX_VALUE
TheCharacterclassalsodefinesaplethoraofstaticmethodsforhandlingvarious
attributesofacharacter,andcaseissuesrelatingtocharacters,asdefinedbytheUnicode
standard:
Clickheretoviewcodeimage
staticintgetNumericValue(charch)
staticbooleanisLowerCase(charch)
staticbooleanisUpperCase(charch)
staticbooleanisTitleCase(charch)
staticbooleanisDigit(charch)
staticbooleanisLetter(charch)
staticbooleanisLetterOrDigit(charch)
staticchartoUpperCase(charch)
staticchartoLowerCase(charch)
staticchartoTitleCase(charch)
Thefollowingcodeconvertsalowercasecharactertoanuppercasecharacter:
Clickheretoviewcodeimage
charch=‘a’;
if(Character.isLowerCase(ch))ch=Character.toUpperCase(ch);
The Class
Inadditiontothecommonutilitymethodsforwrapperclassesdiscussedearlierinthis
section,theBooleanclassdefinesthefollowingwrapperobjectstorepresentthe
primitivevaluestrueandfalse,respectively:
Boolean.TRUE
Boolean.FALSE
ConvertingStringstoBooleanValues
ThewrapperclassBooleandefinesthefollowingstaticmethod,whichreturnsthe
booleanvaluetrueonlyiftheStringargumentisequaltothestring"true",
ignoringthecase;otherwise,itreturnsthebooleanvaluefalse.Notethatthismethod
doesnotthrowanyexceptions,asitsnumericcounterpartsdo.
Clickheretoviewcodeimage
staticbooleanparseBoolean(Stringstr)
Clickheretoviewcodeimage
booleanb1=Boolean.parseBoolean(“TRUE”);//true.
booleanb2=Boolean.parseBoolean(“true”);//true.
booleanb3=Boolean.parseBoolean(“false”);//false.
booleanb4=Boolean.parseBoolean(“FALSE”);//false.
booleanb5=Boolean.parseBoolean(“nottrue”);//false.
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ReviewQuestions
8.4Whichofthefollowingarewrapperclasses?
Selectthethreecorrectanswers.
(a)java.lang.Void
(b)java.lang.Int
(c)java.lang.Boolean
(d)java.lang.Long
(e)java.lang.String
8.5Whichofthefollowingclassesdonotextendthejava.lang.Numberclass?
Selectthetwocorrectanswers.
(a)java.lang.Float
(b)java.lang.Byte
(c)java.lang.Character
(d)java.lang.Boolean
(e)java.lang.Short
8.6Whichoftheseclassesdefineimmutableobjects?
Selectthethreecorrectanswers.
(a)Character
(b)Byte
(c)Number
(d)Short
(e)Object
8.7Whichoftheseclasseshaveasingle-parameterconstructortakingastring?
Selectthetwocorrectanswers.
(a)Void
(b)Integer
(c)Boolean
(d)Character
(e)Object
8.8WhichofthewrapperclasseshaveabooleanValue()method?
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Selecttheonecorrectanswer
(a)Allwrapperclasses
(b)AllwrapperclassesexceptVoid
(c)AllwrapperclassesthatalsoimplementthecompareTo()method
(d)AllwrapperclassesextendingNumber
(e)OnlytheclassBoolean
8.9Whichstatementsaretrueaboutwrapperclasses?
Selectthetwocorrectanswers.
(a)Stringisawrapperclass.
(b)DoublehasacompareTo()method.
(c)CharacterhasaintValue()method.
(d)ByteextendsNumber.
8.10Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
publicclassRQ200A70{
publicstaticvoidmain(String[]args){
Integeri=newInteger(-10);
Integerj=newInteger(-10);
Integerk=-10;
System.out.print((i==j)+“|”);
System.out.print(i.equals(j)+“|”);
System.out.print((i==k)+“|”);
System.out.print(i.equals(k));
}
}
Selecttheonecorrectanswer.
(a)false|true|false|true
(b)true|true|true|true
(c)false|true|true|true
(d)true|true|false|true
(e)Noneoftheabove.
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8.4The Class
Handlingcharactersequencesissupportedthroughthreefinalclasses:String,
StringBuilder,andStringBuffer.TheJavaplatformusesthevariable-length
UTF-16encodingtostorecharactersinchararraysandinthestringhandlingclasses.
TheUTF-16encodingallowscharacterswhoseUnicodevaluesareintherange0000to
10FFFF.ThechartyperepresentsUnicodevaluesintherange0000toFFFF—thatis,
charactersthatcanberepresentedinasingle16-bitword.Asaconsequence,the
supplementarycharactersarerepresentedbymultiplecharvalues,ormultiple16-bit
words,whenthesearestoredinastringorachararray.Thestringhandlingclasses
providemethodstohandlethefullrangeofcharactersintheUTF-16encoding,butwe
willnotdwellonthesubjectinthisbook.
Immutability
TheStringclassimplementsimmutablecharacterstrings,whichareread-onlyoncethe
stringhasbeencreatedandinitialized.OperationsonaStringobjectthatmodifythe
charactersreturnanewStringobject.TheStringBuilderclassimplements
dynamiccharacterstrings.TheStringBufferclassisathread-safeversionofthe
StringBuilderclass.
ThissectiondiscussestheclassStringthatprovidesfacilitiesforcreating,initializing,
andmanipulatingcharacterstrings.ThenextsectiondiscussestheStringBuilderand
StringBufferclasses.
CreatingandInitializingStrings
StringLiteralsRevisited
TheeasiestwaytocreateaStringobjectistouseastringliteral:
Clickheretoviewcodeimage
Stringstr1=“Youcannotchangeme!”;
AstringliteralisareferencetoaStringobject.ThevalueintheStringobjectisthe
charactersequenceenclosedinthedoublequotesofthestringliteral.Sinceastringliteral
isareference,itcanbemanipulatedlikeanyotherStringreference.Thereference
valueofastringliteralcanbeassignedtoanotherStringreference:Thereference
str1willdenotetheStringobjectwiththevalue"Youcannotchangeme!"
aftertheprecedingassignment.Astringliteralcanbeusedtoinvokemethodsonits
Stringobject:
Clickheretoviewcodeimage
intstrLength=“Youcannotchangeme!”.length();//21
Thecompileroptimizeshandlingofstringliterals(andcompile-timeconstantexpressions
thatevaluatetostrings):OnlyoneStringobjectissharedbyallstring-valuedconstant
expressionswiththesamecharactersequence.Suchstringsaresaidtobeinterned,
meaningthattheyshareauniqueStringobjectiftheyhavethesamecontent.The
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Stringclassmaintainsastringliteralpoolwheresuchstringsareinterned.
Clickheretoviewcodeimage
Stringstr2=“Youcannotchangeme!”;//Alreadyinterned.
BothStringreferencesstr1andstr2denotethesameinternedString
objectinitializedwiththecharacterstring:"Youcannotchangeme!"Sodoesthe
referencestr3inthefollowingcode.Thecompile-timeevaluationoftheconstant
expressioninvolvingthetwostringliteralsresultsinastringthatisalreadyinterned:
Clickheretoviewcodeimage
Stringstr3=“Youcannot”+”changeme!”;//Compile-timeconstant
expression
Inthefollowingcode,boththereferencescan1andcan2denotethesameinterned
Stringobject,whichcontainsthestring"7Up":
Clickheretoviewcodeimage
Stringcan1=7+“Up”;//Valueofcompile-timeconstantexpression:“7Up”
Stringcan2=“7Up”;//“7Up”
However,inthefollowingcode,thereferencecan4denotesanewStringobjectthat
willhavethevalue"7Up"atruntime:
Clickheretoviewcodeimage
Stringword=“Up”;
Stringcan4=7+word;//Notacompile-timeconstantexpression.
ThesharingofStringobjectsbetweenstring-valuedconstantexpressionsposesno
problem,sincetheStringobjectsareimmutable.Anyoperationperformedonone
Stringreferencewillneverhaveanyeffectontheusageofotherreferencesdenoting
thesameobject.TheStringclassisalsodeclaredasfinal,sothatnosubclasscan
overridethisbehavior.
StringConstructors
TheStringclasshasnumerousconstructorstocreateandinitializeStringobjects
basedonvarioustypesofarguments.Herewepresentafewselectedconstructors:
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String()
ThisconstructorcreatesanewStringobject,whosecontentistheemptystring,
"".
String(Stringstr)
ThisconstructorcreatesanewStringobject,whosecontentsarethesameas
thoseoftheStringobjectpassedasargument.
Clickheretoviewcodeimage
String(char[]value)
String(char[]value,intoffset,intcount)
TheseconstructorscreateanewStringobject,whosecontentsarecopiedfroma
chararray.Thesecondconstructorallowsextractionofacertainnumberof
characters(count)fromagivenoffsetinthearray.
String(StringBuilderbuilder)
String(StringBufferbuffer)
TheseconstructorsallowinteroperabilitywiththeStringBuilderand
StringBufferclasses,respectively.
Notethatusingaconstructorcreatesabrand-newStringobject;usingaconstructor
doesnotinternthestring.Areferencetoaninternedstringcanbeobtainedbycallingthe
intern()methodintheStringclass—althoughinpractice,thereisusuallynoreason
todoso.
Inthefollowingcode,theStringobjectdenotedbystr4isdifferentfromtheinterned
Stringobjectpassedasanargument:
Clickheretoviewcodeimage
Stringstr4=newString(“Youcannotchangeme!”);
ConstructingStringobjectscanalsobedonefromarraysofbytes,arraysofcharacters,
orstringbuilders:
Clickheretoviewcodeimage
byte[]bytes={97,98,98,97};
char[]characters={‘a’,‘b’,‘b’,‘a’};
StringBuilderstrBuilder=newStringBuilder(“abba”);
//…
StringbyteStr=newString(bytes);//Usingarrayofbytes:“abba”
StringcharStr=newString(characters);//Usingarrayofchars:“abba”
StringbuildStr=newString(strBuilder);//Usingstringbuilder:“abba”
InExample8.3,notethatthereferencestr1doesnotdenotethesameStringobjectas
thereferencesstr4andstr5.UsingthenewoperatorwithaStringconstructor
alwayscreatesanewStringobject.Theexpression"Youcannot"+wordsisnot
aconstantexpressionand,therefore,resultsinthecreationofanewStringobject.The
localreferencesstr2andstr3inthemain()methodandthestaticreferencestr1in
theAuxiliaryclassalldenotethesameinternedstring.Objectvalueequalityishardly
surprisingbetweenthesereferences.Indeed,itmightbetemptingtousetheoperator==
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forobjectvalueequalityofstringliterals,butthisisnotadvisable.
Example8.3StringConstructionandEquality
Clickheretoviewcodeimage
//File:StringConstruction.java
classAuxiliary{
staticStringstr1=“Youcannotchangeme!”;//Interned
}
//______________________________________________________________________________
publicclassStringConstruction{
staticStringstr1=“Youcannotchangeme!”;//Interned
publicstaticvoidmain(String[]args){
StringemptyStr=newString();//””
System.out.println(“emptyStr:"”+emptyStr+“"”);
Stringstr2=“Youcannotchangeme!”;//Interned
Stringstr3=“Youcannot”+”changeme!”;//Interned
Stringstr4=newString(“Youcannotchangeme!”);//NewStringobject
Stringwords=”changeme!”;
Stringstr5=“Youcannot”+words;//NewStringobject
System.out.println(“str1==str2:”+(str1==str2));//(1)
true
System.out.println(“str1.equals(str2):”+str1.equals(str2));//(2)
true
System.out.println(“str1==str3:”+(str1==str3));//(3)
true
System.out.println(“str1.equals(str3):”+str1.equals(str3));//(4)
true
System.out.println(“str1==str4:”+(str1==str4));//(5)
false
System.out.println(“str1.equals(str4):”+str1.equals(str4));//(6)
true
System.out.println(“str1==str5:”+(str1==str5));//(7)
false
System.out.println(“str1.equals(str5):”+str1.equals(str5));//(8)
true
System.out.println(“str1==Auxiliary.str1:”+
(str1==Auxiliary.str1));//(9)true
System.out.println(“str1.equals(Auxiliary.str1):”+
str1.equals(Auxiliary.str1));//(10)true
System.out.println(“"Youcannotchangeme!".length():”+
“Youcannotchangeme!”.length());//(11)21
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
emptyStr:””
str1==str2:true
str1.equals(str2):true
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str1==str3:true
str1.equals(str3):true
str1==str4:false
str1.equals(str4):true
str1==str5:false
str1.equals(str5):true
str1==Auxiliary.str1:true
str1.equals(Auxiliary.str1):true
“Youcannotchangeme!”.length():21
The Interface
Thisinterfacedefinesareadablesequenceofcharvalues.Itisimplementedbyallthree
classes:String,StringBuilder,andStringBuffer.Manymethodsinthese
classesacceptargumentsofthisinterfacetype,andspecifyitastheirreturntype.This
interfacefacilitatesinteroperabilitybetweentheseclasses.Itdefinesthefollowing
methods:
charcharAt(intindex)
AcharacterataparticularindexinasequencecanbereadusingthecharAt()
method.Thefirstcharacterisatindex0andthelastoneatindex1lessthanthe
numberofcharactersinthestring.Iftheindexvalueisnotvalid,an
IndexOutOfBoundsExceptionisthrown.
intlength()
Thismethodreturnsthenumberofcharvaluesinthissequence.
Clickheretoviewcodeimage
CharSequencesubSequence(intstart,intend)
ThismethodreturnsanewCharSequencethatisasubsequenceofthis
sequence.Charactersfromthecurrentsequencearereadfromindexstarttothe
indexend-1,inclusive.
StringtoString()
Thismethodreturnsastringcontainingthecharactersinthissequenceinthesame
orderasthissequence.
ReadingCharactersfromaString
Thefollowingmethodscanbeusedforcharacter-relatedoperationsonastring:
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charcharAt(intindex)
ThismethodisdefinedintheCharSequenceinterface,whichtheStringclass
implements(p.360).
char[]toCharArray()
Thismethodreturnsanewcharacterarray,withlengthequaltothelengthofthis
string,thatcontainsthecharactersinthisstring.
Clickheretoviewcodeimage
voidgetChars(intsrcBegin,intsrcEnd,char[]dst,intdstBegin)
Thismethodcopiescharactersfromthecurrentstringintothedestinationcharacter
array.CharactersfromthecurrentstringarereadfromindexsrcBegintothe
indexsrcEnd-1,inclusive.Theyarecopiedintothedestinationarray(dst),
startingatindexdstBeginandendingatindexdstbegin+(srcEnd-src-
Begin)-1.Thenumberofcharacterscopiedis(srcEnd-srcBegin).An
Index-OutOfBoundsExceptionisthrowniftheindicesdonotmeetthe
criteriafortheoperation.
intlength()
ThismethodisdefinedintheCharSequenceinterface,whichtheStringclass
implements(p.360).
booleanisEmpty()
Thismethodreturnstrueifthelengthofthestringis0,andfalse
otherwise.
Example8.4usessomeofthesemethodsat(3),(4),(5),and(6).Theprogramprintsthe
frequencyofacharacterinastringandillustratescopyingfromastringintoacharacter
array.
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Example8.4ReadingCharactersfromaString
Clickheretoviewcodeimage
publicclassReadingCharsFromString{
publicstaticvoidmain(String[]args){
int[]frequencyData=newint[Character.MAX_VALUE];//(1)
Stringstr=“Youcannotchangeme!”;//(2)
//Countthefrequencyofeachcharacterinthestring.
for(inti=0;i<str.length();i++){//(3)
try{
frequencyData[str.charAt(i)]++;//(4)
}catch(StringIndexOutOfBoundsExceptione){
System.out.println(“Indexerrordetected:“+i+”notinrange.”);
}
}
//Printthecharacterfrequency.
System.out.println(“Characterfrequencyforstring:"”+str+“"”);
for(inti=0;i<frequencyData.length;i++){
if(frequencyData[i]!=0)
System.out.println((char)i+”(code“+i+”):”+frequencyData[i]);
}
System.out.println(“Copyingintoachararray:”);
char[]destination=newchar[str.length()-3];//3characters
less.
str.getChars(0,7,destination,0);//(5)“Youcan”
str.getChars(10,str.length(),destination,7);//(6)”change
me!”
//“not”not
copied.
//Printthecharacterarray.
for(inti=0;i<destination.length;i++){
System.out.print(destination[i]);
}
System.out.println();
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
CharacterFrequencyforstring:“Youcannotchangeme!”
(code32):3
!(code33):1
Y(code89):1
a(code97):2
c(code99):2
e(code101):2
g(code103):1
h(code104):1
m(code109):1
n(code110):3
o(code111):2
t(code116):1
u(code117):1
Copyingintoachararray:
Youcanchangeme!
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InExample8.4,thefrequencyDataarrayat(1)storesthefrequencyofeachcharacter
thatcanoccurinastring.Thestringinquestionisdeclaredat(2).Sinceacharvalueis
promotedtoanintvalueinarithmeticexpressions,itcanbeusedasanindexinanarray.
EachelementinthefrequencyDataarrayfunctionsasafrequencycounterforthe
charactercorrespondingtotheindexvalueoftheelement:
Clickheretoviewcodeimage
frequencyData[str.charAt(i)]++;//(4)
ThecallstothegetChars()methodat(5)and(6)copyparticularsubstringsfromthe
stringintodesignatedplacesinthedestinationarray,beforeprintingthewhole
characterarray.
ComparingStrings
CharactersarecomparedbasedontheirUnicodevalues.
Clickheretoviewcodeimage
booleantest=‘a’<‘b’;//truesince0x61<0x62
Twostringsarecomparedlexicographically,asinadictionaryortelephonedirectory,by
successivelycomparingtheircorrespondingcharactersateachpositioninthetwostrings,
startingwiththecharactersinthefirstposition.Thestring"abba"islessthan"aha",
sincethesecondcharacter'b'inthestring"abba"islessthanthesecondcharacter
'h'inthestring"aha".Thecharactersinthefirstpositionineachofthesestringsare
equal.
Thefollowingpublicmethodscanbeusedforcomparingstrings:
Clickheretoviewcodeimage
booleanequals(Objectobj)
booleanequalsIgnoreCase(Stringstr2)
TheStringclassoverridestheequals()methodfromtheObjectclass.The
Stringclassequals()methodimplementsStringobjectvalueequalityas
twoStringobjectshavingthesamesequenceofcharacters.The
equalsIgnoreCase()methoddoesthesame,butignoresthecaseofthe
characters.
intcompareTo(Stringstr2)
TheStringclassimplementstheComparable<String>interface.The
compareTo()methodcomparesthetwostrings,andreturnsavaluebasedonthe
outcomeofthecomparison:
•Thevalue0,ifthisstringisequaltothestringargument
•Avaluelessthan0,ifthisstringislexicographicallylessthanthestring
argument
•Avaluegreaterthan0,ifthisstringislexicographicallygreaterthanthestring
argument
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Herearesomeexamplesofstringcomparisons:
Clickheretoviewcodeimage
StringstrA=newString(“TheCasewasthrownoutofCourt”);
StringstrB=newString(“thecasewasthrownoutofcourt”);
booleanb1=strA.equals(strB);//false
booleanb2=strA.equalsIgnoreCase(strB);//true
Stringstr1=newString(“abba”);
Stringstr2=newString(“aha”);
intcompVal1=str1.compareTo(str2);//negativevalue=>str1<str2
CharacterCaseinaString
Clickheretoviewcodeimage
StringtoUpperCase()
StringtoUpperCase(Localelocale)
StringtoLowerCase()
StringtoLowerCase(Localelocale)
Notethattheoriginalstringisreturnedifnoneofthecharactersneedsitscase
changed,butanewStringobjectisreturnedifanyofthecharactersneedsits
casechanged.Thesemethodsdelegatethecharacter-by-charactercaseconversion
tocorrespondingmethodsfromtheCharacterclass.
Thesemethodsusetherulesofthe(default)locale(returnedbythemethod
Locale.getDefault()),whichembodiestheidiosyncrasiesofaspecific
geographical,political,orculturalregionregardingnumber/date/currencyformats,
characterclassification,alphabet(includingcaseidiosyncrasies),andother
localizations.
Exampleofcaseinstrings:
Clickheretoviewcodeimage
StringstrA=newString(“TheCasewasthrownoutofCourt”);
StringstrB=newString(“thecasewasthrownoutofcourt”);
StringstrC=strA.toLowerCase();//Caseconversion=>NewStringobject:
//“thecasewasthrownoutofcourt”
StringstrD=strB.toLowerCase();//Nocaseconversion=>SameString
object
StringstrE=strA.toUpperCase();//Caseconversion=>NewStringobject:
//“THECASEWASTHROWNOUTOFCOURT”
booleantest1=strC==strA;//false
booleantest2=strD==strB;//true
booleantest3=strE==strA;//false
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ConcatenationofStrings
Concatenationoftwostringsresultsinanewstringthatconsistsofthecharactersofthe
firststringfollowedbythecharactersofthesecondstring.Theoverloadedoperator+for
stringconcatenationisdiscussedin§5.7,p.169.Inaddition,thefollowingmethodcanbe
usedtoconcatenatetwostrings:
Stringconcat(Stringstr)
Theconcat()methoddoesnotmodifytheStringobjectonwhichitisinvoked,as
Stringobjectsareimmutable.Instead,theconcat()methodreturnsareferencetoa
brand-newStringobject:
Clickheretoviewcodeimage
Stringbillboard=“Just”;
billboard.concat(”lostinspace.”);//(1)Returnedreferencevaluenot
stored.
System.out.println(billboard);//(2)“Just”
billboard=billboard.concat(”advertise”).concat(”here.”);//(3)
Chaining.
System.out.println(billboard);//(4)“Justadvertisehere.”
At(1),thereferencevalueoftheStringobjectreturnedbythemethodconcat()is
notstored.ThisStringobjectbecomesinaccessibleafter(1).Weseethatthereference
billboardstilldenotesthestringliteral"Just"at(2).
At(3),twomethodcallstotheconcat()methodarechained.Thefirstcallreturnsa
referencevaluetoanewStringobject,whosecontentis"Justadvertise".The
secondmethodcallisinvokedonthisStringobjectusingthereferencevaluethatwas
returnedinthefirstmethodcall.ThesecondcallresultsinyetanothernewString
object,whosecontentis"Justadvertisehere.".Thereferencevalueofthis
Stringobjectisassignedtothereferencebillboard.BecauseStringobjectsare
immutable,thecreationofthetemporaryStringobjectwiththecontent"Just
advertise"isinevitableat(3).
Somemoreexamplesofstringconcatenationfollow:
Clickheretoviewcodeimage
Stringmotto=newString(“Programonce”);//(1)
motto+=“,executeeverywhere.”;//(2)
motto=motto.concat(”Don’tbetonit!”);//(3)
NotethatanewStringobjectisassignedtothereferencemottoeachtimeinthe
assignmentsat(1),(2),and(3).TheStringobjectwiththecontents"Program
once"becomesinaccessibleaftertheassignmentat(2).TheStringobjectwiththe
contents"Programonce,executeeverywhere."becomesinaccessibleafter
(3).ThereferencemottodenotestheStringobjectwiththefollowingcontentsafter
executionoftheassignmentat(3):
Clickheretoviewcodeimage
“Programonce,executeeverywhere.Don’tbetonit!”
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Joiningof Objects
Oneoperationcommonlyperformedonasequenceofstringsistoformatthemsothat
eachstringisseparatedfromthenextonebyadelimiter.Forexample,giventhefollowing
sequenceofstrings:
“2014”
“January”
“11”
wewishtoformatthemsothatindividualstringsareseparatedbythedelimiter“/”:
“2014/January/11”
ThefollowingstaticmethodsintheStringclasscanbeusedforthispurpose:
Clickheretoviewcodeimage
staticStringjoin(CharSequencedelimiter,CharSequence…elements)
staticStringjoin(CharSequencedelimiter,
Iterable<?extendsCharSequence>elements)
BothstaticmethodsreturnanewStringcomposedofcopiesofthe
CharSequenceelementsjoinedtogetherwithacopyofthespecified
CharSequencedelimiter.Thus,theresultingstringiscomposedoftextual
representationsoftheelementsseparatedbythetextualrepresentationofthe
specifieddelimiter.
Ifanelementisnull,thestring"null"isaddedasitstextualrepresentation.If
thedelimiterisnull,aNullPointerExceptionisthrown.
NotethatboththeindividualstringsandthedelimiterstringareCharSequenceobjects.
TheexamplesinthissectionuseStringandStringBuilderobjectsthatimplement
theCharSequenceinterface(p.360).
AnIterableprovidesaniteratortotraverseoveritselements.Thefollowingexamples
useanArrayList(§10.1,p.423)thatimplementstheIterableinterface.The
secondjoin()methodisthenabletotraversetheIterableusingtheiterator.This
methodwillacceptonlyanIterablewhoseelementsareeitheroftype
CharSequenceorsubtypesofCharSequence.
ThefirstexampleshowsjoiningofStringobjects.Thefirstjoin()methodiscalled
inthiscase.
Clickheretoviewcodeimage
//(1)JoiningindividualStringobjects:
StringdateStr=String.join(“/”,“2014”,“January”,“11”);
System.out.println(dateStr);//2014/January/11
ThesecondexampleshowsjoiningofelementsinaStringBuilderarray.Againthe
firstjoin()methodiscalled,withthearraybeingpassedasthesecondparameter.
Clickheretoviewcodeimage
//(2)JoiningelementsinaStringBuilderarray:
StringBuilderleft=newStringBuilder(“Left”);
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StringBuilderright=newStringBuilder(“Right”);
StringBuilder[]strBuilders={left,right,left};
Stringmarch=String.join(“—>”,strBuilders);
System.out.println(march);//Left—>Right—>Left
ThethirdexampleshowsjoiningofelementsinanArrayListofStringBuilder.
Thesecondjoin()methodiscalled,withtheArrayListbeingpassedasthesecond
parameter.NotethatsomeoftheelementsoftheArrayListarenull.
Clickheretoviewcodeimage
//(3)JoiningelementsinaStringBuilderlist:
ArrayList<StringBuilder>sbList=newArrayList<>();
sbList.add(right);sbList.add(null);sbList.add(left);sbList.add(null);
StringresultStr=“[”+String.join(“,“,sbList)+“]”;
System.out.println(resultStr);//[Right,null,Left,null]
ThelastexampleshowsjoiningofelementsinanArrayListofCharSequence.
Againthesecondjoin()methodiscalled,withtheArrayListbeingpassedasthe
secondparameter.NotethatelementsoftheArrayListareStringand
StringBuilderobjectsthatarealsooftypeCharSequence.
Clickheretoviewcodeimage
//(4)JoiningelementsinaCharSequencelist:
ArrayList<CharSequence>charSeqList=newArrayList<>();
charSeqList.add(right);charSeqList.add(left);//AddStringBuilder
objects.
charSeqList.add(“Right”);charSeqList.add(“Left”);//AddStringobjects.
StringresultStr2=“<”+String.join(“;“,charSeqList)+“>”;
System.out.println(resultStr2);//<Right;Left;Right;Left>
SearchingforCharactersandSubstrings
Thefollowingoverloadedmethodscanbeusedtofindtheindexofacharacterorthestart
indexofasubstringinastring.Thesemethodssearchforwardtowardtheendofthe
string.Inotherwords,theindexofthefirstoccurrenceofthecharacterorsubstringis
found.Ifthesearchisunsuccessful,thevalue–1isreturned.
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intindexOf(intch)
intindexOf(intch,intfromIndex)
Thefirstmethodfindstheindexofthefirstoccurrenceoftheargumentcharacterin
astring.Thesecondmethodfindstheindexofthefirstoccurrenceoftheargument
characterinastring,startingattheindexspecifiedinthesecondargument.Ifthe
indexargumentisnegative,theindexisassumedtobe0.Iftheindexargumentis
greaterthanthelengthofthestring,itiseffectivelyconsideredtobeequaltothe
lengthofthestring,resultinginthevalue-1beingreturned.
Clickheretoviewcodeimage
intindexOf(Stringstr)
intindexOf(Stringstr,intfromIndex)
Thefirstmethodfindsthestartindexofthefirstoccurrenceofthesubstring
argumentinastring.Thesecondmethodfindsthestartindexofthefirstoccurrence
ofthesubstringargumentinastring,startingattheindexspecifiedinthesecond
argument.
TheStringclassalsodefinesasetofmethodsthatsearchforacharacterorasubstring,
butthesearchisbackwardtowardthestartofthestring.Inotherwords,theindexofthe
lastoccurrenceofthecharacterorsubstringisfound.
Clickheretoviewcodeimage
intlastIndexOf(intch)
intlastIndexOf(intch,intfromIndex)
intlastIndexOf(Stringstr)
intlastIndexOf(Stringstr,intfromIndex)
Thefollowingmethodscanbeusedtocreateastringinwhichalloccurrencesofa
characterorasubsequenceinastringhavebeenreplacedwithanothercharacteror
subsequence:
Clickheretoviewcodeimage
Stringreplace(charoldChar,charnewChar)
Stringreplace(CharSequencetarget,CharSequencereplacement)
ThefirstmethodreturnsanewStringobjectthatistheresultofreplacingall
occurrencesoftheoldCharinthecurrentstringwiththenewChar.Thecurrent
stringisreturnedifnooccurrencesoftheoldCharcanbefound.
ThesecondmethodreturnsanewStringobjectthatistheresultofreplacingall
occurrencesofthecharactersequencetargetinthecurrentstringwiththe
charactersequencereplacement.Thecurrentstringisreturnedifno
occurrencesofthetargetcanbefound.
Thefollowingmethodscanbeusedtotestwhetherastringsatisfiesagivencriterion:
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booleancontains(CharSequencecs)
Thismethodreturnstrueifthecurrentstringcontainsthespecifiedcharacter
sequence,andfalseotherwise.
Clickheretoviewcodeimage
booleanstartsWith(Stringprefix)
Thismethodreturnstrueifthecurrentstringstartswiththecharactersequence
specifiedbyparameterprefix,andfalseotherwise.
Clickheretoviewcodeimage
booleanstartsWith(Stringprefix,intindex)
Thismethodreturnstrueifthesubstringofthecurrentstringatthespecified
indexstartswiththecharactersequencespecifiedbyparameterprefix,and
falseotherwise.
Clickheretoviewcodeimage
booleanendsWith(Stringsuffix)
Thismethodreturnstrueifthecurrentstringendswiththecharactersequence
specifiedbyparametersuffix,andfalseotherwise.
Examplesofsearchandreplacemethods:
Clickheretoviewcodeimage
StringfunStr=“JavaJives”;
//0123456789
intjInd1a=funStr.indexOf(‘J’);//0
intjInd1b=funStr.indexOf(‘J’,1);//5
intjInd2a=funStr.lastIndexOf(‘J’);//5
intjInd2b=funStr.lastIndexOf(‘J’,4);//0
Stringbanner=“Oneman,Onevote”;
//01234567890123456
intsubInd1a=banner.indexOf(“One”);//0
intsubInd1b=banner.indexOf(“One”,3);//9
intsubInd2a=banner.lastIndexOf(“One”);//9
intsubInd2b=banner.lastIndexOf(“One”,10);//9
intsubInd2c=banner.lastIndexOf(“One”,8);//0
intsubInd2d=banner.lastIndexOf(“One”,2);//0
StringnewStr=funStr.replace(‘J’,‘W’);//“WavaWives”
StringnewBanner=banner.replace(“One”,“No”);//“Noman,Novote”
booleanfound1=banner.contains(“One”);//true
booleanfound2=newBanner.contains(“One”);//false
Stringsong=“Startmeup!”;
//012345677890
booleanfound3=song.startsWith(“Start”);//true
booleannotFound1=song.startsWith(“start”);//false
booleanfound4=song.startsWith(“me”,6);//true
booleanfound5=song.endsWith(“up!”);//true
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booleannotFound2=song.endsWith(“up”);//false
ExtractingSubstrings
Stringtrim()
Thismethodcanbeusedtocreateastringwherewhitespace(infact,allcharacters
withvalueslessthanorequaltothespacecharacter'\u0020')hasbeenremoved
fromthefront(leading)andtheend(trailing)ofastring.
Clickheretoviewcodeimage
Stringsubstring(intstartIndex)
Stringsubstring(intstartIndex,intendIndex)
TheStringclassprovidestheseoverloadedmethodstoextractsubstringsfroma
string.AnewStringobjectcontainingthesubstringiscreatedandreturned.The
firstmethodextractsthestringthatstartsatthegivenindexstartIndexand
extendstotheendofthestring.Theendofthesubstringcanbespecifiedbyusinga
secondargumentendIndexthatistheindexofthefirstcharacterafterthe
substring—thatis,thelastcharacterinthesubstringisatindexendIndex-1.If
theindexvalueisnotvalid,anIndexOutOfBoundsExceptionisthrown.
Examplesofextractingsubstrings:
Clickheretoviewcodeimage
Stringutopia=“\t\nJavaNation\n\t”;
utopia=utopia.trim();//“JavaNation”
utopia=utopia.substring(5);//“Nation”
Stringradioactive=utopia.substring(3,6);//“ion”
ConvertingPrimitiveValuesandObjectstoStrings
TheStringclassoverridesthetoString()methodintheObjectclassandreturns
theStringobjectitself:
StringtoString()
ThismethodisdefinedintheCharSequenceinterface,whichtheStringclass
implements(p.360).
TheStringclassalsodefinesasetofstaticoverloadedvalueOf()methodstoconvert
objectsandprimitivevaluesintostrings:
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staticStringvalueOf(Objectobj)
staticStringvalueOf(char[]charArray)
staticStringvalueOf(booleanb)
staticStringvalueOf(charc)
Allofthesemethodsreturnastringrepresentingthegivenparametervalue.Acall
tothemethodwiththeparameterobjisequivalenttoobj.toString()when
objisnotnull;otherwise,the"null"stringisreturned.Thebooleanvalues
trueandfalseareconvertedintothestrings"true"and"false".The
charparameterisconvertedtoastringconsistingofasinglecharacter.
Clickheretoviewcodeimage
staticStringvalueOf(inti)
staticStringvalueOf(longl)
staticStringvalueOf(floatf)
staticStringvalueOf(doubled)
ThestaticvalueOf()method,whichacceptsaprimitivevalueasanargument,is
equivalenttothestatictoString()methodinthecorrespondingwrapperclass
foreachoftheprimitivedatatypes((6a)and(6b)in§8.3,p.347).Notethatthere
arenovalueOf()methodsthatacceptabyteorashort.
Examplesofstringconversions:
Clickheretoviewcodeimage
StringanonStr=String.valueOf(“Makemeastring.”);//“Makemea
string.”
StringcharStr=String.valueOf(newchar[]{‘a’,‘h’,‘a’});//“aha”
StringboolTrue=String.valueOf(true);//“true”
StringdoubleStr=String.valueOf(Math.PI);//
“3.141592653589793”
FormattedStrings
WehaveusedtheSystem.out.printf()methodtoformatvaluesandprintthemto
theterminalwindow(§1.9,p.15).Tojustcreatethestringwiththeformattedvalues,but
notprinttheformattedresult,wecanusethefollowingstaticmethodfromtheString
class.Itacceptsthesameargumentsastheprintf()method,andusesthesameformat
specifications(Table1.2,p.20).
Clickheretoviewcodeimage
staticStringformat(Stringformat,Object…args)
Themethodreturnsastringwiththeresultofformattingthevaluesinthevarargs
parameterargsaccordingtotheStringparameterformat.Theformatstring
containsformatspecificationsthatdeterminehoweachsubsequentvalueinthe
varargsparameterargswillbeformatted.
Anyerrorintheformatstringwillresultinaruntimeexception.
Thefollowingcalltotheformat()methodcreatesaformattedstringwiththethree
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valuesformattedaccordingtothespecifiedformatstring:
Clickheretoviewcodeimage
StringformattedStr=String.format(“Formattedvalues|%5d|%8.3f|%5s|”,
2016,Math.PI,“Hi”);
System.out.println(formattedStr);//Formattedvalues|2016|3.142|Hi|
formattedStr=formattedStr.toUpperCase();
System.out.println(formattedStr);//FORMATTEDVALUES|2016|3.142|HI|
OthermiscellaneousmethodsexistintheStringclassforpatternmatching
(matches()),splittingstrings(split()),andconvertingastringtoanarrayofbytes
(getBytes()).ThemethodhashCode()canbeusedtocomputeahashvaluebased
onthecharactersinthestring.ConsulttheJavaSEplatformAPIdocumentationformore
details.
ReviewQuestions
8.11WhichofthefollowingoperatorscannothaveanoperandoftypeString?
Selectthetwocorrectanswers.
(a)+
(b)-
(c)+=
(d).
(e)&
8.12Whichexpressionwillextractthesubstring"kap",giventhefollowing
declaration:
Stringstr=“kakapo”;
Selecttheonecorrectanswer.
(a)str.substring(2,2)
(b)str.substring(2,3)
(c)str.substring(2,4)
(d)str.substring(2,5)
(e)str.substring(3,3)
8.13Whatwillbetheresultofattemptingtocompileandrunthefollowingcode?
Clickheretoviewcodeimage
classMyClass{
publicstaticvoidmain(String[]args){
Stringstr1=“str1”;
Stringstr2=“str2”;
Stringstr3=“str3”;
str1.concat(str2);
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System.out.println(str3.concat(str1));
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompilebecausetheexpressionstr3.concat(str1)
willnotresultinavalidargumentfortheprintln()method.
(b)Theprogramwillprintstr3str1str2atruntime.
(c)Theprogramwillprintstr3atruntime.
(d)Theprogramwillprintstr3str1atruntime.
(e)Theprogramwillprintstr3str2atruntime.
8.14Whichstatementaboutthetrim()methodoftheStringclassistrue?
Selecttheonecorrectanswer.
(a)Itreturnsastringwheretheleadingwhitespaceoftheoriginalstringhasbeen
removed.
(b)Itreturnsastringwherethetrailingwhitespaceoftheoriginalstringhasbeen
removed.
(c)Itreturnsastringwhereboththeleadingandtrailingwhitespaceoftheoriginal
stringhasbeenremoved.
(d)Itreturnsastringwhereallthewhitespaceoftheoriginalstringhasbeen
removed.
(e)Noneoftheabove.
8.15Whichofthefollowingstatementsaretrue?
Selectthetwocorrectanswers.
(a)Stringobjectsareimmutable.
(b)SubclassesoftheStringclasscanbemutable.
(c)Allwrapperclassesaredeclaredasfinal.
(d)Allobjectshaveapublicmethodnamedclone.
(e)Theexpression((newchar[]{'o','k'})instanceof
String)isalwaystrue.
8.16Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassRefEq{
publicstaticvoidmain(String[]args){
Strings=“ab”+“12”;
Stringt=“ab”+12;
Stringu=newString(“ab12”);
System.out.println((s==t)+””+(s==u));
}
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}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillprintfalsefalseatruntime.
(c)Theprogramwillprintfalsetrueatruntime.
(d)Theprogramwillprinttruefalseatruntime.
(e)Theprogramwillprinttruetrueatruntime.
8.17WhichoftheseparameterlistscanbefoundinaconstructoroftheStringclass?
Selectthefivecorrectanswers.
(a)()
(b)(intcapacity)
(c)(char[]data)
(d)(Stringstr)
(e)(CharSequencecs)
(f)(StringBuildersb)
(g)(charc)
(h)(Objecto)
(i)(Stringstr,intbeginIndex,intendIndex)
(j)(char[]data,intoffset,intcount)
8.18WhichofthefollowingmethodsisnotdefinedintheStringclass?
Selecttheonecorrectanswer.
(a)trim()
(b)length()
(c)concat(String)
(d)hashCode()
(e)reverse()
8.19Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassUppity{
publicstaticvoidmain(String[]args){
Stringstr1=“lower”,str2=“LOWER”,str3=“UPPER”;
str1.toUpperCase();
str1.replace(“LOWER”,“UPPER”);
System.out.println((str1.equals(str2))+””+(str1.equals(str3)));
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}
}
Selecttheonecorrectanswer.
(a)Theprogramwillprintfalsetrue.
(b)Theprogramwillprintfalsefalse.
(c)Theprogramwillprinttruefalse.
(d)Theprogramwillprinttruetrue.
(e)Theprogramwillfailtocompile.
(f)Theprogramwillcompile,butthrowanexceptionatruntime.
8.20Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassFunCharSeq{
privatestaticvoidputO(Strings1){
s1=s1.trim();
s1+=“O”;
}
publicstaticvoidmain(String[]args){
Strings1=”W“;
putO(s1);
s1.concat(“W”);
System.out.println(“|”+s1+“|”);
}
}
Selecttheonecorrectanswer.
(a)|WOW|
(b)|WW|
(c)|WO|
(d)|W|
(e)Theprogramwillfailtocompile.
(f)Theprogramwillcompile,butthrowanexceptionatruntime.
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8.5The and Classes
Thread-Safety
TheclassesStringBuilderandStringBufferimplementmutablesequencesof
characters.Bothclassessupportthesameoperations,buttheStringBufferclassisthe
thread-safeanalogoftheStringBuilderclass.Certainoperationsonastringbuffer
aresynchronized,sothatwhenusedbymultiplethreads,theseoperationsareperformedin
anorderlymanner.NotethataStringobjectisalsothread-safe—becauseitis
immutable,athreadcannotchangeitsstate.Stringbuildersarepreferredwhenheavy
modificationofcharactersequencesisinvolvedandsynchronizationofoperationsisnot
important.
Althoughtherestofthissectionfocusesonstringbuilders,itisequallyapplicableto
stringbuffers.
Mutability
IncontrasttotheStringclass,whichimplementsimmutablecharactersequences,the
StringBuilderclassimplementsmutablecharactersequences.Notonlycanthe
charactersequencesinastringbuilderbechanged,butthecapacityofthestringbuilder
canalsochangedynamically.Thecapacityofastringbuilderisthemaximumnumberof
charactersthatastringbuildercanaccommodatebeforeitssizeisautomatically
augmented.
AlthoughthereisacloserelationshipbetweenobjectsoftheStringand
StringBuilderclasses,thesearetwoindependentfinalclasses,bothdirectly
extendingtheObjectclass.Hence,Stringreferencescannotbestored(orcast)to
StringBuilderreferences,andviceversa.However,bothclassesimplementthe
CharSequenceinterface(p.360).
TheStringBuilderclassprovidesvariousfacilitiesformanipulatingstringbuilders:
•Constructingstringbuilders
•Changing,deleting,andreadingcharactersinstringbuilders
•Constructingstringsfromstringbuilders
•Appending,inserting,anddeletinginstringbuilders
•Controllingstringbuildercapacity
ConstructingStringBuilders
ThefinalclassStringBuilderprovidesfourconstructorsthatcreateandinitialize
StringBuilderobjectsandsettheirinitialcapacity.
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StringBuilder(Stringstr)
StringBuilder(CharSequencecharSeq)
ThecontentsofthenewStringBuilderobjectarethesameasthecontentsof
theStringobjectorthecharactersequencepassedasanargument.Theinitial
capacityofthestringbuilderissettothelengthoftheargumentsequence,plus
roomfor16morecharacters.
Clickheretoviewcodeimage
StringBuilder(intinitialCapacity)
ThenewStringBuilderobjecthasnocontent.Theinitialcapacityofthestring
builderissettothevalueoftheargument,whichcannotbelessthan0.
StringBuilder()
ThisconstructoralsocreatesanewStringBuilderobjectwithnocontent.The
initialcapacityofthestringbuilderissetto16characters.
ExamplesofStringBuilderobjectcreationandinitialization:
Clickheretoviewcodeimage
StringBuilderstrBuilder1=newStringBuilder(“Phew!”);//“Phew!”,capacity
21
StringBuilderstrBuilder2=newStringBuilder(10);//””,capacity10
StringBuilderstrBuilder3=newStringBuilder();//””,capacity16
ReadingandChangingCharactersinStringBuilders
Returnsthenumberofcharactersinthestringbuilder.
Thesemethodsreadandchangethecharacterataspecifiedindexinthestring
builder,respectively.Thefirstcharacterisatindex0,andthelastoneisatindex1
lessthanthenumberofcharactersinthestringbuilder.AIndexOutOfBounds–
Exceptionisthrowniftheindexisnotvalid.
Clickheretoviewcodeimage
CharSequencesubSequence(intstart,intend)
ThismethodisimplementedaspartoftheCharSequenceinterface(p.360).
Thefollowingisanexampleofreadingandchangingstringbuildercontents:
Clickheretoviewcodeimage
StringBuilderstrBuilder=newStringBuilder(“Javv”);//“Javv”,
capacity20
strBuilder.setCharAt(strBuilder.length()-1,strBuilder.charAt(1));//“Java”
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ConstructingStringsfromStringBuilders
TheStringBuilderclassoverridesthetoString()methodfromtheObject
class(seealsotheCharSequenceinterface,p.360).Itreturnsthecontentsofastring
builderinaStringobject.
Clickheretoviewcodeimage
StringfromBuilder=strBuilder.toString();//“Java”
DifferencesbetweentheStringandStringBuilderClasses
SincetheStringBuilderclassdoesnotoverridetheequals()methodfromthe
Objectclass,nordoesitimplementtheComparableinterface,thecontentsofstring
buildersshouldbeconvertedtoStringobjectsforstringcomparison.
TheStringBuilderclassalsodoesnotoverridethehashCode()methodfromthe
Objectclass.Again,astringbuildercanbeconvertedtoaStringobjecttoobtaina
hashvalue.
Appending,Inserting,andDeletingCharactersinStringBuilders
Appending,inserting,anddeletingcharactersautomaticallyresultsinadjustmentofthe
stringbuilder’sstructureandcapacity,ifnecessary.Theindicespassedasargumentsinthe
methodsmustbeequaltoorgreaterthan0.AnIndexOutOfBounds–Exceptionis
thrownifanindexisnotvalid.
Notethatthemethodsinthissubsectionreturnthereferencevalueofthemodifiedstring
builder,makingitconvenienttochaincallstothesemethods.
AppendingCharacterstoaStringBuilder
Theoverloadedmethodappend()canbeusedtoappendcharactersattheendofa
stringbuilder.
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StringBuilderappend(Objectobj)
Theobjargumentisconvertedtoastringasifbythestaticmethodcall
String.valueOf(obj),andthisstringisappendedtothecurrentstring
builder.
Clickheretoviewcodeimage
StringBuilderappend(Stringstr)
StringBuilderappend(CharSequencecharSeq)
StringBuilderappend(CharSequencecharSeq,intstart,intend)
StringBuilderappend(char[]charArray)
StringBuilderappend(char[]charArray,intoffset,intlength)
StringBuilderappend(charc)
Thesemethodsallowcharactersfromvarioussourcestobeappendedtotheendof
thecurrentstringbuilder.
Clickheretoviewcodeimage
StringBuilderappend(booleanb)
StringBuilderappend(inti)
StringBuilderappend(longl)
StringBuilderappend(floatf)
StringBuilderappend(doubled)
Thesemethodsconverttheprimitivevalueoftheargumenttoastringbyapplying
thestaticmethodString.valueOf()totheargument,beforeappendingthe
resulttothestringbuilder.
InsertingCharactersinaStringBuilder
Theoverloadedmethodinsert()canbeusedtoinsertcharactersatagivenpositionin
astringbuilder.
Clickheretoviewcodeimage
StringBuilderinsert(intoffset,Objectobj)
StringBuilderinsert(intdstOffset,CharSequenceseq)
StringBuilderinsert(intdstOffset,CharSequenceseq,intstart,intend)
StringBuilderinsert(intoffset,Stringstr)
StringBuilderinsert(intoffset,char[]charArray)
StringBuilderinsert(intoffset,charc)
StringBuilderinsert(intoffset,booleanb)
StringBuilderinsert(intoffset,inti)
StringBuilderinsert(intoffset,longl)
StringBuilderinsert(intoffset,floatf)
StringBuilderinsert(intoffset,doubled)
Theargumentisconverted,ifnecessary,byapplyingthestaticmethod
String.valueOf().Theoffsetargumentspecifieswherethecharactersare
tobeinsertedinthestringbuilder,andmustbegreaterthanorequalto0.
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DeletingCharactersinaStringBuilder
Thefollowingmethodscanbeusedtodeletecharactersfromspecificpositionsinastring
builder:
Clickheretoviewcodeimage
StringBuilderdeleteCharAt(intindex)
StringBuilderdelete(intstart,intend)
Thefirstmethoddeletesacharacterataspecifiedindexinthestringbuilder,
contractingthestringbuilderbyonecharacter.Thesecondmethoddeletesa
substring,whichisspecifiedbythestartindex(inclusive)andtheendindex
(exclusive),contractingthestringbuilderaccordingly.
AmongothermiscellaneousmethodsincludedintheclassStringBuilderisthe
followingmethod,whichreversesthecontentsofastringbuilder:
StringBuilderreverse()
Examplesofappending,inserting,anddeletinginstringbuilders:
Clickheretoviewcodeimage
StringBuilderbuilder=newStringBuilder(“bananasplit”);//“banana
split”
builder.delete(4,12);//“bana”
builder.append(42);//“bana42”
builder.insert(4,“na”);//“banana42”
builder.reverse();//“24ananab”
builder.deleteCharAt(builder.length()-1);//“24anana”
builder.append(‘s’);//“24ananas”
Allofthepreviouslymentionedmethodsmodifythecontentsofthestringbuilderand
returnareferencevaluedenotingthecurrentstringbuilder.Thisallowschainingof
methodcalls.Themethodcallsinvokedonthestringbuilderdenotedbythereference
buildercanbechainedasfollows,givingthesameresult:
Clickheretoviewcodeimage
builder.delete(4,12).append(42).insert(4,“na”).reverse().
deleteCharAt(builder.length()-1).append(‘s’);//“24ananas”
Themethodcallsinthechainareevaluatedfromlefttoright,sothatthepreviouschainof
callsisinterpretedasfollows:
Clickheretoviewcodeimage
(((((builder.delete(4,12)).append(42)).insert(4,“na”)).reverse()).
deleteCharAt(builder.length()-1)).append(‘s’);//“24ananas”
Eachmethodcallreturnsthereferencevalueofthemodifiedstringbuilder,whichisthen
usedtoinvokethenextmethod.Thestringbuilderremainsdenotedbythereference
builder.
Thecompilerusesstringbuilderstoimplementstringconcatenationwiththe+operatorin
String-valuednon-constantexpressions.Thefollowingcodeillustratesthis
optimization:
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StringtheChosen=“U”;
Stringstr1=4+theChosen+“Only”;//(1)Non-constant
expression.
Theassignmentstatementat(1)isequivalenttothefollowingcodeusingonestring
builder:
Clickheretoviewcodeimage
Stringstr2=newStringBuilder().
append(4).append(theChosen).append(“Only”).toString();//
(2)
Thecodeat(2)doesnotcreateanytemporaryStringobjectswhenconcatenating
severalstrings,sinceasingleStringBuilderobjectismodifiedandfinallyconverted
toaStringobjecthavingthestringcontent"4UOnly".
ControllingStringBuilderCapacity
Thefollowingmethodscanbeusedtocontrolvariouscapacity-relatedaspectsofastring
builder:
intcapacity()
Returnsthecurrentcapacityofthestringbuilder,meaningthenumberofcharacters
thecurrentbuildercanaccommodatewithoutallocatinganew,largerarraytohold
characters.
Clickheretoviewcodeimage
voidensureCapacity(intminCapacity)
EnsuresthatthereisroomforatleastaminCapacitynumberofcharacters.It
expandsthestringbuilder,dependingonthecurrentcapacityofthebuilder.
voidtrimToSize()
Attemptstoreducethestorageusedforthecharactersequence.Itmayaffectthe
capacityofthestringbuilder.
voidsetLength(intnewLength)
Ensuresthattheactualnumberofcharacters—thatis,thelengthofthestring
builder—isexactlyequaltothevalueofthenewLengthargument,whichmustbe
greaterthanorequalto0.Thisoperationcanresultinthestringbeingtruncatedor
paddedwithnullcharacters('\u0000').
Thismethodaffectsthecapacityofthestringbuilderonlyifthevalueofthe
parameternewLengthisgreaterthanthecurrentcapacity.
Oneuseofthismethodistoclearthestringbuilder:
Clickheretoviewcodeimage
builder.setLength(0);//Emptythebuilder.
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ReviewQuestions
8.21Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
Strings=“hello”;
StringBuildersb=newStringBuilder(s);
sb.reverse();
if(s==sb)System.out.println(“a”);
if(s.equals(sb))System.out.println(“b”);
if(sb.equals(s))System.out.println(“c”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butthrowanexceptionatruntime.
(c)Theprogramwillcompile,butwillnotprintanything.
(d)Theprogramwillcompile,andwillprintabc.
(e)Theprogramwillcompile,andwillprintbc.
(f)Theprogramwillcompile,andwillprinta.
(g)Theprogramwillcompile,andwillprintb.
(h)Theprogramwillcompile,andwillprintc.
8.22Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
StringBuildersb=newStringBuilder(“haveaniceday”);
sb.setLength(6);
System.out.println(sb);
}
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompile,becausethereisnomethodnamedsetLength
intheStringBuilderclass.
(b)Thecodewillfailtocompile,becausetheStringBuilderreferencesbis
notalegalargumenttotheprintln()method.
(c)TheprogramwillthrowaStringIndexOutOfBoundsExceptionat
runtime.
(d)Theprogramwillprinthaveanicedayatruntime.
(e)Theprogramwillprinthaveaatruntime.
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(f)Theprogramwillprintcedayatruntime.
8.23Whichoftheseparameterlistscanbefoundinaconstructorofthe
StringBuilderclass?
Selectthefourcorrectanswers.
(a)()
(b)(intcapacity)
(c)(char[]data)
(d)(Stringstr)
(e)(CharSequencecs)
(f)(StringBuildersb)
(g)(charc)
(h)(Objecto)
(i)(Stringstr,intbeginIndex,intendIndex)
(j)(char[]data,intoffset,intcount)
8.24WhichofthefollowingmethodsisnotdefinedintheStringBuilderclass?
Selecttheonecorrectanswer.
(a)trim()
(b)length()
(c)append(String)
(d)reverse()
(e)setLength(int)
8.25Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassPeskyCharSeq{
publicstaticvoidmain(String[]args){
StringBuildersb1=newStringBuilder(“WOW”);
StringBuildersb2=newStringBuilder(sb1);
System.out.println((sb1==sb2)+””+sb1.equals(sb2));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillprintfalsetrue.
(b)Theprogramwillprintfalsefalse.
(c)Theprogramwillprinttruefalse.
(d)Theprogramwillprinttruetrue.
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(e)Theprogramwillfailtocompile.
(f)Theprogramwillcompile,butthrowsanexceptionatruntime.
8.26Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassMoreCharSeq{
publicstaticvoidmain(String[]args){
Strings1=“WOW”;
StringBuilders2=newStringBuilder(s1);
Strings3=newString(s2);
System.out.println((s1.hashCode()==s2.hashCode())+””+
(s1.hashCode()==s3.hashCode()));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillprintfalsetrue.
(b)Theprogramwillprintfalsefalse.
(c)Theprogramwillprinttruefalse.
(d)Theprogramwillprinttruetrue.
(e)Theprogramwillfailtocompile.
(f)Theprogramwillcompile,butthrowanexceptionatruntime.
8.27Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassAppendage{
privatestaticvoidputO(StringBuilders1){
s1.append(“O”);
}
publicstaticvoidmain(String[]args){
StringBuilders1=newStringBuilder(“W”);
putO(s1);
s1.append(“W!”);
System.out.println(s1);
}
}
Selecttheonecorrectanswer.
(a)TheprogramwillprintWW!.
(b)TheprogramwillprintWOW!.
(c)TheprogramwillprintW.
(d)TheprogramwillprintWO.
(e)Theprogramwillfailtocompile.
(f)Theprogramwillcompile,butthrowanexceptionatruntime.
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ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•TheObjectclass,whichisthemostfundamentalclassinJava
•Wrapperclasses,whichnotonlyallowprimitivevaluestobetreatedasobjects,but
alsocontainusefulmethodsforconvertingvalues
•TheStringclass,includinghowimmutablestringsarecreatedandused
•TheStringBuilderclass,includinghowdynamicstringsarecreatedand
manipulated
•ComparisonoftheString,StringBuilder,andStringBufferclasses
ProgrammingExercises
8.1CreateaclassnamedPair,whichaggregatestwoarbitraryobjects.Implementthe
equals()andhashCode()methodsinsuchawaythataPairobjectis
identicaltoanotherPairobjectif,andonlyif,thepairofconstituentobjectsare
identical.MakethetoString()implementationreturnthetextualrepresentation
ofboththeconstituentobjectsinaPairobject.ObjectsofthePairclassshould
beimmutable.
8.2Apalindromeisatextphrasethatisspelledthesamewaybackwardandforward.
Thewordredividerisapalindrome,sincethewordwouldbespelledthesameeven
ifthecharactersequencewerereversed.Writeaprogramthattakesastringasan
argumentandreportswhetherthestringisacase-sensitivepalindrome.For
example,thewordRedividerwouldnotbeapalindromeinthiscase,sincewe
distinguishbetweenuppercaseandlowercaseletters.
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9.ObjectLifetime
9.1GarbageCollection
Efficientmemorymanagementisessentialinaruntimesystem.Storageforobjectsis
allocatedinadesignatedpartofthememorycalledtheheap,whichhasafinitesize.
Garbagecollectionisaprocessofmanagingtheheapefficiently,byreclaimingmemory
occupiedbyobjectsthatarenolongerneededandmakingitavailablefornewobjects.
Javaprovidesautomaticgarbagecollection,meaningthattheruntimeenvironmentcan
takecareofmemorymanagementwithouttheprogramhavingtotakeanyspecialaction.
Objectsallocatedontheheap(throughthenewoperator)areadministeredbythe
automaticgarbagecollector.Theautomaticgarbagecollectionschemeguaranteesthata
referencetoanobjectisalwaysvalidwhiletheobjectisneededbytheprogram.
Specifically,theobjectwillnotbereclaimed,leavingthereferencedangling.
Havinganautomaticgarbagecollectorfreestheprogrammerfromtheresponsibilityof
writingcodefordeletingobjects.Byrelyingontheautomaticgarbagecollector,aJava
programalsoforfeitsanysignificantinfluenceonthegarbagecollectionofitsobjects(p.
393).However,thispriceisinsignificantwhencomparedtothecostofputtingthecode
forobjectmanagementinplaceandpluggingallthememoryleaks.Time-critical
applicationsshouldrecognizethattheautomaticgarbagecollectorrunsasabackground
taskandmayhaveanegativeimpactontheirperformance.
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9.2ReachableObjects
Anautomaticgarbagecollectoressentiallyperformstwotasks:
•Decidesifandwhenmemoryneedstobereclaimed
•Findsobjectsthatarenolongerneededbytheprogramandreclaimstheirstorage
Aprogramhasnoguaranteesthattheautomaticgarbagecollectorwillberunduringits
execution.Consequently,aprogramshouldnotrelyontheschedulingoftheautomatic
garbagecollectorforitsbehavior(p.393).
Tounderstandhowtheautomaticgarbagecollectorfindsobjectswhosestorageshouldbe
reclaimed,weneedtolookattheactivityhappeningintheJVM.Javaprovidesthread-
basedmultitasking,meaningthatseveralthreadscanbeexecutingconcurrentlyinthe
JVM,eachdoingitsowntask.Athreadisanindependentpathofexecutionthroughthe
programcode.Athreadisaliveifithasnotcompleteditsexecution.Eachlivethreadhas
itsownJVMstack,asexplainedin§6.5,p.230.TheJVMstackcontainsactivationframes
ofmethodsthatarecurrentlyactive.Localreferencesdeclaredinamethodcanalwaysbe
foundinthemethod’sactivationframe,storedontheJVMstackassociatedwiththethread
inwhichthemethodiscalled.Objects,incontrast,arealwayscreatedontheheap.Ifan
objecthasafieldreference,thefieldwillbefoundinsidetheobjectintheheap,andthe
objectdenotedbythefieldreferencewillalsobefoundintheheap.
AnexampleofhowmemoryisorganizedduringexecutionisdepictedinFigure9.1,
whichshowstwolivethreads(t1andt2)andtheirrespectiveJVMstackswiththe
activationframes.Thediagramindicateswhichobjectsintheheaparereferencedbylocal
referencesinthemethodactivationframes.Italsoidentifiesfieldreferencesinobjects,
whichrefertootherobjectsintheheap.Someobjectshaveseveralaliases.
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Figure9.1MemoryOrganizationatRuntime
Anobjectintheheapissaidtobereachableifitisreferencedbyanylocalreferenceina
JVMstack.Likewise,anyobjectthatisdenotedbyareferenceinareachableobjectis
saidtobereachable.Reachabilityisatransitiverelationship.Thus,areachableobjecthas
atleastonechainofreachablereferencesfromtheJVMstack.Anyreferencethatmakes
anobjectreachableiscalledareachablereference.Anobjectthatisnotreachableissaid
tobeunreachable.
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Areachableobjectisalive,andisaccessiblebyalivethread.Notethatanobjectcanbe
accessiblebymorethanonethread.Anyobjectthatisnotaccessiblebyalivethreadisa
candidateforgarbagecollection.Whenanobjectbecomesunreachableandiswaitingfor
itsmemorytobereclaimed,itissaidtobeeligibleforgarbagecollection.Anobjectis
eligibleforgarbagecollectionifallreferencesdenotingitareineligibleobjects.Eligible
objectsdonotaffectthefuturecourseofprogramexecution.Whenthegarbagecollector
runs,itfindsandreclaimsthestorageofeligibleobjects,althoughgarbagecollectiondoes
notnecessarilyoccurassoonasanobjectbecomesunreachable.
InFigure9.1,theobjectso4,o5,o11,o12,o14,ando15allhavereachablereferences.
Objectso13ando16havenoreachablereferencesand,therefore,areeligibleforgarbage
collection.
Fromtheprecedingdiscussionwecanconcludethatifacompositeobjectbecomes
unreachable,itsconstituentobjectsalsobecomeunreachable,barringanyreachable
referencestotheconstituentobjects.Althoughtheobjectso1,o2,ando3inFigure9.1
formacircularlist,theydonothaveanyreachablereferences.Thus,theseobjectsareall
eligibleforgarbagecollection.Conversely,theobjectso5,o6,ando7formalinearlist,
buttheyareallreachable,asthefirstobjectinthelist,o5,isreachable.Theobjectso8,
o10,o11,ando9alsoformalinearlist(inthatorder),butnotallobjectsinthelistare
reachable.Onlytheobjectso9ando11arereachable,asobjecto11hasareachable
reference.Theobjectso8ando10areeligibleforgarbagecollection.
Thelifetimeofanobjectisthetimefromitscreationtothetimeitisgarbagecollected.
Undernormalcircumstances,anobjectisaccessiblefromthetimewhenitiscreatedtothe
timewhenitbecomesunreachable.Thelifetimeofanobjectcanalsoincludeaperiod
whenitiseligibleforgarbagecollection,waitingforitsstoragetobereclaimed.The
finalizationmechanism(p.390)inJavadoesprovideameansforresurrectinganobject
afteritiseligibleforgarbagecollection,butthefinalizationmechanismisrarelyusedfor
thispurpose.
9.3FacilitatingGarbageCollection
Theautomaticgarbagecollectordetermineswhichobjectsarenotreachableand,
therefore,eligibleforgarbagecollection.Itwillcertainlygotoworkifthereisan
imminentmemoryshortage.Evenso,automaticgarbagecollectionshouldnotbe
perceivedasalicenseforcreatingaplethoraofobjectsandthenforgettingaboutthem.
Nevertheless,certainprogrammingpracticescanhelpinminimizingtheoverhead
associatedwithgarbagecollectionduringprogramexecution.
Certainobjects,suchasfilesandnetworkconnections,cantieupresourcesandshouldbe
disposedofproperlywhentheyarenolongerneeded.Inmostcases,thetry-with-
resourcesstatement(notinthescopeofthisbook)providesaconvenientfacilityforsuch
purposes,asitwillalwaysensureproperclosingoftheAuto-Closeableresources.
Tooptimizeitsmemoryfootprint,alivethreadshouldretainaccesstoanobjectaslong
foronlyastheobjectisneededforitsexecution.Theprogramcanallowobjectsto
becomeeligibleforgarbagecollectionasearlyaspossiblebyremovingallreferencesto
anobjectwhenitisnolongerneeded.
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Objectsthatarecreatedandaccessedbylocalreferencesinamethodareeligiblefor
garbagecollectionwhenthemethodterminates,unlessreferencevaluestotheseobjects
areexportedoutofthemethod.Thiscanoccurifareferencevalueisreturnedfromthe
method,passedasargumenttoanothermethodthatrecordsthereferencevalue,orthrown
asanexception.However,amethodneednotalwaysleaveobjectstobegarbagecollected
afteritstermination.Itcanfacilitategarbagecollectionbytakingsuitableaction—for
example,bynullingreferences.
Clickheretoviewcodeimage
importjava.io.*;
classWellBehavedClass{
//…
voidwellBehavedMethod(){
FileaFile;
long[]bigArray=newlong[20000];
//…useslocalvariables…
//Doesclean-up(beforestartingsomethingextensive)
aFile=null;//(1)
bigArray=null;//(2)
//Startsomeotherextensiveactivity
//…
}
//…
}
Inthiscode,thelocalvariablesaresettonullafteruseat(1)and(2),beforestarting
someotherextensiveactivity.Thismakestheobjectsdenotedbythelocalvariables
eligibleforgarbagecollectionfromthispointonward,ratherthanafterthemethod
terminates.Thisoptimizationtechniqueofnullingreferencesshouldbeusedonlyasalast
resortwhenresourcesarescarce.
Herearesomeothertechniquestofacilitategarbagecollection:
•Whenamethodreturnsareferencevalueandtheobjectdenotedbythevalueisnot
needed,notassigningthisvaluetoareferencefacilitatesgarbagecollection.
•Ifareferenceisassignedanewvalue,theobjectthatwaspreviouslydenotedbythe
referencecanbecomeeligibleforgarbagecollection.
•Removingreachablereferencestoacompositeobjectcanmaketheconstituent
objectsbecomeeligibleforgarbagecollection,asexplainedearlier.
Example9.1illustrateshowaprogramcaninfluencegarbagecollectioneligibility.The
classHeavyItemrepresentsobjectswithalargememoryfootprint,forwhichwewantto
monitorgarbagecollection.EachcompositeHeavyItemobjecthasareferencetoalarge
array.Theclassoverridesthefinalize()methodfromtheObjectclasstoprintout
anIDwhentheobjectisfinalized.Thismethodisalwayscalledonaneligibleobject
beforeitisdestroyed(p.390).Weuseittoindicateintheoutputifandwhena
HeavyItemisreclaimed.Toillustratetheeffectofgarbagecollectiononobject
hierarchies,eachHeavyItemobjectmayalsohaveareferencetoanotherHeavyItem.
InExample9.1,theclassRecyclingBindefinesamethodcreateHeavyItem()at
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(4).Inthismethod,theHeavyItemcreatedat(5)iseligibleforgarbagecollectionafter
thereassignmentofreferenceitemAat(6),asthisobjectwillthenhavenoreferences.
TheHeavyItemcreatedat(6)isaccessibleonreturnfromthemethod.Itsfatedepends
onthecodethatcallsthismethod.
InExample9.1,theclassRecyclingBinalsodefinesamethodcreateList()at
(8).Itreturnsthereferencevalueinthereferenceitem1,whichdenotesthefirstitemina
listofthreeHeavyItemobjects.Becauseoftheliststructure,noneoftheHeavyItem
objectsinthelistiseligibleforgarbagecollectiononreturnfromthemethod.Again,the
fateoftheobjectsinthelistisdecidedbythecodethatcallsthismethod.Itisenoughfor
thefirstiteminthelisttobecomeunreachable,sothatallobjectsinthelistbecome
eligibleforgarbagecollection(barringanyreachablereferences).
Example9.1GarbageCollectionEligibility
Clickheretoviewcodeimage
//File:RecyclingBin.java
classHeavyItem{//(1)
int[]itemBody;
StringitemID;
HeavyItemnextItem;
HeavyItem(StringID,HeavyItemitemRef){//(2)
itemBody=newint[1_000_000];
itemID=ID;
nextItem=itemRef;
}
@Override
protectedvoidfinalize()throwsThrowable{//(3)
System.out.println(itemID+“:recycled.”);
super.finalize();
}
}
//______________________________________________________________________________
publicclassRecyclingBin{
publicstaticHeavyItemcreateHeavyItem(StringitemID){//(4)
HeavyItemitemA=newHeavyItem(itemID+“:localitem”,null);//(5)
itemA=newHeavyItem(itemID,null);//(6)
System.out.println(“ReturnfromcreatingHeavyItem”+itemID);
returnitemA;//(7)
}
publicstaticHeavyItemcreateList(StringlistID){//(8)
HeavyItemitem3=newHeavyItem(listID+“:item3”,null);//(9)
HeavyItemitem2=newHeavyItem(listID+“:item2”,item3);//(10)
HeavyItemitem1=newHeavyItem(listID+“:item1”,item2);//(11)
System.out.println(“Returnfromcreatinglist”+listID);
returnitem1;//(12)
}
publicstaticvoidmain(String[]args){//(13)
HeavyItemlist=createList(“X”);//(14)
list=createList(“Y”);//(15)
HeavyItemitemOne=createHeavyItem(“One”);//(16)
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HeavyItemitemTwo=createHeavyItem(“Two”);//(17)
itemOne=null;//(18)
createHeavyItem(“Three”);//(19)
createHeavyItem(“Four”);//(20)
System.out.println(“Returnfrommain().”);
}
}
Probableoutputfromtheprogram:
Clickheretoviewcodeimage
ReturnfromcreatinglistX
ReturnfromcreatinglistY
X:item3:recycled.
X:item2:recycled.
X:item1:recycled.
ReturnfromcreatingHeavyItemOne
ReturnfromcreatingHeavyItemTwo
ReturnfromcreatingHeavyItemThree
Three:localitem:recycled.
Three:recycled.
Two:localitem:recycled.
ReturnfromcreatingHeavyItemFour
One:localitem:recycled.
One:recycled.
Returnfrommain().
InExample9.1,themain()methodat(13)intheclassRecyclingBinusesthe
methodscreateHeavyItem()andcreateList().ItcreatesalistXat(14),butthe
referencetoitsfirstitemisreassignedat(15),makingobjectsinlistXeligibleforgarbage
collectionafter(15).ThefirstitemoflistYisstoredinthereferencelist,makingthis
listnon-eligibleforgarbagecollectionduringtheexecutionofthemain()method.
Themain()methodcreatestwoitemsat(16)and(17),storingtheirreferencevaluesin
thereferencesitemOneanditemTwo,respectively.ThereferenceitemOneisnulled
at(18),makingtheHeavyItemobjectwithidentityOneeligibleforgarbagecollection.
