OCA Java SE 8 Programmer I Exam Guide (Exams 1Z0 808) (Certification & Career OMG)

OCA%20Java%20SE%208%20Programmer%20I%20Exam%20Guide%20(Exams%201Z0-808)%20(Certification%20%26%20Career%20-%20OMG)

OCA%20Java%20SE%208%20Programmer%20I%20Exam%20Guide%20(Exams%201Z0-808)

OCA%201Z0-808%20SE%208%20Programmer%20I%20Exam%20Guide%20-%20%20Kathy%20Sierra

2017OCA%20Java%20SE%208%20Examination%20Guide

OCA%201Z0-808%20SE%208%20Programmer%20I%20Exam%20Guide%20Kathy%20Sierra

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CONTRIBUTORS
AbouttheAuthors
KathySierrawasaleaddeveloperfortheSCJPexamforJava5andJava6.KathyworkedasaSun
“mastertrainer,”andin1997,foundedJavaRanch.com(nownamedCoderanch.com),theworld’slargest
Javacommunitywebsite.HerbestsellingJavabookshavewonmultipleSoftwareDevelopment
Magazineawards,andsheisafoundingmemberofOracle’sJavaChampionsprogram.
Thesedays,Kathyisdevelopingadvancedtrainingprogramsinavarietyofdomains(from
horsemanshiptocomputerprogramming),butthethreadthattiesallherprojectstogetherishelping
learnersreducecognitiveload.
BertBateswasaleaddeveloperformanyofSun’sJavacertificationexams,includingtheSCJPforJava
5andJava6.BertwasalsooneoftheleaddevelopersforOracle’sOCA7andOCP7examsanda
contributoronOracle’sOCA8andOCP8exams.HeisaforummoderatoronCoderanch.com(formerly
JavaRanch.com)andhasbeendevelopingsoftwareformorethan30years(argh!).Bertistheco-authorof
severalbestsellingJavabooks,andhe’safoundingmemberofOracle’sJavaChampionsprogram.Now
thatthebookisdone,Bertplanstogowhackafewtennisballsaroundandonceagainstartridinghis
beautifulIcelandichorse,EyrrarosfraGufudal-Fremri.
AbouttheTechnicalReviewTeam
Thisisthefiftheditionofthebookthatwe’vecookedup.ThefirstversionweworkedonwasforJava2.
ThenweupdatedthebookfortheSCJP5,againfortheSCJP6,againfortheOCA7andOCP7exams,
andnowfortheOCA8.Everystepoftheway,wewereunbelievablyfortunatetohavefantastic,
JavaRanch.com-centrictechnicalreviewteamsatoursides.Overthecourseofthelast14years,we’ve
been“evolving”thebookmorethanrewritingit.ManysectionsfromouroriginalworkontheJava2
bookarestillintact.Onthefollowingpages,we’dliketoacknowledgethemembersofthevarious
technicalreviewteamswhohavesavedourbaconovertheyears.
AbouttheJava2TechnicalReviewTeam
JohannesdeJonghasbeentheleaderofourtechnicalreviewteamsforeverandever.(Hehasmore
patiencethananythreepeopleweknow.)FortheJava2book,heledourbiggestteamever.Oursincere
thanksgoouttothefollowingvolunteerswhowereknowledgeable,diligent,patient,andpicky,picky,
picky!
RobRoss,NicholasCheung,JaneGriscti,IljaPreuss,VincentBrabant,KudretSerin,BillSeipel,Jing
Yi,GinuJacobGeorge,Radiya,LuAnnMazza,AnshuMishra,AnandhiNavaneethakrishnan,Didier
Varon,MaryMcCartney,HarshaPherwani,AbhishekMisra,andSumanDas.
AbouttheSCJP5TechnicalReviewTeam
Wedon’tknowwhoburnedthemostmidnightoil,butwecan(anddid)counteverybody’sedits—soin
orderofmosteditsmade,weproudlypresentourSuperstars.
OurtophonorsgotoKristinStromberg—everytimeyouseeasemicolonusedcorrectly,tipyourhat
toKristin.NextupisBurkHufnagelwhofixedmorecodethanwecaretoadmit.BillMietelskiand
GianFrancoCasulacaughteverykindoferrorwethrewatthem—awesomejob,guys!Devender
Tharejamadesurewedidn’tusetoomuchslang,andMarkSpritzlerkeptthehumorcoming.Mikalai
ZaikinandSeemaManivannanmadegreatcatcheseverystepoftheway,andMarilyndeQueirozand
ValentinCrettazbothputinanotherstellarperformance(savingourbuttsyetagain).
MarceloOrtega,JefCumps(anotherveteran),AndrewMonkhouse,andJeroenSterkenroundedout
ourcrewofSuperstars—thankstoyouall.JimYingstwasamemberoftheSunexamcreationteam,and
hehelpeduswriteandreviewsomeofthetwistierquestionsinthebook(bwa-ha-ha-ha).
Asalways,everytimeyoureadacleanpage,thankourreviewers,andifyoudocatchanerror,it’s
mostcertainlybecauseyourauthorsmessedup.Andoh,onelastthankstoJohannes.Yourule,dude!
AbouttheSCJP6TechnicalReviewTeam
SincetheupgradetotheJava6examwaslikeasmallsurgicalstrikewedecidedthatthetechnical
reviewteamforthisupdatetothebookneededtobesimilarlyfashioned.Tothatendwehand-pickedan
elitecrewofJavaRanch’stopgurustoperformthereviewfortheJava6exam.
OurendlessgratitudegoestoMikalaiZaikin.MikalaiplayedahugeroleintheJava5book,andhe
returnedtohelpusoutagainforthisJava6edition.WeneedtothankVolha,Anastasia,andDariafor
lettingusborrowMikalai.Hiscommentsandeditshelpedusmakehugeimprovementstothebook.
Thanks,Mikalai!
MarcPeabodygetsspecialkudosforhelpingusoutonadoubleheader!Inadditiontohelpinguswith
Sun’snewSCWCDexam,Marcpitchedinwithagreatsetofeditsforthisbook—yousavedourbacon
thiswinter,Marc!(BTW,wedidn’tlearnuntillateinthegamethatMarc,BryanBasham,andBertall
shareapassionforultimateFrisbee!)
Likeseveralofourreviewers,notonlydoesFredRosenbergervolunteercopiousamountsofhistime
moderatingatJavaRanch,healsofoundtimetohelpusoutwiththisbook.StaceyandOlivia,youhave
ourthanksforloaningusFredforawhile.
MarcWebermoderatesatsomeofJavaRanch’sbusiestforums.Marcknowshisstuffanduncovered
somereallysneakyproblemsthatwereburiedinthebook.WhilewereallyappreciateMarc’shelp,we
needtowarnyoualltowatchout—he’sgotaPhaser!
Finally,wesendourthankstoChristopheVerre—ifwecanfindhim.ItappearsthatChristophe
performshisJavaRanchmoderationdutiesfromvariouslocationsaroundtheglobe,includingFrance,
Wales,andmostrecentlyTokyo.OnmorethanoneoccasionChristopheprotectedusfromourownlackof
organization.Thanksforyourpatience,Christophe!It’simportanttoknowthattheseguysalldonatedtheir
reviewerhonorariumstoJavaRanch!TheJavaRanchcommunityisinyourdebt.
TheOCA7andOCP7Team
ContributingAuthors
TheOCA7examisprimarilyausefulrepackagingofsomeoftheobjectivesfromtheSCJP6exam.On
theotherhand,theOCP7examintroducedavastarrayofbrand-newtopics.Weenlistedseveraltalented
JavagurustohelpuscoversomeofthenewtopicsontheOCP7exam.ThanksandkudostoTom
McGinnforhisfantasticworkincreatingthemassiveJDBCchapter.SeveralreviewerstoldusthatTom
didanamazingjobchannelingtheinformaltoneweusethroughoutthebook.Next,thankstoJeanne
Boyarsky.Jeannewastrulyarenaissancewomanonthisproject.ShecontributedtoseveralOCP
chapters;shewrotesomequestionsforthemasterexams;sheperformedsomeprojectmanagement
activities;andasifthatwasn’tenough,shewasoneofourmostenergetictechnicalreviewers.Jeanne,we
can’tthankyouenough.OurthanksgotoMattHeimerforhisexcellentworkontheConcurrentchapter.
Areallytoughtopicnicelyhandled!Finally,RoelDeNijsandRobertoPerillomadesomenice
contributionstothebookandhelpedoutonthetechnicalreviewteam—thanks,guys!
TechnicalReviewTeam
Roel,whatcanwesay?Yourworkasatechnicalreviewerisunparalleled.Roelcaughtsomany
technicalerrors,itmadeourheadsspin.BetweentheprintedbookandallthematerialontheCD,we
estimatethatthereareover1,500pagesof“stuff”here.It’shuge!Roelgrindedthroughpageafterpage,
neverlosthisfocus,andmadethisbookbetterincountlessways.Thankyou,Roel!
Inadditiontoherothercontributions,Jeanneprovidedoneofthemostthoroughtechnicalreviewswe
received.(Wethinksheenlistedherteamofkillerrobotstohelpher!)
ItseemslikenoK&BbookwouldbecompletewithouthelpfromouroldfriendMikalaiZaikin.
Somehow,betweenearning812differentJavacertifications,beingahusbandandfather(thankstoVolha,
Anastasia,Daria,andIvan),andbeinga“theoreticalfisherman”[sic],Mikalaimadesubstantial
contributionstothequalityofthebook;we’rehonoredthatyouhelpedusagain,Mikalai.
Nextup,we’dliketothankVijithaKumara,JavaRanchmoderatorandtechreviewerextraordinaire.
Wehadmanyreviewershelpoutduringthelongcourseofwritingthisbook,butVijithawasoneofthe
fewwhostuckwithusfromChapter1allthewaythroughthemasterexamsandontoChapter15.Vijitha,
thankyouforyourhelpandpersistence!
Finally,thankstotherestofourreviewteam:RobertoPerillo(whoalsowrotesomekillerexam
questions),JimYingst(wasthisyourfourthtime?),otherrepeatoffenders:FredRosenberger,
ChristopheVerre,DevakaCooray,MarcPeabody,andnewcomerAmitGhorpade—thanks,guys!
AbouttheOCA8TechnicalReviewTeam
Dueto“piloterror”onyourauthors’part,themembersofthereviewteamforthisbookwereaskedto
workunderatighter-than-idealschedule.Weoweahugethankstooursixreviewersforthisbook,who
leapttoactiononshortnoticeandonceagainmadenumerousfinecontributionstothequalityofthebook.
Ashasbecome“thenorm,”allourreviewersaremoderatorsatthefinestJavacommunitywebsiteinthe
wholeuniverse,Coderanch.com.
OurfirstmentiongoestoCampbellRitchie,long-timeCoderanchmoderator,hikerofsteephills,and,
byourcount,mostprolificreviewerforthisedition.Inotherwords,Campbellfoundthemosterrors.
Amongotherpursuits,Campbellisanexpertinbothpathologyandprogramming,because…whynot?
Everytimeyoureadanerror-freepage,thinkofCampbellandthankhim.
Inavirtualtieforsecondplaceinthe“errorsfound”competition,wepresentPawełBaczyńskiand
FritzWalraven.PawełcallsPolandhishomeandismostproudofhiswife,Ania,andkids,Szymekand
Gabrysia.Heisalmostasproudofthefactthatwehavetouseaspecialkeyboardtospellhisname
correctly!
FritzhailsfromtheNetherlands,andifweunderstandcorrectly,cheershiskidsastheydemonstrate
theirathleticprowessinvarioussportsvenuesaroundAmersfoort.Fritzhasalsovolunteeredatan
orphanageinUganda(wehopeheteachesthosekidsJava!).Fritz,ifweevermeetinperson,Berthereby
challengesyoutoatabletennismatch.
We’dliketogiveaspecialshoutouttoFritzandPawelforstickingwithusfromChapter1alltheway
throughtothesecondpracticeexam.Itwasamarathon,andwethankyouboth.
Next,we’dliketothankreturningreviewerVijithaKumara.That’sright,he’shelpedusbeforeand
yethevolunteeredagain—wow!WhenVijithaisn’ttravelingorhiking,heenjoyswhathereferstoas
“crazyexperiments.”Weloveyou,Vijitha—don’tblowyourselfup!
TimCookewastherewhenweneededhim,atthebeginningoftheprocessandagaintohelpuswrap
thingsup.WelikeTimeventhoughhe’sbeenknowntospendtimewithnefarious“functional
programmers.”(Wesuspectthat’swherehewaswhenhewentmissingforafewofthemiddle-of-the-
bookchapters.)TimlivesinIrelandandstartedprogrammingatanearlyageonanAmstradCPC464.
Nice!
Finally,aspecialthankstoanotherveteranofthetechnicalreviewteam,RobertoPerillo.Thanksfor
comingbacktohelpusagainRoberto.Robertoisafamilymanwholovesspendingtimewithhisson,
Lorenzo.OnceLorenzohasgonetobed,RobertoisknowntoplayabitofguitarorcheerfortheSao
Paulo“FutebolClube”(wethinkthisisasoccerteam).
Youguysareallawesome.Thanksforyourwonderfulassistance.
ForAndi
ForBob
CONTENTSATAGLANCE
1DeclarationsandAccessControl
2ObjectOrientation
3Assignments
4Operators
5FlowControlandExceptions
6Strings,Arrays,ArrayLists,Dates,andLambdas
AAbouttheDownload
Index
CONTENTS
Acknowledgments
Preface
Introduction
1DeclarationsandAccessControl
JavaRefresher
FeaturesandBenefitsofJava(OCAObjective1.5)
IdentifiersandKeywords(OCAObjectives1.2and2.1)
LegalIdentifiers
Oracle’sJavaCodeConventions
DefineClasses(OCAObjectives1.2,1.3,1.4,6.4,and7.5)
SourceFileDeclarationRules
UsingthejavacandjavaCommands
Usingpublicstaticvoidmain(String[]args)
ImportStatementsandtheJavaAPI
StaticImportStatements
ClassDeclarationsandModifiers
Exercise1-1:CreatinganAbstractSuperclassandConcreteSubclass
UseInterfaces(OCAObjective7.5)
DeclaringanInterface
DeclaringInterfaceConstants
DeclaringdefaultInterfaceMethods
DeclaringstaticInterfaceMethods
DeclareClassMembers(OCAObjectives2.1,2.2,2.3,4.1,4.2,6.2,6.3,and6.4)
AccessModifiers
NonaccessMemberModifiers
ConstructorDeclarations
VariableDeclarations
DeclareandUseenums(OCAObjective1.2)
Declaringenums
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
2ObjectOrientation
Encapsulation(OCAObjectives6.1and6.5)
InheritanceandPolymorphism(OCAObjectives7.1and7.2)
TheEvolutionofInheritance
IS-AandHAS-ARelationships
Polymorphism(OCAObjective7.2)
Overriding/Overloading(OCAObjectives6.1and7.2)
OverriddenMethods
OverloadedMethods
Casting(OCAObjectives2.2and7.3)
ImplementinganInterface(OCAObjective7.5)
Java8—NowwithMultipleInheritance!
LegalReturnTypes(OCAObjectives2.2and6.1)
ReturnTypeDeclarations
ReturningaValue
ConstructorsandInstantiation(OCAObjectives6.3and7.4)
ConstructorBasics
ConstructorChaining
RulesforConstructors
DetermineWhetheraDefaultConstructorWillBeCreated
OverloadedConstructors
InitializationBlocks(OCAObjectives1.2and6.3-ish)
Statics(OCAObjective6.2)
StaticVariablesandMethods
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
3Assignments
StackandHeap—QuickReview
Literals,Assignments,andVariables(OCAObjectives2.1,2.2,and2.3)
LiteralValuesforAllPrimitiveTypes
AssignmentOperators
Exercise3-1:CastingPrimitives
Scope(OCAObjective1.1)
VariableScope
VariableInitialization(OCAObjectives2.1,4.1,and4.2)
UsingaVariableorArrayElementThatIsUninitializedandUnassigned
Local(Stack,Automatic)PrimitivesandObjects
PassingVariablesintoMethods(OCAObjective6.6)
PassingObjectReferenceVariables
DoesJavaUsePass-By-ValueSemantics?
PassingPrimitiveVariables
GarbageCollection(OCAObjective2.4)
OverviewofMemoryManagementandGarbageCollection
OverviewofJava’sGarbageCollector
WritingCodeThatExplicitlyMakesObjectsEligibleforCollection
Exercise3-2:GarbageCollectionExperiment
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
4Operators
JavaOperators(OCAObjectives3.1,3.2,and3.3)
AssignmentOperators
RelationalOperators
instanceofComparison
ArithmeticOperators
ConditionalOperator
LogicalOperators
OperatorPrecedence
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
5FlowControlandExceptions
UsingifandswitchStatements(OCAObjectives3.3and3.4)
if-elseBranching
switchStatements
Exercise5-1:Creatingaswitch-caseStatement
CreatingLoopsConstructs(OCAObjectives5.1,5.2,5.3,5.4,and5.5)
UsingwhileLoops
UsingdoLoops
UsingforLoops
Usingbreakandcontinue
UnlabeledStatements
LabeledStatements
Exercise5-2:CreatingaLabeledwhileLoop
HandlingExceptions(OCAObjectives8.1,8.2,8.3,8.4,and8.5)
CatchinganExceptionUsingtryandcatch
Usingfinally
PropagatingUncaughtExceptions
Exercise5-3:PropagatingandCatchinganException
DefiningExceptions
ExceptionHierarchy
HandlinganEntireClassHierarchyofExceptions
ExceptionMatching
ExceptionDeclarationandthePublicInterface
Exercise5-4:CreatinganException
RethrowingtheSameException
CommonExceptionsandErrors(OCAObjective8.5)
WhereExceptionsComeFrom
JVM-ThrownExceptions
ProgrammaticallyThrownExceptions
ASummaryoftheExam’sExceptionsandErrors
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
6Strings,Arrays,ArrayLists,Dates,andLambdas
UsingStringandStringBuilder(OCAObjectives9.2and9.1)
TheStringClass
ImportantFactsAboutStringsandMemory
ImportantMethodsintheStringClass
TheStringBuilderClass
ImportantMethodsintheStringBuilderClass
WorkingwithCalendarData(OCAObjective9.3)
Immutability
FactoryClasses
UsingandManipulatingDatesandTimes
FormattingDatesandTimes
UsingArrays(OCAObjectives4.1and4.2)
DeclaringanArray
ConstructinganArray
InitializinganArray
UsingArrayListsandWrappers(OCAObjectives9.4and2.5)
WhentoUseArrayLists
ArrayListMethodsinAction
ImportantMethodsintheArrayListClass
AutoboxingwithArrayLists
TheJava7“Diamond”Syntax
AdvancedEncapsulation(OCAObjective6.5)
EncapsulationforReferenceVariables
UsingSimpleLambdas(OCAObjective9.5)
CertificationSummary
Two-MinuteDrill
Q&ASelfTest
SelfTestAnswers
AAbouttheDownload
SystemRequirements
DownloadingfromMcGraw-HillProfessional’sMediaCenter
InstallingthePracticeExamSoftware
RunningthePracticeExamSoftware
PracticeExamSoftwareFeatures
RemovingInstallation
Help
TechnicalSupport
Windows8Troubleshooting
McGraw-HillEducationContentSupport
Index
K
ACKNOWLEDGMENTS
athyandBertwouldliketothankthefollowingpeople:
Alltheincrediblyhard-workingfolksatMcGraw-HillEducation:TimGreen(who’s
beenputtingupwithusfor14yearsnow),LisaMcClain,andLeeAnnPickrell.Thanksforallyour
helpandforbeingsoresponsive,patient,flexible,andprofessional,andthenicestgroupofpeople
wecouldhopetoworkwith.
AllourfriendsatKraftur(andourotherhorse-relatedfriends)and,mostespecially,toSherry,
Steinar,Stinaandthegirls,Kacie,DJ,Jec,LeslieandDavid,AnnetteandBruce,Lucy,Cait,and
Jennifer,Gabrielle,andMary.ThankstoPedroandEly.
Someofthesoftwareprofessionalsandfriendswhohelpedusintheearlydays:TomBender,
PeterLoerincs,CraigMatthews,LeonardCoyne,MorganPorter,andMikeKavenaugh.
DaveGustafsonforhiscontinuedsupport,insights,andcoaching.
OurwonderfulcontactandfriendatOracle,YvonnePrefontaine.
Ourgreatfriendsandgurus:SimonRoberts,BryanBasham,andKathyCollina.
Stu,Steve,Burt,andMarcHedlundforinjectingsomefunintotheprocess.
ToEdenandSkyler,forbeinghorrifiedthatadults—outofschool—wouldstudythishardfor
anexam.
TotheCoderanchTrailBoss,PaulWheaton,forrunningthebestJavacommunitysiteonthe
Web,andtoallthegenerousandpatientJavaRanch,er,Coderanchmoderators.
ToallthepastandpresentJavainstructorsforhelpingtomakelearningJavaafunexperience,
including(tonameonlyafew)AlanPetersen,JeanTordella,GeorgiannaMeagher,Anthony
Orapallo,JacquelineJones,JamesCubeta,TeriCubeta,RobWeingruber,JohnNyquist,Asok
Perumainar,SteveStelting,KimberlyBobrow,KeithRatliff,andthemostcaringandinspiringJava
guyontheplanet,JariPaukku.
Ourfurryandfeatheredfriends:Eyra,Kara,Draumur,Vafi,Boi,Niki,andBokeh.
Finally,toEricFreemanandBethRobsonforyourcontinuedinspiration.
T
PREFACE
hisbook’sprimaryobjectiveistohelpyouprepareforandpassOracle’sOCAJavaSE8
ProgrammerIcertificationexam.
Thisbookfollowscloselyboththebreadthandthedepthoftherealexams.Forinstance,
afterreadingthisbook,youprobablywon’temergeasanOOguru,butifyoustudythematerialanddo
wellontheSelfTests,you’llhaveabasicunderstandingofOO,andyou’lldowellontheexam.After
completingthisbook,youshouldfeelconfidentthatyouhavethoroughlyreviewedalltheobjectivesthat
Oraclehasestablishedfortheseexams.
InThisBook
ThisbookisorganizedintosixchapterstooptimizeyourlearningofthetopicscoveredbytheOCA8
exam.Wheneverpossible,we’veorganizedthechapterstoparalleltheOracleobjectives,butsometimes
we’llmixupobjectivesorpartiallyrepeatthemtopresenttopicsinanorderbettersuitedtolearningthe
material.
InEveryChapter
We’vecreatedasetofchaptercomponentsthatcallyourattentiontoimportantitems,reinforceimportant
points,andprovidehelpfulexam-takinghints.Takealookatwhatyou’llfindineverychapter:
EverychapterbeginswiththeCertificationObjectives—whatyouneedtoknowin
ordertopassthesectionontheexamdealingwiththechaptertopic.TheCertification
Objectiveheadingsidentifytheobjectiveswithinthechapter,soyou’llalwaysknowan
objectivewhenyouseeit!
ExamWatchnotescallattentiontoinformationabout,andpotentialpitfallsin,theexam.
Becausewewereontheteamthatcreatedtheseexams,weknowwhatyou’reabouttogo
through!
OntheJobcalloutsdiscusspracticalaspectsofcertificationtopicsthatmightnot
occurontheexam,butthatwillbeusefulintherealworld.
Exercisesareinterspersedthroughoutthechapters.Theyhelpyoumasterskillsthat
arelikelytobeanareaoffocusontheexam.Don’tjustreadthroughtheexercises;theyare
hands-onpracticethatyoushouldbecomfortablecompleting.Learningbydoingisan
effectivewaytoincreaseyourcompetencywithaproduct.
FromtheClassroomsidebarsdescribetheissuesthatcomeupmostofteninthe
trainingclassroomsetting.Thesesidebarsgiveyouavaluableperspectiveinto
certification-andproduct-relatedtopics.Theypointoutcommonmistakesandaddress
questionsthathavearisenfromclassroomdiscussions.
TheCertificationSummaryisasuccinctreviewofthechapterandarestatementof
salientpointsregardingtheexam.
 TheTwo-MinuteDrillattheendofeverychapterisachecklistofthemainpoints
ofthechapter.Itcanbeusedforlast-minutereview.
Q&A TheSelfTestoffersquestionssimilartothosefoundonthecertificationexam,including
multiplechoiceandpseudodrag-and-dropquestions.Theanswerstothesequestions,as
wellasexplanationsoftheanswers,canbefoundattheendofeverychapter.Bytakingthe
SelfTestaftercompletingeachchapter,you’llreinforcewhatyou’velearnedfromthat
chapter,whilebecomingfamiliarwiththestructureoftheexamquestions.
INTRODUCTION
Organization
Thisbookisorganizedinsuchawayastoserveasanin-depthreviewfortheOCA8exam,forboth
experiencedJavaprofessionalsandthoseintheearlystagesofexperiencewithJavatechnologies.Each
chaptercoversatleastonemajoraspectoftheexam,withanemphasisonthe“why”aswellasthe“how
to”ofprogrammingintheJavalanguage.
Practiceexamsoftwarewithtwo80-questionexamsisavailablefordownload.
WhatThisBookIsNot
Youwillnotfindabeginner’sguidetolearningJavainthisbook.All500+pagesofthisbookare
dedicatedsolelytohelpingyoupasstheexam.IfyouarebrandnewtoJava,wesuggestyouspendalittle
timelearningthebasics.Youshouldnotstartwiththisbookuntilyouknowhowtowrite,compile,and
runsimpleJavaprograms.Wedonot,however,assumeanylevelofpriorknowledgeoftheindividual
topicscovered.Inotherwords,foranygiventopic(drivenexclusivelybytheactualexamobjectives),we
startwiththeassumptionthatyouarenewtothattopic.Soweassumeyou’renewtotheindividualtopics,
butweassumethatyouarenotnewtoJava.
Wealsodonotpretendtobebothpreparingyoufortheexamandsimultaneouslymakingyoua
completeJavabeing.Thisisacertificationexamstudyguide,andit’sveryclearaboutitsmission.That’s
nottosaythatpreparingfortheexamwon’thelpyoubecomeabetterJavaprogrammer!Onthecontrary,
eventhemostexperiencedJavadevelopersoftenclaimthathavingtoprepareforthecertificationexam
madethemfarmoreknowledgeableandwell-roundedprogrammersthantheywouldhavebeenwithout
theexam-drivenstudying.
DigitalContent
Formoreinformationaboutthepracticeexamsoftware,pleaseseetheAppendix.
SomePointers
Onceyou’vefinishedreadingthisbook,setasidesometimetodoathoroughreview.Youmightwantto
returntothebookseveraltimesandmakeuseofallthemethodsitoffersforreviewingthematerial:
1.Re-readalltheTwo-MinuteDrills,orhavesomeonequizyou.Youalsocanusethedrillsas
awaytodoaquickcrambeforetheexam.Youmightwanttomakesomeflashcardsoutof3×5
indexcardsthathavetheTwo-MinuteDrillmaterialonthem.
2.Re-readalltheExamWatchnotes.Rememberthatthesenotesarewrittenbyauthorswho
helpedcreatetheexam.Theyknowwhatyoushouldexpect—andwhatyoushouldbeonthelookout
for.
3.Re-taketheSelfTests.Takingthetestsrightafteryou’vereadthechapterisagoodidea
becausethequestionshelpreinforcewhatyou’vejustlearned.However,it’sanevenbetterideato
gobacklateranddoallthequestionsinthebookinonesitting.Pretendthatyou’retakingthelive
exam.(WheneveryoutaketheSelfTests,markyouranswersonaseparatepieceofpaper.That
way,youcanrunthroughthequestionsasmanytimesasyouneedtountilyoufeelcomfortablewith
thematerial.)
4.Completetheexercises.Theexercisesaredesignedtocoverexamtopics,andthere’sno
betterwaytogettoknowthismaterialthanbypracticing.Besureyouunderstandwhyyouare
performingeachstepineachexercise.Ifthereissomethingyouarenotclearon,re-readthatsection
inthechapter.
5.WritelotsofJavacode.We’llrepeatthisadviceseveraltimes.Whenwewrotethisbook,we
wrotehundredsofsmallJavaprogramstohelpusdoourresearch.Wehaveheardfromhundredsof
candidateswhohavepassedtheexam,andinalmosteverycase,thecandidateswhoscored
extremelywellontheexamwrotelotsofcodeduringtheirstudies.Experimentwiththecode
samplesinthebook,createhorrendouslistsofcompilererrors—putawayyourIDE,crankupthe
commandline,andwritecode!
IntroductiontotheMaterialintheBook
TheOCA8examisconsideredoneofthehardestintheITindustry,andwecantellyoufromexperience
thatalargechunkofexamcandidatesgoesintothetestunprepared.Asprogrammers,wetendtolearn
onlywhatweneedtocompleteourcurrentproject,giventheinsanedeadlineswe’reusuallyunder.
ButthisexamattemptstoproveyourcompleteunderstandingoftheJavalanguage,notjustthepartsof
ityou’vebecomefamiliarwithinyourwork.
Experiencealonewillrarelygetyouthroughthisexamwithapassingmark,becauseeventhethings
youthinkyouknowmightworkjustalittledifferentlythanyouimagined.Itisn’tenoughtobeabletoget
yourcodetoworkcorrectly;youmustunderstandthecorefundamentalsinadeepway,andwithenough
breadthtocovervirtuallyanythingthatcouldcropupinthecourseofusingthelanguage.