ThetwocallstothecreateHeavyItem()methodat(19)and(20)returnreference
valuestoHeavyItemobjects,whicharenotstored,makingeachobjecteligiblefor
garbagecollectionimmediatelyaftertheirrespectivemethodcallsreturn.
Theoutputfromtheprogrambearsouttheobservationsmadeearlier.ObjectsinlistY
(accessiblethroughthereferencelist)andtheHeavyItemobjectwithidentityTwo
(accessiblethroughthereferenceitemTwo)remainnon-eligiblewhilethemain()
methodexecutes.AlthoughtheoutputshowsthattheHeavyItemobjectwithidentity
Fourwasnevergarbagecollected,itisnotaccessibleonceitbecomeseligiblefor
garbagecollectionat(20).Anyobjectsintheheapaftertheprogramterminatesare
reclaimedbytheoperatingsystem.
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9.4ObjectFinalization
Objectfinalizationprovidesanobjectwithalastresorttoundertakeanyactionbeforeits
storageisreclaimed.Theautomaticgarbagecollectorcallsthefinalize()methodin
anobjectthatiseligibleforgarbagecollectionbeforeactuallydestroyingtheobject.The
finalize()methodisdefinedintheObjectclass:
Clickheretoviewcodeimage
protectedvoidfinalize()throwsThrowable
Animplementationofthefinalize()methodiscalledafinalizer.Asubclasscan
overridethefinalizerfromtheObjectclasssoastotakemorespecificandappropriate
actionbeforeanobjectofthesubclassisdestroyed.Notethattheoverridingmethod
cannotnarrowthevisibilityoftheoverriddenmethodandmustbedeclaredaseither
protectedorpublic.
Afinalizer,likeanyothermethod,cancatchandthrowexceptions(§6.7,p.238).Whena
finalizeriscalledexplicitlybytheprogramcode,exceptionhandlingisnodifferentduring
executionofafinalizerthanduringexecutionofanyothermethod.However,any
exceptionthrownbutnotcaughtbyafinalizerthatiscalledbythegarbagecollectoris
ignored,andthefinalizationofthisobjectisterminated.Thefinalizeriscalledonlyonce
onanobject,regardlessofwhetheranyexceptionisthrownduringitsexecution.Incase
offinalizationfailure,theobjectremainseligiblefordisposalatthediscretionofthe
garbagecollector(unlessithasbeenresurrected,asexplainedlaterinthissection).Since
thereisnoguaranteethatthegarbagecollectorwilleverrun,thereisalsonoguarantee
thatthefinalizerwilleverbecalled.
Inthefollowingcode,thefinalizerat(1)willtakeappropriateactionifandwhencalledon
objectsoftheclassbeforetheyaregarbagecollected,ensuringthattheresourceisfreed.
Sinceitisnotguaranteedthatthefinalizerwilleverbecalledatall,aprogramshouldnot
relyonthefinalizationtodoanycriticaloperations.
Clickheretoviewcodeimage
publicclassAnotherWellBehavedClass{
SomeResourceobjRef;
//…
@Override
protectedvoidfinalize()throwsThrowable{//(1)
try{//(2)
if(objRef!=null)objRef.close();
}finally{//(3)
super.finalize();//(4)
}
}
}
Thefinalizerinasubclassshouldexplicitlycallthefinalizerinitssuperclassasitslast
action,asshownat(4).Thecalltothefinalizerofthesuperclassisinafinallyblockat
(3),whichisguaranteedtoexecuteregardlessofanyexceptionsthrownbythecodeinthe
tryblockat(2).(AnotherexampleoffinalizerchainingisprovidedinExample9.2inthe
nextsection.)
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Thefinalizerofanobjectcanmaketheobjectnon-eligibleagain(i.e.,resurrectit),thereby
avoidinggarbagecollectionoftheobject.Onesimpletechniqueistoassigntheobject’s
thisreferencetoastaticfield,whichthenbecomesareachablereferencefortheobject.
Sinceafinalizeriscalledonlyonceonanobjectbeforeitisgarbagecollected,anobject
canberesurrectedonlyonce.Inotherwords,iftheobjectagainbecomeseligiblefor
garbagecollectionandthegarbagecollectorruns,thefinalizerwillnotbecalled.Such
objectresurrectionsarenotrecommended,astheyunderminethepurposeofthe
finalizationmechanism.
Notethatanenumtypecannotdeclareafinalizer.Therefore,anenumconstantmaynever
befinalized.
9.5FinalizerChaining
Unlikesubclassconstructors,overriddenfinalizersarenotimplicitlychained(§7.5,p.
282).Chainingoffinalizersrequiresanexplicitcalltotheoverriddenfinalizer.Example
9.2illustratestheprocessofprogrammaticallychainingfinalizers.Itcreatesauser-
specifiednumberoflargeobjectsofauser-specifiedsize;thenumberandsizeare
providedthroughcommand-lineprogramarguments.Theloopat(7)inthemain()
methodcreatesBlobobjects,butdoesnotstoreanyreferencestothem.Objectscreated
areinstancesoftheclassBlobdefinedat(3).TheBlobconstructorat(4)initializesthe
fieldsizebyconstructingalargearrayofintegers.TheBlobclassextendsthe
BasicBlobclass,whichassignseachblobauniquenumber(blobId)andkeepstrack
ofthenumberofblobs(population)notyetgarbagecollected.
CreationofeachBlobobjectbytheconstructorat(4)printstheIDnumberoftheobject
andthemessage"Hello".Thefinalize()methodat(5)iscalledbeforeaBlob
objectisgarbagecollected.ItprintstheinheritedfieldblobIdoftheBlobobjectand
themessage"Bye",beforecallingthefinalize()methodinthesuperclass
BasicBlobat(2),whichdecrementsthepopulationcount.Theprogramoutputshows
thattwoblobswerenotgarbagecollectedatthetimetheprintstatementat(8)was
executed.Itisevidentfromthenumberof"Bye"messagesthatthreeblobsweregarbage
collectedbeforeallfiveblobswerecreatedintheloopat(7).
Example9.2UsingFinalizers
Clickheretoviewcodeimage
//File:Finalizers.java
classBasicBlob{//(1)
privatestaticintidCounter;
privatestaticintpopulation;
protectedintblobId;
BasicBlob(){
blobId=idCounter++;
++population;
}
@Override
protectedvoidfinalize()throwsThrowable{//(2)
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—population;
super.finalize();
}
publicstaticintgetPopulation(){
returnpopulation;
}
}
//______________________________________________________________________________
classBlobextendsBasicBlob{//(3)
privateint[]size;
Blob(intbloatedness){//(4)
size=newint[bloatedness];
System.out.println(blobId+“:Hello”);
}
@Override
protectedvoidfinalize()throwsThrowable{//(5)
System.out.println(blobId+“:Bye”);
super.finalize();
}
}
//______________________________________________________________________________
publicclassFinalizers{
publicstaticvoidmain(String[]args){//(6)
intblobsRequired,blobSize;
try{
blobsRequired=Integer.parseInt(args[0]);
blobSize=Integer.parseInt(args[1]);
}catch(IndexOutOfBoundsExceptione){
System.out.println(“Toofewprogramarguments.”);
System.out.println(“Usage:Finalizers<numberofblobs><blobsize>”);
return;
}catch(NumberFormatExceptione){
System.out.println(“Illegalprogramargument.”);
System.out.println(“Usage:Finalizers<numberofblobs><blobsize>”);
return;
}
for(inti=0;i<blobsRequired;++i){//(7)
newBlob(blobSize);
}
System.out.println(BasicBlob.getPopulation()+”blobsalive”);//(8)
}
}
Probableoutputfromrunningtheprogramwiththefollowingcommand:
>javaFinalizers5500000
0:Hello
1:Hello
2:Hello
0:Bye
1:Bye
2:Bye
3:Hello
4:Hello
2blobsalive
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9.6InvokingGarbageCollectionProgrammatically
AlthoughJavaprovidesfacilitiestoinvokethegarbagecollectionexplicitly,thereareno
guaranteesthatitwillberun.Theprogramcanrequestthatgarbagecollectionbe
performed,butthereisnowaytoforcegarbagecollectiontobeactivated.
TheSystem.gc()methodcanbeusedtorequestgarbagecollection,andthe
System.runFinalization()methodcanbecalledtosuggestthatanypending
finalizersberunforobjectseligibleforgarbagecollection.
staticvoidgc()
Requeststhatgarbagecollectionberun.
staticvoidrunFinalization()
Requeststhatanypendingfinalizersberunforobjectseligibleforgarbage
collection.
Alternatively,correspondingmethodsintheRuntimeclasscanbeused.AJava
applicationhasauniqueRuntimeobjectthatcanbeusedbytheapplicationtointeract
withtheJVM.Anapplicationcanobtainthisobjectbycallingthemethod
Runtime.getRuntime().TheRuntimeclassprovidesseveralmethodsrelatedto
memoryissues:
staticRuntimegetRuntime()
ReturnstheRuntimeobjectassociatedwiththecurrentapplication.
voidgc()
Requeststhatgarbagecollectionberun.However,itisrecommendedtousethe
moreconvenientstaticmethodSystem.gc().
voidrunFinalization()
Requeststhatanypendingfinalizersberunforobjectseligibleforgarbage
collection.Again,itismoreconvenienttousethestaticmethod
System.runFinalization().
longfreeMemory()
Returnstheamountoffreememory(bytes)intheJVMthatisavailablefornew
objects.
longtotalMemory()
Returnsthetotalamountofmemory(bytes)availableintheJVM,includingboth
memoryoccupiedbycurrentobjectsandmemoryavailablefornewobjects.
Example9.3illustratestheprocessofinvokinggarbagecollection.TheclassMemory-
CheckisanadaptationoftheclassFinalizersfromExample9.2.TheRunTime
objectfortheapplicationisobtainedat(7).Thisobjectisusedtogetinformation
regardingtotalmemoryandfreememoryintheJVMat(8)and(9),respectively.Blobsare
createdintheloopat(10).Theamountoffreememoryafterblobcreationisprintedat
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(11).Fromtheprogramoutput,itisapparentthatsomeblobswerealreadygarbage
collectedbeforetheexecutionreached(11).Arequestforgarbagecollectionismadeat
(12).Checkingfreememoryafterthisrequestshowsthatmorememoryhasbecome
available,indicatingthattherequestwashonored.Itisinstructivetoruntheprogram
withoutthemethodcallSystem.gc()at(12)tocomparetheresultswithandwithout
thiscall.
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Example9.3InvokingGarbageCollection
Clickheretoviewcodeimage
classBasicBlob{/*SeeExample9.2.*/}
classBlobextendsBasicBlob{/*SeeExample9.2.*/}
//______________________________________________________________________________
publicclassMemoryCheck{
publicstaticvoidmain(String[]args){//(6)
intblobsRequired,blobSize;
try{
blobsRequired=Integer.parseInt(args[0]);
blobSize=Integer.parseInt(args[1]);
}catch(IndexOutOfBoundsExceptione){
System.out.println(“Toofewprogramarguments.”);
System.out.println(“Usage:MemoryCheck<numberofblobs><blobsize>”);
return;
}catch(NumberFormatExceptione){
System.out.println(“Illegalprogramargument.”);
System.out.println(“Usage:MemoryCheck<numberofblobs><blobsize>”);
return;
}
Runtimeenvironment=Runtime.getRuntime();//(7)
System.out.println(“Totalmemory:”+environment.totalMemory());//(8)
System.out.println(“Freememorybeforeblobcreation:”+
environment.freeMemory());//(9)
for(inti=0;i<blobsRequired;++i){//(10)
newBlob(blobSize);
}
System.out.println(“Freememoryafterblobcreation:”+
environment.freeMemory());//(11)
System.gc();//(12)
System.out.println(“FreememoryafterrequestingGC:”+
environment.freeMemory());//(13)
System.out.println(BasicBlob.getPopulation()+”blobsalive”);//(14)
}
}
Probableoutputfromrunningtheprogramwiththefollowingcommand:
Clickheretoviewcodeimage
>javaMemoryCheck5100000
Totalmemory:2031616
Freememorybeforeblobcreation:1773192
0:Hello
1:Hello
2:Hello
1:Bye
2:Bye
3:Hello
0:Bye
3:Bye
4:Hello
Freememoryafterblobcreation:818760
4:Bye
FreememoryafterrequestingGC:1619656
0blobsalive
Thefollowingpointsregardingautomaticgarbagecollectionshouldbenoted:
•Therearenoguaranteesthatthefinalizersofobjectseligibleforgarbagecollection
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willbeexecuted.Garbagecollectionmightnotevenberuniftheprogramexecution
doesnotwarrantit.Thus,anymemoryallocatedduringprogramexecutionmight
remainallocatedafterprogramtermination,butwilleventuallybereclaimedbythe
operatingsystem.
•Therearealsonoguaranteesabouttheorderinwhichtheobjectswillbegarbage
collected,ortheorderinwhichtheirfinalizerswillbeexecuted.Therefore,the
programshouldnotmakeanyassumptionsbasedonthesecriteria.
•Garbagecollectiondoesnotguaranteethattherewillbeenoughmemoryforthe
programtorun.Aprogramcanrelyonthegarbagecollectortorunwhenmemory
getsverylow,anditcanexpectanOutOfMemoryExceptiontobethrownifits
memorydemandscannotbemet.
ReviewQuestions
9.1Whichofthefollowingstatementsistrue?
Selecttheonecorrectanswer.
(a)Objectscanbeexplicitlydestroyedusingthekeyworddelete.
(b)Anobjectwillbegarbagecollectedimmediatelyafteritbecomesunreachable.
(c)Ifobjectobj1isaccessiblefromobjectobj2,andobjectobj2isaccessible
fromobj1,thenobj1andobj2arenoteligibleforgarbagecollection.
(d)Onceanobjecthasbecomeeligibleforgarbagecollection,itwillremain
eligibleuntilitisdestroyed.
(e)Ifobjectobj1canaccessobjectobj2thatiseligibleforgarbagecollection,
thenobj1isalsoeligibleforgarbagecollection.
9.2Identifythelocationinthefollowingprogramwheretheobject,initiallyreferenced
byarg1,iseligibleforgarbagecollection.
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
Stringmsg;
Stringpre=“Thisprogramwascalledwith“;
Stringpost=”asfirstargument.”;
Stringarg1=newString((args.length>0)?”’”+args[0]+”’”:
”<noargument>”);
msg=arg1;
arg1=null;//(1)
msg=pre+msg+post;//(2)
pre=null;//(3)
System.out.println(msg);
msg=null;//(4)
post=null;//(5)
args=null;//(6)
}
}
Selecttheonecorrectanswer.
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(a)Afterthelinelabeled(1)
(b)Afterthelinelabeled(2)
(c)Afterthelinelabeled(3)
(d)Afterthelinelabeled(4)
(e)Afterthelinelabeled(5)
(f)Afterthelinelabeled(6)
9.3Howmanyobjectsareeligibleforgarbagecollectionwhencontrolreaches(1)?
Clickheretoviewcodeimage
publicclassEligible{
publicstaticvoidmain(String[]args){
for(inti=0;i<5;i++){
Eligibleobj=newEligible();
newEligible();
}
System.gc();//(1)
}
}
Selecttheonecorrectanswer.
(a)0
(b)5
(c)10
(d)Hardtosay
9.4Howmanyobjectsareeligibleforgarbagecollectionwhencontrolreaches(1)?
Clickheretoviewcodeimage
publicclassLink{
privateLinknext;
Link(Linknext){this.next=next;}
publicvoidfinialize(){System.out.print(“X”);}
publicstaticvoidmain(String[]args){
Linkp=null;
for(inti=0;i<5;i++){
p=newLink(p);
}
System.gc();//(1);
}
}
Selecttheonecorrectanswer.
(a)0
(b)5
(c)10
(d)Hardtosay
9.5Whichofthefollowingstatementsistrue?
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Selecttheonecorrectanswer.
(a)Ifanexceptionisthrownduringexecutionofthefinalize()methodofan
eligibleobject,theexceptionisignoredandtheobjectisdestroyed.
(b)Allobjectshaveafinalize()method.
(c)Objectscanbedestroyedbyexplicitlycallingthefinalize()method.
(d)Thefinalize()methodcanbedeclaredwithanyaccessibility.
(e)Thecompilerwillfailtocompilecodethatdefinesanoverriding
finalize()methodthatdoesnotexplicitlycalltheoverridden
finalize()methodfromthesuperclass.
9.6Whichofthefollowingstatementsistrue?
Selecttheonecorrectanswer.
(a)Thecompilerwillfailtocompilecodethatexplicitlytriestocallthe
finalize()method.
(b)Thefinalize()methodcanbeoverridden,butitmustbedeclaredwith
protectedaccessibility.
(c)Anoverridingfinalize()methodinanyclasscanalwaysthrowchecked
exceptions.
(d)Thefinalize()methodcanbeoverloaded.
(e)Thebodyofthefinalize()methodcanaccessonlyotherobjectsthatare
eligibleforgarbagecollection.
9.7Whichmethodheaderswillresultinacorrectimplementationofafinalizerforthe
followingclass?
Clickheretoviewcodeimage
publicclassCurtain{
//(1)INSERTMETHODHEADERHERE…
{
System.out.println(“Finalcurtain”);
super.finalize();
}
}
Selectthetwocorrectanswers.
(a)voidfinalize()throwsThrowable
(b)voidfinalize()throwsException
(c)voidfinalize()
(d)protectedvoidfinalize()throwsThrowable
(e)protectedvoidfinalize()throwsException
(f)protectedvoidfinalize()
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(g)publicvoidfinalize()throwsThrowable
(h)publicvoidfinalize()throwsException
(i)publicvoidfinalize()
(j)privatevoidfinalize()throwsThrowable
(k)privatevoidfinalize()throwsException
(l)privatevoidfinalize()
9.8Whichscenariocandefinitelynotbetheresultofcompilingandrunningthe
followingprogram?
Clickheretoviewcodeimage
publicclassGrade{
privatechargrade;
Grade(chargrade){this.grade=grade;}
publicvoidfinalize()throwsThrowable{
System.out.print(grade);
super.finalize();
}
publicstaticvoidmain(String[]args){
newGrade(‘A’);newGrade(‘F’);
System.gc();
}
}
Selecttheonecorrectanswer.
(a)TheprogrammayprintAF.
(b)TheprogrammayprintFA.
(c)TheprogrammayprintA.
(d)TheprogrammayprintF.
(e)TheprogrammayprintAFA.
(f)Theprogrammaynotprintanything.
9.9Whichscenarioscanbetheresultofcompilingandrunningthefollowing
program?
Clickheretoviewcodeimage
publicclassMyString{
privateStringstr;
MyString(Stringstr){this.str=str;}
publicvoidfinalize()throwsThrowable{
System.out.print(str);
super.finalize();
}
publicvoidconcat(Stringstr2){
this.str.concat(str2);
}
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publicstaticvoidmain(String[]args){
newMyString(“A”).concat(“B”);
System.gc();
}
}
Selectthetwocorrectanswers.
(a)TheprogrammayprintAB.
(b)TheprogrammayprintBA.
(c)TheprogrammayprintA.
(d)TheprogrammayprintB.
(e)Theprogrammaynotprintanything.
9.7Initializers
Initializerscanbeusedtosetinitialvaluesforfieldsinobjectsandclasses.Therearethree
kindsofinitializers:
•Fieldinitializerexpressions
•Staticinitializerblocks
•Instanceinitializerblocks
Subsequentsectionsinthischapterprovidedetailsontheseinitializers,concludingwitha
discussionoftheprocedureinvolvedinconstructingthestateofanobjectwhentheobject
iscreatedbyusingthenewoperator.
9.8FieldInitializerExpressions
Initializationoffieldscanbespecifiedinfielddeclarationstatementsusinginitializer
expressions.Thevalueoftheinitializerexpressionmustbeassignmentcompatiblewith
thedeclaredfield(see§5.6,p.158and§7.9,p.312).Wedistinguishbetweenstaticand
non-staticfieldinitializers.
Clickheretoviewcodeimage
classConstantInitializers{
intminAge=12;//(1)Non-static
staticdoublepensionPoints=10.5;//(2)Static
//…
}
Thefieldsofanobjectareinitializedwiththevaluesofinitializerexpressionswhenthe
objectiscreatedbyusingthenewoperator.Inthepreviousexample,thedeclarationat(1)
willresultinthefieldminAgebeinginitializedto12ineveryobjectoftheclass
ConstantInitializerscreatedwiththenewoperator.Ifnoexplicitinitializer
expressionsarespecified,defaultvalues(§2.4,p.42)areassignedtothefields.
Whenaclassisloaded,itisinitialized,meaningitsstaticfieldsareinitializedwiththe
valuesoftheinitializerexpressions.Thedeclarationat(2)willresultinthestaticfield
pensionPointsbeinginitializedto10.5whentheclassisloadedbytheJVM.Again,
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ifnoexplicitinitializersarespecified,defaultvaluesareassignedtothestaticfields.
Aninitializerexpressionforastaticfieldcannotrefertonon-staticmembersbytheir
simplenames.Thekeywordsthisandsupercannotoccurinastaticinitializer
expression.
Sinceaclassisalwaysinitializedbeforeitcanbeinstantiated,aninstanceinitializer
expressioncanalwaysrefertoanystaticmemberofaclass,regardlessofthemember
declarationorder.Inthefollowingcode,theinstanceinitializerexpressionat(1)refersto
thestaticfieldNO_OF_WEEKSdeclaredandinitializedat(2).Suchaforwardreferenceis
legal.Moreexamplesofforwardreferencesaregiveninthenextsubsection.
Clickheretoviewcodeimage
classMoreInitializers{
intnoOfDays=7*NO_OF_WEEKS;//(1)Non-static
staticintNO_OF_WEEKS=52;//(2)Static
//…
}
Initializerexpressionscanalsobeusedtodefineconstantsininterfaces(§7.6,p.302).
Suchinitializerexpressionsareimplicitlystatic,astheydefinevaluesofstaticfinal
fields.
Initializerexpressionsareusedtoinitializelocalvariablesaswell(§2.3,p.40).Alocal
variableisinitializedwiththevalueoftheinitializerexpressioneverytimethelocal
variabledeclarationisexecuted.
DeclarationOrderofInitializerExpressions
Whenanobjectiscreatedusingthenewoperator,instanceinitializerexpressionsare
executedintheorderinwhichtheinstancefieldsaredeclaredintheclass.
Javarequiresthatthedeclarationofafieldmustoccurbeforeitsusageinanyinitializer
expressionifthefieldisusedontheright-handsideofanassignmentintheinitializer
expression.Thisessentiallymeansthatthedeclarationofafieldmustoccurbeforethe
valueofthefieldisreadinaninitializerexpression.Usingthefieldontheleft-handside
ofanassignmentintheinitializerexpressiondoesnotviolatethedeclaration-before-
readingrule,asthisconstitutesawriteoperation.Thisruleapplieswhentheusageofthe
fieldisbyitssimplename.
Thereisonecaveattothedeclaration-before-readingrule:Itdoesnotapplyifthe
initializerexpressiondefinesananonymousclass,astheusagethenoccursinadifferent
classthathasitsownaccessibilityrulesintheenclosingcontext.Therestrictionsoutlined
earlierhelptodetectinitializationanomaliesatcompiletime.
Inthenextcodeexample,theinitializationat(2)generatesacompile-timeerror,because
thefieldwidthintheinitializerexpressionviolatesthedeclaration-before-readingrule.
Becausetheusageofthefieldwidthintheinitializerexpressionat(2)doesnotoccuron
theleft-handsideoftheassignment,thisisanillegalforwardreference.Toremedythe
error,thedeclarationofthefieldwidthat(4)canbemovedinfrontofthedeclarationat
(2).Inanycase,wecanusethekeywordthisasshownat(3),butitwillreadthedefault
value0inthefieldwidth.
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classNonStaticInitializers{
intlength=10;//(1)
//doublearea=length*width;//(2)NotOK.Illegalforward
reference.
doublearea=length*this.width;//(3)OK,butwidthhasdefaultvalue
0.
intwidth=10;//(4)
intsqSide=height=20;//(5)OK.Legalforwardreference.
intheight;//(6)
}
Theforwardreferenceat(5)islegal.Theusageofthefieldheightintheinitializer
expressionat(5)occursontheleft-handsideoftheassignment.Theinitializerexpression
at(5)isevaluatedas(sqSide=(height=20)).Everyobjectoftheclass
NonStaticInitializerswillhavethefieldsheightandsqSidesettothevalue
20.
Thedeclaration-before-readingruleisequallyapplicabletostaticinitializerexpressions
whenstaticfieldsarereferencedbytheirsimplenames.
Example9.4showswhytheorderoffieldinitializerexpressionscanbeimportant.The
initializerexpressionsinthisexamplearecallstomethodsdefinedintheclass,and
methodsarenotsubjecttothesameaccessrulesasinitializerexpressions.Thecallat(2)
tothemethodinitMaxGuests()definedat(4)isexpectedtoreturnthemaximum
numberofguests,butthefieldoccupancyPerRoomat(3)willnothavebeenexplicitly
initializedatthispoint;therefore,itsdefaultvalue0willbeusedinthemethod
initMaxGuests(),whichwillreturnanincorrectvalue.Theprogramoutputshows
thatafterobjectcreation,theoccupancyperroomiscorrect,butthemaximumnumberof
guestsiswrong.
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Example9.4InitializerExpressionOrderandMethodCalls
Clickheretoviewcodeimage
//File:TestOrder.java
classHotel{
privateintnoOfRooms=12;//(1)
privateintmaxNoOfGuests=initMaxGuests();//(2)
Bug
privateintoccupancyPerRoom=2;//(3)
publicintinitMaxGuests(){//(4)
System.out.println(“occupancyPerRoom:”+occupancyPerRoom);
System.out.println(“maxNoOfGuests:”+noOfRooms*occupancyPerRoom);
returnnoOfRooms*occupancyPerRoom;
}
publicintgetMaxGuests(){returnmaxNoOfGuests;}//(5)
publicintgetOccupancy(){returnoccupancyPerRoom;}//(6)
}
//________________________________________________________________________
publicclassTestOrder{
publicstaticvoidmain(String[]args){
Hotelhotel=newHotel();//(7)
System.out.println(“Afterobjectcreation:“);
System.out.println(“occupancyPerRoom:”+hotel.getOccupancy());//(8)
System.out.println(“maxNoOfGuests:”+hotel.getMaxGuests());//(9)
}
}
Outputfromtheprogram:
occupancyPerRoom:0
maxNoOfGuests:0
Afterobjectcreation:
occupancyPerRoom:2
maxNoOfGuests:0
9.9StaticInitializerBlocks
Javaallowsstaticinitializerblockstobedefinedinaclass.Althoughsuchblockscan
includearbitrarycode,theyareprimarilyusedforinitializingstaticfields.Thecodeina
staticinitializerblockisexecutedonlyonce,whentheclassisloadedandinitialized.
Thesyntaxofastaticinitializerblockcomprisesthekeywordstaticfollowedbya
localblockthatcancontainarbitrarycode,asshownat(3).
Clickheretoviewcodeimage
classStaticInitializers{
staticfinalintROWS=12,COLUMNS=10;//(1)
staticlong[][]matrix=newlong[ROWS][COLUMNS];//(2)
//…
static{//(3)Static
initializer
for(inti=0;i<matrix.length;i++)
for(intj=0;j<matrix[i].length;j++)
matrix[i][j]=2*i+j;
}
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//…
}
WhentheclassStaticInitializersisfirstloadedinthepreviousexample,the
staticfinalfieldsat(1)areinitialized.Thenthearrayofarraysmatrixof
specifiedsizeiscreatedat(2),followedbytheexecutionofthestaticblockat(3).
Ifaclassreliesonnativemethodimplementations,astaticinitializercanbeusedtoload
anyexternallibrariesthattheclassneeds(§4.8,p.137).
Notethatthestaticinitializerblockisnotcontainedinanymethod.Aclasscanhavemore
thanonestaticinitializerblock.Initializerblocksarenotmembersofaclass,andthey
cannothaveareturnstatementbecausetheycannotbecalleddirectly.
Whenaclassisinitialized,theinitializerexpressionsinstaticfielddeclarationsandstatic
initializerblocksareexecutedintheorderinwhichtheyarespecifiedintheclass.Inthe
previousexample,theinitializerexpressionsat(1)and(2)areexecutedbeforethestatic
initializerblockat(3).
Similarrestrictionsapplytostaticinitializerblocksasforstaticinitializerexpressions:
Thekeywordsthisandsupercannotoccurinastaticinitializerblock,becausesucha
blockdefinesastaticcontext.
DeclarationOrderofStaticInitializers
Whenmakingforwardreferencesusingsimplenames,codeinastaticinitializerblockis
alsosubjecttothedeclaration-before-readingrulediscussedintheprevioussubsection.
Example9.5illustratesforwardreferencesandtheorderofexecutionforstaticinitializer
expressionsandstaticinitializerblocks.Anillegalforwardreferenceoccursat(4),where
anattemptismadetoreadthevalueofthefieldsf1beforeitsdeclaration.At(11),the
readoperationoccursafterthedeclaration,soitisallowed.Forwardreferencemadeon
theleft-handsideoftheassignmentisalwaysallowed,asshownat(2),(5),and(7).The
initializersareexecutedintheirdeclarationorder.Astaticfieldhasthevaluethatitwas
lastassignedinaninitializer.Ifthereisnoexplicitassignment,thefieldhasthedefault
valueofitstype.
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Example9.5StaticInitializersandForwardReferences
Clickheretoviewcodeimage
publicclassStaticForwardReferences{
static{//(1)Staticinitializerblock.
sf1=10;//(2)OK.Assignmenttosf1allowed.
//sf1=if1;//(3)NotOK.Non-staticfieldaccessinstatic
context.
//inta=2*sf1;//(4)NotOK.Readoperationbeforedeclaration.
intb=sf1=20;//(5)OK.Assignmenttosf1allowed.
intc=StaticForwardReferences.sf1;//(6)OK.Notaccessedbysimple
name.
}
staticintsf1=sf2=30;//(7)Staticfield.Assignmenttosf2allowed.
staticintsf2;//(8)Staticfield.
intif1=5;//(9)Non-staticfield.
static{//(10)Staticinitializerblock.
intd=2*sf1;//(11)OK.Readoperationafterdeclaration.
inte=sf1=50;//(12)OK.Assignmenttosf1allowed.
}
publicstaticvoidmain(String[]args){
System.out.println(“sf1:”+StaticForwardReferences.sf1);
System.out.println(“sf2:”+StaticForwardReferences.sf2);
}
}
Outputfromtheprogram:
sf1:50
sf2:30
9.10InstanceInitializerBlocks
Justasstaticinitializerblockscanbeusedtoinitializestaticfieldsinanamedclass,so
Javaprovidestheabilitytoinitializefieldsduringobjectcreationusinginstanceinitializer
blocks.Inthisrespect,suchblocksservethesamepurposeasconstructorsduringobject
creation.Thesyntaxofaninstanceinitializerblockisthesameasthatofalocalblock,as
shownat(2)inthefollowingcode.Thecodeinthelocalblockisexecutedeverytimean
instanceoftheclassiscreated.
Clickheretoviewcodeimage
classInstanceInitializers{
long[]squares=newlong[10];//(1)
//…
{//(2)InstanceInitializer
for(inti=0;i<squares.length;i++)
squares[i]=i*i;
}
//…
}
Thearraysquaresofspecifiedlengthisfirstcreatedat(1);itscreationisfollowedby
theexecutionoftheinstanceinitializerblockat(2)everytimeaninstanceoftheclass
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InstanceInitializersiscreated.Notethattheinstanceinitializerblockisnot
containedinanymethod.Aclasscanhavemorethanoneinstanceinitializerblock,and
these(andanyinstanceinitializerexpressionsininstancefielddeclarations)areexecuted
intheordertheyarespecifiedintheclass.
DeclarationOrderofInstanceInitializers
Analogoustotheotherinitializersdiscussedearlier,aninstanceinitializerblockcannot
makeaforwardreferencetoafieldthatviolatesthedeclaration-before-readingrule.In
Example9.6,anillegalforwardreferenceoccursinthecodeat(4),whichattemptstoread
thevalueofthefieldnsf1beforeitisdeclared.Thereadoperationat(11)occursafter
thedeclarationand,therefore,isallowed.Forwardreferencemadeontheleft-handsideof
theassignmentisalwaysallowed,asshownat(2),(3),(5),and(7).
Asinaninstanceinitializerexpression,thekeywordsthisandsupercanbeusedto
refertothecurrentobjectinaninstanceinitializerblock.Asinastaticinitializerblock,
thereturnstatementisnotallowedininstanceinitializerblocks.
Aninstanceinitializerblockcanbeusedtofactoroutcommoninitializationcodethatwill
beexecutedregardlessofwhichconstructorisinvoked.
Example9.6InstanceInitializersandForwardReferences
Clickheretoviewcodeimage
publicclassNonStaticForwardReferences{
{//(1)Instanceinitializerblock.
nsf1=10;//(2)OK.Assignmenttonsf1allowed.
nsf1=sf1;//(3)OK.Staticfieldaccessinnon-static
context.
//inta=2*nsf1;//(4)NotOK.Readoperationbeforedeclaration.
intb=nsf1=20;//(5)OK.Assignmenttonsf1allowed.
intc=this.nsf1;//(6)OK.Notaccessedbysimplename.
}
intnsf1=nsf2=30;//(7)Non-staticfield.Assignmenttonsf2
allowed.
intnsf2;//(8)Non-staticfield.
staticintsf1=5;//(9)Staticfield.
{//(10)Instanceinitializerblock.
intd=2*nsf1;//(11)OK.Readoperationafterdeclaration.
inte=nsf1=50;//(12)OK.Assignmenttonsf1allowed.
}
publicstaticvoidmain(String[]args){
NonStaticForwardReferencesobjRef=newNonStaticForwardReferences();
System.out.println(“nsf1:”+objRef.nsf1);
System.out.println(“nsf2:”+objRef.nsf2);
}
}
Outputfromtheprogram:
nsf1:50
nsf2:30
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9.11ConstructingInitialObjectState
Objectinitializationinvolvesconstructingtheinitialstateofanobjectwhenitiscreated
bythenewoperator.Firstthefieldsareinitializedtotheirdefaultvalues(§2.4,p.42)—
whethertheyaresubsequentlygivennon-defaultinitialvaluesornot—andthenthe
constructorisinvoked.Thiscanleadtolocalchainingofconstructors.Theinvocationof
theconstructorattheendofthelocalchainofconstructorinvocationsresultsinthe
followingactions,beforetheconstructor’sexecutionresumes:
•Implicitorexplicitinvocationofthesuperclassconstructor.Constructorchaining
ensuresthattheinheritedstateoftheobjectisconstructedfirst(§7.5,p.282).
•Initializationoftheinstancefieldsbyexecutingtheirinstanceinitializerexpressions
andanyinstanceinitializerblocks,intheordertheyarespecifiedintheclass
declaration.
Example9.7illustratesobjectinitialization.Thenewoperatorisusedat(8)tocreatean
objectofSubclassB.Theno-argumentconstructorSubclassB()at(2)usesthe
this()constructtolocallychaintothenon-zeroargumentconstructorat(3).This
constructorthenleadstoanimplicitcallofthesuperclassconstructor.Ascanbeseenfrom
theprogramoutput,theexecutionofthesuperclass’sconstructorat(1)reachescompletion
first.Thisisfollowedbytheexecutionoftheinstanceinitializerblockat(4)andthe
instanceinitializerexpressionat(6).Thentheexecutionofthebodyofthenon-zero
argumentconstructorat(3)resumes.Finally,theno-argumentconstructorcompletesits
execution,therebycompletingtheconstructionoftheobjectstate.
Notethattheinstanceinitializersareexecutedintheordertheyarespecifiedintheclass
declaration.Theforwardreferencetothefieldvalueat(5)islegal,becausetheusageof
thefieldvalueisontheleft-handsideoftheassignment(itdoesnotviolatethe
declaration-before-readingrule).Thedefaultvalueofthefieldvalueisoverwrittenby
theinstanceinitializerblockat(5).Thefieldvalueisagainoverwrittenbytheinstance
initializerexpressionat(6),andfinallybythenon-zeroargumentconstructorat(3).
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Example9.7ObjectStateConstruction
Clickheretoviewcodeimage
//File:ObjectConstruction.java
classSuperclassA{
publicSuperclassA(){//(1)
System.out.println(“ConstructorinSuperclassA”);
}
}
//_______________________________________________________________________________
classSubclassBextendsSuperclassA{
SubclassB(){//(2)No-argumentconstructor
this(3);
System.out.println(“No-argumentconstructorinSubclassB”);
}
SubclassB(inti){//(3)Non-zeroargument
constructor
System.out.println(“Non-zeroargumentconstructorinSubclassB”);
value=i;
}
{//(4)Instanceinitializer
block
System.out.println(“InstanceinitializerblockinSubclassB”);
value=2;//(5)
}
intvalue=initializerExpression();//(6)
privateintinitializerExpression(){//(7)
System.out.println(“InstanceinitializerexpressioninSubclassB”);
return1;
}
}
//_______________________________________________________________________________
publicclassObjectConstruction{
publicstaticvoidmain(String[]args){
SubclassBobjRef=newSubclassB();//(8)
System.out.println(“value:”+objRef.value);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
ConstructorinSuperclassA
InstanceinitializerblockinSubclassB
InstanceinitializerexpressioninSubclassB
Non-zeroargumentconstructorinSubclassB
No-argumentconstructorinSubclassB
value:3
Somecareshouldbeexercisedwhenwritingconstructorsfornon-finalclasses,since
theobjectthatisconstructedmightbeasubclassinstance.Example9.8showsasituation
whereuseofoverriddenmethodsinsuperclassinitializersandconstructorscangive
unexpectedresults.Theexampleintentionallyusesthethisreferencetounderlinethat
theinstancemethodsandconstructorsareinvokedonthecurrentobject,andthatthe
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constructorcallresultsintheinitializationoftheobjectstate,asexpected.
TheprogramoutputfromExample9.8showsthatthefieldsuperValueat(1)in
SuperclassAnevergetsinitializedexplicitlywhenanobjectofSubclassBiscreated
at(8).TheSuperclassAconstructorat(2)doeshaveacalltoamethodthathasthe
namedoValueat(3).AmethodwithsuchanameisdefinedinSuperclassAat(4),
butisalsooverriddeninSubclassBat(7).Theprogramoutputindicatesthatthe
methoddoValue()fromSubclassBiscalledat(3)intheSuperclassA
constructor.TheimplementationofthemethoddoValue()at(4)nevergetsexecuted
whenanobjectofSubclassBiscreated.Methodinvocationalwaysdeterminesthe
implementationofthemethodtobeexecuted,basedontheactualtypeoftheobject.
KeepinginmindthatitisanobjectofSubclassBthatisbeinginitialized,thecalltothe
methodnameddoValueat(3)resultsinthemethodfromSubclassBbeingexecuted.
Thiscanleadtounintendedresults.TheoverridingmethoddoValue()at(7)in
SubclassBcanaccessthefieldvaluedeclaredat(5)beforeitsinitializerexpression
hasbeenexecuted;thus,themethodinvokedcanaccessthestateoftheobjectbeforethis
hasbeencompletelyinitialized.Thevalue0isthenprinted,asthefieldvaluehasnot
yetbeeninitializedwiththevalue800whenthesuperclassconstructorisexecuted.
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Example9.8InitializationAnomalyunderObjectStateConstruction
Clickheretoviewcodeimage
//File:ObjectInitialization.java
classSuperclassA{
protectedintsuperValue;//(1)
SuperclassA(){//(2)
System.out.println(“ConstructorinSuperclassA”);
this.doValue();//(3)
}
voiddoValue(){//(4)
this.superValue=911;
System.out.println(“superValue:”+this.superValue);
}
}
//_______________________________________________________________________________
classSubclassBextendsSuperclassA{
privateintvalue=800;//(5)
SubclassB(){//(6)
System.out.println(“ConstructorinSubclassB”);
this.doValue();
System.out.println(“superValue:”+this.superValue);
}
@Override
voiddoValue(){//(7)
System.out.println(“value:”+this.value);
}
}
//_______________________________________________________________________________
publicclassObjectInitialization{
publicstaticvoidmain(String[]args){
System.out.println(“CreatinganobjectofSubclassB.”);
newSubclassB();//(8)
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
CreatinganobjectofSubclassB.
ConstructorinSuperclassA
value:0
ConstructorinSubclassB
value:800
superValue:0
Classinitializationtakesplacebeforeanyinstanceoftheclasscanbecreatedorastatic
methodoftheclasscanbeinvoked.Asuperclassisinitializedbeforeitssubclassesare
initialized.Initializingaclassinvolvesinitializationofthestaticfieldsbyexecutingtheir
staticinitializerexpressionsandanystaticinitializerblocks.
Initializationofaninterfaceinvolvesexecutionofanystaticinitializerexpressionsforthe
staticfieldsdeclaredintheinterface.Aninterfacecannotspecifyinstanceinitializer
expressions,becauseithasnoinstancefields,norcanitspecifyanyinitializerblocks,
becauseitcannotbeinstantiated.
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ReviewQuestions
9.10Giventhefollowingclass,whichofthesestaticinitializerblockscanbe
independentlyinsertedat(1)?
Clickheretoviewcodeimage
publicclassMyClass{
privatestaticintcount=5;
staticfinalintSTEP=10;
booleanalive;
//(1)INSERTSTATICINITIALIZERBLOCKHERE
}
Selectthethreecorrectanswers.
(a)static{alive=true;count=0;}
(b)static{STEP=count;}
(c)static{count+=STEP;}
(d)static;
(e)static{;}
(f)static{count=1;}
9.11Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
MyClassobj=newMyClass(n);
}
staticinti=5;
staticintn;
intj=7;
intk;
publicMyClass(intm){
System.out.println(i+“,”+j+“,”+k+“,”+n+“,”+m);
}
{j=70;n=20;}//Instanceinitializerblock
static{i=50;}//Staticinitializerblock
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompile,becauseoftheinstanceinitializerblock.
(b)Thecodewillfailtocompile,becauseofthestaticinitializerblock.
(c)Thecodewillcompile,andprint50,70,0,20,0atruntime.
(d)Thecodewillcompile,andprint50,70,0,20,20atruntime.
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(e)Thecodewillcompile,andprint5,70,0,20,0atruntime.
(f)Thecodewillcompile,andprint5,70,0,20,20atruntime.
(g)Thecodewillcompile,andprint5,7,0,20,0atruntime.
(h)Thecodewillcompile,andprint5,7,0,20,20atruntime.
9.12Giventhefollowingclass,whichinstanceinitializerblockinsertedindependently
at(1)willallowtheclasstobecompiled?
publicclassFirstClass{
staticintgap=10;
doublelength;
finalbooleanactive;
//(1)INSERTCODEHERE
}
Selecttheonecorrectanswer.
(a)instance{active=true;}
(b)FirstClass{gap+=5;}
(c){gap=5;length=(active?100:200)+gap;}
(d){;}
(e){length=4.2;}
(f){active=(gap>5);length=5.5+gap;}
9.13Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassInitialization{
privatestaticStringmsg(Stringmsg){
System.out.println(msg);
returnmsg;
}
publicInitialization(){m=msg(“1”);}
{m=msg(“2”);}
Stringm=msg(“3”);
publicstaticvoidmain(String[]args){
Objectobj=newInitialization();
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,andprint1,2,and3atruntime.
(c)Theprogramwillcompile,andprint2,3,and1atruntime.
(d)Theprogramwillcompile,andprint3,1,and2atruntime.
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(e)Theprogramwillcompile,andprint1,3,and2atruntime.
9.14Whichofthelabeledlinesinthefollowingcodecanbeindependently
uncommentedbyremovingthe//characters,suchthatthecodewillstillcompile?
Clickheretoviewcodeimage
classGeomInit{
//intwidth=14;/*LineA*/
{
//area=width*height;/*LineB*/
}
intwidth=37;
{
//height=11;/*LineC*/
}
intheight,area;
//area=width*height;/*LineD*/
{
//intwidth=15;/*LineE*/
area=100;
}
}
Selectthetwocorrectanswers.
(a)LineA
(b)LineB
(c)LineC
(d)LineD
(e)LineE
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Automaticgarbagecollection,includingtheworkingsofthegarbagecollectorand
guidelinesforfacilitatinggarbagecollection
•Objectfinalizationandchainingaspartofgarbagecollection
•Staticandinstanceinitializers,bothasinitializerexpressionsandasinitializer
blocks
•Theroleplayedbyinitializersininitializingobjects,classes,andinterfaces
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10.TheArrayList<E>ClassandLambdaExpressions
10.1The Class
Aprogrammanipulatesdata,soorganizingandusingdataefficientlyarenaturally
importantinaprogram.Datastructuresarewaystoorganizedata.Javausestheterm
collectiontomeanadatastructurethatcanmaintainagroupofobjectssothattheobjects
canbemanipulatedasasingleentityorunit.Objectscanbestored,retrieved,and
manipulatedaselementsofacollection.Thetermcontainerisalsousedintheliterature
forsuchdatastructures.Arraysareanexampleofonekindofcollection.Otherexamples
includelists,sets,queues,andstacks,amongmanyothers.
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Lists
Onceanarrayiscreated,itslengthcannotbechanged.Thisinflexibilitycanbea
significantdrawbackwhentheamountofdatatobestoredinanarrayisnotknowna
priori.InJava,thestructuresknownaslistsalleviatethisshortcoming.Listsare
collectionsthatmaintaintheirelementsinorderandcancontainduplicates.Theorderof
elementsinalistispositionalorder,andindividualelementscanbeaccessedaccordingto
theirpositioninthelist.Eachelement,therefore,hasapositioninthelist.Azero-based
indexcanbeusedtoaccesstheelementatthepositiondesignatedbytheindexvalue,
analogoustoaccessingelementsinanarray.However,unlikeinanarray,thepositionof
anelementinalistcanchangeaselementsareinsertedordeletedfromthelist—thatis,as
thelistischangedstructurally.
Sortingimpliesorderingtheelementsinacollectionaccordingtosomerankingcriteria,
usuallybasedonthevaluesoftheelements.However,elementsisanArrayListare
maintainedintheordertheyareinsertedinthelist,knownastheinsertionorder.The
elementsinsuchalistarethereforeordered,buttheyarenotsorted,asitisnotthevalues
oftheelementsthatdeterminetheirrankinginthelist.Thus,orderingdoesnotnecessarily
implysorting.
TheJavaCollectionsFramework
TheCollectioninterfaceinthejava.utilpackage(alsoknownastheJava
CollectionsFramework)definesthegeneraloperationsthatacollectionshouldprovide.
OthersubinterfacesintheJavaCollectionsFrameworkaugmentthisinterfacetoprovide
specificoperationsforparticularkindsofcollections.Thejava.util.Listinterface
extendsthejava.util.Collectioninterfacewiththenecessaryoperationsto
maintainthecollectionasalist(seeFigure10.1).Inadditiontotheoperationsinherited
fromthejava.util.Collectioninterface,thejava.util.Listinterface
definesoperationsthatworkspecificallyonlists:position-basedaccessofthelist
elements,searchinginalist,operationsonpartsofalist(calledopenrange-view
operations),andcreationofcustomizediteratorstotraversealist.
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Figure10.1PartialArrayListInheritanceHierarchy
Theconcreteclassjava.util.ArrayListimplementsthejava.util.List
interface.InFigure10.1,thetypeparameterEinangularbrackets(<>)afterareference
typenameindicatesthatthereferencetypeisagenerictype.ThetypeparameterE
representsthetypeoftheelementinthecollection.Useofagenerictyperequiresa
concretereferencetypetobesubstitutedforthetypeparameterE.Examplesinthissection
willmakeamplyclearhowtouseagenerictype,andinparticular,theclassArrayList.
TheArrayListclassisadynamicallyresizableimplementationoftheListinterface
usingarrays(alsoknownasdynamicarrays),providingfastrandomaccess(i.e.,position-
basedaccessinconstanttime)andfastlisttraversal—verymuchlikeusinganordinary
array.TheArrayListclassisnotthread-safe;thatis,itsintegritycanbejeopardizedby
concurrentaccess.TheJavaCollectionsFrameworkprovidesotherimplementationsofthe
Listinterface,butinmostcasestheArrayListimplementationistheoverallbest
choiceforimplementinglists.
ThissectioncoversthebasicsofusinganArrayList.TheJavaCollectionsFramework
isanextensivetopic,farbeyondthescopeofthisbook.However,divingdeepintothe
JavaCollectionsFrameworkisabeneficialexercisethatishighlyrecommendedforall
Javaprogrammers.
DeclaringReferencesandConstructing s
Inthediscussionthatfollows,weassumethatanyclassorinterfaceusedfromthe
java.utilpackagehasbeenimportedwithanappropriateimportstatement.
Thefollowingdeclarationstatementshowshowwecandeclareareferencethatcanrefer
toanArrayListofaspecificelementtype.Italsoillustrateshowwecancreatean
emptyArrayListofaspecificelementtype,andassignitsreferencevaluetoa
reference:
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ArrayList<String>palindromes=newArrayList<String>();//(1)
Asthiscodeindicates,theelementtypeisspecifiedusingangularbrackets(<>).The
referencepalindromescanrefertoanyArrayListwhoseelementtypeisString.
ThetypeparameterEoftheclassArrayListinFigure10.1isreplacedbytheconcrete
classString.Thecompilerensuresthatthereferencepalindromescanonlyreferto
anArrayListwhoseelementsareoftypeString,andanyoperationsonthislistvia
thisreferencearetype-safe.
ThesimplestwaytoconstructanArrayLististousethedefaultconstructortocreate
anemptyArrayList,asshowninthepreviousdeclaration.Thedefaultconstructor
createsanemptylistwiththeinitialcapacityof10.Thecapacityofalistreferstohow
manyelementsitcancontainatanygiventime,nothowmanyelementsareactuallyinthe
list(calledthesize).Thecapacityofalistanditssizecanchangedynamicallyasthelistis
manipulated.TheArrayListcreatedin(1)cancontainonlyelementsoftypeString.
Theassignmentinthedeclarationstatement(1)isvalidbecausethetypesonbothsides
areassignmentcompatible—anArrayListofString.Thereferencepalindromes
cannowbeusedtomanipulatetheArrayListthatwascreated.
TheDiamondOperator:<>
Theelementtypewithintheangularbrackets(<>)canbeomittedintheArrayList
creationexpressionontheright-handsideofthedeclarationstatement.Inthisparticular
context,thecompilercaninfertheelementtypeoftheArrayListfromthedeclaration
ofthereferencetypeontheleft-handside.
Clickheretoviewcodeimage
ArrayList<String>palindromes=newArrayList<>();//Usingthediamond
operator
Theemptyangularbrackets,<>,arecommonlyreferredtoasthediamondoperator.This
operatormustbeusedwiththenewoperatorwhenconstructinganobjectofageneric
type,likeArrayList,wherethetypeinformationforitsusagecanbeinferredbythe
compilerfromthecontext,asintheprecedingdeclarationstatement.
However,ifthediamondoperatorisomitted,thecompilerwillissueanunchecked
conversionwarning,asshownat(2)inthenextcodesnippet.AnewArrayListis
createdbasedonanArrayListofIntegerthatispassedasanargumenttothe
constructor.TheArrayListofIntegeriscreatedat(1).ThereferencenewList1of
typeArrayList<String>referstoanArrayListwhoseelementtypeis
Integer,notString.Thecodeat(2)compiles,butwegeta
ClassCastExceptionatruntimeat(3)whenweretrieveanelementfromthislist.
Theget()methodcallat(3)expectsaStringintheArrayList,butgetsan
Integer.Ifthediamondoperatorisused,asshownat(4),thecompilerreportsa
compile-timeerror,andtheproblemdescribedat(3)cannotoccuratruntime.Byissuing
anuncheckedconversionwarningat(2),thecompileralertsustothefactthatitcannot
guaranteetype-safetyofthelistcreatedat(2).
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ArrayList<Integer>intList=newArrayList<>();//(1)ArrayListof
Integer
intList.add(10);intList.add(100);intList.add(1000);
ArrayList<String>newList1=newArrayList(intList);//(2)Unchecked
conversion
//warning
System.out.println(newList1.get(0));//(3)
ClassCastException!
ArrayList<String>newList2=newArrayList<>(intList);//(4)Compile-time
error!
Bestpracticesadvocateprogrammingtoaninterface.Inpracticalterms,thismeansusing
referencesofaninterfacetypetomanipulateobjectsofaconcreteclassthatimplement
thisinterface.Sincetheclassjava.util.ArrayListimplementsthe
java.util.Listinterface,thedeclaration(1)canbewrittenasshowninthenext
codesnippet.Thisdeclarationisvalid,sincethereferencevalueofasubtypeobject
(ArrayList<String>)canbeassignedtoareferenceofitssupertype
(List<String>).
Clickheretoviewcodeimage
List<String>palindromes=newArrayList<>();//(2)Listreference
Thisbestpracticeprovidesgreatflexibilityinsubstitutingotherobjectsforataskwhen
necessary.Thecurrentconcreteclasscaneasilybereplacedbyanotherconcreteclassthat
implementsthesameinterface.Onlycodecreatingobjectsneedstobechanged.Asit
happens,theJavaCollectionsFrameworkprovidesanotherimplementationoflists:the
java.util.LinkedListclass,whichalsoimplementstheListinterface.Ifthis
classisfoundtobemoreconduciveformaintainingpalindromesinalist,weneedsimply
changethenameoftheclassindeclaration(2),andcontinueusingthereference
palindromesintheprogram:
Clickheretoviewcodeimage
List<String>palindromes=newLinkedList<>();//Changingimplementation.
TheArrayListclassalsoprovidesaconstructorthatallowsanemptyArrayListto
becreatedwithaspecificinitialcapacity.
Clickheretoviewcodeimage
List<String>palindromes=newArrayList<>(20);//Initialcapacityis20.
TheArrayListclassprovidestheadd(E)methodtoappendanelementtothelist.
Thisobjectisaddedafterthelastelementinthelist,therebyincreasingthelistsizeby1.
Clickheretoviewcodeimage
palindromes.add(“level”);palindromes.add(“Ada”);palindromes.add(“kayak”);
System.out.println(palindromes);
TheprintstatementcallsthetoString()methodintheArrayListclasstoprintthe
elementsinthelist.ThistoString()methodappliesthetoString()methodofthe
individualelementstocreateatextualrepresentationinthefollowingdefaultformat:
[level,Ada,kayak]
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AthirdconstructorallowsanArrayListtobeconstructedfromanothercollection.The
followingcodecreatesalistofwordsfromalistofpalindromes.Theorderofthe
elementsinthenewArrayLististhesameasthatintheArrayListthatwaspassed
asanargumentintheconstructor.
Clickheretoviewcodeimage
List<String>wordList=newArrayList<>(palindromes);
System.out.println(wordList);//[level,Ada,kayak]
wordList.add(“Naan”);
System.out.println(wordList);//[level,Ada,kayak,Naan]
Thenextexamplesillustratethecreationofemptylistsofdifferenttypesofelements.The
compilerensuresthatoperationsontheArrayListaretype-safewithrespecttothe
elementtype.Declaration(3)showshowwecancreatenestedliststructures(i.e.,listof
lists),analogoustoanarrayofarrays.Notethatthediamondoperatorisnotnestedin(3).
Declaration(4)showsthattheelementtypecannotbeaprimitivetype;rather,itmustbea
referencetype.
Clickheretoviewcodeimage
List<StringBuilder>synonyms=newArrayList<>();//ListofStringBuilder
List<Integer>attendance=newArrayList<>();//ListofInteger
List<List<String>>listOfLists=newArrayList<>();//(3)ListofListof
String
List<int>frequencies=newArrayList<>();//(4)Compile-time
error!
WhencomparingarraysandArrayLists,thereisoneothersignificantdifferencethat
concernsthesubtyperelationship.
Clickheretoviewcodeimage
Object[]objArray=newString[10];//(5)OK!
Indeclaration(5),sinceStringisasubtypeofObject,String[]isasubtypeof
Object[].ThuswecanmanipulatethearrayofStringusingtheobjArray
reference.
Clickheretoviewcodeimage
objArray[2]=“Green”;//(6)OK!
objArray[1]=newInteger(2016);//ArrayStoreException!
Theprecedingassignmentrequiresaruntimechecktoguaranteethattheassignmentis
typecompatible.Otherwise,anArrayStoreExceptionisthrownatruntime.
FortheArrayList,thefollowingdeclarationswillnotcompile:
Clickheretoviewcodeimage
ArrayList<Object>objList1=newArrayList<String>();//(7)Compile-time
error!
List<Object>objList2=newArrayList<String>();//(8)Compile-time
error!
AlthoughStringisasubtypeofObject,itisnotthecasethatan
ArrayList<String>isasubtypeofArrayList<Object>.Ifthiswasthecase,
wecouldusetheobjList1referencetoaddothertypesofobjectstotheArrayList
ofString,therebyjeopardizingitstype-safety.Sincethereisnoinformationaboutthe
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elementtypeEavailableatruntimetocarryoutatypecompatibilitycheck,asinthecase
ofarrays,thesubtyperelationshipisnotallowedin(7).Forthesamereason,(8)willnot
compile:ArrayList<String>isnotasubtypeofList<Object>.Ingeneral,the
subtyperelationshipdoesnotholdforgenerictypes.TheJavalanguageprovideswaysto
overcomethisrestriction,butwewillnotpursuethismatterfurther.
TheArrayListconstructorsaresummarizedhere:
Clickheretoviewcodeimage
ArrayList()
ArrayList(intinitialCapacity)
ArrayList(Collection<?extendsE>c)
Thedefaultconstructorcreatesanew,emptyArrayListwithaninitialcapacity
of10.
Thesecondconstructorcreatesanew,emptyArrayListwiththespecifiedinitial
capacity.
ThethirdconstructorcreatesanewArrayListcontainingtheelementsinthe
specifiedcollection.Thedeclarationoftheparametercessentiallymeansthat
parameterccanrefertoanycollectionwhoseelementtypeisEorwhoseelement
typeisasubtypeofE.ThenewArrayListwillretainanyduplicates.The
orderingintheArrayListwillbedeterminedbythetraversalorderofthe
iteratorforthecollectionpassedasanargument.
Modifyingan
Asummaryofselectedmethodsthatcanmodifythecontentsofalistisgivenhere:
Clickheretoviewcodeimage
booleanadd(Eelement)
voidadd(intindex,Eelement)
Thefirstmethodwillappendthespecifiedelementtotheendofthelist.Itreturns
trueifthecollectionwasmodifiedasaresultoftheoperation.
Thesecondmethodinsertsthespecifiedelementatthespecifiedindex.If
necessary,itshiftstheelementpreviouslyatthisindexandanysubsequentelements
onepositiontowardtheendofthelist.Themethodwillthrowan
IndexOutOfBoundsExceptioniftheindexisoutofrange(index<0||
index>size()).
ThetypeparameterErepresentstheelementtypeofthelist.
Clickheretoviewcodeimage
booleanaddAll(Collection<?extendsE>c)
booleanaddAll(intindex,Collection<?extendsE>c)
Thefirstmethodinsertstheelementsfromthespecifiedcollectionattheendofthe
list.Thesecondmethodinsertstheelementsfromthespecifiedcollectionatthe
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specifiedindex;thatis,themethodsplicestheelementsofthespecifiedcollection
intothelistatthespecifiedindex.Themethodsreturntrueifanyelementswere
added.Elementsareinsertedusinganiteratorofthespecifiedcollection.The
secondmethodwillthrowanIndexOutOfBoundsExceptioniftheindexis
outofrange(index<0||index>size()).
Thedeclarationoftheparametercessentiallymeansthatparameterccanreferto
anycollectionwhoseelementtypeisEorwhoseelementtypeisasubtypeofE.
Eset(intindex,Eelement)
Thismethodreplacestheelementatthespecifiedindexwiththespecifiedelement.
Itreturnsthepreviouselementatthespecifiedindex.Themethodthrowsan
IndexOutOfBoundsExceptioniftheindexisoutofrange(index<0||
index>=size()).
Clickheretoviewcodeimage
Eremove(intindex)
booleanremove(Objectelement)
Thefirstmethoddeletesandreturnstheelementatthespecifiedindex.Themethod
throwsanIndexOutOfBoundsExceptioniftheindexisoutofrange(index
<0||index>=size()).
Thesecondmethodremovesthefirstoccurrenceoftheelementfromthelist,using
objectvalueequality.Themethodreturnstrueifthecallwassuccessful.
Bothmethodswillcontractthelistaccordinglyifanyelementsareremoved.
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booleanremoveAll(Collection<?>c)
booleanremoveIf(Predicate<?superE>filter)
TheremoveAll()methodremovesfromthislistallelementsthatarecontained
inthespecifiedcollection.
TheremoveIf()methodremovesfromthislistallelementsthatsatisfythe
filteringcriteriadefinedbyalambdaexpressionthatimplementsthe
Predicate<T>functionalinterface(p.452).
Bothmethodsreturntrueifthecallwassuccessful.Thelistiscontracted
accordinglyifanyelementsareremoved.
voidtrimToSize()
Thismethodtrimsthecapacityofthislisttoitscurrentsize.
voidclear()
Thismethoddeletesallelementsfromthelist.Thelistisemptyafterthecall,soit
hassize0.
AllthecodesnippetsisthissectioncanbefoundinExample10.1,p.427.Themethod
printListWithIndex()at(16)inExample10.1printstheelementsprefixedwith
theirindexinthelist,makingiteasiertoseehowthelistchangesstructurally:
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[0:level,1:Ada,2:Java,3:kayak,4:Bob,5:Rotator,6:Bob]
Wehaveseenthattheadd(E)methodappendsanelementtotheendofthelist.The
followingcodeaddsthestringsfromanarrayofStringtoanArrayListofString.
TheoutputfromExample10.1at(2)showshowtheelementsareaddedattheendofthe
list.
Clickheretoviewcodeimage
System.out.println(“\n(2)Addelementstolist:”);
for(Stringstr:wordArray){
strList.add(str);
printListWithIndex(strList);
}
Wecaninsertanewelementataspecificindexusingtheoverloadedmethodadd(int,
E).Theoutputfromthefollowingcodeshowshowinsertinganelementatindex2shifted
theelementsstructurallyinthelist.
Clickheretoviewcodeimage
//[0:level,1:Ada,2:kayak,3:Bob,4:Rotator,5:Bob]
strList.add(2,“Java”);//Insertanelementatindex2inthelist.
printListWithIndex(strList);//[0:level,1:Ada,2:Java,3:kayak,4:Bob,
//5:Rotator,6:Bob]
Notethatanindexvalueequalto0orthesizeofthelistisalwaysallowedforthemethod
add(int,E).
Clickheretoviewcodeimage
List<String>list1=newArrayList<>();//[]
list1.add(0,“First”);//[First]
list1.add(list1.size(),“Last”);//[First,Last]
Wecanreplaceanelementataspecifiedindexusingtheset(int,E)method.The
methodreturnstheelementthatwasreplaced.
Clickheretoviewcodeimage
System.out.println(“(3)Replacetheelementatindex1:”);
StringoldElement=strList.set(1,“Naan”);
System.out.println(“Elementthatwasreplaced:”+oldElement);//“Ada”
printListWithIndex(strList);//[0:level,1:Naan,2:Java,3:kayak,4:Bob,
//5:Rotator,6:Bob]
Wecanalsoremoveelementsfromalist,withthelistbeingcontractedaccordingly.
Clickheretoviewcodeimage
System.out.println(“(4)Removetheelementatindex0:”);
System.out.println(“Elementremoved:”+strList.remove(0));//“level”
printListWithIndex(strList);//[0:Naan,1:Java,2:kayak,3:Bob,4:Rotator,
5:Bob]
System.out.println(“(5)Removethefirstoccurrenceof"Java":”);
System.out.println(“Elementremoved:”+strList.remove(“Java”));//true
printListWithIndex(strList);//[0:Naan,1:kayak,2:Bob,3:Rotator,4:Bob]
Theremove(int)removestheelementatthespecifiedindex.Themethod
remove(Object)needstosearchthelistandcomparetheargumentobjectwith
elementsinthelistforobjectvalueequality.Thistestrequiresthattheargumentobject
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overridetheequals()methodfromtheObjectclass,whichmerelydetermines
referencevalueequality.TheStringclassprovidestheappropriateequals()method.
However,thefollowingcodewillnotgivetheexpectedresult,becausethe
StringBuilderclassdoesnotprovideitsownequals()method.
Clickheretoviewcodeimage
List<StringBuilder>sbList=newArrayList<>();
for(Stringstr:wordArray)
strList.add(str);
System.out.println(sbList);//[level,Ada,kayak,Bob,Rotator,Bob]
StringBuilderelement=newStringBuilder(“Ada”);
System.out.println(“Elementtoberemoved:”+element);//Ada
System.out.println(“Elementremoved:”+sbList.remove(element));//false
System.out.println(sbList);//[level,Ada,kayak,Bob,Rotator,Bob]
PrimitiveValuesandArrayLists
SinceprimitivevaluescannotbestoredinanArrayList,wecanusethewrapper
classestoboxsuchvaluesfirst.Inthefollowingcode,wecreatealistofIntegerin
whichtheintvaluesareautoboxedinIntegerobjectsandthenaddedtothelist.We
trytodeletetheelementwithvalue1,butendupdeletingtheelementatindex1instead
(i.e,thevalue20).
Clickheretoviewcodeimage
List<Integer>intList=newArrayList<>();
intList.add(10);intList.add(20);intList.add(1);
System.out.println(intList);//[10,20,1]
System.out.println(“Elementtoberemoved:”+1);//1
System.out.println(“Elementremoved:”+intList.remove(1));//20
System.out.println(intList);//[10,1]
Themethodcall
intList.remove(1)
hasthesignature
intList.remove(int)
Thissignaturematchestheoverloadedmethodthatremovestheelementataspecified
index,soitisthismethodthatiscalledatruntime.Wesaythatthismethodisthemost
specificinthiscase.Forthecodetoworkasintended,theprimitivevaluemustbe
explicitlyboxed.
Clickheretoviewcodeimage
System.out.println(intList);//[10,20,1]
System.out.println(“Elementtoberemoved:”+1);//
1
System.out.println(“Elementremoved:”+intList.remove(newInteger(1)));//
true
System.out.println(intList);//[10,20]
Themethodcall
Clickheretoviewcodeimage
intList.remove(newInteger(1))
hasthesignature
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intList.remove(Integer)
Thiscallmatchestheoverloadedremove(Object)method,sinceanIntegerobject
canbepassedtoanObjectparameter.Thismethodisthemostspecificinthiscase,and
isexecuted.
Queryingan
Asummaryofusefulmethodsthatcanbeusedtoqueryalistisprovidedhere:
intsize()
Returnsthenumberofelementscurrentlyinthelist.Inanon-emptylist,thefirst
elementisatindex0andthelastelementisatsize()-1.
booleanisEmpty()
Determineswhetherthelistisempty(i.e.,whetheritssizeis0).
Eget(intindex)
Returnstheelementatthespecifiedpositionalindex.Themethodthrowsan
IndexOutOfBoundsExceptioniftheindexisoutofrange(index<0||
index>=size()).
booleanequals(Objecto)
Comparesthespecifiedobjectwiththislistforobjectvalueequality.Itreturns
trueifandonlyifthespecifiedobjectisalsoalist,bothlistshavethesamesize,
andallcorrespondingpairsofelementsinthetwolistsareequalaccordingtoobject
valueequality.
Clickheretoviewcodeimage
booleancontains(Objectelement)
Determineswhethertheargumentobjectiscontainedinthecollection,usingobject
valueequality.Thisiscalledthemembershiptest.
intindexOf(Objecto)
intlastIndexOf(Objecto)
Returntheindexesofthefirstandlastoccurrencesoftheelementthatareequal
(usingobjectvalueequality)tothespecifiedargument,respectively,ifsuchan
elementexistsinthelist;otherwise,thevalue–1isreturned.Thesemethods
provideelementsearchinthelist.
Themethodget(int)retrievestheelementatthespecifiedindex.
Clickheretoviewcodeimage
System.out.println(“Firstelement:”+strList.get(0);//
Naan
System.out.println(“Lastelement:”+strList.get(strList.size()-1));//Bob
Theequals()methodoftheArrayListclasscanbeusedtocomparetwolistsfor
equalitywithregardtosizeandcorrespondingelementsbeingequalineachlist.
Clickheretoviewcodeimage
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List<String>strList2=newArrayList<>(strList);
booleantrueOrFalse=strList.equals(strList2);//true
Themembershiptestiscarriedoutbythecontains(Object)method.Wecanfind
theindexofaspecifiedelementinthelistbyusingtheindexOf()and
lastIndexOf()methods.
Clickheretoviewcodeimage
booleanfound=strList.contains(“Naan”);//true
intpos=strList.indexOf(“Bob”);//2
pos=strList.indexOf(“BOB”);//-1(Notfound)
pos=strList.lastIndexOf(“Bob”);//4(Lastoccurrence)
Again,thesemethodsrequirethattheelementtypeprovideameaningfulequals()
methodforobjectvalueequalitytesting.
Traversingan
Averycommontaskistotraversealistsoastoperformsomeoperationoneachelement
ofthelist.Wecanusepositionalaccesstotraversealistwiththefor(;;)loop.The
genericmethodprintListWithIndex()inExample10.1usesthefor(;;)loopto
createanewArrayListofStringthatcontainseachelementoftheargumentlist
prefixedwiththeindexoftheelement.
Clickheretoviewcodeimage
publicstatic<E>voidprintListWithIndex(List<E>list){
List<String>newList=newArrayList<>();
for(inti=0;i<list.size();i++){
newList.add(i+“:”+list.get(i));
}
System.out.println(newList);
}
SampleoutputfromthemethodcallprintWithIndex(strList)isshownhere:
Clickheretoviewcodeimage
[0:level,1:Ada,2:kayak,3:Bob,4:Rotator,5:Bob]
ThemethodprintListWithIndex()inExample10.1canprintanylistinthis
format.ItsheaderdeclarationsaysthatitacceptsalistofelementtypeE.Theelement
typeEisdeterminedfromthemethodcall.Intheprecedingexample,Eisdeterminedto
beString,asaListofStringispassedinthemethodcall.
SincetheArrayListclassimplementstheIterableinterface(i.e.,theclassprovides
aniterator),wecanusethefor(:)looptotraversealist.
Clickheretoviewcodeimage
for(Stringstr:strList){
System.out.print(str+”“);
}
TheArrayListalsoprovidesspecializediteratorstotraversealist,andthediligent
readerisencouragedtomaketheiracquaintanceathisorherownleisure.
Onepertinentquestiontoaskishowtoremoveelementsfromthelistwhentraversingthe
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list.Thefor(:)loopdoesnotallowtheliststructuretobemodified:
Clickheretoviewcodeimage
for(Stringstr:strList){
if(str.length()<=3){
strList.remove(str);//Throws
ConcurrentModificationException
}
}
Wecanusepositionalaccessinalooptotraversethelist,butmustbecarefulinupdating
theloopvariable,asthelistcontractswhenanelementisremoved.Abettersolutionisto
usetheArrayListmethodremoveIf(),passingthecriteriaforselectionasan
argument(p.452).Aniteratorcanalsobeusedexplicitlyforthispurpose,butweleave
thatasanexercise.
Convertingan toanArray
ThetwofollowingmethodscanbeusedtoconvertanArrayListtoanarray:
Object[]toArray()
<T>T[]toArray(T[]a)
ThefirstmethodreturnsanarrayoftypeObjectfilledwithalltheelementsofa
collection.
Thesecondmethodisagenericmethodthatstorestheelementsofacollectioninan
arrayoftypeT.Ifthespecifiedarrayisbigenough,theelementsarestoredinthis
array.Ifthereisroomtospareinthearray—thatis,ifthelengthofthearrayis
greaterthanthenumberofelementsinthecollection—theelementfound
immediatelyafterstoringtheelementsofthecollectionissettothenullvalue
beforethearrayisreturned.Ifthearrayistoosmall,anewarrayoftypeTand
appropriatesizeiscreated.IfTisnotasupertypeoftheruntimetypeofevery
elementinthecollection,anArrayStoreExceptionisthrown.
TheactualelementtypeoftheelementsintheObjectarrayreturnedbythefirst
toArray()methodcanbeanysubtypeofObject.Itmaybenecessarytocastthe
Objectreferenceofanelementtotheappropriatetype,asinthefollowingcode:
Clickheretoviewcodeimage
System.out.println(“(14)Convertlisttoarray:”);
Object[]objArray=strList.toArray();//Object[]
System.out.println(“Object[]length:”+objArray.length);//5
System.out.print(“LengthofeachstringintheObjectarray:“);
for(Objectobj:objArray){
Stringstr=(String)obj;//Castrequired.
System.out.print(str.length()+”“);
}
System.out.println();
ThesecondtoArray()methodreturnsanarrayoftypeT,whenitispassedanarrayof
typeTasargument.Inthefollowingcode,thearrayofStringreturnedhasthesame
lengthasthesizeofthelistofString,eventhoughaStringarrayoflength0was
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passedasargument:
Clickheretoviewcodeimage
String[]strArray=strList.toArray(newString[0]);//String[]
System.out.println(“String[]length:”+strArray.length);//5
System.out.print(“LengthofeachstringintheStringarray:“);
for(Stringstr:strArray){
System.out.print(str.length()+”“);
}
System.out.println();
Sortingan
Thefollowingstaticmethodofthejava.util.Collectionsclasscanbeusedto
sortelementsofalist.Notethenameoftheclass.Italsoprovidesmanyusefulutility
methodsforcollections.
Clickheretoviewcodeimage
static<TextendsComparable<?superT>>voidsort(List<T>list)
Thisgenericmethodsortsthespecifiedlistintoascendingorder,accordingtothe
naturalorderingofitselements.Thedeclarationessentiallysaysthatelementsof
thelisthavetobecomparable;inotherwords,theycanbecomparedwiththe
compareTo()methodoftheComparableinterface.
Hereisanexampleofsortingalist:
Clickheretoviewcodeimage
System.out.println(“Unsortedlist:”+strList);//[Naan,kayak,Bob,Rotator,
Bob]
Collections.sort(strList);
System.out.println(“Sortedlist:”+strList);//[Bob,Bob,Naan,Rotator,
kayak]
TheStringclassimplementstheComparableinterface,anditsnaturalorderingis
lexicographicalordering,basedontheUnicodevaluesofthecharactersinthestring.
Arraysversus
Table10.1summarizesthedifferencesbetweenarraysandArrayLists.
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Table10.1SummaryofArraysversusArrayLists
Example10.1UsinganArrayList
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.Collections;
importjava.util.List;
publicclassUsingArrayList{
publicstaticvoidmain(String[]args){
String[]wordArray={“level”,“Ada”,“kayak”,“Bob”,“Rotator”,“Bob”
};
System.out.println(“(1)Createanemptylistofstrings:”);
List<String>strList=newArrayList<>();
printListWithIndex(strList);
System.out.println(“\n(2)Addelementstolist:”);
for(Stringstr:wordArray){
strList.add(str);
printListWithIndex(strList);
}
System.out.println(“Insertanelementatindex2inthelist:”);
strList.add(2,“Java”);
printListWithIndex(strList);
System.out.println(“\n(3)Replacetheelementatindex1:”);
StringoldElement=strList.set(1,“Naan”);
System.out.println(“Elementthatwasreplaced:”+oldElement);
printListWithIndex(strList);
System.out.println(“\n(4)Removetheelementatindex0:”);
System.out.println(“Elementremoved:”+strList.remove(0));
printListWithIndex(strList);
System.out.println(“\n(5)Removethefirstoccurrenceof"Java":”);
System.out.println(“Elementremoved:”+strList.remove(“Java”));
printListWithIndex(strList);
System.out.println(“\n(6)Determinethesizeofthelist:”);
System.out.println(“Thesizeofthelistis”+strList.size());
System.out.println(“\n(7)Determineifthelistisempty:”);
booleanresult=strList.isEmpty();
System.out.println(“Thelist”+(result?“is”:“isnot”)+”empty.”);
System.out.println(“\n(8)Gettheelementatspecificindex:”);
System.out.println(“Firstelement:”+strList.get(0));
System.out.println(“Lastelement:”+strList.get(strList.size()-1));
System.out.println(“\n(9)Comparetwolists:”);
List<String>strList2=newArrayList<>(strList);
booleantrueOrFalse=strList.equals(strList2);
System.out.println(“ThelistsstrListandstrList2are”
+(trueOrFalse?””:”not”)+”equal.”);
strList2.add(null);
printListWithIndex(strList2);
trueOrFalse=strList.equals(strList2);
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System.out.println(“ThelistsstrListandstrList2are”
+(trueOrFalse?””:”not”)+”equal.”);
System.out.println(“\n(10)Membershiptest:”);
booleanfound=strList.contains(“Naan”);
Stringmsg=found?“contains”:“doesnotcontain”;
System.out.println(“Thelist”+msg+”thestring"Naan".”);
System.out.println(“\n(11)Findtheindexofanelement:”);
intpos=strList.indexOf(“Bob”);
System.out.println(“Theindexofstring"Bob"is:”+pos);
pos=strList.indexOf(“BOB”);
System.out.println(“Theindexofstring"BOB"is:”+pos);
pos=strList.lastIndexOf(“Bob”);
System.out.println(“Thelastindexofstring"Bob"is:”+pos);
printListWithIndex(strList);
System.out.println(“\n(12)Traversingthelistusingthefor(;;)loop:”);
for(inti=0;i<strList.size();i++){
System.out.print(i+“:”+strList.get(i)+”“);
}
System.out.println();
System.out.println(“\n(13)Traversingthelistusingthefor(:)loop:”);
for(Stringstr:strList){
System.out.print(str+”“);
//strList.remove(str);//Throws
ConcurrentModificationException.
}
System.out.println();
System.out.println(“\n(14)Convertlisttoarray:”);
Object[]objArray=strList.toArray();
System.out.println(“Object[]length:”+objArray.length);
System.out.print(“LengthofeachstringintheObjectarray:“);
for(Objectobj:objArray){
Stringstr=(String)obj;//Castrequired.
System.out.print(str.length()+”“);
}
System.out.println();
String[]strArray=strList.toArray(newString[0]);
System.out.println(“String[]length:”+strArray.length);
System.out.print(“LengthofeachstringintheStringarray:“);
for(Stringstr:strArray){
System.out.print(str.length()+”“);
}
System.out.println();
System.out.println(“\n(15)Sortingalist:”);
List<StringBuilder>sbList=newArrayList<>();
for(Stringstr:strArray){
sbList.add(newStringBuilder(str));
}
//Collections.sort(sbList);//Compile-timeerror!
System.out.println(“Unsortedlist:”+strList);
Collections.sort(strList);
System.out.println(“Sortedlist:”+strList);
}
/**
*Printtheelementsofalist,togetherwiththeirindex:
*[0:value0,1:value1,…]
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*@paramlistListtoprintwithindex
*/
publicstatic<E>voidprintListWithIndex(List<E>list){//
(16)
List<String>newList=newArrayList<>();
for(inti=0;i<list.size();i++){
newList.add(i+“:”+list.get(i));
}
System.out.println(newList);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
(1)Createanemptylistofstrings:
[]
(2)Addelementstolist:
[0:level]
[0:level,1:Ada]
[0:level,1:Ada,2:kayak]
[0:level,1:Ada,2:kayak,3:Bob]
[0:level,1:Ada,2:kayak,3:Bob,4:Rotator]
[0:level,1:Ada,2:kayak,3:Bob,4:Rotator,5:Bob]
Insertanelementatindex2inthelist:
[0:level,1:Ada,2:Java,3:kayak,4:Bob,5:Rotator,6:Bob]
(3)Replacetheelementatindex1:
Elementthatwasreplaced:Ada
[0:level,1:Naan,2:Java,3:kayak,4:Bob,5:Rotator,6:Bob]
(4)Removetheelementatindex0:
Elementremoved:level
[0:Naan,1:Java,2:kayak,3:Bob,4:Rotator,5:Bob]
(5)Removethefirstoccurrenceof“Java”:
Elementremoved:true
[0:Naan,1:kayak,2:Bob,3:Rotator,4:Bob]
(6)Determinethesizeofthelist:
Thesizeofthelistis5
(7)Determineifthelistisempty:
Thelistisnotempty.
(8)Gettheelementatspecificindex:
Firstelement:Naan
Lastelement:Bob
(9)Comparetwolists:
ThelistsstrListandstrList2areequal.
[0:Naan,1:kayak,2:Bob,3:Rotator,4:Bob,5:null]
ThelistsstrListandstrList2arenotequal.
(10)Membershiptest:
Thelistcontainsthestring“Naan”.
(11)Findtheindexofanelement:
Theindexofstring“Bob”is:2
Theindexofstring“BOB”is:-1
Thelastindexofstring“Bob”is:4
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[0:Naan,1:kayak,2:Bob,3:Rotator,4:Bob]
(12)Traversingthelistusingthefor(;;)loop:
0:Naan1:kayak2:Bob3:Rotator4:Bob
(13)Traversingthelistusingthefor(:)loop:
NaankayakBobRotatorBob
(14)Convertlisttoarray:
Object[]length:5
LengthofeachstringintheObjectarray:45373
String[]length:5
LengthofeachstringintheStringarray:45373
(15)Sortingalist:
Unsortedlist:[Naan,kayak,Bob,Rotator,Bob]
Sortedlist:[Bob,Bob,Naan,Rotator,kayak]
ReviewQuestions
10.1Whichstatementistrueaboutthejava.util.ArrayListclass?
Selecttheonecorrectanswer.
(a)Themethoddelete()canbeusedtodeleteanelementataspecificindexin
anArrayList.
(b)ThemethoddeleteAll()canbeusedtodeleteallelementsinan
ArrayList.
(c)Themethodinsert()canbeusedtoinsertanelementataspecificindexin
anArrayList.
(d)Themethodappend()canbeusedtoappendanelementattheendofan
ArrayList.
(e)Themethodreplace()canbeusedtoreplacetheelementataspecificindex
withanotherelementinanArrayList.
(f)Themethodfind()canbeusedtodeterminewhetheranelementisinan
ArrayList.
(g)Themethodcapacity()canbeusedtodeterminethecurrentcapacityofan
ArrayList.
(h)Noneoftheabove.
10.2Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassRQ12A10{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
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strList.add(“Anna”);strList.add(“Ada”);strList.add(“Ada”);
strList.add(“Bob”);strList.add(“Bob”);strList.add(“Adda”);
for(inti=0;i<strList.size();/*empty*/){
if(strList.get(i).length()<=3){
strList.remove(i);
}else{
++i;
}
}
System.out.println(strList);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowanIndexOutOfBoundsExceptionatruntime.
(c)TheprogramwillthrowaConcurrentModificationExceptionat
runtime.
(d)Theprogramwillnotterminatewhenrun.
(e)Theprogramwillprint:[Anna,Adda].
(f)Theprogramwillprint:[Anna,Ada,Bob,Adda].
10.3Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassRQ12A15{
publicstaticvoidmain(String[]args){
doIt1();doIt2();
}
publicstaticvoiddoIt1(){
List<StringBuilder>sbListOne=newArrayList<>();
sbListOne.add(newStringBuilder(“Anna”));
sbListOne.add(newStringBuilder(“Ada”));
sbListOne.add(newStringBuilder(“Bob”));
List<StringBuilder>sbListTwo=newArrayList<>(sbListOne);
sbListOne.add(null);
sbListTwo.get(1).reverse();
System.out.println(sbListOne);//
(1)
}
publicstaticvoiddoIt2(){
List<String>listOne=newArrayList<>();
listOne.add(“Anna”);listOne.add(“Ada”);listOne.add(“Bob”);
List<String>listTwo=newArrayList<>(listOne);
StringstrTemp=listOne.get(0);
listOne.set(0,listOne.get(listOne.size()-1));
listOne.set(listOne.size()-1,strTemp);
System.out.println(listTwo);//
(2)
}
}
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Selectthetwocorrectanswers.
(a)(1)willprint[Anna,Ada,Bob,null].
(b)(1)willprint[Anna,adA,Bob,null].
(c)(2)willprint[Anna,Ada,Bob].
(d)(2)willprint[Bob,Ada,Anna].
(e)TheprogramwillthrowanIndexOutOfBoundsExceptionatruntime.
10.4Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassRQ12A20{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
strList.add(“Anna”);strList.add(“Ada”);strList.add(null);
strList.add(“Bob”);strList.add(“Bob”);strList.add(“Adda”);
for(inti=0;i<strList.size();++i){
if(strList.get(i).equals(“Bob”)){
System.out.print(i);
}
}
System.out.println();
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowanIndexOutOfBoundsExceptionatruntime.
(c)TheprogramwillthrowaNullPointerExceptionatruntime.
(d)Theprogramwillprint:34.
10.5Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassRQ12A30{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
strList.add(“Anna”);strList.add(“Ada”);
strList.add(“Bob”);strList.add(“Bob”);
for(inti=0;i<strList.size();++i){
if(strList.get(i).equals(“Bob”)){
strList.remove(i);
}
}
System.out.println(strList);
}
}
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Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowanIndexOutOfBoundsExceptionatruntime.
(c)TheprogramwillthrowaNullPointerExceptionatruntime.
(d)TheprogramwillthrowaConcurrentModificationExceptionat
runtime.
(e)Theprogramwillnotterminatewhenrun.
(f)Theprogramwillprint[Anna,Ada,Bob].
(g)Theprogramwillprint[Anna,Ada].
10.6Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassRQ12A40{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
strList.add(“Anna”);strList.add(“Ada”);strList.add(null);
strList.add(“Bob”);strList.add(“Bob”);strList.add(“Adda”);
while(strList.remove(“Bob”));
System.out.println(strList);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowaNullPointerExceptionatruntime.
(c)Theprogramwillnotterminatewhenrun.
(d)Theprogramwillprint:[Anna,Ada,Adda].
(e)Theprogramwillprint:[Anna,Ada,Bob,Adda].
(f)Theprogramwillprint:[Anna,Ada,null,Adda].
(g)Theprogramwillprint:
[Anna,Ada,null,Bob,Adda]
[Anna,Ada,null,Adda]
10.2LambdaExpressions
Inmanyways,Java8representsawatershedinthehistoryofthelanguage.BeforeJava8,
thelanguagesupportedonlyobject-orientedprogramming.Packingstateandbehaviorinto
objectsthatcommunicateinaproceduralmannerwastheorderoftheday.Java8brings
functional-styleprogrammingintothelanguage,wherecoderepresentingfunctionalitycan
bepassedasvaluestotailorthebehaviorofmethods.
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Thissectiongivesatasteofwhatfunctional-styleprogrammingbringstoJava.Itprovides
anintroductiontotwolanguagefeatures(functionalinterfacesandlambdaexpressions)
thatarethebasisforthisprogrammingstyleinJava.Thatsufficesforthepurposesofthis
book.However,functional-styleprogrammingisapowerfulparadigmworthexploring
further,andanyJavaprogrammerwhoignoresitdoessoathisorherownperil.
BehaviorParameterization
Todemonstratehowfunctional-styleprogrammingcanbeuseful,wewillusearunning
examplethatwewillgraduallyrefinetoimplementanefficientandconcisesolutionusing
lambdaexpressionsandfunctionalinterfaces.
Acommonoperationonelementsinacollectionistoselectthoseelementsthatsatisfya
certaincriterion.Thisoperationiscalledfiltering.Example10.2createsalistofstringsat
(1).Wewouldliketofilterthislistforone-wordpalindromes,wordsthatarespelledthe
samewayforwardandbackward.Forexample,"anana"isapalindrome,but
"banana"isnot.
FilteringUsingCustomizedMethods
Astraightforwardandnaivesolutionistoimplementacustomizedmethodthattakesalist
ofwordsandreturnsalistofthewordsthatfulfillthecriteriaforbeingapalindrome.The
methodfilterPalindromes()at(4)inExample10.2doesexactlythat.Ittraverses
thelistandselectselementswiththefollowingcode:
Clickheretoviewcodeimage
if(word.equals(newStringBuilder(word).reverse().toString())){//(5)
result.add(word);
}
Theargumenttotheequals()methodcreatesaStringBuilderbasedonthe
currentwordinthelist.ThecontentsoftheStringBuilderarereversedandconverted
backtoaString.Theoriginalwordiscomparedforobjectvalueequalitywiththe
reversedwordtodeterminewhetheritisapalindrome.Becausetheequalitytestisbased
ontheUnicodevaluesofthecharacters,thiscriteriontakesintoaccountthecaseofthe
letters,ascanbeseenfromtheoutput.
Whatifwewantedtocreateafilterthatwascaseinsensitive?Weneedtoimplementa
newmethod,asshownin(6).WeseethatthemethodequalsIgnoreCase()usedfor
equalitycomparisonat(7)ignoresthecase,andtheoutputbearsthisout.
Clickheretoviewcodeimage
if(word.equalsIgnoreCase(
newStringBuilder(word).reverse().toString())){//(7)
result.add(word);
}
Creatinganewmethodeverytimewewanttofilteronanewcriterioniscertainlynot
viable.Thereislotofcodeduplication,andthemostsignificantchangeoccursonlyinthe
conditionoftheifstatementat(5)and(7).
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Example10.2ImplementingCustomizedMethodsforFilteringanArrayList
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassFunWithPalindromesV0{
publicstaticvoidmain(String[]args){
//Createalistof
words:(1)
List<String>words=newArrayList<>();
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
System.out.println(“Listofwords:”+words);
//Callamethodtofilterthelistforpalindromes(case
sensitive).(2)
List<String>palindromes=filterPalindromes(words);
System.out.println(“Case-sensitivepalindromes:”+palindromes);
//Callamethodtofilterthelistforpalindromes(caseinsensitive).
(3)
List<String>palindromesIgnoreCase=filterPalindromesIgnorecase(words);
System.out.println(“Case-insensitivepalindromes:”+
palindromesIgnoreCase);
}
/**(4)
*Findspalindromesinalistofwords.Usescase-sensitivefiltering.
*@paramwordsListofstrings
*@returnListofpalindromesfound
*/
publicstaticList<String>filterPalindromes(List<String>words){
List<String>result=newArrayList<>();
for(Stringword:words){
if(word.equals(newStringBuilder(word).reverse().toString())){//
(5)
result.add(word);
}
}
returnresult;
}
/**(6)
*Findspalindromesinalistofwords.Usescase-insensitivefiltering.
*@paramwordsListofstrings
*@returnListofpalindromesfound
*/
publicstaticList<String>filterPalindromesIgnorecase(List<String>words)
{
List<String>result=newArrayList<>();
for(Stringword:words){
if(word.equalsIgnoreCase(
newStringBuilder(word).reverse().toString())){//
(7)
result.add(word);
}
}
returnresult;
}
}
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Outputfromtheprogram:
Clickheretoviewcodeimage
Listofwords:[Otto,ADA,Alyla,Bob,HannaH,Java]
Case-sensitivepalindromes:[ADA,HannaH]
Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
FilteringUsinganInterface
InExample10.3,wehavegeneralizedthemethodthatappliesthecriteriaandselectsthe
elements.ThemethodfilterStrings()at(9)takesasinputalistandaselection
criteriaobjectthatimplementsaninterface,andreturnsalistwithelementsthatsatisfythe
selectioncriteria.
Thebestpracticeofprogrammingtoaninterfacecertainlymakessensehere.Thatwaythe
filterStrings()methodcanbeusedforfilteringonanycriteria.Anappropriate
objectforfilteringcanbepassedtothemethodaslongasitimplementstheinterface.This
objectmustimplementtheStrPredicateinterface,declaredat(1),supplyingthe
booleanmethodtest()thatactuallydetermineswhetheranelementisselected.The
test()methodisanexampleofapredicate,afunctionthattakesanargumentand
returnsabooleanvalue.
Examples10.3showstwoapproachestohowthecriteriaobjectcanbecreated.Wecan
defineconcreteclassesthatimplementtheStrPredicateinterface.Theclasses
PalindromeCaseSensitiveandPalindromeCaseInsensitiveat(2)and(3)
inExample10.3,respectively,implementtheStrPredicateinterfaceandprovidean
implementationofthetest()method.Objectsoftheseclassesarepassedasarguments
tothefilterStrings()methodat(5)and(6)tofilterthelist.Againnotethatany
newselectioncriterionimplementedbythisapproachrequiresanewconcreteclass,
resultinginproliferationofclasses,whenallthatneedschangingisthecodeinthebodyof
thetest()method.
Insteadofconcreteclasses,wecanuseanonymousclassestoinstantiatethecriteriaobject,
asshownat(7)and(8)inExample10.3.Wewillnotgointodetailsofexplainingthe
syntaxofananonymousclass.Thebasicideaisthatwecandeclareandinstantiatethe
classatthesametime,whereitisneededinthecode—inourcase,asanargumentinthe
calltothefilterStrings()method.Thenewoperatorcreatesanobjectofthe
anonymousclasswhosebodyresemblesthatofaconcreteclass.Thenameoftheinterface
inthedeclarationensuresthatthisclasswillimplementthemethodsoftheinterface.In
ourcase,theanonymousclassesprovideimplementationofthetest()method.By
usinganonymousclassesweavoidcreatingconcreteclasses,buttheverbosityofdeclaring
anonymousclassestoencapsulateasinglemethodisinescapable.Also,westillhaveto
declareanewanonymousclassforeachselectioncriterion,duplicatingalotofboilerplate
code.
Example10.3ImplementinganInterfaceforFilteringanArrayList
Clickheretoviewcodeimage
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/**Interfacetotesttwostringsaccordingtoacriterion.*/
publicinterfaceStrPredicate{//
(1)
booleantest(Stringstr);
}
/**Testswhetherastringisapalindrome(casesensitive).*/
publicclassPalindromeCaseSensitiveimplementsStrPredicate{//
(2)
@Overridepublicbooleantest(Stringstr){
returnstr.equals(newStringBuilder(str).reverse().toString());
}
}
/**Testswhetherastringisapalindrome(caseinsensitive).*/
publicclassPalindromeCaseInsensitiveimplementsStrPredicate{//
(3)
@Overridepublicbooleantest(Stringstr){
returnstr.equalsIgnoreCase(newStringBuilder(str).reverse().toString());
}
}
importjava.util.ArrayList;
importjava.util.List;
publicclassFunWithPalindromesV1{
publicstaticvoidmain(String[]args){
//Createalistof
words:(4)
List<String>words=newArrayList<>();
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
System.out.println(“Listofwords:”+words);
//Useaclasstofilterforpalindromes(case
sensitive).(5)
List<String>palindromes1=filterStrings(words,
newPalindromeCaseSensitive());
System.out.println(“Case-sensitivepalindromes:”+palindromes1);
//Useaclasstofilterforpalindromes(case
insensitive).(6)
List<String>palindromes2=filterStrings(words,
new
PalindromeCaseInsensitive());
System.out.println(“Case-insensitivepalindromes:”+palindromes2);
//Useananonymousclasstofilterforpalindromes(case
sensitive).(7)
List<String>palindromes3=filterStrings(words,
newStrPredicate(){
@Override
publicbooleantest(Stringstr){
returnstr.equals(newStringBuilder(str).reverse().toString());
}
}
);
System.out.println(“Case-sensitivepalindromes:”+palindromes3);
//Useananonymousclasstofilterforpalindromes(caseinsensitive).
(8)
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List<String>palindromes4=filterStrings(words,
newStrPredicate(){
@Override
publicbooleantest(Stringstr){
returnstr.equalsIgnoreCase(
newStringBuilder(str).reverse().toString());
}
}
);
System.out.println(“Case-insensitivepalindromes:”+palindromes4);
}
/**
*Filtersalistofstringsaccordingtothecriteriaofthepredicate.
*@paramstrListListofstringstofilter
*@parampredicateProvidesthecriteriaforfilteringthestrings
*@returnListofstringsthatmatchthecriteria
*/
publicstaticList<String>filterStrings(List<String>strList,//
(9)
StrPredicatepredicate){
List<String>result=newArrayList<>();
for(Stringstr:strList){
if(predicate.test(str)){//
(10)
result.add(str);
}
}
returnresult;
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Listofwords:[Otto,ADA,Alyla,Bob,HannaH,Java]
Case-sensitivepalindromes:[ADA,HannaH]
Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
Case-sensitivepalindromes:[ADA,HannaH]
Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
FilteringUsingLambdaExpressions
Ideallywewouldliketopassthecodefortheselectioncriteriaasanargumenttothe
filterStrings()methodsothatthemethodcanapplythecriteriatotheelementsin
thelist;thatis,wewouldliketobeabletochangethebehaviorofthe
filterStrings()methoddependingontheselectioncriteria.Example10.4isastep
inthatdirection.
TheStrPredicateinterfaceandthefilterStrings()methodarestillthesame
asinExample10.3,buttheimplementationoftheStrPredicateinterfaceat(2)and
(4)inExample10.4isspecifiedbylambdaexpressions.
TheStrPredicateinterfaceisanexampleofafunctionalinterface(p.442).Suchan
interfacehasexactlyoneabstractmethod.InthecaseoftheStrPredicatemethod,
thismethodisthetest()method,whichtakesaStringobjectasanargumentand
returnsabooleanvalue.KnowingthatsomethingisaStrPredicate,allthe
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informationaboutitssoleabstractmethodcanbeinferred,asitistheonlyabstractmethod
intheinterface;inotherwords,weknowitsname,itsparameters,anyvalueitreturns,and
whetheritthrowsanyexceptions.
Implementationofthesoleabstractmethodofafunctionalinterfacecanbeprovidedbya
lambdaexpression(p.444),incontrasttoaconventionalmethodimplementationin
concreteandanonymousclasses,asseeninExample10.3.Theassignmentat(2)in
Example10.4usesalambdaexpressiontoprovideanimplementationforthe
StrPredicatefunctionalinterface:
Clickheretoviewcodeimage
StrPredicatepredicate1=(Stringstr)->
str.equals(newStringBuilder(str).reverse().toString());//(2)
Thereferencepredicate1ontheleft-handsideisoftypeStrPredicate,anditis
assignedthevalueofthelambdaexpressionontheright-handside.Alambdaexpression
hasthreeparts:
•Aparameterlistthatisanalogoustotheparameterlistofamethod.In(2),the
parameterlisthasonlyoneparameter:
(Stringstr)
•The->operator(thearrow)thatseparatestheparameterlistfromthelambdabody.
•Alambdabodythatiseitherasingleexpressionorastatementblock.In(2),the
lambdabodyisasingleexpression,whosevalueisreturnedwhenthelambda
expressionisevaluated.Thelambdabodyexpressionevaluatestoaboolean
value,sincethecalltotheequals()methodreturnsabooleanvalue.
Clickheretoviewcodeimage
str.equals(newStringBuilder(str).reverse().toString());//Lambdabody
expression
Thelambdaexpressionat(2)definesanamelessfunctionthattakesaStringastheonly
parameter,andreturnsabooleanvalue.Recallthatthetest()methodofthe
StrPredicatefunctionalinterfacedoesexactlythat.
Lambdaexpressionsarealsocalledanonymousfunctions,astheydonothavenames.
Theycanbestoredasvaluesinreferences,asinthereferencepredicate1in(2).The
compilercantypecheckthatthelambdaexpressionisassignabletothereferenceonthe
left-handside,sincetheexpressionrepresentsananonymousfunctionthatiscompatible
withthesoleabstractmethodtest()oftheStrPredicateinterface.
Thelambdaexpressionat(2)ispassedasanargumenttothefilterStrings()
methodviathereferencepredicate1at(3).Itisexecutedonlywhenthetest()
methodiscalledwithaStringargumentinthefilterStrings()methodat(7).
NowwejustneedtopassanewlambdaexpressiontothefilterStrings()method
tofilteralistofstringsbasedonaselectioncriterion.Example10.4ismoreprecise,
concise,andreadablethanthepreviousversions,especiallyasyoubecomecomfortable
withlambdaexpressions.
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Example10.4User-DefinedFunctionalInterfaceforFilteringanArrayList
Clickheretoviewcodeimage
/**Interfacetotesttwostringsaccordingtoacriterion.*/
publicinterfaceStrPredicate{//
(1)
booleantest(Stringstr);
}
importjava.util.ArrayList;
importjava.util.List;
publicclassFunWithPalindromesV2{
publicstaticvoidmain(String[]args){
//Createalistofwords:
List<String>words=newArrayList<>();
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
System.out.println(“Listofwords:”+words);
StrPredicatepredicate1=(Stringstr)->
str.equals(newStringBuilder(str).reverse().toString());//
(2)
List<String>palindromes1=filterStrings(words,predicate1);//
(3)
System.out.println(“Case-sensitivepalindromes:”+palindromes1);
StrPredicatepredicate2=(Stringstr)->
str.equalsIgnoreCase(newStringBuilder(str).reverse().toString());//
(4)
List<String>palindromes2=filterStrings(words,predicate2);//
(5)
System.out.println(“Case-insensitivepalindromes:”+palindromes2);
}
/**//
(6)
*Filtersalistofstringsaccordingtothecriteriaofthepredicate.
*@paramstrListListofstringstofilter
*@parampredicateProvidesthecriteriaforfilteringthestrings
*@returnListofstringsthatmatchthecriteria
*/
publicstaticList<String>filterStrings(List<String>strList,
StrPredicatepredicate){
List<String>result=newArrayList<>();
for(Stringstr:strList){
if(predicate.test(str)){//
(7)
result.add(str);
}
}
returnresult;
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Listofwords:[Otto,ADA,Alyla,Bob,HannaH,Java]
Case-sensitivepalindromes:[ADA,HannaH]
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Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
FilteringUsingthePredicate<T>FunctionalInterface
Example10.4usedauser-definedfunctionalinterfaceStrPredicate.TheJavaSE
platformAPIprovidesfunctionalinterfacesformanytasks(suchastestinganobjectand
creatingobjects).Forimplementingpredicates,wecanusethegeneric
java.util.function.Predicate<T>functionalinterface,whichspecifiesthe
single-parameterbooleanmethodtest(Tt).Itisamoregeneralizedversionofthe
StrPredicatefunctionalinterfacethatwehaveusedearlier.
Example10.5usesthegenericPredicate<T>functionalinterface,wherethetype
parameterTrepresentstheconcretetypeString.InExample10.4,wesimplyneedto
replacethenameStrPredicatewiththenamePredicate<String>in(2),(4),
and(6),andimportthePredicate<T>functionalinterfacefromthe
java.util.functionpackage((1)inExample10.5).Thediscussionoftype
checkingandevaluationoflambdaexpressionsinExample10.4alsoappliestoExample
10.5.
Functionalinterfacesandlambdaexpressionstogetherallowbehaviorparameterization,a
powerfulprogrammingparadigmthatallowscoderepresentingbehaviortobepassed
aroundasvalues,andexecutedwhentheabstractmethodofthefunctionalinterfaceis
invoked.Thisapproachisscalable,requiringonlyalambdaexpressiontorepresentthe
filteringcriteria.
Functional-styleprogrammingisalsobeneficialindevelopingparallelcode.Inthe
multicoreworldinwhichwelive,wecanuseallthehelpwecangettoutilizethe
computingpowerthatnewhardwarearchitecturesareputtingatourdisposal.
Therestofthissectionprovidesmoreinsightintothesetwofeaturesofthelanguage:
functionalinterfacesandlambdaexpressions.
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Example10.5UsingthePredicate<T>FunctionalInterfaceforFilteringan
ArrayList
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;//
(1)
publicclassFunWithPalindromesV3{
publicstaticvoidmain(String[]args){
List<String>words=newArrayList<>();
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
System.out.println(“Listofwords:”+words);
Predicate<String>predicate1=(Stringstr)->
str.equals(newStringBuilder(str).reverse().toString());//
(2)
List<String>palindromes1=filterStrings(words,predicate1);//
(3)
System.out.println(“Case-sensitivepalindromes:”+palindromes1);
Predicate<String>predicate2=(Stringstr)->
str.equalsIgnoreCase(newStringBuilder(str).reverse().toString());//
(4)
List<String>palindromes2=filterStrings(words,predicate2);//
(5)
System.out.println(“Case-insensitivepalindromes:”+palindromes2);
}
/**//
(6)
*Filtersalistofstringsaccordingtothecriteriaofthepredicate.
*@paramstrListListofstringstofilter
*@parampredicateProvidesthecriteriaforfilteringthestrings
*@returnListofstringsthatmatchthecriteria
*/
publicstaticList<String>filterStrings(List<String>strList,
Predicate<String>predicate){
List<String>result=newArrayList<>();
for(Stringstr:strList){
if(predicate.test(str)){//
(7)
result.add(str);
}
}
returnresult;
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Listofwords:[Otto,ADA,Alyla,Bob,HannaH,Java]
Case-sensitivepalindromes:[ADA,HannaH]
Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
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FunctionalInterfaces
Afunctionalinterfacecanhaveonlyoneabstractmethod.Thisabstractmethodiscalled
thefunctionalmethodforthatinterface.Likeanyotherinterface,afunctionalinterfacecan
haveanynumberofstaticanddefaultmethods.Suchaninterfacecanalsoprovideexplicit
publicabstractmethoddeclarationsfornon-finalpublicinstancemethodsinthe
Objectclass,buttheseareexcludedfromthedefinitionofafunctionalinterface.Note
thatabstractmethodsdeclaredinaninterfaceareimplicitlyabstractandpublic.
Interfacesarediscussedin§7.6,p.290.
WedefinedandusedtheStrPredicateinterfaceearlier.Ithasexactlyoneabstract
method.
Clickheretoviewcodeimage
@FunctionalInterface
interfaceStrPredicate{
booleantest(Stringstr);//Solepublicabstract
method.
}
Theannotation@FunctionalInterfaceisusefulwhendefiningfunctional
interfaces.Thecompilerwillissueanerrorifthedeclarationviolatesthedefinitionofa
functionalinterface,asillustratedbythefollowingXStrPredicateinterface.Its
declarationhastwoabstractmethods.
Clickheretoviewcodeimage
@FunctionalInterface
interfaceXStrPredicate{//Compile-time
error!
booleantest(Stringstr);//Abstractmethod.
Stringreverse(Stringstr);//Abstractmethod.
}
ThenextfunctionalinterfaceNewStrPredicatedeclaresonlyoneabstractmethodat
(1).Inaddition,itprovidestheimplementationsofonedefaultmethodandonestatic
methodat(2)and(3),respectively.Theabstractmethoddeclarationat(4)isthatofanon-
finalpublicmethodfromtheObjectclass,butsuchdeclarationsareexcluded
fromthedefinitionofafunctionalinterface.
Clickheretoviewcodeimage
@FunctionalInterface
interfaceNewStrPredicate{
booleantest(Stringstr);//(1)Abstract
method
defaultvoidmsg(Stringstr){System.out.println(str);}//(2)Default
method
staticvoidinfo(){System.out.println(“Testing!”);}//(3)Static
method
@Overridebooleanequals(Objectobj);//(4)FromObject
class
}
TheinterfaceStrFormatprovidestheabstractmethoddeclarationofthenon-final
publicmethodtoString()fromtheObjectclass,butsuchdeclarationsare
excludedfromthedefinitionofafunctionalinterface.Effectively,thereisnoabstract
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methoddeclaredintheStrFormatinterface.Thecodewouldcompileasaninterface
withouttheannotation@FunctionalInterface,butitisnotafunctionalinterface.
Clickheretoviewcodeimage
@FunctionalInterface
interfaceStrFormat{//Compile-time
error!
@OverrideStringtoString();//FromObject
class
}
Earlierinthissection,weusedthegenericfunctionalinterface
java.util.function.Predicate<T>,whichprovidestheabstractmethod
test()toimplementpredicates.Thisfunctionalinterfacealsohasonestaticmethod
(isEqual())andthreedefaultmethods(and(),or(),negate()).Thedefault
methodsimplementshort-circuitlogicaloperatorsthatcanbeusedforcomposing
predicatesonPredicateobjects.
ThefunctionalsubinterfaceIStrPredicatethatfollowsiscustomizedtotheString
typebyextendingthejava.util.function.Predicate<T>functionalinterface,
wherethetypeparameterTisString.Itcanreadilybeusedforimplementingpredicates
onstrings.
Clickheretoviewcodeimage
@FunctionalInterface
interfaceIStrPredicateextendsPredicate<String>{}
FunctionalInterfacesinJavaSEPlatformAPI
TheJavaSEplatformAPIhasmanyfunctionalandnonfunctionalinterfaces,allofwhich
supportthepracticeofprogrammingtointerfaces.Forexample,thejava.langpackage
includesfivefunctionalinterfaces:Runnable,Comparable<T>,AutoCloseable,
Iterable<T>,andReadable.However,themainsupportforfunctionalinterfacesis
foundinthejava.util.functionpackage.Itincludesgeneral-purposefunctional
interfacesthatimplementbasicconceptsinfunctional-styleprogramming(Table10.2).In
addition,thepackageprovidesawiderangeoffunctionalinterfacesforvariouspurposes,
sothatimplementingnewextensionsshouldhardlybenecessary.
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Table10.2SelectedFunctionalInterfacesfromthejava.util.functionPackage
Thejava.util.functionpackageincludesfunctionalinterfacesthatarespecialized
forprimitivevalues.Theiruseenablesprogrammerstoavoidexcessiveboxingand
unboxingofprimitivevalueswhensuchvaluesareusedasobjects.Thefunctional
interfacesIntPredicate,LongPredicate,andDoublePredicateprovidean
abstracttest()methodtoevaluatepredicateswithint,long,anddouble
arguments,respectively.
Clickheretoviewcodeimage
Predicate<Integer>integerPred=(Integeri)->i%2==0;//iasoperand
unboxed.
System.out.println(integerPred.test(2015));//Argumentboxed.
false
IntPredicateintPred=(inti)->i%2==0;
System.out.println(intPred.test(2016));//true
DefiningLambdaExpressions
Lambdaexpressionsimplementfunctionalinterfacesbydefininganonymousfunctions
thatfacilitatebehaviorparameterization.Theycanbepassedandusedasvaluesina
program,withouttheexcessbaggageoffirstbeingpackagedintoobjectsinthesource
code.Thecompilertakescareofwhateverthatneedstobedoneinternallytomaketheir
usagefeasible.
Alambdaexpressionhasthefollowingsyntax:
Clickheretoviewcodeimage
formal_parameter_list->lambda_body
Theparameterlistandthebodyareseparatedbythe->operator.Thelambdaexpression
syntaxresemblesasimplifieddeclarationofamethod,withoutmanyofthebellsand
whistlesofamethoddeclaration.Thatstreamliningisimportant,asitavoidsverbosityand
providesasimpleandsuccinctnotationwithwhichtowritelambdaexpressionsonthefly.
Intherestofthissection,wetakeacloserlookattheparameterlist,thelambdabody,and
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thetypecheckingandevaluationoflambdaexpressions.
LambdaParameters
Theparameterlistofalambdaexpressionisacomma-separatedlistofformalparameters
thatisenclosedinparentheses,(),analogoustotheparameterlistinamethod
declaration.Thereareothershorthandformsaswell,asweshallseeshortly.
Ifthetypesoftheparametersarespecified,theyareknownasdeclared-typeparameters.
Ifthetypesoftheparametersarenotspecified,theyareknownasinferred-type
parameters.Typesoftheinferred-typeparametersarederivedfromthefunctional
interfacetypethatisthetargettypeofthelambdaexpression.
Parametersareeitheralldeclared-typeorallinferred-type.Parenthesesaremandatory
withmultipleparameters,whethertheyaredeclared-typeorinferred-type.Foraparameter
listwithasingleinferred-typeparameter,theparenthesescanbeomitted.Also,only
declared-typeparameterscanhavemodifiers.
Clickheretoviewcodeimage
()->..//Emptyparameterlist
(Integerx,Integery,Integerz)->..//Multipledeclared-type
parameters
(x,y,z)->..//Multipleinferred-type
parameters
(Stringstr)->..//Singledeclared-type
parameter
(str)->..//Singleinferred-type
parameter
str->..//Singleinferred-type
parameter
Stringstr->..//Illegal:Missingparentheses
Integerx,Integery,Integerz->..//Illegal:Missingparentheses
i,j,k->..//Illegal:Missingparentheses
(Stringstr,j)->..//Illegal:Cannotmixinferredanddeclaredtype
(finalinti,intj)->..//OK:Modifierwithdeclared-typeparameter
(finali,j)->..//Illegal:Nomodifierwithinferred-type
parameter
LambdaBody
Alambdabodyiseitherasingleexpressionorastatementblock.Executionofalambda
bodyhaseitheranon-voidreturn(i.e.,returnsavalue)oravoidreturn(i.e.,doesnot
returnavalue),oritsevaluationthrowsanexception.
Asingle-expressionlambdabodyisusedforshortandsuccinctlambdaexpressions.A
single-expressionlambdabodywithavoidreturntypeiscommonlyusedtoachieveside
effects.Thereturnkeywordisnotallowedinasingle-expressionlambdabody.
Intheexamplesthatfollow,thebodyofthelambdaexpressionsisanexpressionwhose
executionreturnsavalue(i.e.,hasanon-voidreturn):
Clickheretoviewcodeimage
()->2015//Expressionbody,non-void
return
()->null//Expressionbody,non-void
return
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(i,j)->i+j//Expressionbody,non-void
return
(i,j)->i<=j?i:j//Expressionbody,non-void
return
str->str.length()>3//Expressionbody,non-void
return
str->str!=null//Expressionbody,non-void
return
&&!str.equals(””)&&str.length()>3
&&str.equals(newStringBuilder(str).reverse().toString())
Inthenextsetofexamples,thelambdabodyisanexpressionstatementthatcanhavea
voidoranon-voidreturn.However,iftheabstractmethodofthefunctionalinterface
returnsvoid,thenon-voidreturnofalambdaexpressionwithanexpressionstatement
asbodycanbeinterpretedasavoidreturn(i.e.,thereturnvalueisignored).
Clickheretoviewcodeimage
val->System.out.println(val)//Methodinvocationstatement,voidreturn
sb->sb.trimToSize()//Methodinvocationstatement,voidreturn
sb->sb.append(“!”)//Methodinvocationstatement,non-void
return
()->newStringBuilder(“?”)//Objectcreationstatement,non-void
return
value->value++//Incrementstatement,non-voidreturn
value->value*=2//Assignmentstatement,non-voidreturn
Thefollowingexamplesarenotlegallambdaexpressions:
Clickheretoviewcodeimage
(inti)->while(i<10)++i//Illegal:notanexpressionbutastatement
(x,y)->returnx+y//Illegal:returnnotallowedinexpression
Thestatementblockcomprisesdeclarationsandstatementsenclosedinbraces({}).The
returnstatementisallowedonlyinablocklambdabody.
Clickheretoviewcodeimage
()->{}//Blockbody,voidreturn
()->{return2015;}//Blockbody,non-voidreturn
()->{return2015}//Illegal:statementterminator(;)inblock
missing
()->{newStringBuilder(“Gonuts.”);}//Blockbody,voidreturn
()->{returnnewStringBuilder(“Gonuts!”);}//Blockbody,non-void
return
(inti)->{while(i<10)++i;}//Blockbody,voidreturn
(i,j)->{if(i<=j)returni;elsereturnj;}//Blockbody,non-void
return
(done)->{//Multiplestatementsinblockbody,void
return
if(done){
System.out.println(“Youdeserveabreak!”);
return;
}
System.out.println(“Stayrighthere!”);
}
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AccessingMembersintheEnclosingClass
Sincealambdaexpressionisnotassociatedwithanyclass,thereisnonotionofathis
referenceassociatedwithit.Ifitisusedinalambdaexpression,thethisreferencerefers
totheenclosingobject,andcanbeusedtoaccessmembersofthisobject.Thenameofa
memberintheenclosingobjecthasthesamemeaningwhenusedinalambdaexpression.
Inotherwords,therearenorestrictionsonaccessingmembersintheenclosingobject.In
thecaseofshadowingmembernames,thekeywordthiscanbeexplicitlyused,andthe
keywordsupercanbeusedtoaccessanymembersinheritedbytheenclosingobject.
InExample10.6,thegetPredicate()methodat(7)definesalambdaexpressionat
(8).ThislambdaexpressionaccessesthestaticfieldstrListandtheinstancefield
bannerintheenclosingclassat(1)and(2),respectively.
Inthemain()methodinExample10.6,anArrayListisassignedtothestatic
fieldstrListat(3)andisinitialized.TheArrayListreferredtobythestaticfield
strListhasthefollowingcontent:
Clickheretoviewcodeimage
[Otto,ADA,Alyla,Bob,HannaH,Java]
AMembersOnlyobjectiscreatedat(4).ItsStringBuilderfieldbanneris
initializedwiththestring"love",andthelocalvariableobjreferstothis
MembersOnlyobject.At(5),aPredicateobjectiscreatedbycallingthe
getPredicate()methodontheMembersOnlyobjectreferredtobythelocal
variableobj.Thispredicateisfirstevaluatedwhenthetest()methodiscalledat(6)
onthePredicateobject,withtheargumentstring"neverdies!".Callingthe
test()methodresultsinthelambdaexpressioncreatedat(5)bythe
getPredicate()methodbeingevaluatedintheenclosingMembersOnlyobject
referredtobythelocalvariableobj.
Theparameterstrofthelambdaexpressionisinitializedwiththestring"never
dies!",theargumenttothetest()method.Inthebodyofthelambdaexpression,the
ArrayListreferredtobythestaticfieldstrListisfirstprintedat(9):
Clickheretoviewcodeimage
List:[Otto,ADA,Alyla,Bob,HannaH,Java]
At(10),theparameterstr(withcontents"neverdies!")isappendedtothe
StringBuilder(withcontents"love")referredtobytheinstancefieldbannerin
theenclosingobject,resultinginthefollowingcontentsinthisStringBuilder:
“loveneverdies!”
Sincethelengthofthestring"neverdies!",referredtobytheparameterstr,is
greaterthan5,thelambdaexpressionreturnstrueat(11).Thisisthevaluereturnedby
thetest()methodcallin(6).
Inthecalltotheprintln()methodat(6),theargument
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p.test(“neverdies!”)+””+obj.banner
nowevaluatesas
Clickheretoviewcodeimage
true+””+“loveneverdies!”
Example10.6AccessingMembersinanEnclosingObject
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;
publicclassMembersOnly{
//Instancevariable
privateStringBuilderbanner;//(1)
//Staticvariable
privatestaticList<String>strList;//(2)
//Constructor
publicMembersOnly(Stringstr){
banner=newStringBuilder(str);
}
//Staticmethod
publicstaticvoidmain(String[]args){
strList=newArrayList<>();//(3)
strList.add(“Otto”);strList.add(“ADA”);strList.add(“Alyla”);
strList.add(“Bob”);strList.add(“HannaH”);strList.add(“Java”);
MembersOnlyobj=newMembersOnly(“love“);//(4)
Predicate<String>p=obj.getPredicate();//(5)
System.out.println(p.test(“neverdies!”)+””+obj.banner);//(6)
}
//Instancemethod
publicPredicate<String>getPredicate(){//(7)
returnstr->{//(8)Lambda
expression
System.out.println(“List:”+strList);//
(9)MembersOnly.strList
banner.append(str);//(10)this.banner
returnstr.length()>5;//(11)booleanvalue
};
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
List:[Otto,ADA,Alyla,Bob,HannaH,Java]
trueloveneverdies!
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AccessingLocalVariablesintheEnclosingContext
Asmentionedearlier,alambdaexpressionisnotassociatedwithaclassoranyofits
members.Apartfromnothavingathisreference,alambdaexpressionalsodoesnot
createitsownscope.Instead,itispartofthescopeoftheenclosingcontext—ithaslexical
orblockscope(§4.4,p.117).Allvariabledeclarationsinalambdaexpressionfollowthe
rulesofblockscoping.Theyarenotaccessibleoutsideofthelambdaexpression.In
addition,wecannotredeclarelocalvariablesalreadydeclaredintheenclosingscope.In
Example10.7,redeclaringthelocalvariablesbannerandwordsat(6)and(7),
respectively,resultsinacompile-timeerror.
Localvariablesdeclaredintheenclosingmethod,includingitsformalparameters,canbe
accessedinalambdaexpressionprovidedtheyareeffectivelyfinal.Thismeansthatoncea
localvariablehasbeenassignedavalue,itsvaluedoesnotchangeinthemethod.Using
thefinalmodifierinthedeclarationofalocalvariableexplicitlyinstructsthecompiler
toensurethatthisisthecase.Thefinalmodifierimplieseffectivelyfinal.Ifthefinal
modifierisomittedandalocalvariableisusedinalambdaexpression,thecompiler
effectivelyperformsthesameanalysisasifthefinalmodifierhadbeenspecified.A
lambdaexpressionmightbeexecutedatalatertime,afterthemethodhasfinished
execution.Atthatpoint,thelocalvariablesusedinthelambdaexpressionarenolonger
accessible.Toensuretheiravailability,copiesoftheirvaluesaremaintainedwiththe
lambdaexpression.Thisiscalledvariablecapture,althoughinessenceitisthevaluesthat
arecaptured.Notethatitisnottheobjectthatiscopiedinthecaseofalocalreference
variable,butratherthereferencevalue.Objectsresideontheheapandareaccessibleviaa
copyofthereferencevalue.Correctexecutionofthelambdaexpressionisguaranteed,
sincetheseeffectivelyfinalvaluescannotchange.Notethatthestateofanobjectreferred
tobyafinaloraneffectivelyfinalreferencecanchange,butnotthereferencevaluein
thereference;thus,suchareferencewillcontinuerefertothesameobjectonceitis
initialized.
InExample10.7,themethodgetPredicate()at(1)hasoneformalparameter
(banner),andalocalvariable(words)declaredat(2).Althoughthestateofthe
ArrayListobject,referredtobythereferencewords,ischangedinthemethod(we
addelementstoit),thereferencevalueinthereferencedoesnotchange;thatis,it
continuestorefertothesameobjectwhosereferencevalueitwasassignedat(2).The
parameterbannerisassignedthereferencevalueoftheargumentobjectwhenthe
methodisinvoked,andcontinuestorefertothisobjectthroughoutthemethod.Bothlocal
variablesareeffectivelyfinal.Theirvaluesarecapturedbythelambdaexpression,and
usedwhenthelambdaexpressionisexecutedafterthecalltothegetPredicate()
methodinthemain()method.
Ifweuncomment(3)and(4)inExample10.7,thenbothlocalvariablesarenoteffectively
final.Theirreferencevaluesarechangedat(3)and(4),respectively.Thecompilernow
flagserrorsin(8)and(9),respectively,becausethesenon-finallocalvariablesareusedin
thelambdaexpression.
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Example10.7AccessingLocalVariablesinanEnclosingMethod
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;
publicclassLocalsOnly{
publicstaticvoidmain(String[]args){
StringBuilderbanner=newStringBuilder(“love“);
LocalsOnlyinstance=newLocalsOnly();
Predicate<String>p=instance.getPredicate(banner);
System.out.println(p.test(“neverdies!”)+””+banner);
}
publicPredicate<String>getPredicate(StringBuilderbanner){//(1)
List<String>words=newArrayList<>();//(2)
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
//banner=newStringBuilder();//(3)Illegal:Noteffectively
final
//words=newArrayList<>();//(4)Illegal:Noteffectively
final
returnstr->{//(5)Lambdaexpression
//Stringbanner=“Don’tredeclareme!”;//(6)Illegal:
Redeclared
//String[]words=newString[6];//(7)Illegal:
Redeclared
System.out.println(“List:”+words);//(8)
banner.append(str);//(9)
returnstr.length()>5;
};
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
List:[Otto,ADA,Alyla,Bob,HannaH,Java]
trueloveneverdies!
TypeCheckingandExecutionofLambdaExpressions
Theuseoflambdaexpressionsistightlycoupledwithfunctionalinterfaces.Alambda
expressioncanbedefinedinacontextwhereafunctionalinterfacecanbeused—for
example,inanassignmentcontext,amethodcallcontext,oracastcontext.Thecompiler
determinesthetargettypethatisrequiredinthecontextwherethelambdaexpressionis
defined.Thistargettypeisalwaysafunctionalinterfacetype.Intheassignmentcontext
thatfollows,thetargettypeisPredicate<Integer>,asitisthetargetofthe
assignmentstatement.NotethatthetypeparameterTofthefunctionalinterfaceis
Integer.
Clickheretoviewcodeimage
Predicate<Integer>p1=i->i%2==0;//(1)Targettype:
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Predicate<Integer>
Themethodtypeofamethoddeclarationcomprisesitstypeparameters,formalparameter
types,returntype,andanyexceptionsthemethodthrows.
Thefunctiontypeofafunctionalinterfaceisthemethodtypeofitssingleabstractmethod.
ThetargettypePredicate<Integer>hasthefollowingmethod,wheretype
parameterTisInteger:
Clickheretoviewcodeimage
publicbooleantest(Integert);//Methodtype:Integer->boolean
ThefunctiontypeofthetargettypePredicate<Integer>isthemethodtypeofthe
thistest()method:
Integer->boolean
Thetypeofthelambdaexpressiondefinedinagivencontextmustbecompatiblewiththe
functiontypeofthetargettype.Ifthelambdaexpressionhasinferred-typeparameters,
theirtypeisinferredfromthefunctiontype,andifnecessaryfromthecontext.Fromthe
functiontypeofthetargettypePredicate<Integer>,thecompilercaninferthatthe
parameteriinthelambdaexpressionat(1)shouldbeoftypeInteger.Thelambda
bodyreturnsabooleanvalue.Thetypeofthelambdaexpressionin(1)is
Integer->boolean
Thetypeofthelambdaexpressioniscompatiblewiththefunctiontypeofthetargettype
Predicate<Integer>.
Inthefollowingassignment,thetargettypeis
java.util.function.IntPredicate:
Clickheretoviewcodeimage
IntPredicatep2=i->i%2==0;//(2)Targettype:IntPredicate
TheIntPredicatefunctionalinterfacehasthefollowingabstractmethod:
Clickheretoviewcodeimage
publicbooleantest(inti);//Methodtype:int->boolean
ThefunctiontypeofthetargettypeIntPredicateisthemethodtypeofitsabstract
method:
int->boolean
Thecompilerinfersthatthetypeoftheinferred-typeparameteriinthelambda
expressionat(2)shouldbeint.Asthelambdabodyreturnsabooleanvalue,thetype
ofthelambdaexpressionin(2)is
int->boolean
Thetypeofthelambdaexpressioniscompatiblewiththefunctiontypeofthetargettype
IntPredicate.
Notethatinbothexamples,thelambdaexpressionisthesame,buttheirtypesaredifferent
inthetwocontexts:Theyrepresenttwodifferentvalues.Thetypeofalambdaexpression
isdeterminedbythecontextinwhichitisdefined.
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System.out.println(p1==p2);//false
Theprocessoftypecheckingalambdaexpressioninagivencontextiscalledtarget
typing.Thepresentationhereissimplified,butsufficesforourpurposestogiveanideaof
whatisinvolved.
Thecompilerdoesthetypecheckingnecessarytouselambdaexpressions,andthe
runtimeenvironmentprovidestherestofthemagictomakeitallwork.Atruntime,the
lambdaexpressionisexecutedwhenthesoleabstractmethodofthefunctionalinterfaceis
invoked.Asmentionedearlier,thisisanexampleofdeferredexecution.Lambda
executionissimilartoinvokingamethodonanobject.Wedefinealambdaexpressionas
afunctionanduseitlikeamethod,lettingthecompilerandtheruntimeenvironmentputit
alltogether.
Clickheretoviewcodeimage
booleanresult1=p1.test(2015);//false
booleanresult2=p2.test(2016);//true
FilteringRevisited:The FunctionalInterface
Example10.8includesthetwopreviousexamplesoffilteringalistforpalindromesat(2)
and(3).Inaddition,itpresentsseveralexamplesoffilteringalistusingdifferentcriteria
definedonpropertiesofastring.Determiningwhetherastringisacase-sensitiveorcase-
insensitivepalindromeisnowdonebythetwostaticmethods
isCaseSensitivePalindrome()andisCaseInsensitivePalindrome(),
respectively.AlsonotethatthefilterStrings()method,whichweusedearlier,is
nowagenericmethodtofilteranylist,notjustalistofstrings,(14).
TheexamplesinExample10.8haveintentionallybeenwrittentoillustratethesyntaxof
lambdaexpressions.Wedrawattentiontothepredicateat(11)thattakesintoaccount
whetherthestringelementhasanullvalue,whetheritisanemptystring,andwhether
itslengthisgreaterthan3,beforetestingitforapalindrome:
Clickheretoviewcodeimage
Predicate<String>predicateE=str->//(11)
str!=null&&!str.equals(””)&&str.length()>3
&&isCaseSensitivePalindrome(str);
ThislambdaexpressionavoidsthrowingaNullPointerException.Thereareother,
moresophisticatedwaysofdealingwithaNullPointerException,buttheyare
beyondthescopeofthisbook.ThePredicate<T>interfacealsoprovidesmethodsto
composecompoundpredicates.
Thefilteringexamplesinthischaptermakeheavyuseoftraversaloveralistusingaloop.
Functional-styleprogrammingfreesusfromthetyrannyofexplicittraversalover
collections,butwehavemerelyscratchedthesurfacehere.Earlierinthischapter(p.424),
wediscussedtheremovalofelementsfromalist.TheremoveIf()methodofthe
ArrayListclassprovidesasafeandconvenientwayofdeletingallelementsthatsatisfy
anycriteriaexpressedasaPredicate<T>.Thus,thetraversalprocessisnowinternal.
Thelambdaexpressionin(11)testswhetherthestringcontainsthecharactera.Allstrings
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satisfyingthiscriterionareremovedfromthelist,asconfirmedbytheoutputfromthe
program.
Clickheretoviewcodeimage
words.removeIf(str->str.indexOf(‘a’)>0);//(12)
Anotherexampleoftheuseofthismethodisgivenin(13),whereitisusedtoremoveall
stringswithlengthgreaterthan3:
Clickheretoviewcodeimage
words.removeIf(str->str.length()>3);//(13)
ItisinstructivetowalkthroughthecodeinExample10.8,andcompareittothesolution
weinitiallywroteinExample10.2,p.434.
Example10.8FilteringanArrayList
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;
publicclassFunWithPalindromesV4{
privatestaticbooleanisCaseSensitivePalindrome(Stringstr){
returnstr.equals(newStringBuilder(str).reverse().toString());
}
privatestaticbooleanisCaseInsensitivePalindrome(Stringstr){
returnstr.equalsIgnoreCase(newStringBuilder(str).reverse().toString());
}
publicstaticvoidmain(String[]args){
//Createalistofwords://
(1)
List<String>words=newArrayList<>();
words.add(“Otto”);words.add(“ADA”);words.add(“Alyla”);
words.add(“Bob”);words.add(“HannaH”);words.add(“Java”);
System.out.println(“Listofwords:”+words);
List<String>palindromes1=filterStrings(words,//
(2)
str->isCaseSensitivePalindrome(str));
System.out.println(“Case-sensitivepalindromes:”+palindromes1);
List<String>palindromes2=filterStrings(words,str->//
(3)
isCaseInsensitivePalindrome(str));
System.out.println(“Case-insensitivepalindromes:”+palindromes2);
Predicate<String>predicate3=str->!isCaseSensitivePalindrome(str);//
(4)
List<String>nonPalindromes=filterStrings(words,predicate3);
System.out.println(“Non-palindromes,casesensitive:”+nonPalindromes);
Predicate<String>predicate4=str->str.length()>3;//
(5)
List<String>strGT3=filterStrings(words,predicate4);
System.out.println(“Wordswithlength>3:”+strGT3);
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Predicate<String>predicate5=str->//
(6)
str.length()>3&&isCaseSensitivePalindrome(str);
List<String>palindromesGT3=filterStrings(words,predicate5);
System.out.println(“Case-sensitivepalindromes,length>3:“
+palindromesGT3);
Predicate<String>predicateA=str->{//
(7)
returnstr.length()>3&&isCaseSensitivePalindrome(str);
};
System.out.println(“Case-sensitivepalindromes,length>3:“
+filterStrings(words,predicateA));
Predicate<String>predicateB=str->{//
(8)
booleanresult1=str.length()>3;
booleanresult2=isCaseSensitivePalindrome(str);
returnresult1&&result2;
};
System.out.println(“Case-sensitivepalindromes,length>3:“
+filterStrings(words,predicateB));
Predicate<String>predicateC=str->{//
(9)
if(str==null||str.equals(””)||str.length()<=3){
returnfalse;
}
StringBuildersb=newStringBuilder(str);
booleanresult=str.equals(sb.reverse().toString());
returnresult;
};
System.out.println(“Case-sensitivepalindromes,length>3:“
+filterStrings(words,predicateC));
Predicate<String>predicateD=str->//
(10)
(str==null||str.equals(””)||str.length()<=3)
?false:isCaseSensitivePalindrome(str);
System.out.println(“Case-sensitivepalindromes,length>3:“
+filterStrings(words,predicateD));
Predicate<String>predicateE=str->//
(11)
str!=null&&!str.equals(””)&&str.length()>3
&&isCaseSensitivePalindrome(str);
System.out.println(“Case-sensitivepalindromes,length>3:“
+filterStrings(words,predicateE));
//Removingelementsfromalist:
words.removeIf(str->str.indexOf(‘a’)>0);//
(12)
System.out.println(“Listofwords,no‘a’:”+words);
words.removeIf(str->str.length()>3);//
(13)
System.out.println(“Listofwords,length<=3:”+words);
}
/**
*Filtersalistaccordingtothecriteriaofthepredicate.
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*@paramlistListtofilter
*@parampredicateProvidesthecriteriaforfilteringthelist
*@returnListofelementsthatmatchthecriteria
*/
publicstatic<E>List<E>filterStrings(List<E>list,//
(14)
Predicate<E>predicate){
List<E>result=newArrayList<>();
for(Eelement:list)
if(predicate.test(element))
result.add(element);
returnresult;
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Listofwords:[Otto,ADA,Alyla,Bob,HannaH,Java]
Case-sensitivepalindromes:[ADA,HannaH]
Case-insensitivepalindromes:[Otto,ADA,Alyla,Bob,HannaH]
Non-palindromes,casesensitive:[Otto,Alyla,Bob,Java]
Wordswithlength>3:[Otto,Alyla,HannaH,Java]
Case-sensitivepalindromes,length>3:[HannaH]
Case-sensitivepalindromes,length>3:[HannaH]
Case-sensitivepalindromes,length>3:[HannaH]
Case-sensitivepalindromes,length>3:[HannaH]
Case-sensitivepalindromes,length>3:[HannaH]
Case-sensitivepalindromes,length>3:[HannaH]
Listofwords,no‘a’:[Otto,ADA,Bob]
Listofwords,length<=3:[ADA,Bob]
ReviewQuestions
10.7Whichstatementistrueaboutfunctionalinterfacesandlambdaexpressions?
Selecttheonecorrectanswer.
(a)Afunctionalinterfacecanbeimplementedonlybylambdaexpressions.
(b)Afunctionalinterfacedeclarationcanhaveonlyonemethoddeclaration.
(c)Inthebodyofalambdaexpression,onlypublicmembersintheenclosing
classcanbeaccessed.
(d)Inthebodyofalambdaexpression,alllocalvariablesintheenclosingscope
canbeaccessed.
(e)Alambdaexpressioninaprogramcanimplementonlyonefunctionalinterface.
(f)Noneoftheabove.
10.8Whichstatementsaretrueaboutthefollowingcode?
Clickheretoviewcodeimage
importjava.util.function.Predicate;
publicclassRQ12A98{
publicstaticfinalStringlock1=“Brinks”;
privatestaticStringlock2=“Yale”;
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publicstaticvoidmain(String[]args){
Predicate<Object>p;
p=lock->{booleanp=lock.equals(“Master”);returnp;
};//(1)
p=lock->{returnlock.toString().equals(“YALE”);
};//(2)
p=lock->{(args.length>0)?lock.equals(args[0]):false;
};//(3)
p=lock->{returnlock.equals(lock1);
};//(4)
p=lock->{returnlock.equals(lock2);
};//(5)
p=lock2->{returnlock2.equals(RQ12A98.lock2);
};//(6)
}
}
Selectthetwocorrectanswers.
(a)(1)willnotcompile.
(b)(2)willnotcompile.
(c)(3)willnotcompile.
(d)(4)willnotcompile.
(e)(5)willnotcompile.
(f)(6)willnotcompile.
10.9Whichstatementsaretrueaboutthefollowingcode?
Clickheretoviewcodeimage
interfaceFunky1{voidabsMethod1(Strings);}
interfaceFunky2{StringabsMethod2(Strings);}
publicclassRQ12A99{
publicstaticvoidmain(String[]args){
Funky1p1;
p1=s->System.out.println(s);//(1)
p1=s->s.length();//(2)
p1=s->s.toUpperCase();//(3)
p1=s->{s.toUpperCase();};//(4)
p1=s->{returns.toUpperCase();};//(5)
Funky2p2;
p2=s->System.out.println(s);//(6)
p2=s->s.length();//(7)
p2=s->s.toUpperCase();//(8)
p2=s->{s.toUpperCase();};//(9)
p2=s->{returns.toUpperCase();};//(10)
}
}
Selectthefourcorrectanswers.
(a)(1)willnotcompile.
(b)(2)willnotcompile.
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(c)(3)willnotcompile.
(d)(4)willnotcompile.
(e)(5)willnotcompile.
(f)(6)willnotcompile.
(g)(7)willnotcompile.
(h)(8)willnotcompile.
(i)(9)willnotcompile.
(j)(10)willnotcompile.
10.10Whichstatementistrueaboutthefollowingprogram?
Clickheretoviewcodeimage
importjava.util.Arrays;
importjava.util.function.IntPredicate;
publicclassRQ12A96{
publicstaticvoidmain(String[]args){
int[]intArray={0,-1,-2,-3,-4,-5,-6,-7,-8,-9};
filterInt(intArray,val->val<0&&val%2==0);
System.out.println(Arrays.toString(intArray));
}
publicstaticvoidfilterInt(int[]intArr,
IntPredicatepredicate){
for(inti=0;i<intArr.length;++i){
if(predicate.test(intArr[i])){
intArr[i]=Math.abs(intArr[i]);
}
}
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillcompile,butwillthrowanexceptionwhenrun.
(c)Theprogramwillcompileandprintthefollowingwhenrun:
[0,1,-2,3,-4,5,-6,7,-8,9]
(d)Theprogramwillcompileandprintthefollowingwhenrun:
[0,-1,2,-3,4,-5,6,-7,8,-9]
(e)Theprogramwillcompileandprintthefollowingwhenrun:
[0,1,2,3,4,5,6,7,8,9]
10.11Whichstatementistrueaboutthefollowingcode?
Clickheretoviewcodeimage
interfaceInterfaceA{voiddoIt();}
interfaceInterfaceBextendsInterfaceA{}
interfaceInterfaceCextendsInterfaceB{
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voiddoIt();
booleanequals(Objectobj);
}
classBetaimplementsInterfaceB{
publicvoiddoIt(){
System.out.print(“Jazz|”);
}
}
publicclassRQ12A999{
publicstaticvoidmain(String[]args){
InterfaceAa=()->System.out.print(“Java|”);//(1)
InterfaceBb=()->System.out.print(“Jive|”);//(2)
InterfaceCc=()->System.out.print(“Jingle|”);//(3)
Objecto=a=c;//(4)
b=newBeta();//(5)
a.doIt();//(6)
b.doIt();//(7)
c.doIt();//(8)
((InterfaceA)o).doIt();//(9)
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowaClassCastException.
(c)Theprogramwillprint:Jingle|Jingle|Jazz|Jingle|
(d)Theprogramwillprint:Jingle|Jazz|Jingle|Jingle|
(e)Theprogramwillprint:Jingle|Jingle|Jingle|Jazz|
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Theconceptofalistasacollection
•TheinheritancerelationshipbetweentheArrayList<E>class,theList<E>
interface,andtheCollection<E>interfaceintheJavaCollectionsFramework
•Useofthediamondoperator(<>)whencreatingobjectsofgenericclasses
•DeclaringandusingreferencesoftheArrayListtype
•Creating,querying,traversing,converting,andsortingArrayLists
•ComparisonofarraysandArrayLists
•Behaviorparameterizationinfunctional-styleprograming
•Theroleoflambdaexpressionsandfunctionalinterfacesinimplementingbehavior
parameterization
•Referencingthegeneral-purposefunctionalinterfacesinthe
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java.util.functionpackage
•ImplementingthePredicate<T>functionalinterfaceusinglambdaexpressions
•DefiningselectioncriteriaaspredicatesforfilteringArrayLists
•Definingandtypecheckinglambdaexpressionsinthecontextofafunctional
interface
•Thedeferredexecutionofalambdaexpression
•Theimplicationsofusingclassmembersfromtheenclosingclass,andofusinglocal
variablesfromtheenclosingmethod
•Passingandassigninglambdavaluesusingfunctionalinterfacereferences
ProgrammingExercise
10.1Findallelementsinalistthatsatisfyallgivenpredicates.
Skeletoncodeforthisproblemisprovidedinthisexercise.Themain()methodat
(1)createsalistofstringsfromanarrayofstrings.Italsocreatesalistof
predicates.Thefollowingpredicatesshouldbeincludedinthislist:
•Astringisnotnull.
•Astringcontainsthesubstring"up"byfirstconvertingthestringtolowercase.
•Thefirstcharacterofastringisaletter.
Themain()methodcallsthemethodsat(2),(4),and(5).Theoutputfromthe
programisshownhere.Thetextualrepresentationofthelambdaexpressionsmight
vary.
Clickheretoviewcodeimage
[Cheerup!,7Upcomingup!,null,Bottomsup!,Getdown!,What’sup?]
[FilterFun$$Lambda$1/12251916@192e0f4,
FilterFun$$Lambda$2/18340259@a418fc,
FilterFun$$Lambda$3/19888781@105068a]
[Cheerup!,Bottomsup!,What’sup?]