WhoCaresAboutCertification?
Employersdo.Headhuntersdo.Programmersdo.Passingthisexamprovesthreeimportantthingstoa
currentorprospectiveemployer:you’resmart;youknowhowtostudyandprepareforachallengingtest;
and,mostofall,youknowtheJavalanguage.Ifanemployerhasachoicebetweenacandidatewhohas
passedtheexamandonewhohasn’t,theemployerknowsthatthecertifiedprogrammerdoesnothaveto
taketimetolearntheJavalanguage.
ButdoesitmeanthatyoucanactuallydevelopsoftwareinJava?Notnecessarily,butit’sagoodhead
start.Toreallydemonstrateyourabilitytodevelop(asopposedtojustyourknowledgeofthelanguage),
youshouldconsiderpursuingtheJavaDeveloperExam,whereyou’regivenanassignmenttobuilda
program,starttofinish,andsubmititforanassessortoevaluateandscore.
TakingtheProgrammer’sExam
Inaperfectworld,youwouldbeassessedforyourtrueknowledgeofasubject,notsimplyhowyou
respondtoaseriesoftestquestions.Butlifeisn’tperfect,anditjustisn’tpracticaltoevaluateeveryone’s
knowledgeonaone-to-onebasis.
Formostofitscertifications,Oracleevaluatescandidatesusingacomputer-basedtestingservice
operatedbyPearsonVUE.Todiscouragesimplememorization,Oracleexamspresentapotentially
differentsetofquestionstodifferentcandidates.Inthedevelopmentoftheexam,hundredsofquestions
arecompiledandrefinedusingbetatesters.Fromthislargecollection,questionsarepulledtogetherfrom
eachobjectiveandassembledintomanydifferentversionsoftheexam.
EachOracleexamhasaspecificnumberofquestions,andthetest’sdurationisdesignedtobe
generous.Thetimeremainingisalwaysdisplayedinthecornerofthetestingscreen.Iftimeexpires
duringanexam,thetestterminates,andincompleteanswersarecountedasincorrect.
Manyexperiencedtest-takersdonotgobackandchangeanswersunlesstheyhaveagood
reasontodoso.Onlychangeananswerwhenyoufeelyoumayhavemisreadormisinterpreted
thequestionthefirsttime.Nervousnessmaymakeyousecond-guesseveryanswerandtalk
yourselfoutofacorrectone.
Aftercompletingtheexam,youwillreceiveane-mailfromOracletellingyouthatyourresultsare
availableontheWeb.Asofwinter2017,yourresultscanbefoundatcertview.oracle.com.Ifyouwanta
printedcopyofyourcertificate,youmustmakeaspecificrequest.
QuestionFormat
Oracle’sJavaexamsposequestionsinmultiple-choiceformat.
Multiple-ChoiceQuestions
Inearlierversionsoftheexam,whenyouencounteredamultiple-choicequestion,youwerenottoldhow
manyanswerswerecorrect;butwitheachversionoftheexam,thequestionshavebecomemoredifficult,
sotoday,eachmultiple-choicequestiontellsyouhowmanyanswerstochoose.TheSelfTestquestionsat
theendofeachchaptercloselymatchtheformat,wording,anddifficultyoftherealexamquestions,with
twoexceptions:
Wheneverwecan,ourquestionswillnottellyouhowmanycorrectanswersexist(wewill
say“Chooseallthatapply”).Wedothistohelpyoumasterthematerial.Somesavvytest-takers
caneliminatewronganswerswhenthenumberofcorrectanswersisknown.It’salsopossible,if
youknowhowmanyanswersarecorrect,tochoosethemostplausibleanswers.Ourjobisto
toughenyouupfortherealexam!
Therealexamtypicallynumberslinesofcodeinaquestion.Sometimeswedonotnumber
linesofcode—mostlysothatwehavethespacetoaddcommentsatkeyplaces.Ontherealexam,
whenacodelistingstartswithline1,itmeansthatyou’relookingatanentiresourcefile.Ifacode
listingstartsatalinenumbergreaterthan1,thatmeansyou’relookingatapartialsourcefile.
Whenlookingatapartialsourcefile,assumethecodeyoucan’tseeiscorrect.(Forinstance,
unlessexplicitlystated,youcanassumethatapartialsourcefilewillhavethecorrectimportand
packagestatements.)
Whenyoufindyourselfstumpedansweringmultiple-choicequestions,useyourscratchpaper
(orwhiteboard)towritedownthetwoorthreeanswersyouconsiderthestrongest,then
underlinetheansweryoufeelismostlikelycorrect.Hereisanexampleofwhatyourscratch
papermightlooklikewhenyou’vegonethroughthetestonce:
21.BorC
33.AorC
Thisisextremelyhelpfulwhenyoumarkthequestionandcontinueon.Youcanthenreturnto
thequestionandimmediatelypickupyourthoughtprocesswhereyouleftoff.Usethis
techniquetoavoidhavingtore-readandrethinkquestions.Youwillalsoneedtouseyour
scratchpaperduringcomplex,text-basedscenarioquestionstocreatevisualimagestobetter
understandthequestion.Thistechniqueisespeciallyhelpfulifyouareavisuallearner.
TipsonTakingtheExam
Thenumberofquestionsandpassingpercentagesforeveryexamaresubjecttochange.Alwayscheck
withOraclebeforetakingtheexam,atwww.Oracle.com.
Youareallowedtoanswerquestionsinanyorder,andyoucangobackandcheckyouranswersafter
you’vegonethroughthetest.Therearenopenaltiesforwronganswers,soit’sbettertoatleastattemptan
answerthantonotgiveoneatall.
Agoodstrategyfortakingtheexamistogothroughonceandanswerallthequestionsthatcometoyou
quickly.Youcanthengobackanddotheothers.Answeringonequestionmightjogyourmemoryforhow
toanswerapreviousone.
Beverycarefulonthecodeexamples.Checkforsyntaxerrorsfirst:countcurlybraces,semicolons,
andparentheses,andthenmakesurethereareasmanyleftonesasrightones.Lookforcapitalization
errorsandothersuchsyntaxproblemsbeforetryingtofigureoutwhatthecodedoes.
Manyofthequestionsontheexamwillhingeonsubtletiesofsyntax.Youwillneedtohaveathorough
knowledgeoftheJavalanguageinordertosucceed.
Thisbringsustoanotherissuethatsomecandidateshavereported.Thetestingcenterissupposedto
provideyouwithsufficientwritingimplementssoyoucanworkproblemsout“onpaper.”Insomecases,
thecentershaveprovidedinadequatemarkersanddry-eraseboardsthataretoosmallandcumbersometo
useeffectively.Werecommendthatyoucallaheadandverifythatyouwillbesuppliedwithasufficiently
largewhiteboard,sufficientlyfine-tippedmarkers,andagooderaser.Whatwe’dreallyliketoencourage
isforeveryonetocomplaintoOracleandPearsonVUEandhavethemprovideactualpencilsandatleast
severalsheetsofblankpaper.
TipsonStudyingfortheExam
Firstandforemost,giveyourselfplentyoftimetostudy.Javaisacomplexprogramminglanguage,and
youcan’texpecttocramwhatyouneedtoknowintoasinglestudysession.Itisafieldbestlearnedover
time,bystudyingasubjectandthenapplyingyourknowledge.Buildyourselfastudyscheduleandstickto
it,butbereasonableaboutthepressureyouputonyourself,especiallyifyou’restudyinginadditionto
yourregulardutiesatwork.
Oneeasytechniquetouseinstudyingforcertificationexamsisthe15-minutes-per-dayeffort.Simply
studyforaminimumof15minuteseveryday.Itisasmallbutsignificantcommitment.Ifyouhaveaday
whereyoujustcan’tfocus,thengiveupat15minutes.Ifyouhaveadaywhereitflowscompletelyfor
you,studylonger.Aslongasyouhavemoreofthe“flowdays,”yourchancesofsucceedingareexcellent.
Westronglyrecommendyouuseflashcardswhenpreparingfortheprogrammer’sexams.Aflashcard
issimplya3×5or4×6indexcardwithaquestiononthefrontandtheanswerontheback.You
constructthesecardsyourselfasyougothroughachapter,capturinganytopicyouthinkmightneedmore
memorizationorpracticetime.Youcandrillyourselfwiththembyreadingthequestion,thinkingthrough
theanswer,andthenturningthecardovertoseeifyou’recorrect.Oryoucangetanotherpersontohelp
youbyholdingupthecardwiththequestionfacingyouandthenverifyingyouranswer.Mostofour
studentshavefoundthesetobetremendouslyhelpful,especiallybecausethey’resoportablethatwhile
you’reinstudymode,youcantakethemeverywhere.Bestnottousethemwhiledriving,though,exceptat
redlights.We’vetakenourseverywhere—thedoctor’soffice,restaurants,theaters,younameit.
Certificationstudygroupsareanotherexcellentresource,andyouwon’tfindalargerormorewilling
communitythanontheCoderanch.comBigMooseSalooncertificationforums.Ifyouhaveaquestion
fromthisbook,oranyothermockexamquestionyoumayhavestumbledupon,postingaquestionina
certificationforumwillgetyouananswerinnearlyallcaseswithinaday—usually,withinafewhours.
Finally,werecommendthatyouwritealotoflittleJavaprograms!Duringthecourseofwritingthis
book,wewrotehundredsofsmallprograms,andifyoulistentowhatthemostsuccessfulcandidatessay
(youknow,thoseguyswhogot98percent),theyalmostalwaysreportthattheywrotealotofcode.
SchedulingYourExam
YoucanpurchaseyourexamvoucherfromeitherOracleorPearsonVUE.VisitOracle.com(followthe
training/certificationlinks)orvisitPearsonVue.comforexamschedulingdetailsandlocationsoftest
centers.
ArrivingattheExam
Aswithanytest,you’llbetemptedtocramthenightbefore.Resistthattemptation.Youshouldknowthe
materialbythispoint,andifyou’regroggyinthemorning,youwon’trememberwhatyoustudiedanyway.
Getagoodnight’ssleep.
Arriveearlyforyourexam;itgivesyoutimetorelaxandreviewkeyfacts.Taketheopportunityto
reviewyournotes.Ifyougetburnedoutonstudying,youcanusuallystartyourexamafewminutesearly.
Wedon’trecommendarrivinglate.Yourtestcouldbecancelled,oryoumightnothaveenoughtimeto
completetheexam.
Whenyouarriveatthetestingcenter,you’llneedtoprovidecurrent,validphotoidentification.Visit
PearsonVue.comfordetailsontheIDrequirements.Theyjustwanttobesurethatyoudon’tsendyour
brilliantJavagurunext-doorneighborwhoyou’vepaidtotaketheexamforyou.
Asidefromabrainfulloffacts,youdon’tneedtobringanythingelsetotheexamroom.Infact,your
brainisaboutallyou’reallowedtotakeintotheexam!
Allthetestsareclosedbook,meaningyoudon’tgettobringanyreferencematerialswithyou.You’re
alsonotallowedtotakeanynotesoutoftheexamroom.Thetestadministratorwillprovideyouwitha
smallmarkerboard.Ifyou’reallowedto,wedorecommendthatyoubringawaterbottleorajuicebottle
(callaheadfordetailsofwhat’sallowed).Theseexamsarelongandhard,andyourbrainfunctionsmuch
betterwhenit’swellhydrated.Intermsofhydration,theidealapproachistotakefrequent,smallsips.
Youshouldalsoverifyhowmany“bio-breaks”you’llbeallowedtotakeduringtheexam!
Leaveyourphoneinthecar.Itwillonlyaddstresstothesituation,sincetheyarenotallowedinthe
examroom,andcansometimesstillbeheardiftheyringoutsideoftheroom.Purses,books,andother
materialsmustbeleftwiththeadministratorbeforeenteringtheexam.
Onceinthetestingroom,you’llbebriefedontheexamsoftware.Youmightbeaskedtocompletea
survey.Thetimeyouspendonthesurveyisnotdeductedfromyouractualtesttime—nordoyougetmore
timeifyoufilloutthesurveyquickly.Also,rememberthequestionsyougetontheexamwillnotchange
dependingonhowyouanswerthesurveyquestions.Onceyou’redonewiththesurvey,therealclock
startstickingandthefunbegins.
Thetestingsoftwareallowsyoutomoveforwardandbackwardbetweenquestions.Mostimportant,
thereisaMarkcheckboxonthescreen—thiswillprovetobeacriticaltool,asexplainedinthenext
section.
Test-TakingTechniques
Withoutaplanofattack,candidatescanbecomeoverwhelmedbytheexamorbecomesidetrackedand
runoutoftime.Forthemostpart,ifyouarecomfortablewiththematerial,theallottedtimeismorethan
enoughtocompletetheexam.Thetrickistokeepthetimefromslippingawayduringanyoneparticular
problem.
Yourobviousgoalistoanswerthequestionscorrectlyandquickly,butotherfactorscandistractyou.
Herearesometipsfortakingtheexammoreefficiently.
SizeUptheChallenge
First,takeaquickpassthroughallthequestionsintheexam.“Cherry-pick”theeasyquestions,answering
themonthespot.Brieflyreadeachquestion,noticingthetypeofquestionandthesubject.Asaguideline,
trytospendlessthan25percentofyourtestingtimeinthispass.
Thisstepletsyouassessthescopeandcomplexityoftheexam,andithelpsyoudeterminehowtopace
yourtime.Italsogivesyouanideaofwheretofindpotentialanswerstosomeofthequestions.
Sometimesthewordingofonequestionmightlendcluesorjogyourthoughtsforanotherquestion.
Ifyou’renotentirelyconfidentinyouranswertoaquestion,answeritanyway,butchecktheMarkbox
toflagitforlaterreview.Intheeventyourunoutoftime,atleastyou’veprovideda“firstguess”answer,
ratherthanleavingitblank.
Second,gobackthroughtheentiretest,usingtheinsightyougainedfromthefirstgo-through.For
example,iftheentiretestlooksdifficult,you’llknowbetterthantospendmorethanaminuteortwoon
eachquestion.Createapacingwithsmallmilestones—forexample,“Ineedtoanswer10questionsevery
15minutes.”
Atthisstage,it’sprobablyagoodideatoskippastthetime-consumingquestions,markingthemforthe
nextpass.Trytofinishthisphasebeforeyou’re50to60percentthroughthetestingtime.
Third,gobackthroughallthequestionsyoumarkedforreview,usingtheReviewMarkedbuttoninthe
questionreviewscreen.Thisstepincludestakingasecondlookatallthequestionsyouwereunsureofin
previouspasses,aswellastacklingthetime-consumingonesyoudeferreduntilnow.Chiselawayatthis
groupofquestionsuntilyou’veansweredthemall.
Ifyou’remorecomfortablewithapreviouslymarkedquestion,unmarktheReviewMarkedbutton
now.Otherwise,leaveitmarked.Workyourwaythroughthetime-consumingquestionsnow,especially
thoserequiringmanualcalculations.Unmarkthemwhenyou’resatisfiedwiththeanswer.
Bytheendofthisstep,you’veansweredeveryquestioninthetest,despitehavingreservationsabout
someofyouranswers.Ifyourunoutoftimeinthenextstep,atleastyouwon’tlosepointsforlackofan
answer.You’reingreatshapeifyoustillhave10to20percentofyourtimeremaining.
ReviewYourAnswers
Nowyou’recruising!You’veansweredallthequestions,andyou’rereadytodoaqualitycheck.Takeyet
anotherpass(yes,onemore)throughtheentiretest,brieflyre-readingeachquestionandyouranswer.
Carefullylookoverthequestionsagaintocheckfor“trick”questions.Beparticularlywaryofthose
thatincludeachoiceof“Doesnotcompile.”Bealertforlast-minuteclues.You’reprettyfamiliarwith
nearlyeveryquestionatthispoint,andyoumayfindafewcluesthatyoumissedbefore.
TheGrandFinale
Whenyou’reconfidentwithallyouranswers,finishtheexambysubmittingitforgrading.Afteryoufinish
yourexam,you’llreceiveane-mailfromOraclegivingyoualinktoapagewhereyourexamresultswill
beavailable.Asofthiswriting,youmustaskforahardcopycertificatespecificallyoronewillnotbe
senttoyou.
Retesting
Ifyoudon’tpasstheexam,don’tbediscouraged.Trytohaveagoodattitudeabouttheexperience,andget
readytotryagain.Consideryourselfalittlemoreeducated.You’llknowtheformatofthetestalittle
better,andyou’llhaveagoodideaofthedifficultylevelofthequestionsyou’llgetnexttimearound.
Ifyoubouncebackquickly,you’llprobablyrememberseveralofthequestionsyoumighthavemissed.
Thiswillhelpyoufocusyourstudyeffortsintherightarea.
Ultimately,rememberthatOraclecertificationsarevaluablebecausethey’rehardtoget.Afterall,if
anyonecouldgetone,whatvaluewouldithave?Intheend,ittakesagoodattitudeandalotofstudying,
butyoucandoit!
ObjectivesMap
Thefollowingtabledescribestheexamobjectivesandwhereyouwillfindcoveragefortheminthe
book.(Note:WehavesummarizedsomeofthedescriptionsyouwillfindontheOracle.comwebsite.)
OracleCertifiedAssociateJavaSE8Programmer(Exam1Z0-808)
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1
DeclarationsandAccessControl
CERTIFICATIONOBJECTIVES
•JavaFeaturesandBenefits
•IdentifiersandKeywords
•javac,java,main(),andImports
•DeclareClassesandInterfaces
•DeclareClassMembers
•DeclareConstructorsandArrays
•CreatestaticClassMembers
•Useenums
Two-MinuteDrill
Q&ASelfTest
eassumethatbecauseyou’replanningonbecomingcertified,youalreadyknowthe
basicsofJava.Ifyou’recompletelynewtothelanguage,thischapter—andtherestofthe
book—willbeconfusing;sobesureyouknowatleastthebasicsofthelanguagebefore
divingintothisbook.Thatsaid,we’restartingwithabrief,high-levelrefreshertoput
youbackintheJavamood,incaseyou’vebeenawayforawhile.
JavaRefresher
AJavaprogramismostlyacollectionofobjectstalkingtootherobjectsbyinvokingeachother’s
methods.Everyobjectisofacertaintype,andthattypeisdefinedbyaclassoraninterface.MostJava
programsuseacollectionofobjectsofmanydifferenttypes.Followingisalistofafewusefultermsfor
thisobject-oriented(OO)language:
ClassAtemplatethatdescribesthekindsofstateandbehaviorthatobjectsofitstypesupport.
ObjectAtruntime,whentheJavaVirtualMachine(JVM)encountersthenewkeyword,itwill
usetheappropriateclasstomakeanobjectthatisaninstanceofthatclass.Thatobjectwillhave
itsownstateandaccesstoallofthebehaviorsdefinedbyitsclass.
State(instancevariables)Eachobject(instanceofaclass)willhaveitsownuniquesetof
instancevariablesasdefinedintheclass.Collectively,thevaluesassignedtoanobject’sinstance
variablesmakeuptheobject’sstate.
Behavior(methods)Whenaprogrammercreatesaclass,shecreatesmethodsforthatclass.
Methodsarewheretheclass’slogicisstoredandwheretherealworkgetsdone.Theyarewhere
algorithmsgetexecutedanddatagetsmanipulated.
IdentifiersandKeywords
AlltheJavacomponentswejusttalkedabout—classes,variables,andmethods—neednames.InJava,
thesenamesarecalledidentifiers,and,asyoumightexpect,therearerulesforwhatconstitutesalegal
Javaidentifier.Beyondwhat’slegal,though,Java(andOracle)programmershavecreatedconventions
fornamingmethods,variables,andclasses.
Likeallprogramminglanguages,Javahasasetofbuilt-inkeywords.Thesekeywordsmustnotbeused
asidentifiers.Laterinthischapterwe’llreviewthedetailsofthesenamingrules,conventions,andthe
Javakeywords.
Inheritance
CentraltoJavaandotherOOlanguagesistheconceptofinheritance,whichallowscodedefinedinone
classorinterfacetobereusedinotherclasses.InJava,youcandefineageneral(moreabstract)
superclassandthenextenditwithmorespecificsubclasses.Thesuperclassknowsnothingoftheclasses
thatinheritfromit,butallofthesubclassesthatinheritfromthesuperclassmustexplicitlydeclarethe
inheritancerelationship.Asubclassthatinheritsfromasuperclassisautomaticallygivenaccessible
instancevariablesandmethodsdefinedbythesuperclass,butthesubclassisalsofreetooverride
superclassmethodstodefinemorespecificbehavior.Forexample,aCarsuperclasscoulddefinegeneral
methodscommontoallautomobiles,butaFerrarisubclasscouldoverridetheaccelerate()method
thatwasalreadydefinedintheCarclass.
Interfaces
Apowerfulcompaniontoinheritanceistheuseofinterfaces.Interfacesareusuallylikea100percent
abstractsuperclassthatdefinesthemethodsasubclassmustsupport,butnothowtheymustbesupported.
Inotherwords,forexample,anAnimalinterfacemightdeclarethatallAnimalimplementationclasses
haveaneat()method,buttheAnimalinterfacedoesn’tsupplyanylogicfortheeat()method.That
meansit’suptotheclassesthatimplementtheAnimalinterfacetodefinetheactualcodeforhowthat
particularAnimaltypebehaveswhenitseat()methodisinvoked.Note:AsofJava8,interfacescan
nowincludeconcrete,inheritablemethods.WewilltalkmuchmoreaboutthiswhenwediveintoOOin
thenextchapter.
CERTIFICATIONOBJECTIVE
FeaturesandBenefitsofJava(OCAObjective1.5)
1.5CompareandcontrastthefeaturesandcomponentsofJavasuchas:platformindependence,object
orientation,encapsulation,etc.
Perhapsagreattopictostartwith,onourofficialcoverageoftheOCA8,istodiscussthevarious
benefitsthatJavaprovidestoprogrammers.Javaisnowover20yearsold(wow!)andremainsoneofthe
mostin-demandprogramminglanguagesintheworld.Somewhatconfusinglythereisasimilarlynamed
language,“JavaScript”(animplementationoftheECMAstandard),whichisalsoaverypopular
language.JavaandJavaScripthavesomeaspectsincommon,buttheyarenottobeconfused;theyare
quitedistinct.Let’slookatsomeofthebenefitsthatJavaprovidestoprogrammersandcomparethem
(whenappropriate)tosomeofJava’scompetitors.Acaveathere,manyofthesebenefitsarebasedon
extremelycomplextopics.Thesedescriptionsarebynomeansdefinitive,butthey’resufficientforthe
exam:
ObjectorientedAssoftwaresystemsgetlarger,theygetmoredifficulttotestandenhance.
Forthelastseveraldecades,object-oriented(OO)programminghasbeenthedominantsoftware
designapproachforlargesystems,becausewell-designedOOsystemsremaintestableand
enhanceable,evenastheygrowintohugeapplicationswithmillionsoflinesofcode.OOdesign
alsooffersanaturalwaytothinkabouthowthecomponentsinasystemshouldbeconstructedand
howtheyshouldinteract.Theclasses,objects,systemstate,andbehaviorsinwell-designedOO
systemsareeasytomapconceptuallytotheircounterpartsintherealworld.
EncapsulationEncapsulationisakeyconceptinOOprogramming.Encapsulationallowsa
softwarecomponenttohideitsdatafromothercomponents,protectingthedatafrombeingupdated
withoutthecomponent’sapprovalorknowledge.Javamakesencapsulationfareasiertoachieve
thaninnon-OOlanguages.
MemorymanagementUnlikesomeofitscompetitors(CandC++),Javaprovidesautomatic
memorymanagement.Inlanguagesthatdon’tprovideautomaticmemorymanagement,keepingtrack
ofmemorythroughpointersisquitecomplex.Further,trackingdownbugsrelatedtomemory
management(oftencalledmemoryleaks)isacommon,error-prone,andtime-consumingprocess.
HugelibraryJavahasanenormouslibraryofprewritten,well-tested,andwell-supported
code.ThiscodeiseasytoincludeinyourJavaapplicationsandiswelldocumentedviatheJava
API.Throughoutthisbookwewillexploresomeofthemostused(andmostuseful)membersof
Java’sstandardcorelibrary.
SecurebydesignWhencompiledJavacodeisexecuted,itrunsinsidetheJavaVirtual
Machine(JVM).TheJVMprovidesasecure“sandbox”foryourJavacodetorunin,andtheJVM
makessurethatnefariousprogrammerscannotwriteJavacodethatwillcausetroubleonother
people’smachineswhenitruns.
Writeonce,runanywhere(cross-platformexecution)Oneofthegoals(largely,butnot
perfectlyachieved)ofJavaisthatmuchoftheJavacodeyouwritecanrunonmanyplatforms,
rangingfromtinyInternet-of-Things(IoT)devices,tophones,tolaptopcomputers,tolargeservers.
Anothercommonphraseforthisabilitytorunonmanydevicesiscross-platform.
StronglytypedAstronglytypedlanguageusuallyrequirestheprogrammertoexplicitly
declarethetypesofthedataandobjectsbeingusedintheprogram.StrongtypingallowstheJava
compilertocatchmanypotentialprogrammingerrorsbeforeyourcodeevencompiles.Attheother
endofthespectrumaredynamicallytypedlanguages.Dynamicallytypedlanguagescanbeless
verbose,fastertocodeinitially,andareoftenpreferredinenvironmentswheresmallteamsand
rapidprototypingarethenorm.ButstronglytypedlanguageslikeJavacomeintotheirowninlarge
softwareshopswithmanyteamsofprogrammersandtheneedformorereliable,testable,
production-qualitycode.
MultithreadedJavaprovidesbuilt-inlanguagefeaturesandAPIsthatallowprogramstouse
manyoperating-systemprocesses(hence,many“cores”)atthesametime.Assystemsgrowto
handlemorecomputationallyintensiveproblemsandlargerdatasets,theabilitytouseallofa
computer’scoreprocessorsbecomesessential.Multithreadedprogrammingisneversimple,but
Javaprovidesarichtoolkittomakeitaseasyaspossible.
DistributedcomputingAnotherwaytotacklebigprogrammingproblemsistodistributethe
workloadacrossmanymachines.TheJavaAPIprovidesseveralwaystosimplifytasksrelatedto
distributedcomputing.Onesuchexampleisserialization,aprocessinwhichaJavaobjectis
convertedtoaportableform.Serializedobjectscanbesenttoothermachines,deserialized,and
thenusedasanormalJavaobject.
Again,we’vejustscratchedthesurfaceofthesecomplextopics,butifyouunderstandthesebrief
descriptions,youshouldbepreparedtohandleanyquestionsforthisobjective.Somuchforthetheory,
let’sgetintodetails…
CERTIFICATIONOBJECTIVE
IdentifiersandKeywords(OCAObjectives1.2and2.1)
1.2DefinethestructureofaJavaclass.
2.1Declareandinitializevariables(includingcastingofprimitivedatatypes).
RememberthatwhenwelistoneormoreCertificationObjectivesinthebook,aswejustdid,itmeans
thatthefollowingsectioncoversatleastsomepartofthatobjective.Someobjectiveswillbecoveredin
severaldifferentchapters,soyou’llseethesameobjectiveinmorethanoneplaceinthebook.For
example,thissectioncoversdeclarationsandidentifiers,butusingthethingsyoudeclareiscovered
primarilyinlaterchapters.
So,we’llstartwithJavaidentifiers.ThetwoaspectsofJavaidentifiersthatwecoverhereare
LegalidentifiersTherulesthecompilerusestodeterminewhetheranameislegal.
Oracle’sJavaCodeConventionsOracle’srecommendationsfornamingclasses,variables,
andmethods.Wetypicallyadheretothesestandardsthroughoutthebook,exceptwhenwe’retrying
toshowyouhowatrickyexamquestionmightbecoded.Youwon’tbeaskedquestionsaboutthe
JavaCodeConventions,butwestronglyrecommendyouusethem.
LegalIdentifiers
Technically,legalidentifiersmustbecomposedofonlyUnicodecharacters,numbers,currencysymbols,
andconnectingcharacters(suchasunderscores).Theexamdoesn’tdiveintothedetailsofwhichranges
oftheUnicodecharactersetqualifyaslettersanddigits.So,forexample,youwon’tneedtoknowthat
Tibetandigitsrangefrom\u0420to\u0f29.Herearetherulesyoudoneedtoknow:
Identifiersmuststartwithaletter,acurrencycharacter($),oraconnectingcharactersuchas
theunderscore(_).Identifierscannotstartwithadigit!
Afterthefirstcharacter,identifierscancontainanycombinationofletters,currency
characters,connectingcharacters,ornumbers.
Inpractice,thereisnolimittothenumberofcharactersanidentifiercancontain.
Youcan’tuseaJavakeywordasanidentifier.Table1-1listsalltheJavakeywords.
TABLE1-1 CompleteListofJavaKeywords(assertaddedin1.4,enumaddedin1.5)
IdentifiersinJavaarecasesensitive;fooandFOOaretwodifferentidentifiers.