[Cheerup!,Bottomsup!,What’sup?]
[Cheerup!,Bottomsup!,What’sup?]
Completetheimplementationofthemain()method,andatleastimplementthe
methodsat(2)and(3).
Toimplementthemethodsat(4)and(5),seethemethodsand()andnegate()
providedbythejava.util.function.Predicate<T>interface.
Themethodsat(4)and(5)alsomakeuseoftheArrayList.removeIf()
method.
Themethodat(5)isastraightforwardgenericversionofthemethodat(4),sothat
anylistcanbefilteredthisway.
Clickheretoviewcodeimage
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importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;
publicclassFilterFun{
publicstaticvoidmain(String[]args){//
(1)
//Createalistofstrings:
String[]strings={“Cheerup!”,“7Upcomingup!”,null,
“Bottomsup!”,“Getdown!”,“What’sup?”};
List<String>strList=newArrayList<>();
/*WRITECODETOPOPULATETHELISTOFSTRINGSHERE.*/
System.out.println(strList);
//Createalistofpredicates.
List<Predicate<String>>predList=newArrayList<>();
/*WRITECODETOPOPULATETHELISTOFPREDICATESHERE.*/
System.out.println(predList);
//Applyfiltering.
applyAllPredicates(strList,predList);
applyAllPredicatesAlt(strList,predList);
applyAllPredicatesGenAlt(strList,predList);
}
/**
*Printsallthestringsinthespecifiedlistthatsatisfyallthe
*predicatesinthelistofpredicates.
*ItusestheandPredicates()methodat(3).
*@paramlistListofstringstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstaticvoidapplyAllPredicates(List<String>list,//
(2)
List<Predicate<String>>predicates){
/*IMPLEMENTTHISMETHOD*/
}
/**
*Determineswhetherastringsatisfiesallthepredicates.
*@paramstrStringtoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*@returntrueonlyifthestringsatisfiesallthepredicates
*/
publicstaticbooleanandPredicates(Stringstr,//
(3)
List<Predicate<String>>predicates){
/*IMPLEMENTTHISMETHOD*/
}
/**
*Removesalltheelementsinthespecifiedlistthatdonotsatisfyall
the
*predicatesinthelistofpredicates,andprintstheremainingelements
*thatdo.
*UsesPredicate.and(),Predicate.negate(),andList.removeIf()methods.
*@paramlistListofstringstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstaticvoidapplyAllPredicatesAlt(List<String>list,//
(4)
List<Predicate<String>>
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predicates){
/*IMPLEMENTTHISMETHOD*/
}
/**Genericversion.
*Removesalltheelementsinthespecifiedlistthatdonotsatisfyall
the
*predicatesinthelistofpredicates,andprintstheremainingelements
*thatdo.
*UsesPredicate.and(),Predicate.negate(),andList.removeIf()methods.
*@paramlistListofelementstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstatic<T>voidapplyAllPredicatesGenAlt(List<T>list,//
(5)
List<Predicate<T>>predicates){
/*IMPLEMENTTHISMETHOD*/
}
}
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11.DateandTime
11.1BasicDateandTimeConcepts
Java8introducedanewcomprehensiveAPIforhandlingtemporalconcepts.Inthis
chapter,weprimarilyconcentrateonclassesthatrepresentthedate,time,andperiod.The
Java8APIalsoprovidessupportforformattingandparsingtemporalobjects.These
classesareallbasedontheISOcalendarsystem,whichisthedefactoworldcalendar.The
DateandTimeAPIisfairlycomprehensive,andincludessupportforothertemporal
conceptssuchasinstants,durations,offsets,timezones,anddifferentcalendars—topics
thatarebeyondthescopeofthisbook.
Wewillusethetermtemporalobjectstomeanobjectsofclassesthatrepresenttemporal
concepts.Twopackagesprovidethemainsupportforhandling,formatting,andparsing
temporalobjects:
•Thejava.timepackageprovidestheclassesLocalDate,LocalTime,
LocalDateTime,andPeriodthatrepresentadate,a(clock)time,acombined
date-time,andaperiod,respectively.
LocalDate:Thisclassrepresentsadateintermsofdate-basedvalues(year,
month,day).Dateobjectshavenotime-basedvaluesoratimezone.
LocalTime:Thisclassrepresentstimeina24-hourdayintermsoftime-based
values(hours,minutes,seconds,nanoseconds).Timeobjectshavenodate-based
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valuesoratimezone.
LocalDateTime:Thisclassrepresentstheconceptofdateandtimecombined,in
termsofbothdate-basedandtime-basedvalues.Date-timeobjectshavenotime
zone.
Period:Thisclassrepresentsadirectedamountorquantityoftimeintermsof
numberofdays,months,andyears,whichcanbenegative.Periodobjectshaveno
notionofaclocktime,adate,oratimezone.
•Thejava.time.formatpackageprovidestheclassDateTimeFormatter
forformattingandparsingtemporalobjects.
DateTimeFormatter:Thisclassprovidesimplementationofformatters:
predefinedformatters(e.g.,DateTimeFormatter.ISO_LOCAL_TIME),and
localizedformattersthatusepredefinedformatstyles(e.g.,
FormatStyle.SHORT),andcustomizedformattersthatuseletterpatterns(e.g.,
"MM/dd/uuuu").Aformattercanbeusedtoobtainastringrepresentationofa
temporalobject(calledformatting),andconverselytoobtainatemporalobjectfrom
astring(calledparsing).
Thischapterprovidesexamplesdemonstratinghowtocreate,combine,convert,query,
compare,format,andparsetemporalobjects,includingtheuseoftemporalarithmetic.
11.2WorkingwithTemporalClasses
Thetemporalclassesimplementimmutableandthread-safetemporalobjects.Thestateof
animmutableobjectcannotbechanged.Anymethodthatissupposedtomodifysuchan
objectreturnsamodifiedcopyofthetemporalobject.Itisacommonmistaketoignore
thenewobject,thinkingthatthecurrentobjecthasbeenmodified,whichcanleadto
incorrectresults.Thread-safetyguaranteesthatthestateofsuchanobjectisnotaffected
byconcurrentaccess.
Anothercommonmistakeistoaccess,format,orparseatemporalobjectthatdoesnot
havetherequiredtemporalvalues.Forexample,aLocalTimeobjecthasonlytime-
basedvalues,sotryingtoformatitwithaformatterfordate-basedvalueswillresultina
java.time.DateTimeException.Manymethodswillalsothrowanexceptionifan
invalidoranout-of-rangeargumentispassedinthemethodcall.Itisimportanttokeepin
mindwhichtemporalvaluesconstitutethestateofatemporalobject.
Hereweignoretheimplicationsofatimezone,sincethetemporalobjectsdiscussedin
thischapteraretimezoneagnostic.However,thecoverageherewillfacilitatethe
inquisitivereadertofurtherexplorethefeaturesoftheDateandTimeAPI.
BeforedivingintotheDateandTimeAPI,weprovideanoverviewofthemethodnaming
conventionsusedinthetemporalclasses(Table11.1).Thismethodnamingconvention
makesiteasytousetheAPI,asitensuresmethodnamingisstandardizedacrossall
temporalclasses.
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Table11.1SelectedCommonMethodPrefixoftheTemporalClasses
ThetemporalclassesLocalTime,LocalDate,andLocalDateTimeareallfoundin
thejava.timepackage.Anappropriateimportstatementshouldbeincludedinthe
sourcefiletouseanyoftheseclasses.
CreatingTemporalObjects
Thetemporalclassesdonotprovideanypublicconstructorstocreatetemporalobjects.
Instead,theyprovideoverloadedstaticfactorymethodsnamedofwithwhichtocreate
temporalobjectsfromconstituenttemporalvalues.Weusethetermtemporalvaluesto
meanbothtime-basedanddate-basedvalues.Theof()methodscheckthatthevaluesof
theargumentsareinrange.Anyinvalidargumentresultsina
java.time.DateTimeException.
AllcodesnippetsinthissubsectioncanbefoundinExample11.1,p.467,readyfor
runningandexperimenting.
Thedeclarationstatementsthatfollowshowexamplesofcreatinginstancesofthe
LocalTimeclasstorepresenttimeona24-hourclockintermsofhours,minutes,
seconds,andnanoseconds.Therangesofvaluesforthehours(0–23),minutes(0–
59),seconds(0–59),andnanoseconds(0–999,999,999)aredefinedbytheISOstandard.
ThetoString()methodoftheclasswillformatthetime-basedvaluesaccordingtothe
ISOstandardasHH:mm:ss.SSSSSSSSS.Omittingtheseconds(ss)andfractionsof
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seconds(SSSSSSSSS)impliesthattheirvalueiszero.(Moreonformattingin§11.4,p.
495.)Intheseconddeclarationstatement,thesecondsandthenanosecondsarenot
specifiedinthemethodcall,resultingintheirvaluesbeingsettozero.Inthethird
statement,thehourvalue(25)isoutofrange,andifuncommented,willresultina
DateTimeException.
Clickheretoviewcodeimage
LocalTimetime1=LocalTime.of(8,15,35,900);//08:15:35.000000900
LocalTimetime2=LocalTime.of(16,45);//16:45
//LocalTimetime3=LocalTime.of(25,13,30);//DateTimeException
CreatinginstancesoftheLocalDateclassisanalogoustocreatinginstancesofthe
LocalTimeclass.Theof()methodoftheLocalDateclassispasseddate-based
values:theyear,monthoftheyear,anddayofthemonth.Therangesofthevaluesforthe
year,themonth,andthedayare(–999,999,999to+999,999,999),(1–12),and(1–31),
respectively.Themonthcanalsobespecifiedusingtheenumconstantsofthe
java.time.Monthclass,asintheseconddeclarationstatementinthenextsetof
examples.ADateTimeExceptionisthrownifthevalueofanyparameterisoutof
range,orifthedayisinvalidforthespecifiedmonthoftheyear.Inthethirddeclaration,
themonthvalue13isoutofrange.Inthelastdeclaration,themonthofFebruarycannot
have29days,sincetheyear2015isnotaleapyear.
Clickheretoviewcodeimage
LocalDatedate1=LocalDate.of(1969,7,20);//1969-07-20
LocalDatedate2=LocalDate.of(-3113,Month.AUGUST,11);//-3113-08-11
//LocalDatedate3=LocalDate.of(2015,13,11);//DateTimeException
//LocalDatedate4=LocalDate.of(2015,2,29);//DateTimeException
TheyearisrepresentedasaprolepticyearintheISOstandard,whichcanbenegative.A
yearinCE(CurrentEra,orAD)hasthesamevalueasaprolepticyear;forexample,2015
CEissameastheprolepticyear2015.However,forayearinBCE(BeforeCurrentEra,or
BC),theprolepticyear0correspondsto1BCE,theprolepticyear–1correspondsto2
BCE,andsoon.ThetoString()methodoftheclasswillformatthedate-basedvalues
accordingtotheISOstandardasuuuu-MM-dd(seealso§11.4,p.495).Inthesecond
declarationintheprecedingsetofexamples,thedate-3113-08-11correspondsto11
August3114BCE.
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//LocalTime
staticLocalTimeof(inthour,intminute)
staticLocalTimeof(inthour,intminute,intsecond)
staticLocalTimeof(inthour,intminute,intsecond,intnanoOfSecond)
ThisoverloadedstaticfactorymethodintheLocalTimeclassreturnsaninstance
ofLocalTimefromthespecifiedtime-basedvalues.Thesecondandnanosecond
valuesaresettozero,ifnotspecified.
Clickheretoviewcodeimage
//LocalDate
staticLocalDateof(intyear,intmonth,intdayOfMonth)
staticLocalDateof(intyear,Monthmonth,intdayOfMonth)
ThisoverloadedstaticfactorymethodintheLocalDateclassreturnsaninstance
ofLocalDatefromthespecifieddate-basedvalues.Thejava.time.Month
enumtypeallowsmonthstobereferredbyname—forexample,Month.MARCH.
Notethatmonthnumberingstartswith1(Month.JANUARY).
Clickheretoviewcodeimage
//LocalDateTime
staticLocalDateTimeof(intyear,intmonth,intdayOfMonth,
inthour,intminute)
staticLocalDateTimeof(intyear,intmonth,intdayOfMonth,
inthour,intminute,intsecond)
staticLocalDateTimeof(intyear,intmonth,intdayOfMonth,inthour,
intminute,intsecond,intnanoOfSecond)
staticLocalDateTimeof(intyear,Monthmonth,intdayOfMonth,
inthour,intminute,intsecond)
staticLocalDateTimeof(intyear,Monthmonth,intdayOfMonth,
inthour,intminute)
staticLocalDateTimeof(intyear,Monthmonth,intdayOfMonth,
inthour,intminute,intsecond,intnanoOfSecond)
staticLocalDateTimeof(LocalDatedate,LocalTimetime)
ThisoverloadedstaticfactorymethodintheLocalDateTimeclassreturnsan
instanceofLocalDateTimefromthespecifiedtimeanddate-basedvalues.The
secondandnanosecondvaluesaresettozero,ifnotspecified.The
java.time.Monthenumtypeallowsmonthstobereferredbyname—for
example,Month.MARCH(i.e.,month3intheyear).
TheclassLocalDateTimeallowsthedateandthetimetobecombinedintooneentity,
whichisusefulforrepresentingsuchconceptsasappointments.Theof()methodsinthe
LocalDateTimeclassarecombinationsoftheof()methodsfromtheLocalTime
andLocalDateclasses,takingbothtime-basedanddate-basedvaluesasarguments.The
toString()methodofthisclasswillformatthetemporalvaluesaccordingtotheISO
standardasuuuu-MM-dd'T'HH:mm:ss.SSSSSSSSS.TheletterTseparatesthedate-
basedvaluesfromthetime-basedvalues(§11.4,p.495).
Clickheretoviewcodeimage
//2015-04-28T12:15
LocalDateTimedt1=LocalDateTime.of(2015,4,28,12,15);
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//2015-08-19T14:00
LocalDateTimedt2=LocalDateTime.of(2015,Month.AUGUST,19,14,0);
TheLocalDateTimeclassalsoprovidesanof()methodthatcombinesa
LocalDateobjectandaLocalTimeobject.Thefirstdeclarationinthenextcode
snippetcombinesadateandatime.ThestaticfieldLocalTime.NOONdefinesthetime
atnoon.Inaddition,theLocalTimeclassprovidestheinstancemethodatDate(),
whichtakesaspecifieddateasanargumentandreturnsaLocalDateTimeobject.The
seconddeclarationcombinesthetimeatnoonwiththedatereferredtobythereference
date1.Conversely,theLocalDateclassprovidestheoverloadedinstancemethod
atTime()tocombineadatewithaspecifiedtime.Inthelasttwodeclarations,the
atTime()methodispassedaLocalTimeobjectandspecifictime-basedvalues,
respectively.
Clickheretoviewcodeimage
//1969-07-20T12:00
LocalDateTimedt3=LocalDateTime.of(date1,LocalTime.NOON);
LocalDateTimedt4=LocalTime.of(12,0).atDate(date1);
LocalDateTimedt5=date1.atTime(LocalTime.NOON);
LocalDateTimedt6=date1.atTime(12,0);
Asaconvenience,eachtemporalclassprovidesastaticmethodnow()thatreadsthe
systemclockandreturnstherelevanttemporalvaluesinaninstanceofthetargetclass.
Clickheretoviewcodeimage
LocalTimecurrentTime=LocalTime.now();
LocalDatecurrentDate=LocalDate.now();
LocalDateTimecurrentDateTime=LocalDateTime.now();
Example11.1includesthedifferentwaystocreatetemporalobjectsthatwehave
discussedsofar.
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//LocalTime
LocalDateTimeatDate(LocalDatedate)
ReturnsaLocalDateTimethatcombinesthistimewiththespecifieddate.
Clickheretoviewcodeimage
//LocalDate
LocalDateTimeatTime(inthour,intminute)
LocalDateTimeatTime(inthour,intminute,intsecond)
LocalDateTimeatTime(inthour,intminute,intsecond,intnanoOfSecond)
LocalDateTimeatTime(LocalTimetime)
ReturnsaLocalDateTimethatcombinesthisdatewiththespecifiedtime-based
values.Thesecondandnanosecondvaluesaresettozero,ifnotspecified.
Clickheretoviewcodeimage
//LocalTime,LocalDate,LocalDateTime
staticTemporalTypenow()
Eachtemporalclasshasthisstaticfactorymethod,whichreturnseitherthecurrent
time,date,ordate-timefromthesystemclockinthedefaulttimezone,where
TemporalTypeiseitherLocalTime,LocalDate,orLocalDateTime,
respectively.
Example11.1CreatingTemporalObjects
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.Month;
publicclassCreatingTemporals{
publicstaticvoidmain(String[]args){
//Creatingaspecifictimefromtime-basedvalues:
LocalTimetime1=LocalTime.of(8,15,35,900);//08:15:35.000000900
LocalTimetime2=LocalTime.of(16,45);//16:45
//LocalTimetime3=LocalTime.of(25,13,30);//DateTimeException
System.out.println(“Surveillancestarttime:”+time1);
System.out.println(“Closingtime:”+time2);
//Creatingaspecificdatefromdate-basedvalues:
LocalDatedate1=LocalDate.of(1969,7,20);//1969-07-20
LocalDatedate2=LocalDate.of(-3113,Month.AUGUST,11);//-3113-08-11
//LocalDatedate3=LocalDate.of(2015,13,11);//
DateTimeException
//LocalDatedate4=LocalDate.of(2015,2,29);//
DateTimeException
System.out.println(“Dateoflunarlanding:”+date1);
System.out.println(“StartDateofMayanCalendar:”+date2);
//Creatingaspecificdate-timefromdate-andtime-basedvalues.
//2015-04-28T12:15
LocalDateTimedt1=LocalDateTime.of(2015,4,28,12,15);
//2015-08-17T14:00
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LocalDateTimedt2=LocalDateTime.of(2015,Month.AUGUST,17,14,0);
System.out.println(“Carserviceappointment:”+dt1);
System.out.println(“Hospitalappointment:”+dt2);
//Combiningdateandtimeobjects.
//1969-07-20T12:00
LocalDateTimedt3=LocalDateTime.of(date1,LocalTime.NOON);
LocalDateTimedt4=LocalTime.of(12,0).atDate(date1);
LocalDateTimedt5=date1.atTime(LocalTime.NOON);
LocalDateTimedt6=date1.atTime(12,0);
System.out.println(“Factorydate-timecombo:”+dt3);
System.out.println(“Timewithdatecombo:”+dt4);
System.out.println(“Datewithtimecombo:”+dt5);
System.out.println(“Datewithexplicittimecombo:”+dt6);
//Currenttime:
LocalTimecurrentTime=LocalTime.now();
System.out.println(“Currenttime:”+currentTime);
//Currentdate:
LocalDatecurrentDate=LocalDate.now();
System.out.println(“Currentdate:”+currentDate);
//Currentdateandtime:
LocalDateTimecurrentDateTime=LocalDateTime.now();
System.out.println(“Currentdate-time:”+currentDateTime);
}
}
Possibleoutputfromtheprogram:
Clickheretoviewcodeimage
Surveillancestarttime:08:15:35.000000900
Closingtime:16:45
Dateoflunarlanding:1969-07-20
StartDateofMayanCalendar:-3113-08-11
Carserviceappointment:2015-04-28T12:15
Hospitalappointment:2015-08-17T14:00
Factorydate-timecombo:1969-07-20T12:00
Timewithdatecombo:1969-07-20T12:00
Datewithtimecombo:1969-07-20T12:00
Datewithexplicittimecombo:1969-07-20T12:00
Currenttime:10:32:03.069
Currentdate:2015-08-21
Currentdate-time:2015-08-21T10:32:03.083
QueryingTemporalObjects
Atemporalobjectprovidesgetmethodsthataretailoredtoaccessthespecifictemporal
valuesthatconstituteitsstate.TheLocalTimeclassprovidesgetmethodsforthetime-
basedvalues,andtheLocalDateclassprovidesgetmethodsforthedate-basedvalues.
Notsurprisingly,theLocalDateTimeclassprovidesgetmethodsforbothtime-and
date-basedvalues.Usageofthegetmethodsisstraightforward,asshowninExample11.2.
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//LocalTime,LocalDateTime
intgetHour()
intgetMinute()
intgetSecond()
intgetNano()
Getstheappropriatetime-basedvaluefromthecurrentLocalTimeorLocal-
DateTimeobject.
//LocalDate,LocalDateTime
intgetDayOfMonth()
DayOfWeekgetDayOfWeek()
intgetDayOfYear()
MonthgetMonth()
intgetMonthValue()
intgetYear()
Getstheappropriatedate-basedvaluefromthecurrentLocalDateor
LocalDateTimeobject.TheenumtypeDayOfWeekallowsdaysoftheweekto
bereferredtobyname—forexample,DayOfWeek.MONDAYisday1oftheweek.
TheenumtypeMonthallowsmonthsoftheyeartobereferredbyname—for
example,Month.JANUARY.Themonthvalueisfrom1(Month.JANUARY)to
12(Month.DECEMBER).
ThetemporalclassLocalDateTimealsoprovidestwomethodstoobtainthedateand
thetimeastemporalobjects,incontrasttoaccessingindividualdate-andtime-based
values.
Clickheretoviewcodeimage
LocalDateTimedoomsday=LocalDateTime.of(1945,8,6,8,15);
LocalDatedate=doomsday.toLocalDate();//1945-08-06
LocalTimetime=doomsday.toLocalTime();//08:15
//LocalDateTime
LocalDatetoLocalDate()
LocalTimetoLocalTime()
ThesemethodscanbeusedtogettheLocalDateandLocalTimepartofthis
date-time,respectively.
Itisalsopossibletocheckwhetheratemporalobjectrepresentsapointintimebeforeor
afteranothertemporalobjectofthesametype.Inaddition,theLocalDateand
LocalDateTimeclassesprovideanisEqual()methodthatdetermineswhethera
temporalobjectisequaltoanothertemporalobjectofthesametype.Incontrast,the
equals()methodallowsequalitycomparisonwithanarbitraryobject.
Clickheretoviewcodeimage
LocalDated1=LocalDate.of(-1004,3,1);//-1004-03-01
LocalDated2=LocalDate.of(1004,3,1);//1004-03-01
booleanresult1=d1.isBefore(d2);//true
booleanresult2=d2.isAfter(d1);//true
booleanresult3=d1.isAfter(d1);//false
booleanresult4=d1.isEqual(d2);//false
booleanresult5=d1.isEqual(d1);//true
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booleanresult6=d2.isLeapYear();//true
TheisLeapYear()methodoftheLocalDateclasscheckswhetherayearinadateis
aleapyear.
Clickheretoviewcodeimage
//LocalTime
booleanisAfter(LocalTimeother)
booleanisBefore(LocalTimeother)
ThesemethodsdeterminewhetherthisLocalTimerepresentsapointonthelocal
time-lineafterorbeforetheothertime,respectively.
Clickheretoviewcodeimage
//LocalDate
booleanisAfter(ChronoLocalDateother)
booleanisBefore(ChronoLocalDateother)
booleanisEqual(ChronoLocalDateother)
booleanisLeapYear()
ThefirsttwomethodsdeterminewhetherthisLocalDaterepresentsapointon
thelocaltimelineafterorbeforetheotherdate,respectively.TheLocalDate
classimplementstheChronoLocalDateinterface.
Thethirdmethoddetermineswhetherthisdateisequaltothespecifieddate.
ThelastmethodchecksforaleapyearaccordingtotheISOprolepticcalendar
systemrules.
Clickheretoviewcodeimage
//LocalDateTime
booleanisAfter(ChronoLocalDateTime<?>other)
booleanisBefore(ChronoLocalDateTime<?>other)
booleanisEqual(ChronoLocalDateTime<?>other)
ThefirsttwomethodsdeterminewhetherthisLocalDateTimerepresentsapoint
onthelocaltimelineafterorbeforethespecifieddate-time,respectively.The
LocalDateTimeclassimplementstheChronoLocalDateTimeinterface.
Thethirdmethoddetermineswhetherthisdate-timerepresentsthesamepointon
thelocaltimelineastheotherdate-time.
ComparingTemporalObjects
ThetemporalclassesimplementtheComparableinterface,providingthe
compareTo()methodsothattemporalobjectscanbecomparedinameaningfulway.
Thetemporalclassesalsooverridetheequals()methodoftheObjectclass.These
methodsmakeitpossibletobothsearchforandsorttemporalobjects.
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Clickheretoviewcodeimage
intcompareTo(LocalTimeother)//LocalTime
intcompareTo(ChronoLocalDateother)//LocalDate
intcompareTo(ChronoLocalDateTime<?>other)//LocalDateTime
Thesemethodscomparethistemporalobjecttoanothertemporalobject.Thethree
temporalclassesimplementtheComparablefunctionalinterface.The
compareTo()methodreturns0ifthetwotemporalobjectsareequal,anegative
valueifthistemporalobjectislessthantheothertemporalobject,andapositive
valueifthistemporalobjectisgreaterthantheothertemporalobject.
Clickheretoviewcodeimage
booleanequals(Objectobj)//LocalTime,LocalDate,LocalDateTime
Thethreetemporalclassesoverridetheequals()methodoftheObjectclass.
Themethodcheckswhetherthistemporalobjectisequaltoanotherobject.The
specifiedobjectmustbeofthesametypeasthistemporalobject;otherwise,the
resultisfalse.
CreatingModifiedCopiesofTemporalObjects
Animmutableobjectdoesnotprovideanysetmethodsthatcanchangeitsstate.Instead,it
usuallyprovideswhatareknownaswithmethods(or“withers”)thatreturnacopyofthe
originalobjectwhereexactlyonepropertyhasbeensettoanewvalue.TheLocalTime
andLocalDateclassesprovidewithmethodstosetatime-ordate-basedvalue,
respectively.Notsurprisingly,theLocalDateTimeclassprovideswithmethodstoset
bothtime-anddate-basedvaluesindividually.Awithmethodchangesaspecific
propertyinanabsoluteway,whichisreflectedinthestateofthenewtemporalobject;the
originalobject,however,isnotaffected.Suchwithmethodsarealsocalledabsolute
adjusters,incontrasttotherelativeadjustersthatwewillmeetlater(p.474).
Clickheretoviewcodeimage
LocalDatedate2=LocalDate.of(2015,3,1);//2015-03-01
date2=date2.withYear(2016).withMonth(2).withDayOfMonth(28);//2016-02-28
TheprecedingcodelinesarefromExample11.2.Inthesecondassignmentstatement,the
methodcallsarechained.ThreeinstancesoftheLocalDateclassarecreated
consecutively,aseachwithmethodiscalledtosetaspecificdate-basedvalue.
Clickheretoviewcodeimage
date2=date2.withYear(2016);//2016-03-01
date2.withMonth(2).withDayOfMonth(28);//date2isstill2016-03-
01.
Thiscodecontainsalogicalerror,suchthatthelasttwoLocalDateinstancesreturned
bythewithmethodsareignored,andthereferencedate2nevergetsupdated.
Inthenextcodeexamples,eachcalltoawithmethodthrowsa
DateTimeException.TheminuteandhourvaluesareoutofrangeforaLocalTime
object.Certainlythemonthvalue13isoutofrangeforaLocalDateobject.Thedayof
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themonthvalue31isnotvalidfortheAprilmonth,whichhas30days.Thedayofthe
yearvalue366isoutofrangeaswell,sincetheyear2015isnotaleapyear.
Clickheretoviewcodeimage
LocalTimetime=LocalTime.of(14,45);//14:45
time=time.withMinute(100);//Outofrange.DateTimeException.
time=time.withHour(25);//Outofrange.DateTimeException.
LocalDatedate=LocalDate.of(2015,4,30);//2015-04-30
date=date.withMonth(13);//Outofrange.DateTimeException.
date=date.withDayOfMonth(31);//Outofrangeformonth.
DateTimeException.
date=date.withDayOfYear(366);//Outofrangeforyear.
DateTimeException.
ThenextcodesnippetillustrateshowthewithYear()andthewithMonth()methods
adjustthedayofthemonth,ifnecessary,whentheyearorthemonthischanged,
respectively.Theyearinthedate2016-02-29ischangedto2017,resultinginthe
followingdate:2017-02-29.Sincetheyear2017isnotaleapyear,themonthofFebruary
cannothave29days.ThewithYear()methodadjuststhedayofthemonthtothelast
validdayofthemonth,28.Similarly,themonthinthedate2015-03-31ischangedto4
(i.e.,April),resultinginthefollowingdate:2015-04-31.SincethemonthAprilhas30
days,thewithMonth()methodadjuststhedayofthemonthtothelastvaliddayofthe
month,30.
Clickheretoviewcodeimage
LocalDatedate3=LocalDate.of(2016,2,29);//Original:2016-02-29
date3=date3.withYear(2017);//Expected:2017-02-29
System.out.println(“Date3:”+date3);//Adjusted:2017-02-28
LocalDatedate4=LocalDate.of(2015,3,31);//Original:2015-03-31
date4=date4.withMonth(4);//Expected:2015-04-31
System.out.println(“Date4:”+date4);//Adjusted:2015-04-30
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Clickheretoviewcodeimage
//LocalTime,LocalDateTime
LocalTime/LocalDateTimewithHour(inthour)
LocalTime/LocalDateTimewithMinute(intminute)
LocalTime/LocalDateTimewithSecond(intsecond)
LocalTime/LocalDateTimewithNano(intnanoOfSecond)
ReturnsacopyofthisLocalTimeorLocalDateTimewiththeappropriate
time-basedvaluechangedtothespecifiedvalue.ADateTimeExceptionis
throwniftheargumentvalueisoutofrange.
Clickheretoviewcodeimage
//LocalDate,LocalDateTime
LocalDate/LocalDateTimewithYear(intyear)
LocalDate/LocalDateTimewithMonth(intmonth)
LocalDate/LocalDateTimewithDayOfMonth(intdayOfMonth)
LocalDate/LocalDateTimewithDayOfYear(intdayOfYear)
ReturnsacopyofthisLocalDateorLocalDateTimewiththeappropriate
date-basedvaluechangedtothespecifiedvalue.ADateTimeExceptionis
thrown,ifthespecifiedvalueisoutofrangeorisinvalidincombinationwithother
time-ordate-basedvaluesinthetemporalobject.
Thefirstandsecondmethodswilladjustthedayofthemonthtothelastvalidday
ofthemonth,ifthedayofmonthbecomesinvalidwhentheyearorthemonthis
changed(e.g.,themonthvalue2willchangethedate2016-03-31to2016-02-29).
Incontrast,thethirdmethodwillthrowaDateTimeExceptionifthespecified
dayofthemonthisinvalidforthemonth-yearcombination(e.g.,thedayofmonth
29isinvalidforFebruary2015),aswillthelastmethodifthedayoftheyearis
invalidfortheyear(e.g.,thedayofyear366isinvalidfortheyear2015).
Example11.2UsingTemporalObjects
Clickheretoviewcodeimage
importjava.time.DayOfWeek;
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.Month;
publicclassUsingTemporals{
publicstaticvoidmain(String[]args){
//Date-Time:1945-08-06T08:15
LocalDateTimedoomsday=LocalDateTime.of(1945,8,6,8,15);
LocalDatedate=doomsday.toLocalDate();//1945-08-06
LocalTimetime=doomsday.toLocalTime();//08:15
System.out.println(“Date-Time:”+doomsday);
//Time:08:15
inthour=time.getHour();//8
intminute=time.getMinute();//15
intsecond=time.getSecond();//0
System.out.println(“Time:”+time);
System.out.println(“Hour:”+hour);
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System.out.println(“Min:”+minute);
System.out.println(“Sec:”+second);
//Date:1945-08-06
intyear=date.getYear();//1945
intmonthValue=date.getMonthValue();//8
Monthmonth=date.getMonth();//AUGUST
DayOfWeekdow=date.getDayOfWeek();//MONDAY
intday=date.getDayOfMonth();//6
System.out.println(“Date:”+date);
System.out.println(“Year:”+year);
System.out.println(“Monthvalue:”+monthValue);
System.out.println(“Month:”+month);
System.out.println(“DoW:”+dow);
System.out.println(“DoM:”+day);
//Ordering
LocalDated1=LocalDate.of(-1004,3,1);//-1004-03-01
LocalDated2=LocalDate.of(1004,3,1);//1004-03-01
booleanresult1=d1.isBefore(d2);//true
booleanresult2=d2.isAfter(d1);//true
booleanresult3=d1.isAfter(d1);//false
booleanresult4=d1.isEqual(d2);//false
booleanresult5=d1.isEqual(d1);//true
booleanresult6=d2.isLeapYear();//true
System.out.println(d1+”isbefore“+d2+“:”+result1);
System.out.println(d2+”isafter“+d1+“:”+result2);
System.out.println(d1+”isafter“+d1+“:”+result3);
System.out.println(d1+”isequalto”+d2+“:”+result4);
System.out.println(d1+”isequalto”+d1+“:”+result5);
System.out.println(d2.getYear()+”isaleapyear:”+result6);
//Absoluteadjusters:
LocalDatedate2=LocalDate.of(2015,3,1);
System.out.println(“Datebeforeadjusting:”+date2);//2015-03-01
date2=date2.withYear(2016).withMonth(2).withDayOfMonth(28);
System.out.println(“Dateafteradjusting:”+date2);//2016-02-28
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Date-Time:1945-08-06T08:15
Time:08:15
Hour:8
Min:15
Sec:0
Date:1945-08-06
Year:1945
Monthvalue:8
Month:AUGUST
DoW:MONDAY
DoM:6
-1004-03-01isbefore1004-03-01:true
1004-03-01isafter-1004-03-01:true
-1004-03-01isafter-1004-03-01:false
-1004-03-01isequalto1004-03-01:false
-1004-03-01isequalto-1004-03-01:true
1004isaleapyear:true
Datebeforeadjusting:2015-03-01
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Dateafteradjusting:2016-02-28
TemporalArithmetic
Thetemporalclassesprovideplusandminusmethodsthatreturnacopyoftheoriginal
objectthathasbeenincrementedordecrementedbyaspecificamountoftime—for
example,bynumberofhoursorbynumberofmonths.
TheLocalTimeandLocalDateclassesprovideplus/minusmethodsto
increment/decrementatimeoradatebyaspecificamountintermsofatimeunit(for
example,hours,minutes,andseconds)oradateunit(forexample,years,months,and
days),respectively.TheLocalDateTimeclassprovidesplus/minusmethodsto
increment/decrementadate-timebyanamountthatisspecifiedintermsofeitheratime
unitoradateunit.Forexample,theplusMonths()methodintheLocalDateclass
returnsanewLocalDateobjectafteraddingthespecifiednumberofmonthspassedas
anargumenttothemethod.Similarly,theminusMinutes()methodinthe
LocalTimeclassreturnsanewLocalTimeobjectaftersubtractingthespecified
numberofminutespassedasanargumenttothemethod.Thechangeisrelative,and
reflectedinthenewtemporalobjectthatisreturned.Suchplus/minusmethodsare
alsocalledrelativeadjusters,incontrasttoabsoluteadjusters(p.470).
Example11.3demonstrateswhatwecancalltemporalarithmetic,whereaLocalDate
objectismodifiedbyaddingorsubtractinganamountspecifiedasdays,weeks,or
months.Notehowthedate-basedvaluesareadjustedaftereachoperation.Thedate
2015-10-23iscreatedat(1),and10months,3weeks,and40daysaresuccessively
addedtothenewdateobjectreturnedbyeachplusmethodcallat(2),(3),and(4),
respectively,resultinginthedate2016-10-23.Wethensubtract2days,4weeks,and
11monthssuccessivelyfromthenewdateobjectreturnedbyeachminusmethodcallat
(5),(6),and(7),respectively,resultinginthedate2015-10-23.InExample11.3,
severalassignmentstatementsareusedtoprinttheintermediatedates,butthecodecanbe
mademorecompactbymethodchaining.
Clickheretoviewcodeimage
LocalDatedate=LocalDate.of(2015,10,23);//2015-10-23
date=date.plusMonths(10).plusWeeks(3).plusDays(40);//Methodchaining
System.out.println(date);//2016-10-23
date=date.minusDays(2).minusWeeks(4).minusMonths(11);//Methodchaining
System.out.println(date);//2015-10-23
Thefollowingcodesnippetillustratesthewrappingoftimearoundmidnight,asone
wouldexpectona24-hourclock.EachmethodcallreturnsanewLocalTimeobject.
Clickheretoviewcodeimage
LocalTimewitchingHour=LocalTime.MIDNIGHT//00:00
.plusHours(14)//14:00
.plusMinutes(45)//14:45
.plusMinutes(30)//15:15
.minusHours(15)//00:15
.minusMinutes(15);//00:00
ThenextcodesnippetillustrateshowtheplusYears()methodadjuststhedayofthe
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month,ifnecessary,whentheyearvalueischanged.Theyearinthedate2016-02-29is
changedto2017byadding1year,resultinginthefollowingdate:2017-02-29.The
plusYears()methodadjuststhedayofthemonthtothelastvaliddayofthe
month,28;astheyear2017isnotaleapyear,themonthofFebruarycannothave29
days.
Clickheretoviewcodeimage
LocalDatedate5=LocalDate.of(2016,2,29);//Original:2016-02-29
date5=date5.plusYears(1);//Expected:2017-02-29
System.out.println(“Date5:”+date5);//Adjusted:2017-02-28
Clickheretoviewcodeimage
//LocalTime,LocalDateTime
LocalTime/LocalDateTimeminusHours/plusHours(longhours)
LocalTime/LocalDateTimeminusMinutes/plusMinutes(longminutes)
LocalTime/LocalDateTimeminusSeconds/plusSeconds(longseconds)
LocalTime/LocalDateTimeminusNanos/plusNanos(longnanos)
ThesemethodsreturnacopyofthisLocalTimeorLocalDateTimeobject
withthespecifiedamounteithersubtractedoraddedtoaspecifictime-basedvalue.
Thecalculationalwayswrapsaroundmidnight.
ForthemethodsoftheLocalDateTimeclass,aDateTimeExceptionis
throwniftheresultexceedsthedaterange.
Clickheretoviewcodeimage
//LocalDate,LocalDateTime
LocalDate/LocalDateTimeminusYears/plusYears(longyears)
LocalDate/LocalDateTimeminusMonths/plusMonths(longmonths)
LocalDate/LocalDateTimeminusWeeks/plusWeeks(longweeks)
LocalDate/LocalDateTimeminusDays/plusDays(longdays)
ThesemethodsreturnacopyofthisLocalDateorLocalDateTimewiththe
specifiedamounteithersubtractedoraddedtoaspecificdate-basedvalue.
AllmethodsthrowaDateTimeExceptioniftheresultexceedsthedaterange.
Thefirstandsecondmethodswillchangethedayofthemonthtothelastvalidday
ofthemonthifnecessary,whenthedayofmonthbecomesinvalidasaresultofthe
operation.
Thethirdandlastmethodswilladjustthemonthandyearfieldsasnecessaryto
ensureavalidresult.
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Example11.3TemporalArithmetic
Clickheretoviewcodeimage
importjava.time.LocalDate;
publicclassTemporalArithmetic{
publicstaticvoidmain(String[]args){
LocalDatedate=LocalDate.of(2015,10,23);//(1)
System.out.println(“Date:”+date);//2015-10-23
date=date.plusMonths(10);//(2)
System.out.println(“10monthsafter:”+date);//2016-08-23
date=date.plusWeeks(3);//(3)
System.out.println(“3weeksafter:”+date);//2016-09-13
date=date.plusDays(40);//(4)
System.out.println(“40daysafter:”+date);//2016-10-23
date=date.minusDays(2);//(5)
System.out.println(“2daysbefore:”+date);//2016-10-21
date=date.minusWeeks(4);//(6)
System.out.println(“4weeksbefore:”+date);//2016-09-23
date=date.minusMonths(11);//(7)
System.out.println(“11monthsbefore:”+date);//2015-10-23
}
}
Outputfromtheprogram:
Date:2015-10-23
10monthsafter:2016-08-23
3weeksafter:2016-09-13
40daysafter:2016-10-23
2daysbefore:2016-10-21
4weeksbefore:2016-09-23
11monthsbefore:2015-10-23
11.3WorkingwithPeriods
Forrepresentinganamountoftime,theDateandTimeAPIprovidesthetwoclasses
PeriodandDuration.WewillbrieflymentiontheDurationclass,andconcentrate
onthePeriodclass.
TheDurationclassmodelsanamountoftimeintermsofsecondsandnanoseconds,but
aDurationobjectcanalsobeaccessedintermsofdays,hours,andminutes.It
essentiallyrepresentsatime-basedamountoftime,whereasthePeriodclassrepresents
adate-basedamountoftimeintermsofyears,months,anddays.Thetime-based
DurationclasscanbeusedwiththeLocalTimeclass,andnotsurprisingly,thedate-
basedPeriodclasscanbeusedwiththeLocalDateclass.Ofcourse,the
LocalDateTimeclasscanusebothclasses.
ThePeriodclassisinthesamepackage(java.time)asthetemporalclasses,andits
repertoireofmethodsshouldalsolookfamiliar,asitsharesmanyofthemethodprefixes
withthetemporalclasses(Table11.1,p.463).
Themantraofimmutableandthread-safeobjectsalsoappliestothePeriodclass.
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CreatingPeriods
Likethetemporalclasses,thePeriodclassdoesnotprovideanypublicconstructors,
butratherprovidesanoverloadedstaticfactorymethodof()toconstructperiodsof
differentlengths,basedonadateunit.
Clickheretoviewcodeimage
Periodp=Period.of(2,4,8);//(1)
System.out.println(p);//(2)P2Y4M8D(2Years,4Months,8
Days)
Periodp1=Period.ofYears(10);//P10Y,periodof10years.
Periodp2=Period.ofMonths(14);//P14M,periodof14months.
Periodp3=Period.ofDays(40);//P40D,periodof40days.
Periodp4=Period.ofWeeks(2);//P14D,periodof14days(2weeks).
Themostversatileof()methodrequirestheamountoftimeforalldateunits:years,
months,anddays,asin(1).ThetoString()methodofthePeriodclassreturnsa
textualrepresentationofaPeriodobjectintheISOstandard.Theoutputfrom(2),
P2Y4M8D,indicatesaperiodof2years,4months,and8days.Otherof()methods
createaperiodbasedonaparticulardateunit,asshowninthepreviousexamples.
Thenextcodesnippetdoesnotcreateaperiodof3years,4months,and5days.Thefirst
methodcallusestheclassname,andthesubsequentmethodcallsusethePeriodobject
returnedasaconsequenceofthepreviouscall.Theof()methodcreatesanewPeriod
objectbasedonitsargument.
Clickheretoviewcodeimage
Periodperiod=Period.ofYears(3).ofMonths(4).ofDays(5);//P5D.Logical
error.
Aswewouldexpect,wecancreateaperiodthatrepresentstheamountoftimebetween
twodatesbycallingthestaticmethodbetween()ofthePeriodclass.
Clickheretoviewcodeimage
LocalDated1=LocalDate.of(2015,3,1);//2015-03-01
LocalDated2=LocalDate.of(2016,3,1);//2016-03-01
Periodperiod12=Period.between(d1,d2);//P1Y
Periodperiod21=Period.between(d2,d1);//P-1Y
ThePeriodclassalsoprovidesthestaticmethodparse()tocreateaperiodfroma
stringthatcontainsatextualrepresentationofaperiodintheISOstandard.Iftheformatis
notcorrect,ajava.time.format.DateTimeParseExceptionisthrown.
Clickheretoviewcodeimage
Periodperiod2=Period.parse(“P1Y15M20D”);//1year,15months,20days
Periodperiod3=Period.parse(“P20D”);//20days
Periodperiod4=Period.parse(“P5W”);//35days(5weeks)
//PeriodpX=Period.parse(“P24H”);//
java.time.format.DateTimeParseException
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Clickheretoviewcodeimage
staticPeriodof(intyears,intmonths,intdays)
staticPeriodofYears(intyears)
staticPeriodofMonths(intmonths)
staticPeriodofWeeks(intweeks)
staticPeriodofDays(intdays)
ThesestaticfactorymethodsreturnaPeriodrepresentinganamountoftime
equaltothespecifiedvalueofadateunit.Date-basedvaluesimplicitlyimpliedare
settozero.Aweekisequalto7days.Theargumentvaluecanbenegative.
Clickheretoviewcodeimage
staticPeriodbetween(LocalDatestartDateInclusive,
LocalDateendDateExclusive)
ThisstaticmethodreturnsaPeriodconsistingofthenumberofyears,months,
anddaysbetweentwodates.Thecalculationexcludestheenddate.
Clickheretoviewcodeimage
staticPeriodparse(CharSequencetext)
ThisstaticmethodreturnsaPeriodparsedfromacharactersequence—for
example,"P3Y10M2D"(3years,10months,2days).A
java.time.format.DateTimeParseExceptionisthrownifthetext
cannotbeparsedtoaperiod.
QueryingPeriods
ThePeriodclassprovidestheobviousgetmethodstoreadthedate-basedpartsofa
Periodobject.Theclassalsohasmethodstocheckifanydate-basedpartofaperiodis
negativeorifalldate-basedpartsofaperiodarezero.
Clickheretoviewcodeimage
Periodperiod5=Period.of(2,4,-10);
System.out.println(“Period:”+period5);//Period:P2Y4M-10D
System.out.println(“Years:”+period5.getYears());//Years:2
System.out.println(“Months:”+period5.getMonths());//Months:4
System.out.println(“Days:”+period5.getDays());//Days:-10
System.out.println(“Totalmonths:”+period5.toTotalMonths());//28months
System.out.println(period5.isNegative());//true
System.out.println(period5.isZero());//false
TheclassPeriodprovidesthemethodtoTotalMonths()toderivethetotalnumber
ofmonthsinaperiod.However,thiscalculationissolelybasedonthenumberofyears
andmonthsintheperiod;thenumberofdaysisnotconsidered.APeriodjustrepresents
anamountoftime,soithasnonotionofadate.Conversionbetweenmonthsandyearsis
notaproblem,as1yearis12months.However,conversionbetweenthenumbersofdays
andtheotherdateunitsisproblematic.Thenumberofdaysinayearandinamonthare
verymuchdependentonwhethertheyearisaleapyearandonaparticularmonthinthe
year,respectively.APeriodisobliviousaboutboththeyearandthemonthintheyear.
ThePeriodclassoverridestheequals()methodoftheObjectclass.Eachdate-
basedpartiscomparedindividually,andmusthavethesamevaluetobeconsideredequal.
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Aperiodof1yearand14monthsisnotequaltoaperiodof2yearsand2months,ortoa
periodof26months.
Clickheretoviewcodeimage
Periodpx=Period.of(1,14,0);
Periodpy=Period.of(2,2,0);
Periodpz=Period.ofMonths(26);
System.out.println(px.equals(py));//false
System.out.println(px.equals(pz));//false
System.out.println(px.equals(Period.ZERO));//false
intgetYears()
intgetMonths()
intgetDays()
Returnsthevalueofaspecificdateunitofthisperiod.
booleanisNegative()
Determineswhetheranyofthedate-basedvaluesofthisperiodarenegative.
booleanisZero()
Determineswhetheralldate-basedvaluesofthisperiodarezero.
longtoTotalMonths()
Returnsthetotalnumberofmonthsinthisperiod,basedontheyearsandthe
monthsvalue.Thedaysvalueisnotconsidered.
booleanequals(Objectobj)
Determineswhetherthisperiodisequaltoanotherperiod,meaningthateachdate
unithasthesamevalue.
CreatingModifiedCopiesofPeriods
ThePeriodclassprovideswithmethodstosetanewvalueforeachdateunit
individually,whilethevaluesoftheotherdateunitsremainunchanged.Notethateach
methodcallreturnsanewPeriodobject,andchainingmethodcallsworksasexpected.
Clickheretoviewcodeimage
Periodp5=Period.of(2,1,30)//P2Y1M30D
.withYears(3)//P3Y1M30D,setsthenumberofyears
.withMonths(16)//P3Y16M30D,setsthenumberofmonths
.withDays(1);//P3Y16M1D,setsthenumberofdays
PeriodwithYears(intyears)
PeriodwithMonths(intmonths)
PeriodwithDays(intdays)
Returnsacopyofthisperiodwhereaspecificdateunitissettothevalueofthe
argument.Thevaluesoftheotherdateunitsarenotaffected.
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MoreTemporalArithmetic
ThePeriodclassprovidesplusandminusmethodsthatreturnacopyoftheoriginal
objectthathasbeenincrementedordecrementedbyaspecificamountspecifiedinterms
ofadateunit—forexample,asanumberofyears,months,ordays.Asthenextcode
snippetshows,onlythevalueofaspecificdateunitischanged;theotherdate-based
valuesareunaffected.Thereisnoimplicitnormalizationperformed,unlessthe
normalized()methodiscalled.Thismethodnormalizesonlythemonths,adjusting
thevaluesofthemonthsandyearsasnecessary.
Clickheretoviewcodeimage
Periodp6=Period.of(2,10,30)//P2Y10M30D
.plusDays(10)//P2Y10M40D
.plusMonths(8)//P2Y18M40D
.plusYears(1)//P3Y18M40D
.normalized();//P4Y6M40D
Wecandosimplearithmeticwithperiods.Thenextcodeexamplesusetheplus()and
minus()methodsofthePeriodclassthattakeaTemporalAmountasanargument.
BoththePeriodandtheDurationclassesimplementtheTemporalAmount
interface.Inthelastassignmentstatement,wehaveshownthestateofbothnewPeriod
objectsthatarecreated.
Clickheretoviewcodeimage
Periodp7=Period.of(1,1,1);//P1Y1M1D
Periodp8=Period.of(2,12,30);//P2Y12M30D
Periodp9=p8.minus(p7);//P1Y11M29D
p8=p8.plus(p7).plus(p8);//P3Y13M31D,P5Y25M61D
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PeriodminusYears/plusYears(longyears)
PeriodminusMonths/plusMonths(longmonths)
PeriodminusDays/plusDays(longdays)
Returnsacopyofthisperiod,withthespecifieddate-basedvaluesubtractedor
added.Theotherdate-basedvaluesareunaffected.
Clickheretoviewcodeimage
Periodminus/plus(TemporalAmountamount)
Returnsacopyofthisperiod,withthespecifiedamountsubtractedoradded.The
amountisoftheinterfacetypeTemporalAmountthatisimplementedbythe
classesPeriodandDuration.Thereisnonormalizationperformed.A
DateTimeExceptionisthrowniftheoperationcannotbeperformed.
Periodnormalized()
Returnsacopyofthisperiodwheretheyearsandmonthsarenormalized.The
numberofdaysisnotaffected.
Periodnegated()
ReturnsanewinstanceofPeriodwhereeachdate-basedvalueinthisperiodis
individuallynegated.
Clickheretoviewcodeimage
PeriodmultipliedBy(intscalar)
Returnsanewinstancewhereeachdate-basedvalueinthisperiodisindividually
multipliedbythespecifiedscalar.
Wecanalsodosimplearithmeticwithdatesandperiods.Thefollowingcodeusesthe
plus()andminus()methodsoftheLocalDateclassthattakea
TemporalAmountasanargument:
Clickheretoviewcodeimage
Periodp10=Period.of(1,1,1);//P1Y1M1D
LocalDatedate1=LocalDate.of(2015,4,1);//2015-04-01
LocalDatedate2=date1.plus(p10);//2016-05-02
date1=date1.minus(p10);//2014-02-28
WecanaddandsubtractperiodsfromLocalDateandLocalDateTimeobjects,but
notfromLocalTimeobjects,asaLocalTimeobjecthasonlytime-basedvalues.
Clickheretoviewcodeimage
LocalTimetime=LocalTime.NOON;
time=time.plus(p10);//
java.time.temporal.UnsupportedTemporalTypeException
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//LocalTime,LocalDate,LocalDateTime
TemporalTypeminus(TemporalAmountamount)
TemporalTypeplus(TemporalAmountamount)
Eachtemporalclassprovidesthesetwomethods,whichreturnacopyofthis
temporalobjectwiththespecifiedamounteithersubtractedoradded,where
TemporalTypeiseitherLocalTime,LocalDate,orLocalDateTime.
TheamountisoftheinterfacetypeTemporalAmount,whichisimplementedby
theclassesPeriodandDuration.
BothmethodsthrowaDateTimeExceptioniftheoperationcannotbe
performed.
Clickheretoviewcodeimage
//LocalDate
Perioduntil(ChronoLocalDateendDateExclusive)
Thismethodcalculatestheamountoftimebetweenthisdateandanotherdateasa
Period.Thecalculationexcludestheenddate.
Example11.4isasimpleexampletoillustrateimplementingperiod-basedloops.The
methodreserveDates()at(1)isastubforreservingcertaindates,dependingonthe
periodpassedasanargument.Thefor(;;)loopat(2)usesthe
LocalDate.isBefore()methodtoterminatetheloop,andthe
LocalDate.plus()methodtoincrementthecurrentdatewiththespecifiedperiod.
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Example11.4Period-BasedLoop
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.Period;
publicclassPeriodBasedLoop{
publicstaticvoidmain(String[]args){
reserveDates(Period.ofDays(7),
LocalDate.of(2015,10,20),LocalDate.of(2015,11,20));
System.out.println();
reserveDates(Period.ofMonths(1),
LocalDate.of(2015,10,20),LocalDate.of(2016,1,20));
System.out.println();
reserveDates(Period.of(0,1,7),
LocalDate.of(2015,10,20),LocalDate.of(2016,1,21));
}
publicstaticvoidreserveDates(Periodperiod,//(1)
LocalDatefromDate,
LocalDatetoDateExclusive){
System.out.println(“Startdate:”+fromDate);
for(LocalDatedate=fromDate.plus(period);//(2)
date.isBefore(toDateExclusive);
date=date.plus(period)){
System.out.println(“Reserved(”+period+“):”+date);
}
System.out.println(“Enddate:”+toDateExclusive);
}
}
Outputfromtheprogram:
Startdate:2015-10-20
Reserved(P7D):2015-10-27
Reserved(P7D):2015-11-03
Reserved(P7D):2015-11-10
Reserved(P7D):2015-11-17
Enddate:2015-11-20
Startdate:2015-10-20
Reserved(P1M):2015-11-20
Reserved(P1M):2015-12-20
Enddate:2016-01-20
Startdate:2015-10-20
Reserved(P1M7D):2015-11-27
Reserved(P1M7D):2016-01-03
Enddate:2016-01-21
WeconcludethissectionwithExample11.5,whichbringstogethersomeofthemethods
oftheDateandTimeAPI.Givenadateofbirth,themethodbirthdayInfo()at(1)
calculatestheageandthetimetonextbirthday.Theageiscalculatedat(2)usingthe
Period.between()method,whichcomputestheperiodbetweentwodates.Thedate
fornextbirthdayissetat(3)asthebirthdatewiththecurrentyear.Theifstatementat
(4)adjuststhenextbirthdaydateby1yearat(5),ifthebirthdayhasalreadypassed.The
statementat(6)calculatesthetimeuntilnextbirthdaybycallingthe
LocalDate.until()method.WecouldalsohaveusedthePeriod.between()
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methodat(6).Thechoicebetweenthesemethodsreallydependsonwhichmethodmakes
thecodemorereadableinagivencontext.
Example11.5MoreTemporalArithmetic
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.Month;
importjava.time.Period;
publicclassActYourAge{
publicstaticvoidmain(String[]args){
birthdayInfo(LocalDate.of(1981,Month.AUGUST,19));
birthdayInfo(LocalDate.of(1935,Month.JANUARY,8));
}
publicstaticvoidbirthdayInfo(LocalDatedateOfBirth){//(1)
LocalDatetoday=LocalDate.now();
System.out.println(“Today:”+today);
System.out.println(“DateofBirth:”+dateOfBirth);
Periodp1=Period.between(dateOfBirth,today);//(2)
System.out.println(“Age:”+
p1.getYears()+”years,”+
p1.getMonths()+”months,and”+
p1.getDays()+”days”);
LocalDatenextBirthday=dateOfBirth.withYear(today.getYear());//(3)
if(nextBirthday.isBefore(today)||//(4)
nextBirthday.isEqual(today)){
nextBirthday=nextBirthday.plusYears(1);//(5)
}
Periodp2=today.until(nextBirthday);//(6)
System.out.println(“Birthdayin”+p2.getMonths()+”monthsand”+
p2.getDays()+”days”);
}
}
Possibleoutputfromtheprogram:
Clickheretoviewcodeimage
Today:2015-11-20
DateofBirth:1981-08-19
Age:34years,3months,and1days
Birthdayin8monthsand30days
Today:2015-11-20
DateofBirth:1935-01-08
Age:80years,10months,and12days
Birthdayin1monthsand19days
ReviewQuestions
11.1WhichstatementistrueabouttheDateandTimeAPI?
Selecttheonecorrectanswer.
(a)TheclassesLocalDateandLocalDateTimeprovidethe
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isLeapYear()methodtocheckforaleapyear.
(b)TheclassesLocalTime,LocalDate,andLocalDateTimeprovidethe
isEqual()methodtotestwhethertwotemporalobjectsofthesametypeare
equal.
(c)TheclassPeriodprovidesthewithWeeks()method,whichreturnsacopy
ofthisperiod,wherethenumberofdaysissetaccordingtothenumberofweeks
specified.
(d)TheclassesLocalTime,LocalDate,andPeriodprovidethe
plusWeeks()method,whichreturnsanewobject,wherethenumberofdays
correspondingtothespecifiednumberofweekshasbeenadded.
(e)Noneoftheabove.
11.2Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.LocalDate;
publicclassRQ11A05{
publicstaticvoidmain(String[]args){
LocalDatedate=LocalDate.of(2016,3,1);
date.withMonth(4);
System.out.println(date.getYear()+“|”+
date.getMonth()+“|”+date.getDayOfMonth());
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexceptionwhenrun.
(c)Theprogramwillprint2016|APRIL|1.
(d)Theprogramwillprint2016|4|1.
(e)Theprogramwillprint2016|MARCH|1.
(f)Theprogramwillprint2016|3|1.
(g)Theprogramwillprint2016|JULY|1.
(h)Theprogramwillprint2016|7|1.
11.3Whichdeclarationswillcorrectlycreatethedate13August2009?
Selectthefourcorrectanswers.
(a)LocalDatedate0=LocalDate.of(2009,7,13);
(b)LocalDatedate1=LocalDate.of(2009,8,13);
(c)LocalDatedate2=LocalDate.of(2009,Month.AUGUST,
13);
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(d)LocalDatedate3=LocalDate.of(0,0,
0).withYear(2009).withMonth(8).withDayOfMonth(13);
(e)LocalDatedate4=LocalDate.of(2008,7,
12).plusYears(1).plusMonths(1).plusDays(1);
(f)LocalDatedate5=newLocalDate(2009,8,13);
(g)LocalDatedate6=LocalDate.of(1,1,
1).plus(Period.of(2008,7,12));
11.4WhichdeclarationswillcorrectlyassignaninstanceoftheLocalTimeclassto
thedeclaredreference?
Selectthethreecorrectanswers.
(a)LocalTimetime1=LocalTime.of(12,60);
(b)LocalTimetime2=newLocalTime(12,60);
(c)LocalTimetime3=LocalTime.NOON.plusHours(-3);
(d)LocalTimetime4=LocalTime.NOON.minusHours(12);
(e)LocalTimetime5=LocalTime.MIDNIGHT.withHours(12);
(f)LocalTimetime6=
LocalTime.of(12,00).plusMinutes(-15);
11.5Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.LocalTime;
publicclassRQ11A20{
publicstaticvoidmain(String[]args){
LocalTimetime=LocalTime.NOON;
time=time.plusHours(10).plusMinutes(120);
System.out.println(time);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexceptionwhenrun.
(c)Theprogramwillprint00:00.
(d)Theprogramwillprint24:00.
(e)Noneoftheabove.
11.6Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.Period;
publicclassRQ11A55{
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publicstaticvoidmain(String[]args){
Periodp1=Period.of(1,1,1);
Periodp2=Period.of(2,12,30);
p1=p1.plus(p2).plus(p1);
System.out.println(p1);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexceptionwhenrun.
(c)TheprogramwillprintP6Y26M62D.
(d)TheprogramwillprintP4Y14M32D.
(e)Noneoftheabove.
11.7Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.LocalDate;
publicclassRQ11A30{
publicstaticvoidmain(String[]args){
LocalDatedate=LocalDate.of(2015,1,1);
date=date.withYear(5).plusMonths(14);
System.out.println(date);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexceptionwhenrun.
(c)Theprogramwillprint0006-03-01.
(d)Theprogramwillprint2021-03-01.
(e)Theprogramwillprint0005-15-01.
(f)Noneoftheabove.
11.8Whichexpressions,wheninsertedat(1),willresultinthefollowingoutput:
date2isafterdate1?
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.Period;
publicclassRQ11A45{
publicstaticvoidmain(String[]args){
LocalDatedate1=LocalDate.of(2015,8,19);
LocalDatedate2=LocalDate.of(2015,10,23);
if(/*(1)INSERTCODEHERE*/){
System.out.println(“date2isafterdate1”);
}
}
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}
Selectthefivecorrectanswers.
(a)date2.isAfter(date1)
(b)date1.isAfter(date2)
(c)date2.isBefore(date1)
(d)date1.isBefore(date2)
(e)Period.between(date2,date1).isNegative()
(f)Period.between(date1,date2).isNegative()
(g)date2.until(date1).isNegative()
(h)date1.until(date2).isNegative()
(i)date1.compareTo(date2)<0
(j)date2.compareTo(date1)<0
11.4FormattingandParsing
Aformatterhastwoprimaryfunctions.Thefirstistocreateahuman-readabletextual
representationofanobject,aprocesscalledformattinganobject.Thesecondistocreate
anobjectfromastringcontainingatextualrepresentationofanobject,whichisthe
inverseofformatting.Thisprocessiscalledparsingastring.
Inthissectionwetakeacloserlookatformattingandparsingoftemporalobjects.In
particular,weconsiderthefollowingformatters,whichprovideincreasingflexibilityin
customizingformattingandparsingoftemporalobjects:
•DefaultformattersareimplicitlyusedbysuchmethodsasthetoString()
methodofthetemporalclasses.
•Predefinedformattersareready-madeformattersprovidedasconstantsbythe
java.time.format.DateTimeFormatterclass,suchasthosethatadhere
totheISOstandard(Table11.2,p.489).
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Table11.2SelectedISO-BasedPredefinedFormattersforDateandTime
•Localizedformattersarelocale-sensitiveformattersthatusetheformatstyles
definedbytheconstantsofthejava.time.format.FormatStyleenumtype
(Table11.3,p.490).Theseformattersarecreatedbythestaticfactorymethods
ofLocalizedType()oftheDateTimeFormatterclass,whereTypeis
eitherTime,Date,orDateTime(Table11.4,p.491).
Table11.3FormatStylesforDateandTime
•Customizedformattersusecustomizedformatstylesdefinedbypatternletters
(Table11.5,p.496).Theseformattersarecreatedbythestaticfactorymethod
ofPattern()oftheDateTimeFormatterclass.
Theidiomforusingaformatteristoobtainaformatterfirst,andthenpassittothe
methodsresponsibleforformattingandparsing.TheDateTimeFormatterclass
providesfactorymethodsforobtainingaformatter.Eachofthetemporalclasses
LocalTime,LocalDate,andLocalDateTimeprovidesthefollowingmethods:an
instancemethodformat()andastaticmethodparse().Thesetwomethodsdothe
formattingandtheparsingaccordingtotherulesoftheformatterthatispassedas
argument,respectively.Analogousmethodsforformattingandparsingarealsoprovided
bytheDateTimeFormatterclass,butarenotconsideredhere.
Itisagaintimetochantthemantraofimmutabilityandthread-safety.Formatterssupplied
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bytheDateTimeFormatterclassalsosubscribetothiscreed.Fromthemethod
headersoftheformat()andtheparse()methodsofthetemporalclasses,wecansee
thatthesemethodswillreadilycompilewithanyDateTimeFormatter.Thevalidityof
theformatterforagiventemporalobjectisresolvedatruntime,resultinginaresounding
exceptionifitisnotvalid.
Clickheretoviewcodeimage
//LocalTime,LocalDate,LocalDateTime
Stringformat(DateTimeFormatterformatter)
Thismethodformatsthetemporalobjectusingthespecifiedformatter,andreturns
theresultingstring.Eachtemporalclassprovidesthismethod.Thetemporalobject
isformattedaccordingtotherulesoftheformatter.Themethodthrowsa
java.time.DateTimeExceptionifformattingisunsuccessful.
Clickheretoviewcodeimage
//LocalTime,LocalDate,LocalDateTime
staticTemporalTypeparse(CharSequencetext)
staticTemporalTypeparse(CharSequencetext,DateTimeFormatterformatter)
Eachtemporalclassprovidesthesetwostaticmethods,whereTemporalType
canbeanyofthetemporalclassesLocalTime,LocalDate,or
LocalDateTime.
ThefirstmethodreturnsaninstanceoftheTemporalTypefromacharacter
sequence,usingthedefaultparsingrulesfortheTemporalType.
ThesecondmethodobtainsaninstanceoftheTemporalTypefromacharacter
sequence,usingthespecifiedformatter.
Bothmethodsreturnanobjectofaspecifictemporalclass,andboththrowa
java.time.format.DateTimeParseExceptionifparsingis
unsuccessful.
DefaultFormatters
SofarinthischapterwehavereliedonthetoString()methodoftheindividual
temporalclassesforcreatingatextualrepresentationofatemporalobject.Thedefault
formatterusedbythetoString()methodappliestheformattingrulesdefinedbythe
ISOstandard.Inthefollowingcode,theresultofformattingaLocalTimeobjectis
shownat(1):
Clickheretoviewcodeimage
LocalTimetime=LocalTime.of(12,30,15,99);
StringstrTime=time.toString();//(1)12:30:15.000000099
LocalTimeparsedTime=LocalTime.parse(strTime);//(2)
System.out.println(time.toString().equals(parsedTime.toString()));//true
Eachtemporalclassprovidesastaticmethodparse(CharSequencetext)that
parsesacharactersequenceusingadefaultformatterthatcomplieswiththeISOstandard.
Intheprecedingcode,thetextualrepresentationcreatedin(1)isparsedin(2)toobtaina
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newLocalTimeobject.Notsurprisingly,thetextualrepresentationsofthetwo
LocalTimeobjectsreferredtobythereferencestimeandparsedTimeareequal.
Thenextlineofcodeshowsthattheargumentstringpassedtotheparse()methodis
notinaccordancewiththeISOstandard,resultinginaruntimeexception:
Clickheretoviewcodeimage
LocalTimebadTime=LocalTime.parse(“12.30.15”);//DateTimeParseException
Example11.6showsexamplesofformattingandparsingobjectsofthethreetemporal
classesLocalTime,LocalDate,andLocalDateTime.Itisworthstudyingthe
outputfromthisexampletofamiliarizeyourselfwiththeISOstandardforformatting
temporalobjects.Tosummarize,thisstandardisemployedbythetoString()
andparse(CharSequencetext)methodswhenformattingandparsingtemporal
objects,respectively.
Example11.6UsingDefaultDateandTimeFormatters
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
publicclassDefaultFormattingParsing{
publicstaticvoidmain(String[]args){
System.out.printf(“%70s%n”,“Defaultformatting|Defaultparsing”);
//LocalTime
LocalTimetime=LocalTime.of(12,30,15,99);
StringstrTime=time.toString();//(1)
12:30:15.000000099
LocalTimeparsedTime=LocalTime.parse(strTime);//(2)
System.out.printf(“LocalTime:%33s|%s%n”,strTime,parsedTime);
//LocalDate
LocalDatedate=LocalDate.of(2015,4,28);
StringstrDate=date.toString();//2015-04-28
LocalDateparsedDate=LocalDate.parse(strDate);
System.out.printf(“LocalDate:%33s|%s%n”,strDate,parsedDate);
//LocalDateTime
LocalDateTimedateTime=LocalDateTime.of(date,time);
StringstrDateTime=dateTime.toString();//2015-04-
28T12:30:15.000000099
LocalDateTimeparsedDateTime=LocalDateTime.parse(strDateTime);
System.out.printf(“LocalDateTime:%23s|%s%n”,strDateTime,
parsedDateTime);
}
}
Outputfromtheprogram:
Clickheretoviewcodeimage
Defaultformatting|Defaultparsing
LocalTime:12:30:15.000000099|12:30:15.000000099
LocalDate:2015-04-28|2015-04-28
LocalDateTime:2015-04-28T12:30:15.000000099|2015-04-28T12:30:15.000000099
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PredefinedFormatters
TheDateTimeFormatterclassprovidesamyriadofpredefinedformattersfor
temporalobjects,themajorityofwhichcomplywiththeISOstandard.Table11.2shows
fourISO-basedpredefinedformattersfromthisclass.Wehavealsoindicatedwhich
temporalclassestheycanbeusedwithforformattingandparsing;withcertainclasses,
however,theycanbeusedonlyforeitherformattingorparsing.
AnexampleofusinganISO-basedpredefinedformatterisgivennext.Notethatthe
formatterobtainedat(1)isaformatterfordate-basedvalues.Itcanbeusedonlywith
temporalobjectsthathavedate-basedvalues—inotherwords,theLocalDateand
LocalDateTimeclasses.Thisformatterispassedat(2)totheformat()method,to
createatextualrepresentationofadate.Theresultingstringisparsedat(3)bythe
parse()methodthatusesthesameformatter.Theresultingdateisalsoformattedusing
thesameformatterat(4).Itishardlysurprisingthatthetextualrepresentationsofboth
datesareequal.
Clickheretoviewcodeimage
DateTimeFormatterdf=DateTimeFormatter.ISO_LOCAL_DATE;//(1)
LocalDatedate=LocalDate.of(1935,1,8);
StringstrDate=date.format(df);//(2)1935-01-
08
LocalDateparsedDate=LocalDate.parse(strDate,df);//(3)
System.out.println(strDate+“|”+
parsedDate.format(df));//(4)1935-01-08|1935-
01-08
Asthiscodeshows,aformattercanbereused,bothforformattingandforparsing.The
codeat(4)inthenextexampleappliestheformatterfrom(1)intheprecedingcode
snippettoformataLocalDateTimeobject.Itshouldnotcomeasasurprisethatthe
resultingtextualrepresentationoftheLocalDateTimeobjectpertainstoonlydate-
basedvaluesintheobject;thetime-basedvaluesoftheLocalDateTimeobjectare
ignored.Parsingthistextualrepresentationbackwiththesameformatterat(5)willyield
onlyaLocalDateobject.
Clickheretoviewcodeimage
LocalDateTimedateTime=LocalDateTime.of(1935,1,8,12,45);
StringstrDate2=dateTime.format(df);//(4)1935-01-
08
LocalDateparsedDate2=LocalDate.parse(strDate2,df);//(5)LocalDate
Tosummarize,theDateTimeFormatter.ISO_LOCAL_DATEcanbeusedtoformat
andparseaLocalDate,butcanonlyformatthedatepartofaLocalDateTime
object.
Usingthisdate-basedformatterwithaLocalTimeobjectiscourtingdisaster,asshown
bythefollowingcode.Formattingwiththisformatterresultsina
java.time.temporal.UnsupportedTemporalTypeException,andparsing
resultsinajava.time.format.DateTimeParseException.
Clickheretoviewcodeimage
StringtimeStr2=LocalTime.NOON.format(df);//
UnsupportedTemporalTypeException
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LocalTimetime2=LocalTime.parse(“12:00”,df);//DateTimeParseException
LocalizedFormatters
FormoreflexibleformattersthanthepredefinedISO-basedformatters,the
DateTimeFormatterclassprovidesthestaticfactorymethods
ofLocalizedType(),whereTypeiseitherTime,Date,orDateTime.These
methodscreateformattersthatusealocale-specificformatstyle.However,theformat
stylecannotbechangedaftertheformatteriscreated.Formatstylesaredefinedbythe
enumtypejava.time.format.FormatStyle,andareshowninTable11.3.The
stylesdefinelocale-specificformatpatternsthatvaryintheirdegreeofverbosity.
Clickheretoviewcodeimage
staticDateTimeFormatterofLocalizedTime(FormatStyletimeStyle)
staticDateTimeFormatterofLocalizedDate(FormatStyledateStyle)
staticDateTimeFormatterofLocalizedDateTime(FormatStyledateTimeStyle)
staticDateTimeFormatterofLocalizedDateTime(FormatStyledateStyle,
FormatStyletimeStyle)
ThesestaticfactorymethodsoftheDateTimeFormattercreatealocale-
specificformatterthatwillformatorparseatime,adate,oradate-time,
respectively,usingthespecifiedformatstyle.
Inthecodethatfollows,thedateformattercreatedat(1)isusedat(3)toparsetheinput
stringfrom(2).
Clickheretoviewcodeimage
DateTimeFormatterdf=
DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT);//(1)
StringinputStr=“2/29/15”;//(2)
LocalDatedate=LocalDate.parse(inputStr,df);//(3)
System.out.println(date.format(df));//(4)2/28/15
System.out.println(date);//(5)2015-02-28
Inthiscode,theinputstring"2/29/15"isspecifiedintheshortstyleofthedefault
locale(whichinourcaseistheUnitedStates).Theinputstringisparsedbythedate
formatter(usingtheshortformatstyle)tocreateanewLocalDateobject.Althoughthe
value29isinvalidforthenumberofdaysinFebruaryfortheyear2015,theoutputshows
thatitwasadjustedcorrectly.Theformatstyleinthedateformatter(inthiscase,
FormatStyle.SHORT)andthecontentsoftheinputstring(inthiscase,"2/29/15")
mustbecompatible.Ifthisisnotthecase,aDateTimeParseExceptionisthrown.
TheLocalDateobjectparsedfromtheinputstringisformattedat(4)usingthesame
formatter.Notethatintheprintstatementat(5),theLocalDateobjectfromtheparsing
isconvertedtoastringbytheLocalDate.toString()methodusingtheimplicit
ISO-basedformatter.
Table11.4showswhichtemporalclassescanbeformattedandparsedbyacombinationof
aformatstylefromTable11.3andanofLocalizedType()methodofthe
DateTimeFormatterclass,whereTypeiseitherTime,Date,orDateTime.The
tablealsoindicatesthatcertainclassescanonlybeeitherformattedorparsed,depending
ontheformatterreturnedbythemethod.Forexample,inTable11.4wecanseethatthe
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methodcallDateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)
willreturnaformatterthatcanbeusedtobothformatandparseinstancesofthe
LocalDateclass,butitwillonlyformatthedatepartofaLocalDateTimeobject.
Thisformatterwillusethespecifiedformatstyle,FormatStyle.SHORT.
Table11.4CombinationofFormatStylesandLocalizedFormatters
Example11.7,Example11.8,andExample11.9illustratetheuseoftheformatters
suppliedbytheDateTimeFormatterclass.Itisrecommendedtostudytheoutput
fromtheseexamples,togetherwithTable11.2,Table11.3,andTable11.4.
Example11.7illustratesformattersreturnedbytheofLocalizedTime()method(the
secondcolumninTable11.4).Combinationsofthismethodandvalidformatstylesare
createdinaDateTimeFormatterarrayat(1).Theseformatterscanbeusedtoformat
andparsetemporalobjectscomprisingonlytime-basedvalues(i.e.,LocalTimeobjects),
asshownat(4)and(5),respectively.Inaddition,theseformatterscanbeusedtoformat
onlythetimepartofaLocalDateTimeobject,asshownat(6).Theseformatterscannot
beusedforLocalDateobjects,astheydonotdealwithdate-basedvalues.
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Example11.7UsingPredefinedFormatStyleswithTime-BasedValues
Clickheretoviewcodeimage
importjava.time.DateTimeException;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
importjava.time.format.DateTimeParseException;
importjava.time.format.FormatStyle;
publicclassFormattingParsingTime{
publicstaticvoidmain(String[]args){
//Createsometimeformatters:
DateTimeFormatter[]timeFormatters={//(1)
DateTimeFormatter.ISO_LOCAL_TIME,
DateTimeFormatter.ofLocalizedTime(FormatStyle.SHORT),
DateTimeFormatter.ofLocalizedTime(FormatStyle.MEDIUM),
//ThefollowingtwocombinationsresultinaDateTimeExceptionat
runtime:
//DateTimeFormatter.ofLocalizedTime(FormatStyle.LONG),
//DateTimeFormatter.ofLocalizedTime(FormatStyle.FULL)
};
String[]formatStyles={“ISO”,“SHORT”,“MEDIUM”,“LONG”,“FULL”};
//Formattingandparsingatime:
LocalTimetime=LocalTime.of(14,15,30);//
(2)
LocalDateTimedateTime=LocalDateTime.of(2015,12,1,14,15,30);//
(3)
inti=0;
System.out.println(“StyleFormattingoftime,date-time|Parsingof
time”);
for(DateTimeFormattertf:timeFormatters)
try{
StringstrTime=time.format(tf);//(4)
LocalTimeparsedTime=LocalTime.parse(strTime,tf);//(5)
StringstrTime2=dateTime.format(tf);//(6)
System.out.printf(“%-7s”,formatStyles[i++]);
System.out.printf(“%14s|%14s|%14s%n”,
strTime,strTime2,parsedTime.format(tf));
}catch(DateTimeParseExceptionpe){
System.out.println(pe);
return;
}catch(DateTimeExceptiondte){
System.out.println(dte);
return;
}
}
}
Outputfromtheprogram(defaultlocaleistheUnitedStates):
Clickheretoviewcodeimage
StyleFormattingoftime,date-time|Parsingoftime
ISO14:15:30|14:15:30|14:15:30
SHORT2:15PM|2:15PM|2:15PM
MEDIUM2:15:30PM|2:15:30PM|2:15:30PM
Example11.8illustratesformattersreturnedbytheofLocalizedDate()method(the
thirdcolumninTable11.4,p.491).Combinationsofthismethodandavalidformatstyle
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arecreatedinaDateTimeFormatterarrayat(1).Theseformatterscanbeusedto
formatandparsetemporalobjectscomprisingonlydate-basedvalues(i.e.,LocalDate
objects),asshownat(4)and(5),respectively.Inaddition,theseformatterscanbeusedto
formatonlythedatepartofaLocalDateTimeobject,asshownat(6).Theseformatters
cannotbeusedforLocalTimeobjects,astheydonotdealwithtime-basedvalues.
Example11.8UsingPredefinedFormatStyleswithDate-BasedValues
Clickheretoviewcodeimage
importjava.time.DateTimeException;
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.format.DateTimeFormatter;
importjava.time.format.DateTimeParseException;
importjava.time.format.FormatStyle;
publicclassFormattingParsingDate{
publicstaticvoidmain(String[]args){
//Createsomedateformatters:
DateTimeFormatter[]dateFormatters={//(1)
DateTimeFormatter.BASIC_ISO_DATE,
DateTimeFormatter.ISO_LOCAL_DATE,
DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT),
DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM),
DateTimeFormatter.ofLocalizedDate(FormatStyle.LONG),
DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL)
};
String[]formatStyles={“BASIC”,“ISO”,“SHORT”,“MEDIUM”,“LONG”,
“FULL”};
//Formattingandparsingadate:
LocalDatedate=LocalDate.of(2015,12,1);//
(2)
LocalDateTimedateTime=LocalDateTime.of(2015,12,1,14,15,30);//
(3)
inti=0;
System.out.printf(“%s%39s%30s%n”,“Style”,“Formattingofdate,date-
time”,
”|Parsingofdate”);
for(DateTimeFormatterdf:dateFormatters)
try{
StringstrDate=date.format(df);//(4)
LocalDateparsedDate=LocalDate.parse(strDate,df);//(5)
StringstrDate2=dateTime.format(df);//(6)
System.out.printf(“%-6s”,formatStyles[i++]);
System.out.printf(“%25s|%25s|%s%n”,
strDate,strDate2,parsedDate.format(df));
}catch(DateTimeParseExceptionpe){
System.out.println(pe);
return;
}catch(DateTimeExceptiondte){
System.out.println(dte);
return;
}
}
}
Outputfromtheprogram(defaultlocaleistheUnitedStates,outputeditedtofitonpage):
Clickheretoviewcodeimage
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StyleFormattingofdate,date-time|Parsingofdate
BASIC20151201|20151201|20151201
ISO2015-12-01|2015-12-01|2015-12-01
SHORT12/1/15|12/1/15|12/1/15
MEDIUMDec1,2015|Dec1,2015|Dec1,2015
LONGDecember1,2015|December1,2015|December1,2015
FULLTuesday,December1,2015|Tuesday,December1,2015|Tuesday,December1,
2015
Example11.9illustratesformattersreturnedbytheofLocalizedDateTime()
method(thefourthcolumninTable11.4,p.491).Combinationsofthismethodandvalid
formatstylesarecreatedinaDateTimeFormatterarrayat(1).Theseformatterscan
beusedtoformatandparsetemporalobjectscomprisingbothtime-basedanddate-based
values(i.e.,LocalDateTimeobjects),asshownat(3)and(4),respectively.Inaddition,
theseformatterscanbeusedtoparsethetimepartorthedatepartofadate-timetextual
representationtoobtainaLocalTimeobjectoraLocalDateobject,asshownat(5)
and(6),respectively.TheseformatterscannotbeusedtoformateitheraLocalTime
objectoraLocalDateobject,astheyrequirebothtime-basedanddate-basedvalues
whenusedforformatting.
Example11.9UsingPredefinedFormatStyleswithDateandTime-BasedValues
Clickheretoviewcodeimage
importjava.time.DateTimeException;
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
importjava.time.format.DateTimeParseException;
importjava.time.format.FormatStyle;
publicclassFormattingParsingDateTime{
publicstaticvoidmain(String[]args){
//Createsomedate-timeformatters:
DateTimeFormatter[]dtFormatters={//
(1)
DateTimeFormatter.ISO_LOCAL_DATE_TIME,
DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT),
DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM),
DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM,
FormatStyle.SHORT),
DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT,
FormatStyle.MEDIUM),
//ThefollowingtwocombinationsresultinaDateTimeExceptionat
runtime:
//DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG),
//DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL)
};
String[]formatStyles={“ISO”,“SHORT”,“MEDIUM”,
“MEDIUM,SHORT”,“SHORT,MEDIUM”};
//Formattingandparsingadate-time:
LocalDateTimedateTime=LocalDateTime.of(2015,12,1,14,15,30);//
(2)
inti=0;
System.out.printf(“%s%31s%38s%n”,“Style”,“Formattingofdate-time|”,
“Parsingofdate-time,date,time”);
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for(DateTimeFormatterdtf:dtFormatters)
try{
StringstrDateTime=dateTime.format(dtf);//(3)
LocalDateTimeparsedDateTime
=LocalDateTime.parse(strDateTime,dtf);//(4)
LocalTimeparsedTime=LocalTime.parse(strDateTime,dtf);//(5)
LocalDateparsedDate=LocalDate.parse(strDateTime,dtf);//(6)
System.out.printf(“%-12s”,formatStyles[i++]);
System.out.printf(“%23s|%22s|%s|%8s%n”,strDateTime,
parsedDateTime.format(dtf),parsedDate,
parsedTime);
}catch(DateTimeParseExceptionpe){
System.out.println(pe);
return;
}catch(DateTimeExceptiondte){
System.out.println(dte);
return;
}
}
}
Outputfromtheprogram(defaultlocaleistheUnitedStates):
Clickheretoviewcodeimage
StyleFormattingofdate-time|Parsingofdate-time,date,time
ISO2015-12-01T14:15:30|2015-12-01T14:15:30|2015-12-
01|14:15:30
SHORT12/1/152:15PM|12/1/152:15PM|2015-12-
01|14:15
MEDIUMDec1,20152:15:30PM|Dec1,20152:15:30PM|2015-12-
01|14:15:30
MEDIUM,SHORTDec1,20152:15PM|Dec1,20152:15PM|2015-12-
01|14:15
SHORT,MEDIUM12/1/152:15:30PM|12/1/152:15:30PM|2015-12-
01|14:15:30
CustomizedFormatters
Formorefine-grainedformattingandparsingcapabilitiesfordate/time-basedvalues,we
canusetheofPattern()methodoftheDateTimeFormatterclass.Thismethod
createsformattersthatinterpretdate/time-basedvaluesaccordingtoastringpatternthatis
definedusingthepatternlettersshowninTable11.5.
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Table11.5SelectedDate/TimePatternLetters
Clickheretoviewcodeimage
staticDateTimeFormatterofPattern(Stringpattern)
Thisstaticmethodcreatesaformatterusingthespecifiedpattern.Thesetof
temporalobjectsitcanbeusedwithdependsonthepatternlettersusedinthe
specificationofthepattern.Theletterpatterndefinestherulesusedbythe
formatter.ThemethodthrowsanIllegalArgumentExceptionifthepattern
isinvalid.
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Table11.5providesanoverviewofselectedpatternletters.Alllettersarereservedwhen
usedinaletterpattern.Asequenceofcharacterscanbeescapedbyenclosingitinsingle
quotes(e.g.,"EEEE'at'HH:mm").Non-lettercharactersinthestringareinterpreted
verbatimandneednotbeescapedusingsinglequotes(e.g.,"uuuu.MM.dd@
HH:mm:ss").Thenumberoftimesapatternletterisrepeatedcanhaveabearingonthe
interpretationofthecorrespondingdate/time-basedvalue.TheuppercaseletterM(Month
oftheyear)shouldnotbeconfusedwiththelowercaseletterm(minutesinthehour).
Aletterpatterncanbeusedtoformatatemporalobjectifthetemporalobjecthasthe
temporalvaluesrequiredbythepattern.Thepattern"'Hour':HH"canbeusedto
formatthehourpartofanyLocalTimeobjectoraLocalDateTimeobject,butnota
LocalDate.
Aletterpatterncanbeusedtoparseastringifthestringmatchesthepatternandtheletter
patternspecifiesthemandatorypartsneededtoconstructatemporalobject.Thepattern
"MM/dd/uuuu"canbeusedtoparsethestring"08/13/2009"toobtaina
LocalDateobject,butnotaLocalDateTimeobject.Thelatterrequiresthetimepart
aswell.
Example11.10demonstratesbothformattingandparsingoftemporalobjectsusingletter
patterns.
•Thecodeat(1)and(2)demonstratesusingaletterpatternforthetimeparttoboth
formatandparseaLocalTimeobject,respectively.Thesamepatternisusedat(3)
toformatonlythetimepartofaLocalDateTimeobject.
•Thecodeat(4)and(5)demonstratesusingaletterpatternforthedateparttoboth
formatandparseaLocalDateobject,respectively.Thesamepatternisusedat(6)
toformatonlythedatepartofaLocalDateTimeobject.
•Thecodeat(7)and(8)demonstratesusingaletterpatternforthedateandtimeparts
tobothformatandparseaLocalDateTimeobject,respectively.Thesamepattern
isusedat(9)and(10)toparsethetextualrepresentationofaLocalDateTimeto
obtainaLocalDateandaLocalTimeobject,respectively.
TheusageofletterpatternswiththeofPattern()methodinExample11.10is
analogoustotheusageofthepredefinedformatstyleswiththeofLocalizedType()
methods(Table11.4,p.491).Themaindifferenceisthatletterpatternsprovidegreat
flexibilityincreatingcustomizedformatstyles.
Example11.10FormattingandParsingwithLetterPatterns
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassFormattingParsingWithPatterns{
publicstaticvoidmain(String[]args){
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LocalTimetime=LocalTime.of(12,30,15,99);
LocalDatedate=LocalDate.of(2015,4,28);
LocalDateTimedateTime=LocalDateTime.of(date,time);
System.out.printf(“Time:%s%n”,time);
System.out.printf(“Date:%s%n”,date);
System.out.printf(“DateTime:%s%n%n”,dateTime);
//Timepart
StringtimePattern=“HH::mm::ss:SSS”;
DateTimeFormattertimeFormatter=
DateTimeFormatter.ofPattern(timePattern);
StringstrTime=time.format(timeFormatter);//(1)
LocalTimeparsedTime=LocalTime.parse(strTime,timeFormatter);//(2)
StringstrTime2=dateTime.format(timeFormatter);//(3)
System.out.printf(“Timepattern:%s%n”,timePattern);
System.out.printf(“LocalTime:%s|%s%n”,
strTime,parsedTime.format(timeFormatter));
System.out.printf(“LocalDateTime(formattedtimepart):%s%n%n”,
strTime2);
//Datepart
StringdatePattern=“EEEE,uuuu/MMMM/dd”;
DateTimeFormatterdateFormatter=
DateTimeFormatter.ofPattern(datePattern);
StringstrDate=date.format(dateFormatter);//(4)
LocalDateparsedDate=LocalDate.parse(strDate,dateFormatter);//(5)
StringstrDate2=dateTime.format(dateFormatter);//(6)
System.out.printf(“Datepattern:%s%n”,datePattern);
System.out.printf(“LocalDate:%s|%s%n”,
strDate,parsedDate.format(dateFormatter));
System.out.printf(“LocalDateTime(formatteddatepart):%s%n%n”,
strDate2);
//Dateandtimeparts
StringdtPattern=“EE,HH::mm::ss‘on’uuuu/MM/dd”;
DateTimeFormatterdtFormatter=DateTimeFormatter.ofPattern(dtPattern);
StringstrDateTime=dateTime.format(dtFormatter);//
(7)
LocalDateTimeparsedDateTime=LocalDateTime.parse(strDateTime,//
(8)
dtFormatter);
LocalDateparsedDate3=LocalDate.parse(strDateTime,dtFormatter);//
(9)
LocalTimeparsedTime3=LocalTime.parse(strDateTime,dtFormatter);//
(10)
System.out.printf(“DateTimepattern:%s%n”,dtPattern);
System.out.printf(“LocalDateTime:%s|%s%n”,
strDateTime,parsedDateTime.format(dtFormatter));
System.out.printf(“LocalDate(parseddatepart):%s%n”,
parsedDate3.format(dateFormatter));
System.out.printf(“LocalTime(parsedtimepart):%s%n”,
parsedTime3.format(timeFormatter));
}
}
Probableoutputfromtheprogram:
Clickheretoviewcodeimage
Time:12:30:15.000000099
Date:2015-04-28
DateTime:2015-04-28T12:30:15.000000099
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Timepattern:HH::mm::ss:SSS
LocalTime:12::30::15:000|12::30::15:000
LocalDateTime(formattedtimepart):12::30::15:000
Datepattern:EEEE,uuuu/MMMM/dd
LocalDate:Tuesday,2015/April/28|Tuesday,2015/April/28
LocalDateTime(formatteddatepart):Tuesday,2015/April/28
DateTimepattern:EE,HH::mm::ss‘on’uuuu/MM/dd
LocalDateTime:Tue,12::30::15on2015/04/28|Tue,12::30::15on2015/04/28
LocalDate(parseddatepart):Tuesday,2015/April/28
LocalTime(parsedtimepart):12::30::15:000
Example11.11demonstratestheversatilityofletterpatternsforformattingtemporal
objects.Notehowapatterncanbeusedtoformatspecificpartsofadate-time,andhow
theinterpretationofapatternletterchangeswiththenumberoftimesitisrepeated.
Example11.11FormattingwithDate/TimeLetterPatterns
Clickheretoviewcodeimage
importjava.time.LocalDateTime;
importjava.time.format.DateTimeFormatter;
publicclassDateTimeFormattingPatterns{
staticpublicvoidmain(String[]args){
//DateTimetoformat.
LocalDateTimedateTime=LocalDateTime.of(1972,12,2,14,45,30);
//Formattingpatterns.
String[]patterns={
“dd/MM/uu”,
“u/M/d”,
“dMMMMuuuu”,
“‘Anniversary’:dMMMM”,
“uuuu.MM.dd”,
“uuuu.MM.dd@hh:mm:ss”,
“uuuu.MMMM.ddhh:mma”,
“EEE,MMMd’th’,”uu”,
“EEEEdMMMMuuuu”,
“EEEdMMMuuu”,
“EEdMMuu”,
“EdMu”,
“h:ma”,
“hh:mm”,
“HH:mm”,
“HH:mm:ss”,
“‘Hour’:HH”,
“EEEathh:mm”,//IllegalArgumentException-Unknownpatternletter:
t
“hh::mmm”,//IllegalArgumentException-Toomanypattern
letters:m
};
System.out.println(“Formattingdate/time(”+dateTime+“)”+
”accordingtodifferentpatterns:”);
for(Stringpattern:patterns){
Stringoutput;
try{
DateTimeFormatterformatter=DateTimeFormatter.ofPattern(pattern);
output=dateTime.format(formatter);
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}catch(IllegalArgumentExceptione){
output=String.format(“%s-%s”,e.getClass().getSimpleName(),
e.getMessage());
}
System.out.printf(“%25s%s%n”,pattern,output);
}
}
}
Probableoutputfromtheprogram(defaultlocaleistheUnitedStates):
Clickheretoviewcodeimage
Formattingdate/time(1972-12-02T14:45:30)accordingtodifferentpatterns:
dd/MM/uu02/12/72
u/M/d1972/12/2
dMMMMuuuu2December1972
‘Anniversary’:dMMMMAnniversary:2December
uuuu.MM.dd1972.12.02
uuuu.MM.dd@hh:mm:ss1972.12.02@02:45:30
uuuu.MMMM.ddhh:mma1972.December.0202:45PM
EEE,MMMd’th’,”uuSat,Dec2th,‘72
EEEEdMMMMuuuuSaturday2December1972
EEEdMMMuuuSat2Dec1972
EEdMMuuSat21272
EdMuSat2121972
h:ma2:45PM
hh:mm02:45
HH:mm14:45
HH:mm:ss14:45:30
‘Hour’:HHHour:14
EEEathh:mmIllegalArgumentException-Unknownpatternletter:
t
hh::mmmIllegalArgumentException-Toomanypattern
letters:m
ReviewQuestions
11.9Whichstatementistrueaboutformattingandparsingoftemporalobjects?
Selecttheonecorrectanswer.
(a)TheDateTimeFormatterclassprovidesonlyfactorymethodstoobtain
predefinedformatters.
(b)Thestylesdefinedbythejava.time.format.FormatStyleenumtype
arebasedontheISOstandard.
(c)TheofLocalizedDate()methodoftheDateTimeFormatterclass
returnsaformatterthatisbasedonaletterpatternpassedasargumenttothe
method.
(d)Thepattern"yy-mm-dd"canbeusedtocreateaformatterthatcanformata
LocalDateobject.
(e)Noneoftheabove.
11.10Whichcode,wheninsertedat(1),willmaketheprogramcompileandexecute
normally?
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Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassRQ11A75{
publicstaticvoidmain(String[]args){
Stringpattern=“MM/dd/yyyy‘at’HH:mm:ss”;
StringinputStr=“02/29/2015at00:15:30”;
DateTimeFormatterdtf=DateTimeFormatter.ofPattern(pattern);
//(1)INSERTCODEHERE.
}
}
Selectthefourcorrectanswers.
(a)LocalTimetime=LocalTime.parse(inputStr,dtf);
(b)LocalDatedate=LocalDate.parse(inputStr,dtf);
(c)LocalDateTimedateTime=
LocalDateTime.parse(inputStr,dtf);
(d)StringtimeStr=LocalTime.MIDNIGHT.format(dtf);
(e)StringdateStr=LocalDate.of(2015,12,
24).format(dtf);
(f)StringdateTimeStr=LocalDateTime.of(2015,12,24,
0,0).format(dtf);
11.11Whichcode,wheninsertedat(1),willmaketheprogramcompileandexecute
normally?
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassRQ11A85{
publicstaticvoidmain(String[]args){
Stringpattern=“‘Date:’dd|MM|yyyy”;
StringinputStr=“Date:02|12|2015”;
DateTimeFormatterdtf=DateTimeFormatter.ofPattern(pattern);
//(1)INSERTCODEHERE.
}
}
Selectthethreecorrectanswers.
(a)LocalTimetime=LocalTime.parse(inputStr,dtf);
(b)LocalDatedate=LocalDate.parse(inputStr,dtf);
(c)LocalDateTimedateTime=
LocalDateTime.parse(inputStr,dtf);
(d)StringtimeStr=LocalTime.MIDNIGHT.format(dtf);
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(e)StringdateStr=LocalDate.of(2015,12,
24).format(dtf);
(f)StringdateTimeStr=LocalDateTime.of(2015,12,24,
0,0).format(dtf);
11.12Whichcode,wheninsertedat(1),willresultinthefollowingoutput:
Clickheretoviewcodeimage
5minutespast9
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassRQ11A96{
publicstaticvoidmain(String[]args){
//(1)INSERTCODEHERE.
StringinputStr=“5minutespast9”;
DateTimeFormatterformatter=DateTimeFormatter.ofPattern(pattern);
LocalTimetime=LocalTime.parse(inputStr,formatter);
System.out.println(time.format(formatter));
}
}
Selecttheonecorrectanswer.
(a)Stringpattern="m'minutespast'h";
(b)Stringpattern="M'minutespast'h";
(c)Stringpattern="m'minutespast'ha";
(d)Stringpattern="m'minutespast'Ha";
(e)Stringpattern="m'minutespast'H";
(f)Stringpattern="mm'minutespast'H";
(g)Stringpattern="M'minutespast'H";
11.13Whichcode,wheninsertedat(1),willmaketheprogramcompileandexecute
normally?
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
importjava.time.format.FormatStyle;
publicclassRQ11A90{
publicstaticvoidmain(String[]args){
//(1)INSERTCODEHERE.
StringtimeStr=LocalTime.NOON.format(dtf);
StringdateStr=LocalDate.of(2015,12,24).format(dtf);
StringdateTimeStr=LocalDateTime.of(2015,12,24,12,
0).format(dtf);
}
}
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Selecttheonecorrectanswer.
(a)
Clickheretoviewcodeimage
DateTimeFormatterdtf=
DateTimeFormatter.ofLocalizedTime(FormatStyle.SHORT);
(b)
Clickheretoviewcodeimage
DateTimeFormatterdtf=
DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT);
(c)
Clickheretoviewcodeimage
DateTimeFormatterdtf=
DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT);
(d)Noneoftheabove.
ChapterSummary
Thefollowingtopicswerecoveredinthischapter:
•Anoverviewofclassesinthejava.timepackagethatrepresentapointintime:a
(clock)time(LocalTime),adate(LocalDate),adate-timecombination
(LocalDateTime),andaclassthatrepresentsanamountoftimeinyears,months,
anddays(Period).
•Theconsequencesofimmutabilityforobjectsthatrepresenttemporalconcepts
•Thenamingconventionsusedformethodsinthetemporalclassesthatfacilitatetheir
usage
•Creating,querying,converting,andcomparingtemporalobjects
•Writingcodetodotemporalarithmetic
•Formattingandparsingusingdefaultformatters
•FormattingandparsingusingISO-basedpredefinedformattersofthe
DateTimeFormatterclass
•Formattingandparsingusinglocalizedformattersthatarebasedonthepredefined
formatstylesdefinedbythejava.time.format.FormatStyleenumtype
•Formattingandparsingusingcustomizedformattersthatarebasedonletterpatterns
•Theexceptionsthrownwhenformattingandparsingtemporalobjects
ProgrammingExercise
11.1Printstatisticsaboutastronautswhohavestayedonaspacestation.
Skeletoncodefortheproblemisprovidedinthisexercise.TheclassAstronaut
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representsstatisticsaboutanastronaut:
•Thenameoftheastronaut(astronautName)
•Thedateandtimewhentheastronautarrivedatthespacestation
(arrivalDateTime)
•ThescheduleddateofreturntoEarthfortheastronaut
(scheduledReturnDate)
•Theactualperiodofstayatthespacestation(actualPeriodOfStay),which
canbelongerorshorterthanthescheduledlengthofstay
Theprogramshouldprintthefollowingreport:
Clickheretoviewcodeimage
NameArr.DateSched.ReturnAct.ReturnStatusAct.Stay
Sched.StayDiff
AstroAli2010/03/012010/05/012010/06/01
DelayedP3MP2MP1M
LailaLightyear2015/02/012015/06/302015/08/30Delayed
P210DP4M29DP2M
OrbitOrwell2014/03/012014/09/012014/09/01On
timeP6MP6MP0D
Rocket
Rogers2013/07/312013/09/302013/09/29EarlyP60DP1M30DP-
1D
Sam
Spacey2009/01/012009/11/012009/04/01EarlyP90DP10MP-
7M
Eachrowcontainsthename(Namecolumn),thearrivaldateatthespacestation
(Arr.Datecolumn),andthescheduledreturndate(Sched.Returncolumn)foreach
astronaut.Inaddition,eachrowcontainstheactualdateofreturn(Act.Returncolumn)
determinedbytheactuallengthofstay(Act.Staycolumn);whethertheastronautwas
Delayed,Ontime,orEarlyinreturningtoEarth(Statuscolumn);thescheduled
lengthofstaythatwasoriginallyplanned(Sched.Staycolumn);andthedifference
betweenthescheduledreturndateandtheactualreturndate(Diffcolumn).
Furtherinformationisprovidedinthedocumentationincludedinthecode.Implementthe
fourmethods(getActualReturnDate(),getReturnStatus(),
getPlannedPeriodOfStay(),getDiffPeriodOfStay())oftheAstronaut
class,andcompletetheimplementationoftheprintReport()methodinthe
SpaceStationStatsclass.
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.Period;
/**Classrepresentsstatisticsaboutanastronaut.*/
publicclassAstronaut{
privatefinalStringastronautName;
privatefinalLocalDateTimearrivalDateTime;
privatefinalLocalDatescheduledReturnDate;
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privatefinalPeriodactualPeriodOfStay;
publicAstronaut(Stringname,LocalDateTimearrival,Periodperiod,
LocalDatereturnDate){
astronautName=name;
arrivalDateTime=arrival;
actualPeriodOfStay=period;
scheduledReturnDate=returnDate;
}
publicStringgetAstronautName(){returnastronautName;}
publicLocalDateTimegetArrivalDateTime(){returnarrivalDateTime;}
publicPeriodgetActualPeriodOfStay(){returnactualPeriodOfStay;}
publicLocalDategetScheduledReturnDate(){returnscheduledReturnDate;}
/**@returnLocalDateTheactualdateofreturn.*/
publicLocalDategetActualReturnDate(){
/*IMPLEMENTTHISMETHOD.*/
}
/**
*Returnsstatusoftheactualreturncomparedtothescheduledreturn,
*whetheritwasontime,delayed,orearly.
*@returnStringIndicating“Ontime”,“Delayed”,or“Early”.
*/
publicStringgetReturnStatus(){
/*IMPLEMENTTHISMETHOD.*/
}
/**@returnPeriodTheplannedstayaccordingtothescheduledreturn.*/
publicPeriodgetPlannedPeriodOfStay(){
/*IMPLEMENTTHISMETHOD.*/
}
/**
*@returnPeriodThedifferencebetweentheactualreturndateand
*thescheduledreturndate.*/
publicPeriodgetDiffPeriodOfStay(){
/*IMPLEMENTTHISMETHOD.*/
}
}
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.Period;
importjava.time.format.DateTimeFormatter;
publicclassSpaceStationStats{
publicstaticvoidmain(String[]args){
//Astronautdata
Astronaut[]astronauts={
newAstronaut(“AstroAli”,
LocalDateTime.of(2010,3,1,10,45),Period.ofMonths(3),
LocalDate.of(2010,5,1)),
newAstronaut(“LailaLightyear”,
LocalDateTime.of(2015,2,1,17,0),Period.ofWeeks(30),
LocalDate.of(2015,6,30)),
newAstronaut(“OrbitOrwell”,
LocalDateTime.of(2014,3,1,20,20),Period.ofMonths(6),
LocalDate.of(2014,9,1)),
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newAstronaut(“RocketRogers”,
LocalDateTime.of(2013,7,31,15,30),Period.ofDays(60),
LocalDate.of(2013,9,30)),
newAstronaut(“SamSpacey”,
LocalDateTime.of(2009,1,1,12,15),Period.ofDays(90),
LocalDate.of(2009,11,1)),
};
printReport(astronauts);
}
/**
*Methodprintsstatisticsaboutstayonaspacestation.
*Seetheexercisetextfortheformatofthereport.
*@paramastronautsThearraywithastronautdata
*/
privatestaticvoidprintReport(Astronaut[]astronauts){
System.out.println(“NameArr.DateSched.Return”
+”Act.ReturnStatusAct.StaySched.StayDiff”);
StringreportFormatStr=“%-16s%10s%12s%12s%8s%6s%10s%9s%n”;
/*IMPLEMENTTHERESTOFTHEMETHOD.*/
}
}
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AppendixA.TakingtheJavaSE8ProgrammerIExam
PleasenotethatallinformationpresentedinthisappendixwasvalidasofJanuary2016.It
isimperativetovisitthewebsitesmentionedinthisappendixregularly,asOracleis
knowntochangethepracticalinformationabouttheexamandtheexamobjectives
intermittently.
TheprimaryfocusofthisbookistheJavaSE8ProgrammerIExam(1Z0-808)required
toqualifyasanOracleCertifiedAssociate(OCA),JavaSE8Programmer(OCAJP8).
Pertinentinformationaboutthisexamcanbefoundhere:
http://education.oracle.com/pls/web_prod-plq-dad/db_pages.getpage?
page_id=5001&get_params=p_exam_id:1Z0-808
ThisexamisthefirstoftworequiredtoobtainOracleCertifiedProfessional(OCP),Java
SE8ProgrammerCertification(OCPJP8).Thesecondexamrequiredforthisprofessional
certificationistheJavaSE8ProgrammerIIExam(1Z0-809).Wewillnotgointothe
detailsaboutthesecondexaminthisappendix,butmoreinformationaboutthesecond
examcanbefoundhere:
https://education.oracle.com/pls/web_prod-plq-dad/db_pages.getpage?
page_id=5001&get_params=p_exam_id:1Z0-809
ForauthoritativeinformationaboutthecertificationpathforJavaSE,consultthe
followingwebpage:
https://education.oracle.com/pls/web_prod-plq-dad/db_pages.getpage?
page_id=653&get_params=p_id:357#tabs-1-1
AppendixBcontainsspecificinformationabouttheexamobjectivesfortheJavaSE8
ProgrammerIexam.
A.1PreparingfortheExam
ThegoaloftheexamistotestpracticalknowledgeandusageoftheJavaprogramming
language.Theexamrequiresthoroughunderstandingofboththesyntaxandthesemantics
ofthelanguage.Theexamcoversawidevarietyoftopics,asdefinedintheobjectivesfor
theexam(AppendixB).Centraltotheexamislanguageconstructs,usageofthecoreAPI,
andspecifictopics,withheavyemphasisoninterpretingcodescenarios.
Theneedforreal-worldexperiencepriortotakingtheexamcannotbestressedenough.It
requiresverythoroughpreparationtopasstheexamwithouthavingsomeactual
experienceprogramminginJava.Simplyreadingstraightthroughthisbookisnot
recommended.Readersshouldtaketimetotryoutwhattheyhavelearnedalongevery
stepoftheway.Readersareencouragedtotesttheirnewlyacquiredknowledgeusingthe
reviewquestionsprovidedaftereverymajortopic.
Experimentingwiththeexamplesandworkingthroughtheprogrammingexercisesinthis
bookwillservetogivethereaderamuchbetterchanceofpassingtheexam.Numbered
examplesinthebookarecompleteJavaprograms,thesourcecodeforwhichisavailable
fromthebookwebsite.WhetheroneusesthetoolsintheJDKoranIDE(integrated
developmentenvironment),itisthehands-onprogrammingthatisimportant.Toolsinthe
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JDKaretobepreferredaspreparationfortheexam,asthereislessrelianceon
programmingsupportprovidedbyanIDE.
Whenthereaderfeelsready,sheshouldtestherskillsonthemockexamprovidedin
AppendixE.Thiswillgiveanindicationofhowwellthereaderispreparedfortheexam,
andwhichtopicsneedfurtherstudy.Thestructureofthebookshouldmakeiteasyforthe
readertofocusonsingletopics,ifnecessary.
Theexamisconsideredtobedifficult,andrequiresafairamountofstudyingonthepart
ofthecandidate.EvenseasonedJavaprogrammersshouldinvestsometimeinpreparing
fortheexam.Simplyhavingreal-worldexperienceisalsonotenoughtopasstheexam.
Devisingastudyplanishighlyrecommended,incorporatingtheactivitieswementioned
previously.Disruptionsinthestudyplanshouldbeavoid,asthiscanresultinrevisingthe
materialalreadycoveredwhenyoupickupthethread.Theexamshouldbescheduled
immediatelyafterthestudyperiod,whenthemomentumfromthepreparationisatits
maximum.
WealsohighlyrecommendjoininganonlineJavacertificationcommunity,suchas
CodeRanch(www.coderanch.com),whichhasdedicatedactiveforumsforthe
differentJavacertifications.AllthingsJavaarediscussedinafriendlymanneraroundthe
bonfireatthisranch,wheregreenhornsarespeciallywelcome.TheforumforOCAJPcan
befoundhere:
www.coderanch.com/forums/f-117/ocajp
A.2RegisteringfortheExam
Thewebsitefortheexamprovidesalltherelevantinformationontheprocedurefor
registering,paying,andschedulingtheexam.
ItisagoodideatohaveanOraclewebaccountandauthenticatetheaccountinthe
CertViewcertificationportal,wheretheexamresultswillbemadeavailable.Thesesteps
canbeaccomplishedatcertview.oracle.com.
ContactInformation
BothOracleandPearsonVUEhaveofficesandassociatesaroundtheworldthatcan
provideinformationabouttheexam.Theycanbecontactedtopurchaseavoucherforthe
exambeforesigningupforthetestatalocaltestingcenter.
Thebestwaytofindcontactinformationandlocaltestingcentersistovisittheirwebsites.
OracleUniversity
http://education.oracle.com/
PearsonVUE
http://pearsonvue.com/oracle/
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ObtaininganExamVoucher
ExamvouchersaresoldbyOracleandPearsonVUE.Besuretoobtainthecorrect
voucherfortheexam.Creditcardinformationisrequiredtoarrangepayment.Thecostof
thevouchermayvarydependingonthecountryyoulivein.ForU.S.residents,itcost
$245atthetimeofthisbook’swriting.
Notethatthevoucherhasanexpirationdate,usually6monthsafteritisacquired.Neither
OraclenorPearsonVUEwillreplacelostorexpiredvouchers,norwilltheyofferrefunds
forunusedvouchers.
SigningUpfortheTest
Afterobtainingtheexamvoucher,PearsonVUEcanbecontactedtosignupforthetestby
makinganappointmentatoneofthelocaltestingcenters.Theexamcanberescheduled
withoutpenaltyupto24hoursbeforetheappointmenttime.
AfterTakingtheExam
ThecandidatewillimmediatelyreceiveanemailfromOracleinformingherthattheexam
resultsareavailableinCertView.Onpassingtheexam,thecandidatewillreceiveanemail
informingherthataneCertificateisavailableinCertView.Instructionsforrequestinga
printedcopyofthecertificateareincludedinthisemail.
Acandidatewhofailstheexamcanregistertoretaketheexamaftera14-daywaiting
period.
A.3HowtheExamIsConducted
TheTestingLocations
Tobeonthesafeside,acandidateshouldbringtwoformsofIDonthedayoftheexam.
Whenacandidateshowsupatthelocaltestingcenterattheappointedtime,shewillbe
escortedtoherownlittlecubiclewithadesktopcomputer.Thetestwillbeconductedin
thiscubicle,usingatestingprogramonthecomputer.Theprogramwillaskquestions,
recordanswers,andtabulatescores.
Candidateswillnotbeallowedtobringanypersonalbelongingsorfoodwiththemtothe
cubicle.Acandidatewillbeprovidedwitheithererasableornonerasableboards.During
theexam,candidatescanusetheboardtomakenotes,buttheywillnotbeallowedtotake
anythingwiththemaftertheexam.Quiteoftentheexamareaisfittedwithsecurity
cameras.
UtilizingtheAllottedTime
Theexamconsistsofafixednumberofquestionsthatmustbeansweredwithinthe
allocatedtime.Someofthesequestionsmaybeunscored,meaningtheydonotcontribute
tothefinalscore,whethertheyareansweredcorrectlyornot.Thereisalsonowayto
distinguishwhetheraquestionisscoredorunscoredontheexam.
Thequestionsvaryindifficulty.Someareeasyandsomearedifficult.Withlimitedtime
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toanswereachquestion,thecandidatecannotaffordtogetstuckonthehardquestions.If
theanswerdoesnotbecomeapparentwithinareasonabletime,itisadvisabletomoveon
tothenextquestion.Timepermitting,itispossibletoreturntotheunansweredquestions
later.Itisimportanttoanswerallquestion.Awronganswerandablankanswercarrythe
samepenalty:lossofpoints.Thereforeitisbettertoguessananswerthantoleaveit
blank,andhopethattheguessisright.Theprocessofeliminationcansometimesbeuseful
innarrowingdowntheanswertoaquestion.Eliminatingobviousincorrectchoices
increasesthechancesofarrivingattherightanswer.
AnexperiencedJavaprogrammerusedtotakingexamsshouldbeabletocompletethe
examwithintheallottedtime.Anyremainingtimeisbestusedinreviewingtheanswers.
TheExamProgram
Thecomputerprogramusedtoconducttheexamwillselectasetofquestionsatrandom,
andpresentthemthroughagraphicaluserinterface.Theinterfaceisdesignedinsucha
waythatcandidatesareabletomovebackandforththroughthequestionsforreviewing
purposes.Questionscanbetemporarilyleftunanswered,andthecandidatecanreturnto
themlater.Questionscanalsobemarkedforreviewattheendoftheexam.
Beforetheexamstarts,thecandidateisallowedatestrunwiththecomputerprogram.A
demotestthathasnothingtodowiththeJavaexamisused.Itssolepurposeistoacquaint
thecandidatewiththeprogrambeingusedtoconducttheexam.
TheExamResult
Aftertakingtheexam,thecandidateshouldlogontotheCertViewcertificationportalto
seetheresult.Thecandidatewillbepresentedwiththefollowinginformation:
•Anindicationofwhetherthecandidatepassedorfailed.
•Thetotalscore.Onlythescoredquestionsontheexamcontributetothefinalscore.
Allthescoredquestionsareweightedequally,andthescoreiscalculatedbasedon
thepercentageofcorrectanswers.Nocreditisgivenforpartiallycorrectanswersfor
thescoredquestions.
•Indicationsonhowwellthecandidatedidoneachofthecategoriesoftheobjectives.
Candidateswhofailtheexamshouldpaycloseattentiontothisinformation.Ifthe
candidateisplanningtoretaketheexam,itmaygiveagoodindicationofwhich
topicsneedcloserattention.
Theresultwillnotdivulgewhichquestionswereansweredcorrectly.
A.4TheQuestions
AssumptionsabouttheExamQuestions
ThewebsitefortheJavaSE8ProgrammerIExamlistscertainassumptionsaboutthe
examquestions.Inthefollowinglist,weprovideashortexplanationoftheseassumptions.
•Missingpackageandimportstatements
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Unlessexplicitlyprovided,stated,orreferredtointhequestion,assumethatthe
codeisinthesamepackageandallnecessaryimportstatementsaregiven.
•Nofileordirectorypathnamesforclasses
Inthiscase,assumethateitherallclassesareinthesamefileoreachclassisina
separatefileandthesefilesareinthesamedirectory.
•Unintendedlinebreaks
Linebreaksthatmakethecodelinesappeartobewrappedunintentionallyshouldbe
ignored,andthecodeassumedtocompilewithouterrors.
•Codefragments
Assumethatthenecessarycontextexiststocompileandexecutethecode,ifsuch
contextisnotexplicitlyspecifiedbythequestion.
•Descriptivecomments
Suchcommentsshouldbetakenattheirfacevalue,providingtheintentdescribedin
thecomment.
TypesofQuestionsAsked
Mostofthequestionsfollowacommonformatthatrequirescandidatestoapplytheir
knowledgeinaspecialway.
•Analyzingprogramcode.Thequestionprovidesasourcecodesnippet,andasksa
specificquestionpertainingtothesnippet.Willrunningtheprogramprovidethe
expectedresult?Whatwillbewrittentothestandardoutputwhentheprogramis
run?Willthecodecompile?
•Identifyingtrueorfalsestatements.
Whenanalyzingprogramcode,itisusefultotrytoapplythesamerulesasthecompiler:
examiningtheexactsyntaxused,ratherthanmakingassumptionsonwhatthecodetriesto
accomplish.
Thewordingofthequestionsisprecise,andtheresponsesselectedinmultiple-choice
questionsarelikewiseexpectedtobeprecise.Thisoftencausesthetesttobeperceivedas
fastidious.Closeattentionshouldbepaidtothewordingoftheresponsesinamultiple-
choicequestion.
Noneofthequestionsisintentionallymeanttobeatrickquestion.Examquestionshave
beenreviewedbybothJavaexpertsandlanguageexpertstoremoveasmuchambiguity
fromtheirwordingaspossible.
Sincetheprogramusedintheexamwillselectandpresentthequestionsinarandom
fashion,thereisnopointintryingtoguesstheformofthequestions.Theorderofthe
answersinmultiple-choicequestionshasbeenrandomizedand,therefore,hasno
significance.
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TypesofAnswersExpected
Allexamquestionsaremultiplechoice.Thecorrectnumberofalternativestoselectis
designatedinthequestion,andallmustbeselectedforthequestionasawholetobe
consideredcorrectlyanswered.
Thereshouldbenoproblemidentifyingwhichformofanswereachquestionrequires.The
wordingofthequestionswillindicatethis,andthesoftwareusedwillpresentthe
candidatewithaninputmethodcorrespondingtotheformofanswerexpected.
Formultiple-choicequestions,theprogramwillaskthecandidatetoselectaspecific
numberofanswersfromalist.Whereasinglecorrectanswerisexpected,radiobuttons
willallowtheselectionofonlyoneanswer.Themostappropriateresponseshouldbe
selected.
Inquestionswhereallappropriateresponsesshouldbeselected,checkboxeswillallow
theselectionofeachresponseindividually.Inthiscase,allchoicesshouldbeconsidered
ontheirownmerits;thatis,responsesshouldnotbeweighedagainsteachother.Itcanbe
helpfultothinkofeachofthechoicesasanindividualtrue–falsequestion.
TopicsCoveredbytheQuestions
Topicscoveredbytheexamarebasicallyderivedfromthesetofexamobjectivesdefined
byOracle.TheobjectivesfortheJavaSE8ProgrammerIexamareincludedinAppendix
B,withreferencestowherethetopicsarecoveredinthebook.
TheultimategoalofanexamistodistinguishexperiencedJavaprogrammersfromthe
rest.Tothisend,someofthequestionsareaimedattopicsthatnewJavaprogrammers
usuallyfinddifficult.Suchtopicsinclude:
•Castingandconversion
•Polymorphism,overriding,andoverloading
•Exceptionhandlingwithtry-catch-finallyblocks
KnowledgeobtainedfromstudyingotherlanguagessuchasC++shouldbeusedwithcare.
SomeofthequestionsoftenseemtoleadastrayC++programmerswhohavenotgrasped
themanydifferencesbetweenC++andJava.ThosewithaC++backgroundshouldpay
specialattentiontothefollowingJavatopics:
•Usingnull,notNULL
•Usingtrueandfalse,not1and0
•Wideningconversions
•Conditionalandbooleanlogicoperators
•Accessibilityrules
•Polymorphism
SomeofthequestionsmayrequireintimateknowledgeofthecoreJavaAPI.Thisbook
coversthemostimportantclassesandmethodsoftheJavaSEplatformAPI,butitdoes
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notgoasfaraslistingeverymemberofeveryclass.TheJavaSEplatformAPI
documentationshouldbeconsulted.Itisessentialthatreadersfamiliarizethemselveswith
therelevantpartsoftheAPIdocumentation.
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AppendixB.ExamTopics:JavaSE8ProgrammerI
PleasenotethatallinformationpresentedinthisappendixwasvalidasofJanuary2016.It
isimperativetovisittheexamwebsitementionedinthisappendixregularlywhileyouare
preparingfortheexam,asOracleisknowntochangetheexamobjectivesintermittently.
TheJavaSE8ProgrammerIexamisrequiredtoqualifyasOracleCertifiedAssociate,
JavaSE8Programmer(OCAJP8).Pertinentinformationaboutthisexamcanbefoundat:
http://education.oracle.com/pls/web_prod-plq-dad/db_pages.getpage?
page_id=5001&get_params=p_exam_id:1Z0-808
ThewebpagealsoprovideslinkstotheexamtopicsdefinedbyOracle.Thetopicsare
organizedinsections,andeachsectionisreproducedverbatiminthisappendix.Foreach
section,wehaveprovidedreferencestowhereinthebooktheexamtopicsinthesection
arecovered.Inaddition,theextensiveindexattheendofthebookcanbeusedtolookup
specifictopics.
GeneralinformationabouttakingtheexamcanbefoundinAppendixA.Oraclehasalso
specifiedcertainimportantassumptionsabouttheexamquestions,whichcanfoundin
AppendixA,p.511.
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AppendixC.AnnotatedAnswerstoReviewQuestions
1BasicsofJavaProgramming
1.1(d)
Amethodisanoperationdefiningthebehaviorforaparticularabstraction.Java
implementsabstractionsusingclassesthathavepropertiesandbehavior.Behavior
isdefinedbytheoperationsoftheabstraction.
1.2(b)
Anobjectisaninstanceofaclass.Objectsarecreatedfromclassesthatimplement
abstractions.Theobjectsthatarecreatedareconcreterealizationsofthose
abstractions.Anobjectisneitherareferencenoravariable.
1.3(b)
(2)isthefirstlineofaconstructordeclaration.AconstructorinJavaisdeclared
likeamethod,butdoesnotspecifyareturnvalue.(1)istheheaderofaclass
declaration,(3)isthefirststatementintheconstructorbody,and(4),(5)and(6)are
instancemethoddeclarations.
1.4(b)and(f)
Twoobjectsandthreereferencesarecreatedbythecode.Objectsarenormally
createdbyusingthenewoperator.Thedeclarationofareferencecreatesavariable
regardlessofwhetherareferencevalueisassignedtoit.
1.5(d)
Aninstancememberisafieldoraninstancemethod.Thesemembersbelongtoan
instanceoftheclassratherthantotheclassasawhole.Membersthatarenot
explicitlydeclaredasstaticinaclassdeclarationareinstancemembers.
1.6(c)
Anobjectcommunicateswithanotherobjectbycallinganinstancemethodofthe
otherobject.
1.7(d)and(f)
GiventhedeclarationclassBextendsA{...},wecanconcludethat
classBextendsclassA,classAisthesuperclassofclassB,classBisasubclassof
classA,andclassBinheritsfromclassA,whichmeansthatobjectsofclassBwill
inheritthefieldvalue1fromclassA.
1.8(b),(d),and(g)
ATrainobjectcanshareboththeTrainDriveranditsCarriageobjects
withotherTrainobjects,whenitisnotusingthem.Inotherwords,theycan
outlivetheTrainobject.Thisisanexampleofaggregation.However,aTrain
objectownsthearrayobjectusedforhandlingitscarriages.Thelifetimeofanarray
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objectisnestedinthelifetimeofitsTrainobject.Thisisanexampleof
composition.
1.9(d)
ThecompilersuppliedwiththeJDKisnamedjavac.Thenamesofthesource
filestobecompiledarelistedonthecommandlineafterthecommandjavac.
1.10(a)
JavaprogramsareexecutedbytheJavaVirtualMachine(JVM).IntheJDK,the
commandjavaisusedtostarttheexecutionbytheJVM.Thejavacommand
requiresthenameofaclassthathasavalidmain()method.TheJVMstartsthe
programexecutionbycallingthemain()methodofthegivenclass.Theexact
nameoftheclassshouldbespecified,ratherthanthenameoftheclassfile;thatis,
the.classextensionintheclassfilenameshouldnotbespecified.
1.11(e)
(a):TheJVMmustbecompatiblewiththeJavaPlatformonwhichtheprogram
wasdeveloped.
(b):TheJITfeatureoftheJVMtranslatesbytecodetomachinecode.
(c):Otherlanguages,suchasScala,alsocompiletobytecodeandcanbeexecuted
byaJVM.
(d):AJavaprogramcanonlycreateobjects;destroyingobjectsoccursatthe
discretionoftheautomaticgarbagecollector.
2LanguageFundamentals
2.1(c)
52pickupisnotalegalidentifier.Thefirstcharacterofanidentifiercannotbea
digit.Anunderscoreistreatedasaletterinidentifiernames.
2.2(b),(c),(d),and(f)
In(b),theunderscoreisnotbetweendigits.In(c),digit9isnotvalidinanoctal
literal.In(d),theunderscoreisnotbetweendigits.In(f),thereisnosuchescape
sequence.
2.3(e)
InJava,theidentifiersdelete,thrown,exit,unsigned,andnextarenot
keywords.Javahasagotokeyword,butitisreservedandnotcurrentlyused.
2.4(e)
Everythingfromthestartsequence(/*)ofamultiple-linecommenttothefirst
occurrenceoftheendsequence(*/)ofamultiple-linecommentisignoredbythe
compiler.Everythingfromthestartsequence(//)ofasingle-linecommenttothe
endofthelineisignoredbythecompiler.In(e),themultiple-linecommentends
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withthefirstoccurrenceoftheendsequence(*/),leavingthesecondoccurrence
oftheendsequence(*/)unmatched.
2.5(a)and(d)
Stringisaclass,and"hello"and"t"denoteStringobjects.Javahasthe
followingprimitivedatatypes:boolean,byte,short,char,int,long,
float,anddouble.
2.6(a),(c),and(e)
(a)isabooleandatatype,while(c)and(e)arefloating-pointdatatypes.
2.7(c)
Thebitrepresentationofintis32bitswideandcanholdvaluesintherange–231
through231–1.
2.8(a),(c),and(d)
The\uxxxxnotationcanbeusedanywhereinthesourcetorepresentUnicode
characters.
2.9(c)
Localvariableiisdeclaredbutnotinitialized.Thefirstlineofcodedeclaresthe
localvariablesiandj.Thesecondlineofcodeinitializesthelocalvariablej.
Localvariableiremainsuninitialized.
2.10(c)
Thelocalvariableoftypefloatwillremainuninitialized.Fieldsandstatic
variablesareinitializedwithadefaultvalue.Aninstancevariableoftypeint[]is
areferencevariablethatwillbeinitializedwiththenullvalue.Localvariables
remainuninitializedunlessexplicitlyinitialized.Thetypeofthevariabledoesnot
affectwhetheravariableisinitialized.
2.11(e)
Theprogramwillcompile.Thecompilercanfigureoutthatthelocalvariable
pricewillalwaysbeinitialized,sincethevalueoftheconditionintheif
statementistrue.Thetwoinstancevariablesandthetwostaticvariablesareall
initializedtotherespectivedefaultvalueoftheirtype.
3Declarations
3.1(b)
Only(b)isavalidmethoddeclaration.Methodsmustspecifyareturntypeormust
bedeclaredasvoid.Thismakes(d)and(e)invalid.Methodsmustspecifyalistof
zeroormorecomma-separatedparametersenclosedbyparentheses,().The
keywordvoidcannotbeusedtospecifyanemptyparameterlist.Thismakes(a)
and(c)invalid.
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3.2(a),(b),and(e)
Non-staticmethodshaveanimplicitthisobjectreference.Thethisreference
cannotbechanged,asin(c).Thethisreferencecanbeusedinanon-static
contexttorefertobothinstanceandstaticmembers.However,itcannotbeusedto
refertolocalvariables,asin(d).
3.3(a)and(e)
Thefirstandthirdpairsofmethodswillcompile.Thesecondpairofmethodswill
notcompile,sincetheirmethodsignaturesdonotdiffer.Thecompilerhasnoway
ofdifferentiatingbetweenthetwomethods.Notethatthereturntypeandthenames
oftheparametersarenotapartofthemethodsignature.Bothmethodsinthefirst
pairarenamedflyandhavedifferentnumbersofparameters,thusoverloading
thismethodname.Themethodsinthelastpairdonotoverloadthemethodname
glide,sinceonlyonemethodhasthatname.ThemethodnamedGlideis
distinctfromthemethodnamedglide,asidentifiersarecasesensitiveinJava.
3.4(a)
Aconstructorcannotspecifyanyreturntype,notevenvoid.Aconstructorcannot
befinal,static,orabstract.
3.5(b)and(e)
Aconstructorcanbedeclaredasprivate,butthismeansthatthisconstructorcan
beusedonlywithintheclass.Constructorsneednotinitializeallthefieldswhena
classisinstantiated.Afieldwillbeassignedadefaultvalueifnotexplicitly
initialized.Aconstructorisnon-staticand,assuch,itcandirectlyaccessboththe
staticandnon-staticmembersoftheclass.
3.6(c)
Acompile-timeerrorwilloccurat(3),sincetheclassdoesnothaveaconstructor
acceptingasingleargumentoftypeint.Thedeclarationat(1)declaresamethod,
notaconstructor,sinceitisdeclaredasvoid.Themethodhappenstohavethe
samenameastheclass,butthatisirrelevant.Theclasshasadefaultconstructor,
sincetheclasscontainsnoconstructordeclarations.Thisconstructorwillbe
invokedtocreateaMyClassobjectat(2).
3.7(d)
InJava,arraysareobjects.Eacharrayobjecthasapublicfinalfieldnamed
lengththatstoresthesizeofthearray.
3.8(a)
Javaallowsarraysoflengthzero.Suchanarrayispassedasanargumenttothe
main()methodwhenaJavaprogramisrunwithoutanyprogramarguments.
3.9(c)
The[]notationcanbeplacedbothafterthetypenameandafterthevariablename
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inanarraydeclaration.Multidimensionalarraysarecreatedbyconstructingarrays
thatcancontainreferencestootherarrays.Theexpressionnewint[4][]will
createanarrayoflength4,whichcancontainreferencestoarraysofintvalues.
Theexpressionnewint[4][4]willalsocreateatwo-dimensionalarray,but
willinadditioncreatefourmoreone-dimensionalarrays,eachoflength4andof
thetypeint[].Referencestoeachofthesearraysarestoredinthetwo-
dimensionalarray.Theexpressionint[][4]willnotwork,becausethearrays
forthedimensionsmustbecreatedfromlefttoright.
3.10(b)and(e)
Thesizeofthearraycannotbespecified,asin(b)and(e).Thesizeofthearrayis
givenimplicitlybytheinitializationcode.Thesizeofthearrayisneverspecifiedin
thedeclarationofanarrayreference.Thesizeofanarrayisalwaysassociatedwith
thearrayinstance(ontheright-handside),notthearrayreference(ontheleft-hand
side).
3.11(e)
Thearraydeclarationisvalid,andwilldeclareandinitializeanarrayoflength20
containingintvalues.Allthevaluesofthearrayareinitializedtotheirdefault
valueof0.Thefor(;;)loopwillprintallthevaluesinthearray;thatis,itwill
print0twentytimes.
3.12(d)
Theprogramwillprint0false0nullatruntime.Alltheinstancevariables,
andthearrayelement,willbeinitializedtotheirdefaultvalues.Whenconcatenated
withastring,thevaluesareconvertedtotheirstringrepresentation.Noticethatthe
nullpointerisconvertedtothestring“null",ratherthanthrowinga
NullPointerException.
3.13(b)
Evaluationoftheactualparameteri++yields0,andincrementsito1inthe
process.Thevalue0iscopiedintotheformalparameteriofthemethod
addTwo()duringmethodinvocation.However,theformalparameterislocalto
themethod,andchangingitsvaluedoesnotaffectthevalueintheactualparameter.
Thevalueofthevariableiinthemain()methodremains1.
3.14(d)
Thevariablesaandbarelocalvariablesthatcontainprimitivevalues.Whenthese
variablesarepassedasargumentstoanothermethod,themethodreceivescopiesof
theprimitivevaluesinthevariables.Theactualvariablesareunaffectedby
operationsperformedonthecopiesoftheprimitivevalueswithinthecalled
method.ThevariablebArrcontainsareferencevaluethatdenotesanarrayobject
containingprimitivevalues.Whenthevariableispassedasaparametertoanother
method,themethodreceivesacopyofthereferencevalue.Usingthisreference
value,themethodcanmanipulatetheobjectthatthereferencevaluedenotes.This
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allowstheelementsinthearrayobjectreferencedbybArrtobeaccessedand
modifiedinthemethodinc2().
3.15(a)and(f)
Avaluecanbeassignedtoafinalvariableonlyonce.Afinalformal
parameterisassignedthevalueoftheactualparameteratmethodinvocation.
Withinthemethodbody,itisillegaltoreassignormodifythevaluestoredina
finalparameter.Thiscausesa++andc=dtofail.Whethertheactual
parameterisfinaldoesnotconstraintheclientthatinvokedthemethod,sincethe
actualparametervaluesareassignedtotheformalparameters.
3.16(a),(d),and(f)
Theellipses(...)mustbespecifiedbeforetheparametername.Onlyonevariable
arityparameterispermitted,anditmustbethelastparameterintheformal
parameterlist.
3.17(c)
In(a)and(b),theargumentsareencapsulatedaselementsintheimplicitlycreated
arraythatispassedtothemethod.In(c),theintarrayobjectitselfisencapsulated
asanelementintheimplicitlycreatedarraythatispassedtothemethod.(a),(b)
and(c)arefixedaritycalls.Notethatint[]isnotasubtypeofObject[].In
(d),(e),and(f),theargumentisasubtypeofObject[],andtheargumentitselfis
passedwithouttheneedofanimplicitlycreatedarray;thatis,thesearefixedarity
methodcalls.However,in(d)and(e),thecompilerissuesawarningthatbothfixed
arityandvariablearitymethodcallsarefeasible,butchoosesfixedaritymethod
calls.
4AccessControl
4.1(a)and(c)
Bytecodeofallreferencetypedeclarationsinthefileisplacedinthedesignated
package,andallreferencetypedeclarationsinthefilecanaccesstheimported
types.
4.2(e)
Bothclassesareinthesamepackageapp,sothefirsttwoimportstatementsare
unnecessary.Thepackagejava.langisalwaysimportedinallcompilationunits,
sothenexttwoimportstatementsareunnecessary.Thelaststaticimportstatement
isnecessarytoaccessthestaticvariableframeintheWindowclassbyitssimple
name.
4.3(b),(c),(d),and(e)
(a):Theimportstatementimportstypesfromthemainpkgpackage,butWindow
isnotoneofthem.
(b):Theimportstatementimportstypesfromthemainpkg.subpkg1package,
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andWindowisoneofthem.
(c):Theimportstatementimportstypesfromthemainpkg.subpkg2package,
andWindowisoneofthem.
(d):Thefirstimportstatementistype-import-on-demandandthesecondimport
statementissingle-type-import.BothimportthetypeWindow.Thesecond
overridesthefirstone.
(e):Thefirstimportstatementissingle-type-importandthesecondimport
statementistype-import-on-demand.BothimportthetypeWindow.Thefirst
overridesthesecondone.
(f):BothimportstatementsimportthetypeWindow,makingtheimport
ambiguous.
(g):Bothsingle-type-importstatementsimportthetypeWindow.Thesecond
importstatementcausesaconflictwiththefirst.
4.4(c)and(e)
Thenameoftheclassmustbefullyqualified.Aparameterlistafterthemethod
nameisnotpermitted.(c)illustratessinglestaticimportand(e)illustratesstatic
importondemand.
4.5(b),(d),and(f)
In(a),thefileA.classwillbeplacedinthesamedirectoryasthefileA.java.
Thereisno-Doptionforthejavaccommand,asin(c).Thecompilermapsthe
packagestructuretothefilesystem,creatingthenecessary(sub)directories.
4.6(b)and(d)
In(a)and(c),classAcannotbefound.In(e)and(f),classBcannotbefound—
thereisnopackageunderthecurrentdirectory/top/wrk/pkgtosearchforclass
B.Notethatspecifyingpkgintheclasspathin(d)issuperfluous.Theparent
directoryofthepackagemustbespecified,meaningthelocationofthepackage.
4.7(d)and(f)
Theparentdirectory(orlocation)ofthepackagemustbespecified.Only(d)and
(f)dothat.(d)specifiesthecurrentdirectoryfirst,butthereisnofile
top/sub/A.classunderthecurrentdirectory.Searchingunder../bin(i.e.,
/proj/bin)findsthefiletop/sub/A.class.
4.8(c)and(d)
Aclassorinterfacenamecanbereferredtobyusingeitheritsfullyqualifiedname
oritssimplename.Usingthefullyqualifiednamewillalwayswork,buttousethe
simplenameithastobeimported.Whennet.basemaster.*isimported,all
thetypenamesfromthepackagenet.basemasterwillbeimportedandcan
nowbereferredtousingsimplenames.Importingnet.*willnotimportthe
subpackagebasemaster.
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4.9(c)
Anynon-finalclasscanbedeclaredasabstract.Aclasscannotbeinstantiatedif
theclassisdeclaredasabstract.Thedeclarationofanabstractmethod
cannotprovideanimplementation.Thedeclarationofanonabstractmethod
mustprovideanimplementation.Ifanymethodinaclassisdeclaredas
abstract,thentheclassmustbedeclaredasabstract,so(a)isinvalid.The
declarationin(b)isnotvalid,sinceitomitsthekeywordabstractinthemethod
declaration.Thedeclarationin(d)isnotvalid,sinceitomitsthekeywordclass.
In(e),thereturntypeofthemethodismissing.
4.10(e)
Onlyafinalclasscannotbeextended,asin(d).(c)and(e)willalsonotcompile.
Thekeywordnativecanbeusedonlyformethods,notforclassesandfields.A
classcannotbedeclaredasbothfinalandabstract.
4.11(b)
Outsidethepackage,thememberjisaccessibletoanyclass,whereasthemember
kisaccessibleonlytosubclassesofMyClass.
Thefieldihaspackageaccessibility,andisaccessibletoonlyclassesinsidethe
package.Thefieldjhaspublicaccessibility,andisaccessiblefromanywhere.The
fieldkhasprotectedaccessibility,andisaccessiblefromanyclassinsidethe
packageandfromsubclassesanywhere.Thefieldlhasprivateaccessibility,andis
accessibleonlywithinitsownclass.
4.12(c)
Thedefaultaccessibilityformembersismorerestrictivethanprotected
accessibility,butlessrestrictivethanprivateaccessibility.Memberswithdefault
accessibilityareaccessibleonlywithintheclassitselfandfromclassesinthesame
package.Protectedmembersare,inaddition,accessiblefromsubclassesanywhere.
Memberswithprivateaccessibilityareaccessibleonlywithintheclassitself.
4.13(b)
Aprivatememberisaccessibleonlywithintheclassofthemember.Ifno
accessibilitymodifierhasbeenspecifiedforamember,thememberhasdefault
accessibility,alsoknownaspackageaccessibility.Thekeyworddefaultisnotan
accessibilitymodifier.Amemberwithpackageaccessibilityisaccessibleonlyfrom
classesinthesamepackage.Subclassesinotherpackagescannotaccessamember
withdefaultaccessibility.
4.14(a),(c),(d),(e),and(h)
Thelines(1),(3),(4),(5),and(8)willcompile.Aprotectedmemberofa
superclassisalwaysinheritedbyasubclass.Directaccesstotheprotectedfield
pfispermittedinsubclassesBandCat(1)and(5),respectively.
Asubclassinanotherpackagecanaccessprotectedmembersinthesuperclassonly
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viareferencesofitsowntypeoritssubtypes.InpackageB,thesubclassBcan
accesstheprotectedfieldpfinthesuperclasspackageA.Aviareferencesoftype
B(i.e.,parameterobj2)andreferencesofitssubclassC(i.e.,parameterobj3).
However,thesubclassCcanaccesstheprotectedfieldpfinthesuperclass
packageA.AonlyviareferencesoftypeC(i.e.,parameterobj3).Thisisthe
caseat(3),(4),and(8).
TheclassDdoesnothaveanyinheritancerelationshipwithanyoftheotherclasses,
andthereforetheprotectedfieldpfisnotaccessibleintheclassD.Thisrules
outthelinesfrom(9)to(12).
4.15(b)and(e)
Ifnoaccessibilitymodifier(public,protected,orprivate)isgiveninthe
memberdeclarationofaclass,thememberisaccessibleonlytoclassesinthesame
package.
Asubclassdoesnothaveaccesstomemberswithdefaultaccessibilitydeclaredina
superclass,unlesstheyareinthesamepackage.
Localvariablescannotbedeclaredasstaticorhaveanaccessibilitymodifier.
4.16(c)
Line(3)voidk(){i++;}canbereinsertedwithoutintroducingerrors.
Reinsertingline(1)willcausethecompilationtofail,sinceMyOtherClasswill
trytooverrideafinalmethod.Reinsertingline(2)willfail,since
MyOtherClasswillnolongerhavethe(no-argument)defaultconstructor.The
main()methodneedstocalltheno-argumentconstructor.Reinsertingline(3)
willworkwithoutanyproblems,butreinsertingline(4)willfail,sincethemethod
willtrytoaccessaprivatememberofthesuperclass.
4.17(b)
Thekeywordthiscanbeusedonlyinnon-staticcode,asinnon-staticmethods,
constructors,andinstanceinitializerblocks.Onlyoneoccurrenceofeachstatic
variableofaclassiscreated,whentheclassisloadedbytheJVM.Thisoccurrence
issharedamongalltheobjectsoftheclass(andforthatmatter,byotherclients).
Localvariablesareaccessibleonlywithintheblockscope,regardlessofwhether
theblockscopeisdefinedwithinastaticcontext.
4.18(c)
Thedeclarationin(c)isnotlegal,asvariablescannotbedeclaredasabstract.