Examplesoflegalandillegalidentifiersfollow.Firstsomelegalidentifiers:
Thefollowingareillegal(it’syourjobtorecognizewhy):
Oracle’sJavaCodeConventions
Oracleestimatesthatoverthelifetimeofastandardpieceofcode,20percentoftheeffortwillgointothe
originalcreationandtestingofthecode,and80percentoftheeffortwillgointothesubsequent
maintenanceandenhancementofthecode.Agreeingon,andcodingto,asetofcodestandardshelpsto
reducetheeffortinvolvedintesting,maintaining,andenhancinganypieceofcode.Oraclehascreateda
setofcodingstandardsforJavaandpublishedthosestandardsinadocumentcleverlytitled“JavaCode
Conventions,”whichyoucanfindifyoustartatjava.oracle.com.It’sagreatdocument,short,andeasy
toread,andwerecommendithighly.
Thatsaid,you’llfindthatmanyofthequestionsintheexamdon’tfollowthecodeconventionsbecause
ofthelimitationsinthetestenginethatisusedtodelivertheexaminternationally.Oneofthegreatthings
abouttheOraclecertificationsisthattheexamsareadministereduniformlythroughouttheworld.To
achievethat,thecodelistingsthatyou’llseeintherealexamareoftenquitecrampedanddonotfollow
Oracle’scodestandards.Totoughenyouupfortheexam,we’lloftenpresentcodelistingsthathavea
similarlycrampedlookandfeel,oftenindentingourcodeonlytwospacesasopposedtotheOracle
standardoffour.
We’llalsojamourcurlybracestogetherunnaturally,andwe’llsometimesputseveralstatementsonthe
sameline…ouch!Forexample:
Consideryourselfforewarned—you’llseelotsofcodelistings,mockquestions,andrealexam
questionsthatarethissickandtwisted.Nobodywantsyoutowriteyourcodelikethis—notyour
employer,notyourcoworkers,notus,notOracle,andnottheexamcreationteam!Codelikethiswas
createdonlysothatcomplexconceptscouldbetestedwithinauniversaltestingtool.Theonlystandards
thatarefollowedasmuchaspossibleintherealexamarethenamingstandards.Herearethenaming
standardsthatOraclerecommendsandthatweuseintheexamandinmostofthebook:
ClassesandinterfacesThefirstlettershouldbecapitalized,andifseveralwordsarelinked
togethertoformthename,thefirstletteroftheinnerwordsshouldbeuppercase(aformatthat’s
sometimescalled“CamelCase”).Forclasses,thenamesshouldtypicallybenouns.Herearesome
examples:
Dog
Account
PrintWriter
Forinterfaces,thenamesshouldtypicallybeadjectives,likethese:
Runnable
Serializable
MethodsThefirstlettershouldbelowercase,andthennormalCamelCaserulesshouldbe
used.Inaddition,thenamesshouldtypicallybeverb-nounpairs.Forexample:
getBalance
doCalculation
setCustomerName
VariablesLikemethods,theCamelCaseformatshouldbeused,butstartingwithalowercase
letter.Oraclerecommendsshort,meaningfulnames,whichsoundsgoodtous.Someexamples:
buttonWidth
accountBalance
myString
ConstantsJavaconstantsarecreatedbymarkingvariablesstaticandfinal.Theyshouldbe
namedusinguppercaseletterswithunderscorecharactersasseparators:
MIN_HEIGHT
CERTIFICATIONOBJECTIVE
DefineClasses(OCAObjectives1.2,1.3,1.4,6.4,and7.5)
1.2DefinethestructureofaJavaclass.
1.3CreateexecutableJavaapplicationswithamainmethod;runaJavaprogramfromthecommand
line;includingconsoleoutput.(sic)
1.4ImportotherJavapackagestomakethemaccessibleinyourcode.
6.4Applyaccessmodifiers.
7.5Useabstractclassesandinterfaces.
WhenyouwritecodeinJava,you’rewritingclassesorinterfaces.Withinthoseclasses,asyouknow,
arevariablesandmethods(plusafewotherthings).Howyoudeclareyourclasses,methods,and
variablesdramaticallyaffectsyourcode’sbehavior.Forexample,apublicmethodcanbeaccessedfrom
coderunninganywhereinyourapplication.Markthatmethodprivate,though,anditvanishesfrom
everyone’sradar(excepttheclassinwhichitwasdeclared).
Forthisobjective,we’llstudythewaysinwhichyoucandeclareandmodify(ornot)aclass.You’ll
findthatwecovermodifiersinanextremelevelofdetail,andalthoughweknowyou’realreadyfamiliar
withthem,we’restartingfromtheverybeginning.MostJavaprogrammersthinktheyknowhowallthe
modifierswork,butoncloserstudytheyoftenfindoutthattheydon’t(atleastnottothedegreeneededfor
theexam).Subtledistinctionsareeverywhere,soyouneedtobeabsolutelycertainyou’recompletely
solidoneverythinginthissection’sobjectivesbeforetakingtheexam.
SourceFileDeclarationRules
Beforewedigintoclassdeclarations,let’sdoaquickreviewoftherulesassociatedwithdeclaring
classes,importstatements,andpackagestatementsinasourcefile:
Therecanbeonlyonepublicclasspersourcecodefile.
Commentscanappearatthebeginningorendofanylineinthesourcecodefile;theyare
independentofanyofthepositioningrulesdiscussedhere.
Ifthereisapublicclassinafile,thenameofthefilemustmatchthenameofthepublic
class.Forexample,aclassdeclaredaspublicclassDog{}mustbeinasourcecodefile
namedDog.java.
Iftheclassispartofapackage,thepackagestatementmustbethefirstlineinthesource
codefile,beforeanyimportstatementsthatmaybepresent.
Ifthereareimportstatements,theymustgobetweenthepackagestatement(ifthereisone)
andtheclassdeclaration.Ifthereisn’tapackagestatement,thentheimportstatement(s)mustbe
thefirstline(s)inthesourcecodefile.Iftherearenopackageorimportstatements,theclass
declarationmustbethefirstlineinthesourcecodefile.
importandpackagestatementsapplytoallclasseswithinasourcecodefile.Inother
words,there’snowaytodeclaremultipleclassesinafileandhavethemindifferentpackagesor
usedifferentimports.
Afilecanhavemorethanonenon-publicclass.
Fileswithnopublicclassescanhaveanamethatdoesnotmatchanyoftheclassesinthe
file.
UsingthejavacandjavaCommands
Inthisbook,we’regoingtotalkaboutinvokingthejavacandjavacommandsabout1000times.
Althoughintherealworldyou’llprobablyuseanintegrateddevelopmentenvironment(IDE)mostofthe
time,youcouldseeafewquestionsontheexamthatusethecommandlineinstead,sowe’regoingto
reviewthebasics.(Bytheway,wedidNOTuseanIDEwhilewritingthisbook.Westillhaveaslight
preferenceforthecommandlinewhilestudyingfortheexam;allIDEsdotheirbesttobe“helpful,”and
sometimesthey’llfixyourproblemswithouttellingyou.That’sniceonthejob,butmaybenotsogreat
whenyou’restudyingforacertificationexam!)
Compilingwithjavac
ThejavaccommandisusedtoinvokeJava’scompiler.Youcanspecifymanyoptionswhenrunning
javac.Forexample,thereareoptionstogeneratedebugginginformationorcompilerwarnings.Here’s
thestructuraloverviewforjavac:
javac[options][sourcefiles]
Thereareadditionalcommand-lineoptionscalled@argfiles,butthey’rerarelyused,andyouwon’t
needtostudythemfortheexam.Boththe[options]andthe[sourcefiles]areoptionalpartsofthe
command,andbothallowmultipleentries.Thefollowingarelegaljavaccommands:
javac-help
javac-versionFoo.javaBar.java
Thefirstinvocationdoesn’tcompileanyfiles,butprintsasummaryofvalidoptions.Thesecond
invocationpassesthecompileranoption(-version,whichprintstheversionofthecompileryou’re
using)andpassesthecompilertwo.javafilestocompile(Foo.javaandBar.java).Wheneveryou
specifymultipleoptionsand/orfiles,theyshouldbeseparatedbyspaces.
LaunchingApplicationswithjava
ThejavacommandisusedtoinvoketheJavaVirtualMachine(JVM).Here’sthebasicstructureofthe
command:
java[options]class[args]
The[options]and[args]partsofthejavacommandareoptional,andtheycanbothhavemultiple
values.Youmustspecifyexactlyoneclassfiletoexecute,andthejavacommandassumesyou’retalking
abouta.classfile,soyoudon’tspecifythe.classextensiononthecommandline.Here’sanexample:
java-showversionMyClassx1
Thiscommandcanbeinterpretedas“ShowmetheversionoftheJVMbeingused,andthenlaunchthefile
namedMyClass.classandsendittwoStringargumentswhosevaluesarexand1.”Let’slookatthe
followingcode:
It’scompiledandtheninvokedasfollows:
javaMyClassx1
Theoutputwillbe
x1
We’llbegettingintoarraysindepthlater,butfornowit’senoughtoknowthatargs—likeallarrays—
useazero-basedindex.Inotherwords,thefirstcommand-lineargumentisassignedtoargs[0],the
secondargumentisassignedtoargs[1],andsoon.
Usingpublicstaticvoidmain(String[]args)
Theuseofthemain()methodisimpliedinmostofthequestionsontheexam,andontheOCAexamitis
specificallycovered.Forthe.0001percentofyouwhodon’tknow,main()isthemethodthattheJVM
usestostartexecutionofaJavaprogram.
Firstoff,it’simportantforyoutoknowthatnamingamethodmain()doesn’tgiveitthesuperpowers
wenormallyassociatewithmain().AsfarasthecompilerandtheJVMareconcerned,theonlyversion
ofmain()withsuperpowersisthemain()withthissignature:
publicstaticvoidmain(String[]args)
Otherversionsofmain()withothersignaturesareperfectlylegal,butthey’retreatedasnormalmethods.
Thereissomeflexibilityinthedeclarationofthe“special”main()method(theoneusedtostartaJava
application):theorderofitsmodifierscanbealteredalittle;theStringarraydoesn’thavetobenamed
args;anditcanbedeclaredusingvar-argssyntax.Thefollowingarealllegaldeclarationsforthe
“special”main():
FortheOCA8exam,theonlyotherthingthat’simportantforyoutoknowisthatmain()canbe
overloaded.We’llcoveroverloadingindetailinthenextchapter.
ImportStatementsandtheJavaAPI
ThereareagazillionJavaclassesintheworld.TheJavaAPIhasthousandsofclasses,andtheJava
communityhaswrittentherest.We’llgooutonalimbandcontendthatallJavaprogrammerseverywhere
useacombinationofclassestheywroteandclassesthatotherprogrammerswrote.Supposewecreated
thefollowing:
Thisisaperfectlylegalclass,butasitturnsout,oneofthemostcommonlyusedclassesintheJavaAPI
isalsonamedArrayList,orsoitseems….TheAPIversion’sactualnameisjava.util.ArrayList.
That’sitsfullyqualifiedname.TheuseoffullyqualifiednamesiswhathelpsJavadevelopersmakesure
thattwoversionsofaclasslikeArrayListdon’tgetconfused.Sonowlet’ssaythatIwanttousethe
ArrayListclassfromtheAPI:
(Firstoff,trustusonthe<String>syntax;we’llgettothatlater.)Althoughthisislegal,it’salsoaLOT
ofkeystrokes.Sinceweprogrammersarebasicallylazy(there,wesaidit),weliketouseotherpeople’s
classesaLOT,ANDwehatetotype.Ifwehadalargeprogram,wemightendupusingArrayListsmany
times.
importstatementstotherescue!Insteadoftheprecedingcode,ourclasscouldlooklikethis:
Wecaninterprettheimportstatementassaying,“IntheJavaAPIthereisapackagecalled‘util’,and
inthatpackageisaclasscalled‘ArrayList’.Wheneveryouseetheword‘ArrayList’inthisclass,it’s
justshorthandfor:‘java.util.ArrayList’.”(Note:Lotsmoreonpackagestocome!)Ifyou’reaC
programmer,youmightthinkthattheimportstatementissimilartoan#include.Notreally.AllaJava
importstatementdoesissaveyousometyping.That’sit.
Aswejustimplied,apackagetypicallyhasmanyclasses.Theimportstatementoffersyetanother
keystroke-savingcapability.Let’ssayyouwantedtouseafewdifferentclassesfromthejava.util
package:ArrayListandTreeSet.Youcanaddawildcardcharacter(*)toyourimportstatementthat
means,“Ifyouseeareferencetoaclassyou’renotsureof,youcanlookthroughtheentirepackagefor
thatclass,”likeso:
WhenthecompilerandtheJVMseethiscode,they’llknowtolookthroughjava.utilforArrayList
andTreeSet.Fortheexam,thelastthingyou’llneedtorememberaboutusingimportstatementsinyour
classesisthatyou’refreetomixandmatch.It’sokaytosaythis:
StaticImportStatements
DearReader,Wereallystruggledwithwheretoincludethisdiscussionofstaticimports.Fromalearning
perspective,thisisprobablynottheideallocation,butfromareferenceperspective,wethoughtitmade
sense.Asyou’relearningthematerialforthefirsttime,youmightbeconfusedbysomeoftheideasinthis
section.Ifthat’sthecase,weapologize.Putastickynoteonthispageandcirclebackaroundafteryou’re
finishedwithChapter3.Ontheotherhand,onceyou’repastthelearningstageandyou’reusingthisbook
asareference,wethinkputtingthissectionherewillbequiteuseful.Now,ontostaticimports.
Sometimesclasseswillcontainstaticmembers.(We’lltalkmoreaboutstaticclassmemberslater,but
sincewe’reonthetopicofimportswethoughtwe’dtossinstaticimportsnow.)Staticclassmemberscan
existintheclassesyouwriteandinalotoftheclassesintheJavaAPI.
Aswesaidearlier,ultimatelytheonlyvalueimportstatementshaveisthattheysavetypingandthey
canmakeyourcodeeasiertoread.InJava5(alongtimeago),theimportstatementwasenhancedto
provideevengreaterkeystroke-reductioncapabilities,althoughsomewouldarguethatthiscomesatthe
expenseofreadability.Thisfeatureisknownasstaticimports.Staticimportscanbeusedwhenyouwant
to“savetyping”whileusingaclass’sstaticmembers.(YoucanusethisfeatureonclassesintheAPIand
onyourownclasses.)Here’sa“beforeandafter”exampleusingafewstaticclassmembersprovidedby
acommonlyusedclassintheJavaAPI,java.lang.Integer.Thisexamplealsousesastaticmember
thatyou’veusedathousandtimes,probablywithoutevergivingitmuchthought;theoutfieldinthe
Systemclass.
Beforestaticimports:
Afterstaticimports:
Bothclassesproducethesameoutput:
2147483647
2a
Let’slookatwhat’shappeninginthecodethat’susingthestaticimportfeature:
1.Eventhoughthefeatureiscommonlycalled“staticimport,”thesyntaxMUSTbeimport
staticfollowedbythefullyqualifiednameofthestaticmemberyouwanttoimport,ora
wildcard.Inthiscase,we’redoingastaticimportontheSystemclassoutobject.
2.Inthiscase,wemightwanttouseseveralofthestaticmembersofthejava.lang.Integer
class.Thisstaticimportstatementusesthewildcardtosay,“IwanttodostaticimportsofALLthe
staticmembersinthisclass.”
3.Nowwe’refinallyseeingthebenefitofthestaticimportfeature!Wedidn’thavetotypethe
SysteminSystem.out.println!Wow!Second,wedidn’thavetotypetheIntegerin
Integer.MAX_VALUE.Sointhislineofcodewewereabletouseashortcutforastaticmethod
ANDaconstant.
4.Finally,wedoonemoreshortcut,thistimeforamethodintheIntegerclass.
We’vebeenalittlesarcasticaboutthisfeature,butwe’renottheonlyones.We’renotconvincedthat
savingafewkeystrokesisworthpossiblymakingthecodealittlehardertoread,butenoughdevelopers
requesteditthatitwasaddedtothelanguage.
Hereareacoupleofrulesforusingstaticimports:
Youmustsayimportstatic;youcan’tsaystaticimport.
Watchoutforambiguouslynamedstaticmembers.Forinstance,ifyoudoastaticimportfor
boththeIntegerclassandtheLongclass,referringtoMAX_VALUEwillcauseacompilererror,
becausebothIntegerandLonghaveaMAX_VALUEconstantandJavawon’tknowwhich
MAX_VALUEyou’rereferringto.
Youcandoastaticimportonstaticobjectreferences,constants(rememberthey’restatic
andfinal),andstaticmethods.
Asyou’veseen,whenusingimportandimportstaticstatements,sometimesyoucanusethe
wildcardcharacter*todosomesimplesearchingforyou.(Youcansearchwithinapackageor
withinaclass.)Asyousawearlier,ifyouwantto"searchthroughthejava.utilpackagefor
classnames,"youcansaythis:
Inasimilarvein,ifyouwantto"searchthroughthejava.lang.Integerclassforstatic
members,"youcansaythis:
Butyoucan’tcreatebroadersearches.Forinstance,youCANNOTuseanimporttosearch
throughtheentireJavaAPI:
ClassDeclarationsandModifiers
Theclassdeclarationswe’lldiscussinthissectionarelimitedtotop-levelclasses.Inadditiontotop-
levelclasses,Javaprovidesforanothercategoryofclassknownasnestedclassesorinnerclasses.Inner
classesareincludedontheOCPexam,butnottheOCAexam.WhenyoubecomeanOCPcandidate,
you’regoingtolovelearningaboutinnerclasses.No,really.Seriously.
Thefollowingcodeisabare-bonesclassdeclaration:
classMyClass{}
Thiscodecompilesjustfine,butyoucanalsoaddmodifiersbeforetheclassdeclaration.Ingeneral,
modifiersfallintotwocategories:
Accessmodifiers(public,protected,private)
Nonaccessmodifiers(includingstrictfp,final,andabstract)
We’lllookataccessmodifiersfirst,soyou’lllearnhowtorestrictorallowaccesstoaclassyou
create.AccesscontrolinJavaisalittletrickybecausetherearefouraccesscontrols(levelsofaccess)
butonlythreeaccessmodifiers.Thefourthaccesscontrollevel(calleddefaultorpackageaccess)is
whatyougetwhenyoudon’tuseanyofthethreeaccessmodifiers.Inotherwords,everyclass,method,
andinstancevariableyoudeclarehasanaccesscontrol,whetheryouexplicitlytypeoneornot.Although
allfouraccesscontrols(whichmeansallthreemodifiers)workformostmethodandvariable
declarations,aclasscanbedeclaredwithonlypublicordefaultaccess;theothertwoaccesscontrol
levelsdon’tmakesenseforaclass,asyou’llsee.
Javaisapackage-centriclanguage;thedevelopersassumedthatforgoodorganizationandname
scoping,youwouldputallyourclassesintopackages.Theywereright,andyoushould.Imaginethis
nightmare:Threedifferentprogrammers,inthesamecompanybutworkingondifferentpartsofa
project,writeaclassnamedUtilities.IfthosethreeUtilitiesclasseshavenotbeendeclaredin
anyexplicitpackageandareintheclasspath,youwon’thaveanywaytotellthecompilerorJVM
whichofthethreeyou’retryingtoreference.Oraclerecommendsthatdevelopersusereverse
domainnames,appendedwithdivisionand/orprojectnames.Forexample,ifyourdomainnameis
geeksanonymous.comandyou’reworkingontheclientcodefortheTwelvePointOStepsprogram,
youwouldnameyourpackagesomethinglikecom.geeksanonymous.steps.client.Thatwould
essentiallychangethenameofyourclasstocom.geeksanonymous.steps.client.Utilities.You
mightstillhavenamecollisionswithinyourcompanyifyoudon’tcomeupwithyourownnaming
schemes,butyou’reguaranteednottocollidewithclassesdevelopedoutsideyourcompany
(assumingtheyfollowOracle’snamingconvention,andiftheydon’t,well,ReallyBadThingscould
happen).
ClassAccess
Whatdoesitmeantoaccessaclass?Whenwesaycodefromoneclass(classA)hasaccesstoanother
class(classB),itmeansclassAcandooneofthreethings:
CreateaninstanceofclassB.
ExtendclassB(inotherwords,becomeasubclassofclassB).
AccesscertainmethodsandvariableswithinclassB,dependingontheaccesscontrolofthose
methodsandvariables.
Ineffect,accessmeansvisibility.IfclassAcan’tseeclassB,theaccesslevelofthemethodsand
variableswithinclassBwon’tmatter;classAwon’thaveanywaytoaccessthosemethodsand
variables.
DefaultAccessAclasswithdefaultaccesshasnomodifierprecedingitinthedeclaration!It’sthe
accesscontrolyougetwhenyoudon’ttypeamodifierintheclassdeclaration.Thinkofdefaultaccessas
package-levelaccess,becauseaclasswithdefaultaccesscanbeseenonlybyclasseswithinthesame
package.Forexample,ifclassAandclassBareindifferentpackages,andclassAhasdefaultaccess,
classBwon’tbeabletocreateaninstanceofclassAorevendeclareavariableorreturntypeofclassA.
Infact,classBhastopretendthatclassAdoesn’tevenexistorthecompilerwillcomplain.Lookatthe
followingsourcefile:
Asyoucansee,thesuperclass(Beverage)isinadifferentpackagefromthesubclass(Tea).The
importstatementatthetopoftheTeafileistrying(fingerscrossed)toimporttheBeverageclass.The
Beveragefilecompilesfine,butwhenwetrytocompiletheTeafile,wegetsomethinglikethis:
(Note:Forvariousreasons,theerrormessagesweshowthroughoutthisbookmightnotmatchthe
errormessagesyouget.Don’tworry,therealpointistounderstandwhenyou’reapttogetanerror
ofsomesort.)
Teawon’tcompilebecauseitssuperclass,Beverage,hasdefaultaccessandisinadifferentpackage.
Youcandooneoftwothingstomakethiswork.Youcouldputbothclassesinthesamepackage,oryou
coulddeclareBeverageaspublic,asthenextsectiondescribes.
Whenyouseeaquestionwithcomplexlogic,besuretolookattheaccessmodifiersfirst.Thatway,if
youspotanaccessviolation(forexample,aclassinpackageAtryingtoaccessadefaultclassinpackage
B),you’llknowthecodewon’tcompilesoyoudon’thavetobotherworkingthroughthelogic.It’snotas
ifyoudon’thaveanythingbettertodowithyourtimewhiletakingtheexam.Justchoosethe“Compilation
fails”answerandzoomontothenextquestion.
PublicAccess
Aclassdeclarationwiththepublickeywordgivesallclassesfromallpackagesaccesstothepublic
class.Inotherwords,allclassesintheJavaUniverse(JU)haveaccesstoapublicclass.Don’tforget,
though,thatifapublicclassyou’retryingtouseisinadifferentpackagefromtheclassyou’rewriting,
you’llstillneedtoimportthepublicclass.
Intheexamplefromtheprecedingsection,wemaynotwanttoplacethesubclassinthesamepackage
asthesuperclass.Tomakethecodework,weneedtoaddthekeywordpublicinfrontofthesuperclass
(Beverage)declaration,asfollows:
packagecert;
publicclassBeverage{}
ThischangestheBeverageclasssoitwillbevisibletoallclassesinallpackages.Theclasscannow
beinstantiatedfromallotherclasses,andanyclassisnowfreetosubclass(extendfrom)it—unless,that
is,theclassisalsomarkedwiththenonaccessmodifierfinal.Readon.
Other(Nonaccess)ClassModifiers
Youcanmodifyaclassdeclarationusingthekeywordfinal,abstract,orstrictfp.Thesemodifiers
areinadditiontowhateveraccesscontrolisontheclass,soyoucould,forexample,declareaclassas
bothpublicandfinal.Butyoucan’talwaysmixnonaccessmodifiers.You’refreetousestrictfpin
combinationwithfinal,forexample,butyoumustnever,ever,evermarkaclassasbothfinaland
abstract.You’llseewhyinthenexttwosections.
Youwon’tneedtoknowhowstrictfpworks,sowe’refocusingonlyonmodifyingaclassasfinal
orabstract.Fortheexam,youneedtoknowonlythatstrictfpisakeywordandcanbeusedtomodify
aclassoramethod,butneveravariable.Markingaclassasstrictfpmeansthatanymethodcodeinthe
classwillconformstrictlytotheIEEE754standardrulesforfloatingpoints.Withoutthatmodifier,
floatingpointsusedinthemethodsmightbehaveinaplatform-dependentway.Ifyoudon’tdeclareaclass
asstrictfp,youcanstillgetstrictfpbehavioronamethod-by-methodbasisbydeclaringamethodas
strictfp.Ifyoudon’tknowtheIEEE754standard,now’snotthetimetolearnit.Youhave,astheysay,
biggerfishtofry.
FinalClasses
Whenusedinaclassdeclaration,thefinalkeywordmeanstheclasscan’tbesubclassed.Inother
words,nootherclasscaneverextend(inheritfrom)afinalclass,andanyattemptstodosowillresult
inacompilererror.
Sowhywouldyouevermarkaclassfinal?Afterall,doesn’tthatviolatethewholeOOnotionof
inheritance?Youshouldmakeafinalclassonlyifyouneedanabsoluteguaranteethatnoneofthe
methodsinthatclasswilleverbeoverridden.Ifyou’redeeplydependentontheimplementationsof
certainmethods,thenusingfinalgivesyouthesecuritythatnobodycanchangetheimplementationout
fromunderyou.
You’llnoticemanyclassesintheJavacorelibrariesarefinal.Forexample,theStringclasscannot
besubclassed.Imaginethehavocifyoucouldn’tguaranteehowaStringobjectwouldworkonany
givensystemyourapplicationisrunningon!IfprogrammerswerefreetoextendtheStringclass(and
thussubstitutetheirnewStringsubclassinstanceswherejava.lang.Stringinstancesareexpected),
civilization—asweknowit—couldcollapse.Sousefinalforsafety,butonlywhenyou’recertainthat
yourfinalclasshasindeedsaidallthateverneedstobesaidinitsmethods.Markingaclassfinal
means,inessence,yourclasscan’teverbeimprovedupon,orevenspecialized,byanotherprogrammer.
There’sabenefittohavingnonfinalclassesinthisscenario:Imaginethatyoufindaproblemwitha
methodinaclassyou’reusing,butyoudon’thavethesourcecode.Soyoucan’tmodifythesourceto
improvethemethod,butyoucanextendtheclassandoverridethemethodinyournewsubclassand
substitutethesubclasseverywheretheoriginalsuperclassisexpected.Iftheclassisfinal,though,
you’restuck.
Let’smodifyourBeverageexamplebyplacingthekeywordfinalinthedeclaration:
Nowlet’strytocompiletheTeasubclass:
Inpractice,you’llalmostnevermakeafinalclass.AfinalclassobliteratesakeybenefitofOO—
extensibility.Unlessyouhaveaserioussafetyorsecurityissue,assumethatsomedayanotherprogrammer
willneedtoextendyourclass.Ifyoudon’t,thenextprogrammerforcedtomaintainyourcodewillhunt
youdownand<insertreallyscarything>.
AbstractClassesAnabstractclasscanneverbeinstantiated.Itssolepurpose,missioninlife,
raisond’être,istobeextended(subclassed).(Note,however,thatyoucancompileandexecutean
abstractclass,aslongasyoudon’ttrytomakeaninstanceofit.)Whymakeaclassifyoucan’tmake
objectsoutofit?Becausetheclassmightbejusttoo,well,abstract.Forexample,imagineyouhavea
classCarthathasgenericmethodscommontoallvehicles.Butyoudon’twantanyoneactuallycreatinga
genericabstractCarobject.Howwouldtheyinitializeitsstate?Whatcolorwoulditbe?Howmany
seats?Horsepower?All-wheeldrive?Ormoreimportantly,howwoulditbehave?Inotherwords,how
wouldthemethodsbeimplemented?
No,youneedprogrammerstoinstantiateactualcartypessuchasBMWBoxsterandSubaruOutback.
We’llbettheBoxsterownerwilltellyouhiscardoesthingstheSubarucando“onlyinitsdreams.”Take
alookatthefollowingabstractclass:
Theprecedingcodewillcompilefine.However,ifyoutrytoinstantiateaCarinanotherbodyofcode,
you’llgetacompilererrorsomethinglikethis:
Noticethatthemethodsmarkedabstractendinasemicolonratherthancurlybraces.
Lookforquestionswithamethoddeclarationthatendswithasemicolon,ratherthancurlybraces.If
themethodisinaclass—asopposedtoaninterface—thenboththemethodandtheclassmustbemarked
abstract.Youmightgetaquestionthataskshowyoucouldfixacodesamplethatincludesamethod
endinginasemicolonbutwithoutanabstractmodifierontheclassormethod.Inthatcase,youcould
eithermarkthemethodandclassabstractorchangethesemicolontocode(likeacurlybracepair).
Rememberthatifyouchangeamethodfromabstracttononabstract,don’tforgettochangethe
semicolonattheendofthemethoddeclarationintoacurlybracepair!
We’lllookatabstractmethodsinmoredetaillaterinthisobjective,butalwaysrememberthatif
evenasinglemethodisabstract,thewholeclassmustbedeclaredabstract.Oneabstractmethod
spoilsthewholebunch.Youcan,however,putnonabstractmethodsinanabstractclass.Forexample,
youmighthavemethodswithimplementationsthatshouldn’tchangefromCartypetoCartype,suchas
getColor()orsetPrice().Byputtingnonabstractmethodsinanabstractclass,yougiveallconcrete
subclasses(concretejustmeansnotabstract)inheritedmethodimplementations.Thegoodnewsthereis
thatconcretesubclassesgettoinheritfunctionalityandneedtoimplementonlythemethodsthatdefine
subclass-specificbehavior.