Thekeywordsstaticandfinalarevalidmodifiersforbothfieldandmethod
declarations.Themodifiersabstractandnativearevalidformethods,but
nottogether.Theycannotbespecifiedforfields.
4.19(a)and(c)
Abstractclassescandeclarebothfinalmethodsandnon-abstractmethods.
Non-abstractclassescannot,however,containabstractmethods.Norcan
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abstractclassesbefinal.Onlymethodscanbedeclarednative.
4.20(a)
Thekeywordtransientsignifiesthatthefieldsshouldnotbestoredwhen
objectsareserialized.Constructorscannotbedeclaredasabstract.Whenan
arrayobjectiscreated,asin(c),theelementsinthearrayobjectareassignedthe
defaultvaluecorrespondingtothetypeoftheelements.Whetherthereference
variabledenotingthearrayobjectisalocalvariableoramembervariableis
irrelevant.Abstractmethodsfromasuperclassneednotbeimplementedbya
subclass,butthesubclassmustthenbedeclaredasabstract.Staticmethodscan
alsobeaccessedinanon-staticcontext—forexample,ininstancemethods,
constructors,andinstanceinitializerblocks.
5OperatorsandExpressions
5.1(a)
Avalueoftypecharcanbeassignedtoavariableoftypeint.Awidening
conversionwillconvertthevaluetoanint.
5.2(d)
Anassignmentstatementisanexpressionstatement.Thevalueoftheexpression
statementisthevalueoftheexpressionontheright-handside.Sincethe
assignmentoperatorisrightassociative,thestatementa=b=c=20is
evaluatedasfollows:(a=(b=(c=20))).Thisresultsinthevalue20
beingassignedtoc,thenthesamevaluebeingassignedtobandfinallytoa.The
programwillcompile,andprint20atruntime.
5.3(c)
Stringsareobjects.Thevariablesa,b,andcarereferencesthatcandenotesuch
objects.Assigningtoareferenceonlychangesthereferencevalue;itdoesnot
createacopyofthesourceobjectorchangetheobjectdenotedbytheoldreference
valueinthetargetreference.Inotherwords,assignmenttoreferencesaffectsonly
whichobjectthetargetreferencedenotes.Thereferencevalueofthe"cat"object
isfirstassignedtoa,thentob,andlatertoc.Theprogramprintsthestring
denotedbyc,"cat".ThelocalfinalStringvariablebisinitializedonly
onceinthecode.
5.4(a),(d),and(e)
Abinaryexpressionwithanyfloating-pointoperandwillbeevaluatedusing
floating-pointarithmetic.Expressionssuchas2/3,wherebothoperandsare
integers,willuseintegerarithmeticandevaluatetoanintegervalue.In(e),the
resultof(0x10*1L)ispromotedtoafloating-pointvalue.
5.5(b)
The/operatorhashigherprecedencethanthe+operator.Thismeansthatthe
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expressionisevaluatedas((1/2)+(3/2)+0.1).Theassociativityofthe
binaryoperatorsisfromlefttoright,giving(((1/2)+(3/2))+0.1).
Integerdivisionresultsin((0+1)+0.1),whichevaluatesto1.1.
5.6(e)
0x10isahexadecimalliteralequivalenttothedecimalvalue16.10isadecimal
literal.010isanoctalliteralequivalenttothedecimalvalue8.0b10isabinary
literalequivalenttothedecimalvalue2.Theprintln()methodwillprintthe
sumofthesevalues,whichis36,indecimalform.
5.7(b),(c),and(f)
Theunary+and-operatorswithrightassociativityareusedinthevalid
expressions(b),(c),and(f).Expression(a)triestouseanonexistentunary-
operatorwithleftassociativity,expression(d)triestouseadecrementoperator(--
)onanexpressionthatdoesnotresolvetoavariable,andexpression(e)triestouse
anonexistentunary*operator.(c)compilesbecausetheunaryoperatorscannotbe
interpretedasincrement(++)ordecrement(--)operators:(+(-(+(-(+
(-1)))))).
5.8(b)
Theexpressionevaluatesto–6.Thewholeexpressionisevaluatedas(((-(-1))
-((3*10)/5))-1)accordingtotheprecedenceandassociativity
rules.
5.9(a),(b),(d),and(e)
In(a),theconditionsforimplicitnarrowingconversionarefulfilled:Thesourceisa
constantexpressionoftypeint,thedestinationtypeisoftypeshort,andthe
valueofthesource(12)isintherangeofthedestinationtype.Theassignmentsin
(b),(d),and(e)arevalid,sincethesourcetypeisnarrowerthanthetargettypeand
animplicitwideningconversionwillbeapplied.Theexpression(c)isnotvalid.
Valuesoftypebooleancannotbeconvertedtoothertypes.
5.10(a),(c),and(d)
Theleftassociativityofthe+operatormakestheevaluationof(1+2+"3")
proceedasfollows:(1+2)+"3"→3+"3"→"33".Evaluationofthe
expression("1"+2+3),however,willproceedasfollows:("1"+2)+
3→"12"+3→"123".(4+1.0f)evaluatesas4.0f+1.0f→
5.0fand(10/9)performsintegerdivision,resultinginthevalue1.Theoperand
'a'intheexpression('a'+1)willbepromotedtoint,andtheresulting
valuewillbeoftypeint.
5.11(d)
Theexpression++k+k++++kisevaluatedas((++k)+(k++))+
(+k)→((2)+(2)+(3)),resultinginthevalue7.
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5.12(d)
Thetypescharandintarebothintegral.Acharvaluecanbeassignedtoan
intvariablesincetheinttypeiswiderthanthechartypeandanimplicit
wideningconversionwillbedone.Aninttypecannotbeassignedtoachar
variablebecausethechartypeisnarrowerthantheinttype.Thecompilerwill
reportanerroraboutapossiblelossofprecisionin(4).
5.13(c)
Variablesofthetypebytecanstorevaluesintherange–128to127.The
expressionontheright-handsideofthefirstassignmentistheintliteral128.Had
thisliteralbeenintherangeofthebytetype,animplicitnarrowingconversion
wouldhavebeenappliedtoconvertittoabytevalueduringassignment.Since
128isoutsidetherangeofthetypebyte,theprogramwillnotcompile.
5.14(a)
First,theexpression++iisevaluated,resultinginthevalue2.Nowthevariablei
alsohasthevalue2.Thetargetoftheassignmentisnowdeterminedtobethe
elementarray[2].Evaluationoftheright-handexpression,--i,resultsinthe
value1.Thevariableinowhasthevalue1.Thevalueoftheright-hand
expression,1,isthenassignedtothearrayelementarray[2],causingthearray
contentstobecome{4,8,1}.Theprogramcomputesandprintsthesumof
thesevalues,13.
5.15(a)and(c)
In(a)and(e),bothoperandsareevaluated,with(a)yieldingtrue,but(e)yielding
false.Thenullliteralcanbecompared,so(null!=null)yieldsfalse.
Theexpression(4<=4)istrue.(!true)isfalse.
5.16(c)and(e)
Theremainderoperator%isnotlimitedtointegralvalues,butcanalsobeapplied
tofloating-pointoperands.Short-circuitevaluationoccursonlywiththeconditional
operators(&&,||).Theoperators*,/,and%havethesamelevelofprecedence.
Thedatatypeshortisa16-bitsignedtwo’scomplementinteger,sotherangeof
valuesisfrom-32768to+32767,inclusive.(+15)isalegalexpressionusing
theunary+operator.
5.17(a),(c),and(e)
The!=and^operators,whenusedonbooleanoperands,willreturntrueifand
onlyifoneoperandistrue,andfalseotherwise.Thismeansthatdandeinthe
programwillalwaysbeassignedthesamevalue,givenanycombinationoftruth
valuesinaandb.Theprogramwill,therefore,printtruefourtimes.
5.18(b)
Theelementreferencedbya[i]isdeterminedbasedonthecurrentvalueofi,
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whichis0—thatis,theelementa[0].Theexpressioni=9willevaluatetothe
value9,whichwillbeassignedtothevariablei.Thevalue9isalsoassignedtothe
arrayelementa[0].Aftertheexecutionofthestatement,thevariableiwill
containthevalue9,andthearrayawillcontainthevalues9and6.Theprogram
willprint996atruntime.
5.19(c)and(d)
Notethatthelogicalandconditionaloperatorshavelowerprecedencethanthe
relationaloperators.Unlikethe&and|operators,the&&and||operatorsshort-
circuittheevaluationoftheiroperandsiftheresultoftheoperationcanbe
determinedfromthevalueofthefirstoperand.Thesecondoperandofthe||
operatorintheprogramisneverevaluatedbecauseofshort-circuiting.Allthe
operandsoftheotheroperatorsareevaluated.Variableiendsupwiththevalue3,
whichisthefirstdigitprinted,andjendsupwiththevalue1,whichisthesecond
digitprinted.
5.20(d)and(f)
&&=and%%arenotoperatorsinJava.Theoperators%,&&,%=,<=,and->are
calledremainder,conditionalAND,remaindercompoundassignment,relational
lessthanorequal,andarrowoperator,respectively.
5.21(c),(e),and(f)
In(a),thethirdoperandhasthetypedouble,whichisnotassignmentcompatible
withthetypeintofthevariableresult1.Blocksarenotlegaloperandsinthe
conditionaloperator,asin(b).In(c),thelasttwooperandsresultinwrapperobjects
withtypeIntegerandDouble,respectively,whichareassignmentcompatible
withthetypeNumberofthevariablenumber.Theevaluationoftheconditional
expressionresultsinthereferencevalueofanIntegerobject,withvalue20
beingassignedtothenumbervariable.Allthreeoperandsoftheoperatorare
mandatory,whichisnotthecasein(d).In(e),thelasttwooperandsareoftype
int,andtheevaluationoftheconditionalexpressionresultsinanintvalue(21),
whosestringrepresentationisprinted.In(f),thevalueofthesecondoperandis
boxedintoaBoolean.Theevaluationoftheconditionalexpressionresultsina
stringliteral("inotequaltoj"),whichisprinted.Theprintln()
methodcreatesandprintsastringrepresentationofanyobjectwhosereference
valueispassedasparameter.
5.22(d)
Theconditionintheouterconditionalexpressionisfalse.Theconditioninthe
nestedconditionalexpressionistrue,resultinginthevalueofm1(i.e.,20)being
printed.
6ControlFlow
6.1(d)
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Theprogramwilldisplaytheletterbwhenrun.Thesecondifstatementis
evaluatedsincethebooleanexpressionofthefirstifstatementistrue.The
elseclausebelongstothesecondifstatement.Sincethebooleanexpressionof
thesecondifstatementisfalse,theifblockisskippedandtheelseclauseis
executed.
6.2(a),(b),and(e)
Theconditionofanifstatementcanbeanyexpression,includingmethodcalls,as
longasitevaluatesorcanbeunboxedtoavalueoftypeboolean.Theexpression
(a=b)doesnotcomparethevariablesaandb,butassignsthevalueofbtothe
variablea.Theresultoftheexpressionisthevaluebeingassigned.Sinceaandb
areeitherbooleanorBooleanvariables,thevaluereturnedbytheexpressionis
alsoeitherbooleanorBoolean.Thisallowstheexpressiontobeusedasthe
conditionforanifstatement.Anifstatementmustalwayshaveanifblock,but
theelseclauseisoptional.Theexpressionif(false);else;islegal.
Inthiscase,boththeifblockandtheelseblockaresimplytheemptystatement.
6.3(f)
Thereisnothingwrongwiththecode.Thecaseanddefaultlabelsdonothave
tobespecifiedinanyspecificorder.Theuseofthebreakstatementisnot
mandatory,andwithoutitthecontrolflowwillsimplyfallthroughthelabelsofthe
switchstatement.
6.4(c)
Thecaselabelvalue2*iLocisaconstantexpressionwhosevalueis6,the
sameastheswitchexpression.Fall-throughresultsintheprogramoutputshown
in(c).
6.5(b)
Theswitchexpression,whenunboxed,hasthevalue5.Thestatementassociated
withthedefaultlabelisexecuted,andthefall-throughcontinuesuntilthe
breakstatement.
6.6(a),(b),(f),and(j)
In(a),(b),(f),and(j),thestringexpressioninvolvesconstantvaluesandevaluates
to"TomTom".Programoutputis"Hi,TomTom!"In(i),theconstantstring
expressionevaluatesto"304Tom"(84+111+109+"Tom").Thefirstthree
literalsareoftypechar,andtheirintvaluesareaddedbeforebeing
concatenatedwithlastStringoperand.Programoutputis"Whatever!"In(c),
(d),(e),(g),and(h),thecaselabelisnotaconstantstringexpression,andthe
programwillnotcompile.
6.7(e)
Theloopbodyisexecutedtwiceandtheprogramwillprint3.Thefirsttimethe
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loopisexecuted,thevariableichangesfrom1to2andthevariablebchanges
fromfalsetotrue.Thentheloopconditionisevaluated.Sincebistrue,the
loopbodyisexecutedagain.Thistimethevariableichangesfrom2to3andthe
variablebchangesfromtruetofalse.Theloopconditionisthenevaluated
again.Sincebisnowfalse,theloopterminatesandthecurrentvalueofiis
printed.
6.8(b)and(e)
Boththefirstandsecondnumbersprintedwillbe10.Boththeloopbodyandthe
updateexpressionwillbeexecutedexactly10times.Eachexecutionoftheloop
bodywillbedirectlyfollowedbyanexecutionoftheupdateexpression.Afterward,
theconditionj<10isevaluatedtoseewhethertheloopbodyshouldbe
executedagain.
6.9(c)
Only(c)containsavalidforloop.Theinitializationinafor(;;)statementcan
containeitherdeclarationsoralistofexpressionstatements,butnotbothas
attemptedin(a).Theloopconditionmustbeoftypeboolean.(b)triestousean
assignmentofanintvalue(noticetheuseof=ratherthan==)asaloopcondition
and,therefore,isnotvalid.Theloopconditionintheforloop(d)triestousethe
uninitializedvariablei,andthefor(;;)loopin(e)issyntacticallyinvalid,as
thereisonlyonesemicolon.
6.10(f)
Thecodewillcompilewithouterror,butwillneverterminatewhenrun.Allthe
sectionsintheforheaderareoptionalandcanbeomitted(butnotthe
semicolons).Anomittedloopconditionisinterpretedasbeingtrue.Thus,a
for(;;)loopwithanomittedloopconditionwillneverterminate,unlessan
appropriatecontroltransferstatementisencounteredintheloopbody.Theprogram
willenteraninfiniteloopat(4).
6.11(b),(d),and(e)
Theloopconditioninawhilestatementisnotoptional.Itismissingin(a).Itis
notpossibletobreakoutoftheifstatementin(c).Noticethatifthisifstatement
hadbeenplacedwithinaswitchstatementoraloop,theusageofbreakwould
bevalid.Insidealabeledblock,alabeledbreakstatementwouldberequired.
6.12(a)and(d)
“i=1,j=0”and“i=2,j=1”arepartoftheoutput.Thevariableiiterates
throughthevalues0,1,and2intheouterloop,whilejtogglesbetweenthevalues
0and1intheinnerloop.Ifthevaluesofiandjareequal,theprintingofthe
valuesisskippedandtheexecutioncontinueswiththenextiterationoftheouter
loop.Thefollowingcanbededucedwhentheprogramisrun:Variablesiandjare
both0andtheexecutioncontinueswiththeupdateexpressionoftheouterloop.
“i=1,j=0”isprintedandthenextiterationoftheinnerloopstarts.Variablesi
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andjareboth1andtheexecutioncontinueswiththeupdateexpressionofthe
outerloop.“i=2,j=0”isprintedandthenextiterationoftheinnerloopstarts.
“i=2,j=1”isprinted,jisincremented,j<2isfalse,andtheinnerloop
ends.Variableiisincremented,i<3isfalse,andtheouterloopends.
6.13(b)
Thecodewillfailtocompile,sincetheconditionoftheifstatementisnotoftype
boolean.Thevariableiisoftypeint.Thereisnoconversionbetween
booleanandotherprimitivetypes.
6.14(c)and(d)
Theelementtypeofthearraynumsmustbeassignmentcompatiblewiththetype
oftheloopvariable,int.Onlytheelementtypein(c),Integer,canbe
automaticallyunboxedtoanint.Theelementtypein(d)isint.
6.15(d)and(e)
Intheheaderofafor(:)loop,wecandeclareonlyonelocalvariable.Thisrules
out(a)and(b),astheyspecifytwolocalvariables.Alsothearrayexpressionin(a),
(b),and(c)isnotvalid.Only(d)and(e)specifyalegalfor(:)header.
6.16(d)
Theprogramwillprint1,4,and5,inthatorder.Theexpression5/kwillthrowan
ArithmeticException,sincekequals0.Controlistransferredtothefirst
catchclause,sinceitisthefirstclausethatcanhandlethearithmeticexceptions.
Thisexceptionhandlersimplyprints1.Theexceptionhasnowbeencaughtand
normalexecutioncanresume.Beforeleavingthetrystatement,thefinally
clauseisexecuted.Thisclauseprints4.Thelaststatementofthemain()method
prints5.
6.17(b)and(e)
Ifrunwithnoarguments,theprogramwillprintTheend.Ifrunwithone
argument,theprogramwillprintthegivenargumentfollowedby"Theend".
Thefinallyclausewillalwaysbeexecuted,nomatterhowcontrolleavesthe
tryblock.
6.18(c)and(d)
Normalexecutionwillresumeonlyiftheexceptioniscaughtbythemethod.The
uncaughtexceptionwillpropagateuptheJVMstackuntilsomemethodhandlesit.
Anoverridingmethodneedsimplydeclarethatitcanthrowasubsetofthechecked
exceptionsthattheoverriddenmethodcanthrow.Themain()methodcandeclare
thatitthrowscheckedexceptionsjustlikeanyothermethod.Thefinallyclause
willalwaysbeexecuted,nomatterhowcontrolleavesthetryblock.
6.19(a)
Theprogramwillprint2andthrowanInterruptedException.An
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InterruptedExceptionisthrowninthetryblock.Thereisnocatch
clausetohandletheexception,soitwillbesenttothecallerofthemain()
method—thatis,tothedefaultexceptionhandler.Beforethishappens,the
finallyclauseisexecuted.Thecodetoprint3isneverreached.
6.20(b)
Theonlythingthatiswrongwiththecodeistheorderingofthecatchand
finallyclauses.Ifpresent,thefinallyclausemustalwaysappearlastina
try-catch-finallyconstruct.
6.21(a)
Overridingmethodscanspecifyall,none,orasubsetofthecheckedexceptions
thattheoverriddenmethoddeclaresinitsthrowsclause.The
InterruptedExceptionistheonlycheckedexceptionspecifiedinthe
throwsclauseoftheoverriddenmethod.Theoverridingmethodcompute()
neednotspecifytheInterruptedExceptionfromthethrowsclauseofthe
overriddenmethod,becausetheexceptionisnotthrownhere.
7Object-OrientedProgramming
7.1(a)and(b)
Theextendsclauseisusedtospecifythataclassextendsanotherclass.A
subclasscanbedeclaredasabstractregardlessofwhetherthesuperclasswas
declaredasabstract.Private,overridden,andhiddenmembersfromthe
superclassarenotinheritedbythesubclass.Aclasscannotbedeclaredasboth
abstractandfinal,sinceanabstractclassneedstobeextendedtobe
useful,andafinalclasscannotbeextended.Theaccessibilityoftheclassisnot
limitedbytheaccessibilityofitsmembers.Aclasswithallthemembersdeclared
privatecanstillbedeclaredaspublic.
7.2(b)and(e)
TheObjectclasshasapublicmethodnamedequals,butitdoesnothave
anymethodnamedlength.SinceallclassesaresubclassesoftheObjectclass,
theyallinherittheequals()method.Thus,allJavaobjectshaveapublic
methodnamedequals.InJava,aclasscanextendonlyasinglesuperclass,but
thereisnolimitonhowmanysubclassescanextendasuperclass.
7.3(a),(b),and(d)
BarisasubclassofFoothatoverridesthemethodg().Thestatementa.j=5
isnotlegal,sincethememberjintheclassBarcannotbeaccessedthroughaFoo
reference.Thestatementb.i=3isnotlegaleither,sincetheprivatemember
icannotbeaccessedfromoutsideoftheclassFoo.
7.4(g)
ItisnotpossibletoinvokethedoIt()methodinAfromaninstancemethodin
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classC.ThemethodinCneedstocallamethodinasuperclasstwolevelsupinthe
inheritancehierarchy.Thesuper.super.doIt()strategywillnotwork,since
superisakeywordandcannotbeusedasanordinaryreference,norcanitbe
accessedlikeafield.Ifthemembertobeaccessedhadbeenafield,thesolution
wouldbetocastthethisreferencetotheclassofthefieldandusetheresulting
referencetoaccessthefield.Fieldaccessisdeterminedbythedeclaredtypeofthe
reference,whereastheinstancemethodtoexecuteisdeterminedbytheactualtype
oftheobjectdenotedbythereferenceatruntime.
7.5(e)
Thecodewillcompilewithouterrors.Noneofthecallstoamax()methodare
ambiguous.Whentheprogramisrun,themain()methodwillcallthemax()
methodontheCobjectreferredtobythereferencebwiththeparameters13and
29.Thismethodwillcallthemax()methodinBwiththeparameters23and39.
Themax()methodinBwillinturncallthemax()methodinAwiththe
parameters39and23.Themax()methodinAwillreturn39tothemax()
methodinB.Themax()methodinBwillreturn29tothemax()methodinC.
Themax()methodinCwillreturn29tothemain()method.
7.6(c)
ThesimplestwaytoprintthemessageintheclassMessagewouldbetouse
msg.text.Themain()methodcreatesaninstanceofMyClass,whichresults
inthecreationofaMessageinstance.ThefieldmsgdenotesthisMessage
objectinMySuperclassandisinheritedbytheMyClassobject,asthisfield
hasdefaultaccessibility.Thus,themessageintheMessageobjectcanbeaccessed
directlybymsg.textintheprint()methodofMyClass,andalsoby
this.msg.textandsuper.msg.text.
7.7(g)
IntheclassCar,thestaticmethodgetModelName()hidesthestaticmethodof
thesamenameinthesuperclassVehicle.IntheclassCar,theinstancemethod
getRegNo()overridestheinstancemethodofthesamenameinthesuperclass
Vehicle.Thedeclaredtypeofthereferencedeterminesthemethodtoexecute
whenastaticmethodiscalled,buttheactualtypeoftheobjectatruntime
determinesthemethodtoexecutewhenanoverriddenmethodiscalled.
7.8(e)
TheclassMySuperdoesnothaveano-argumentconstructor.Thismeansthat
constructorsinsubclassesmustexplicitlycallthesuperclassconstructorand
providetherequiredparameters.Thesuppliedconstructoraccomplishesthisby
callingsuper(num)initsfirststatement.Additionalconstructorscanaccomplish
thiseitherbycallingthesuperclassconstructordirectlyusingthesuper()call,or
bycallinganotherconstructorinthesameclassusingthethis()call,whichin
turncallsthesuperclassconstructor.(a)and(b)arenotvalid,sincetheydonotcall
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thesuperclassconstructorexplicitly.(d)fails,sincethesuper()callmustalways
bethefirststatementintheconstructorbody.(f)fails,sincethesuper()and
this()callscannotbecombined.
7.9(b)
Inasubclasswithoutanydeclaredconstructors,thedefaultconstructorwillcall
super().Theuseofthesuper()andthis()statementsarenotmandatoryas
longasthesuperclasshasadefaultconstructor.Ifneithersuper()northis()
isdeclaredasthefirststatementinthebodyofaconstructor,thenthedefault
super()willimplicitlybethefirststatement.Aconstructorbodycannothave
bothasuper()andathis()statement.Callingsuper()willnotalways
work,sinceasuperclassmightnothaveadefaultconstructor.
7.10(d)
Theprogramwillprint12followedbyTest.Whenthemain()methodis
executed,itwillcreateanewinstanceofBbypassing"Test"asanargument.
ThisresultsinacalltotheconstructorofB,whichhasoneStringparameter.The
constructordoesnotexplicitlycallanysuperclassconstructororanyoverloaded
constructorinBusingathis()call;instead,theno-argumentconstructorofthe
superclassAiscalledimplicitly.Theno-argumentconstructorofAcallsthe
constructorinAthathastwoStringparameters,passingittheargumentlist
("1","2").ThisconstructorcallstheconstructorwithoneStringparameter,
passingtheargument"12".Thisconstructorprintstheargument,afterimplicitly
invokingtheno-argumentconstructorofthesuperclassObject.Nowthe
executionofalltheconstructorsinAiscompleted,andexecutioncontinuesinthe
constructorofB.Thisconstructornowprintstheoriginalargument"Test"and
returnstothemain()method.
7.11(b)and(c)
Interfacedeclarationsdonotprovideanymethodimplementationsandpermitonly
multipleinterfaceinheritance.Aninterfacecanextendanynumberofinterfaces
andcanbeextendedbyanynumberofinterfaces.Fieldsininterfacesarealways
static,andcanbedeclaredasstaticexplicitly.Abstractmethoddeclarations
ininterfacesarealwaysnon-static,andcannotbedeclaredstatic.
Interfacesallowonlymultipleinterfaceinheritance.Aninterfacecanextendany
numberofinterfaces,andcanbeextendedbyanynumberofinterfaces.Fieldsin
interfacesarealwaysstatic,andcanbedeclaredasstaticexplicitly.Static
methods,ofcourse,canbedeclaredasstatic.Abstractmethoddeclarationsin
interfacesarealwaysnon-static,andcannotbedeclaredasstatic.
7.12(a),(d),(e),and(f)
Thekeywordsprotected,private,andfinalcannotbeappliedtointerface
methods.Thekeywordpublicisimplied,butcanbespecifiedforallinterface
methods.Thekeywordsdefault,abstract,andstaticcanbespecifiedfor
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default,abstract,andstaticmethods,respectively.Thekeywordsdefaultand
staticarerequiredfordefaultandstaticmethods,respectively,butthekeyword
abstractisoptionalandisimplicitlyimpliedforabstractmethods.
7.13(a),(f),and(g)
Onlythekeywordspublic,static,andfinalareimplicitlyimpliedfor
interfacevariables.
7.14(e)
(1):Thefinalstaticconstantisnotinitialized.
(2):Theabstractmethodcannothaveanimplementation.
(3):Thestaticmethodismissingtheimplementation.
(4):Thedefaultmethodcannotbefinal.
7.15(b)and(c)
ThedefaultinstancemethodprintSlogan()isinheritedbytheclass
Company.
(a):Itcanbecalledfromanon-staticcontext(instancemethodtestSlogan())
byitssimplename,butnotfromastaticcontext(staticmethodmain()).
(b),(c):Aninstancemethodcanbeinvokedonaninstanceviaareference,
regardlessofwhetheritisinastaticornon-staticcontext.
(d),(e):Aninstancemethodcannotbeinvokedviaareferencetype,butonlyonan
instanceviaareference;thatis,youcannotmakeastaticreferencetoanon-static
method.
7.16(e)
ThestaticmethodprintSlogan()isnotinheritedbytheclassFirm.Itcan
beinvokedbyusingastaticreference,thenameoftheinterfaceinwhichitis
declared,regardlessofwhetherthecallisinastaticoranon-staticcontext.
7.17(c)
Theinstancemethodat(3)overridesthedefaultmethodat(1).Thestaticmethodat
(2)isnotinheritedbytheclassRaceA.Theinstancemethodat(4)doesnot
overridethestaticmethodat(2).
Themethodtobeinvokedbythecallat(5)isdeterminedatruntimebytheobject
typeofthereference,whichinthiscaseisAthlete,resultinginthemethodat(3)
beinginvoked.Similarly,thecallat(6)willinvoketheinstancemethodat(4).
7.18(a)
Theprogramwillnotcompile,becausetheoverridingmethodat(2)cannothave
narroweraccessibilitythantheoverriddenmethodat(1).Themethodat(1)has
publicaccessibility,whereasthemethodat(2)haspackageaccessibility.
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7.19(a),(c),and(d)
Fieldsininterfacesdeclarenamedconstants,andarealwayspublic,static,
andfinal.Noneofthesemodifiersismandatoryinaconstantdeclaration.All
namedconstantsmustbeexplicitlyinitializedinthedeclaration.
7.20(a)and(d)
Thekeywordimplementsisusedwhenaclassimplementsaninterface.The
keywordextendsisusedwhenaninterfaceinheritsfromanotherinterfaceora
classinheritsfromanotherclass.
7.21(d)
Thecodewillcompilewithouterrors.TheclassMyClassdeclaresthatit
implementstheinterfacesInterface1andInterface2.Sincetheclassis
declaredasabstract,itdoesnotneedtoimplementallabstractmethod
declarationsdefinedintheseinterfaces.Anynon-abstractsubclassesof
MyClassmustprovidethemissingmethodimplementations.Thetwointerfaces
shareacommonabstractmethoddeclarationvoidg().MyClassprovidesan
implementationforthisabstractmethoddeclarationthatsatisfiesboth
Interface1andInterface2.Bothinterfacesprovidedeclarationsof
constantsnamedVAL_B.ThiscanleadtoambiguitywhenreferringtoVAL_Bby
itssimplenamefromMyClass.Theambiguitycanberesolvedbyusingthe
qualifiednames:Interface1.VAL_BandInterface2.VAL_B.However,
therearenoproblemswiththecodeasitstands.
7.22(a)and(c)
Declaration(b)fails,sinceitcontainsanillegalforwardreferencetoitsownnamed
constant.Thetypeoftheconstantismissingindeclaration(d).Declaration(e)tries
(illegally)tousetheprotectedmodifier,eventhoughnamedconstantsalways
havepublicaccessibility.Suchconstantsareimplicitlypublic,static,and
final.
7.23(c)
Theprogramwillthrowajava.lang.ClassCastExceptioninthe
assignmentat(3)atruntime.Thestatementat(1)willcompile,sincethe
assignmentisdonefromasubclassreferencetoasuperclassreference.Thecastat
(2)assuresthecompilerthatarrAreferstoanobjectthatcanbecasttotypeB[].
Thiswillworkwhenrun,sincearrAwillrefertoanobjectoftypeB[].Thecast
at(3)assuresthecompilerthatarrAreferstoanobjectthatcanbecasttotype
B[].Thiswillnotworkwhenrun,sincearrAwillrefertoanobjectoftypeA[].
7.24(d)and(f)
(4)and(6)willcauseacompile-timeerror,sinceanattemptismadetoassigna
referencevalueofasupertypeobjecttoareferenceofasubtype.Thetypeofthe
sourcereferencevalueisMyClassandthetypeofthedestinationreferenceis
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MySubclass.(1)and(2)willcompile,sincethereferenceisassignedareference
valueofthesametype.(3)willalsocompile,sincethereferenceisassigneda
referencevalueofasubtype.
7.25(e)
OnlytheassignmentI1b=obj3isvalid.Theassignmentisallowed,sinceC3
extendsC1,whichimplementsI1.Theassignmentobj2=obj1isnotlegal,
sinceC1isnotasubclassofC2.Theassignmentsobj3=obj1andobj3=
obj2arenotlegal,sinceneitherC1norC2isasubclassofC3.Theassignment
I1a=obj2isnotlegal,sinceC2doesnotimplementI1.AssignmentI2c
=obj1isnotlegal,sinceC1doesnotimplementI2.
7.26(b)
Thecompilerwillallowthestatement,asthecastisfromthesupertype(Super)to
thesubtype(Sub).However,ifatruntimethereferencexdoesnotdenoteanobject
ofthetypeSub,aClassCastExceptionwillbethrown.
7.27(b)
Theexpression(oinstanceofB)willreturntrueiftheobjectreferredto
byoisoftypeBorasubtypeofB.Theexpression(!(oinstanceofC))
willreturntrueunlesstheobjectreferredtobyoisoftypeCorasubtypeofC.
Thus,theexpression(oinstanceofB)&&(!(oinstanceofC))
willreturntrueonlyiftheobjectisoftypeBorasubtypeofBthatisnotCora
subtypeofC.GivenobjectsoftheclassesA,B,andC,thisexpressionwillreturn
trueonlyforobjectsofclassB.
7.28(d)
TheprogramwillprintallthelettersI,J,C,andDatruntime.Theobjectreferred
tobythereferencexisofclassD.ClassDextendsclassCandimplementsJ,and
classCimplementsinterfaceI.ThismakesI,J,andCsupertypesofclassD.The
referencevalueofanobjectofclassDcanbeassignedtoanyreferenceofits
supertypesand,therefore,isaninstanceofthesetypes.
7.29(a)
Thesignaturesyingyang(Integer[])andyingyang(Integer…)are
equivalentand,therefore,arenotpermittedinthesameclass.
7.30(c)
Thecallstothecompute()methodinthemethoddeclarationsat(2)andat(3)
aretothecompute()methoddeclarationat(1),astheargumentisalwaysan
int[].
Themethodcallat(4)callsthemethodat(2).Thesignatureofthecallat(4)is
compute(int[],int[])
whichmatchesthesignatureofthemethodat(2).Noimplicitarrayiscreated.
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Themethodcallin(5)callsthemethodat(1).Animplicitarrayofintiscreatedto
storetheargumentvalues.
Themethodcallsin(6)and(7)callthemethodin(3).Notethetypeofthevariable
arityparameterin(3):anint[][].Thesignatureofthecallsat(6)and(7)is
compute(int[],int[][])
whichmatchesthesignatureofthemethodat(3).Noimplicitarrayiscreated.
7.31(e)
Theprogramwillprint2whenSystem.out.println(ref2.f())is
executed.Theobjectreferencedbyref2isofclassC,butthereferenceisoftype
B.SinceBcontainsamethodf(),themethodcallwillbeallowedatcompiletime.
DuringexecutionitisdeterminedthattheobjectisofclassC,anddynamicmethod
lookupwillcausetheoverridingmethodinCtobeexecuted.
7.32(c)
Theprogramwillprint1whenrun.Thef()methodsinAandBareprivate,
andarenotaccessiblebythesubclasses.Becauseofthis,thesubclassescannot
overloadoroverridethesemethods,butsimplydefinenewmethodswiththesame
signature.TheobjectbeingcalledisofclassC.Thereferenceusedtoaccessthe
objectisoftypeB.SinceBcontainsamethodg(),themethodcallwillbeallowed
atcompiletime.DuringexecutionitisdeterminedthattheobjectisofclassC,and
dynamicmethodlookupwillcausetheoverridingmethodg()inBtobeexecuted.
Thismethodcallsamethodnamedf.Itcanbedeterminedduringcompilationthat
thiscanrefertoonlythef()methodinB,sincethemethodisprivateand
cannotbeoverridden.Thismethodreturnsthevalue1,whichisprinted.
7.33(b),(c),and(d)
Thecodeasitstandswillcompile.Theuseofinheritanceinthiscodedefinesa
Planetis-aStarrelationship.Thecodewillfailifthenameofthefield
starNameischangedintheStarclass,sincethesubclassPlanettriesto
accessitusingthenamestarName.AninstanceofPlanetisnotaninstanceof
HeavenlyBody.NeitherPlanetnorStarimplementsHeavenlyBody.
7.34(b)
Thecodewillcompile.Thecodewillnotfailtocompileifthenameofthefield
starNameischangedintheStarclass,sincethePlanetclassdoesnottryto
accessthefieldbyname,butinsteadusesthepublicmethoddescribe()in
theStarclassforthatpurpose.AninstanceofPlanetisnotaninstanceof
HeavenlyBody,sinceitneitherimplementsHeavenlyBodynorextendsa
classthatimplementsHeavenlyBody.
7.35(e)
(a)to(f)arealltrue;therefore(e)isnot.
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8FundamentalClasses
8.1(b)
ThemethodhashCode()intheObjectclassreturnsahashcodevalueoftype
int.
8.2(e)
AllarraysaregenuineobjectsandinheritallthemethodsdefinedintheObject
class,includingtheclone()method.NeitherthehashCode()methodnorthe
equals()methodisdeclaredasfinalintheObjectclass,anditcannotbe
guaranteedthatimplementationsofthesemethodswilldifferentiateamongall
objects.
8.3(a)
Theclone()methodoftheObjectclasswillthrowa
CloneNotSupportedExceptioniftheclassoftheobjectdoesnotimplement
theCloneableinterface.
8.4(a),(c),and(d)
Theclassjava.lang.Voidisconsideredawrapperclass,althoughitdoesnot
wrapanyvalue.Thereisnoclassnamedjava.lang.Int,butthereisawrapper
classnamedjava.lang.Integer.Aclassnamedjava.lang.Stringalso
exists,butitisnotawrapperclasssinceallstringsinJavaareobjects.
8.5(c)and(d)
TheclassesCharacterandBooleanarenon-numericwrapperclassesanddo
notextendtheNumberclass.TheclassesByte,Short,Integer,Long,
Float,andDoublearenumericwrapperclassesthatextendtheabstract
Numberclass.
8.6(a),(b),and(d)
Allinstancesofconcretewrapperclassesareimmutable.TheNumberclassisan
abstractclass.
8.7(b)and(c)
AllinstancesofwrapperclassesexceptVoidandCharacterhaveaconstructor
thatacceptsasingleStringparameter.TheclassObjecthasonlyano-
argumentconstructor.
8.8(e)
WhileallnumericwrapperclasseshavethemethodsbyteValue(),
doubleValue(),floatValue(),intValue(),longValue(),and
shortValue(),onlytheBooleanclasshasthebooleanValue()method.
Likewise,onlytheCharacterclasshasthecharValue()method.
8.9(b)and(d)
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Stringisnotawrapperclass.AllwrapperclassesexceptVoidhavea
compareTo()method.OnlythenumericwrapperclasseshaveanintValue()
method.TheByteclass,likeallothernumericwrapperclasses,extendsthe
Numberclass.
8.10(a)
Usingthenewoperatorcreatesanewobject.Boxingalsocreatesanewobjectif
oneisnotalreadyinternedfrombefore.
8.11(b)and(e)
Theoperators-and&cannotbeusedinconjunctionwithaStringobject.The
operators+and+=performconcatenationonstrings,andthedotoperatoraccesses
membersoftheStringobject.
8.12(d)
Theexpressionstr.substring(2,5)willextractthesubstring"kap".The
methodextractsthecharactersfromindex2toindex4,inclusive.
8.13(d)
Theprogramwillprintstr3str1whenrun.Theconcat()methodwillcreate
andreturnanewStringobject,whichistheconcatenationofthecurrent
StringobjectandtheStringobjectgivenasanargument.Theexpression
statementstr1.concat(str2)createsanewStringobject,butitsreference
valueisnotstoredaftertheexpressionisevaluated.ThereforethisStringobject
getsdiscarded.
8.14(c)
Thetrim()methodoftheStringclassreturnsastringwhereboththeleading
andtrailingwhitespaceoftheoriginalstringhavebeenremoved.
8.15(a)and(c)
TheStringclassandallwrapperclassesaredeclaredasfinaland,therefore,
cannotbeextended.Theclone()methodisdeclaredasprotectedinthe
Objectclass.Stringobjectsandwrapperclassobjectsareimmutableand,
therefore,cannotbemodified.TheclassStringandchararraytypesare
unrelated,resultinginacompile-timeerror.
8.16(d)
Theconstantexpressions"ab"+"12"and"ab"+12will,atcompiletime,
beevaluatedtothestring-valuedconstant"ab12".Bothvariablessandtare
assignedareferencetothesameinternedStringobjectcontaining"ab12".The
variableuisassignedanewStringobject,createdbyusingthenewoperator.
8.17(a),(c),(d),(f),and(j)
TheStringclasshasconstructorswiththeparameterlistsgivenin(a),(c),(d),
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(f),and(j).
8.18(e)
TheStringclasshasnoreverse()method.
8.19(b)
Thereferencevalueinthereferencestr1neverchanges;italwaysreferstothe
stringliteral"lower".ThecallstotoUpperCase()andreplace()returna
newStringobjectwhosereferencevalueisignored.
8.20(d)
ThecalltotheputO()methoddoesnotchangetheStringobjectreferredtoby
thes1referenceinthemain()method.Thereferencevaluereturnedbythecall
totheconcat()methodisignored.
8.21(a)
Thecodewillfailtocompile,sincetheexpression(s==sb)isillegal.It
comparesreferencesoftwoclassesthatarenotrelated.
8.22(e)
Theprogramwillcompilewithouterrorsandwillprinthaveawhenrun.The
contentsofthestringbufferaretruncatedto6charactersbythemethodcall
sb.setLength(6).
8.23(a),(b),(d),and(e)
TheStringBuilderclasshasonlyconstructorswiththeparameterlistsgiven
in(a),(b),(d),and(e).
8.24(a)
TheStringBuilderclassdoesnotdefineatrim()method.
8.25(b)
Thereferencessb1andsb2arenotaliases.TheStringBuilderclassdoesnot
overridetheequals()method;hencetheansweris(b).
8.26(a)
TheStringBuilderclassdoesnotoverridethehashCode()method,butthe
Stringclassdoes.Thereferencess1ands2refertoaStringobjectanda
StringBuilderobject,respectively.Thehashvaluesoftheseobjectsare
computedbythehashCode()methodintheStringandtheObjectclass,
respectively—givingdifferentresults.Thereferencess1ands3refertotwo
differentStringobjectsthatareequal;hencetheyhavethesamehashvalue.
8.27(b)
ThecalltotheputO()methodchangestheStringBuilderobjectreferredto
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bythes1referenceinthemain()method.Sodoesthecalltotheappend()
method.
9ObjectLifetime
9.1(e)
Anobjectiseligibleforgarbagecollectiononlyifallremainingreferencestothe
objectarefromotherobjectsthatarealsoeligibleforgarbagecollection.Therefore,
ifanobjectobj2iseligibleforgarbagecollectionandobjectobj1containsa
referencetoit,thenobjectobj1mustalsobeeligibleforgarbagecollection.Java
doesnothaveakeyworddelete.Anobjectwillnotnecessarilybegarbage
collectedimmediatelyafteritbecomesunreachable,buttheobjectwillbeeligible
forgarbagecollection.Circularreferencesdonotpreventobjectsfrombeing
garbagecollected;onlyreachablereferencesdo.Anobjectisnoteligiblefor
garbagecollectionaslongastheobjectcanbeaccessedbyanylivethread.An
objectthatiseligibleforgarbagecollectioncanbemadenon-eligibleifthe
finalize()methodoftheobjectcreatesareachablereferencetotheobject.
9.2(b)
Before(1),theStringobjectinitiallyreferencedbyarg1isdenotedbyboth
msgandarg1.After(1),theStringobjectisdenotedbyonlymsg.At(2),
referencemsgisassignedanewreferencevalue.Thisreferencevaluedenotesa
newStringobjectcreatedbyconcatenatingthecontentsofseveralother
Stringobjects.After(2),therearenoreferencestotheStringobjectinitially
referencedbyarg1.TheStringobjectisnoweligibleforgarbagecollection.
9.3(d)
Itisdifficulttosayhowmanyobjectsareeligibleforgarbagecollectionwhen
controlreaches(1),becausesomeoftheeligibleobjectsmayhavealreadybeen
finalized.
9.4(a)
Alltheobjectscreatedinthelooparereachableviap,whencontrolreaches(1).
9.5(b)
TheObjectclassdefinesaprotectedfinalize()method.Allclasses
inheritfromObject;thus,allobjectshaveafinalize()method.Classescan
overridethefinalize()methodand,aswithalloverriding,thenewmethod
mustnotreducetheaccessibility.Thefinalize()methodofaneligibleobject
iscalledbythegarbagecollectortoallowtheobjecttodoanycleaningupbefore
theobjectisdestroyed.Whenthegarbagecollectorcallsthefinalize()
method,itwillignoreanyexceptionsthrownbythefinalize()method.Ifthe
finalize()methodiscalledexplicitly,normalexceptionhandlingoccurswhen
anexceptionisthrownduringtheexecutionofthefinalize()method;thatis,
exceptionsarenotsimplyignored.Callingthefinalize()methoddoesnotin
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itselfdestroytheobject.Chainingofthefinalize()methodisnotenforcedby
thecompiler,anditisnotmandatorytocalltheoverriddenfinalize()method.
9.6(d)
Thefinalize()methodislikeanyothermethod:Itcanbecalledexplicitlyifit
isaccessible.However,suchamethodisintendedtobecalledbythegarbage
collectortocleanupbeforeanobjectisdestroyed.Overloadingthefinalize()
methodisallowed,butonlythemethodwiththeoriginalsignaturewillbecalledby
thegarbagecollector.Thefinalize()methodintheObjectclassis
protected.Thismeansthatanyoverridingmethodmustbedeclaredaseither
protectedorpublic.Thefinalize()methodintheObjectclass
specifiesaThrowableobjectinitsthrowsclause.Anoverridingdefinitionof
thismethodcanthrowanytypeofThrowable.Overridingmethodscanlimitthe
rangeofthrowablestouncheckedexceptionsorspecifynoexceptionsatall.Further
overridingdefinitionsofthismethodinsubclasseswillthennotbeabletothrow
checkedexceptions.
9.7(d)and(g)
(a),(b),(c),(j),(k),and(l)reducethevisibilityoftheinheritedmethod.In(e),(f),
(h),and(i),thecalltothefinalize()methodofthesuperclasscanthrowa
Throwable,whichisnothandledbythemethod.TheThrowablesuperclassis
notassignabletotheExceptionsubclass.
9.8(e)
Itisnotguaranteedifandwhengarbagecollectionwilloccur,norinwhichorder
theobjectswillbefinalized.However,itisguaranteedthatthefinalizationofan
objectwillberunonlyonce.Hence,(e)cannotpossiblybearesultfromrunning
theprogram.
9.9(c)and(e)
Itisnotguaranteedifandwhengarbagecollectionwilloccur,norinwhichorder
theobjectswillbefinalized.Thus,theprogrammaynotprintanything.Ifgarbage
collectiondoestakeplace,theMyStringobjectcreatedintheprogrammayget
finalizedbeforetheprogramterminates.Inthatcase,thefinalize()method
willprintA,asthestringinthefieldstrisnotchangedbytheconcat()
method.KeepinmindthataStringobjectisimmutable.
9.10(c),(e),and(f)
Thestaticinitializerblocks(a)and(b)arenotlegal,sincethefieldsaliveand
STEParenon-staticandfinal,respectively.(d)isnotasyntacticallylegal
staticinitializerblock.Thestaticblockin(e)willhavenoeffect,asitexecutesthe
emptystatement.Thestaticblockin(f)willchangethevalueofthestaticfield
countfrom5to1.
9.11(c)
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Theprogramwillcompile,andprint50,70,0,20,0atruntime.Allfields
aregivendefaultvaluesunlesstheyareexplicitlyinitialized.Fieldiisassignedthe
value50inthestaticinitializerblockthatisexecutedwhentheclassisinitialized.
Thisassignmentwilloverridetheexplicitinitializationoffieldiinitsdeclaration
statement.Whenthemain()methodisexecuted,thestaticfieldiis50andthe
staticfieldnis0.Whenaninstanceoftheclassiscreatedusingthenewoperator,
thevalueofstaticfieldn(i.e.,0)ispassedtotheconstructor.Beforethebodyof
theconstructorisexecuted,theinstanceinitializerblockisexecuted,whichassigns
thevalues70and20tothefieldsjandn,respectively.Whenthebodyofthe
constructorisexecuted,thefieldsi,j,k,andnandtheparametermhavethe
values50,70,0,20,and0,respectively.
9.12(f)
Thisclasshasablankfinalbooleaninstancevariableactive.Thisvariable
mustbeinitializedwhenaninstanceisconstructed,orelsethecodewillnot
compile.Thisalsoappliestoblankfinalstaticvariables.Thekeyword
staticisusedtosignifythatablockisastaticinitializerblock.Nokeywordis
usedtosignifythatablockisaninstanceinitializerblock.(a)and(b)arenot
instanceinitializersblocks,and(c),(d),and(e)failtoinitializetheblankfinal
variableactive.
9.13(c)
Theprogramwillcompile,andprint2,3,and1atruntime.Whentheobjectis
createdandinitialized,theinstanceinitializerblockisexecutedfirst,printing2.
Thentheinstanceinitializerexpressionisexecuted,printing3.Finally,the
constructorbodyisexecuted,printing1.Theforwardreferenceintheinstance
initializerblockislegal,astheuseofthefieldmisontheleft-handsideofthe
assignment.
9.14(c)and(e)
LineAwillcauseanillegalredefinitionofthefieldwidth.LineBusesanillegal
forwardreferencetothefieldswidthandheight.TheassignmentinLineCis
legal.LineDisanassignmentstatement,soitisillegalinthiscontext.LineE
declaresalocalvariableinsideaninitializerblock,withthesamenameasthe
instancevariablewidth,whichisallowed.Thesimplenameinthisblockwill
refertothelocalvariable.Toaccesstheinstancevariablewidth,thethis
referencemustbeusedinthisblock.
10TheArrayList<E>ClassandLambdaExpressions
10.1(h)
Themethodremove()canbeusedtodeleteanelementataspecificindexinan
ArrayList.
Themethodclear()canbeusedtodeleteallelementsinanArrayList.
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Themethodadd(int,E)canbeusedtoinsertanelementataspecificindexin
anArrayList.
Themethodadd()canbeusedtoappendanelementattheendofan
ArrayList.
Themethodset()canbeusedtoreplacetheelementataspecificindexwith
anotherelementinanArrayList.
Themethodcontains()canbeusedtodeterminewhetheranelementisinan
ArrayList.
ThereisnomethodtodeterminethecurrentcapacityofanArrayList.
10.2(e)
Thefor(;;)loopcorrectlyincrementstheloopvariablesothatalltheelements
inthelistaretraversed.Removingelementsusingthefor(;;)loopdoesnot
throwaConcurrentModificationExceptionatruntime.
10.3(b)and(c)
InthemethoddoIt1(),oneofthecommonelements("Ada")betweenthetwo
listsisreversed.Thevaluenullisaddedtoonlyoneofthelistsbutnottheother.
InthemethoddoIt2(),thetwolistshavecommonelements.Swappingthe
elementsinonelistdoesnotchangetheirpositionsintheotherlist.
10.4(c)
Theelementatindex2hasthevaluenull.Callingtheequals()methodonthis
elementthrowsaNullPointerException.
10.5(f)
Deletingelementswhentraversingalistrequirescare,asthesizechangesandany
elementstotherightofthedeletedelementareshiftedleft.Incrementingtheloop
variableafterdeletinganelementwillmissthenextelement,asisthecasewiththe
lastoccurrenceof"Bob".Removingelementsusingthefor(;;)loopdoesnot
throwaConcurrentModificationExceptionatruntime.
10.6(f)
Thewhileloopwillexecuteaslongastheremove()methodsreturnstrue—
thatis,aslongasthereisanelementwiththevalue"Bob"inthelist.Thewhile
loopbodyistheemptystatement.Theremove()methoddoesnotthrowan
exceptionifanelementvalueisnull,orifitispassedanullvalue.
10.7(f)
Afunctionalinterfacecanbeimplementedbylambdaexpressionsandclasses.
Afunctionalinterfacedeclarationcanhaveonlyoneabstractmethoddeclaration.
Inthebodyofalambdaexpression,allmembersintheenclosingclasscanbe
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Inthebodyofalambdaexpression,onlyeffectivelyfinallocalvariablesinthe
enclosingscopecanbeaccessed.
Alambdaexpressioninaprogramcanimplementmorethanonefunctional
interface.Forexample,thelambdaexpression(i->i%2==0)canbethe
targettypeofboththefunctionalinterfacesIntPredicateand
Predicate<Integer>.
10.8(a)and(c)
(1)redeclaresthelocalvariablepfromtheenclosingscope,whichisnotlegal.
In(2),theequals()methodoftheStringclassiscalled,becauseitisinvoked
onthetextualrepresentationoftheparameter.Intheotherstatements,the
equals()methodoftheobjectreferredtobytheparameteriscalled.
Thelambdabodyin(3)isastatementblockwithanexpressionwhosevaluemust
bereturnedbythereturnstatement.
(4)and(5)accessstaticmembersintheclass,whichislegal.
In(6),theparameternamelock2shadowsthestaticvariablebythesamename,
butisalocalvariableinthelambdaexpression.Thestaticvariableisreferredto
usingtheclassname.
10.9(e),(f),(g),and(i)
Assignmentsin(5),(6),(7),and(9)willnotcompile.Wemustcheckwhetherthe
functiontypeofthetargettypeandthetypeofthelambdaexpressionare
compatible.Thefunctiontypeofthetargettypep1intheassignmentstatements
from(1)to(5)isString->void,oravoidreturn.Thefunctiontypeofthe
targettypep2intheassignmentstatementsfrom(6)to(10)isString->
String,oranon-voidreturn.Inthefollowingcode,thefunctionaltypeofthe
targettypeisshowninacommentwiththeprefixLHS(left-handside),andthe
typeofthelambdaexpressionforeachassignmentfrom(1)to(10)isshownina
commentwiththeprefixRHS(right-handside).
Clickheretoviewcodeimage
Funky1p1;//LHS:String->void
p1=s->System.out.println(s);//(1)RHS:String->void
p1=s->s.length();//(2)RHS:String->int
p1=s->s.toUpperCase();//(3)RHS:String->
String
p1=s->{s.toUpperCase();};//(4)RHS:String->void
//p1=s->{returns.toUpperCase();};//(5)RHS:String->
String
Funky2p2;//LHS:String->
String
//p2=s->System.out.println(s);//(6)RHS:String->void
//p2=s->s.length();//(7)RHS:String->int
p2=s->s.toUpperCase();//(8)RHS:String->
String
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//p2=s->{s.toUpperCase();};//(9)RHS:String->void
p2=s->{returns.toUpperCase();};//(10)RHS:String->
String
Thenon-voidreturnofalambdaexpressionwithanexpressionstatementasthe
bodycanbeinterpretedasavoidreturn,ifthefunctiontypeofthetargettype
returnsvoid.Thisisthecasein(2)and(3).Thereturnvalueisignored.Thetype
String->Stringofthelambdaexpressionin(5)isnotcompatiblewiththe
functiontypeString->voidofthetargettypep1.
Thetypeofthelambdaexpressionin(6),(7),and(9)isnotcompatiblewiththe
functiontypeString->Stringofthetargettypep2.
10.10(d)
Thelambdaexpressionfiltersallintegervaluesthatarebothnegativeandeven
numbers.Thesevaluesarereplacedwiththeirabsolutevaluesintheintegerarray.
Thefunctionalinterfacejava.util.function.IntPredicatehasthe
abstractmethod:booleantest(inti).
10.11(d)
Thethreeinterfacesarefunctionalinterfaces.InterfaceBexplicitlyprovidesan
abstractmethoddeclarationofthepublicmethodequals()fromtheObject
class,butsuchdeclarationsareexcludedfromthedefinitionofafunctional
interface.ThusInterfaceBeffectivelyhasonlyoneabstractmethod.A
functionalinterfacecanbeimplementedbyaconcreteclass,suchasBeta.The
functiontypeofthetargettypeintheassignments(1)to(3)isvoid->void.
Thetypeofthelambdaexpressionin(1)to(3)isalsovoid->void.The
assignments(1)to(3)arelegal.
Theassignmentin(4)islegal.Subtypereferencesareassignedtosupertype
references.Referenceso,a,andcrefertothelambdaexpressionin(3).
Theassignmentin(5)islegal.ThereferencebhasthetypeInterfaceB,and
classBetaimplementsthisinterface.
(6),(7),and(8)invokethemethoddoIt().(6)evaluatesthelambdaexpressionin
(3),printingJingle|.(7)invokesthedoIt()methodonanobjectofclass
Beta,printingJazz|.(8)alsoevaluatesthelambdaexpressionin(3),printing
Jingle|.
In(9),thereferenceoiscastdowntoInterfaceA.Thereferenceoactually
referstothelambdaexpressionin(3),whichhastargettypeInterfaceC.This
interfaceisasubtypeofInterfaceA.Thesubtypeiscasttoasupertype,which
isallowed,sonoClassCastExceptionisthrownatruntime.Invokingthe
doIt()methodagainresultsinevaluationofthelambdaexpressionin(3),
printingJingle|.
Apartfromthedeclarationsofthelambdaexpressions,therestofthecodeisplain-
vanillaJava.Notealsothatthefollowingassignmentthatdefinesalambda
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expressionwouldnotbevalid,sincetheObjectclassisnotafunctionalinterface
andthereforecannotprovideatargettypeforthelambdaexpression:
Clickheretoviewcodeimage
Objectobj=()->System.out.println(“Jingle”);//Compile-time
error!
11DateandTime
11.1(e)
TheLocalDateTimeclassdoesnotprovidetheisLeapYear()method.
TheLocalTimeclassdoesnotprovidetheisEqual()method.
ThePeriodclassdoesnotprovidethewithWeeks()method,butdoesprovide
theofWeeks()staticmethod.
BoththePeriodandLocalTimeclassesdonotprovidetheplusWeeks()
method.
11.2(e)
Thedatereferencenevergetsupdated,asthereturnvalueisignored.Ifithad
beenupdated,thecorrectanswerwouldhavebeen(c).The
LocalDate.getMonth()methodreturnsaMonthenumconstant—inthis
case,Month.MARCH.TheLocalDate.getMonthValue()methodreturns
themonthasavaluebetween1and12—inthiscase,3.
11.3(b),(c),(e),and(g)
(a):Themonthnumbersstartwith1.Augusthasmonthvalue8.
(d):Invalidmonth(0)andday(0)argumentsinthecalltotheof()methodresult
inaDateTimeExceptionbeingthrownatruntime.
(f):TheLocalDateclassdoesnotprovideapublicconstructor.
11.4(c),(d),and(f)
(a):Invalidargumentfortheminutes(0–59).
(b):TheLocalTimeclassdoesnotprovideapublicconstructor.
(c):Thetimeassignedis09:00.
(d):Thetimeassignedis00:00.
(e):ThereisnowithHours()method,butthereisawithHour()methodin
theLocalTimeclass.
(f):Thetimeassignedis11:45.
11.5(c)
Boththehourandminutesarenormalizedbytheplusmethods,andthetimeof
daywrapsaroundmidnight.Thecalculationof
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time.plusHours(10).plusMinutes(120)proceedsasfollows:
Clickheretoviewcodeimage
12:00+10hours==>22:00+120min(i.e.,2hrs.)==>00:00
11.6(d)
Thecalculationofp1.plus(p2).plus(p1)proceedsasfollows:
Clickheretoviewcodeimage
P1Y1M1D+P2Y12M30D==>P3Y13M31D+P1Y1M1D==>P4Y14M32D
11.7(c)
Thecalculationofdate.withYear(5).plusMonths(14)proceedsas
follows:
Clickheretoviewcodeimage
2015-01-01withyear5==>0005-01-01+14months(i.e.,1year2months)
==>
0006-03-01
11.8(a),(d),(e),(g),and(i)
Thebetween()anduntil()methodsreturnaPeriod,whichcanbe
negative.TheisAfter(),isBefore(),between(),anduntil()methods
arestrictinthesensethattheenddateisexcluded.ThecompareTo()method
returns0ifthetwodatesareequal,anegativevalueifdate1islessthandate2,
andapositivevalueifdate1isgreaterthandate2.
11.9(e)
(a):TheDateTimeFormatterclassprovidesfactorymethodstoobtainboth
predefinedandcustomizedformatters.
(b):Thestylesdefinedbythejava.time.format.FormatStyleenumtype
arelocalesensitive.
(c):TheofLocalizedDate()methodoftheDateTimeFormatterclass
returnsaformatterthatisbasedonaformatstyle(aconstantofthe
FormatStyleenumtype)passedasanargumenttothemethod.
(d):Thepattern"yy-mm-dd"cannotbeusedtocreateaformatterthatcanformat
aLocalDateobject.Thelettermstandsforminutesofthehour,whichisnota
partofadate.
11.10(a),(b),(c),and(f)
(a),(b),(c):Theinputstringmatchesthepattern.Theinputstringspecifiesthe
mandatorypartsofbothadateandatime,neededbytherespectivemethodto
constructeitheraLocalTime,aLocalDate,oraLocalDateTime.
Tousethepatternforformatting,thetemporalobjectmustprovidetheparts
correspondingtothepatternlettersinthepattern.TheLocalTimeobjectin(d)
doesnothavethedatepartrequiredbythepattern.TheLocalDateobjectin(e)
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doesnothavethetimepartrequiredbythepattern.Both(d)and(e)willthrowan
UnsupportedTemporalTypeException.OnlytheLocalDateTime
objectin(f)hasboththedateandtimepartsrequiredbythepattern.
11.11(b),(e),and(f)
Theinputstringmatchesthepattern.Itspecifiesthedate-basedvaluesthatcanbe
usedtoconstructaLocalDateobjectin(b),basedonthedate-relatedpattern
lettersinthepattern.Notime-basedvaluescanbeinterpretedfromtheinputstring,
asthispatternhasonlydate-relatedpatternletters.(a)and(c),whichrequireatime
part,willthrowaDateTimeParseException.
Tousethepatternforformatting,thetemporalobjectmustprovidevaluesforthe
partscorrespondingtothepatternlettersinthepattern.TheLocalTimeobjectin
(d)doesnothavethedatepartrequiredbythepattern.(d)willthrowan
UnsupportedTemporalTypeException.TheLocalDateobjectin(e)has
thedatepartrequiredbythepattern,asdoestheLocalDateTimeobjectin(f).In
(f),onlythedatepartoftheLocalDateTimeobjectisformatted.
11.12(e)
(a),(b),(c),(d),and(f)resultinaDateTimeParseExceptionwhenparsing.
(a):Thepatternletterhrepresentshourintheday,butrequiresAM/PM
informationtoresolvethehourina24-hourclock(i.e.,patternlettera),whichis
missing.
(b):ThepatternletterMisinterpretedcorrectlyasmonthoftheyear(value5).
Matchingthepatternletterhistheproblem,asexplainedfor(a).
(c),(d):Thepatternletteracannotberesolvedfromtheinputstring,asanAM/PM
markerismissingintheinputstring.
(e):Theparsesucceeds,withtheLocalTimeobjecthavingthevalue09:05.
Formattingthisobjectwiththeformatterresultsintheoutputstring:5minutes
past9.
(f):Theletterpatternmmcannotberesolved,astheminutesvaluehasonlyone
digit(i.e.,5)intheinputstring.
(g):Theparsesucceeds,withtheresultingLocalTimeobjecthavingthevalue
09:00.Themonthvalue5isignored.Formattingthisobjectwiththeformatter
resultsinanUnsupportedTemporalTypeException,becausenowthe
patternletterMrequiresamonthvalue,whichisnotpartofaLocalTimeobject.
11.13(d)
(a):TheformatterwillformataLocalTimeobject,orthetimepartofa
LocalDateTimeobject,butnotaLocalDateobject,asitknowsnothingabout
formattingthedatepart.
(b):TheformatterwillformataLocalDateobject,orthedatepartofa
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LocalDateTimeobject,butnotaLocalTimeobject,asitknowsnothingabout
formattingthetimepart.
(c):TheformatterwillformataLocalDateTimeobject,butnotaLocalDate
objectoraLocalTimeobject,asitwillformatonlytemporalobjectswithboth
dateandtimeparts.
Theprogramthrowsa
java.time.temporal.UnsupportedTemporalTypeExceptioninall
cases.
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AppendixD.SolutionstoProgrammingExercises
1BasicsofJavaProgramming
1.1TheprintStackElements()methodofthePrintableCharStackclass
doesnotpoptheelements.
Clickheretoviewcodeimage
//File:CharStack.java
publicclassCharStack{
//Instancevariables:
protectedchar[]stackArray;//Thearrayimplementingthestack.
protectedinttopOfStack;//Thetopofthestack.
//Staticvariable
privatestaticintcounter;//(1)
//Constructornowincrementsthecounterforeachobjectcreated.
publicCharStack(intcapacity){//(2)
stackArray=newchar[capacity];
topOfStack=-1;
counter++;
}
//Instancemethods:
publicvoidpush(charelement){stackArray[++topOfStack]=element;}
publiccharpop(){returnstackArray[topOfStack—];}
publiccharpeek(){returnstackArray[topOfStack];}
publicbooleanisEmpty(){returntopOfStack==-1;}
publicbooleanisFull(){returntopOfStack==stackArray.length
-1;}
//Staticmethod(3)
publicstaticintgetInstanceCount(){returncounter;}
}
Clickheretoviewcodeimage
//File:PrintableCharStack.java
publicclassPrintableCharStackextendsCharStack{//
(1)
//Instancemethod
publicvoidprintStackElements(){//
(2)
for(inti=0;i<=topOfStack;i++)
System.out.print(stackArray[i]);//printeachcharonterminal
System.out.println();
}
//Constructorcallstheconstructorofthesuperclassexplicitly.
PrintableCharStack(intcapacity){super(capacity);}//
(3)
}
Clickheretoviewcodeimage
//File:Client.java
publicclassClient{
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publicstaticvoidmain(String[]args){
//Createaprintablecharacterstack.
PrintableCharStackstack=newPrintableCharStack(40);
//Createastringtopushonthestack:
Stringstr=“!notisotnuferaskcatS”;
System.out.println(“Originalstring:”+str);
intlength=str.length();
//Pushthestringcharbycharontothestack:
for(inti=0;i<length;i++){
stack.push(str.charAt(i));
}
System.out.print(“Stackcontents:“);
stack.printStackElements();
System.out.print(“Reversedstring:“);
//Popandprinteachcharfromthestack:
while(!stack.isEmpty()){
System.out.print(stack.pop());
}
System.out.println();
System.out.print(“Stackcontents:“);
stack.printStackElements();
}
}
2LanguageFundamentals
2.1Thefollowingprogramcompilesandrunswithouterrors:
Clickheretoviewcodeimage
//File:Temperature.java
/*IdentifiersandkeywordsinJavaarecasesensitive.Therefore,the
thenameofthepublicclassmustmatchthenameofthefile,and
keywordsmust
allbewritteninlowercase.ThenameoftheStringclasshasan
uppercaseS.Themainmethodmustbestaticandtakesanarrayof
Stringobjectsasanargument.*/
publicclassTemperature{
publicstaticvoidmain(String[]args){//Correctmethodsignature
doublefahrenheit=62.5;
//Amultiple-linecomment,whichcanspanseverallines,startswith
//thecharactersequence/*andendswiththecharactersequence*/.
/*Convert*/
doublecelsius=f2c(fahrenheit);
//Characterliteralsareenclosedinsinglequotes;
//stringliteralsareenclosedindoublequotes.
//Onlythefirstcharacterliteralisquotedasastringtoavoid
addition.
//Thesecondcharliteralisimplicitlyconvertedtoitsstring
//representation,asstringconcatenationisperformedby
//thelast+operator.
//Javaiscasesensitive.ThenameCelsiusshouldbechangedto
//thevariablenamecelsius.
System.out.println(fahrenheit+“F”+”=”+celsius+‘C’);
}
/*Methodshouldbedeclaredstatic.*/
staticdoublef2c(doublefahr){//Noteparametertypeshouldbe
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double.
return(fahr-32.0)*5.0/9.0;
}
}
3Declarations
3.1
Clickheretoviewcodeimage
publicclassQuizGrader{
/**Enumtypetorepresenttheresultofansweringaquestion.*/
enumResult{CORRECT,WRONG,UNANSWERED}
privatestaticfinalintPASS_MARK=5;
privatestaticString[]correctAnswers={“C”,“A”,“B”,“D”,
“B”,“C”,“C”,“A”};
publicstaticvoidmain(String[]args){
System.out.println(“QuestionSubmittedAns.CorrectAns.Result”);
//Countersformiscellaneousstatistics:
intnumOfCorrectAnswers=0;
intnumOfWrongAnswers=0;
intnumOfUnanswered=0;
//Loopthroughsubmittedanswersandcorrectanswers:
for(inti=0;i<args.length;i++){
StringsubmittedAnswer=args[i];
StringcorrectAnswer=correctAnswers[i];
Resultresult=determineResult(submittedAnswer,correctAnswer);
//Printreportforcurrentquestion.
System.out.printf(“%5d%10s%15s%15s%n”,
i+1,submittedAnswer,correctAnswer,result);
//Accumulatestatistics:
switch(result){
caseCORRECT:numOfCorrectAnswers++;break;
caseWRONG:numOfWrongAnswers++;break;
caseUNANSWERED:numOfUnanswered++;break;
}
}
//Printsummaryofstatistics:
System.out.println(“No.ofcorrectanswers:”+
numOfCorrectAnswers);
System.out.println(“No.ofwronganswers:”+
numOfWrongAnswers);
System.out.println(“No.ofquestionsunanswered:”+
numOfUnanswered);
System.out.println(“Thecandidate”+
(numOfCorrectAnswers>=PASS_MARK?“PASSED.”:
“FAILED.”));
}
/**Determinestheresultofanswertoaquestion.*/
publicstaticResultdetermineResult(StringsubmittedAnswer,
StringcorrectAnswer){
Resultresult=null;
if(submittedAnswer.equals(correctAnswer))
result=Result.CORRECT;
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elseif(submittedAnswer.equals(“X”))
result=Result.UNANSWERED;
else
result=Result.WRONG;
returnresult;
}
}
4AccessControl
4.1
Clickheretoviewcodeimage
//File:Account.java
packagecom.megabankcorp.records;
publicclassAccount{}
//File:Database.java
//Specifypackage.
packagecom.megabankcorp.system;
//RefertotheAccountclassbyusingitssimplename.
importcom.megabankcorp.records.Account;
//Classmustbeabstractsinceithasabstractmethods.
publicabstractclassDatabase{
//Abstractandaccessibletoallclassesinanypackage.
publicabstractvoiddeposit(Accountacc,doubleamount);
//Abstractandaccessibletoallclassesinanypackage.
publicabstractvoidwithdraw(Accountacc,doubleamount);
//Abstractandaccessibletoallclasseswithinitsownpackage
//andtosubclassesinotherpackages.
protectedabstractdoublebalance(Accountacc);
//Cannotbeoverriddenbyasubclassandaccessibleonly
//toclasseswithinitsownpackage.
finalvoidtransfer(Accountfrom,Accountto,doubleamount){
withdraw(from,amount);
deposit(to,amount);
}
}
5OperatorsandExpressions
5.1
Clickheretoviewcodeimage
//File:SunlightSolution.java
publicclassSunlightSolution{
publicstaticvoidmain(String[]args){
//Distancefromsun(150millionkilometers)
/*Themaxvalueforintis2_147_483_647,sousingintherewill
work.*/
intkmFromSun=150_000_000;
//Again,usingintforthisvalueisOK.
intlightSpeed=299_792_458;//Meterspersecond
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//Convertdistancetometers.