(Bytheway,ifyouthinkwemisusedraisond’êtreearlier,don’tsendane-mail.We’dliketoseeyou
workitintoaprogrammercertificationbook.)
Codingwithabstractclasstypes(includinginterfaces,discussedlaterinthischapter)letsyoutake
advantageofpolymorphismandgivesyouthegreatestdegreeofflexibilityandextensibility.You’lllearn
moreaboutpolymorphisminChapter2.
Youcan’tmarkaclassasbothabstractandfinal.Theyhavenearlyoppositemeanings.An
abstractclassmustbesubclassed,whereasafinalclassmustnotbesubclassed.Ifyouseethis
combinationofabstractandfinalmodifiersusedforaclassormethoddeclaration,thecodewillnot
compile.
EXERCISE1-1
CreatinganAbstractSuperclassandConcreteSubclass
Thefollowingexercisewilltestyourknowledgeofpublic,default,final,andabstractclasses.
CreateanabstractsuperclassnamedFruitandaconcretesubclassnamedApple.Thesuperclass
shouldbelongtoapackagecalledfoodandthesubclasscanbelongtothedefaultpackage(meaningit
isn’tputintoapackageexplicitly).Makethesuperclasspublicandgivethesubclassdefaultaccess.
1.Createthesuperclassasfollows:
packagefood;
publicabstractclassFruit{/*anycodeyouwant*/}
2.Createthesubclassinaseparatefileasfollows:
importfood.Fruit;
classAppleextendsFruit{/*anycodeyouwant*/}
3.Createadirectorycalledfoodoffthedirectoryinyourclasspathsetting.
4.Attempttocompilethetwofiles.IfyouwanttousetheAppleclass,makesureyouplacethe
Fruit.classfileinthefoodsubdirectory.
CERTIFICATIONOBJECTIVE
UseInterfaces(OCAObjective7.5)
7.6Useabstractclassesandinterfaces.
DeclaringanInterface
Ingeneral,whenyoucreateaninterface,you’redefiningacontractforwhataclasscando,without
sayinganythingabouthowtheclasswilldoit.
Note:AsofJava8,youcannowalsodescribethehow,butyouusuallywon’t.Untilwegettothenew
interface-relatedfeaturesofJava8—defaultandstaticmethods—wewilldiscussinterfacesfroma
traditionalperspective,whichisagain,definingacontractforwhataclasscando.
Aninterfaceisacontract.YoucouldwriteaninterfaceBounceable,forexample,thatsaysineffect,
“ThisistheBounceableinterface.Anyconcreteclasstypethatimplementsthisinterfacemustagreeto
writethecodeforthebounce()andsetBounceFactor()methods.”
BydefininganinterfaceforBounceable,anyclassthatwantstobetreatedasaBounceablethingcan
simplyimplementtheBounceableinterfaceandprovidecodefortheinterface’stwomethods.
Interfacescanbeimplementedbyanyclass,fromanyinheritancetree.Thisletsyoutakeradically
differentclassesandgivethemacommoncharacteristic.Forexample,youmightwantbothaBallanda
Tiretohavebouncebehavior,butBallandTiredon’tshareanyinheritancerelationship;Ballextends
ToywhileTireextendsonlyjava.lang.Object.ButbymakingbothBallandTireimplement
Bounceable,you’resayingthatBallandTirecanbetreatedas“Thingsthatcanbounce,”whichinJava
translatesto,“Thingsonwhichyoucaninvokethebounce()andsetBounceFactor()methods.”Figure
1-1illustratestherelationshipbetweeninterfacesandclasses.
FIGURE1-1 Therelationshipbetweeninterfacesandclasses
Thinkofatraditionalinterfaceasa100percentabstractclass.Likeanabstractclass,aninterface
definesabstractmethodsthattakethefollowingform:
Butalthoughanabstractclasscandefinebothabstractandnonabstractmethods,aninterface
generallyhasonlyabstractmethods.Anotherwayinterfacesdifferfromabstractclassesisthat
interfaceshaveverylittleflexibilityinhowthemethodsandvariablesdefinedintheinterfaceare
declared.Theserulesarestrict:
Interfacemethodsareimplicitlypublicandabstract,unlessdeclaredasdefaultor
static.Inotherwords,youdonotneedtoactuallytypethepublicorabstractmodifiersinthe
methoddeclaration,butthemethodisstillalwayspublicandabstract.
Allvariablesdefinedinaninterfacemustbepublic,static,andfinal—inotherwords,
interfacescandeclareonlyconstants,notinstancevariables.
Interfacemethodscannotbemarkedfinal,strictfp,ornative.(Moreonthesemodifiers
laterinthechapter.)
Aninterfacecanextendoneormoreotherinterfaces.
Aninterfacecannotextendanythingbutanotherinterface.
Aninterfacecannotimplementanotherinterfaceorclass.
Aninterfacemustbedeclaredwiththekeywordinterface.
Interfacetypescanbeusedpolymorphically(seeChapter2formoredetails).
Thefollowingisalegalinterfacedeclaration:
publicabstractinterfaceRollable{}
Typingintheabstractmodifierisconsideredredundant;interfacesareimplicitlyabstractwhether
youtypeabstractornot.Youjustneedtoknowthatbothofthesedeclarationsarelegalandfunctionally
identical:
publicabstractinterfaceRollable{}
publicinterfaceRollable{}
Thepublicmodifierisrequiredifyouwanttheinterfacetohavepublicratherthandefaultaccess.
We’velookedattheinterfacedeclaration,butnowwe’lllookcloselyatthemethodswithinan
interface:
Typinginthepublicandabstractmodifiersonthemethodsisredundant,though,sinceallinterface
methodsareimplicitlypublicandabstract.Giventhatrule,youcanseethatthefollowingcodeis
exactlyequivalenttotheprecedinginterface:
Youmustrememberthatallinterfacemethodsnotdeclareddefaultorstaticarepublicand
abstractregardlessofwhatyouseeintheinterfacedefinition.
Lookforinterfacemethodsdeclaredwithanycombinationofpublic,abstract,ornomodifiers.For
example,thefollowingfivemethoddeclarations,ifdeclaredwithintheirowninterfaces,arelegaland
identical!
Thefollowinginterfacemethoddeclarationswon’tcompile:
DeclaringInterfaceConstants
You’reallowedtoputconstantsinaninterface.Bydoingso,youguaranteethatanyclassimplementing
theinterfacewillhaveaccesstothesameconstant.Byplacingtheconstantsrightintheinterface,any
classthatimplementstheinterfacehasdirectaccesstotheconstants,justasiftheclasshadinherited
them.
Youneedtorememberonekeyruleforinterfaceconstants.Theymustalwaysbe
publicstaticfinal
Sothatsoundssimple,right?Afterall,interfaceconstantsarenodifferentfromanyotherpublicly
accessibleconstants,sotheyobviouslymustbedeclaredpublic,static,andfinal.Butbeforeyou
breezepasttherestofthisdiscussion,thinkabouttheimplications:Becauseinterfaceconstantsare
definedinaninterface,theydon’thavetobedeclaredaspublic,static,orfinal.Theymustbe
public,static,andfinal,butyoudon’tactuallyhavetodeclarethemthatway.Justasinterface
methodsarealwayspublicandabstractwhetheryousaysointhecodeornot,anyvariabledefinedin
aninterfacemustbe—andimplicitlyis—apublicconstant.Seeifyoucanspottheproblemwiththe
followingcode(assumetwoseparatefiles):
Youcan’tchangethevalueofaconstant!Oncethevaluehasbeenassigned,thevaluecanneverbe
modified.Theassignmenthappensintheinterfaceitself(wheretheconstantisdeclared),sothe
implementingclasscanaccessitanduseit,butasaread-onlyvalue.SotheBAR=27assignmentwill
notcompile.
Lookforinterfacedefinitionsthatdefineconstants,butwithoutexplicitlyusingtherequired
modifiers.Forexample,thefollowingareallidentical:
Anycombinationoftherequired(butimplicit)modifiersislegal,asisusingnomodifiersat
all!Ontheexam,youcanexpecttoseequestionsyouwon’tbeabletoanswercorrectlyunless
youknow,forexample,thataninterfacevariableisfinalandcanneverbegivenavaluebythe
implementing(oranyother)class.
DeclaringdefaultInterfaceMethods
AsofJava8,interfacescanincludeinheritable*methodswithconcreteimplementations.(*Thestrict
definitionof“inheritance”hasgottenalittlefuzzywithJava8;we’lltalkmoreaboutinheritancein
Chapter2.)Theseconcretemethodsarecalleddefaultmethods.Inthenextchapterwe’lltalkalotabout
thevariousOO-relatedrulesthatareimpactedbecauseofdefaultmethods.Fornowwe’lljustcoverthe
simpledeclarationrules:
defaultmethodsaredeclaredbyusingthedefaultkeyword.Thedefaultkeywordcanbe
usedonlywithinterfacemethodsignatures,notclassmethodsignatures.
defaultmethodsarepublicbydefinition,andthepublicmodifierisoptional.
defaultmethodscannotbemarkedasprivate,protected,static,final,orabstract.
defaultmethodsmusthaveaconcretemethodbody.
Herearesomeexamplesoflegalandillegaldefaultmethods:
DeclaringstaticInterfaceMethods
AsofJava8,interfacescanincludestaticmethodswithconcreteimplementations.Aswithinterface
defaultmethods,thereareOOimplicationsthatwe’lldiscussinChapter2.Fornow,we’llfocusonthe
basicsofdeclaringandusingstaticinterfacemethods:
staticinterfacemethodsaredeclaredbyusingthestatickeyword.
staticinterfacemethodsarepublicbydefault,andthepublicmodifierisoptional.
staticinterfacemethodscannotbemarkedasprivate,protected,final,orabstract.
staticinterfacemethodsmusthaveaconcretemethodbody.
Wheninvokingastaticinterfacemethod,themethod’stype(interfacename)MUSTbe
includedintheinvocation.
Herearesomeexamplesoflegalandillegalstaticinterfacemethodsandtheiruse:
whichproducesthisoutput:
42
42
Aswesaidearlier,we’llreturntoourdiscussionofdefaultmethodsandstaticmethodsforinterfaces
inChapter2.
CERTIFICATIONOBJECTIVE
DeclareClassMembers(OCAObjectives2.1,2.2,2.3,4.1,
4.2,6.2,6.3,and6.4)
2.1Declareandinitializevariables(includingcastingofprimitivedatatypes).
2.2Differentiatebetweenobjectreferencevariablesandprimitivevariables.
2.3Knowhowtoreadorwritetoobjectfields.
4.1Declare,instantiate,initialize,anduseaone-dimensionalarray.
4.2Declare,instantiate,initialize,andusemultidimensionalarray.(sic)
6.2Applythestatickeywordtomethodsandfields.
6.3Createandoverloadconstructors;includingimpactondefaultconstructors.(sic)
6.4Applyaccessmodifiers.
We’velookedatwhatitmeanstouseamodifierinaclassdeclaration,andnowwe’lllookatwhatit
meanstomodifyamethodorvariabledeclaration.
Methodsandinstance(nonlocal)variablesarecollectivelyknownasmembers.Youcanmodifya
memberwithbothaccessandnonaccessmodifiers,andyouhavemoremodifierstochoosefrom(and
combine)thanwhenyou’redeclaringaclass.
AccessModifiers
Becausemethodandvariablemembersareusuallygivenaccesscontrolinexactlythesameway,we’ll
coverbothinthissection.
Whereasaclasscanusejusttwoofthefouraccesscontrollevels(defaultorpublic),memberscan
useallfour:
public
protected
default
private
Defaultprotectioniswhatyougetwhenyoudon’ttypeanaccessmodifierinthememberdeclaration.
Thedefaultandprotectedaccesscontroltypeshavealmostidenticalbehavior,exceptforonedifference
thatwewillmentionlater.
Note:AsofJava8,theworddefaultcanALSObeusedtodeclarecertainmethodsininterfaces.
Whenusedinaninterface’smethoddeclaration,defaulthasadifferentmeaningthanwhatweare
describingfortherestofthischapter.
It’scrucialthatyouknowaccesscontrolinsideandoutsidefortheexam.Therewillbequiteafew
questionswhereaccesscontrolplaysarole.Somequestionstestseveralconceptsofaccesscontrolatthe
sametime,sonotknowingonesmallpartofaccesscontrolcouldmeanyoublowanentirequestion.
Whatdoesitmeanforcodeinoneclasstohaveaccesstoamemberofanotherclass?Fornow,ignore
anydifferencesbetweenmethodsandvariables.IfclassAhasaccesstoamemberofclassB,itmeans
thatclassB’smemberisvisibletoclassA.Whenaclassdoesnothaveaccesstoanothermember,the
compilerwillslapyoufortryingtoaccesssomethingthatyou’renotevensupposedtoknowexists!
Youneedtounderstandtwodifferentaccessissues:
Whethermethodcodeinoneclasscanaccessamemberofanotherclass
Whetherasubclasscaninheritamemberofitssuperclass
Thefirsttypeofaccessoccurswhenamethodinoneclasstriestoaccessamethodoravariableof
anotherclass,usingthedotoperator(.)toinvokeamethodorretrieveavariable.Forexample:
Thesecondtypeofaccessrevolvesaroundwhich,ifany,membersofasuperclassasubclasscan
accessthroughinheritance.We’renotlookingatwhetherthesubclasscan,say,invokeamethodonan
instanceofthesuperclass(whichwouldjustbeanexampleofthefirsttypeofaccess).Instead,we’re
lookingatwhetherthesubclassinheritsamemberofitssuperclass.Remember,ifasubclassinheritsa
member,it’sexactlyasifthesubclassactuallydeclaredthememberitself.Inotherwords,ifasubclass
inheritsamember,thesubclasshasthemember.Here’sanexample:
Figure1-2comparesaclassinheritingamemberofanotherclassandaccessingamemberofanother
classusingareferenceofaninstanceofthatclass.
FIGURE1-2 Comparisonofinheritancevs.dotoperatorformemberaccess
Muchofaccesscontrol(bothtypes)centersonwhetherthetwoclassesinvolvedareinthesameor
differentpackages.Don’tforget,though,thatifclassAitselfcan’tbeaccessedbyclassB,thenno
memberswithinclassAcanbeaccessedbyclassB.
Youneedtoknowtheeffectofdifferentcombinationsofclassandmemberaccess(suchasadefault
classwithapublicvariable).Tofigurethisout,firstlookattheaccessleveloftheclass.Iftheclass
itselfwillnotbevisibletoanotherclass,thennoneofthememberswillbevisibleeither,evenifthe
memberisdeclaredpublic.Onceyou’veconfirmedthattheclassisvisible,thenitmakessensetolook
ataccesslevelsonindividualmembers.
PublicMembers
Whenamethodorvariablememberisdeclaredpublic,itmeansallotherclasses,regardlessofthe
packagetheybelongto,canaccessthemember(assumingtheclassitselfisvisible).
Lookatthefollowingsourcefile:
Asyoucansee,GooandSludgeareindifferentpackages.However,Goocaninvokethemethodin
SludgewithoutproblemsbecauseboththeSludgeclassanditstestIt()methodaremarkedpublic.
Forasubclass,ifamemberofitssuperclassisdeclaredpublic,thesubclassinheritsthatmember
regardlessofwhetherbothclassesareinthesamepackage:
TheRooclassdeclaresthedoRooThings()memberaspublic.SoifwemakeasubclassofRoo,any
codeinthatRoosubclasscancallitsowninheriteddoRooThings()method.
NoticeinthefollowingcodethatthedoRooThings()methodisinvokedwithouthavingtoprefaceit
withareference:
Remember,ifyouseeamethodinvoked(oravariableaccessed)withoutthedotoperator(.),itmeans
themethodorvariablebelongstotheclasswhereyouseethatcode.Italsomeansthatthemethodor
variableisimplicitlybeingaccessedusingthethisreference.Sointheprecedingcode,thecallto
doRooThings()intheClooclasscouldalsohavebeenwrittenasthis.doRooThings().Thereference
thisalwaysreferstothecurrentlyexecutingobject—inotherwords,theobjectrunningthecodewhere
youseethethisreference.Becausethethisreferenceisimplicit,youdon’tneedtoprefaceyour
memberaccesscodewithit,butitwon’thurt.Someprogrammersincludeittomakethecodeeasierto
readfornew(ornon)Javaprogrammers.
BesidesbeingabletoinvokethedoRooThings()methodonitself,codefromsomeotherclasscan
calldoRooThings()onaClooinstance,asinthefollowing:
PrivateMembers
Membersmarkedprivatecan’tbeaccessedbycodeinanyclassotherthantheclassinwhichthe
privatememberwasdeclared.Let’smakeasmallchangetotheRooclassfromanearlierexample:
ThedoRooThings()methodisnowprivate,sonootherclasscanuseit.Ifwetrytoinvokethe
methodfromanyotherclass,we’llrunintotrouble:
IfwetrytocompileUseARoo,wegetacompilererrorsomethinglikethis:
cannotfindsymbol
symbol:methoddoRooThings()
It’sasifthemethoddoRooThings()doesn’texist,andasfarasanycodeoutsideoftheRooclassis
concerned,thisistrue.Aprivatememberisinvisibletoanycodeoutsidethemember’sownclass.
Whataboutasubclassthattriestoinheritaprivatememberofitssuperclass?Whenamemberis
declaredprivate,asubclasscan’tinheritit.Fortheexam,youneedtorecognizethatasubclasscan’t
see,use,oreventhinkabouttheprivatemembersofitssuperclass.Youcan,however,declarea
matchingmethodinthesubclass.Butregardlessofhowitlooks,itisnotanoverridingmethod!Itis
simplyamethodthathappenstohavethesamenameasaprivatemethod(whichyou’renotsupposedto
knowabout)inthesuperclass.Therulesofoverridingdonotapply,soyoucanmakethisnewly-
declared-but-just-happens-to-matchmethoddeclarenewexceptions,orchangethereturntype,ordo
anythingelseyouwantittodo.
ThedoRooThings()methodisnowofflimitstoallsubclasses,eventhoseinthesamepackageasthe
superclass:
IfwetrytocompilethesubclassCloo,thecompilerisdelightedtospitoutanerrorsomethinglike
this:
Canaprivatemethodbeoverriddenbyasubclass?That’saninterestingquestion,buttheansweris
no.Becausethesubclass,aswe’veseen,cannotinheritaprivatemethod,it,therefore,cannotoverride
themethod—overridingdependsoninheritance.We’llcovertheimplicationsofthisinmoredetailalittle
laterinthissectionaswellasinChapter2,butfornow,justrememberthatamethodmarkedprivate
cannotbeoverridden.Figure1-3illustratestheeffectsofthepublicandprivatemodifiersonclasses
fromthesameordifferentpackages.
FIGURE1-3 Effectsofpublicandprivateaccess
ProtectedandDefaultMembers
Note:Justareminder,inthenextseveralsections,whenweusetheword“default,”we’retalkingabout
accesscontrol.We’reNOTtalkingaboutthenewkindofJava8interfacemethodthatcanbedeclared
default.
Theprotectedanddefaultaccesscontrollevelsarealmostidentical,butwithonecriticaldifference.
Adefaultmembermaybeaccessedonlyiftheclassaccessingthememberbelongstothesamepackage,
whereasaprotectedmembercanbeaccessed(throughinheritance)byasubclassevenifthesubclass
isinadifferentpackage.Takealookatthefollowingtwoclasses:
Inanothersourcecodefileyouhavethefollowing:
Asyoucansee,thetestIt()methodinthefirstfilehasdefault(thinkpackage-level)access.Notice
alsothatclassOtherClassisinadifferentpackagefromtheAccessClass.WillAccessClassbeable
tousethemethodtestIt()?Willitcauseacompilererror?WillDanielevermarryFrancesca?Stay
tuned.
NomethodmatchingtestIt()foundinclass
certification.OtherClass.o.testIt();
Fromtheprecedingresults,youcanseethatAccessClasscan’tusetheOtherClassmethod
testIt()becausetestIt()hasdefaultaccessandAccessClassisnotinthesamepackageas
OtherClass.SoAccessClasscan’tseeit,thecompilercomplains,andwehavenoideawhoDanieland
Francescaare.
Defaultandprotectedbehaviordifferonlywhenwetalkaboutsubclasses.Iftheprotectedkeyword
isusedtodefineamember,anysubclassoftheclassdeclaringthemembercanaccessitthrough
inheritance.Itdoesn’tmatterifthesuperclassandsubclassareindifferentpackages;theprotected
superclassmemberisstillvisibletothesubclass(althoughvisibleonlyinaveryspecificway,aswe’ll
seealittlelater).Thisisincontrasttothedefaultbehavior,whichdoesn’tallowasubclasstoaccessa
superclassmemberunlessthesubclassisinthesamepackageasthesuperclass.
Whereasdefaultaccessdoesn’textendanyspecialconsiderationtosubclasses(you’reeitherinthe
packageoryou’renot),theprotectedmodifierrespectstheparent-childrelationship,evenwhenthe
childclassmovesaway(andjoinsanewpackage).Sowhenyouthinkofdefaultaccess,thinkpackage
restriction.Noexceptions.Butwhenyouthinkprotected,thinkpackage+kids.Aclasswitha
protectedmemberismarkingthatmemberashavingpackage-levelaccessforallclasses,butwitha
specialexceptionforsubclassesoutsidethepackage.
Butwhatdoesitmeanforasubclass-outside-the-packagetohaveaccesstoasuperclass(parent)
member?Itmeansthesubclassinheritsthemember.Itdoesnot,however,meanthesubclass-outside-the-
packagecanaccessthememberusingareferencetoaninstanceofthesuperclass.Inotherwords,
protected=inheritance.Protecteddoesnotmeanthatthesubclasscantreattheprotectedsuperclass
memberasthoughitwerepublic.Soifthesubclass-outside-the-packagegetsareferencetothesuperclass
(by,forexample,creatinganinstanceofthesuperclasssomewhereinthesubclass’scode),thesubclass
cannotusethedotoperatoronthesuperclassreferencetoaccesstheprotectedmember.Toasubclass-
outside-the-package,aprotectedmembermightaswellbedefault(orevenprivate),whenthe
subclassisusingareferencetothesuperclass.Thesubclasscanseetheprotectedmemberonly
throughinheritance.
Areyouconfused?Hanginthereanditwillallbecomeclearerwiththenextbatchofcodeexamples.
ProtectedDetails
Let’stakealookataprotectedinstancevariable(remember,aninstancevariableisamember)ofa
superclass.
Theprecedingcodedeclaresthevariablexasprotected.Thismakesthevariableaccessibletoall
otherclassesinsidethecertificationpackage,aswellasinheritablebyanysubclassesoutsidethe
package.
Nowlet’screateasubclassinadifferentpackageandattempttousethevariablex(thatthesubclass
inherits):
Theprecedingcodecompilesfine.Notice,though,thattheChildclassisaccessingtheprotected
variablethroughinheritance.Rememberthatanytimewetalkaboutasubclasshavingaccesstoa
superclassmember,wecouldbetalkingaboutthesubclassinheritingthemember,notsimplyaccessing
thememberthroughareferencetoaninstanceofthesuperclass(thewayanyothernonsubclasswould
accessit).WatchwhathappensifthesubclassChild(outsidethesuperclass’spackage)triestoaccessa
protectedvariableusingaParentclassreference:
Thecompilerismorethanhappytoshowustheproblem:
Sofar,we’veestablishedthataprotectedmemberhasessentiallypackage-levelordefaultaccessto
allclassesexceptforsubclasses.We’veseenthatsubclassesoutsidethepackagecaninheritaprotected
member.Finally,we’veseenthatsubclassesoutsidethepackagecan’tuseasuperclassreferenceto
accessaprotectedmember.Forasubclassoutsidethepackage,theprotectedmembercanbe
accessedonlythroughinheritance.
Butthere’sstillonemoreissuewehaven’tlookedat:Whatdoesaprotectedmemberlookliketo
otherclassestryingtousethesubclass-outside-the-packagetogettothesubclass’sinheritedprotected
superclassmember?Forexample,usingourpreviousParent/Childclasses,whathappensifsomeother
class—Neighbor,say—inthesamepackageastheChild(subclass)hasareferencetoaChildinstance
andwantstoaccessthemembervariablex?Inotherwords,howdoesthatprotectedmemberbehave
oncethesubclasshasinheritedit?DoesitmaintainitsprotectedstatussuchthatclassesintheChild’s
packagecanseeit?
No!Oncethesubclass-outside-the-packageinheritstheprotectedmember,thatmember(asinherited
bythesubclass)becomesprivatetoanycodeoutsidethesubclass,withtheexceptionofsubclassesofthe
subclass.SoifclassNeighborinstantiatesaChildobject,thenevenifclassNeighborisinthesame
packageasclassChild,classNeighborwon’thaveaccesstotheChild’sinherited(butprotected)
variablex.Figure1-4illustratestheeffectofprotectedaccessonclassesandsubclassesinthesameor
differentpackages.
Whew!Thatwrapsupprotected,themostmisunderstoodmodifierinJava.Again,it’susedonlyin
veryspecialcases,butyoucancountonitshowingupontheexam.Nowthatwe’vecoveredthe
protectedmodifier,we’llswitchtodefaultmemberaccess,apieceofcakecomparedtoprotected.
FIGURE1-4 Effectsofprotectedaccess
DefaultDetails
Let’sstartwiththedefaultbehaviorofamemberinasuperclass.We’llmodifytheParent’smemberxto
makeitdefault.
Noticewedidn’tplaceanaccessmodifierinfrontofthevariablex.Rememberthatifyoudon’ttype
anaccessmodifierbeforeaclassormemberdeclaration,theaccesscontrolisdefault,whichmeans
packagelevel.We’llnowattempttoaccessthedefaultmemberfromtheChildclassthatwesawearlier.
WhenwetrytocompiletheChild.javafile,wegetanerrorlikethis:
Thecompilergivesthesameerroraswhenamemberisdeclaredasprivate.ThesubclassChild(ina
differentpackagefromthesuperclassParent)can’tseeorusethedefaultsuperclassmemberx!Now,
whataboutdefaultaccessfortwoclassesinthesamepackage?
Andinthesecondclassyouhavethefollowing:
Theprecedingsourcefilecompilesfine,andtheclassChildrunsanddisplaysthevalueofx.Just
rememberthatdefaultmembersarevisibletosubclassesonlyifthosesubclassesareinthesamepackage
asthesuperclass.
LocalVariablesandAccessModifiers
Canaccessmodifiersbeappliedtolocalvariables?NO!
Thereisneveracasewhereanaccessmodifiercanbeappliedtoalocalvariable,sowatchoutfor
codelikethefollowing:
Youcanbecertainthatanylocalvariabledeclaredwithanaccessmodifierwillnotcompile.Infact,
thereisonlyonemodifierthatcaneverbeappliedtolocalvariables—final.
Thataboutdoesitforourdiscussiononmemberaccessmodifiers.Table1-2showsallthe
combinationsofaccessandvisibility;youreallyshouldspendsometimewithit.Next,we’regoingtodig
intotheother(nonaccess)modifiersthatyoucanapplytomemberdeclarations.
TABLE1-2 DeterminingAccesstoClassMembers
NonaccessMemberModifiers
We’vediscussedmemberaccess,whichreferstowhethercodefromoneclasscaninvokeamethod(or
accessaninstancevariable)fromanotherclass.Thatstillleavesaboatloadofothermodifiersyoucan
useonmemberdeclarations.Twoyou’realreadyfamiliarwith—finalandabstract—becausewe
appliedthemtoclassdeclarationsearlierinthischapter.Butwestillhavetotakeaquicklookat
transient,synchronized,native,strictfp,andthenalonglookattheBigOne,static,muchlater
inthechapter.
We’lllookfirstatmodifiersappliedtomethods,followedbyalookatmodifiersappliedtoinstance
variables.We’llwrapupthissectionwithalookathowstaticworkswhenappliedtovariablesand
methods.
FinalMethods
Thefinalkeywordpreventsamethodfrombeingoverriddeninasubclassandisoftenusedtoenforce
theAPIfunctionalityofamethod.Forexample,theThreadclasshasamethodcalledisAlive()that
checkswhetherathreadisstillactive.IfyouextendtheThreadclass,though,thereisreallynowaythat
youcancorrectlyimplementthismethodyourself(itusesnativecode,foronething),sothedesigners
havemadeitfinal.Justasyoucan’tsubclasstheStringclass(becauseweneedtobeabletotrustin
thebehaviorofaStringobject),youcan’toverridemanyofthemethodsinthecoreclasslibraries.This
can’t-be-overriddenrestrictionprovidesforsafetyandsecurity,butyoushoulduseitwithgreatcaution.
PreventingasubclassfromoverridingamethodstiflesmanyofthebenefitsofOO,includingextensibility
throughpolymorphism.Atypicalfinalmethoddeclarationlookslikethis:
It’slegaltoextendSuperClass,sincetheclassisn’tmarkedfinal,butwecan’toverridethefinal
methodshowSample(),asthefollowingcodeattemptstodo:
Attemptingtocompiletheprecedingcodegivesussomethinglikethis:
FinalArguments
Methodargumentsarethevariabledeclarationsthatappearinbetweentheparenthesesinamethod
declaration.Atypicalmethoddeclarationwithmultipleargumentslookslikethis:
publicRecordgetRecord(intfileNumber,intrecNumber){}
Methodargumentsareessentiallythesameaslocalvariables.Intheprecedingexample,thevariables
fileNumberandrecNumberwillbothfollowalltherulesappliedtolocalvariables.Thismeansthey
canalsohavethemodifierfinal:
publicRecordgetRecord(intfileNumber,finalintrecNumber){}
Inthisexample,thevariablerecNumberisdeclaredasfinal,which,ofcourse,meansitcan’tbe
modifiedwithinthemethod.Inthiscase,“modified”meansreassigninganewvaluetothevariable.In
otherwords,afinalparametermustkeepthesamevalueastheargumenthadwhenitwaspassedinto
themethod.
AbstractMethods
Anabstractmethodisamethodthat’sbeendeclared(asabstract)butnotimplemented.Inother
words,themethodcontainsnofunctionalcode.Andifyourecallfromtheearliersection“Abstract
Classes,”anabstractmethoddeclarationdoesn’tevenhavecurlybracesforwheretheimplementation
codegoes,butinsteadcloseswithasemicolon.Inotherwords,ithasnomethodbody.Youmarka
methodabstractwhenyouwanttoforcesubclassestoprovidetheimplementation.Forexample,ifyou
writeanabstractclassCarwithamethodgoUpHill(),youmightwanttoforceeachsubtypeofCarto
defineitsowngoUpHill()behavior,specifictothatparticulartypeofcar.
publicabstractvoidshowSample();
Noticethattheabstractmethodendswithasemicoloninsteadofcurlybraces.Itisillegaltohave
evenasingleabstractmethodinaclassthatisnotexplicitlydeclaredabstract!Lookatthe
followingillegalclass:
Youcan,however,haveanabstractclasswithnoabstractmethods.Thefollowingexamplewill
compilefine:
Intheprecedingexample,goodMethod()isnotabstract.Threedifferentcluestellyouit’snotan
abstractmethod:
Themethodisnotmarkedabstract.
Themethoddeclarationincludescurlybraces,asopposedtoendinginasemicolon.Inother
words,themethodhasamethodbody.
Themethodmightprovideactualimplementationcodeinsidethecurlybraces.
Anyclassthatextendsanabstractclassmustimplementallabstractmethodsofthesuperclass,
unlessthesubclassisalsoabstract.Theruleisthis:
Thefirstconcretesubclassofanabstractclassmustimplementallabstractmethodsofthe
superclass.
Concretejustmeansnonabstract,soifyouhaveanabstractclassextendinganotherabstractclass,
theabstractsubclassdoesn’tneedtoprovideimplementationsfortheinheritedabstractmethods.
Soonerorlater,though,somebody’sgoingtomakeanonabstractsubclass(inotherwords,aclassthatcan
beinstantiated),andthatsubclasswillhavetoimplementalltheabstractmethodsfromupthe
inheritancetree.Thefollowingexampledemonstratesaninheritancetreewithtwoabstractclassesand
oneconcreteclass:
SohowmanymethodsdoesclassMinihave?Three.ItinheritsboththegetType()and
doCarThings()methodsbecausethey’republicandconcrete(nonabstract).ButbecausegoUpHill()is
abstractinthesuperclassVehicleandisneverimplementedintheCarclass(soitremainsabstract),
itmeansclassMini—asthefirstconcreteclassbelowVehicle—mustimplementthegoUpHill()
method.Inotherwords,classMinican’tpassthebuck(ofabstractmethodimplementation)tothenext
classdowntheinheritancetree,butclassCarcan,sinceCar,likeVehicle,isabstract.Figure1-5
illustratestheeffectsoftheabstractmodifieronconcreteandabstractsubclasses.
FIGURE1-5 Theeffectsoftheabstractmodifieronconcreteandabstractsubclasses
Lookforconcreteclassesthatdon’tprovidemethodimplementationsforabstractmethodsofthe
superclass.Thefollowingcodewon’tcompile:
ClassBwon’tcompilebecauseitdoesn’timplementtheinheritedabstractmethodfoo().Although
thefoo(intI)methodinclassBmightappeartobeanimplementationofthesuperclass’sabstract
method,itissimplyanoverloadedmethod(amethodusingthesameidentifier,butdifferentarguments),
soitdoesn’tfulfilltherequirementsforimplementingthesuperclass’sabstractmethod.We’lllookat
thedifferencesbetweenoverloadingandoverridingindetailinChapter2.
Amethodcannever,ever,everbemarkedasbothabstractandfinal,orbothabstractand
private.Thinkaboutit—abstractmethodsmustbeimplemented(whichessentiallymeansoverridden
byasubclass),whereasfinalandprivatemethodscannoteverbeoverriddenbyasubclass.Orto
phraseitanotherway,anabstractdesignationmeansthesuperclassdoesn’tknowanythingabouthow
thesubclassesshouldbehaveinthatmethod,whereasafinaldesignationmeansthesuperclassknows
everythingabouthowallsubclasses(howeverfardowntheinheritancetreetheymaybe)shouldbehave
inthatmethod.Theabstractandfinalmodifiersarevirtuallyopposites.Becauseprivatemethods
cannotevenbeseenbyasubclass(letaloneinherited),they,too,cannotbeoverridden,sothey,too,
cannotbemarkedabstract.
Finally,youneedtoknowthat—fortop-levelclasses—theabstractmodifiercanneverbecombined
withthestaticmodifier.We’llcoverstaticmethodslaterinthisobjective,butfornowjustremember
thatthefollowingwouldbeillegal:
abstractstaticvoiddoStuff();
Anditwouldgiveyouanerrorthatshouldbefamiliarbynow:
MyClass.java:2:illegalcombinationofmodifiers:abstractandstatic
abstractstaticvoiddoStuff();
SynchronizedMethods
Thesynchronizedkeywordindicatesthatamethodcanbeaccessedbyonlyonethreadatatime.When
youarestudyingforyourOCP8,you’llstudythesynchronizedkeywordextensively,butfornow...all
we’reconcernedwithisknowingthatthesynchronizedmodifiercanbeappliedonlytomethods—not
variables,notclasses,justmethods.Atypicalsynchronizeddeclarationlookslikethis:
publicsynchronizedRecordretrieveUserInfo(intid){}
Youshouldalsoknowthatthesynchronizedmodifiercanbematchedwithanyofthefouraccess
controllevels(whichmeansitcanbepairedwithanyofthethreeaccessmodifierkeywords).
NativeMethods
Thenativemodifierindicatesthatamethodisimplementedinplatform-dependentcode,ofteninC.You
don’tneedtoknowhowtousenativemethodsfortheexam,otherthanknowingthatnativeisa
modifier(thusareservedkeyword)andthatnativecanbeappliedonlytomethods—notclasses,not
variables,justmethods.Notethatanativemethod’sbodymustbeasemicolon(;)(likeabstract
methods),indicatingthattheimplementationisomitted.
StrictfpMethods
Welookedearlieratusingstrictfpasaclassmodifier,butevenifyoudon’tdeclareaclassas
strictfp,youcanstilldeclareanindividualmethodasstrictfp.Remember,strictfpforcesfloating
points(andanyfloating-pointoperations)toadheretotheIEEE754standard.Withstrictfp,youcan
predicthowyourfloatingpointswillbehaveregardlessoftheunderlyingplatformtheJVMisrunningon.
Thedownsideisthatiftheunderlyingplatformiscapableofsupportinggreaterprecision,astrictfp
methodwon’tbeabletotakeadvantageofit.
You’llwanttostudytheIEEE754ifyouneedsomethingtohelpyoufallasleep.Fortheexam,
however,youdon’tneedtoknowanythingaboutstrictfpotherthanwhatit’susedfor—thatitcan
modifyaclassormethoddeclaration,andthatavariablecanneverbedeclaredstrictfp.
MethodswithVariableArgumentLists(var-args)
Javaallowsyoutocreatemethodsthatcantakeavariablenumberofarguments.Dependingonwhereyou
look,youmighthearthiscapabilityreferredtoas“variable-lengthargumentlists,”“variablearguments,”
“var-args,”“varargs,”orourpersonalfavorite(fromthedepartmentofobfuscation),“variablearity
parameters.”They’reallthesamething,andwe’llusetheterm“var-args”fromhereonout.
Asabitofbackground,we’dliketoclarifyhowwe’regoingtousetheterms“argument”and
“parameter”throughoutthisbook:
argumentsThethingsyouspecifybetweentheparentheseswhenyou’reinvokingamethod:
parametersThethingsinthemethod’ssignaturethatindicatewhatthemethodmustreceive
whenit’sinvoked:
Let’sreviewthedeclarationrulesforvar-args:
Var-argtypeWhenyoudeclareavar-argparameter,youmustspecifythetypeofthe
argument(s)thisparameterofyourmethodcanreceive.(Thiscanbeaprimitivetypeoranobject
type.)
BasicsyntaxTodeclareamethodusingavar-argparameter,youfollowthetypewithan
ellipsis(...),aspace,andthenthenameofthearraythatwillholdtheparametersreceived.
OtherparametersIt’slegaltohaveotherparametersinamethodthatusesavar-arg.
Var-arglimitsThevar-argmustbethelastparameterinthemethod’ssignature,andyou
canhaveonlyonevar-arginamethod.
Let’slookatsomelegalandillegalvar-argdeclarations:
Legal:
Illegal:
ConstructorDeclarations
InJava,objectsareconstructed.Everytimeyoumakeanewobject,atleastoneconstructorisinvoked.
Everyclasshasaconstructor,althoughifyoudon’tcreateoneexplicitly,thecompilerwillbuildonefor
you.Therearetonsofrulesconcerningconstructors,andwe’resavingourdetaileddiscussionfor
Chapter2.Fornow,let’sfocusonthebasicdeclarationrules.Here’sasimpleexample:
Thefirstthingtonoticeisthatconstructorslookanawfullotlikemethods.Akeydifferenceisthata
constructorcan’tever,ever,ever,haveareturntype…ever!Constructordeclarationscan,however,have
allofthenormalaccessmodifiers,andtheycantakearguments(includingvar-args),justlikemethods.
TheotherBIGRULEtounderstandaboutconstructorsisthattheymusthavethesamenameastheclassin
whichtheyaredeclared.Constructorscan’tbemarkedstatic(theyare,afterall,associatedwithobject
instantiation),andtheycan’tbemarkedfinalorabstract(becausetheycan’tbeoverridden).Hereare
somelegalandillegalconstructordeclarations:
VariableDeclarations
TherearetwotypesofvariablesinJava:
PrimitivesAprimitivecanbeoneofeighttypes:char,boolean,byte,short,int,long,
double,orfloat.Onceaprimitivehasbeendeclared,itsprimitivetypecanneverchange,
althoughinmostcasesitsvaluecanchange.
ReferencevariablesAreferencevariableisusedtoreferto(oraccess)anobject.A
referencevariableisdeclaredtobeofaspecifictype,andthattypecanneverbechanged.A
referencevariablecanbeusedtorefertoanyobjectofthedeclaredtypeorofasubtypeofthe
declaredtype(acompatibletype).We’lltalkalotmoreaboutusingareferencevariabletoreferto
asubtypeinChapter2,whenwediscusspolymorphism.
DeclaringPrimitivesandPrimitiveRanges
Primitivevariablescanbedeclaredasclassvariables(statics),instancevariables,methodparameters,
orlocalvariables.Youcandeclareoneormoreprimitives,ofthesameprimitivetype,inasingleline.In
Chapter3wewilldiscussthevariouswaysinwhichtheycanbeinitialized,butfornowwe’llleaveyou
withafewexamplesofprimitivevariabledeclarations:
OnpreviousversionsoftheexamyouneededtoknowhowtocalculaterangesforalltheJava
primitives.Forthecurrentexam,youcanskipsomeofthatdetail,butit’sstillimportanttounderstandthat
fortheintegertypesthesequencefromsmalltobigisbyte,short,int,andlong,andthatdoublesare
biggerthanfloats.
Youwillalsoneedtoknowthatthenumbertypes(bothintegerandfloating-pointtypes)areallsigned
andhowthataffectstheirranges.First,let’sreviewtheconcepts.
AllsixnumbertypesinJavaaremadeupofacertainnumberof8-bitbytesandaresigned,meaning
theycanbenegativeorpositive.Theleftmostbit(themostsignificantdigit)isusedtorepresentthesign,
wherea1meansnegativeand0meanspositive,asshowninFigure1-6.Therestofthebitsrepresentthe
value,usingtwo’scomplementnotation.
FIGURE1-6 Thesignbitforabyte
TABLE1-3 RangesofNumericPrimitives
Table1-3showstheprimitivetypeswiththeirsizesandranges.Figure1-6showsthatwithabyte,for
example,thereare256possiblenumbers(or28).Halfofthesearenegative,andhalf–1arepositive.The
positiverangeisonelessthanthenegativerangebecausethenumber0isstoredasapositivebinary
number.Weusetheformula–2(bits–1)tocalculatethenegativerange,andweuse2(bits–1)–1forthe
positiverange.Again,ifyouknowthefirsttwocolumnsofthistable,you’llbeingoodshapeforthe
exam.
Therangeforfloating-pointnumbersiscomplicatedtodetermine,butluckilyyoudon’tneedtoknow
thesefortheexam(althoughyouareexpectedtoknowthatadoubleholds64bitsandafloat32).
Thereisnotarangeofbooleanvalues;abooleancanbeonlytrueorfalse.Ifsomeoneasksyou
forthebitdepthofaboolean,lookthemstraightintheeyeandsay,“That’svirtual-machinedependent.”
They’llbeimpressed.
Thechartype(acharacter)containsasingle,16-bitUnicodecharacter.AlthoughtheextendedASCII
setknownasISOLatin-1needsonly8bits(256differentcharacters),alargerrangeisneededto
representcharactersfoundinlanguagesotherthanEnglish.Unicodecharactersareactuallyrepresentedby
unsigned16-bitintegers,whichmeans216possiblevalues,rangingfrom0to65535(216–1).You’lllearn
inChapter3thatbecauseacharisreallyanintegertype,itcanbeassignedtoanynumbertypelarge
enoughtohold65535(whichmeansanythinglargerthanashort;althoughbothcharsandshortsare
16-bittypes,rememberthatashortuses1bittorepresentthesign,sofewerpositivenumbersare
acceptableinashort).
DeclaringReferenceVariables
Referencevariablescanbedeclaredasstaticvariables,instancevariables,methodparameters,orlocal
variables.Youcandeclareoneormorereferencevariables,ofthesametype,inasingleline.InChapter
3wewilldiscussthevariouswaysinwhichtheycanbeinitialized,butfornowwe’llleaveyouwitha
fewexamplesofreferencevariabledeclarations:
InstanceVariables
Instancevariablesaredefinedinsidetheclass,butoutsideofanymethod,andareinitializedonlywhen
theclassisinstantiated.Instancevariablesarethefieldsthatbelongtoeachuniqueobject.Forexample,
thefollowingcodedefinesfields(instancevariables)forthename,title,andmanagerforemployee
objects:
TheprecedingEmployeeclasssaysthateachemployeeinstancewillknowitsownname,title,and
manager.Inotherwords,eachinstancecanhaveitsownuniquevaluesforthosethreefields.Forthe
exam,youneedtoknowthatinstancevariables
Canuseanyofthefouraccesslevels(whichmeanstheycanbemarkedwithanyofthethree
accessmodifiers)
Canbemarkedfinal
Canbemarkedtransient
Cannotbemarkedabstract
Cannotbemarkedsynchronized
Cannotbemarkedstrictfp
Cannotbemarkednative
Cannotbemarkedstaticbecausethenthey’dbecomeclassvariables
We’vealreadycoveredtheeffectsofapplyingaccesscontroltoinstancevariables(itworksthesame
wayasitdoesformembermethods).Alittlelaterinthischapterwe’lllookatwhatitmeanstoapplythe
finalortransientmodifiertoaninstancevariable.First,though,we’lltakeaquicklookatthe
differencebetweeninstanceandlocalvariables.Figure1-7comparesthewayinwhichmodifierscanbe
appliedtomethodsversusvariables.
FIGURE1-7 Comparisonofmodifiersonvariablesvs.methods
Local(Automatic/Stack/Method)Variables
Alocalvariableisavariabledeclaredwithinamethod.Thatmeansthevariableisnotjustinitialized
withinthemethod,butalsodeclaredwithinthemethod.Justasthelocalvariablestartsitslifeinsidethe
method,it’salsodestroyedwhenthemethodhascompleted.Localvariablesarealwaysonthestack,not
theheap.(We’lltalkmoreaboutthestackandtheheapinChapter3.)Althoughthevalueofthevariable
mightbepassedinto,say,anothermethodthatthenstoresthevalueinaninstancevariable,thevariable
itselflivesonlywithinthescopeofthemethod.
Justdon’tforgetthatwhilethelocalvariableisonthestack,ifthevariableisanobjectreference,the
objectitselfwillstillbecreatedontheheap.Thereisnosuchthingasastackobject,onlyastack
variable.You’lloftenhearprogrammersusethephrase“localobject,”butwhattheyreallymeanis,
“locallydeclaredreferencevariable.”Soifyouhearaprogrammerusethatexpression,you’llknowthat
he’sjusttoolazytophraseitinatechnicallypreciseway.Youcantellhimwesaidthat—unlessheknows
wherewelive.
Localvariabledeclarationscan’tusemostofthemodifiersthatcanbeappliedtoinstancevariables,
suchaspublic(ortheotheraccessmodifiers),transient,volatile,abstract,orstatic,butaswe
sawearlier,localvariablescanbemarkedfinal.Andasyou’lllearninChapter3(buthere’sa
preview),beforealocalvariablecanbeused,itmustbeinitializedwithavalue.Forinstance:
Typically,you’llinitializealocalvariableinthesamelineinwhichyoudeclareit,althoughyoumight
stillneedtoreassignitlaterinthemethod.Thekeyistorememberthatalocalvariablemustbe
initializedbeforeyoutrytouseit.Thecompilerwillrejectanycodethattriestousealocalvariablethat
hasn’tbeenassignedavaluebecause—unlikeinstancevariables—localvariablesdon’tgetdefault
values.
Alocalvariablecan’tbereferencedinanycodeoutsidethemethodinwhichit’sdeclared.Inthe
precedingcodeexample,itwouldbeimpossibletorefertothevariablecountanywhereelseintheclass
exceptwithinthescopeofthemethodlogIn().Again,that’snottosaythatthevalueofcountcan’tbe
passedoutofthemethodtotakeonanewlife.Butthevariableholdingthatvalue,count,can’tbe
accessedoncethemethodiscomplete,asthefollowingillegalcodedemonstrates:
Itispossibletodeclarealocalvariablewiththesamenameasaninstancevariable.It’sknownas
shadowing,asthefollowingcodedemonstrates:
WhyonEarth(ortheplanetofyourchoice)wouldyouwanttodothat?Normally,youwon’t.Butone
ofthemorecommonreasonsistonameaparameterwiththesamenameastheinstancevariabletowhich
theparameterwillbeassigned.
Thefollowing(wrong)codeistryingtosetaninstancevariable’svalueusingaparameter:
Soyou’vedecidedthat—foroverallreadability—youwanttogivetheparameterthesamenameasthe
instancevariableitsvalueisdestinedfor,buthowdoyouresolvethenamingcollision?Usethekeyword
this.Thekeywordthisalways,always,alwaysreferstotheobjectcurrentlyrunning.Thefollowing
codeshowsthisinaction:
ArrayDeclarations
InJava,arraysareobjectsthatstoremultiplevariablesofthesametypeorvariablesthatareall
subclassesofthesametype.Arrayscanholdeitherprimitivesorobjectreferences,butanarrayitself
willalwaysbeanobjectontheheap,evenifthearrayisdeclaredtoholdprimitiveelements.Inother
words,thereisnosuchthingasaprimitivearray,butyoucanmakeanarrayofprimitives.
Fortheexam,youneedtoknowthreethings:
Howtomakeanarrayreferencevariable(declare)
Howtomakeanarrayobject(construct)
Howtopopulatethearraywithelements(initialize)
Forthisobjective,youonlyneedtoknowhowtodeclareanarray;we’llcoverconstructingand
initializingarraysinChapter5.
Arraysareefficient,butmanytimesyou’llwanttouseoneoftheCollectiontypesfromjava.util
(includingHashMap,ArrayList,andTreeSet).Collectionclassesoffermoreflexiblewaysto
accessanobject(forinsertion,deletion,reading,andsoon)and,unlikearrays,canexpandor
contractdynamicallyasyouaddorremoveelements.ThereareCollectiontypesforawiderangeof
needs.Doyouneedafastsort?Agroupofobjectswithnoduplicates?Awaytoaccessaname-value
pair?JavaprovidesawidevarietyofCollectiontypestoaddressthesesituations,buttheonly
CollectiontypeontheexamisArrayList,andChapter5discussesArrayListinmoredetail.
Arraysaredeclaredbystatingthetypeofelementsthearraywillhold(anobjectoraprimitive),
followedbysquarebracketstoeithersideoftheidentifier.
DeclaringanArrayofPrimitives:
DeclaringanArrayofObjectReferences:
Whendeclaringanarrayreference,youshouldalwaysputthearraybracketsimmediatelyafterthe
declaredtype,ratherthanaftertheidentifier(variablename).Thatway,anyonereadingthecode
caneasilytellthat,forexample,keyisareferencetoanintarrayobject,notanintprimitive.
Wecanalsodeclaremultidimensionalarrays,whichare,infact,arraysofarrays.Thiscanbedonein
thefollowingmanner:
String[][][]occupantName;
String[]managerName[];
Thefirstexampleisathree-dimensionalarray(anarrayofarraysofarrays),andthesecondisatwo-
dimensionalarray.Noticeinthesecondexample,wehaveonesquarebracketbeforethevariablename
andoneafter.Thisisperfectlylegaltothecompiler,provingonceagainthatjustbecauseit’slegal
doesn’tmeanit’sright.
Itisneverlegaltoincludethesizeofthearrayinyourdeclaration.Yes,weknowyoucando
thatinsomeotherlanguages,whichiswhyyoumightseeaquestionortwothatincludecode
similartothefollowing:
int[5]scores;
Theprecedingcodewon’tcompile.Remember,theJVMdoesn’tallocatespaceuntilyou
actuallyinstantiatethearrayobject.That’swhensizematters.
InChapter6,we’llspendalotoftimediscussingarrays,howtoinitializeandusethemandhowto
dealwithmultidimensionalarrays…staytuned!
FinalVariables
Declaringavariablewiththefinalkeywordmakesitimpossibletoreassignthatvariableonceithas
beeninitializedwithanexplicitvalue(noticewesaid“explicit”ratherthan“default”).Forprimitives,
thismeansthatoncethevariableisassignedavalue,thevaluecan’tbealtered.Forexample,ifyou
assign10totheintvariablex,thenxisgoingtostay10,forever.Sothat’sstraightforwardfor
primitives,butwhatdoesitmeantohaveafinalobjectreferencevariable?Areferencevariable
markedfinalcanneverbereassignedtorefertoadifferentobject.Thedatawithintheobjectcanbe
modified,butthereferencevariablecannotbechanged.Inotherwords,afinalreferencestillallows
youtomodifythestateoftheobjectitrefersto,butyoucan’tmodifythereferencevariabletomakeit
refertoadifferentobject.Burnthisin:therearenofinalobjects,onlyfinalreferences.We’llexplain
thisinmoredetailinChapter3.
We’venowcoveredhowthefinalmodifiercanbeappliedtoclasses,methods,andvariables.Figure
1-8highlightsthekeypointsanddifferencesofthevariousapplicationsoffinal.
FIGURE1-8 Effectoffinalonvariables,methods,andclasses
TransientVariables
Ifyoumarkaninstancevariableastransient,you’retellingtheJVMtoskip(ignore)thisvariablewhen
youattempttoserializetheobjectcontainingit.SerializationisoneofthecoolestfeaturesofJava;itlets
yousave(sometimescalled“flatten”)anobjectbywritingitsstate(inotherwords,thevalueofits
instancevariables)toaspecialtypeofI/Ostream.Withserialization,youcansaveanobjecttoafileor
evenshipitoverawireforreinflating(deserializing)attheotherendinanotherJVM.Wewerehappy
whenserializationwasaddedtotheexamasofJava5,butwe’resadtosaythatasofJava7,
serializationisnolongerontheexam.
VolatileVariables
ThevolatilemodifiertellstheJVMthatathreadaccessingthevariablemustalwaysreconcileitsown
privatecopyofthevariablewiththemastercopyinmemory.Saywhat?Don’tworryaboutit.Forthe
exam,allyouneedtoknowaboutvolatileisthatitexists.
Thevolatilemodifiermayalsobeappliedtoprojectmanagers!
StaticVariablesandMethods
Note:ThediscussionofstaticinthissectionDOESNOTincludethenewstaticinterfacemethod
discussedearlierinthischapter.Don’tyoujustlovehowtheJava8folksreusedimportantJavaterms?
Thestaticmodifierisusedtocreatevariablesandmethodsthatwillexistindependentlyofany
instancescreatedfortheclass.Allstaticmembersexistbeforeyouevermakeanewinstanceofaclass,
andtherewillbeonlyonecopyofastaticmemberregardlessofthenumberofinstancesofthatclass.
Inotherwords,allinstancesofagivenclasssharethesamevalueforanygivenstaticvariable.We’ll
coverstaticmembersingreatdetailinthenextchapter.
Thingsyoucanmarkasstatic:
Methods
Variables
Aclassnestedwithinanotherclass,butnotwithinamethod(notontheOCA8exam)
Initializationblocks
Thingsyoucan’tmarkasstatic:
Constructors(makesnosense;aconstructorisusedonlytocreateinstances)
Classes(unlesstheyarenested)
Interfaces(unlesstheyarenested)
Methodlocalinnerclasses(notontheOCA8exam)
Innerclassmethodsandinstancevariables(notontheOCA8exam)
Localvariables
CERTIFICATIONOBJECTIVE
DeclareandUseenums(OCAObjective1.2)
1.2DefinethestructureofaJavaclass.
Note:Duringthecreationofthisbook,OracleadjustedsomeoftheobjectivesfortheOCAexam.
We’renot100percentsurethatthetopicofenumsisincludedintheOCAexam,butwe’vedecidedthat
it’sbettertobesafethansorry,sowerecommendthatOCAcandidatesstudythissection.Inanycase,
you’relikelytoencountertheuseofenumsintheJavacodeyouread,solearningaboutthemwillpayoff
regardless.
Declaringenums
Javaletsyourestrictavariabletohavingoneofonlyafewpredefinedvalues—inotherwords,onevalue
fromanenumeratedlist.(Theitemsintheenumeratedlistarecalled,surprisingly,enums.)
Usingenumscanhelpreducethebugsinyourcode.Forinstance,imagineyou’recreatinga
commercial-coffee-establishmentapplication,andinyourcoffeeshopapplication,youmightwantto
restrictyourCoffeeSizeselectionstoBIG,HUGE,andOVERWHELMING.IfyouletanorderforaLARGE
oraGRANDEslipin,itmightcauseanerror.enumstotherescue.Withthefollowingsimpledeclaration,
youcanguaranteethatthecompilerwillstopyoufromassigninganythingtoaCoffeeSizeexceptBIG,
HUGE,orOVERWHELMING:
enumCoffeeSize{BIG,HUGE,OVERWHELMING};
Fromthenon,theonlywaytogetaCoffeeSizewillbewithastatementsomethinglikethis:
CoffeeSizecs=CoffeeSize.BIG;
It’snotrequiredthatenumconstantsbeinallcaps,butborrowingfromtheOraclecodeconventionthat
constantsarenamedincaps,it’sagoodidea.
Thebasiccomponentsofanenumareitsconstants(thatis,BIG,HUGE,andOVERWHELMING),althoughin
aminuteyou’llseethattherecanbealotmoretoanenum.enumscanbedeclaredastheirownseparate
classorasaclassmember;however,theymustnotbedeclaredwithinamethod!
Here’sanexampledeclaringanenumoutsideaclass:
Theprecedingcodecanbepartofasinglefile(or,ingeneral,enumclassescanexistintheirownfile
likeCoffeeSize.java).Butremember,inthiscasethefilemustbenamedCoffeeTest1.javabecause
that’sthenameofthepublicclassinthefile.Thekeypointtorememberisthatanenumthatisn’t
enclosedinaclasscanbedeclaredwithonlythepublicordefaultmodifier,justlikeanon-innerclass.
Here’sanexampleofdeclaringanenuminsideaclass:
Thekeypointstotakeawayfromtheseexamplesarethatenumscanbedeclaredastheirownclassor
enclosedinanotherclass,andthatthesyntaxforaccessinganenum’smembersdependsonwherethe
enumwasdeclared.
ThefollowingisNOTlegal:
Tomakeitmoreconfusingforyou,theJavalanguagedesignersmadeitoptionaltoputasemicolonat
theendoftheenumdeclaration(whennootherdeclarationsforthisenumfollow):
Sowhatgetscreatedwhenyoumakeanenum?Themostimportantthingtorememberisthatenumsare
notStringsorints!EachoftheenumeratedCoffeeSizevaluesisactuallyaninstanceofCoffeeSize.