/*Theresultofthisequationwillnotfitinanint,
soweusealonginstead.Weneedtoensurethatthevaluesthat
aremultipliedareactuallymultipliedusinglong
datatypes,andnotmultipliedasintdatatypesandlater
convertedtolong.TheLsuffixonthe1000Linteger
literalensuresthis.ThevalueofthevariablekmFromSunwill
implicitlybeconvertedfrominttolongtomatchthe
datatypeoftheotherfactor.Theconversioncanbedone
implicitlybythecompilersincetheconversionrepresents
awideningofthedatatype.*/
longmFromSun=kmFromSun*1000L;
/*Weknowthattheresultvaluewillfitinanint.
However,thenarrowingconversiononassignmentfromlongtoint
inthiscaserequiresacast.*/
intseconds=(int)(mFromSun/lightSpeed);
System.out.print(“Lightwilluse“);
printTime(seconds);
System.out.println(”totravelfromthesuntotheearth.”);
}
/*Nochangesnecessaryinthismethod.*/
publicstaticvoidprintTime(intsec){
intmin=sec/60;
sec=sec-(min*60);
System.out.print(min+”minute(s)and”+sec+”second(s)”);
}
}
6ControlFlow
6.1Findingprimesusingforloops.
Clickheretoviewcodeimage
//File:ForPrimes.java
publicclassForPrimes{
privatestaticfinalintMAX=100;
publicstaticvoidmain(String[]args){
numbers:
for(intnum=1;num<MAX;num++){
intdivLim=(int)Math.sqrt(num);
for(intdiv=2;div<=divLim;div++){
if((num%div)==0){
continuenumbers;
}
}
System.out.println(num);
}
}
}
Findingprimesusingwhileloops.
Clickheretoviewcodeimage
//File:WhilePrimes.java
publicclassWhilePrimes{
privatestaticfinalintMAX=100;
publicstaticvoidmain(String[]args){
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intnum=1;
numbers:
while(num<MAX){
intnumber=num++;
intdivLim=(int)Math.sqrt(number);
intdiv=2;
while(div<=divLim){
if((number%div++)==0){
continuenumbers;
}
}
System.out.println(number);
}
}
}
6.2
Clickheretoviewcodeimage
packageenergy;
/**APowerPlantwithareactorcore.
ThesolutionpresentedhereisprovidedbyJennieYip.*/
publicclassPowerPlant{
/**Eachpowerplanthasareactorcore.
ThisfieldhaspackageaccessibilitysothattheControlclass,
definedinthesamepackage,canaccessit.*/
finalReactorcore;
/**Initializesthepowerplant,createsareactorcore.*/
PowerPlant(){
core=newReactor();
}
/**Soundsthealarmtoevacuatethepowerplant.*/
publicvoidsoundEvacuateAlarm(){
//…implementationunspecified…
}
/**@returnthelevelofreactoroutputthatismostdesirableatthis
time.
(Unitsareunspecified.)*/
publicintgetOptimalThroughput(){
//…implementationunspecified…
return0;
}
/**Themainentrypointoftheprogram:setsupaPowerPlantobject
andaControlobjectandletstheControlobjectrunthepower
plant.*/
publicstaticvoidmain(String[]args){
PowerPlantplant=newPowerPlant();
Controlctrl=newControl(plant);
ctrl.runSystem();
}
}
//______________________________________________________________________________
/**Areactorcorethathasathroughputthatcanbeeitherdecreasedor
increased.*/
classReactor{
/**@returnthecurrentthroughputofthereactor.(Unitsare
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unspecified.)*/
publicintgetThroughput(){
//…implementationunspecified…
return0;
}
/**@returntrueifthereactorstatusiscritical,falseotherwise.*/
publicbooleanisCritical(){
//…implementationunspecified…
returnfalse;
}
/**Asksthereactortoincreasethroughput.*/
voidincreaseThroughput()throwsReactorCritical{
//…implementationunspecified…
}
/**Asksthereactortodecreasethroughput.*/
voiddecreaseThroughput(){
//…implementationunspecified…
}
}
//______________________________________________________________________________
/**Thisexceptionclassshouldbeusedtoreportthatthereactorstatus
is
critical.*/
classReactorCriticalextendsException{}
//______________________________________________________________________________
/**Acontrollerthatwillmanagethepowerplanttomakesurethatthe
reactorrunswithoptimalthroughput.*/
classControl{
privatefinalPowerPlantthePlant;
staticfinalintTOLERANCE=10;
/**@parampthepowerplanttocontrol*/
publicControl(PowerPlantp){
thePlant=p;
}
/**Runthepowerplantbycontinuouslymonitoringthe
optimalthroughputandtheactualthroughputofthereactor.
Ifthethroughputsdifferbymorethan10units(i.e.tolerance),
adjustthereactorthroughput.
Ifthereactorgoescritical,theevacuatealarmis
soundedandthereactorisshutdown.
TherunSystem()methodcallsthemethodsneedAdjustment(),
adjustThroughput(),andshutdown().*/
publicvoidrunSystem(){
try{
while(true){//infiniteloop
intoptimalThroughput=thePlant.getOptimalThroughput();
if(needAdjustment(optimalThroughput)){
adjustThroughput(optimalThroughput);
}
}
}catch(ReactorCriticalrc){
thePlant.soundEvacuateAlarm();
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}finally{
shutdown();
}
}
/**Reportswhetherthethroughputofthereactorneedsadjusting.
Thismethodshouldalsomonitorandreportifthereactorgoes
critical.
@paramtargetthedesiredthroughput.
@returntrueiftheoptimalandactualthroughputvaluesdifferby
morethan10units.
@throwsReactorCriticalifthereactorgoescritical*/
publicbooleanneedAdjustment(inttarget)throwsReactorCritical{
/*Weaddedthethrowsclausetothemethoddeclarationsothat
themethodcanthrowaReactorCriticalexceptionifthereactor
goescritical.*/
if(thePlant.core.isCritical()){
thrownewReactorCritical();
}
returnMath.abs(thePlant.core.getThroughput()-target)>TOLERANCE;
}
/**Adjuststhethroughputofthereactorbycalling
increaseThroughput()
anddecreaseThroughput()methodsuntiltheactualthroughputis
within
10unitsofthetargetthroughput.
@paramtargetthedesiredthroughput.
@throwsReactorCriticalifthereactorgoescritical.*/
publicvoidadjustThroughput(inttarget)throwsReactorCritical{
/*Weaddedthethrowsclausetothemethoddeclarationbecause
thismethoddoesnotwanttohandleanyReactorCriticalexception
thrownbytheincreaseThroughput()method.*/
while(needAdjustment(target)){
if((thePlant.core.getThroughput()-target)>TOLERANCE){
thePlant.core.increaseThroughput();
}else{
thePlant.core.decreaseThroughput();
}
}
}
/**Shutsdownthereactorbyloweringthethroughputto0.*/
publicvoidshutdown(){
while(thePlant.core.getThroughput()>0){
thePlant.core.decreaseThroughput();
}
}
}
7Object-OrientedProgramming
7.1
Clickheretoviewcodeimage
//File:Exercise1.java
packagechap07pe1;
interfaceFunction{
intevaluate(intarg);
}
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classHalfimplementsFunction{
@Overridepublicintevaluate(intarg){
returnarg/2;
}
}
publicclassExercise1{
publicstaticint[]applyFunction(int…arrIn){
intlength=arrIn.length;
int[]arrOut=newint[length];
Functionfunc=newHalf();
for(inti=0;i<length;i++){
arrOut[i]=func.evaluate(arrIn[i]);
}
returnarrOut;
}
publicstaticvoidmain(String[]args){
//Halvethevalues.
int[]myArr=applyFunction(2,4,6,8);
System.out.println(“Arraywithvalueshalvedbyintegerdivision:”);
for(intvalue:myArr){
System.out.println(value);
}
}
}
7.2
Clickheretoviewcodeimage
//File:Exercise2.java
packagechap07pe2;
interfaceFunction{
intevaluate(intarg);
}
classHalfimplementsFunction{
@Overridepublicintevaluate(intarg){
returnarg/2;
}
}
classPrintimplementsFunction{
@Overridepublicintevaluate(intarg){
System.out.println(arg);
returnarg;
}
}
publicclassExercise2{
publicstaticint[]applyFunction(Functionfunc,int…arrIn){
intlength=arrIn.length;
int[]arrOut=newint[length];
for(inti=0;i<length;i++){
arrOut[i]=func.evaluate(arrIn[i]);
}
returnarrOut;
}
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publicstaticvoidmain(String[]args){
//Createaprintfunction.
Functionprint=newPrint();
System.out.println(“Originalvalues:”);
int[]myArr=applyFunction(print,2,4,6,8);
//Halvethearrayvalues.
myArr=applyFunction(newHalf(),myArr);
System.out.println(“Halvedvalues:”);
applyFunction(print,myArr);
}
}
8FundamentalClasses
8.1
Clickheretoviewcodeimage
/**
*Aggregate(non-generic)pairsofarbitraryobjects.
*/
publicfinalclassPair{
privatefinalObjectfirst,second;
/**ConstructaPairobject.*/
publicPair(Objectone,Objecttwo){
first=one;
second=two;
}
/**@returnthefirstconstituentobject.*/
publicObjectgetFirst(){returnfirst;}
/**@returnthesecondconstituentobject.*/
publicObjectgetSecond(){returnsecond;}
/**@returntrueifthepairofobjectsareidentical.*/
@Override
publicbooleanequals(Objectother){
if(this==other)returntrue;
if(!(otherinstanceofPair))returnfalse;
PairotherPair=(Pair)other;
returnfirst.equals(otherPair.first)&&
second.equals(otherPair.second);
}
/**@returnahashcodefortheaggregatepair.*/
@Override
publicinthashCode(){
//XORingthehashcodestocreateahashcodeforthepair.
returnfirst.hashCode()^second.hashCode();
}
/**@returnatextualrepresentationoftheaggregatedobject.*/
@Override
publicStringtoString(){
return“[”+first+“,”+second+“]”;
}
}
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8.2
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/**Determinewhetherastringisacase-sensitivepalindrome.*/
publicclassPalindrome{
publicstaticvoidmain(String[]args){
if(args.length!=1){
System.out.println(“Usage:javaPalindrome<word>”);
return;
}
Stringword=args[0];
StringBuilderreverseWord=newStringBuilder(word).reverse();
booleanisPalindrome=word.equals(reverseWord.toString());
System.out.println(“Theword”+word+”is”+
(isPalindrome?””:“not“)+“apalindrome”);
}
}
9ObjectLifetime
Noprogrammingexercises.
10TheArrayList<E>ClassandLambdaExpressions
10.1
Clickheretoviewcodeimage
/*Findallelementsinalistthatsatisfyallpredicates.*/
importjava.util.ArrayList;
importjava.util.List;
importjava.util.function.Predicate;
publicclassFilterFunSolution{
publicstaticvoidmain(String[]args)
{//(1)
//Createalistofstrings:
String[]strings={“Cheerup!”,“7Upcomingup!”,null,
“Bottomsup!”,“Getdown!”,“What’sup?”};
List<String>strList=newArrayList<>();
/*WRITECODETOPOPULATETHELISTOFSTRINGSHERE.*/
for(Stringstr:strings)
strList.add(str);
System.out.println(strList);
//Createalistofpredicates.
List<Predicate<String>>predList=newArrayList<>();
/*WRITECODETOPOPULATETHELISTOFPREDICATESHERE.*/
predList.add(str->str!=null);
predList.add(str->str.toLowerCase().contains(“up”));
predList.add(str->Character.isLetter(str.charAt(0)));
System.out.println(predList);
//Applyfiltering.
applyAllPredicates(strList,predList);
applyAllPredicatesAlt(strList,predList);
applyAllPredicatesGenAlt(strList,predList);
}
/**
*Printsallthestringsinthespecifiedlistthatsatisfyallthe
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*predicatesinthelistofpredicates.
*ItusestheandPredicates()methodat(3).
*@paramlistListofstringstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstaticvoidapplyAllPredicates(List<String>
list,//(2)
List<Predicate<String>>
predicates){
/*IMPLEMENTTHISMETHOD*/
List<String>resultList=newArrayList<>();
for(Stringstr:list){
if(andPredicates(str,predicates)){
resultList.add(str);
}
}
System.out.println(resultList);
}
/**
*Determineswhetherastringsatisfiesallthepredicates.
*@paramstrStringtoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*@returntrueonlyifthestringsatisfiesallthe
predicates
*/
publicstaticbooleanandPredicates(String
str,//(3)
List<Predicate<String>>predicates)
{
/*IMPLEMENTTHISMETHOD*/
booleanproceed=true;
for(Predicate<String>p:predicates){
proceed=proceed&&p.test(str);
if(!proceed)break;
}
returnproceed;
}
/**
*Removesalltheelementsinthespecifiedlistthatdonotsatisfy
allthe
*predicatesinthelistofpredicates,andprintstheremaining
elements
*thatdo.
*UsesPredicate.and(),Predicate.negate(),andList.removeIf()
methods.
*@paramlistListofstringstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstaticvoidapplyAllPredicatesAlt(List<String>
list,//(4)
List<Predicate<String>>
predicates){
/*IMPLEMENTTHISMETHOD*/
Predicate<String>compPred=s->true;
for(Predicate<String>p:predicates){
compPred=compPred.and(p);
}
list.removeIf(compPred.negate());
System.out.println(list);
}
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/**Genericversion.
*Removesalltheelementsinthespecifiedlistthatdonotsatisfy
allthe
*predicatesinthelistofpredicates,andprintstheremaining
elements
*thatdo.
*UsesPredicate.and(),Predicate.negate(),andList.removeIf()
methods.
*@paramlistListofelementstoapplythepredicateson
*@parampredicatesListofpredicatestoapply
*/
publicstatic<T>voidapplyAllPredicatesGenAlt(List<T>
list,//(5)
List<Predicate<T>>predicates){
/*IMPLEMENTTHISMETHOD*/
Predicate<T>compPred=s->true;
for(Predicate<T>p:predicates){
compPred=compPred.and(p);
}
list.removeIf(compPred.negate());
System.out.println(list);
}
}
11DateandTime
11.1
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.Period;
/**Classrepresentsstatisticsaboutanastronaut.*/
publicclassAstronaut{
privatefinalStringastronautName;
privatefinalLocalDateTimearrivalDateTime;
privatefinalLocalDatescheduledReturnDate;
privatefinalPeriodactualPeriodOfStay;
publicAstronaut(Stringname,LocalDateTimearrival,Periodperiod,
LocalDatereturnDate){
astronautName=name;
arrivalDateTime=arrival;
actualPeriodOfStay=period;
scheduledReturnDate=returnDate;
}
publicStringgetAstronautName(){returnastronautName;}
publicLocalDateTimegetArrivalDateTime(){returnarrivalDateTime;}
publicPeriodgetActualPeriodOfStay(){returnactualPeriodOfStay;
}
publicLocalDategetScheduledReturnDate(){returnscheduledReturnDate;
}
/**@returnLocalDateTheactualdateofreturn.*/
publicLocalDategetActualReturnDate(){
/*IMPLEMENTTHISMETHOD.*/
returnarrivalDateTime.toLocalDate().plus(actualPeriodOfStay);
}
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/**
*Returnsstatusoftheactualreturncomparedtothescheduled
return,
*whetheritwasontime,delayed,orearly.
*@returnStringIndicating“Ontime”,“Delayed”,or“Early”.
*/
publicStringgetReturnStatus(){
/*IMPLEMENTTHISMETHOD.*/
Stringstatus=“Ontime”;
LocalDateactualReturnDate=getActualReturnDate();
if(scheduledReturnDate.isBefore(actualReturnDate)){
status=“Delayed”;
}elseif(scheduledReturnDate.isAfter(actualReturnDate)){
status=“Early”;
}
returnstatus;
}
/**@returnPeriodTheplannedstayaccordingtothescheduled
return.*/
publicPeriodgetPlannedPeriodOfStay(){
/*IMPLEMENTTHISMETHOD.*/
returnPeriod.between(arrivalDateTime.toLocalDate(),
scheduledReturnDate);
}
/**
*@returnPeriodThedifferencebetweentheactualreturndateand
*thescheduledreturndate.*/
publicPeriodgetDiffPeriodOfStay(){
/*IMPLEMENTTHISMETHOD.*/
returnscheduledReturnDate.until(getActualReturnDate());
//Followingcodehaslogicalerror,asaperiodhasnonotionofa
date.
//returnactualPeriodOfStay.minus(getPlannedPeriodOfStay());
}
}
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.Period;
importjava.time.format.DateTimeFormatter;
publicclassSpaceStationStats{
publicstaticvoidmain(String[]args){
//Astronautdata
Astronaut[]astronauts={
newAstronaut(“AstroAli”,
LocalDateTime.of(2010,3,1,10,45),Period.ofMonths(3),
LocalDate.of(2010,5,1)),
newAstronaut(“LailaLightyear”,
LocalDateTime.of(2015,2,1,17,0),Period.ofWeeks(30),
LocalDate.of(2015,6,30)),
newAstronaut(“OrbitOrwell”,
LocalDateTime.of(2014,3,1,20,20),Period.ofMonths(6),
LocalDate.of(2014,9,1)),
newAstronaut(“RocketRogers”,
LocalDateTime.of(2013,7,31,15,30),Period.ofDays(60),
LocalDate.of(2013,9,30)),
newAstronaut(“SamSpacey”,
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LocalDateTime.of(2009,1,1,12,15),Period.ofDays(90),
LocalDate.of(2009,11,1)),
};
printReport(astronauts);
}
/**
*Methodprintsstatisticsaboutstayonaspacestation.
*Seetheexercisetextfortheformatofthereport.
*@paramastronautsThearraywithastronautdata
*/
privatestaticvoidprintReport(Astronaut[]astronauts){
System.out.println(“NameArr.DateSched.Return”
+”Act.ReturnStatusAct.StaySched.StayDiff”);
StringreportFormatStr=“%-16s%10s%12s%12s%8s%6s%10s%9s%n”;
/*IMPLEMENTTHERESTOFTHEMETHOD.*/
DateTimeFormatterdf=DateTimeFormatter.ofPattern(“uuuu/MM/dd”);
for(Astronautastro:astronauts){
StringastronautName=astro.getAstronautName();
LocalDatearrivalDate=astro.getArrivalDateTime().toLocalDate();
LocalDatescheduledReturnDate=astro.getScheduledReturnDate();
LocalDateactualReturnDate=astro.getActualReturnDate();
Stringstatus=astro.getReturnStatus();
PeriodperiodOfStay=astro.getActualPeriodOfStay();
PeriodplannedPeriodOfStay=astro.getPlannedPeriodOfStay();
PerioddiffPeriodOfStay=astro.getDiffPeriodOfStay();
System.out.printf(reportFormatStr,astronautName,
arrivalDate.format(df),
scheduledReturnDate.format(df),
actualReturnDate.format(df),status,
periodOfStay,plannedPeriodOfStay,
diffPeriodOfStay);
}
}
}
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AppendixE.MockExam:JavaSE8ProgrammerI
ThisisamockexamfortheJavaSE8ProgrammerIexam.Itcomprisesbrand-new
questions,whicharesimilartothequestionsthatcanbeexpectedontherealexam.
Workingthroughthisexamwillgiveyouagoodindicationofhowwellyouareprepared
fortherealexam,andwhetheranytopicsneedfurtherstudy.Annotatedanswerstothe
questionscanbefoundinAppendixF.
Questions
Q1Whichexpressionstatements,wheninsertedat(1),willcausethefollowingclassto
compilewithouterrors?
publicclassQ6db8{
privateinta;
privateintb=0;
privatestaticintc;
publicvoidm(){
intd;
inte=0;
//(1)INSERTCODEHERE.
}
}
Selectthefourcorrectanswers.
(a)a++;
(b)b++;
(c)c++;
(d)d++;
(e)e++;
Q2Whatwillbeprintedwhenthefollowingprogramisrun?
Clickheretoviewcodeimage
publicclassQd803{
publicstaticvoidmain(String[]args){
Stringword=“restructure”;
System.out.println(word.substring(2,3));
}
}
Selecttheonecorrectanswer.
(a)est
(b)es
(c)str
(d)st
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(e)s
Q3Whatwillbeprintedwhenthefollowingprogramisrun?
Clickheretoviewcodeimage
publicclassQ8929{
publicstaticvoidmain(String[]args){
for(inti=12;i>0;i-=3)
System.out.print(i);
System.out.println(””);
}
}
Selecttheonecorrectanswer.
(a)12
(b)129630
(c)12963
(d)36912
(e)Noneoftheabove.
Q4Whatwillbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassQ275d{
privatestaticinta;
privateintb;
publicQ275d(){
intc;
c=a;//(1)
a++;//(2)
b+=c;//(3)
a-=b;//(4)
}
publicstaticvoidmain(String[]args){
newQ275d();
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompilebecauseofthelinemarked(1).
(b)Theprogramwillfailtocompilebecauseofthelinemarked(2).
(c)Theprogramwillfailtocompilebecauseofthelinemarked(3).
(d)Theprogramwillfailtocompilebecauseofthelinemarked(4).
(e)Theprogramwillcompileandrunwithoutanyproblems.
Q5Whatwillbeprintedwhenthefollowingprogramisrun?
Clickheretoviewcodeimage
classBase{
protectedinti;
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Base(){add(1);}
voidadd(intv){i+=v;}
voidprint(){System.out.println(i);}
}
classExtensionextendsBase{
Extension(){add(2);}
voidadd(intv){i+=v*2;}
}
publicclassQd073{
publicstaticvoidmain(String[]args){
bogo(newExtension());
}
staticvoidbogo(Baseb){
b.add(8);
b.print();
}
}
Selecttheonecorrectanswer.
(a)9
(b)11
(c)13
(d)21
(e)22
Q6Whichisthefirstlineinthefollowingcodeafterwhichtheobjectcreatedintheline
marked(0)willbeacandidateforgarbagecollection,assumingnocompiler
optimizationsaredone?
Clickheretoviewcodeimage
classWidget{
privateStringmessage;
Widget(Stringmessage){
this.message=message;
}
publicStringtoString(){
returnthis.message;
}
}
publicclassQ76a9{
staticWidgetfiddle(){
Widgeta=newWidget(“hello”);
Widgetb=newWidget(“bye”);//(0)
Widgetc=newWidget(b+“!”);//(1)
Widgetd=b;//(2)
b=a;//(3)
d=a;//(4)
returnc;//(5)
}
publicstaticvoidmain(String[]args){
Widgetgiz=fiddle();
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System.out.println(giz);//(6)
}
}
Selecttheonecorrectanswer.
(a)Thelinemarked(1)
(b)Thelinemarked(2)
(c)Thelinemarked(3)
(d)Thelinemarked(4)
(e)Thelinemarked(5)
(f)Thelinemarked(6)
Q7WhichmethodsfromtheStringortheStringBuilderclassmodifythe
objectonwhichtheyareinvoked?
Selectthetwocorrectanswers.
(a)ThecharAt()methodoftheStringclass
(b)ThetoUpperCase()methodoftheStringclass
(c)Thereplace()methodoftheStringclass
(d)Thereplace()methodoftheStringBuilderclass
(e)Thereverse()methodoftheStringBuilderclass
(f)ThecharAt()methodoftheStringBuilderclass
Q8Whichofthefollowingstatements,wheninsertedindependentlyat(1),willthrowa
runtimeexception?
Clickheretoviewcodeimage
classA{}
classBextendsA{}
classCextendsA{}
publicclassQ3ae4{
publicstaticvoidmain(String[]args){
Ax=newA();
By=newB();
Cz=newC();
//(1)INSERTCODEHERE.
}
}
Selecttheonecorrectanswer.
(a)x=y;
(b)z=x;
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(c)y=(B)x;
(d)z=(C)y;
(e)y=(A)y;
Q9Giventhefollowingprogram:
Clickheretoviewcodeimage
publicclassQ400A60{
publicstaticvoidmain(String[]args){
Stringstr=“loopornottoloop”;
String[]strs={“loop”,“or”,“not”,“to”,“loop”};
//(1)INSERTLOOPHERE.
}
}
Whichcode,wheninsertedindependentlyat(1),willcompilewithouterrors?
Selectthefourcorrectanswers.
(a)
for(charch:str)
System.out.print(ch);
(b)
Clickheretoviewcodeimage
for(charch:str.toCharArray())
System.out.print(ch);
(c)
Clickheretoviewcodeimage
for(Characterch:str.toCharArray())
System.out.print(ch);
(d)
Clickheretoviewcodeimage
for(Characterch:str.toCharArray())
System.out.print(ch.charValue());
(e)
for(Stringstr:strs)
System.out.print(str);
(f)
for(Stringelt:strs[])
System.out.print(elt);
(g)
for(Stringelt:strs)
System.out.print(elt);
(h)
Clickheretoviewcodeimage
for(Characterch:strs[strs.length-1].toArray())
System.out.print(ch);
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Q10Whichcodeinitializesthetwo-dimensionalarraymatrixsothatmatrix[3]
[2]isavalidelement?
Selectthetwocorrectanswers.
(a)
int[][]matrix={
{0,0,0},
{0,0,0}
};
(b)
Clickheretoviewcodeimage
intmatrix[][]=newint[4][];
for(inti=0;i<matrix.length;i++)matrix[i]=newint[3];
(c)
intmatrix[][]={
0,0,0,0,
0,0,0,0,
0,0,0,0,
0,0,0,0
};
(d)intmatrix[3][2];
(e)int[]matrix[]={{0,0,0},{0,0,0},{0,0,0},
{0,0,0}};
Q11Whatwillbetheresultofattemptingtorunthefollowingprogram?
Clickheretoviewcodeimage
publicclassQaa75{
publicstaticvoidmain(String[]args){
String[][][]arr={
{{},null},
{{“1”,“2”},{“1”,null,“3”}},
{},
{{“1”,null}}
};
System.out.println(arr.length+arr[1][2].length);
}
}
Selecttheonecorrectanswer.
(a)TheprogramwillthrowanArrayIndexOutOfBoundsExceptionand
terminate.
(b)TheprogramwillthrowaNullPointerExceptionandterminate.
(c)Theprogramwillprint4.
(d)Theprogramwillprint6.
(e)Theprogramwillprint7.
Q12Whichexpressionswillevaluatetotrueifprecededbythefollowingcode?
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Stringa=“hello”;
Stringb=newString(a);
Stringc=a;
char[]d={‘h’,‘e’,‘l’,‘l’,‘o’};
Selectthetwocorrectanswers.
(a)(a=="Hello")
(b)(a==b)
(c)(a==c)
(d)a.equals(b)
(e)a.equals(d)
Q13Whichstatementsaretrueaboutthevalueofafield,whennoexplicitinitialvalue
hasbeenassigned?
Selectthetwocorrectanswers.
(a)Thevalueofafieldoftypeintisundetermined.
(b)Thevalueofafieldofanynumerictypeis0.
(c)Thecompilermayissueanerrorifthefieldisusedinamethodbeforeitis
initialized.
(d)AfieldoftypeStringwilldenotetheemptystring("").
(e)Thevalueofallfieldsthatarereferencesisnull.
Q14Whichmain()methodwillsucceedinprintingthelastprogramargumentand
terminatenormallywithnooutput,ifnoprogramargumentsarespecified?
Selecttheonecorrectanswer.
(a)
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args){
if(args.length!=0)
System.out.println(args[args.length-1]);
}
(b)
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args){
try{System.out.println(args[args.length]);}
catch(ArrayIndexOutOfBoundsExceptione){}
}
(c)
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args){
intix=args.length;
Stringlast=args[ix];
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if(ix!=0)System.out.println(last);
}
(d)
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args){
intix=args.length-1;
if(ix>0)System.out.println(args[ix]);
}
(e)
Clickheretoviewcodeimage
publicstaticvoidmain(String[]args){
try{System.out.println(args[args.length-1]);}
catch(NullPointerExceptione){}
}
Q15Whatwillbeprintedwhenthefollowingprogramisexecuted?
Clickheretoviewcodeimage
publicclassQcb90{
privateinta;
privateintb;
publicvoidf(){
a=0;
b=0;
int[]c={0};
g(b,c);
System.out.println(a+””+b+””+c[0]+”“);
}
publicvoidg(intb,int[]c){
a=1;
b=1;
c[0]=1;
}
publicstaticvoidmain(String[]args){
Qcb90obj=newQcb90();
obj.f();
}
}
Selecttheonecorrectanswer.
(a)000
(b)001
(c)010
(d)100
(e)101
Q16Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassQ28fd{
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publicstaticvoidmain(String[]args){
intcounter=0;
l1:
for(inti=0;i<10;i++){
l2:
intj=0;
while(j++<10){
if(j>i)breakl2;
if(j==i){
counter++;
continuel1;
}
}
}
System.out.println(counter);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillnotterminatenormally.
(c)Theprogramwillprint10andterminatenormally.
(d)Theprogramwillprint0andterminatenormally.
(e)Theprogramwillprint9andterminatenormally.
Q17Giventhefollowinginterfacedeclaration,whichdeclarationisvalid?
interfaceI{
voidsetValue(intval);
intgetValue();
}
Selecttheonecorrectanswer.
(a)
Clickheretoviewcodeimage
classAextendsI{
intvalue;
voidsetValue(intval){value=val;}
intgetValue(){returnvalue;}
}
(b)
interfaceBextendsI{
voidincrement();
}
(c)
Clickheretoviewcodeimage
abstractclassCimplementsI{
intgetValue(){return0;}
abstractvoidincrement();
}
(d)
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interfaceDimplementsI{
voidincrement();
}
(e)
Clickheretoviewcodeimage
classEimplementsI{
intvalue;
publicvoidsetValue(intval){value=val;}
}
Q18Whatwillbetheresultofattemptingtocompileandrunthefollowingcode?
Clickheretoviewcodeimage
publicclassQ6b0c{
publicstaticvoidmain(String[]args){
inti=4;
floatf=4.3;
doubled=1.8;
intc=0;
if(i==f)c++;
if(((int)(f+d))==((int)f+(int)d))c+=2;
System.out.println(c);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillprint0.
(c)Theprogramwillprint1.
(d)Theprogramwillprint2.
(e)Theprogramwillprint3.
Q19Whichoperatorswillalwaysevaluatealltheoperands?
Selectthetwocorrectanswers.
(a)||
(b)+
(c)&&
(d)?:
(e)%
Q20Whichstatementabouttheswitchconstructistrue?
Selecttheonecorrectanswer.
(a)Allswitchstatementsmusthaveadefaultlabel.
(b)Astatementwithinaswitchstatementcanhaveonlyonecaselabel.
(c)Thekeywordcontinuecanneveroccurwithinthebodyofaswitch
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statement.
(d)Nocaselabelmayfollowadefaultlabelinasingleswitchstatement.
(e)Acharacterliteralcanbeusedasavalueforacaselabel.
Q21Whatwillbeprintedwhenthefollowingprogramisrun?
Clickheretoviewcodeimage
publicclassQ03e4{
publicstaticvoidmain(String[]args){
Stringspace=”“;
Stringcomposite=space+“hello”+space+space;
composite.concat(“world”);
Stringtrimmed=composite.trim();
System.out.println(trimmed.length());
}
}
Selecttheonecorrectanswer.
(a)5
(b)6
(c)7
(d)12
(e)13
Q22Whichmethoddeclarations,wheninsertedat(1),willcorrectlyoverloadthe
methodsum()?
Clickheretoviewcodeimage
publicclassQdd1f{
publiclongsum(longa,longb){returna+b;}
//(1)INSERTCODEHERE.
}
Selectthetwocorrectanswers.
(a)publicintsum(inta,intb){returna+b;}
(b)publicintsum(longa,longb){return0;}
(c)abstractintsum();
(d)privatelongsum(longa,longb){returna+b;}
(e)publiclongsum(longa,intb){returna+b;}
Q23Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
publicclassQ200A80{
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publicstaticvoidmain(String[]args){
callType(10);
}
privatestaticvoidcallType(Numbernum){
System.out.println(“Numberpassed”);
}
privatestaticvoidcallType(Objectobj){
System.out.println(“Objectpassed”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramcompilesandprints:Objectpassed.
(b)Theprogramcompilesandprints:Numberpassed.
(c)Theprogramfailstocompile,becausethecalltothecallType()methodis
ambiguous.
(d)Noneoftheabove.
Q24Whichofthesemethoddeclarationsarevaliddeclarationsofthemain()method
thatwouldbecalledbytheJVMtostarttheexecutionofaJavaapplication?
Selectthethreecorrectanswers.
(a)staticvoidmain(String[]args){/*...*/}
(b)publicstaticintmain(String[]args){/*...*/}
(c)publicstaticvoidmain(Stringargs){/*...*/}
(d)finalpublicstaticvoidmain(String[]arguments){/*
...*/}
(e)publicintmain(Strings[]args,intargc){/*...*/
}
(f)staticpublicvoidmain(Stringargs[]){/*...*/}
(g)staticpublicvoidmain(String…args){/*...*/}
Q25Giventheclass
Clickheretoviewcodeimage
publicclassArgs{
publicstaticvoidmain(String[]args){
System.out.println(args[0]+””+args[args.length-1]);
}
}
whatwouldbetheresultofexecutingthefollowingcommandline?
Clickheretoviewcodeimage
>javaArgsInpoliticsstupidityisnotahandicap
Selecttheonecorrectanswer.
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(a)TheprogramwillthrowanArrayIndexOutOfBoundsException.
(b)Theprogramwillprintjavahandicap.
(c)TheprogramwillprintArgshandicap.
(d)TheprogramwillprintInhandicap.
(e)TheprogramwillprintArgsa.
(f)TheprogramwillprintIna.
Q26Whichstatementaboutthefollowingprogramistrue?
Clickheretoviewcodeimage
classMyClass{
publicstaticvoidmain(String[]args){
String[]numbers={“one”,“two”,“three”,“four”};
if(args.length==0){
System.out.println(“noarguments”);
}else{
System.out.println(numbers[args.length]+”arguments”);
}
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)TheprogramwillthrowaNullPointerExceptionwhenrunwithno
programarguments.
(c)Theprogramwillprintnoargumentsandtwoargumentswhencalled
withzeroandthreeprogramarguments,respectively.
(d)Theprogramwillprintnoargumentsandthreeargumentswhen
calledwithzeroandthreeprogramarguments,respectively.
(e)Theprogramwillprintnoargumentsandfourargumentswhencalled
withzeroandthreeprogramarguments,respectively.
(f)Theprogramwillprintoneargumentsandfourargumentswhencalled
withzeroandthreeprogramarguments,respectively.
Q27Whichstatementsaretrueabouttheimportstatement?
Selectthetwocorrectanswers.
(a)Staticimportfromaclassautomaticallyimportsthenamesofstaticmembersof
anynestedtypesdeclaredinthatclass.
(b)Staticmembersofthedefaultpackagecannotbeimported.
(c)Staticimportstatementsmustbespecifiedafteranytypeimportstatements.
(d)Inthecaseofanameconflict,thenameinthelaststaticimportstatementis
chosen.
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(e)Adeclarationofanameinacompilationunitcanshadowanamethatis
imported.
Q28Whatwouldbetheresultofcompilingandrunningthefollowingprogram?
Clickheretoviewcodeimage
classMyClass{
staticMyClassref;
String[]arguments;
publicstaticvoidmain(String[]args){
ref=newMyClass();
ref.func(args);
}
publicvoidfunc(String[]args){
ref.arguments=args;
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile,sincethestaticmethodmain()cannothavea
calltothenon-staticmethodfunc().
(b)Theprogramwillfailtocompile,sincethenon-staticmethodfunc()cannot
accessthestaticvariableref.
(c)Theprogramwillfailtocompile,sincetheargumentargspassedtothestatic
methodmain()cannotbepassedtothenon-staticmethodfunc().
(d)Theprogramwillcompile,butwillthrowanexceptionwhenrun.
(e)Theprogramwillcompileandrunsuccessfully.
Q29Giventhefollowingmemberdeclarations,whichstatementistrue?
Clickheretoviewcodeimage
inta;//(1)
staticinta;//(2)
intf(){returna;}//(3)
staticintf(){returna;}//(4)
Selecttheonecorrectanswer.
(a)Declarations(1)and(3)cannotoccurinthesameclassdeclaration.
(b)Declarations(2)and(4)cannotoccurinthesameclassdeclaration.
(c)Declarations(1)and(4)cannotoccurinthesameclassdeclaration.
(d)Declarations(2)and(3)cannotoccurinthesameclassdeclaration.
Q30Whichofthesecombinationsofswitchexpressiontypesandcaselabelvalue
typesarelegalwithinaswitchstatement?
Selectthethreecorrectanswers.
(a)switchexpressionoftypeintandcaselabelvalueoftypechar
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(b)switchexpressionoftypefloatandcaselabelvalueoftypeint
(c)switchexpressionoftypebyteandcaselabelvalueoftypefloat
(d)switchexpressionoftypecharandcaselabelvalueoftypelong
(e)switchexpressionoftypebooleanandcaselabelvalueoftypeboolean
(f)switchexpressionoftypeByteandcaselabelvalueoftypebyte
(g)switchexpressionoftypebyteandcaselabelvalueoftypeByte
(h)switchexpressionoftypeStringandcaselabelvalueoftypeString
Q31Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassSwitcheroo{
publicstaticvoidmain(String[]args){
finalintiLoc=3;
finalIntegeriFour=4;
IntegeriRef=4;
switch(iRef){
caseiFour:
System.out.println(“It’sOK.”);
case1:
caseiLoc:
case2*iLoc:
System.out.println(“IamnotOK.”);
default:
System.out.println(“YouareOK.”);
}
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butwillthrowaruntimeexception.
(c)Theprogramwillcompilecorrectlyandwillprintthefollowingatruntime:
It’sOK.
IamnotOK.
YouareOK.
(d)Theprogramwillcompilecorrectlyandwillprintthefollowingatruntime:
It’sOK.
IamnotOK.
(e)Theprogramwillcompilecorrectlyandwillprintthefollowingatruntime:
It’sOK.
Q32Whichofthefollowingimplementationsofamax()methodwillcorrectlyreturn
thelargestvalue?
Clickheretoviewcodeimage
//(1)
intmax(intx,inty){
return(if(x>y){x;}else{y;});
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}
//(2)
intmax(intx,inty){
return(if(x>y){returnx;}else{returny;});
}
//(3)
intmax(intx,inty){
switch(x<y){
casetrue:
returny;
default:
returnx;
};
}
//(4)
intmax(intx,inty){
if(x>y)returnx;
returny;
}
Selecttheonecorrectanswer.
(a)Implementationlabeled(1)
(b)Implementationlabeled(2)
(c)Implementationlabeled(3)
(d)Implementationlabeled(4)
Q33Giventhefollowingcode,whichstatementistrue?
Clickheretoviewcodeimage
classMyClass{
publicstaticvoidmain(String[]args){
intk=0;
intl=0;
for(inti=0;i<=3;i++){
k++;
if(i==2)break;
l++;
}
System.out.println(k+“,”+l);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillprint3,3atruntime.
(c)Theprogramwillprint4,3atruntime,ifthebreakstatementisreplacedby
thecontinuestatement.
(d)Theprogramwillfailtocompileifthebreakstatementisreplacedbythe
returnstatement.
(e)Theprogramwillfailtocompileifthebreakstatementisreplacedbyanempty
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statement.
Q34Whichstatementsaretrue?
Selectthetwocorrectanswers.
(a){{}}isavalidblockstatement.
(b){continue;}isavalidblockstatement.
(c)block:{breakblock;}isavalidblockstatement.
(d)block:{continueblock;}isavalidblockstatement.
(e)Thebreakstatementcanbeusedonlyinaloop(while,do-whileorfor)
oraswitchstatement.
Q35Giventhedeclaration:
Clickheretoviewcodeimage
int[][]nums={{20},{30},{40}};
Whichcodewillcompileandprint90atruntime?
Selecttheonecorrectanswer.
(a)
{
intsum=0;
for(int[]row:nums[])
for(intval:nums[row])
sum+=val;
System.out.println(sum);
}
(b)
{
intsum=0;
for(int[]row:nums[][])
for(intval:nums[row])
sum+=val;
System.out.println(sum);
}
(c)
{
intsum=0;
for(int[]row:nums)
for(intval:nums[row])
sum+=val;
System.out.println(sum);
}
(d)
{
intsum=0;
for(int[]row:nums)
for(intval:row)
sum+=val;
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System.out.println(sum);
}
(e)
{
intsum=0;
for(Integer[]row:nums)
for(intval:row)
sum+=val;
System.out.println(sum);
}
Q36Whichdigits,andinwhatorder,willbeprintedwhenthefollowingprogramis
compiledandrun?
Clickheretoviewcodeimage
publicclassMyClass{
publicstaticvoidmain(String[]args){
try{
interruptForLunch();
}catch(InterruptedExceptione){
System.out.println(“1”);
thrownewRuntimeException();
}catch(RuntimeExceptione){
System.out.println(“2”);
return;
}catch(Exceptione){
System.out.println(“3”);
}finally{
System.out.println(“4”);
}
System.out.println(“5”);
}
//InterruptedExceptionisadirectsubclassofException.
staticvoidinterruptForLunch()throwsInterruptedException{
thrownewInterruptedException(“Timeforlunch.”);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillprint5.
(b)Theprogramwillprint1and4,inthatorder.
(c)Theprogramwillprint1,2,and4,inthatorder.
(d)Theprogramwillprint1,4,and5,inthatorder.
(e)Theprogramwillprint1,2,4,and5,inthatorder.
(f)Theprogramwillprint3and5,inthatorder.
Q37Howmanyobjectsarereachablewhencontrolreaches(1)?
Clickheretoviewcodeimage
publicclassNullify{
privatestaticvoidnullify(Object[]array){array=null;}
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publicstaticvoidmain(String[]args){
args=null;
Object[]array=newObject[4];
for(inti=0;i<4;i++){
array[i]=newObject();
}
nullify(array);
System.gc();//(1);
}
}
Selecttheonecorrectanswer.
(a)0
(b)1
(c)4
(d)5
(e)Itisdifficulttosay.
Q38Whichstatementdescribestheguaranteedbehaviorofthegarbagecollectionand
finalizationmechanisms?
Selecttheonecorrectanswer.
(a)Objectswillnotbedestroyeduntiltheyhavenoreferencestothem.
(b)Anobjecteligibleforgarbagecollectionwilleventuallybedestroyedbythe
garbagecollector.
(c)IfobjectAbecameeligibleforgarbagecollectionbeforeobjectB,thenobjectA
willbedestroyedbeforeobjectB.
(d)Anobject,onceeligibleforgarbagecollection,canneverbecomeaccessibleby
alivethread.
(e)Noneoftheabove.
Q39Whichoftheseexpressionsarelegal?
Selectthefourcorrectanswers.
(a)"co".concat("ol")
(b)("co"+"ol")
(c)('c'+'o'+'o'+'l')
(d)("co"+newString('o'+'l'))
(e)("co"+newString("co"))
Q40WhichstatementaboutthecharAt()methodoftheStringclassistrue?
Selecttheonecorrectanswer.
(a)ThecharAt()methodtakesacharvalueasanargument.
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(b)ThecharAt()methodreturnsaCharacterobject.
(c)Theexpression("abcdef").charAt(3)isillegal.
(d)Theexpression"abcdef".charAt(3)evaluatestothecharacter'd'.
(e)Theindexofthefirstcharacteris1.
Q41Whichexpressionwillevaluatetotrue?
Selecttheonecorrectanswer.
(a)"hello:there!".equals("hellothere")
(b)"HELLOTHERE".equals("hellothere")
(c)("hello".concat("there")).equals("hellothere")
(d)"HelloThere".compareTo("hellothere")==0
(e)"Hellothere".toLowerCase().equals("hellothere")
Q42Whatwillthefollowingprogramprintwhenrun?
Clickheretoviewcodeimage
publicclassSearch{
publicstaticvoidmain(String[]args){
Strings=“Contentment!”;
intmiddle=s.length()/2;
Stringnt=s.substring(middle-1,middle+1);
System.out.println(s.lastIndexOf(nt,middle));
}
}
Selecttheonecorrectanswer.
(a)2
(b)4
(c)5
(d)7
(e)9
(f)11
(g)Noneoftheabove.
Q43Whatwillbetheresultofattemptingtocompileandrunthefollowingprogram?
Clickheretoviewcodeimage
publicclassStringMethods{
publicstaticvoidmain(String[]args){
Stringstr=newString(“eeny”);
str.concat(”meeny”);
StringBuilderstrBuilder=newStringBuilder(”miny”);
strBuilder.append(”mo”);
System.out.println(str+strBuilder);
}
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}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillprinteenymeenyminymoatruntime.
(c)Theprogramwillprintmeenyminymoatruntime.
(d)Theprogramwillprinteenyminymoatruntime.
(e)Theprogramwillprinteenymeenyminyatruntime.
Q44Whatwillbetheresultofattemptingtocompilethiscode,containedinasourcefile
namedAClass.java?
importjava.util.*;
packagecom.acme.toolkit;
publicclassAClass{
publicOtheranInstance;
}
classOther{
intvalue;
}
Selecttheonecorrectanswer.
(a)Thecodewillfailtocompile,sincetheclassOtherhasnotyetbeendeclared
whenreferencedintheclassAClass.
(b)Thecodewillfailtocompile,sinceanimportstatementcannotoccurasthe
firststatementinasourcefile.
(c)Thecodewillfailtocompile,sincethepackagedeclarationcannotoccurafter
animportstatement.
(d)Thecodewillfailtocompile,sincetheclassOthermustbedefinedinafile
calledOther.java.
(e)Thecodewillfailtocompile,sincetheclassOthermustbedeclaredas
public.
(f)Theclasswillcompilewithouterrors.
Q45Whichcodecanbeinsertedat(1)sothattheexceptionthrownbytheprogramis
caughtbythecatchclause?
Clickheretoviewcodeimage
publicclassQ1408a{
publicstaticvoidmain(String[]args){
try{
String[][]trio={null,{null},{“Tom”},{},{“Dick”,“Harry”}};
Stringsubstr=trio[3][0].substring(1,2);
}catch(/*(1)INSERTCODEHERE*/){
System.out.println(“Mindtheindex!”);
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}
}
}
Selectthetwocorrectanswers.
(a)ArrayIndexOutOfBoundsExceptione
(b)IndexOutOfBoundsExceptione
(c)StringIndexOutOfBoundsExceptione
(d)IllegalIndexFoundExceptione
(e)NullPointerExceptione
Q46Whatistheoutputfromrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassQ1408b{
publicstaticvoidmain(String[]args){
inti=0;
while(++i==i){
System.out.println(i++);
}
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillexecuteandterminatenormally,butwillnotprintanything.
(b)Theprogramwillexecuteindefinitely,printingallnumbersfrom1andupward.
(c)Theprogramwillexecuteindefinitely,printingallnumbersfrom2andupward.
(d)Theprogramwillexecuteindefinitely,printingallevennumbersfrom2and
upward.
(e)Theprogramwillexecuteindefinitely,printingalloddnumbersfrom1and
upward.
(f)Theprogramwillexecuteindefinitely,printingalloddnumbersfrom3and
upward.
Q47Whatistheoutputfromrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassRemainderFun{
publicstaticvoidmain(String[]args){
inti=24,k=7;
System.out.print(i%k+“|”);
System.out.print(i%-k+“|”);
System.out.print(-i%k+“|”);
System.out.println(-i%-k);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
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(b)Theprogramwillcompile,butwillthrowaruntimeexception.
(c)3|-3|-3|3
(d)3|3|-3|-3
(e)3|-3|-3|-3
(f)3|-3|3|-3
Q48Whatistheoutputfromrunningthefollowingprogram?
Clickheretoviewcodeimage
publicclassThingy{
privateStringname;
publicThingy(Stringname){
this.name=name;
}
publicstaticvoidmain(String[]args){
Thingything1=newThingy(“thing1”);
Thingything2=newThingy(“thing2”);
System.out.print(thing1.equals(thing2)+“,”);
thing2.name=“thing1”;
System.out.print(thing1.equals(thing2)+“,”);
thing2=thing1;
System.out.println(thing1.equals(thing2));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butwillthrowaruntimeexception.
(c)false,true,true
(d)false,false,false
(e)false,false,true
(f)false,true,false
Q49Whichstatementistrueaboutthefollowingprogram?
Clickheretoviewcodeimage
publicclassSwitchy{
publicstaticvoidmain(String[]args){
finalStrings1=“January”;
finalStringyr=”2014”;
s1.concat(yr);
switch(s1){
default:
System.out.println(“Sorry.”);
case“January”+yr:cases1+”2015”:
System.out.println(“OK.”);
}
}
}
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Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillcompile.Whenrun,itwillprint:
Sorry.
OK.
(c)Theprogramwillcompile.Whenrun,itwillprint:
Sorry.
(d)Theprogramwillcompile.Whenrun,itwillprint:
OK.
Q50Whichstatementswillcompilewithouterrors?
Selectthethreecorrectanswers.
(a)IntegeriRef=0b111_000;
(b)byteb=0B1111_1111;
(c)doubled=0B1111_1111D;
(d)DoubledRef=3_____141.592_653_589_793e-3;
(e)intdate1=Integer.parseInt("_2014_01_11");
(f)intdate2=_2014_01_11;
(g)longdate3=2014_01_11_L;
Q51Whicharraydeclarationswillnotcompile?
Selectthethreecorrectanswers.
(a)int[]array1=newint[0];
(b)int[]array2={};
(c)int[]array3=newint[]{};
(d)int[]array4=newint[4]{};
(e)int[]array5=newint[4]{0,1,2,3};
(f)int[]array6=newint[]{0,1,2,3};
(g)int[]arr2d1[]=newint[4][];
(h)int[][]arr2d2=newint[4][];
(i)int[][]arr2d3=newint[][4];
(j)int[][]arr2d4=newint[4][0];
Q52Whichstatements,whenconsideredindividually,aretrueaboutthefollowingcode?
Clickheretoviewcodeimage
publicclassOverloading{
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privateStringxqt(inti){returnnull;}//(1)
publicvoidxqt(){}//(2)
publicIntegerxqt(intj){return1;}//(3)
}
Selectthetwocorrectanswers.
(a)Themethodsat(1)and(2)arecorrectlyoverloaded.
(b)Themethodsat(1)and(3)arecorrectlyoverloaded.
(c)Themethodsat(2)and(3)arecorrectlyoverloaded.
(d)Allmethodsarecorrectlyoverloaded.
Q53Whatistheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
publicclassGizmo{
privateStringBuildername;
privatedoubleweight;
publicGizmo(){
name=newStringBuilder(“MyGizmo”);
weight=10.0;
}
publicStringtoString(){return“Name:”+name+“,Weight:”+
weight;}
privatestaticvoidchangeName(StringBuildersb){sb.append(“2014”);}
privatestaticvoidchangeWeight(doubleweight){weight=2*weight;
}
publicstaticvoidmain(String[]arguments){
Gizmogiz=newGizmo();
changeName(giz.name);
changeWeight(giz.weight);
System.out.println(giz);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butwillthrowaruntimeexception.
(c)Name:MyGizmo,Weight:10.0
(d)Name:MyGizmo2014,Weight:10.0
(e)Name:MyGizmo,Weight:20.0
(f)Name:MyGizmo2014,Weight:20.0
Q54Whichcodecanbeinsertedindependentlyat(1)sothattheprogramprintsthe
following:FreeMeal|FreeMeal|FreeMeal|.
Clickheretoviewcodeimage
publicclassLoopDeLoop{
publicstaticvoidmain(String[]args){
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StringBuildermeals[]={
newStringBuilder(),newStringBuilder(),newStringBuilder()
};
//(1)INSERTCODEHERE
for(StringBuildermeal:meals)System.out.print(meal+“|”);
}
}
Selectthefourcorrectanswers.
(a)for(StringBuildermeal:meals)meal.insert(0,"Free
Meal");
(b)for(StringBuildermeal:meals)meal=meal.insert(0,
"FreeMeal");
(c)
Clickheretoviewcodeimage
StringBuilderfreeMeal=newStringBuilder(“FreeMeal”);
for(StringBuildermeal:meals)meal=freeMeal;
(d)for(StringBuildermeal:meals)meal=new
StringBuilder("FreeMeal");
(e)for(StringBuildermeal:meals)meal.append("Free
Meal");
(f)for(StringBuildermeal:meals)meal=
meal.append("FreeMeal");
Q55Whatistheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
importjava.util.ArrayList;
publicclassWeekend{
publicstaticvoidmain(String[]args){
ArrayList<String>longWeekend=newArrayList<>();
longWeekend.add(“Friday”);
longWeekend.add(2,“Saturday”);
longWeekend.add(“Sunday”);
longWeekend.remove(0);
System.out.println(longWeekend);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillcompile.Whenrun,itwillprint[Saturday,Sunday].
(c)Theprogramwillcompile.Whenrun,itwillprint[Sunday,Saturday].
(d)Theprogramwillcompile.Whenrun,itwillthrowa
java.lang.IndexOutOfBoundsException.
Q56Whatistheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
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classRoom{
staticintnumOfGuests;
}
publicclassHotel{
publicstaticvoidmain(String[]args){
Roomr1=newRoom();
Roomr2=newRoom();
r1.numOfGuests=2;
r2.numOfGuests=3;
System.out.println(“Numberofguests:”+r1.numOfGuests+
r2.numOfGuests);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillfailtocompile.
(b)Theprogramwillcompile,butwillthrowaruntimeexception.
(c)Numberofguests:4
(d)Numberofguests:5
(e)Numberofguests:6
(f)Numberofguests:22
(g)Numberofguests:33
Q57Inwhichscenarioswouldado-whileloopbepreferabletoawhileloop?
Selectthethreecorrectanswers.
(a)Admitoneticket-holderatatimeintoamovietheateruntiltherearenomore
ticket-holderstoadmit.
(b)Guessanansweruntiltheansweriscorrect.
(c)Addalittlesalttothefooduntilthefoodtastesright.Assumethefoodhastoo
littlesalttostartwith.
(d)Addcandlestoabirthdaycakeuntiltherearetherightnumberofcandles.
(e)Letthemiceplayalittlewhilethecatisaway.
Q58Whatistheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
publicclassCodeMe{
publicstaticvoidmain(String[]args){
booleanflag=false;
if(false)//(1)
flag=!flag;
System.out.println(flag);
}
}
Selectthetwocorrectanswers.
(a)Theprogram,asitstands,doesnotcompile.
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(b)Theprogramcompileswithouterrors.Whenrun,itwillprintfalse.
(c)Theprogramcompileswithouterrors.Whenrun,itwillprinttrue.
(d)Ifthekeywordifat(1)isreplacedwiththekeywordwhile,theprogram
compileswithouterrors.Whenrun,itwillprintfalse.
(e)Ifthekeywordifat(1)isreplacedwiththekeywordwhile,theprogram
compileswithouterrors.Whenrun,itwillprinttrue.
(f)Ifthekeywordifat(1)isreplacedwiththekeywordwhile,theprogramwill
notcompile.
Q59Whatcodecanbeinsertedindependentlyat(1)sothattheprogramprintsthevalue
2014?
Clickheretoviewcodeimage
publicclassCastMe{
publicstaticvoidmain(String[]args){
Numbernum=2014;
//(1)INSERTCODEHERE.
System.out.println(iRef);
}
}
Selecttheonecorrectanswer.
(a)IntegeriRef=((Integer)num).intValue();
(b)IntegeriRef=(Integer)num.intValue();
(c)IntegeriRef=(Integer)(num.intValue());
(d)IntegeriRef=num.intValue();
(e)Anyoneoftheabovestatementscanbeinsertedat(1)
(f)Noneoftheabove.
Q60Whatistheoutputfromthefollowingprogram?
Clickheretoviewcodeimage
classPerson{
publicvoidcompare(Personp){
System.out.print(“Personsareequal.”);
}
}
classStudentextendsPerson{
publicvoidcompare(Students){
System.out.print(“Studentsareequal.”);
}
}
publicclassCalling{
publicstaticvoidmain(String[]args){
Personp1=newPerson();
Students1=newStudent();
Students2=newStudent();
Personp2=s2;
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p1.compare(s1);p1.compare(p2);System.out.println();
p2.compare(s1);p2.compare(p1);System.out.println();
s1.compare(p1);s1.compare(p2);s1.compare(s2);System.out.println();
}
}
Selecttheonecorrectanswer.
(a)
Clickheretoviewcodeimage
Personsareequal.Personsareequal.
Studentsareequal.Personsareequal.
Personsareequal.Personsareequal.Studentsareequal.
(b)
Clickheretoviewcodeimage
Personsareequal.Personsareequal.
Personsareequal.Personsareequal.
Personsareequal.Personsareequal.Studentsareequal.
(c)
Clickheretoviewcodeimage
Personsareequal.Personsareequal.
Studentsareequal.Personsareequal.
Personsareequal.Studentsareequal.Studentsareequal.
(d)
Clickheretoviewcodeimage
Personsareequal.Personsareequal.
Personsareequal.Personsareequal.
Personsareequal.Studentsareequal.Studentsareequal.
Q61Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A56{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
strList.add(0,“Ada”);
strList.add(“Alyla”);
strList.set(strList.size()-1,“Otto”);
strList.add(strList.size()-1,“Anna”);
System.out.println(strList);//(1)
intsize=strList.size();
for(inti=0;i<size;++i){
strList.add(strList.get(size-1-i));
}
System.out.println(strList);//(2)
}
}
Selectthetwocorrectanswers.
(a)(1)willprint[Ada,Alyla,Anna].
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(b)(1)willprint[Ada,Anna,Otto].
(c)(1)willprint[Ada,Otto,Alyla].
(d)(2)willprint[Ada,Alyla,Anna,Anna,Alyla,Ada].
(e)(2)willprint[Ada,Anna,Otto,Otto,Anna,Ada].
(f)(2)willprint[Ada,Otto,Alyla,Alyla,Otto,Ada].
Q62Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A70{
publicstaticvoidmain(String[]args){
List<String>list1=newArrayList<>(20);
list1.add(“Ada”);
List<String>list2=newArrayList<>(list1);
list2.add(null);
System.out.print(list1.size()==list2.size());
System.out.print(“-”+(list1==list2));
System.out.println(“-”+list1.equals(list2));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowanexception.
(c)Theprogramwillprintfalse-true-true.
(d)Theprogramwillprinttrue-false-true.
(e)Theprogramwillprintfalse-false-false.
Q63Whichlineswillbeintheoutputwhenthefollowingprogramiscompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A99{
publicstaticvoidmain(String[]args){
List<Integer>numList=newArrayList<>();
numList.add(3);numList.add(1);numList.add(1,4);
numList.add(null);numList.add(0);
System.out.println(“(1)prints”+numList.get(3));
System.out.println(“(2)prints”+numList.set(1,3));
System.out.println(“(3)prints”+numList.lastIndexOf(3));
System.out.println(“(4)prints”+numList.contains(3));
System.out.println(“(5)prints”+numList.remove(3));
System.out.println(“(6)prints”+numList.indexOf(3));
System.out.println(“(7)prints”+numList.remove(newInteger(4)));
}
}
Selectthesixcorrectanswers.
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(a)(1)printsnull
(b)(2)prints3
(c)(2)prints4
(d)(3)prints2
(e)(3)prints1
(f)(4)printstrue
(g)(5)printsnull
(h)(5)prints1
(i)(6)prints1
(j)(7)printstrue
(k)(7)printsfalse
Q64Whichlineswillresultinacompile-timeerror?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A80{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>(0);//
(1)
List<Object>objList=strList;//
(2)
List<Number>numList=newArrayList<>();//
(3)
numList.add(1);//
(4)
numList.add(1.5);//
(5)
List<>intList=newArrayList<Integer>();//
(6)
ArrayList<ArrayList<Integer>>loList1=newArrayList<Integer>();//
(7)
ArrayList<ArrayList<Integer>>loList2=newArrayList();//
(8)
}
}
Selectthethreecorrectanswers.
(a)Compile-timeerrorin(1)
(b)Compile-timeerrorin(2)
(c)Compile-timeerrorin(3)
(d)Compile-timeerrorin(4)
(e)Compile-timeerrorin(5)
(f)Compile-timeerrorin(6)
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(g)Compile-timeerrorin(7)
(h)Compile-timeerrorin(8)
Q65Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A85{
publicstaticvoidmain(String[]args){
List<String>list=newArrayList<>(20);
list.add(“Taco”);
for(inti=0;i<3;++i){
list.add(””+list);
}
System.out.println(list);//(1)
System.out.println(list.size());//(2)
}
}
Selectthetwocorrectanswers.
(a)(1)willprint:
[Taco,Taco,Taco,Taco,Taco,Taco,Taco,Taco]
(b)(1)willprint:
[Taco,[Taco,Taco],[Taco,Taco,Taco],[Taco,Taco,Taco,Taco]]
(c)(1)willprint:
[Taco,[Taco],Taco,[Taco],Taco,[Taco],Taco,[Taco]]
(d)(1)willprint:
[Taco,[Taco],[Taco,[Taco]],[Taco,[Taco],[Taco,[Taco]]]]
(e)(2)willprint4.
(f)(2)willprint8.
Q66Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.util.ArrayList;
importjava.util.List;
publicclassMEQ12A55{
publicstaticvoidmain(String[]args){
List<String>strList=newArrayList<>();
strList.add(strList.size(),“Anna”);
strList.add(strList.size()-1,“Ada”);
strList.add(strList.size()-1,“Otto”);
strList.add(0,“Alyla”);
System.out.println(strList);
intsize=strList.size();
for(inti=0;i<size/2;++i){
StringstrTemp=strList.get(i);
strList.set(i,strList.get(size-1-i));
strList.set(size-1-i,strTemp);
}
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System.out.println(strList);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)TheprogramwillthrowanIndexOutOfBoundsException.
(c)TheprogramwillthrowaNullPointerException.
(d)Theprogramwillprint:
[Alyla,Ada,Otto,Anna]
[Anna,Otto,Ada,Alyla]
(e)Theprogramwillprint:
[Ada,Otto,Alyla,Anna]
[Anna,Alyla,Otto,Ada]
Q67Whichstatementsaretrueaboutlambdaexpressions?
Selectthetwocorrectanswers.
(a)Areturnstatementismandatoryinalambdaexpressionifthelambdabodyis
astatementblock.
(b)Areturnstatementismandatoryinalambdaexpressionifthelambdabodyis
asingleexpressionthatreturnsavalue.
(c)Theformalparametersofalambdaexpressionarelocalvariablesintheblock
scopeofthelambdaexpression.
(d)Alocalvariabledeclarationintheblockscopeofalambdaexpressioncan
shadowaclassmemberwiththesamenameintheenclosingclass.
(e)Alocalvariabledeclarationintheblockscopeofalambdaexpressioncan
shadowalocalvariablewiththesamenameintheenclosingmethod.
Q68Whichstatementsaretrueaboutthefollowingcode?
Clickheretoviewcodeimage
importjava.util.function.Predicate;
publicclassMEQ12A92{
publicstaticvoidmain(String[]args){
Predicate<String>p1=Stringobj->obj.equals(“Java”);//
(1)
Predicate<String>p2=(finalStringobj)->obj.equals(“Java”);//
(2)
Predicate<String>p3=(finalobj)->obj.equals(“Java”);//
(3)
Predicate<String>p4=(Stringobj)->obj.equals(“Java”);//
(4)
Predicate<String>p5=(obj)->obj.equals(“Java”);//
(5)
Predicate<String>p6=obj->obj.equals(“Java”);//
(6)
Predicate<String>p7=obj->returnobj.equals(“Java”);//
(7)
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Predicate<String>p8=obj->{obj.equals(“Java”)};//
(8)
Predicate<String>p9=obj->{obj.equals(“Java”);};//
(9)
Predicate<String>p10=obj->{returnobj.equals(“Java”);};//
(10)
Predicate<Object>p11=obj->obj.equals(“Java”);//
(11)
}
}
Selectthefivecorrectanswers.
(a)(1)willnotcompile.
(b)(2)willnotcompile.
(c)(3)willnotcompile.
(d)(4)willnotcompile.
(e)(5)willnotcompile.
(f)(6)willnotcompile.
(g)(7)willnotcompile.
(h)(8)willnotcompile.
(i)(9)willnotcompile.
(j)(10)willnotcompile.
(k)(11)willnotcompile.
Q69Whichstatementsaretrueaboutthefollowingcode?
Clickheretoviewcodeimage
importjava.util.function.Predicate;
publicclassMEQ12A95{
publicstaticvoidmain(String[]args){
finalStringlock3=“Trio”;
Stringlock4=“Chubb”;
Predicate<Object>p;
p=lock->{lock=lock.toString();returnlock.equals(“TRIO”);
};//(1)
p=lock->{if(args.length>0)returnlock.equals(args[0]);
};//(2)
p=lock->{Stringlock3=“CHUBB”;returnlock.equals(lock3);
};//(3)
p=lock->{returnlock.equals(lock3);
};//(4)
p=lock->{returnlock.equals(lock4);
};//(5)
p=lock4->{returnlock4.equals(“Chubb”);
};//(6)
}
}
Selectthethreecorrectanswers.
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(a)(1)willcompile.
(b)(2)willcompile.
(c)(3)willcompile.
(d)(4)willcompile.
(e)(5)willcompile.
(f)(6)willcompile.
Q70Giventhefollowingprogram:
Clickheretoviewcodeimage
importjava.util.function.Predicate;
publicclassMEQ12A97{
publicstaticvoidmain(String[]args){
int[]intArray={-12,12,-123,123,-1234,1234};
filterInt(intArray,/*(1)INSERTCODEHERE*/);
}
privatestaticvoidfilterInt(int[]intArr,Predicate<Integer>
predicate){
for(inti=0;i<intArr.length;++i){
intintValue=intArr[i];
if(predicate.test(intValue)){
System.out.print(intValue+”“);
}
}
System.out.println();
}
}
Whichlambdaexpressionscanbeinsertedat(1)sothattheprogramprintsallodd
numbersthathavethreedigits(i.e.,-123and123)?
Selectthefourcorrectanswers.
(a)val->val%2!=0&&
String.valueOf(Math.abs(val)).length()==3
(b)val->returnval%2!=0&&
String.valueOf(Math.abs(val)).length()==3
(c)val->val%2!=0&&(""+Math.abs(val)).length()
==3
(d)val->{val%2!=0&&(""+Math.abs(val)).length()
==3;}
(e)val->val%2!=0&&new
String(Math.abs(val)).length()==3
(f)val->{returnval%2!=0&&new
String(Math.abs(val)).length()==3;}
(g)val->val%2!=0&&
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Integer.toString(Math.abs(val)).length()==3
(h)val->val%2!=0&&new
Integer(Math.abs(val)).toString().length()==3
Q71Whichstatementistrueaboutthefollowinginterfaces?
Clickheretoviewcodeimage
interfaceIA{booleanequals(Objectobj);}
interfaceIBextendsIA{booleandoIt(Stringstr);}
interfaceICextendsIB{booleandoIt(Stringstr);}
interfaceIDextendsIC{booleanequals(Objectobj);}
Selecttheonecorrectanswer.
(a)IAisafunctionalinterface.
(b)IBisafunctionalinterface.
(c)ICisnotafunctionalinterface.
(d)IDisnotafunctionalinterface.
(e)Noneoftheabove.
Q72WhichstatementistrueabouttheDateandTimeAPI?
Selecttheonecorrectanswer.
(a)TheisBefore()andisAfter()methodsoftheLocalTime,
LocalDate,andLocalDateTimeclassesalwaysreturntruewhenthis
objectisthesameastheobjectpassedasanargument.
(b)ThewithmethodsoftheLocalDateandLocalDateTimeclasseswill
adjustthedayofthemonthtothelastvaliddayofthemonth,ifthedayofmonth
becomesinvalidwhentheirdate-basedvaluesarechanged.
(c)ThetoTotalDays()methodofthePeriodclassreturnsthelengthofthe
periodasthetotalnumberofdays.
(d)TheclassesLocalTime,LocalDate,LocalDateTime,andPeriod
implementtheComparableinterface,providinganimplementationofthe
compareTo()methodsothattwotemporalobjectsofthesametypecanbe
compared.
(e)Noneoftheabove.
Q73Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.LocalTime;
publicclassMEQ11A25{
publicstaticvoidmain(String[]args){
LocalTimetime=LocalTime.NOON;
time=time.withHour(10).plusMinutes(120);
System.out.println(time);
}
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}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexception.
(c)Theprogramwillprint00:00.
(d)Theprogramwillprint12:00.
(e)Theprogramwillprint24:00.
(f)Noneoftheabove.
Q74Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.LocalDate;
publicclassMEQ11A35{
publicstaticvoidmain(String[]args){
LocalDatedate=LocalDate.of(2015,1,1);
date.withYear(5);
System.out.println(date.plusMonths(12));
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexception.
(c)Theprogramwillprint0006-01-01.
(d)Theprogramwillprint2021-01-01.
(e)Theprogramwillprint2015-01-01.
(f)Theprogramwillprint2016-01-01.
(g)Noneoftheabove.
Q75Whatwillthefollowingprogramprintwhencompiledandrun?
Clickheretoviewcodeimage
importjava.time.Period;
publicclassMEQ11A40{
publicstaticvoidmain(String[]args){
Periodperiod=Period.ofYears(10).ofMonths(16);
System.out.println(period);
}
}
Selecttheonecorrectanswer.
(a)Theprogramwillnotcompile.
(b)Theprogramwillthrowaruntimeexception.
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(c)TheprogramwillprintP16M.
(d)TheprogramwillprintP1Y4M.
(e)TheprogramwillprintP11Y4M.
(f)TheprogramwillprintP10Y16M.
(g)Noneoftheabove.
Q76Whichcode,wheninsertedat(1),willmaketheprogramcompileandexecute
normally?
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassMEQ11A90{
publicstaticvoidmain(String[]args){
//(1)INSERTCODEHERE.
StringtimeStr=LocalTime.of(12,5).format(dtf);
StringdateStr=LocalDate.of(2016,4,1).format(dtf);
StringdateTimeStr=LocalDateTime.of(2016,4,1,12,5).format(dtf);
}
}
Selecttheonecorrectanswer.
(a)DateTimeFormatterdtf=
DateTimeFormatter.ISO_LOCAL_TIME;
(b)DateTimeFormatterdtf=
DateTimeFormatter.ISO_LOCAL_DATE;
(c)DateTimeFormatterdtf=
DateTimeFormatter.ISO_LOCAL_DATE_TIME;
(d)Noneoftheabove.
Q77Whichcode,wheninsertedat(1),willmaketheprogramcompileandexecute
normally?
Clickheretoviewcodeimage
importjava.time.LocalDate;
importjava.time.LocalDateTime;
importjava.time.LocalTime;
importjava.time.format.DateTimeFormatter;
publicclassMEQ11A95{
publicstaticvoidmain(String[]args){
StringinputStr=“Thetimeis15minutespast10PM.”;
Stringpattern=“‘Thetimeis‘m’minutespast‘ha.”;
DateTimeFormatterdtf=DateTimeFormatter.ofPattern(pattern);
//(1)INSERTCODEHERE.
}
}
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Selectthethreecorrectanswers.