Inotherwords,BIGisoftypeCoffeeSize.Thinkofanenumasakindofclassthatlookssomething(but
notexactly)likethis:
Noticehoweachoftheenumeratedvalues,BIG,HUGE,andOVERWHELMING,isaninstanceoftype
CoffeeSize.They’rerepresentedasstaticandfinal,which,intheJavaworld,isthoughtofasa
constant.Alsonoticethateachenumvalueknowsitsindexorposition—inotherwords,theorderin
whichenumvaluesaredeclaredmatters.YoucanthinkoftheCoffeeSizeenumsasexistinginanarray
oftypeCoffeeSize,andasyou’llseeinalaterchapter,youcaniteratethroughthevaluesofanenumby
invokingthevalues()methodonanyenumtype.(Don’tworryaboutthatinthischapter.)
DeclaringConstructors,Methods,andVariablesinanenum
Becauseanenumreallyisaspecialkindofclass,youcandomorethanjustlisttheenumeratedconstant
values.Youcanaddconstructors,instancevariables,methods,andsomethingreallystrangeknownasa
constantspecificclassbody.Tounderstandwhyyoumightneedmoreinyourenum,thinkaboutthis
scenario:Imagineyouwanttoknowtheactualsize,inounces,thatmaptoeachofthethreeCoffeeSize
constants.Forexample,youwanttoknowthatBIGis8ounces,HUGEis10ounces,andOVERWHELMINGis
awhopping16ounces.
Youcouldmakesomekindofalookuptableusingsomeotherdatastructure,butthatwouldbeapoor
designandhardtomaintain.Thesimplestwayistotreatyourenumvalues(BIG,HUGE,and
OVERWHELMING)asobjects,eachofwhichcanhaveitsowninstancevariables.Thenyoucanassignthose
valuesatthetimetheenumsareinitializedbypassingavaluetotheenumconstructor.Thistakesalittle
explaining,butfirstlookatthefollowingcode:
whichproduces:
8
BIG8
HUGE10
OVERWHELMING16
Note:Everyenumhasastaticmethod,values(),thatreturnsanarrayoftheenum’svaluesintheorder
they’redeclared.
Thekeypointstorememberaboutenumconstructorsare
YoucanNEVERinvokeanenumconstructordirectly.Theenumconstructorisinvoked
automatically,withtheargumentsyoudefineaftertheconstantvalue.Forexample,BIG(8)invokes
theCoffeeSizeconstructorthattakesanint,passingtheintliteral8totheconstructor.(Behind
thescenes,ofcourse,youcanimaginethatBIGisalsopassedtotheconstructor,butwedon’thave
toknow—orcare—aboutthedetails.)
Youcandefinemorethanoneargumenttotheconstructor,andyoucanoverloadtheenum
constructors,justasyoucanoverloadanormalclassconstructor.Wediscussconstructorsinmuch
moredetailinChapter2.ToinitializeaCoffeeSizewithboththenumberofouncesand,say,alid
type,you’dpasstwoargumentstotheconstructorasBIG(8,“A”),whichmeansyouhavea
constructorinCoffeeSizethattakesbothanintandaString.
And,finally,youcandefinesomethingreallystrangeinanenumthatlookslikeananonymousinner
class.It’sknownasaconstantspecificclassbody,andyouuseitwhenyouneedaparticularconstantto
overrideamethoddefinedintheenum.
Imaginethisscenario:Youwantenumstohavetwomethods—oneforouncesandoneforlidcode(a
String).Nowimaginethatmostcoffeesizesusethesamelidcode,“B”,buttheOVERWHELMINGsizeuses
type“A”.YoucandefineagetLidCode()methodintheCoffeeSizeenumthatreturns“B”,butthenyou
needawaytooverrideitforOVERWHELMING.Youdon’twanttodosomehard-to-maintainif/thencode
inthegetLidCode()method,sothebestapproachmightbetosomehowhavetheOVERWHELMING
constantoverridethegetLidCode()method.
Thislooksstrange,butyouneedtounderstandthebasicdeclarationrules:
CERTIFICATIONSUMMARY
Afterabsorbingthematerialinthischapter,youshouldbefamiliarwithsomeofthenuancesoftheJava
language.Youmayalsobeexperiencingconfusionaroundwhyyoueverwantedtotakethisexaminthe
firstplace.That’snormalatthispoint.Ifyouhearyourselfasking,“WhatwasIthinking?”justliedown
untilitpasses.Wewouldliketotellyouthatitgetseasier…thatthiswasthetoughestchapterandit’sall
downhillfromhere.
Let’sbrieflyreviewwhatyou’llneedtoknowfortheexam:
Therewillbemanyquestionsdealingwithkeywordsindirectly,sobesureyoucanidentifywhichare
keywordsandwhicharen’t.
Youneedtounderstandtherulesassociatedwithcreatinglegalidentifiersandtherulesassociated
withsourcecodedeclarations,includingtheuseofpackageandimportstatements.
Youlearnedthebasicsyntaxforthejavaandjavaccommand-lineprograms.
Youlearnedaboutwhenmain()hassuperpowersandwhenitdoesn’t.
Wecoveredthebasicsofimportandimportstaticstatements.It’stemptingtothinkthatthere’s
moretothemthansavingabitoftyping,butthereisn’t.
Younowhaveagoodunderstandingofaccesscontrolasitrelatestoclasses,methods,andvariables.
You’velookedathowaccessmodifiers(public,protected,andprivate)definetheaccesscontrolof
aclassormember.
Youlearnedthatabstractclassescancontainbothabstractandnonabstractmethods,butthatif
evenasinglemethodismarkedabstract,theclassmustbemarkedabstract.Don’tforgetthata
concrete(nonabstract)subclassofanabstractclassmustprovideimplementationsforalltheabstract
methodsofthesuperclass,butthatanabstractclassdoesnothavetoimplementtheabstractmethods
fromitssuperclass.Anabstractsubclasscan“passthebuck”tothefirstconcretesubclass.
Wecoveredinterfaceimplementation.Rememberthatinterfacescanextendanotherinterface(even
multipleinterfaces),andthatanyclassthatimplementsaninterfacemustimplementallmethodsfromall
theinterfacesintheinheritancetreeoftheinterfacetheclassisimplementing.
You’vealsolookedattheothermodifiers,includingstatic,final,abstract,synchronized,and
soon.You’velearnedhowsomemodifierscanneverbecombinedinadeclaration,suchasmixing
abstractwitheitherfinalorprivate.
KeepinmindthattherearenofinalobjectsinJava.Areferencevariablemarkedfinalcanneverbe
changed,buttheobjectitreferstocanbemodified.You’veseenthatfinalappliedtomethodsmeansa
subclasscan’toverridethem,andwhenappliedtoaclass,thefinalclasscan’tbesubclassed.
Methodscanbedeclaredwithavar-argparameter(whichcantakefromzerotomanyargumentsofthe
declaredtype),butthatyoucanhaveonlyonevar-argpermethod,anditmustbethemethod’slast
parameter.
Makesureyou’refamiliarwiththerelativesizesofthenumericprimitives.Rememberthatwhilethe
valuesofnonfinalvariablescanchange,areferencevariable’stypecanneverchange.
Youalsolearnedthatarraysareobjectsthatcontainmanyvariablesofthesametype.Arrayscanalso
containotherarrays.
Rememberwhatyou’velearnedaboutstaticvariablesandmethods,especiallythatstaticmembers
areper-classasopposedtoper-instance.Don’tforgetthatastaticmethodcan’tdirectlyaccessan
instancevariablefromtheclassit’sinbecauseitdoesn’thaveanexplicitreferencetoanyparticular
instanceoftheclass.
Finally,wecoveredenums.Anenumisasafeandflexiblewaytoimplementconstants.Becausethey
areaspecialkindofclass,enumscanbedeclaredverysimply,ortheycanbequitecomplex—including
suchattributesasmethods,variables,constructors,andaspecialtypeofinnerclasscalledaconstant
specificclassbody.
Beforeyouhurlyourselfatthepracticetest,spendsometimewiththefollowingoptimisticallynamed
“Two-MinuteDrill.”Comebacktothisparticulardrilloftenasyouworkthroughthisbookand
especiallywhenyou’redoingthatlast-minutecramming.Because—andhere’stheadviceyouwished
yourmotherhadgivenyoubeforeyouleftforcollege—it’snotwhatyouknow,it’swhenyouknowit.
Fortheexam,knowingwhatyoucan’tdowiththeJavalanguageisjustasimportantasknowingwhat
youcando.Givethesamplequestionsatry!They’reverysimilartothedifficultyandstructureofthereal
examquestionsandshouldbeaneyeopenerforhowdifficulttheexamcanbe.Don’tworryifyougeta
lotofthemwrong.Ifyoufindatopicthatyouareweakin,spendmoretimereviewingandstudying.Many
programmersneedtwoorthreeseriouspassesthroughachapter(oranindividualobjective)beforethey
cananswerthequestionsconfidently.
TWO-MINUTEDRILL
Rememberthatinthischapter,whenwetalkaboutclasses,we’rereferringtonon-innerclasses,inother
words,top-levelclasses.
JavaFeaturesandBenefits(OCAObjective1.5)
WhileJavaprovidesmanybenefitstoprogrammers,fortheexamyoushouldrememberthat
Javasupportsobject-orientedprogrammingingeneral,encapsulation,automaticmemory
management,alargeAPI(library),built-insecurityfeatures,multiplatformcompatibility,strong
typing,multithreading,anddistributedcomputing.
Identifiers(OCAObjective2.1)
Identifierscanbeginwithaletter,anunderscore,oracurrencycharacter.
Afterthefirstcharacter,identifierscanalsoincludedigits.
Identifierscanbeofanylength.
ExecutableJavaFilesandmain()(OCAObjective1.3)
YoucancompileandexecuteJavaprogramsusingthecommand-lineprogramsjavacand
java,respectively.Bothprogramssupportavarietyofcommand-lineoptions.
Theonlyversionsofmain()methodswithspecialpowersarethoseversionswithmethod
signaturesequivalenttopublicstaticvoidmain(String[]args).
main()canbeoverloaded.
Imports(OCAObjective1.4)
Animportstatement’sonlyjobistosavekeystrokes.
Youcanuseanasterisk(*)tosearchthroughthecontentsofasinglepackage.
Althoughreferredtoas“staticimports,”thesyntaxisimportstatic….
YoucanimportAPIclassesand/orcustomclasses.
SourceFileDeclarationRules(OCAObjective1.2)
Asourcecodefilecanhaveonlyonepublicclass.
Ifthesourcefilecontainsapublicclass,thefilenamemustmatchthepublicclassname.
Afilecanhaveonlyonepackagestatement,butitcanhavemultipleimports.
Thepackagestatement(ifany)mustbethefirst(noncomment)lineinasourcefile.
Theimportstatements(ifany)mustcomeafterthepackagestatement(ifany)andbeforethe
firstclassdeclaration.
Ifthereisnopackagestatement,importstatementsmustbethefirst(noncomment)statements
inthesourcefile.
packageandimportstatementsapplytoallclassesinthefile.
Afilecanhavemorethanonenonpublicclass.
Fileswithnopublicclasseshavenonamingrestrictions.
ClassAccessModifiers(OCAObjective6.4)
Therearethreeaccessmodifiers:public,protected,andprivate.
Therearefouraccesslevels:public,protected,default,andprivate.
Classescanhaveonlypublicordefaultaccess.
Aclasswithdefaultaccesscanbeseenonlybyclasseswithinthesamepackage.
Aclasswithpublicaccesscanbeseenbyallclassesfromallpackages.
Classvisibilityrevolvesaroundwhethercodeinoneclasscan
Createaninstanceofanotherclass
Extend(orsubclass)anotherclass
Accessmethodsandvariablesofanotherclass
ClassModifiers(Nonaccess)(OCAObjectives1.2,7.1,and7.5)
Classescanalsobemodifiedwithfinal,abstract,orstrictfp.
Aclasscannotbebothfinalandabstract.
Afinalclasscannotbesubclassed.
Anabstractclasscannotbeinstantiated.
Asingleabstractmethodinaclassmeansthewholeclassmustbeabstract.
Anabstractclasscanhavebothabstractandnonabstractmethods.
Thefirstconcreteclasstoextendanabstractclassmustimplementallofitsabstract
methods.
InterfaceImplementation(OCAObjective7.5)
Usually,interfacesarecontractsforwhataclasscando,buttheysaynothingaboutthewayin
whichtheclassmustdoit.
Interfacescanbeimplementedbyanyclassfromanyinheritancetree.
Usually,aninterfaceislikea100percentabstractclassandisimplicitlyabstractwhether
ornotyoutypetheabstractmodifierinthedeclaration.
Usuallyinterfaceshaveonlyabstractmethods.
Interfacemethodsarebydefaultpublicandusuallyabstract—explicitdeclarationofthese
modifiersisoptional.
Interfacescanhaveconstants,whicharealwaysimplicitlypublic,static,andfinal.
Interfaceconstantdeclarationsofpublic,static,andfinalareoptionalinany
combination.
AsofJava8,interfacescanhaveconcretemethodsdeclaredaseitherdefaultorstatic.
Note:ThissectionusessomeconceptsthatweHAVENOTyetcovered.Don’tpanic:onceyou’veread
throughallofthebook,thissectionwillmakesenseasareference.
Alegalnonabstractimplementingclasshasthefollowingproperties:
Itprovidesconcreteimplementationsfortheinterface’smethods.
Itmustfollowalllegaloverriderulesforthemethodsitimplements.
Itmustnotdeclareanynewcheckedexceptionsforanimplementationmethod.
Itmustnotdeclareanycheckedexceptionsthatarebroaderthantheexceptionsdeclared
intheinterfacemethod.
Itmaydeclareruntimeexceptionsonanyinterfacemethodimplementationregardlessof
theinterfacedeclaration.
Itmustmaintaintheexactsignature(allowingforcovariantreturns)andreturntypeofthe
methodsitimplements(butdoesnothavetodeclaretheexceptionsoftheinterface).
Aclassimplementinganinterfacecanitselfbeabstract.
Anabstractimplementingclassdoesnothavetoimplementtheinterfacemethods(butthe
firstconcretesubclassmust).
Aclasscanextendonlyoneclass(nomultipleinheritance),butitcanimplementmany
interfaces.
Interfacescanextendoneormoreotherinterfaces.
Interfacescannotextendaclassorimplementaclassorinterface.
Whentakingtheexam,verifythatinterfaceandclassdeclarationsarelegalbeforeverifying
othercodelogic.
MemberAccessModifiers(OCAObjective6.4)
Methodsandinstance(nonlocal)variablesareknownas“members.”
Memberscanuseallfouraccesslevels:public,protected,default,andprivate.
Memberaccesscomesintwoforms:
Codeinoneclasscanaccessamemberofanotherclass.
Asubclasscaninheritamemberofitssuperclass.
Ifaclasscannotbeaccessed,itsmemberscannotbeaccessed.
Determineclassvisibilitybeforedeterminingmembervisibility.
publicmemberscanbeaccessedbyallotherclasses,eveninotherpackages.
Ifasuperclassmemberispublic,thesubclassinheritsit—regardlessofpackage.
Membersaccessedwithoutthedotoperator(.)mustbelongtothesameclass.
this.alwaysreferstothecurrentlyexecutingobject.
this.aMethod()isthesameasjustinvokingaMethod().
privatememberscanbeaccessedonlybycodeinthesameclass.
privatemembersarenotvisibletosubclasses,soprivatememberscannotbeinherited.
Defaultandprotectedmembersdifferonlywhensubclassesareinvolved:
Defaultmemberscanbeaccessedonlybyclassesinthesamepackage.
protectedmemberscanbeaccessedbyotherclassesinthesamepackage,plus
subclasses,regardlessofpackage.
protected=package+kids(kidsmeaningsubclasses).
Forsubclassesoutsidethepackage,theprotectedmembercanbeaccessedonlythrough
inheritance;asubclassoutsidethepackagecannotaccessaprotectedmemberbyusinga
referencetoasuperclassinstance.(Inotherwords,inheritanceistheonlymechanismfora
subclassoutsidethepackagetoaccessaprotectedmemberofitssuperclass.)
Aprotectedmemberinheritedbyasubclassfromanotherpackageisnotaccessibleto
anyotherclassinthesubclasspackage,exceptforthesubclass’sownsubclasses.
LocalVariables(OCAObjectives2.1and6.4)
Local(method,automatic,orstack)variabledeclarationscannothaveaccessmodifiers.
finalistheonlymodifieravailabletolocalvariables.
Localvariablesdon’tgetdefaultvalues,sotheymustbeinitializedbeforeuse.
OtherModifiers—Members(OCAObjectives7.1and7.5)
finalmethodscannotbeoverriddeninasubclass.
abstractmethodsaredeclaredwithasignature,areturntype,andanoptionalthrows
clause,buttheyarenotimplemented.
abstractmethodsendinasemicolon—nocurlybraces.
Threewaystospotanonabstractmethod:
Themethodisnotmarkedabstract.
Themethodhascurlybraces.
ThemethodMIGHThavecodebetweenthecurlybraces.
Thefirstnonabstract(concrete)classtoextendanabstractclassmustimplementallofthe
abstractclass’sabstractmethods.
Thesynchronizedmodifierappliesonlytomethodsandcodeblocks.
synchronizedmethodscanhaveanyaccesscontrolandcanalsobemarkedfinal.
abstractmethodsmustbeimplementedbyasubclass,sotheymustbeinheritable.Forthat
reason
abstractmethodscannotbeprivate.
abstractmethodscannotbefinal.
Thenativemodifierappliesonlytomethods.
Thestrictfpmodifierappliesonlytoclassesandmethods.
Methodswithvar-args(OCAObjective1.2)
Methodscandeclareaparameterthatacceptsfromzerotomanyarguments,aso-calledvar-
argmethod.
Avar-argparameterisdeclaredwiththesyntaxtype...name;forinstance:
doStuff(int...x){}.
Avar-argmethodcanhaveonlyonevar-argparameter.
Inmethodswithnormalparametersandavar-arg,thevar-argmustcomelast.
Constructors(OCAObjectives1.2,and6.3)
Constructorsmusthavethesamenameastheclass
Constructorscanhavearguments,buttheycannothaveareturntype.
Constructorscanuseanyaccessmodifier(evenprivate!).
VariableDeclarations(OCAObjective2.1)
Instancevariablescan
Haveanyaccesscontrol
Bemarkedfinalortransient
Instancevariablescan’tbeabstract,synchronized,native,orstrictfp.
Itislegaltodeclarealocalvariablewiththesamenameasaninstancevariable;thisis
called“shadowing.”
finalvariableshavethefollowingproperties:
finalvariablescannotbereassignedonceassignedavalue.
finalreferencevariablescannotrefertoadifferentobjectoncetheobjecthasbeen
assignedtothefinalvariable.
finalvariablesmustbeinitializedbeforetheconstructorcompletes.
Thereisnosuchthingasafinalobject.AnobjectreferencemarkedfinaldoesNOTmean
theobjectitselfcan’tchange.
Thetransientmodifierappliesonlytoinstancevariables.
Thevolatilemodifierappliesonlytoinstancevariables.
ArrayDeclarations(OCAObjectives4.1and4.2)
Arrayscanholdprimitivesorobjects,butthearrayitselfisalwaysanobject.
Whenyoudeclareanarray,thebracketscanbetotheleftortotherightofthevariablename.
Itisneverlegaltoincludethesizeofanarrayinthedeclaration.
AnarrayofobjectscanholdanyobjectthatpassestheIS-A(orinstanceof)testforthe
declaredtypeofthearray.Forexample,ifHorseextendsAnimal,thenaHorseobjectcangointo
anAnimalarray.
StaticVariablesandMethods(OCAObjective6.2)
Theyarenottiedtoanyparticularinstanceofaclass.
Noclassinstancesareneededinordertousestaticmembersoftheclassorinterface.
Thereisonlyonecopyofastaticvariable/class,andallinstancesshareit.
staticmethodsdonothavedirectaccesstononstaticmembers.
enums(OCAObjective1.2)
Anenumspecifiesalistofconstantvaluesassignedtoatype.
AnenumisNOTaStringoranint;anenumconstant’stypeistheenumtype.Forexample,
SUMMERandFALLareoftheenumtypeSeason.
Anenumcanbedeclaredoutsideorinsideaclass,butNOTinamethod.
AnenumdeclaredoutsideaclassmustNOTbemarkedstatic,final,abstract,
protected,orprivate.
enumscancontainconstructors,methods,variables,andconstant-specificclassbodies.
enumconstantscansendargumentstotheenumconstructor,usingthesyntaxBIG(8),where
theintliteral8ispassedtotheenumconstructor.
enumconstructorscanhaveargumentsandcanbeoverloaded.
enumconstructorscanNEVERbeinvokeddirectlyincode.Theyarealwayscalled
automaticallywhenanenumisinitialized.
Thesemicolonattheendofanenumdeclarationisoptional.Thesearelegal:
enumFoo{ONE,TWO,THREE}enumFoo{ONE,TWO,THREE};
MyEnum.values()returnsanarrayofMyEnum’svalues.
SELFTEST
Thefollowingquestionswillhelpyoumeasureyourunderstandingofthematerialpresentedinthis
chapter.Readallthechoicescarefully,astheremaybemorethanonecorrectanswer.Chooseallcorrect
answersforeachquestion.Stayfocused.
Ifyouhavearoughtimewiththeseatfirst,don’tbeatyourselfup.Bepositive.Repeatnice
affirmationstoyourselflike,“Iamsmartenoughtounderstandenums”and“OK,sothatotherguyknows
enumsbetterthanIdo,butIbethecan’t<insertsomethingyouaregoodat>likeme.”
1.Whicharetrue?(Chooseallthatapply.)
A.“XextendsY”iscorrectifandonlyifXisaclassandYisaninterface
B.“XextendsY”iscorrectifandonlyifXisaninterfaceandYisaclass
C.“XextendsY”iscorrectifXandYareeitherbothclassesorbothinterfaces
D.“XextendsY”iscorrectforallcombinationsofXandYbeingclassesand/orinterfaces
2.Given:
Whicharetrue?(Chooseallthatapply.)
A.Asthecodestands,theoutputisbang
B.Asthecodestands,theoutputissh-bang
C.Asthecodestands,compilationfails
D.IflineAisuncommented,theoutputisbangbang
E.IflineAisuncommented,theoutputissh-bangbang
F.IflineAisuncommented,compilationfails.
3.Giventhattheforloop’ssyntaxiscorrect,andgiven:
Andthecommandline:
java_-A.
Whatistheresult?
A.-A
B.A.
C.-A.
D._A.
E._-A.
F.Compilationfails
G.Anexceptionisthrownatruntime
4.Given:
Whatistheresult?
A.woofburble
B.Multiplecompilationerrors
C.Compilationfailsduetoanerroronline2
D.Compilationfailsduetoanerroronline3
E.Compilationfailsduetoanerroronline4
F.Compilationfailsduetoanerroronline9
5.Giventwofiles:
Whatistheresult?(Chooseallthatapply.)
A.567
B.5followedbyanexception
C.Compilationfailswithanerroronline7
D.Compilationfailswithanerroronline8
E.Compilationfailswithanerroronline9
F.Compilationfailswithanerroronline10
6.Given:
Whatistheresult?(Chooseallthatapply.)
A.Compilationsucceeds
B.Compilationfailswithanerroronline1
C.Compilationfailswithanerroronline3
D.Compilationfailswithanerroronline5
E.Compilationfailswithanerroronline7
F.Compilationfailswithanerroronline9
7.Given:
Whatistheresult?(Chooseallthatapply.)
A.Compilationsucceeds
B.Compilationfailswithanerroronline6
C.Compilationfailswithanerroronline7
D.Compilationfailswithanerroronline8
E.Compilationfailswithanerroronline9
8.Given:
Whatistheresult?(Chooseallthatapply.)
A.TUE
B.WED
C.Theoutputisunpredictable
D.Compilationfailsduetoanerroronline4
E.Compilationfailsduetoanerroronline6
F.Compilationfailsduetoanerroronline8
G.Compilationfailsduetoanerroronline9
9.Given:
Whatistheresult?
A.13
B.Compilationfailsduetomultipleerrors
C.Compilationfailsduetoanerroronline6
D.Compilationfailsduetoanerroronline7
E.Compilationfailsduetoanerroronline11
10.Given:
Whicharetrue?(Chooseallthatapply.)
A.TheclassTabletwillNOTcompile
B.TheinterfaceGadgetwillNOTcompile
C.Theoutputwillbepluginshowbook
D.TheabstractclassElectronicwillNOTcompile
E.TheclassTabletCANNOTbothextendandimplement
11.GiventhattheIntegerclassisinthejava.langpackageandgiven:
Which,insertedindependentlyatline1,compiles?(Chooseallthatapply.)
A.importstaticjava.lang;
B.importstaticjava.lang.Integer;
C.importstaticjava.lang.Integer.*;
D.staticimportjava.lang.Integer.*;
E.importstaticjava.lang.Integer.MAX_VALUE;
F.Noneoftheabovestatementsarevalidimportsyntax
12.Given:
Whichlinesofcode—insertedindependentlyatinsertcodehere—willcompile?(Chooseall
thatapply.)
A.publicstaticm1(){;}
B.defaultvoidm2(){;}
C.abstractintm3();
D.finalshortm4(){return5;}
E.defaultlongm5();
F.staticvoidm6(){;}
13.Whicharetrue?(Chooseallthatapply.)
A.Javaisadynamicallytypedprogramminglanguage
B.Javaprovidesfine-grainedcontrolofmemorythroughtheuseofpointers
C.Javaprovidesprogrammerstheabilitytocreateobjectsthatarewellencapsulated
D.JavaprovidesprogrammerstheabilitytosendJavaobjectsfromonemachinetoanother
E.JavaisanimplementationoftheECMAstandard
F.Java’sencapsulationcapabilitiesprovideitsprimarysecuritymechanism
SELFTESTANSWERS
1. Ciscorrect.
Aisincorrectbecauseclassesimplementinterfaces,theydon’textendthem.Bisincorrect
becauseinterfacesonly“inheritfrom”otherinterfaces.Disincorrectbasedontheprecedingrules.
(OCAObjective7.5)
2. BandFarecorrect.SinceRocket.blastOff()isprivate,itcan’tbeoverridden,andit
isinvisibletoclassShuttle.
A,C,D,andEareincorrectbasedontheabove.(OCAObjective6.4)
3. Biscorrect.Thisquestionisusingvalid(butinappropriateandweird)identifiers,static
imports,main(),andpre-incrementinglogic.(Note:Youmightgetacompilerwarningwhen
compilingthiscode.)
A,C,D,E,F,andGareincorrectbasedontheabove.(OCAObjective1.2)
4. Aiscorrect;enumscanhaveconstructorsandvariables.
B,C,D,E,andFareincorrect;theselinesallusecorrectsyntax.(OCAObjective1.2)
5. DandEarecorrect.Variableahasdefaultaccess,soitcannotbeaccessedfromoutside
thepackage.VariablebhasprotectedaccessinpkgA.
A,B,C,andFareincorrectbasedontheaboveinformation.(OCAObjectives1.4and6.5)
6. Aiscorrect;allofthesearelegaldeclarations.
B,C,D,E,andFareincorrectbasedontheaboveinformation.(OCAObjective7.5)
7. CandDarecorrect.Variablenamescannotbeginwitha#,andanarraydeclarationcan’t
includeasizewithoutaninstantiation.Therestofthecodeisvalid.
A,B,andEareincorrectbasedontheabove.(OCAObjective2.1)
8. Biscorrect.Everyenumcomeswithastaticvalues()methodthatreturnsanarrayof
theenum’svaluesintheorderinwhichtheyaredeclaredintheenum.
A,C,D,E,F,andGareincorrectbasedontheaboveinformation.(OCPObjective1.2)
9. Discorrect.ThecountGold()methodcannotbeinvokedfromastaticcontext.
A,B,C,andEareincorrectbasedontheaboveinformation.(OCAObjective6.2)
10. Aiscorrect.Bydefault,aninterface’smethodsarepublicsotheTablet.doStuff
methodmustbepublic,too.Therestofthecodeisvalid.
B,C,D,andEareincorrectbasedontheabove.(OCAObjective7.5)
11. CandEarecorrectsyntaxforstaticimports.Line4isn’tmakinguseofstaticimports,
sothecodewillalsocompilewithnoneoftheimports.
A,B,D,andFareincorrectbasedontheabove.(OCAObjective1.4)
12. B,C,andFarecorrect.AsofJava8,interfacescanhavedefaultandstaticmethods.
A,D,andEareincorrect.Ahasnoreturntype;Dcannothaveamethodbody;andEneedsa
methodbody.(OCAObjective7.5)
13. CandDarecorrect.
AisincorrectbecauseJavaisastaticallytypedlanguage.Bisincorrectbecauseitdoesnot
providepointers.EisincorrectbecauseJavaScriptisanimplementationoftheECMAstandard,
notJava.FisincorrectbecausetheuseofbytecodeandtheJVMprovideJava’sprimarysecurity
mechanisms.
B
2
ObjectOrientation
CERTIFICATIONOBJECTIVES
•DescribeEncapsulation
•ImplementInheritance
•UseIS-AandHAS-ARelationships(OCP)
•UsePolymorphism
•UseOverridingandOverloading
•UnderstandCasting
•UseInterfaces
•UnderstandandUseReturnTypes
•DevelopConstructors
•UsestaticMembers
Two-MinuteDrill
Q&ASelfTest
einganOracleCertifiedAssociate(OCA)8meansyoumustbeatonewiththeobject-
orientedaspectsofJava.Youmustdreamofinheritancehierarchies;thepowerof
polymorphismmustflowthroughyou;andencapsulationmustbecomesecondnaturetoyou.