(a)LocalTimetime=LocalTime.parse(inputStr,dtf);
(b)LocalDatedate=LocalDate.parse(inputStr,dtf);
(c)LocalDateTimedateTime=LocalDateTime.parse(inputStr,
dtf);
(d)StringtimeStr=LocalTime.of(9,20).format(dtf);
(e)StringdateStr=LocalDate.of(2015,12,
24).format(dtf);
(f)StringdateTimeStr=LocalDateTime.of(2015,12,24,
22,15).format(dtf);
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AppendixF.AnnotatedAnswerstoMockExamI
ThisappendixprovidesannotatedanswerstothequestionsinthemockexamfortheJava
SE8ProgrammerIcertificationfoundinAppendixE.
AnnotatedAnswers
Q1(a),(b),(c),and(e)
Onlylocalvariablesneedtobeexplicitlyinitializedbeforeuse.Fieldsareassigneda
defaultvalueifnotexplicitlyinitialized.
Q2(e)
Givingparameters(2,3)tothemethodsubstring()constructsastring
consistingofthecharactersfromindex2(inclusive)toindex3(exclusive)ofthe
originalstring;thatis,thestringreturnedcontainsthecharacteratindex2.Thefirst
characterisatindex0andthelastcharacterisatindex1lessthanthenumberof
charactersinthestring.
Q3(c)
Theloopprintsoutthevalues12,9,6,and3beforeterminating.Theloop
terminateswhenthevalueintheloopvariableibecomeslessthanorequalto0.
Thishappensafterthevalue3hasbeenprinted.
Q4(e)
Thefactthatafieldisstaticdoesnotmeanthatitisnotaccessiblefromnon-static
methodsandconstructors.Allfieldsareassignedadefaultvalueifnoinitializeris
specified.Staticfieldsareinitializedwhentheclassisloaded,andinstancefieldsare
initializedwhentheclassisinstantiated.Onlylocalvariablesmustbeexplicitly
initializedbeforeuse.
Q5(e)
AnobjectoftheclassExtensioniscreated.Thefirstthingtheconstructorof
ExtensiondoesisinvoketheconstructorofBase,usinganimplicitsuper()
call.Allcallstothemethodvoidadd(int)aredynamicallyboundtothe
add()methodintheExtensionclass,sincetheactualobjectisoftype
Extension.Therefore,thismethodiscalledbytheconstructorofBase,the
constructorofExtension,andthebogo()methodwiththeparameters1,2,and
8,respectively.Theinstancefieldichangesvalueaccordingly:2,6,and22.The
finalvalueof22isprinted.
Q6(d)
At(1),anewWidgetobjectisconstructedwiththemessagethatisaconcatenation
ofthemessage"bye"intheWidgetobjectdenotedbybandthestring"!".
Afterline(2),dandbarealiases.Afterline(3),bandaarealiases,butdstill
denotestheWidgetobjectwiththemessage"bye"fromline(0).Afterline(4),d
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andaarealiases.ReferencednolongerdenotestheWidgetobjectcreatedinline
(0).ThisWidgetobjecthasnoreferencesthatrefertoitand,therefore,isa
candidateforgarbagecollection.
Q7(d)and(e)
Stringobjectsareimmutable.NoneofthemethodsoftheStringclassmodifies
aStringobject.MethodstoUpperCase()andreplace()intheString
classwillreturnanewStringobjectthatcontainsthemodifiedstring.However,
StringBuilderobjectsaremutable.ThecharAt()methodofthe
StringBuilderclassisagetmethod,returningthecharacterataspecificindex,
withoutmodifyingthecontentsoftheStringBuilderobject.
Q8(c)
Statement(a)willexecutewithoutproblem,but(b),(d),and(e)willcausecompile-
timeerrors.Statements(b)and(e)willcausecompile-timeerrorsbecauseitisnot
possibletoconvertfromthesuperclassAtothesubclassesCandB,respectively.
Statement(d)willcausecompile-timeerrorsbecauseacastfromBtoCisinvalid.
BeinganinstanceofBexcludesthepossibilityofbeinganinstanceofC.Statement
(c)willcompile,butwillthrowaruntimeexceptionbecausetheobjectthatiscastto
BisnotaninstanceofB.
Q9(b),(c),(d),and(g)
In(a),aStringisneitheranarraynoranIterable.Themethod
toCharArray()oftheStringclassreturnsanarrayoftypechar.Achar
valueisassignabletothelocalvariableoftypecharin(b),andafterautoboxing,
assignabletothelocalvariableoftypeCharacterin(c)and(d).In(e),thelocal
variablestrisredeclared.In(f),theoccurrenceofthearrayoperator[]isnot
permitted.In(g),thearraystrsispermissibleinthefor(:)loop.In(h),the
StringclassdoesnothaveamethodnamedtoArray,butithasamethodnamed
toCharArray.
Q10(b)and(e)
Fortheexpressionmatrix[3][2]toaccessavalidelementofatwo-dimensional
array,thearraymusthaveatleastfourrowsandthefourthrowmusthaveatleast
threeelements.(a)producesa2×3array.(c)triestoinitializeatwo-dimensional
arrayasaone-dimensionalarray.(d)triestospecifyarraydimensionsinthetypeof
thearrayreferencedeclaration.
Q11(a)
Theexpressionarr.lengthwillevaluateto4.Theexpressionarr[1]will
accessthearray{{"1","2"},{"1",null,"3"}},andarr[1]
[2]willtrytoaccessthethirdelementofthisarray.Thisresultsinan
ArrayIndexOutOfBoundsException,sincethisarrayhasonlytwoelements.
Q12(c)and(d)
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Stringobjectscanhaveidenticalsequencesofcharacters.The==operator,when
usedonStringobjectreferences,willreturntrueifandonlyifbothreferences
denotethesameobject(i.e.,arealiases).Theequals()methodwillreturntrue
wheneverthecontentsoftheStringobjectsareidentical.Anarrayofcharanda
Stringaretwototallydifferenttypes,andwhencomparedusingtheequals()
methodoftheStringclass,thevaluereturnedwillbefalse.
Q13(b)and(e)
Unlikelocalvariables,allfieldsareinitializedwithdefaultinitialvalues.All
numericfieldsareinitializedtozero,booleanfieldstofalse,charfieldsto
'\u0000',andallreferencefieldstonull.
Q14(a)
Themain()methodin(b)willalwaysthrowandcatchan
ArrayIndexOutOfBounds-Exception,sinceargs.lengthisanillegal
indexintheargsarray.Themain()methodin(c)willalwaysthrowan
ArrayIndexOutOfBoundsExceptionsinceitalsousesargs.lengthas
anindex,butthisexceptionisnevercaught.Themain()methodin(d)willfailto
printtheargumentifonlyoneprogramargumentissupplied.Themain()method
in(e)willthrowanuncaughtArrayIndexOutOfBoundsExceptionifno
programargumentsarespecified.
Q15(e)
Methodg()modifiesthefielda.Methodg()modifiestheparameterb,notthe
fieldb,sincetheparameterdeclarationshadowsthefield.Variablesarepassedby
value,sothechangeofvalueinparameterbisconfinedtothemethodg().Method
g()modifiesthearraywhosereferencevalueispassedasaparameter.Changeto
thefirstelementisvisibleafterreturnfromthemethodg().
Q16(a)
Theprogramwillfailtocompilesincethelabell2cannotprecedethedeclaration
intj=0.Foralabeltobeassociatedwithaloop,itmustimmediatelyprecede
theloopconstruct.Iflabell2precededthewhileloop(insteadofthedeclaration
ofj),theprogramwouldcompileandprint9.
Q17(b)
Classescannotextendinterfaces;theymustimplementthem.Interfacescanextend
otherinterfaces,butcannotimplementthem.Aclassmustbedeclaredas
abstractifitdoesnotprovideanimplementationforallabstractmethodsofthe
interfacesthatitimplements.Methodsdeclaredininterfacesareimplicitlypublic
andabstract.Classesthatimplementthesemethodsmustexplicitlydeclarethese
methodstobepublic.
Q18(a)
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Thecodewillfailtocompilebecausetheliteral4.3hasthetypedouble.
Assignmentofadoublevaluetoafloatvariablewithoutanexplicitcastisnot
allowed.Thecodewouldcompileandprint0atruntime,iftheliteral4.3was
replacedwith4.3F.
Q19(b)and(e)
The&&and||operatorsexhibitshort-circuitbehavior.Thefirstoperandofthe
ternaryoperator(?:)isalwaysevaluated.Basedontheresultofthisevaluation,
eitherthesecondorthirdoperandisevaluated.
Q20(e)
Nolabelsaremandatory(includingthedefaultlabel),andlabelscanbeplacedin
anyorderwithintheswitchbody.Thekeywordcontinuemayoccurwithinthe
bodyofaswitchstatementaslongasitpertainstoaloop.Anenumconstant,a
non-longintegralconstantexpression,orastringconstantexpressioncanbeused
forcaselabelsaslongasthetypeiscompatiblewiththeexpressionintheswitch
expression.
Q21(a)
Stringsareimmutable,sotheconcat()methodhasnoeffectontheoriginal
Stringobject.Thestringonwhichthetrim()methodiscalledconsistsofeight
characters,wherethefirstandtwolastcharactersarespaces("hello").The
trim()methodreturnsanewStringobjectinwhichthewhitespacecharacters
ateachendhavebeenremoved.Thisleavesthefivecharactersoftheword
"hello".
Q22(a)and(e)
Methodoverloadingrequiresthatthemethodsignaturesaredifferent,butthemethod
nameisthesame.Thereturntypeisirrelevantinthisregard.Thesignatureofthe
existingmethodissum(long,long).
Thesignatureofthemethodin(a)issum(int,int).Thesignatureofthe
methodin(e)issum(long,int).Bothsignaturesaredifferentfromthe
signatureoftheexistingmethod.
Declarations(b)and(d)fail,sincethemethodsignatureisidenticaltotheexisting
method.Declaration(c)fails,sinceitdeclaresanabstractmethodinanon-
abstractclass.
Q23(b)
Themethodwiththemostspecificsignatureischosen.Inthiscasetheint
argument10isboxedtoanInteger,whichispassedtotheNumberformal
parameter,astypeNumberismorespecificthanObject.
Q24(d),(f),and(g)
Themain()methodmustbedeclaredaspublicandstatic,withreturntype
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void,andtakesasinglearrayofStringobjectsasargument.Theorderofthe
staticandpublickeywordsisirrelevant.Also,declaringthemethodfinalis
irrelevantinthisrespect.
Q25(d)
Thelengthofthearraypassedtothemain()methodisequaltothenumberof
programargumentsspecifiedinthecommandline.Unlikesomeotherprogramming
languages,theelementatindex0doesnotcontainthenameoftheprogram.The
firstprogramargumentspecifiedisretrievedusingargs[0],andthelastprogram
argumentspecifiedisretrievedusingargs[args.length-1],when
args.lengthisgreaterthan0.Aprogramargumentisastring,andseveral
argumentsareseparatedbyspacesonthecommandline.Topassseveralarguments
asoneargument,thesemustbeenclosedindoublequotes.
Q26(e)
Whentheprogramiscalledwithnoprogramarguments,theargsarraywillbeof
length0.Theprogramwillinthiscaseprintnoarguments.Whentheprogramis
calledwiththreearguments,theargsarraywillhavelength3.Usingtheindex3in
thenumbersarraywillretrievethestring"four",becausethestartindexis0.
Q27(b)and(e)
Staticimportfromaclassdoesnotautomaticallyimportstaticmembersofany
nestedtypesdeclaredinthatclass.Theorderoftheimportstatementsisarbitraryas
longastheyaredeclaredafteranypackagestatementandbeforeanytype
declaration.Nameconflictsmustbedisambiguatedexplicitly.
Q28(e)
Anobjectreferenceisneededtoaccessnon-staticmembers.Staticmethodsdonot
havetheimplicitobjectreferencethis,andmustalwayssupplyanexplicitobject
referencewhenreferringtonon-staticmembers.Thestaticmethodmain()legally
referstothenon-staticmethodfunc(),usingthereferencevariableref.Static
membersareaccessiblefrombothstaticandnon-staticmethods,usingtheirsimple
names.NoNullPointerExceptionisthrown,asrefreferstoaninstanceof
MyClass.
Q29(c)
Declaration(4)definesastaticmethodthattriestoaccessavariablenameda,which
isnotlocallydeclared.Sincethemethodisstatic,thisaccesswillbevalidonlyif
variableaisdeclaredasstaticwithintheclass.Therefore,declarations(1)and
(4)cannotoccurinthesameclassdeclaration,whiledeclarations(2)and(4)can.
Q30(a),(f),and(h)
ThetypeoftheswitchexpressionmustbeeitheranenumtypeorStringtype
oroneofthefollowing:byte,char,short,int,orthecorrespondingwrapper
typefortheseprimitivetypes.Thisexcludes(b)and(e).Thetypeofthecaselabels
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mustbeassignabletothetypeoftheswitchexpression.Thisexcludes(c)and(d).
Thecaselabelvaluemustbeaconstantexpression,whichisnottruein(g),where
thecaselabelvalueisoftypeByte.
Q31(a)
ThevalueofthecaselabeliFourisnotaconstantexpressionand,therefore,the
codewillnotcompile.
Q32(d)
Implementation(4)willcorrectlyreturnthelargestvalue.Theifstatementdoesnot
returnanyvalueand,therefore,cannotbeusedasanexpressionstatementin
implementations(1)and(2).Implementation(3)isinvalidsinceneitherthe
switchexpressionnorthecaselabelvaluescanbeoftypeboolean.
Q33(c)
Asitstands,theprogramwillcompilecorrectlyandwillprint3,2atruntime.If
thebreakstatementisreplacedwithacontinuestatement,theloopwillperform
allfouriterationsandwillprint4,3.Ifthebreakstatementisreplacedwitha
returnstatement,thewholemethodwillendwheniequals2,beforeanythingis
printed.Ifthebreakstatementissimplyremoved,leavingtheemptystatement(;),
theloopwillcompleteallfouriterationsandwillprint4,4.
Q34(a)and(c)
Theblockstatement{}isacompoundstatement.Thecompoundstatementcan
containzeroormorearbitrarystatements.Thus,{{}}isalegalcompound
statement,containingonestatementthatisalsoacompoundstatement.Thisinner
compoundstatementhasnostatements.Theblock{continue;}byitselfis
notvalid,sincethecontinuestatementcannotbeusedoutsidethecontextofa
loop.(c)isavalidexampleofbreakingoutofalabeledblock.(d)isnotvalidforthe
samereasonsthat(b)wasnotvalid.Thestatementat(e)isnottrue,sincethe
breakstatementcanalsobeusedtobreakoutoflabeledblocks,asillustratedby
(c).
Q35(d)
Thetypeofnumsisint[][].Theouterloopiteratesovertherows,sothetypeof
theloopvariableintheouterloopmustbeint[],andtheloopexpressionisnums.
Theinnerloopiteratesovereachrow,int[].Theloopvariableintheinnerloop
mustbeint,andtheloopexpressionintheinnerloopisarowgivenbytheloop
variableoftheouterloop.Onlyintheloopheadersin(d)arebothelementtypes
compatible.
Q36(b)
Theprogramwillprint1and4,inthatorder.AnInterruptedExceptionis
handledinthefirstcatchclause.Insidethisclause,anewRuntimeException
isthrown.Thisexceptionwasnotthrowninsidethetryblockandwillnotbe
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handledbythecatchclauses,butwillbesenttothecallerofthemain()method.
Beforethishappens,thefinallyclauseisexecuted.Thecodetoprint5isnever
reached,sincetheRuntimeExceptionremainsuncaughtaftertheexecutionof
thefinallyclause.
Q37(d)
Themethodnullify()doesnotaffectthearrayreferenceinthemain()
method.Thearrayreferencedbyargsisnolongerreachablewhencontrolreaches
(1).OnlythearrayobjectanditsfourObjectelements(i.e.,fiveobjects)are
reachablewhencontrolreaches(1).
Q38(e)
Anobjectcanbeeligibleforgarbagecollectioneveniftherearereferencesdenoting
theobject,aslongastheobjectsowningthesereferencesarealsoeligiblefor
garbagecollection.Thereisnoguaranteethatthegarbagecollectorwilldestroyan
eligibleobjectbeforetheprogramterminates.Theorderinwhichtheobjectsare
destroyedisnotguaranteed.Athreadcannotaccessanobjectonceitbecomes
eligibleforgarbagecollection.
Q39(a),(b),(c),and(e)
Theexpressions('c'+'o'+'o'+'l')and('o'+'l')areoftype
intduetonumericpromotion.Theirevaluationwouldresultinthevalues429and
219,respectively.Expression(d)isillegal,sincetheStringclasshasno
constructortakingasingleintparameter.Expression(a)islegal,sincestring
literalsarereferencesthatdenoteStringobjects.
Q40(d)
Theexpression"abcdef".charAt(3)evaluatestothecharacter'd'.The
charAt()methodtakesanintvalueasanargumentandreturnsacharvalue.
Theexpression("abcdef").charAt(3)islegal;italsoevaluatestothe
character'd'.Theindexofthefirstcharacterinastringis0.
Q41(e)
Theexpression"Hellothere".toLowerCase().equals("hello
there")willevaluatetotrue.Theequals()methodintheStringclasswill
returntrueonlyifthetwostringshavethesamesequenceofcharacters.The
compareTo()methodintheStringclasswillreturn0onlyifthetwostrings
havethesamesequenceofcharacters.Thestringcomparisonbythesetwomethods
iscasesensitive,beingbasedontheUnicodevalueofthecharactersinthestrings.
Q42(c)
Thevariablemiddleisassignedthevalue6.Thevariablentisassignedthestring
"nt".Thesubstring"nt"occursthreetimesinthestring"Contentment!",
startingatindices2,5,and9.Thecalls.lastIndexOf(nt,middle)returns
thestartindexofthelastoccurrenceof"nt",searchingbackwardfromposition6.
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Q43(d)
TheprogramwillconstructanimmutableStringobjectcontaining"eeny"anda
mutableStringBuilderobjectcontaining"miny".Theconcat()method
returnsareferencevaluetoanewimmutableStringobjectcontaining"eeny
meeny",butthereferencevalueisnotstored;consequently,thisStringobject
cannotbereferenced.Theappend()methodappendsthestring"mo"tothe
stringbuilder.
Q44(c)
Thecodewillfailtocompile,sincethepackagedeclarationcannotoccurafteran
importstatement.Thepackageandimportstatements,ifpresent,mustalways
precedeanytypedeclarations.Ifafilecontainsbothimportstatementsanda
packagestatement,thepackagestatementmustoccurbeforetheimport
statements.
Q45(a)and(b)
NotethatArrayIndexOutOfBoundsExceptionand
StringIndexOutOfBoundsExceptionaresubclassesof
IndexOutOfBoundsException.Theelementsofthearrayareinitializedas
follows:
Clickheretoviewcodeimage
trio[0]=null;
trio[1][0]=null;
trio[2][0]=“Tom”;
trio[3]=newString[0];//{},i.e.,zero-lengtharray
trio[4][0]=“Dick”;
trio[4][1]=“Harry”;
Elementtrio[3][0]doesnotexistbecausethearraytrio[3]isofzerolength,
resultinginanArrayIndexOutOfBoundsExceptionbeingthrown;this
exceptionisalsoasubtypeofIndexOutOfBoundsException.
IllegalIndexFoundExceptionisnotdefined.
Q46(e)
Theloopcondition++i==iisalwaystrue,aswearecomparingthevalueofito
itself,andtheloopwillexecuteindefinitely.Theevaluationoftheloopcondition
proceedsasfollows:((++i)==i),withtheoperandshavingthesamevalue.For
eachiteration,theloopvariableiisincrementedtwice:onceintheloopcondition
andasecondtimeintheparameterexpressioni++.However,thevalueofiis
printedbeforeitisincrementedthesecondtime,resultinginoddnumbersfrom1
andupwardbeingprinted.Iftheprefixoperatorisalsousedintheprintln
statement,allevennumbersfrom2andupwardwouldbeprinted.
Q47(d)
Theexpressioni%kevaluatestotheremaindervalue3.Theexpressioni%-k
alsoevaluatestotheremaindervalue3.Weignorethesignoftheoperands,and
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negatetheremainderonlyifthedividend(jinthiscase)isnegative.
Q48(e)
TheclassThingydoesnotoverridetheequals()method,sotheequals()
methodfromtheObjectclassisexecutedeachtime.ThemethodintheObject
classcomparesthereferencevalueforequalitywiththe==operator.Havingthe
samenameinthesecondcalltotheequals()methoddoesnotmaketheThingy
objectsequal.Inthelastcalltotheequals()method,thetworeferencesare
aliases;thatis,theyhavethesamereferencevalue.
Q49(b)
Stringsareimmutable,sothemethodconcat()doesnotchangethestateofthe
s1string.Thedefaultcaseisexecutedintheswitchstatement.Becauseofthe
fall-throughintheswitchstatement,thelastprintstatementisalsoexecuted.
Q50(a),(d),and(e)
In(b),theright-handsideintvalue255requiresacasttoconverttoabyte.In
(c),onlyintegerliteralscanbespecifiedinbinarynotation,notfloating-pointvalues.
(e)willcompile,butwillthrowaNumberFormatExceptionatruntime.In(f),
anunderscorecannotoccuratthestartofanintegervalue.Thecompilerwill
interpretitasanidentifier.In(g),anunderscorecannotoccurbeforeorafterany
typedesignator(L).
Q51(d),(e),and(i)
In(d)and(e),eitherthearraylengthortheinitializerblockcanbespecified,asin
(a),(c),and(f).In(i),thelengthoftheleftmostdimensionmustbespecified;the
otherdimensionsareoptional,asin(g)and(h).
Q52(a)and(c)
Themethodsat(1)and(3)differonlyinthereturntype,whichisnotsufficientfor
correctoverloading.Methodoverloadingrequiresthatthemethodsignaturesare
different,butthemethodnameisthesame.Thereturntypeisirrelevantinthis
regard.Thecodewillnotcompile.
Q53(d)
StringBuilderismutable.Thereferencevalueingiz.nameiscopiedtothe
formalparametersbwhenthemethodiscalled.Referencesgiz.nameandsbare
aliasestothesamestringbuilder.Changesmadetothestringbuilderinthemethod
areapparentwhenthecallreturns.Incontrast,thedoublevalueingiz.weight
iscopiedtotheformalparameterweight,whosevalueischangedinthemethod,
butthisdoesnotaffectthevalueintheactualparameter,whichremainsunchanged.
Q54(a),(b),(e),and(f)
StringBuilderismutable.Themethodsinsert()andappend()ofthe
StringBuilderreturnthereferencevalueofthestringbuilder,inadditionto
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modifyingit.Theassignmentin(b)and(f)issuperfluous.In(c)and(d),onlythe
localvariablemealisassignedareferencevalueofastringbuilder,butitdoesit
notchangethestringbuilderinthearraymeals.
Q55(d)
Afterthefirstcalltotheoverloadedadd()method,thesizeofthearraylistis1.
Tryingtoinsertanelementatindex2inthesecondcalltotheadd()method
resultsinajava.lang.IndexOutOfBoundsException,becauseindex2is
strictlygreaterthanthecurrentsizeofthearraylist.
Q56(g)
ThefieldnumOfGuestsisstatic,meaningthefieldbelongstotheclassRoom
andnottoanyobjectoftheclass.Suchafieldcanbereferencedbyareference
whosetypeisthesameastheclass.Thetworeferencesr1andr2refertothesame
staticfieldnumOfGuests,whichhasthevalue3.Becauseofstringconcatenation,
theexpression"Numberofguests:"+r1.numOfGuests+
r2.numOfGuestsevaluatesto"Numberofguests:33".
Q57(b),(c),and(d)
In(a),thetheatercannotadmitanyoneunlessthereisaticket-holder,sothetestto
seewhetherthereisaticket-holdercomesfirst.
In(b),atleastoneguesshastobemade,sothetestcanbedoneaftermakingthe
guess.
In(c),somesalthastobeadded,asthefoodhastoolittlesaltinitially.Thetestto
seeifthefoodtastesrightcanbedoneaftersomesalthasbeenadded.
In(d),atleastonecandlehastobeadded,sothetestfortherightnumberofcandles
canbedoneafteraddingacandle.
In(e),itagoodideatocheckfirstwhetherthecatisawaybeforelettingthemice
playalittle.
Q58(b)and(f)
Inbothcases,thecodeintheifstatementandthewhileloopisunreachable,soit
canneverbeexecuted.Incaseofthewhileloop,thecompilerflagsanerror.The
ifstatementistreatedasaspecialcasebythecompilertosimulateconditional
compilation,allowingcodethatshouldnotbeexecuted.
Q59(e)
Thevalue2014isboxedintoanInteger.ThesubclassIntegeroverridesthe
abstractmethodintValue()fromthesuperclassNumber,sothatnocastor
explicitparenthesesarenecessary.However,ifthiswasnotthecase,onlythesyntax
withthecastin(a)wouldbecorrect.
Q60(b)
Thethingtonoteisthatthemethodcompare()isoverloadedinthesubclass
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Student,andnotoverridden.ThusobjectsoftheclassStudenthavetwo
methodswiththesamenamecompare.Foroverloadedmethods,themethodtobe
executedisdeterminedatcompiletime,dependingonthetypeofthereferenceused
toinvokethemethod,andthetypeoftheactualparameters.Whenthetypeofthe
referenceisPerson(asisthecaseforp1andp2),themethodcompare()in
Personwillalwaysbeexecuted.ThemethoddefinedinthesubclassStudentis
executedonlybythelastcalls1.compare(s2)inthemain()method.
Q61(b)and(e)
Theadd(element)methodaddsanelementattheendofthelist.The
add(index,element)methodaddstheelementatthespecifiedindexinthe
list,shiftingelementstotherightfromthespecifiedindex.Theindexsatisfies
(index>=0&&index<=size()).Theset(index,element)
methodreplacestheelementatthespecifiedindexinthelistwiththespecified
element.Theindexsatisfies(index>=0&&index<size()).The
for(;;)loopaddstheelementscurrentlyinthelistattheendofthelist.Thelist
changesasfollows:
Clickheretoviewcodeimage
[Ada]
[Ada,Alyla]
[Ada,Otto]
[Ada,Anna,Otto]
[Ada,Anna,Otto,Otto,Anna,Ada]
Q62(e)
Elementswiththenullvaluecounttowardthesizeofthelist.Thelistshave
differentsizes.Thelistsaretwodistinctlists,havinguniquereferencevalues.The
equals()testfailsbecausethelistshavedifferentsizes.
Q63(a),(c),(e),(f),(g),and(k)
Theprogramprintsthefollowing,wherethelistcontentsareshownbeforeandafter
eachprintstatement.NotethereturnvaluefromtheArrayListmethods.
[3,4,1,null,0]
(1)printsnull
[3,4,1,null,0]
(2)prints4
[3,3,1,null,0]
(3)prints1
[3,3,1,null,0]
(4)printstrue
[3,3,1,null,0]
(5)printsnull
[3,3,1,0]
(6)prints0
[3,3,1,0]
(7)printsfalse
[3,3,1,0]
Q64(b),(f),and(g)
(1):Theinitialcapacitycanbe0.Thecapacitycanchangeasthelistchanges
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structurally.
(2):List<String>isnotasubtypeofList<Object>.Assignmentisnot
allowed.
(3),(4),(5):AlthoughtheNumberclassisabstract,wecancreateanArrayList
ofanabstractclass.However,onlyreferencevaluesofobjectsofitsconcrete
subtypescanbestoredinsuchalist.
(6):Thediamondoperatorcanbeusedonlywiththenewoperator.
(7):ArrayList<Integer>isnotasubtypeof
ArrayList<ArrayList<Integer>>.Assignmentisnotallowed.The
ArrayListcreationexpressionmustdeclarethefullelementtypeorusethe
diamondoperator.
(8):Thedeclarationstatementcompiles,butanuncheckedconversionwarningis
issuedbythecompiler.Allbetsareoffregardingthetype-safetyofthe
ArrayList.
Q65(d)and(e)
Textualrepresentationofthecurrentcontentsofthelistisaddedasastringoneach
iterationoftheloop.Theloopbodyisexecuted3times.Thedefaulttextual
representationofalistisenclosedinbrackets([]),wheretextualrepresentationof
eachelementisseparatedbyacomma(,).
Q66(d)
Theadd(index,element)methodacceptsanindexthatsatisfiesthe
condition(index>=0&&index<=size()).Thefor(;;)loopswaps
elementstoreversetheelementsinthelist.
Q67(c)and(d)
(a):Thereturnstatementismandatoryinalambdaexpressiononlyifthelambda
bodyisastatementblockthathasanon-voidreturn.
(b):Areturnstatementisillegalinalambdaexpressionifthelambdabodyisa
singleexpression.
(e):Alocalvariabledeclarationintheblockscopeofalambdaexpressioncannot
shadoworredeclarealocalvariablewiththesamenameintheenclosingmethod.
Q68(a),(c),(g),(h),and(i)
Thedeclarationsat(1),(3),(7),(8),and(9)willnotcompile.In(1),thedeclared-
typeparametermustbeinparentheses.In(3),thefinalmodifiercanbeapplied
onlytodeclared-typeparameters.In(7),thereturnkeywordcannotbeusedwhen
thelambdabodyisasingleexpression.In(8)and(9),thereturnkeywordis
requiredforanon-voidreturnfromalambdaexpressionwithastatementblock.In
(8),thestatementterminator(;)isalsomissing.
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Q69(a),(d),and(e)
Thedeclarationsat(1),(4),and(5)willcompile.In(1),theparameterlockislocal
intheblockscopeofthelambdaexpression.In(2),thelambdaexpressionmust
explicitlyreturnavalue,regardlessofwhethertheifstatementisexecuted.In(3),
thelocalfinalvariablelock3intheenclosingscopecannotberedeclared.In(4)
and(5),localvariableslock3andlock4intheenclosingscopecanbeaccessed
inthelambdaexpression,astheyarebothfinalandeffectivelyfinal,respectively.
In(6),thelocalvariablelock4intheenclosingscopecannotberedeclaredas
parameter.
Q70(a),(c),(g),and(h)
In(b),thereturnkeywordcannotbeusedinalambdaexpressionwithasingle
expressionbody.In(d),thereturnkeywordmustbeusedinalambdaexpression
withanon-voidstatementblockbody.In(e)and(f),theclassStringdoesnot
haveaconstructorthattakesanintegervalue.
Q71(b)
Afunctionalinterfaceisaninterfacethathasonlyoneabstractmethod,asidefrom
theabstractmethoddeclarationsofpublicmethodsfromtheObjectclass.This
singleabstractmethoddeclarationcanbetheresultofinheritingmultiple
declarationsoftheabstractmethodfromsuperinterfaces.
AllexceptIAarefunctionalinterfaces.IAdoesnotdefineanabstractmethod,asit
providesonlyanabstractmethoddeclarationoftheconcretepublicmethod
equals()fromtheObjectclass.IBdefinesasingleabstractmethod,doIt().
ICoverridestheabstractmethodfromIB,soeffectivelyithasonlyoneabstract
method.ICinheritstheabstractmethoddoIt()fromIBandoverridesthe
equals()methodfromIA,soeffectivelyitalsohasonlyoneabstractmethod.
Q72(e)
(a):TheisBefore()andisAfter()methodsarestrictintheircomparison.
(b):ThewithYear()andwithMonth()methodswilladjusttheday.The
withDayOfMonth()andwithDayOfYear()methodswillthrowa
DateTimeExceptioniftheargumentvaluewillresultinaninvaliddate.
(c):ThePeriodclassdoesnothavethemethodtoTotalDays(),butitdoes
havethemethodtoTotalMonths()thatconsidersonlytheyearsandthe
months.ThePeriodclasshasnonotionoftimeofdayordateintheyear.
(d):ThePeriodclassdoesnotimplementtheComparableinterface.
Q73(d)
Thecalculationoftime.withHour(10).plusMinutes(120)proceedsas
follows:
Clickheretoviewcodeimage
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12:00with10hour==>10:00+120min(i.e.,2hours)==>12:00
Q74(f)
Thedatevalue2015-01-01inthedatereferenceneverchanges.The
withYear()methodreturnsanewLocalDateobject(withthedatevalue
0005-01-01)thatisignored.TheplusMonths()methodalsoreturnsanew
LocalDateobjectwhosevalueisprinted.Thecalculationof
date.plusMonths(12)proceedsasfollows:
Clickheretoviewcodeimage
2015-01-01+12months(i.e.,1year)==>2016-01-01
Q75(c)
ThestaticmethodcallPeriod.ofYears(10)returnsaPeriodwiththevalue
P10Y.ThisPeriodobjectisusedtoinvokethestaticmethodofMonths()with
theargumentvalueof16months,resultinginanewPeriodobjectwiththeperiod
valueP16M.Itsreferencevalueisassignedtotheperiodreference.Theof()
methodsdonotnormalizethedate-basedvaluesofaPeriod.
Q76(d)
(a):TheformatterwillformataLocalTimeobjectandthetimepartofa
LocalDateTimeobject,butnotaLocalDateobject,asitknowsnothingabout
formattingthedatepart.ItwillusetheISOstandard.
(b):TheformatterwillformataLocalDateobjectandthedatepartofa
LocalDateTimeobject,butnotaLocalTimeobject,asitknowsnothingabout
formattingthetimepart.ItwillusetheISOstandard.
(c):TheformatterwillformataLocalDateTimeobject,butneithera
LocalDateobjectnoraLocalTimeobject,asitrequiresboththedateandthe
timeparts.ItwillusetheISOstandard.
Q77(a),(d),and(f)
Theinputstringmatchesthepattern.Theinputstringspecifiesthetime-basedvalues
thatcanbeusedtoconstructaLocalTimeobjectin(a)byaformatter,basedon
thetime-relatedpatternlettersinthepattern.Nodate-basedvaluescanbeinterpreted
fromtheinputstring,asthispatternhasonlytime-relatedpatternletters.(b)and(c),
whichrequireadatepart,willthrowaDateTimeParseException.
Tousethepatternforformatting,thetemporalobjectmustprovidevaluesforthe
partscorrespondingtothepatternlettersinthepattern.TheLocalTimeobjectin
(d)hasthetimepartrequiredbythepattern.TheLocalDateobjectin(e)doesnot
havethetimepartrequiredbythepattern,soan
UnsupportedTemporalTypeExceptionwillbethrown.The
LocalDateTimeobjectin(f)hasthetimepartrequiredbythepattern.In(f),only
thetimepartoftheLocalDateTimeobjectisformatted.
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Index
Symbols
-169
--176
^184,189
^=185
_32
;50
:110
!184
!=181,342
?:194
.7,72,97,108
...81,85
‘32,33
"34
[]59,61,195
{}50,60,117
@FunctionalInterface442,443
@Override270
@param49,56
@return225
@throws253
*100,163,167
*=172
/167
/*and*/35
/**and*/36
//35
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/=172
\33
&184,189
&&186
&=185
%167,168
%=172
+169,174
+concatenation364
++176
+=172
<180
<=180
<>414,415,416
-=172
=158
==181,342,351,359
>195,439,444
>180
>=180
|184,189
|=185
||186
~189
A
abilityinterfaces
seemarkerinterfaces
abruptmethodcompletion232
absoluteadjusters470
abstract
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classes120
interfaces290
methods136,291,442
abstract120,136,290,291,442
abstractmethoddeclarations442
ininterfaces290,291
abstraction2,10
accessibility7,17,114
default118,127
members114,120,123
modifiers118
package118
private128
protected126
public124
UMLnotation124
accessibilitymodifiers48,53
activationframe384
seemethodexecution230
actualparameter72
actualparameterlist72,315
addingtoclass264
additiveoperators169
aggregation10,12,267
hierarchy267
versusinheritance331
aliases6,75,182,183
seealsoreferences
ambiguouscall316
analyzingprogramcode512
andoperator189
annotations
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@Override270
anonymousarrays63,66
[]63
anonymousclasses436
anonymousfunctions439
API(applicationprogramminginterface)22
apostrophe33
application16
architectureneutral23
argument
seeactualparameter
argumentstomainmethod85
arithmeticcompoundassignmentoperators172
ArithmeticException236
arity81
arraycreationexpression59,63
arrayinitializer60,63,66
arraystorecheck311
arraytypes
seearrays
ArrayIndexOutOfBoundsException61,236
ArrayList366,414
addcollection419
addelement417,419
autoboxing421
capacity416
clearlist420
comparisonwitharrays425
constructing415
constructors418
convertingtoarray424
elementsearch423
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elementtype415
filtering434
import415
inheritancehierarchy415
initialcapacity417
insertionorder414
listoflists417
membershiptest422
modifying419
nestedlists417
objectvalueequality422
openrange-viewoperations414
ordered414
position-basedaccess415
positionalindex422
positionalorder414
positionalretrieve422
querying422
references415
removecollection420
removeelement419
replaceelement419
size416,422
sorting425
subtyperelationship418
textualrepresentation417
traversing423
trimtosize420
type-safety416,417,418
uncheckedconversionwarning416
zero-basedindex414
arrays58,342
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[]59,61
{}60
anonymous63,66
arraycreationexpression59
arrayinitializelist60,63
arrayinitializer60,66
arrayname59
arraysize60
arraystorecheck311
ArrayIndexOutOfBoundsException61
bounds61
construction59
declarations59
defaultinitialization59,60
dynamic415
elementaccessexpression61
elementdefaultvalue310
elementtype59
elements58,61
index58
indexexpression61
initialization60,65
iteratingover217
length58
multidimensional63,65
objects342
ragged65
reference59,62,311
searching69
sorting68
subtypecovariance309
traverseanarray62
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using61
arraysofarrays59,65
multidimensional65
ArrayStoreException311,418,424
arrow>195,439,444
ASCII32,38
AssertionError237
assignable147,314
assignmentcompatible148,314,416
assignmentconversions147
assignmentoperator5
assignments
arithmeticcompoundoperators172
bitwise192
cascading159
compoundoperators185,192
expressionstatement159
implicitnarrowing160
multiple159
numericconversions160
operator151,158
primitivevalues159
references159
wideningreferenceconversions267
association12
aggregation267
composition267
realization296
associativity152
atomicvalues13
attributesseeproperties
autoboxing68,348
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for(:)statement218
AutoCloseable387
automaticgarbagecollection6,384
automaticvariablesseelocalvariables
B
backslash33
backspace33
base30,349,352
baseclass264
basicforstatement215
BeforeCurrentEra(BCE)464
behavior433
behaviorparameterization434,441
binary
numericpromotion150
operators151
binarysearch
arrays69
bitmask190
bitpatterns154
bitwise
andoperator189
assignment192
complement189
compoundassignment192
operators189
oroperator189
xor189
bitwiseAND
&189
bitwisecomplement
~189
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bitwiseexclusiveOR
^189
bitwiseOR
|189
bitwiseXOR
^189
blankfinalvariable80,134
blocks49,50,117
scope117,448
try240
boilerplatecode436
Boolean
condition200
Booleanwrapperclass355
booleans37,39
casting149
expressions180
literals32
boxingconversions145,146
breakstatement205,206,221
BSseebackspace
buildingabstractions10
byte30,38
bytecode16,23
C
C137
C++23,137
cache139
callbyreference77
callbyvalue77
callchaining72
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callsignature316
callstack
seeJVMstack230
callee72
caller72,224
capacity416
carriagereturn33,35
cascadingassignments159
cascadingif-elsestatements203
caselabels203,205
casesensitivity28
castoperator145,148,151,162,172,182,320
casting147,148,149
seealsoconversions
catchclause240
uni-239
catchingexceptions230
catch-or-declare251
CertView509
chaining406
constructors287,406
finalizers391
char38
charactercase364
charactersequences
seestringsandstringbuilders
characterset
ASCII32,38
ISOLatin-132,38
Unicode32,38
Characterwrapperclass354
characters38
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literals32
searchingfor367
CharSequenceinterface360,365,369
checkedexceptions237
childclass264
choosingbetweenStringandStringBuilderclass374
Classclass343
classfile16
classhierarchy
seeinheritancehierarchy
classinheritance
seeimplementationinheritance
classmethod10
classmodifiers48
classpath107
absolutepathnames110
entriesorder110
entry-separatorcharacter110
fullyqualifiedpackagename109
path-separatorcharacter110
relativepathnames110
searchinanamedpackage109
searchingforclasses107
whitespace110
classsearchpath
seeclasspath
classvariable10
ClassCastException236,321
classes
abstract120
accessibility118
addingto264
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base264
body48
child264
cohesion335
concrete121,122,436
constructors53,282
coupling336
declarations48,96
definitions2,5
derived264
diagram8,9
encapsulation335
extending122,264
final122
finalvs.abstract122
fullyqualifiedname107
fullyqualifiedpackagename98
generalized266
grouping97
header48
implementinginterfaces291
initialization409
instancemembers48
instances4
members7
methods132
modifiers120
name97
normal121
Object342
parent264
runtime343
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scope114
searchingfor107
specialized266
staticmembers48
subclass10,264
superclass10,264
variables132
wrappers342,346
ClassLoaderclass342
ClassNotFoundException235
CLASSPATHenvironmentvariable
seeclasspath
-classpathoption
seeclasspath
clauses
catch240
extends264
finally240,245
implements291
throws251
cleaningup386
clean-upcode245
client7,16
Cloneableinterface343
CloneNotSupportedException343
cloningobjects343
codeoptimizations134
codereuse23,264,334
CodeRanch508
cohesion335
coincidental335
functional335
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high335
coincidentalcohesion335
Collection414
collections414
assingleentity414
elements414
ordered414
sorting414
command17
java17
javac17
commandline17,86
commandprompt17
comments35
communication7,72
Comparableinterface350,363,376,425
comparingobjects342
comparingstrings363
comparison180
compilationunit98
compilingJavasourcecode17
complement
~189
completesabruptly
seeexceptionpropagation232
compositeobject10
composition12,267
compoundstatement50
concatenationofstrings364
concatenationoperator174
concreteclasses436
concretemethod134
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ConcurrentModificationException424
condition
Boolean200
expressions200
conditional180
and186
operators186,194
or186
statements200
conditionalexpressions194
associativity195
nested195
precedence194
short-circuitevaluation194
sideeffects194
conditions180
connectingpunctuationcharacter28
const29
constantdeclarations290
constantexpression147,160,161,176
constantfieldvalues
caselabels208
constantstringexpressions208
constantvalues30,133
constantvariable161
constants302
constituentobjects10
constituents12
constructingarray59
constructorchaining283,287,406
constructors3,53,282
accessibility124
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accessibilitymodifier53
body53
chaining283,287
classname53
declaration48
default54
header53
implicitdefault54
localdeclarations53
no-argument53,54,283,287
non-zeroargument55,287,288
overloading56
superclassconstructor54
constructs28
high-level28
loopsseeiterationstatements
container
seecollections
containscharacters368
continuestatement223
contract2,291,293,334,335
controlflow
break205,221
continue223
do-while214
for(;;)215
for(:)217
if200
if-else201
iterationseeiterationstatements
loopsseeiterationstatements
return224
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statements50,200
switch203
throw249
transferstatements219
while213
controltransfer219
conversioncategories147
conversioncontexts147
conversions144,311
assignment147
contexts147
identity172
implicitnarrowing173
methodinvocation148
narrowingreference320
numbersystems157
numericpromotions149
parameters73
referencecasting320
stringconcatenation175
tostrings369
truncation161
type-safe315
unsafecasts321
wideningreference267,320
convertingnumbersystems157
convertingvalues348,349,350,352,353,355,369
counter-controlledloops215
coupling336
loose336
covariantreturn269,273
-cpoption
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seeclasspath
CRseecarriagereturn
crab217
creating
objects195
criteriaobject436
currencysymbol28
currentdirectory
.108
CurrentEra(CE)464
currentobject50
D
-doption106
danglingreferences384
datastructures414
datatypesseetypes
date
seetemporalobjects
dateunits474
date/timeformatters
customized486,495
formatstyles490
formatting487
immutability487
ISO-baseddefault486,487
ISO-basedpredefined486,488
letterpattern495
localized486,490
parsing487
patternletters495,496
thread-safety487
date-basedvalues462
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date-time
seetemporalobjects
DateTimeException463
DateTimeFormatterclass
seedate/timeformatters
DateTimeParseException477,491
DayOfWeekenumtype468
declarationstatement4,41,171,177,187,216
declarations
arrays59,196
classes48,96
interfaces96
local50
mainmethod85
methods49
multidimensionalarrays63
packages96,98
statements50
variablearitymethod81
declaredtype268,274,275,315
declared-typeparameters445
declaringseedeclarations
decoupling330
decrementoperator176
deepcopying343
default
accessibility118,124,127
constructor54
exceptionhandler232
method297
values42,400,406
default297
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label204
method297
defaultconstructor54
defaultmethod297,442,443
multipleinheritance298
overriding298
defaultpackage98
deferredexecution451
definitions
inheritance296
interfaces290
delegatingrequests334
derivedclass264
destinationdirectory106
destinationstream18
destroyingobjects390
diagrams
class3
object5
seealsoUML
diamondoperator(<>)416
dictionaryorder363
distributed23
divide-and-conqueralgorithm69
dividend168
division
floating-point167
integer167
divisionoperator
/167
divisor168
documentation35
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documentationcomment35,36
tags36
documentingseedocumentation
dot97
double31,39
doublequote33
do-whilestatement214
downcasting145
duplicatingobjects343
Durationclass476
time-based476
dynamic23
dynamicarrays415
dynamicbinding
seedynamicmethodlookup
dynamicmethodlookup277,329,330
dynamictype268,274,275
E
effectivelyfinal448
elementtype59,415
elements58,414
eligibleforgarbagecollection385
ellipsis81
elseclausematching203
embeddedapplications22
emptystatement50
emptystring358
encapsulation22,97,335
encapsulationofimplementation334
endswithcharacters368
enhancedforloop213
enterpriseapplications22
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enumconstant87
symbolicnames87
values87
enumtypes87,103,209,303
declaring87
finalization391
namedconstants87
naturalorder90
ordinalvalue90,209
switchexpression204
using88
enumeratedtypes
seeenumtypes
EOFException235
equality181,342
equalsmethod183,342
objectvalue183
objects183
primitivevalues181
referencevalues182
equalsmethod183,342
Error237
escapesequences33
evaluationorder152,187
arithmeticexpressions164
evaluationshort-circuits187
exam507
multiple-choice513
program510
questions511
registration508
result511
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testinglocations510
voucher509
examobjectives
OCAJP8515
Exceptionclass236
exceptionhandler230
seealsoexceptions
exceptionhandling
advantages254
exceptions230,239
customized238
defaulthandler232
handler230
ignored390
propagation230
situations235
throw249
throwingseethrowingexceptions
thrownbyJVM235
thrownbymethod49
thrownprogrammatically235
throws251
types233
uncaught232
unchecked237
exchanginginformation72
explicit
garbagecollection393
explicittraversal452
exponent31
expressionstatements50,159,177,216,217,446
expressions205
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actualparameters72
boolean180
caselabels205
conditional194
deterministicevaluation150
label205
return224
statements50
throw249
extending
classes264
interfaces294
extendsclause
seeextending
extensions
.class16
.java16
externallibraries403
extractingsubstrings369
F
fall-through204,205
falseliteral32
FFseeformfeed
fielddeclarations48
fieldhiding275
fieldinitialization406
fields2
filename96
filepath105
separatorcharacter105
filtering434
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final
classes122
members133
parameters80
finalization385
finalizationmechanism385
finalizemethod343,390
finalizerchaining391
finalizerseefinalizemethod
finallyclause240,245
fixedaritymethod81
fixedaritymethodcall84
float31,39
floating-point37
double39
float39
literals31
floating-pointarithmetic165
strictfp166
floating-pointdatatypes31
floating-pointdivision167
floating-pointremainder169
flowcontrolseecontrolflow
for(;;)statement215
backward216
forward215
traversearray62
for(:)statement217
traversearray62
for-eachloop213
formfeed33,35
formalparameters49,53,72,117,315
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modifier49
name49
type49
formaltypeparameter290
formatspecifications18,370
formatspecifier19
formatstyles486,490
FormatStyleenumtype486,490
formattedoutput18
formatspecifier19
formattedstring370
formatting35,462,486
forwardreference400,401,403,405,406
fractionalsignednumbers37
fullyqualifiedclassname107
fullyqualifiedpackagename97,98,100
fullyqualifiedtypename97,101
function451
functiontype450
functionalcohesion335
functionalinterface438
@FunctionalInterface442,443
abstractmethod442
functiontype450
functionalmethod442
general-purpose443
generic441
Predicate<T>440,451
primitivevalues444
targettype450
seealsolambdaexpressions
functionalmethod442
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functionalprogramming24
functionality433
functional-styleprogramming433
G
garbagecollection387,389,390,393
automatic384
facilitate387
generalabstractions266
generalloops215
generalization10
generalizedclasses266
genericmethod423
generictype414
goto29,220
grammarrules28
grouping97
H
handlesseereferences
has-arelationship267
hashcode52,343
hashtables52
heap384
hidinginternals335
highcohesion335
high-performance24
horizontaltab33
hotspots24
HTseehorizontaltab
I
IDE(integrateddevelopmentenvironment)508
identifiers28
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predefined29
reserved29
variable40
identityconversion146,172
identityofobject5
IEEE754-198538
ifblock200
if-elsestatement201
ignoredexceptions390
IllegalArgumentException236,495
immediatesuperclass285
immutable462
immutableobjects346,357
immutablestrings357
implementationinheritance264
implementations2,266,335
inheritancehierarchy122
implementing
interfaces291
implementsclause291
implicit
inheritance264
narrowingconversions173
implicitdefaultconstructor54
import
declaration100
seealsostaticimport
single-type-importdeclaration100
statement96
type-import-on-demanddeclaration100
importing
enumconstants103
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referencetypes99
staticmembers101
incrementoperator176
index58
indexexpression61
IndexOutOfBoundsException361,369,375,376,419,422
individualarrayelements61
inequality181
seealsoequality
inferred-typeparameters445
infiniteloop217
infinity165,349
negative165
positive165
informationhiding335
inheritance10,267
hierarchy266
supertype–subtyperelationship267
initialcapacity417
initialstateofobject406
initialization
arrays60,65
code60
defaultvalues42
forstatement215
objects5
references41
variables41
initializer399
declaration-before-readingrule401
static400,401,405,409
initializerblock
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instance404
static402
initializerexpression400
initializers
non-staticblock48
non-staticfield48
staticblock48
staticfield48
initializingseeinitialization
insertionorder414
insertionpoint69
instance
members9,48
methods9,49,50
variableinitialization42
variables9,44
seealsoobject
instanceinitializerblock404
instancemethods6
instancevariables6,406
instanceofoperator195,320,321
instantiation4
int30,38
integerarithmetic165
integerbitwiseoperators189
integerconstantexpressions148
integerdatatypes30
integerdivision167
integerremainderoperation168
integers38
andoperator189
byte38
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complement189
datatypes38
int38
literals30
long38
oroperator189
representation154
short38
types38
xor189
integraltypes37,38,144
interfaceconstantantipattern102
interfaces290
abstract290
abstractmethods291
accessibility118
body290
constants302
declarations96
defaultmethods297
extending294
header290
implementing291
initialization409
marker291
realization296
references296
staticmethods300
subinterfaces294
superinterfaces294
UML295
variables302
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internaltraversal452
internedstrings358,359
internedvalues351
interpackageaccessibility335
interpreter17
intraclassdependencies336
invocationstack
seeJVMstack
invoker224
invokinggarbagecollection393
IOException235
is-arelationship266,267,334
ISOLatin-132,38
ISOstandard486,487,488
Iterableinterface366,424
iteration215
iterationstatements213
nextiteration223
termination213,222
iterators414,424
J
Java
NativeInterfaceseeJNI
java17
Javabytecode16
JavaCollectionsFramework414
Javacompiler17
JavaDevelopmentKit(JDK)21
Javaecosystem21
JavaEE(EnterpriseEdition)22
JavaME(MicroEdition)22
JavaNativeInterfaceseeJNI
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JavaPlatforms22
JavaRuntimeEnvironment(JRE)22
JavaSE(StandardEdition)22
JavaVirtualMachineseeJVM
java.timepackage462
java.time.formatpackage462
java.utilpackage414
java.util.function<T>package444
javac17
Javadoccomment35
@paramtag49,56
@returntag225
@throwstag253
javadocutility36
JDK17,508
JNI137
just-in-time(JIT)23
JVM17,22,384,393
JVMstack230,384
K
key69
keywords29
abstract120,136,291,442
boolean39
breakstatement221
byte38
case203
catch240
char38
class48,290
const29
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continue223
default204,297
do214
double39
else201
extends264
final80,122,133
finally245
float39
for215,217
if200
implements291
import100
instanceof195,320,321
int38
interface290
long38
native137
newseenewoperator
null149,183,320
package98
private128
protected126
public124
reservedwords29
return224
short38
static17,101,132,300,402
strictfp166
super54,272,276,285,299
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switch203
synchronized136
this50
throw249
throws251
transient138
try240
unusedwords29
void17,347
volatile139
while213,214
L
labeledbreakstatement222
labels220,222
break222
case203
default204
expressions205
labeledstatement220
switchstatement203
lambdabody439,444,445
lambdaexpressions433,438,444
accessclassmembers446
anonymousfunctions439
arrow>439,444
asvalues439
blocks
scope448
declared-typeparameters445
deferredexecution451
expression445
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expressionstatements446
function451
inferred-typeparameters445
lambdabody439,444,445
lambdaparameters445
non-voidreturn445
parameterlist439,444
singleexpression439,445
statementblock439,446
targettype450
targettyping451
typechecking450
variablecapture449
voidreturn445
lambdaparameters445
latebinding
seedynamicmethodlookup
leastsignificantbit155
leftassociativity152
legalassignments314
lengthmethod361
letterpattern495
lexicalscope
seeblocks:scope
lexicaltokens28
lexicographicalordering363,425
LFseelinefeed
libraries403
lifetime385
seescope44
lineseparator19
lineterminator35
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linearimplementationinheritance266
linefeed33
LinkageError237
LinkedList417
List414
lists
seeArrayList
literals30
boolean32
character32
defaulttype30,31
double31
escapesequences33
false32
float31
floating-point31
integer30
null30
predefined29
prefix30
quoting32
scientificnotation31
string34
suffix30,31
true32
litmustest
designbyinheritance266
local43
chainingofconstructors283,406
variables44,117
localdeclarations49,50
localvariables53
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blocks
scope448
LocalDateclass
seetemporalobjects
LocalDateTimeclass
seetemporalobjects
locale364,490,492
localizinginformation335
LocalTimeclass
seetemporalobjects
locations
seeclasspath
logicalAND
&184
logicalcomplement
!184
logicalexclusiveOR
^184
logicalinclusiveOR
|184
logicalXOR
^184
long30,38
suffix30
loopbody213,215
loopcondition213,215
loopsseeiterationstatements
loosecoupling336
lossofprecision144
M
magnitude144
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mainmethod17,18,85
arguments86
modifiers85
manifestconstant134
markerinterfaces291
Mathclass52
MAX_VALUEconstant351
memberdeclarations48,290
members3,114
access50
accessibility120,123
defaultvalues42
final133
inheritance264
instance48
modified264
modifiers131
ofobjects7
scope114
short-hand51
static7,48,132
terminology9
variablesseefields
memorymanagement384
memoryorganization384
message
receiver7
methodcall7,49,72
chaining376,378
fixedarity84
variablearity84
methodchaining471,474,479
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methoddeclaration48
methodheader136,137
methodinvocationconversions148,315
methodmodifiers49
methodoverloading52,273
methodoverriding268,273,407
methodsignature49,269
methodtype450
methods3
@Override270
abstract136,291,442
abstractmethoddeclarations291
accessibility49
ambiguouscall316
automaticvariablesseelocalvariables
behavior433
blocks49
body49,117
callchaining72
callseemethodcall
callingvariablearitymethod82
chained365
clone343
concrete134
declaration49,72
default297
dynamiclookup330
equals183,342
exceptions49
final134
finalize343,390
fixedarity81
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functional442
getClass343
header49
implementation136
invocationseemethodcall
localdeclarations49
localvariables
mainseemainmethod
methodinvocationconversions315
methodtype450
modifiers49
mostspecific316,422
name72
native137,251
objects50
overloadedresolution316
overloadingseemethodoverloading
overridingseemethodoverriding
overridingvs.overloading273
parameters49
recursive237
return224
returnvalue49
signature49,52,273
static132,300
synchronized136
termination224
throwsclause251
toString343
variablearity81
MIN_VALUEconstant351
minimizingoverhead386
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mobileapplications22
modifiers
abstract120,136,291
accessibility118,123
classes120
default297
final133
members131
native137
static132,300
strictfp166
synchronized136
transient138
volatile139
Monthenumtype465
mostspecificmethod316,422
multicore441
multicorearchitectures24
multidimensionalarrays63,65
multipleassignments159
multiplecatchclauses239
multipleimplementationinheritance290
multipleinheritance298
multipleinterfaceinheritance290
multiple-linecomment35
multiplicationoperator
*167
multiplicativeoperators167
multithreaded24
mutablecharactersequences374
mutuallycomparable68,69
mutuallyexclusive
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actions202
MVC335
N
name28
namedconstants134
namespaces53
NaN166,349
narrowerrange144
narrowingconversions
primitive144
reference145
narrowingreferenceconversions320
nativelibraries403
nativemethods137,251
header137
naturalordering68,69,425
negativezero165
nestedlists417
nestedloops66
newoperator5,53,59,195,406
newlineseelinefeed
NLseenewline
no-argumentconstructor53,54,283,287
non-associativity151
non-staticcode48
seenon-staticcontext48
non-staticcontext48
non-staticfield9
non-staticfieldinitializers48
non-staticinitializerblock48
non-varargscall
seefixedaritycall
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non-voidreturn445
non-zeroargumentconstructor55,287,288
normalclass121
normalexecution232
notifyingthreads344
nullreference30
casting320
nullreferenceliteral
casting149
equalitycomparison183
nullingreferences387
NullPointerException236
Numberclass351
numbersystems
base30
converting157
decimal30
hexadecimal30
octal30
radix30
NumberFormatException236,347,348
numericpromotions149
assignment160
binary150
unary149
numericwrapperclasses351
numericalliterals
usingunderscore32
O
object4
Objectclass266,342
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objecthierarchy267
objectreferences4,40
objectstate6,53,77,406
object-orienteddesign334
cohesion335
object-orientedparadigm22
object-orientedprogramming2
objects13
aggregate12
alive385
arrays58
callee72
caller72
Classclass343
cleaningup386
cloning343
communication72
comparing342
composite385
constituent12,385
constructing406
contract335
decoupling330
destroying390
eligible387
equality183,342
exchanginginformation72
finalization385
garbagecollection384
identity5
immutable346
implementation335
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initialstate406
initialization5,53
initializerblock404
internals335
lifetime385
members7
methods50
Objectclass342
persistence138
reachable384,385
resurrection385
services335
state133
seeobjectstate
valueequality183
OCAJP8507
examobjectives515
examquestionassumptions511
OCPJP8507
one-dimensionalarrays59
operands148
evaluationorder152
operations2
operators150
-163,169
--176
^184,189
^=185,192
!184
!=181,182
?:194
.7,97
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[]61,195
*163,167
*=172
/163,167
/=172
&184,189
&&186
&=185,192
%163,167,168
%=172
+163,169,174
++176
+=172
<180
<=180
-=172
=158
==181,182
>195
>180
>=180
|184,189
|=185,192
||186
~189
arithmeticcompoundassignment172
assignment151,158
associativity150
binary151
bitwise189
boolean180,181,184
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cast151
comparisons180
compoundassignment185,192
conditional186,194
decrement176
dot7
equality181
executionorder152
floating-point165
floating-pointdivision167
floating-pointremainder169
increment176
instanceof195,320,321
integer189
integerarithmetic165
integerdivision167
integerremainder168
logical184
multiplicative167
newseenewoperator
overflow165
overloaded164,167
postfix151
precedence150
relational180
short-circuited186
stringconcatenation174
ternary151
unary150,151,167
unary-167
unary+167
optimizations24
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oroperator189
OracleUniversity509
ordinalvalue90,209
OutOfMemoryException395
output18
overflow155,165
overloaded164
overloadedmethodresolution316
overloading
constructors56
methodresolution316
methods52,273
overloadingvs.overriding273
overriding253
equals183
finalizers390
methods268,273
toString175
overridingmethods
covariantreturn273
overridingvs.overloading273
ownership12
P
packageaccessibility118,124
packagedirectory106
packagestatement96,98
packages97
accessibilityseepackageaccessibility
declaration96
definition98
destinationdirectory106
hierarchy97
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java.lang342
members97
namingscheme98
packagedirectory106
runningcodefrom106
short-hand100
statementseepackagestatement
subpackages97
unnamed98
using99
palindromes382,434
parallelcode441
parameter
variablearity81
parameterlist439,444
parameterlistseeformalparameters
parameterpassing
byvalue72
variablearity81
parameters49
actual72
arrayelements78
final80
fixedarity81
formalseeformalparameters
implicit50
mainmethod86
passing72
primitives73
program86
references75
this50
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variablearity81
parentclass264
parentheses150
parseTypemethod352
parsing462,486
parsingnumericvalues352
partialimplementation293
passbyvalue72
passing
parameters72
references75
variablearityparameter81
paths
seeclasspath
path-separatorcharacter110
patternletters486,495,496
PearsonVUE509
performance24
period462,476
creating476
date-based476
equality478
getmethods478
immutable476
normalization479
parsing477
period-basedloop481
plus/minusmethods479
querying478
temporalarithmetic479
textualrepresentation477
thread-safe476
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withmethods479
Periodclass
seeperiod
persistentobjects138
polymorphism311,329,334
portability23
positionalorder414
positivezero165
postfixoperators151
precedencerules151
precision160
predefinedidentifiers29
predefinedliterals29
predicate436
Predicate<T>440,451
prefix30
030
0x30
primitivedatatypes
seeprimitivetypes
primitivetypes13,144
autoboxing348
unboxing350
seealsoprimitivevalues
primitivevalues
assignment159
equality181
passing73
printingvalues18
private11
privatemembers128
processofelimination510
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program
application16
arguments86
commandline86
compiling17
formatting35
running17
programarguments86
programoutput18
programmingtoaninterface417
prolepticyear464
promotion149
properties2
seealsoclassmembers
protected11
protectedmembers126
public17
publicmembers124
punctuators29
Q
quotationmark33,34
quotient168
R
radix
prefix30
seebase349
raggedarrays65
range
charactervalues38
floating-pointvalues39
integervalues38
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rangeofdate-basedvalues464
rangeoftime-basedvalues464
rankingcriteria414
realization296
reclaimingmemory384
reducingcomplexity335
referencetypes41,267
classes48
enumtypes87
referencevalues4
referencevariables40
references4,9,40,41,72
abstracttypes121
aliases75,183
array59,62,311
assignment159
casting149,320
dangling384
declaredtype268
downcasting145
dynamictype268
equality182
field385
interfacetype296
local384
narrowingconversions145
nullseenullreference
passing75
reachable384,385
super276
this50
upcasting145
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wideningconversions145
relationaloperators180
relativeadjusters474
reliability24
remainder168
remainderoperator
%168
removewhitespace369
replacingcharacters367
reservedidentifiers29
reservedkeywords29
const29
goto220
reservedliterals
false32
nullseenullreference
true32
resources387
resurrectingobjects385,391
returnstatement224
@returntag225
returntype
covariant
returnvalue7
reuseofcode264,334
rightassociativity152
rightmostbit155
rippleeffect334
robustness24,254
rolerelationship334
root
seeinheritancehierarchy
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runningaJavaapplication17
runtime
boundschecking61
runtimechecks148,418
Runtimeclass342,393
runtimeclass343
runtimeenvironment384
runtimestack
seeJVMstack
RuntimeException236
S
scientificnotation31
scope114
block117
catchclause244
class114
disjoint118
searching
arrays69
searchinginstring367
secure24
SecurityManagerclass342
selectionstatements200
semanticdefinition28
semicolon50
separators29,151
serialization138
services335
shadowing446
shallowcopying343
short30,38
short-circuit186
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evaluation187
signature52,273
simple
assignmentoperator158
if200
statement50
simpletypename97
simplicity23
singleexpression439
singleimplementationinheritance266,290,296
singlequote(‘)32,33
singlestaticimport101
single-linecomment3,35
skeletalsourcefile96
sortingarrays68
source
file15,98
filename96
filestructure96
spaces35
specialcharactervalues33
specialization10
specializedclasses266
stack3
stackframe
seemethodexecution
stacktrace232,235
seemethodexecution
StackOverflowError237
standarderrorstream235
standardout18
startswithcharacters368
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stateseeobjectstate
statementblock439,446
statements50
break221
compound50
conditional200
continue223
controlflow50,200
controltransfer219
declaration171,177,187
declarations50
do-while214
empty50
expression50,177
for(;;)215
for(:)217
if200
if-else201
iteration213
labeled220
return224
selection200
simple50
simpleif200
switch203
throw249
transfer219
try240
while213
static
membersseestaticmembers
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methods7,10,49
variableinitialization42
variablesseestaticvariables
static101,132,300
staticcode
seestaticcontext48
staticcontext48
staticfield10
staticfieldinitializers48
staticimport101
conflicts104
ondemand101
shadowstaticmembers103
singlestaticimport101
staticinitializerblock48,137,402
statickeyword402
staticmembers7,9,10,48
statictype
seedeclaredtype
staticvariables7,10,44
storingobjects138
strictfp166
stringbuilders176
appending376
capacity374,378
constructing374
deleting376
differenceswithstrings376
individualcharacters375
inserting376
joining366
length375
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thread-safety374
Stringclass
seestrings
stringconversion146,175
stringconversions370
stringliteralpool358
interned358
stringliterals357
caselabels208
hashvalue208
interned358
StringBufferclass374
seestringbuilders
thread-safe374
StringBuilderclass374,434
seestringbuilders
strings
appending376
buffers374
builders374
capacity378
changingcase364
compareTo363
comparing363
concatenation174,364
concatenationoperator+176
constructing374
contains368
conversions370
converttocharacterarray361
copyingcharacters361
creating357
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deleting376
differenceswithstringbuilders376
empty358
endswith368
equals363
extractingsubstrings369
findingindex367
formatted370
ignoringcaseincomparison363
immutable357
individualcharacters361,375
initializing357
inserting376
interned358
joining365,366
length361,375
lexicographicalordering363
literals34,357
mutable374
readcharacteratindex361
replacing367
searching367
startswith368
stringliteralpool358
substrings369
trimming369
stronglytypedlanguage148
subclass10,11,264
subinterface294
subpackages97
subsequence361
substringsearching367
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substrings367,369
subtypecovariance309,310
subtyperelationship418
subtypes293
subtype–supertyperelationship145
suffix
D31
F31
L30
super299
construct285
keyword272,276
reference276
superclass10,11,264
superclassconstructor54
superclass–subclassrelationship266
superinterfaces294
supertypes293
supertype–subtyperelationship267
supplementarycharacters357
suppressedexceptions235
switchstatement203
break205,206
defaultclause204
enumtypes209
usingstrings208
synchronized
methods136
syntacticallylegal28
System
out18
Systemclass342
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systemclock466
T
TABseehorizontaltab
tabs35
tabulators35
tags36
targettype450
targettyping451
telephonedirectoryorder363
temporalarithmetic474,479
temporalobjects
before/aftermethods469
combiningdateandtime466
commonmethodprefix463
comparing470
creatingwithfactorymethods464
date462
dateunits474
date-basedvalues462
date-time462
formatting486
getmethods468
immutable462
methodnamingconvention463
parsing486
plus/minusmethods474
querying468
rangeofdate-basedvalues464
rangeoftime-basedvalues464
temporalarithmetic474,479,480
temporalvalues464
thread-safe462
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time462
timeunits474
time-basedvalues462
withmethods470
temporalvalues464
TemporalAmountinterface479
terminatingloops221
ternaryconditionalexpressions
seealsoconditionalexpressions194
ternaryconditionaloperator151,194
textualrepresentation343
this
reference50
this()constructorcall282,406
ThreadDeath237
threads24,342,384
death232
exceptionpropagation232
JVMstack385
live384
notifying344
synchronization136
waiting344
thread-safe357,374,415,462
throwstatement249
Throwable233,342
throw-and-catchparadigm230
throwingexceptions230
throwsclause251
time
seetemporalobjects
timeunits474
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time-basedvalues462
tokens28
toStringmethod343,349
transferstatements219
transientvariables138
transitiverelation267
trimmethod369
trueliteral32
truth-values32,39
tryblock240
try-catch-finallyconstruct238
two’scomplement154
type
declared274
dynamic274
typecast148
typecastexpression320
typechecking450
typedeclarations96
typehierarchy145,267
typeimport
seeimport
typeparameter290,414,441
types
boolean37,39
byte30,38
casting148
char38
classesseeclasses
comparing321
compatibility148
double39
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exceptions233
float39
floating-point37,38
int30,38
integers38
integraltypes37
interface290
long30,38
parsing352
short30,38
wrappers346
seealsoclasses
type-safe315
type-safety416,417,418
typeValuemethod350,352
U
UML2
accessibility124
aggregation12
associations12
classes3
composition12
inheritance10
seealsodiagrams
unaryarithmeticoperators167
unarynumericpromotion149
unaryoperators150,151
unboxing350
do-whilestatement214
for(;;)statement215
for(:)statement218
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ifstatement200
if-elsestatement202
switchstatement204
whilestatement213
unboxingconversions145,146
uncaughtexceptions232
unchangeablevariables134
uncheckedconversionwarning416
uncheckedconversions146
uncheckedexceptions237
uncheckedwarnings145
underflow155,165
uni-catchclause239
Unicode32,38,354,357,363,434
UnifiedModelingLanguageseeUML
unreachablecode244
unsafecasts321
unsignedinteger353
UnsupportedTemporalTypeException480,490
unusedkeywords29
upcasting145
updateexpression215
usingarrays61
usingpackages99
usingvariables41
UTF-16357
supplementarycharacters357
V
valueOfmethod348,349,369
values155
constants30
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overflow155
underflow155
wrap-around155
seealsovariables
varargs81
variablearitycall84
variablearitymethod81
variablearityparameter81
variablecapture449
variabledeclarations41,117
variableinitialization8,43
variables4,41
blankfinal80,134
constantvalues133
defaultvalues42
effectivelyfinal448
final133
identifiers40
ininterfaces302
initializationseevariableinitialization
lifetime44
local117
parameters49,72
referencevariable41
references41
static7
storing138
transient138
volatile139
virtualmethodinvocation
seedynamicmethodlookup
VirtualMachineError237
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void17,224,347
voidreturn445
Voidwrapperclass346
volatilevariables139
voucher509
W
waitingthreads344
whilestatement213
whitespace35,369
whole–partrelationship267
wideningconversions
primitive144
references145
wideningreferenceconversions267,320
widerrange144
withers470
wrapperclasses38,342,343,346,347
internedvalues351
wrappertype146,164,177
X
xor189
Z
zero
negative165
positive165
zero-basedindex414
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