(Coupling,cohesion,composition,anddesignpatternswillbecomeyourbreadandbutter
whenyou’reanOracleCertifiedProfessional[OCP]8.)Thischapterwillprepareyouforalltheobject-
orientedobjectivesandquestionsyou’llencounterontheexam.WehaveheardofmanyexperiencedJava
programmerswhohaven’treallybecomefluentwiththeobject-orientedtoolsthatJavaprovides,sowe’ll
startatthebeginning.
CERTIFICATIONOBJECTIVE
Encapsulation(OCAObjectives6.1and6.5)
6.1Createmethodswithargumentsandreturnvalues;includingoverloadedmethods.
6.5Applyencapsulationprinciplestoaclass.
Imagineyouwrotethecodeforaclassandanotherdozenprogrammersfromyourcompanyallwrote
programsthatusedyourclass.Nowimaginethatlateron,youdidn’tlikethewaytheclassbehaved,
becausesomeofitsinstancevariableswerebeingset(bytheotherprogrammersfromwithintheircode)
tovaluesyouhadn’tanticipated.Theircodebroughtouterrorsinyourcode.(Relax,thisisjust
hypothetical.)Well,itisaJavaprogram,soyoushouldbeabletoshipoutanewerversionoftheclass,
whichtheycouldreplaceintheirprogramswithoutchanginganyoftheirowncode.
Thisscenariohighlightstwoofthepromises/benefitsofanobject-oriented(OO)language:flexibility
andmaintainability.Butthosebenefitsdon’tcomeautomatically.Youhavetodosomething.Youhaveto
writeyourclassesandcodeinawaythatsupportsflexibilityandmaintainability.SowhatifJava
supportsOO?Itcan’tdesignyourcodeforyou.Forexample,imagineyoumadeyourclasswithpublic
instancevariables,andthoseotherprogrammersweresettingtheinstancevariablesdirectly,asthe
followingcodedemonstrates:
Andnowyou’reintrouble.Howareyougoingtochangetheclassinawaythatletsyouhandlethe
issuesthatcomeupwhensomebodychangesthesizevariabletoavaluethatcausesproblems?Your
onlychoiceistogobackinandwritemethodcodetoadjustsize(asetSize(inta)method,for
example)andtheninsulatethesizevariablewith,say,aprivateaccessmodifier.Butassoonasyou
makethatchangetoyourcode,youbreakeveryoneelse’s!
Theabilitytomakechangesinyourimplementationcodewithoutbreakingthecodeofotherswhouse
yourcodeisakeybenefitofencapsulation.Youwanttohideimplementationdetailsbehindapublic
programminginterface.Byinterface,wemeanthesetofaccessiblemethodsyourcodemakesavailable
forothercodetocall—inotherwords,yourcode’sAPI.Byhidingimplementationdetails,youcan
reworkyourmethodcode(perhapsalsoalteringthewayvariablesareusedbyyourclass)withoutforcing
achangeinthecodethatcallsyourchangedmethod.
Ifyouwantmaintainability,flexibility,andextensibility(and,ofcourse,youdo),yourdesignmust
includeencapsulation.Howdoyoudothat?
Keepinstancevariableshidden(withanaccessmodifier,oftenprivate).
Makepublicaccessormethods,andforcecallingcodetousethosemethodsratherthan
directlyaccessingtheinstancevariable.Theseso-calledaccessormethodsallowusersofyour
classtosetavariable’svalueorgetavariable’svalue.
Fortheseaccessormethods,usethemostcommonnamingconventionofset<SomeProperty>
andget<SomeProperty>.
Figure2-1illustratestheideathatencapsulationforcescallersofourcodetogothroughmethods
ratherthanaccessingvariablesdirectly.
FIGURE2-1 Thenatureofencapsulation
Wecalltheaccessmethodsgettersandsetters,althoughsomepreferthefanciertermsaccessorsand
mutators.(Personally,wedon’tliketheword“mutate.”)Regardlessofwhatyoucallthem,they’re
methodsthatotherprogrammersmustgothroughinordertoaccessyourinstancevariables.Theylook
simple,andyou’veprobablybeenusingthemforever:
Waitaminute.Howusefulisthepreviouscode?Itdoesn’tevendoanyvalidationorprocessing.What
benefitcantherebefromhavinggettersandsettersthataddnofunctionality?Thepointis,youcanchange
yourmindlaterandaddmorecodetoyourmethodswithoutbreakingyourAPI.Eveniftodayyoudon’t
thinkyoureallyneedvalidationorprocessingofthedata,goodOOdesigndictatesthatyouplanforthe
future.Tobesafe,forcecallingcodetogothroughyourmethodsratherthangoingdirectlytoinstance
variables.Always.Thenyou’refreetoreworkyourmethodimplementationslater,withoutriskingthe
wrathofthosedozenprogrammerswhoknowwhereyoulive.
Note:InChapter6we’llrevisitthetopicofencapsulationasitappliestoinstancevariablesthatare
alsoreferencevariables.It’strickierthanyoumightthink,sostaytuned!(Also,we’llwaituntilChapter6
tochallengeyouwithencapsulation-themedmockquestions.)
Lookoutforcodethatappearstobeaskingaboutthebehaviorofamethod,whentheproblem
isactuallyalackofencapsulation.Lookatthefollowingexample,andseeifyoucanfigure
outwhat’sgoingon:
Nowconsiderthisquestion:Isthevalueofrightalwaysgoingtobeone-thirdthevalueofleft?
Itlookslikeitwill,untilyourealizethatusersoftheFooclassdon’tneedtousethesetLeft()
method!Theycansimplygostraighttotheinstancevariablesandchangethemtoany
arbitraryintvalue.
CERTIFICATIONOBJECTIVE
InheritanceandPolymorphism(OCAObjectives7.1and
7.2)
7.1Describeinheritanceanditsbenefits.
7.2Developcodethatdemonstratestheuseofpolymorphism;includingoverridingandobjecttype
versusreferencetype(sic).
InheritanceiseverywhereinJava.It’ssafetosaythatit’salmost(almost?)impossibletowriteeven
thetiniestJavaprogramwithoutusinginheritance.Toexplorethistopic,we’regoingtousethe
instanceofoperator,whichwe’lldiscussinmoredetailinChapter4.Fornow,justrememberthat
instanceofreturnstrueifthereferencevariablebeingtestedisofthetypebeingcomparedto.This
code
producesthisoutput:
they’renotequal
t1’sanObject
Wheredidthatequalsmethodcomefrom?Thereferencevariablet1isoftypeTest,andthere’sno
equalsmethodintheTestclass.Oristhere?Thesecondiftestaskswhethert1isaninstanceofclass
Object,andbecauseitis(moreonthatsoon),theiftestsucceeds.
Holdon…howcant1beaninstanceoftypeObject,whenwejustsaiditwasoftypeTest?I’msure
you’rewayaheadofushere,butitturnsoutthateveryclassinJavaisasubclassofclassObject(except,
ofcourse,classObjectitself).Inotherwords,everyclassyou’lleveruseoreverwritewillinheritfrom
classObject.You’llalwayshaveanequalsmethod,aclonemethod,notify,wait,andothers
availabletouse.Wheneveryoucreateaclass,youautomaticallyinheritallofclassObject’smethods.
Why?Let’slookatthatequalsmethodforinstance.Java’screatorscorrectlyassumedthatitwouldbe
verycommonforJavaprogrammerstowanttocompareinstancesoftheirclassestocheckforequality.If
classObjectdidn’thaveanequalsmethod,you’dhavetowriteoneyourself—youandeveryotherJava
programmer.Thatoneequalsmethodhasbeeninheritedbillionsoftimes.(Tobefair,equalshasalso
beenoverriddenbillionsoftimes,butwe’regettingaheadofourselves.)
TheEvolutionofInheritance
UpuntilJava8,whenthetopicofinheritancewasdiscussed,itusuallyrevolvedaroundsubclasses
inheritingmethodsfromtheirsuperclasses.Whilethissimplificationwasneverperfectlycorrect,it
becamelesscorrectwiththenewfeaturesavailableinJava8.Asthefollowingtableshows,it’snow
possibletoinheritconcretemethodsfrominterfaces.Thisisabigchange.Fortherestofthechapter,
whenwetalkaboutinheritancegenerally,wewilltendtousetheterms“subtypes”and“supertypes”to
acknowledgethatbothclassesandinterfacesneedtobeaccountedfor.Wewilltendtousetheterms
“subclass”and“superclass”whenwe’rediscussingaspecificexamplethat’sunderdiscussion.
Inheritanceisakeyaspectofmostofthetopicswe’llbediscussinginthischapter,sobepreparedfor
LOTSofdiscussionabouttheinteractionsbetweensupertypesandsubtypes!
Asyoustudythefollowingtable,you’llnoticethatasofJava8interfacescancontaintwotypesof
concretemethods,staticanddefault.We’lldiscusstheseimportantadditionslaterinthischapter.
Table2-1summarizestheelementsofclassesandinterfacesrelativetoinheritance.
TABLE2-1 InheritableElementsofClassesandInterfaces
Fortheexam,you’llneedtoknowthatyoucancreateinheritancerelationshipsinJavabyextendinga
classorbyimplementinganinterface.It’salsoimportanttounderstandthatthetwomostcommonreasons
touseinheritanceare
Topromotecodereuse
Tousepolymorphism
Let’sstartwithreuse.Acommondesignapproachistocreateafairlygenericversionofaclasswith
theintentionofcreatingmorespecializedsubclassesthatinheritfromit.Forexample:
outputs:
displayingshape
movinggamepiece
NoticethatthePlayerPiececlassinheritsthegenericdisplayShape()methodfromtheless-
specializedclassGameShapeandalsoaddsitsownmethod,movePiece().Codereusethrough
inheritancemeansthatmethodswithgenericfunctionality—suchasdisplayShape(),whichcouldapply
toawiderangeofdifferentkindsofshapesinagame—don’thavetobereimplemented.Thatmeansall
specializedsubclassesofGameShapeareguaranteedtohavethecapabilitiesofthemoregeneral
superclass.Youdon’twanttohavetorewritethedisplayShape()codeineachofyourspecialized
componentsofanonlinegame.
Butyouknewthat.You’veexperiencedthepainofduplicatecodewhenyoumakeachangeinone
placeandhavetotrackdownalltheotherplaceswherethatsame(orverysimilar)codeexists.
Thesecond(andrelated)useofinheritanceistoallowyourclassestobeaccessedpolymorphically—
acapabilityprovidedbyinterfacesaswell,butwe’llgettothatinaminute.Let’ssaythatyouhavea
GameLauncherclassthatwantstoloopthroughalistofdifferentkindsofGameShapeobjectsandinvoke
displayShape()oneachofthem.Atthetimeyouwritethisclass,youdon’tknoweverypossiblekindof
GameShapesubclassthatanyoneelsewilleverwrite.Andyousuredon’twanttohavetoredoyourcode
justbecausesomebodydecidedtobuildadiceshapesixmonthslater.
Thebeautifulthingaboutpolymorphism(“manyforms”)isthatyoucantreatanysubclassof
GameShapeasaGameShape.Inotherwords,youcanwritecodeinyourGameLauncherclassthatsays,“I
don’tcarewhatkindofobjectyouareaslongasyouinheritfrom(extend)GameShape.AndasfarasI’m
concerned,ifyouextendGameShape,thenyou’vedefinitelygotadisplayShape()method,soIknowI
cancallit.”
ImaginewenowhavetwospecializedsubclassesthatextendthemoregenericGameShapeclass,
PlayerPieceandTilePiece:
NowimagineatestclasshasamethodwithadeclaredargumenttypeofGameShape,whichmeansit
cantakeanykindofGameShape.Inotherwords,anysubclassofGameShapecanbepassedtoamethod
withanargumentoftypeGameShape.Thiscode
outputs:
displayingshape
displayingshape
ThekeypointisthatthedoShapes()methodisdeclaredwithaGameShapeargumentbutcanbe
passedanysubtype(inthisexample,asubclass)ofGameShape.Themethodcantheninvokeanymethod
ofGameShape,withoutanyconcernfortheactualruntimeclasstypeoftheobjectpassedtothemethod.
Thereareimplications,though.ThedoShapes()methodknowsonlythattheobjectsareatypeof
GameShapesincethat’showtheparameterisdeclared.Andusingareferencevariabledeclaredastype
GameShape—regardlessofwhetherthevariableisamethodparameter,localvariable,orinstance
variable—meansthatonlythemethodsofGameShapecanbeinvokedonit.Themethodsyoucancallona
referencearetotallydependentonthedeclaredtypeofthevariable,nomatterwhattheactualobjectis,
thatthereferenceisreferringto.Thatmeansyoucan’tuseaGameShapevariabletocall,say,the
getAdjacent()methodeveniftheobjectpassedinisoftypeTilePiece.(We’llseethisagainwhenwe
lookatinterfaces.)
IS-AandHAS-ARelationships
Note:AsofWinter2017,theOCA8examwon’taskyoudirectlyaboutIS-AandHAS-Arelationships.
ButunderstandingIS-AandHAS-ArelationshipswillhelpOCA8candidateswithmanyofthequestions
ontheexam.
IS-A
InOO,theconceptofIS-Aisbasedoninheritance(orinterfaceimplementation).IS-Aisawayofsaying,
“Thisthingisatypeofthatthing.”Forexample,aMustangisatypeofHorse,soinOOtermswecansay,
“MustangIS-AHorse.”SubaruIS-ACar.BroccoliIS-AVegetable(notaveryfunone,butitstillcounts).
YouexpresstheIS-ArelationshipinJavathroughthekeywordsextends(forclassinheritance)and
implements(forinterfaceimplementation).
ACarisatypeofVehicle,sotheinheritancetreemightstartfromtheVehicleclassasfollows:
InOOterms,youcansaythefollowing:
VehicleisthesuperclassofCar.
CaristhesubclassofVehicle.
CaristhesuperclassofSubaru.
SubaruisthesubclassofVehicle.
CarinheritsfromVehicle.
SubaruinheritsfrombothVehicleandCar.
SubaruisderivedfromCar.
CarisderivedfromVehicle.
SubaruisderivedfromVehicle.
SubaruisasubtypeofbothVehicleandCar.
ReturningtoourIS-Arelationship,thefollowingstatementsaretrue:
“CarextendsVehicle”means“CarIS-AVehicle.”
“SubaruextendsCar”means“SubaruIS-ACar.”
Andwecanalsosay:
“SubaruIS-AVehicle”
becauseaclassissaidtobe“atypeof”anythingfurtherupinitsinheritancetree.Iftheexpression(Foo
instanceofBar)istrue,thenclassFooIS-ABar,evenifFoodoesn’tdirectlyextendBar,butinstead
extendssomeotherclassthatisasubclassofBar.Figure2-2illustratestheinheritancetreeforVehicle,
Car,andSubaru.Thearrowsmovefromthesubclasstothesuperclass.Inotherwords,aclass’sarrow
pointstowardtheclassfromwhichitextends.
FIGURE2-2 InheritancetreeforVehicle,Car,Subaru
HAS-A
HAS-Arelationshipsarebasedonuse,ratherthaninheritance.Inotherwords,classAHAS-ABifcode
inclassAhasareferencetoaninstanceofclassB.Forexample,youcansaythefollowing:
AHorseIS-AAnimal.AHorseHAS-AHalter.
Thecodemightlooklikethis:
Inthiscode,theHorseclasshasaninstancevariableoftypeHalter(ahalterisapieceofgearyoumight
haveifyouhaveahorse),soyoucansaythata“HorseHAS-AHalter.”Inotherwords,Horsehasa
referencetoaHalter.HorsecodecanusethatHalterreferencetoinvokemethodsontheHalterand
getHalterbehaviorwithouthavingHalter-relatedcode(methods)intheHorseclassitself.Figure2-3
illustratestheHAS-ArelationshipbetweenHorseandHalter.
FIGURE2-3 HAS-ArelationshipbetweenHorseandHalter
HAS-ArelationshipsallowyoutodesignclassesthatfollowgoodOOpracticesbynothaving
monolithicclassesthatdoagazilliondifferentthings.Classes(andtheirresultingobjects)shouldbe
specialists.AsourfriendAndrewsays,“Specializedclassescanactuallyhelpreducebugs.”Themore
specializedtheclass,themorelikelyitisthatyoucanreusetheclassinotherapplications.Ifyouputall
theHalter-relatedcodedirectlyintotheHorseclass,you’llendupduplicatingcodeintheCowclass,
UnpaidInternclass,andanyotherclassthatmightneedHalterbehavior.BykeepingtheHaltercode
inaseparate,specializedHalterclass,youhavethechancetoreusetheHalterclassinmultiple
applications.
UsersoftheHorseclass(thatis,codethatcallsmethodsonaHorseinstance)thinkthattheHorse
classhasHalterbehavior.TheHorseclassmighthaveatie(LeadRoperope)method,forexample.
UsersoftheHorseclassshouldneverhavetoknowthatwhentheyinvokethetie()method,theHorse
objectturnsaroundanddelegatesthecalltoitsHalterclassbyinvokingmyHalter.tie(rope).The
scenariojustdescribedmightlooklikethis:
InOO,wedon’twantcallerstoworryaboutwhichclassorobjectisactuallydoingtherealwork.To
makethathappen,theHorseclasshidesimplementationdetailsfromHorseusers.Horseusersaskthe
Horseobjecttodothings(inthiscase,tieitselfup),andtheHorsewilleitherdoitor,asinthisexample,
asksomethingelse(likeperhapsaninheritedAnimalclassmethod)todoit.Tothecaller,though,it
alwaysappearsthattheHorseobjecttakescareofitself.UsersofaHorseshouldnotevenneedtoknow
thatthereissuchathingasaHalterclass.
FROMTHECLASSROOM
Object-OrientedDesign
IS-AandHAS-ArelationshipsandencapsulationarejustthetipoftheicebergwhenitcomestoOO
design.Manybooksandgraduatetheseshavebeendedicatedtothistopic.Thereasonforthe
emphasisonproperdesignissimple:money.Thecosttodeliverasoftwareapplicationhasbeen
estimatedtobeasmuchastentimesmoreexpensiveforpoorlydesignedprograms.
EventhebestOOdesigners(oftencalled"architects")makemistakes.Itisdifficulttovisualizethe
relationshipsbetweenhundreds,oreventhousands,ofclasses.Whenmistakesarediscoveredduring
theimplementation(codewriting)phaseofaproject,theamountofcodethatmustberewrittencan
sometimesmeanprogrammingteamshavetostartoverfromscratch.
Thesoftwareindustryhasevolvedtoaidthedesigner.Visualobjectmodelinglanguages,suchas
theUnifiedModelingLanguage(UML),allowdesignerstodesignandeasilymodifyclasseswithout
havingtowritecodefirstbecauseOOcomponentsarerepresentedgraphically.Thisallowsdesigners
tocreateamapoftheclassrelationshipsandhelpsthemrecognizeerrorsbeforecodingbegins.
AnotherinnovationinOOdesignisdesignpatterns.DesignersnoticedthatmanyOOdesignsapply
consistentlyfromprojecttoprojectandthatitwasusefultoapplythesamedesignsbecauseitreduced
thepotentialtointroducenewdesignerrors.OOdesignersthenstartedtosharethesedesignswith
eachother.NowtherearemanycatalogsofthesedesignpatternsbothontheInternetandinbookform.
AlthoughpassingtheJavacertificationexamdoesnotrequireyoutounderstandOOdesignthis
thoroughly,hopefullythisbackgroundinformationwillhelpyoubetterappreciatewhythetestwriters
chosetoincludeencapsulationandIS-AandHAS-Arelationshipsontheexam.
—JonathanMeeks,
SunCertifiedJavaProgrammer
CERTIFICATIONOBJECTIVE
Polymorphism(OCAObjective7.2)
7.2Developcodethatdemonstratestheuseofpolymorphism;includingoverridingandobjecttype
versusreferencetype(sic).
RememberthatanyJavaobjectthatcanpassmorethanoneIS-Atestcanbeconsideredpolymorphic.
OtherthanobjectsoftypeObject,allJavaobjectsarepolymorphicinthattheypasstheIS-Atestfor
theirowntypeandforclassObject.
Remember,too,thattheonlywaytoaccessanobjectisthroughareferencevariable.Thereareafew
keythingsyoushouldknowaboutreferences:
Areferencevariablecanbeofonlyonetype,andoncedeclared,thattypecanneverbe
changed(althoughtheobjectitreferencescanchange).
Areferenceisavariable,soitcanbereassignedtootherobjects(unlessthereferenceis
declaredfinal).
Areferencevariable’stypedeterminesthemethodsthatcanbeinvokedontheobjectthe
variableisreferencing.
Areferencevariablecanrefertoanyobjectofthesametypeasthedeclaredreference,or—
thisisthebigone—itcanrefertoanysubtypeofthedeclaredtype!
Areferencevariablecanbedeclaredasaclasstypeoraninterfacetype.Ifthevariableis
declaredasaninterfacetype,itcanreferenceanyobjectofanyclassthatimplementstheinterface.
EarlierwecreatedaGameShapeclassthatwasextendedbytwootherclasses,PlayerPieceand
TilePiece.Nowimagineyouwanttoanimatesomeoftheshapesonthegameboard.Butnotallshapes
areabletobeanimated,sowhatdoyoudowithclassinheritance?
Couldwecreateaclasswithananimate()methodandhaveonlysomeoftheGameShapesubclasses
inheritfromthatclass?Ifwecan,thenwecouldhavePlayerPiece,forexample,extendboththe
GameShapeclassandAnimatableclass,whereastheTilePiecewouldextendonlyGameShape.Butno,
thiswon’twork!Javasupportsonlysingleclassinheritance!Thatmeansaclasscanhaveonlyone
immediatesuperclass.Inotherwords,ifPlayerPieceisaclass,thereisnowaytosaysomethinglike
this:
classPlayerPieceextendsGameShape,Animatable{//NO!
//morecode
}
Aclasscannotextendmorethanoneclass:thatmeansoneparentperclass.Aclasscanhavemultiple
ancestors,however,sinceclassBcouldextendclassA,andclassCcouldextendclassB,andsoon.So
anygivenclassmighthavemultipleclassesupitsinheritancetree,butthat’snotthesameassayingaclass
directlyextendstwoclasses.
Somelanguages(suchasC++)allowaclasstoextendmorethanoneotherclass.Thiscapabilityis
knownas“multipleinheritance.”ThereasonthatJava’screatorschosenottoallowmultipleclass
inheritanceisthatitcanbecomequitemessy.Inanutshell,theproblemisthatifaclassextended
twootherclasses,andbothsuperclasseshad,say,adoStuff()method,whichversionofdoStuff()
wouldthesubclassinherit?Thisissuecanleadtoascenarioknownasthe“DeadlyDiamondof
Death,”becauseoftheshapeoftheclassdiagramthatcanbecreatedinamultipleinheritance
design.ThediamondisformedwhenclassesBandCbothextendA,andbothBandCinherita
methodfromA.IfclassDextendsbothBandC,andbothBandChaveoverriddenthemethodinA,
classDhas,intheory,inheritedtwodifferentimplementationsofthesamemethod.Drawnasa
classdiagram,theshapeofthefourclasseslookslikeadiamond.
Soifthatdoesn’twork,whatelsecouldyoudo?Youcouldsimplyputtheanimate()codein
GameShape,andthendisablethemethodinclassesthatcan’tbeanimated.Butthat’sabaddesignchoice
formanyreasons—it’smoreerrorprone;itmakestheGameShapeclasslesscohesive;anditmeansthe
GameShapeAPI“advertises”thatallshapescanbeanimatedwhen,infact,that’snottruesinceonlysome
oftheGameShapesubclasseswillbeabletoruntheanimate()methodsuccessfully.
Toreiterate,asofJava8,interfacescanhaveconcretemethods(calleddefaultmethods).This
allowsforaformofmultipleinterhance,whichwe’lldiscusslaterinthechapter.
Sowhatelsecouldyoudo?Youalreadyknowtheanswer—createanAnimatableinterface,andhave
onlytheGameShapesubclassesthatcanbeanimatedimplementthatinterface.Here’stheinterface:
Andhere’sthemodifiedPlayerPiececlassthatimplementstheinterface:
SonowwehaveaPlayerPiecethatpassestheIS-AtestforboththeGameShapeclassandthe
Animatableinterface.ThatmeansaPlayerPiececanbetreatedpolymorphicallyasoneoffourthingsat
anygiventime,dependingonthedeclaredtypeofthereferencevariable:
AnObject(sinceanyobjectinheritsfromObject)
AGameShape(sincePlayerPieceextendsGameShape)
APlayerPiece(sincethat’swhatitreallyis)
AnAnimatable(sincePlayerPieceimplementsAnimatable)
Thefollowingarealllegaldeclarations.Lookclosely:
There’sonlyoneobjecthere—aninstanceoftypePlayerPiece—buttherearefourdifferenttypesof
referencevariables,allreferringtothatoneobjectontheheap.Popquiz:Whichofthepreceding
referencevariablescaninvokethedisplayShape()method?Hint:Onlytwoofthefourdeclarationscan
beusedtoinvokethedisplayShape()method.
Rememberthatmethodinvocationsallowedbythecompilerarebasedsolelyonthedeclaredtypeof
thereference,regardlessoftheobjecttype.Solookingatthefourreferencetypesagain—Object,
GameShape,PlayerPiece,andAnimatable—whichofthesefourtypesknowaboutthe
displayShape()method?
Youguessedit—boththeGameShapeclassandthePlayerPiececlassareknown(bythecompiler)to
haveadisplayShape()method,soeitherofthosereferencetypescanbeusedtoinvoke
displayShape().Rememberthattothecompiler,aPlayerPieceIS-AGameShape,sothecompiler
says,“IseethatthedeclaredtypeisPlayerPiece,andsincePlayerPieceextendsGameShape,that
meansPlayerPieceinheritedthedisplayShape()method.Therefore,PlayerPiececanbeusedto
invokethedisplayShape()method.”
WhichmethodscanbeinvokedwhenthePlayerPieceobjectisbeingreferredtousingareference
declaredastypeAnimatable?Onlytheanimate()method.Ofcourse,thecoolthinghereisthatany
classfromanyinheritancetreecanalsoimplementAnimatable,sothatmeansifyouhaveamethodwith
anargumentdeclaredastypeAnimatable,youcanpassinPlayerPieceobjects,SpinningLogo
objects,andanythingelsethat’saninstanceofaclassthatimplementsAnimatable.Andyoucanusethat
parameter(oftypeAnimatable)toinvoketheanimate()method,butnotthedisplayShape()method
(whichitmightnotevenhave),oranythingotherthanwhatisknowntothecompilerbasedonthe
referencetype.Thecompileralwaysknows,though,thatyoucaninvokethemethodsofclassObjecton
anyobject,sothosearesafetocallregardlessofthereference—classorinterface—usedtorefertothe
object.
We’veleftoutonebigpartofallthis,whichisthateventhoughthecompileronlyknowsaboutthe
declaredreferencetype,theJavaVirtualMachine(JVM)atruntimeknowswhattheobjectreallyis.And
thatmeansthatevenifthePlayerPieceobject’sdisplayShape()methodiscalledusingaGameShape
referencevariable,ifthePlayerPieceoverridesthedisplayShape()method,theJVMwillinvokethe
PlayerPieceversion!TheJVMlooksattherealobjectattheotherendofthereference,“sees”thatit
hasoverriddenthemethodofthedeclaredreferencevariabletype,andinvokesthemethodoftheobject’s
actualclass.Butthereisoneotherthingtokeepinmind:
Polymorphicmethodinvocationsapplyonlytoinstancemethods.Youcanalwaysrefertoanobjectwithamoregeneral
referencevariabletype(asuperclassorinterface),butatruntime,theONLYthingsthataredynamicallyselectedbasedonthe
actualobject(ratherthanthereferencetype)areinstancemethods.Notstaticmethods.Notvariables.Onlyoverridden
instancemethodsaredynamicallyinvokedbasedontherealobject’stype.
Becausethisdefinitiondependsonaclearunderstandingofoverridingandthedistinctionbetweenstatic
methodsandinstancemethods,we’llcoverthoselaterinthechapter.
CERTIFICATIONOBJECTIVE
Overriding/Overloading(OCAObjectives6.1and7.2)
6.1Createmethodswithargumentsandreturnvalues;includingoverloadedmethods.
7.2Developcodethatdemonstratestheuseofpolymorphism;includingoverridingandobjecttype
versusreferencetype(sic).
Theexamwilluseoverriddenandoverloadedmethodsonmany,manyquestions.Thesetwoconcepts
areoftenconfused(perhapsbecausetheyhavesimilarnames?),buteachhasitsownuniqueandcomplex
setofrules.It’simportanttogetreallyclearaboutwhich“over”useswhichrules!
OverriddenMethods
Anytimeatypeinheritsamethodfromasupertype,youhavetheopportunitytooverridethemethod
(unless,asyoulearnedearlier,themethodismarkedfinal).Thekeybenefitofoverridingistheability
todefinebehaviorthat’sspecifictoaparticularsubtype.ThefollowingexampledemonstratesaHorse
subclassofAnimaloverridingtheAnimalversionoftheeat()method:
Forabstractmethodsyouinheritfromasupertype,youhavenochoice:Youmustimplementthe
methodinthesubtypeunlessthesubtypeisalsoabstract.Abstractmethodsmustbeimplementedbythe
firstconcretesubclass,butthisisalotlikesayingtheconcretesubclassoverridestheabstractmethodsof
thesupertype(s).Soyoucouldthinkofabstractmethodsasmethodsyou’reforcedtooverride—
eventually.
TheAnimalclasscreatormighthavedecidedthatforthepurposesofpolymorphism,allAnimal
subtypesshouldhaveaneat()methoddefinedinauniqueway.Polymorphically,whenanAnimal
referencerefersnottoanAnimalinstance,buttoanAnimalsubclassinstance,thecallershouldbeable
toinvokeeat()ontheAnimalreference,buttheactualruntimeobject(say,aHorseinstance)willrunits
ownspecificeat()method.Markingtheeat()methodabstractistheAnimalprogrammer’swayof
sayingtoallsubclassdevelopers,“Itdoesn’tmakeanysenseforyournewsubtypetouseagenericeat()
method,soyouhavetocomeupwithyourowneat()methodimplementation!”A(nonabstract)example
ofusingpolymorphismlookslikethis:
Intheprecedingcode,thetestclassusesanAnimalreferencetoinvokeamethodonaHorseobject.
Remember,thecompilerwillallowonlymethodsinclassAnimaltobeinvokedwhenusingareference
toanAnimal.Thefollowingwouldnotbelegalgiventheprecedingcode:
Toreiterate,thecompilerlooksonlyatthereferencetype,nottheinstancetype.Polymorphismletsyou
useamoreabstractsupertype(includinganinterface)referencetooneofitssubtypes(includinginterface
implementers).
Theoverridingmethodcannothaveamorerestrictiveaccessmodifierthanthemethodbeing
overridden(forexample,youcan’toverrideamethodmarkedpublicandmakeitprotected).Think
aboutit:IftheAnimalclassadvertisesapubliceat()methodandsomeonehasanAnimalreference
(inotherwords,areferencedeclaredastypeAnimal),thatsomeonewillassumeit’ssafetocalleat()
ontheAnimalreferenceregardlessoftheactualinstancethattheAnimalreferenceisreferringto.Ifa
subtypewereallowedtosneakinandchangetheaccessmodifierontheoverridingmethod,thensuddenly
atruntime—whentheJVMinvokesthetrueobject’s(Horse)versionofthemethodratherthanthe
referencetype’s(Animal)version—theprogramwoulddieahorribledeath.(Nottomentionthe
emotionaldistressfortheonewhowasbetrayedbytheroguesubtype.)
Let’smodifythepolymorphicexamplewesawearlierinthissection:
Ifthiscodecompiled(whichitdoesn’t),thefollowingwouldfailatruntime:
ThevariablebisoftypeAnimal,whichhasapubliceat()method.Butrememberthatatruntime,
Javausesvirtualmethodinvocationtodynamicallyselecttheactualversionofthemethodthatwillrun,
basedontheactualinstance.AnAnimalreferencecanalwaysrefertoaHorseinstance,becauseHorse
IS-A(n)Animal.Whatmakesthatsupertypereferencetoasubtypeinstancepossibleisthatthesubtypeis
guaranteedtobeabletodoeverythingthesupertypecando.WhethertheHorseinstanceoverridesthe
inheritedmethodsofAnimalorsimplyinheritsthem,anyonewithanAnimalreferencetoaHorse
instanceisfreetocallallaccessibleAnimalmethods.Forthatreason,anoverridingmethodmustfulfill
thecontractofthesuperclass.
Note:InChapter5wewillexploreexceptionhandlingindetail.Onceyou’vestudiedChapter5,you’ll
appreciatethissinglehandylistofoverridingrules.Therulesforoverridingamethodareasfollows:
Theargumentlistmustexactlymatchthatoftheoverriddenmethod.Iftheydon’tmatch,you
canendupwithanoverloadedmethodyoudidn’tintend.
Thereturntypemustbethesameas,orasubtypeof,thereturntypedeclaredintheoriginal
overriddenmethodinthesuperclass.(Moreonthisinafewpageswhenwediscusscovariant
returns.)
Theaccesslevelcan’tbemorerestrictivethanthatoftheoverriddenmethod.
TheaccesslevelCANbelessrestrictivethanthatoftheoverriddenmethod.
Instancemethodscanbeoverriddenonlyiftheyareinheritedbythesubtype.Asubtypewithin
thesamepackageastheinstance’ssupertypecanoverrideanysupertypemethodthatisnotmarked
privateorfinal.Asubtypeinadifferentpackagecanoverrideonlythosenonfinalmethods
markedpublicorprotected(sinceprotectedmethodsareinheritedbythesubtype).
TheoverridingmethodCANthrowanyunchecked(runtime)exception,regardlessofwhether
theoverriddenmethoddeclarestheexception.(MoreinChapter5.)
TheoverridingmethodmustNOTthrowcheckedexceptionsthatareneworbroaderthan
thosedeclaredbytheoverriddenmethod.Forexample,amethodthatdeclaresa
FileNotFoundExceptioncannotbeoverriddenbyamethodthatdeclaresaSQLException,
Exception,oranyothernonruntimeexceptionunlessit’sasubclassofFileNotFoundException.
Theoverridingmethodcanthrownarrowerorfewerexceptions.Justbecauseanoverridden
method“takesrisks”doesn’tmeanthattheoverridingsubtype’sexceptiontakesthesamerisks.
Bottomline:anoverridingmethoddoesn’thavetodeclareanyexceptionsthatitwillneverthrow,
regardlessofwhattheoverriddenmethoddeclares.
Youcannotoverrideamethodmarkedfinal.
Youcannotoverrideamethodmarkedstatic.We’lllookatanexampleinafewpageswhen
wediscussstaticmethodsinmoredetail.
Ifamethodcan’tbeinherited,youcannotoverrideit.Rememberthatoverridingimpliesthat
you’rereimplementingamethodyouinherited!Forexample,thefollowingcodeisnotlegal,and
evenifyouaddedaneat()methodtoHorse,itwouldn’tbeanoverrideofAnimal’seat()
method:
InvokingaSupertypeVersionofanOverriddenMethod
Often,you’llwanttotakeadvantageofsomeofthecodeinthesupertypeversionofamethod,yetstill
overrideittoprovidesomeadditionalspecificbehavior.It’slikesaying,“Runthesupertypeversionof
themethod,andthencomebackdownhereandfinishwithmysubtypeadditionalmethodcode.”(Note
thatthere’snorequirementthatthesupertypeversionrunbeforethesubtypecode.)It’seasytodoincode
usingthekeywordsuperasfollows:
Inasimilarway,youcanaccessaninterface’soverriddenmethodwiththesyntax:
InterfaceX.super.doStuff();
Note:Usingsupertoinvokeanoverriddenmethodappliesonlytoinstancemethods.(Rememberthat
staticmethodscan’tbeoverridden.)Andyoucanusesuperonlytoaccessamethodinatype’s
supertype,notthesupertypeofthesupertype—thatis,youcannotsaysuper.super.doStuff()andyou
cannotsay:InterfaceX.super.super.doStuff().
Ifamethodisoverriddenbutyouuseapolymorphic(supertype)referencetorefertothe
subtypeobjectwiththeoverridingmethod,thecompilerassumesyou’recallingthesupertype
versionofthemethod.Ifthesupertypeversiondeclaresacheckedexception,buttheoverriding
subtypemethoddoesnot,thecompilerstillthinksyouarecallingamethodthatdeclaresan
exception(moreinChapter5).Let’slookatanexample:
ThiscodewillnotcompilebecauseoftheexceptiondeclaredontheAnimaleat()method.
Thishappenseventhough,atruntime,theeat()methodusedwouldbetheDogversion,which
doesnotdeclaretheexception.
ExamplesofIllegalMethodOverrides
Let’stakealookatoverridingtheeat()methodofAnimal:
publicclassAnimal{
publicvoideat(){}
}
Table2-2listsexamplesofillegaloverridesoftheAnimaleat()method,giventhepreceding
versionoftheAnimalclass.
TABLE2-2 ExamplesofIllegalOverrides
OverloadedMethods
Overloadedmethodsletyoureusethesamemethodnameinaclass,butwithdifferentarguments(and,
optionally,adifferentreturntype).Overloadingamethodoftenmeansyou’rebeingalittlenicertothose
whocallyourmethodsbecauseyourcodetakesontheburdenofcopingwithdifferentargumenttypes
ratherthanforcingthecallertodoconversionspriortoinvokingyourmethod.Therulesaren’ttoo
complex:
OverloadedmethodsMUSTchangetheargumentlist.
OverloadedmethodsCANchangethereturntype.
OverloadedmethodsCANchangetheaccessmodifier.
OverloadedmethodsCANdeclareneworbroadercheckedexceptions.
Amethodcanbeoverloadedinthesametypeorinasubtype.Inotherwords,ifclassA
definesadoStuff(inti)method,thesubclassBcoulddefineadoStuff(Strings)method
withoutoverridingthesuperclassversionthattakesanint.Sotwomethodswiththesamename
butindifferenttypescanstillbeconsideredoverloadedifthesubtypeinheritsoneversionofthe
methodandthendeclaresanotheroverloadedversioninitstypedefinition.
LessexperiencedJavadevelopersareoftenconfusedaboutthesubtledifferencesbetween
overloadedandoverriddenmethods.Becarefultorecognizewhenamethodisoverloaded
ratherthanoverridden.Youmightseeamethodthatappearstobeviolatingarulefor
overriding,butthatisactuallyalegaloverload,asfollows:
It’stemptingtoseetheIOExceptionastheproblembecausetheoverriddendoStuff()method
doesn’tdeclareanexceptionandIOExceptionischeckedbythecompiler.ButthedoStuff()
methodisnotoverridden!SubclassBaroverloadsthedoStuff()methodbyvaryingthe
argumentlist,sotheIOExceptionisfine.
LegalOverloads
Let’slookatamethodwewanttooverload:
InvokingOverloadedMethods
InChapter6wewilllookathowboxingandvar-argsimpactoverloading.(Youstillhavetopayattention
towhat’scoveredhere,however.)
Whenamethodisinvoked,morethanonemethodofthesamenamemightexistfortheobjecttype
you’reinvokingamethodon.Forexample,theHorseclassmighthavethreemethodswiththesamename
butwithdifferentargumentlists,whichmeansthemethodisoverloaded.
Decidingwhichofthematchingmethodstoinvokeisbasedonthearguments.Ifyouinvokethemethod
withaStringargument,theoverloadedversionthattakesaStringiscalled.Ifyouinvokeamethodof
thesamenamebutpassitafloat,theoverloadedversionthattakesafloatwillrun.Ifyouinvokethe
methodofthesamenamebutpassitaFooobject,andthereisn’tanoverloadedversionthattakesaFoo,
thenthecompilerwillcomplainthatitcan’tfindamatch.Thefollowingareexamplesofinvoking
overloadedmethods:
InthisTestAddercode,thefirstcalltoa.addThem(b,c)passestwointstothemethod,sothefirst
versionofaddThem()—theoverloadedversionthattakestwointarguments—iscalled.Thesecondcall
toa.addThem(22.5,9.3)passestwodoublestothemethod,sothesecondversionofaddThem()—the
overloadedversionthattakestwodoublearguments—iscalled.
Invokingoverloadedmethodsthattakeobjectreferencesratherthanprimitivesisalittlemore
interesting.SayyouhaveanoverloadedmethodsuchthatoneversiontakesanAnimalandonetakesa
Horse(subclassofAnimal).IfyoupassaHorseobjectinthemethodinvocation,you’llinvokethe
overloadedversionthattakesaHorse.Orsoitlooksatfirstglance:
Theoutputiswhatyouexpect:
IntheAnimalversion
IntheHorseversion
ButwhatifyouuseanAnimalreferencetoaHorseobject?
AnimalanimalRefToHorse=newHorse();
ua.doStuff(animalRefToHorse);
Whichoftheoverloadedversionsisinvoked?Youmightwanttoanswer,“TheonethattakesaHorse
sinceit’saHorseobjectatruntimethat’sbeingpassedtothemethod.”Butthat’snothowitworks.The
precedingcodewouldactuallyprintthis:
intheAnimalversion
EventhoughtheactualobjectatruntimeisaHorseandnotanAnimal,thechoiceofwhichoverloaded
methodtocall(inotherwords,thesignatureofthemethod)isNOTdynamicallydecidedatruntime.
Justrememberthatthereferencetype(nottheobjecttype)determineswhichoverloadedmethodis
invoked!
Tosummarize,whichoverriddenversionofthemethodtocall(inotherwords,fromwhichclass
intheinheritancetree)isdecidedatruntimebasedonobjecttype,butwhichoverloadedversionof
themethodtocallisbasedonthereferencetypeoftheargumentpassedatcompiletime.
IfyouinvokeamethodpassingitanAnimalreferencetoaHorseobject,thecompilerknowsonly
abouttheAnimal,soitchoosestheoverloadedversionofthemethodthattakesanAnimal.Itdoesnot
matterthat,atruntime,aHorseisactuallybeingpassed.
Canmain()beoverloaded?
Absolutely!Buttheonlymain()withJVMsuperpowersistheonewiththesignatureyou’ve
seenabout100timesalreadyinthisbook.
PolymorphisminOverloadedandOverriddenMethodsHowdoespolymorphismwork
withoverloadedmethods?Fromwhatwejustlookedat,itdoesn’tappearthatpolymorphismmatters
whenamethodisoverloaded.IfyoupassanAnimalreference,theoverloadedmethodthattakesan
Animalwillbeinvoked,eveniftheactualobjectpassedisaHorse.OncetheHorsemasqueradingas
Animalgetsintothemethod,however,theHorseobjectisstillaHorsedespitebeingpassedintoa
methodexpectinganAnimal.Soit’struethatpolymorphismdoesn’tdeterminewhichoverloadedversion
iscalled;however,polymorphismdoescomeintoplaywhenthedecisionisaboutwhichoverridden
versionofamethodiscalled.Butsometimesamethodisbothoverloadedandoverridden.Imaginethat
theAnimalandHorseclasseslooklikethis:
NoticethattheHorseclasshasbothoverloadedandoverriddentheeat()method.Table2-3shows
whichversionofthethreeeat()methodswillrundependingonhowtheyareinvoked.
TABLE2-3 ExamplesofLegalandIllegalOverrides
Don’tbefooledbyamethodthat’soverloadedbutnotoverriddenbyasubclass.It’sperfectly
legaltodothefollowing:
TheBarclasshastwodoStuff()methods:theno-argversionitinheritsfromFoo(anddoes
notoverride)andtheoverloadeddoStuff(Strings)definedintheBarclass.Codewitha
referencetoaFoocaninvokeonlytheno-argversion,butcodewithareferencetoaBarcan
invokeeitheroftheoverloadedversions.
Table2-4summarizesthedifferencebetweenoverloadedandoverriddenmethods.
TABLE2-4 DifferencesBetweenOverloadedandOverriddenMethods
We’llcoverconstructoroverloadinglaterinthechapter,wherewe’llalsocovertheotherconstructor-
relatedtopicsthatareontheexam.Figure2-4illustratesthewayoverloadedandoverriddenmethods
appearinclassrelationships.
FIGURE2-4 Overloadedandoverriddenmethodsinclassrelationships
CERTIFICATIONOBJECTIVE
Casting(OCAObjectives2.2and7.3)
2.2Differentiatebetweenobjectreferencevariablesandprimitivevariables.
7.3Determinewhencastingisnecessary.
You’veseenhowit’sbothpossibleandcommontousegeneralreferencevariabletypestoreferto
morespecificobjecttypes.It’sattheheartofpolymorphism.Forexample,thislineofcodeshouldbe
secondnaturebynow:
Animalanimal=newDog();
Butwhathappenswhenyouwanttousethatanimalreferencevariabletoinvokeamethodthatonly
classDoghas?Youknowit’sreferringtoaDog,andyouwanttodoaDog-specificthing?Inthefollowing
code,we’vegotanarrayofAnimals,andwheneverwefindaDoginthearray,wewanttodoaspecial
Dogthing.Let’sagreefornowthatallthiscodeisokay,exceptthatwe’renotsureaboutthelineofcode
thatinvokestheplayDeadmethod.
Whenwetrytocompilethiscode,thecompilersayssomethinglikethis:
cannotfindsymbol
Thecompilerissaying,“Hey,classAnimaldoesn’thaveaplayDead()method.”Let’smodifytheif
codeblock:
Thenewandimprovedcodeblockcontainsacast,whichinthiscaseissometimescalledadowncast,
becausewe’recastingdowntheinheritancetreetoamorespecificclass.Nowthecompilerishappy.
BeforewetrytoinvokeplayDead,wecasttheanimalvariabletotypeDog.Whatwe’resayingtothe
compileris,“Weknowit’sreallyreferringtoaDogobject,soit’sokaytomakeanewDogreference
variabletorefertothatobject.”Inthiscasewe’resafe,becausebeforeweevertrythecast,wedoan
instanceoftesttomakesure.
It’simportanttoknowthatthecompilerisforcedtotrustuswhenwedoadowncast,evenwhenwe
screwup:
Itcanbemaddening!Thiscodecompiles!Butwhenwetrytorunit,we’llgetanexception,something
likethis:
java.lang.ClassCastException
Whycan’twetrustthecompilertohelpusouthere?Can’titseethatanimalisoftypeAnimal?All
thecompilercandoisverifythatthetwotypesareinthesameinheritancetree,sothatdependingon
whatevercodemighthavecomebeforethedowncast,it’spossiblethatanimalisoftypeDog.The
compilermustallowthingsthatmightpossiblyworkatruntime.However,ifthecompilerknowswith
certaintythatthecastcouldnotpossiblywork,compilationwillfail.Thefollowingreplacementcode
blockwillNOTcompile:
Animalanimal=newAnimal();
Dogd=(Dog)animal;
Strings=(String)animal;//animalcan’tEVERbeaString
Inthiscase,you’llgetanerrorlikethis:
inconvertibletypes
Unlikedowncasting,upcasting(castinguptheinheritancetreetoamoregeneraltype)works
implicitly(thatis,youdon’thavetotypeinthecast)becausewhenyouupcastyou’reimplicitly
restrictingthenumberofmethodsyoucaninvoke,asopposedtodowncasting,whichimpliesthatlateron,
youmightwanttoinvokeamorespecificmethod.Here’sanexample:
BothofthepreviousupcastswillcompileandrunwithoutexceptionbecauseaDogIS-A(n)Animal,
whichmeansthatanythinganAnimalcando,aDogcando.ADogcandomore,ofcourse,butthepointis
thatanyonewithanAnimalreferencecansafelycallAnimalmethodsonaDoginstance.TheAnimal
methodsmayhavebeenoverriddenintheDogclass,butallwecareaboutnowisthataDogcanalways
doatleasteverythinganAnimalcando.ThecompilerandJVMknowit,too,sotheimplicitupcastis
alwayslegalforassigninganobjectofasubtypetoareferenceofoneofitssupertypeclasses(or
interfaces).IfDogimplementsPetandPetdefinesbeFriendly(),thenaDogcanbeimplicitlycasttoa
Pet,buttheonlyDogmethodyoucaninvokethenisbeFriendly(),whichDogwasforcedtoimplement
becauseDogimplementsthePetinterface.
Onemorething…ifDogimplementsPet,then,ifBeagleextendsDogbutBeagledoesnotdeclarethat
itimplementsPet,BeagleisstillaPet!BeagleisaPetsimplybecauseitextendsDog,andDog’s
alreadytakencareofthePetpartsforitselfandforallitschildren.TheBeagleclasscanalways
overrideanymethoditinheritsfromDog,includingmethodsthatDogimplementedtofulfillitsinterface
contract.
Andjustonemorething…ifBeagledoesdeclarethatitimplementsPet,justsothatotherslookingat
theBeagleclassAPIcaneasilyseethatBeagleIS-APetwithouthavingtolookatBeagle’s
superclasses,Beaglestilldoesn’tneedtoimplementthebeFriendly()methodiftheDogclass
(Beagle’ssuperclass)hasalreadytakencareofthat.Inotherwords,ifBeagleIS-ADog,andDogIS-A
Pet,thenBeagleIS-APetandhasalreadymetitsPetobligationsforimplementingthebeFriendly()
methodsinceitinheritsthebeFriendly()method.Thecompilerissmartenoughtosay,“IknowBeagle
alreadyISaDog,butit’sokaytomakeitmoreobviousbyaddingacast.”
Sodon’tbefooledbycodethatshowsaconcreteclassthatdeclaresitimplementsaninterfacebut
doesn’timplementthemethodsoftheinterface.Beforeyoucantellwhetherthecodeislegal,youmust
knowwhatthesupertypesofthisimplementingclasshavedeclared.Ifanysupertypeinitsinheritancetree
hasalreadyprovidedconcrete(thatis,nonabstract)methodimplementations,thenregardlessofwhether
thesupertypedeclaresthatitimplementstheinterface,thesubclassisundernoobligationtoreimplement
(override)thosemethods.
Theexamcreatorswilltellyouthatthey’reforcedtojamtonsofcodeintolittlespaces
"becauseoftheexamengine."Althoughthat’spartiallytrue,theyALSOliketoobfuscate.The
followingcode
canbereplacedwiththiseasy-to-readbitoffun:
Inthiscasethecompilerneedsallthoseparentheses;otherwise,itthinksit’sbeenhandedan
incompletestatement.
CERTIFICATIONOBJECTIVE
ImplementinganInterface(OCAObjective7.5)
7.5Useabstractclassesandinterfaces.
Whenyouimplementaninterface,you’reagreeingtoadheretothecontractdefinedintheinterface.
Thatmeansyou’reagreeingtoprovidelegalimplementationsforeveryabstractmethoddefinedinthe
interface,andthatanyonewhoknowswhattheinterfacemethodslooklike(nothowthey’reimplemented,
buthowtheycanbecalledandwhattheyreturn)canrestassuredthattheycaninvokethosemethodsonan
instanceofyourimplementingclass.
Forexample,ifyoucreateaclassthatimplementstheRunnableinterface(sothatyourcodecanbe
executedbyaspecificthread),youmustprovidethepublicvoidrun()method.Otherwise,thepoor
threadcouldbetoldtogoexecuteyourRunnableobject’scodeand—surprise,surprise—thethreadthen
discoverstheobjecthasnorun()method!(Atwhichpoint,thethreadwouldblowupandtheJVM
wouldcrashinaspectacularyethorribleexplosion.)Thankfully,Javapreventsthismeltdownfrom
occurringbyrunningacompilercheckonanyclassthatclaimstoimplementaninterface.Iftheclasssays
it’simplementinganinterface,itdarnwellbetterhaveanimplementationforeachabstractmethodinthe
interface(withafewexceptionsthatwe’lllookatinamoment).
AssuminganinterfaceBounceablewithtwomethods,bounce()andsetBounceFactor(),the
followingclasswillcompile:
Okay,weknowwhatyou’rethinking:“Thishasgottobetheworstimplementationclassinthehistoryof
implementationclasses.”Itcompiles,though.Anditruns.Theinterfacecontractguaranteesthataclass
willhavethemethod(inotherwords,otherscancallthemethodsubjecttoaccesscontrol),butitnever
guaranteedagoodimplementation—orevenanyactualimplementationcodeinthebodyofthemethod.
(Keepinmind,though,thatiftheinterfacedeclaresthatamethodisNOTvoid,yourclass’s
implementationcodehastoincludeareturnstatement.)Thecompilerwillneversay,“Um,excuseme,but
didyoureallymeantoputnothingbetweenthosecurlybraces?HELLO.Thisisamethodafterall,so
shouldn’titdosomething?”
Implementationclassesmustadheretothesamerulesformethodimplementationasaclassextending
anabstractclass.Tobealegalimplementationclass,anonabstractimplementationclassmustdothe
following:
Provideconcrete(nonabstract)implementationsforallabstractmethodsfromthedeclared
interface.
Followalltherulesforlegaloverrides,suchasthefollowing:
Declarenocheckedexceptionsonimplementationmethodsotherthanthosedeclaredby
theinterfacemethodorsubclassesofthosedeclaredbytheinterfacemethod.
Maintainthesignatureoftheinterfacemethod,andmaintainthesamereturntype(ora
subtype).(Butitdoesnothavetodeclaretheexceptionsdeclaredintheinterfacemethod
declaration.)
Butwait,there’smore!Animplementationclasscanitselfbeabstract!Forexample,thefollowingis
legalforaclassBallimplementingBounceable:
abstractclassBallimplementsBounceable{}
Noticeanythingmissing?Weneverprovidedtheimplementationmethods.Andthat’sokay.Ifthe
implementationclassisabstract,itcansimplypassthebucktoitsfirstconcretesubclass.Forexample,
ifclassBeachBallextendsBall,andBeachBallisnotabstract,thenBeachBallhastoprovidean
implementationforalltheabstractmethodsfromBounceable:
ImplementationclassesareNOTrequiredtoimplementaninterface’sstaticordefault
methods.We’lldiscussthisinmoredepthlaterinthechapter.
Lookforclassesthatclaimtoimplementaninterfacebutdon’tprovidethecorrectmethod
implementations.Unlesstheimplementingclassisabstract,theimplementingclassmustprovide
implementationsforallabstractmethodsdefinedintheinterface.
Youneedtoknowtwomorerules,andthenwecanputthistopictosleep(orputyoutosleep;we
alwaysgetthosetwoconfused):
1.Aclasscanimplementmorethanoneinterface.It’sperfectlylegaltosay,forexample,the
following:
publicclassBallimplementsBounceable,Serializable,Runnable{...}
Youcanextendonlyoneclass,butyoucanimplementmanyinterfaces(which,asofJava8,
meansaformofmultipleinheritance,whichwe’lldiscussshortly).Inotherwords,subclassing
defineswhoandwhatyouare,whereasimplementingdefinesaroleyoucanplayorahatyoucan
wear,despitehowdifferentyoumightbefromsomeotherclassimplementingthesameinterface
(butfromadifferentinheritancetree).Forexample,aPersonextendsHumanBeing(althoughfor
some,that’sdebatable).ButaPersonmayalsoimplementProgrammer,Snowboarder,Employee,
Parent,orPersonCrazyEnoughToTakeThisExam.
2.Aninterfacecanitselfextendanotherinterface.Thefollowingcodeisperfectlylegal:
publicinterfaceBounceableextendsMoveable{}//ok!
Whatdoesthatmean?Thefirstconcrete(nonabstract)implementationclassofBounceablemust
implementalltheabstractmethodsofBounceable,plusalltheabstractmethodsofMoveable!The
subinterface,aswecallit,simplyaddsmorerequirementstothecontractofthesuperinterface.You’llsee
thisconceptappliedinmanyareasofJava,especiallyJavaEE,whereyou’lloftenhavetobuildyour
owninterfacethatextendsoneoftheJavaEEinterfaces.
Holdon,though,becausehere’swhereitgetsstrange.Aninterfacecanextendmorethanoneinterface!
Thinkaboutthatforamoment.Youknowthatwhenwe’retalkingaboutclasses,thefollowingisillegal:
publicclassProgrammerextendsEmployee,Geek{}//Illegal!
Aswementionedearlier,aclassisnotallowedtoextendmultipleclassesinJava.Aninterface,however,
isfreetoextendmultipleinterfaces:
Inthenextexample,BallisrequiredtoimplementBounceable,plusallabstractmethodsfromthe
interfacesthatBounceableextends(includinganyinterfacesthoseinterfacesextend,andsoon,untilyou
reachthetopofthestack—orisitthebottomofthestack?).SoBallwouldneedtolooklikethe
following:
IfclassBallfailstoimplementanyoftheabstractmethodsfromBounceable,Moveable,or
Spherical,thecompilerwilljumpupanddownwildly,redintheface,untilitdoes.Unless,thatis,
classBallismarkedabstract.Inthatcase,Ballcouldchoosetoimplementany,all,ornoneofthe
abstractmethodsfromanyoftheinterfaces,thusleavingtherestoftheimplementationstoaconcrete
subclassofBall,asfollows:
Figure2-5comparesconcreteandabstractexamplesofextendsandimplementsforbothclassesand
interfaces.
FIGURE2-5 Comparingconcreteandabstractexamplesofextendsandimplements
Java8—NowwithMultipleInheritance!
Itmighthavealreadyoccurredtoyouthatsinceinterfacescannowhaveconcretemethodsandclasses
canimplementmultipleinterfaces,thespectreofmultipleinheritanceandtheDeadlyDiamondofDeath
canrearitsuglyhead!Well,you’repartlycorrect.AclassCANimplementinterfaceswithduplicate,
concretemethodsignatures!Butthegoodnewsisthatthecompiler’sgotyourback,andifyouDOwantto
implementbothinterfaces,you’llhavetoprovideanoverridingmethodinyourclass.Let’slookatthe
followingcode:
Lookforillegalusesofextendsandimplements.Thefollowingshowsexamplesoflegaland
illegalclassandinterfacedeclarations:
Burnthesein,andwatchforabusesinthequestionsyougetontheexam.Regardlessofwhat
thequestionappearstobetesting,therealproblemmightbetheclassorinterfacedeclaration.
Beforeyougetcaughtupin,say,tracingacomplexthreadingflow,checktoseeifthecodewill
evencompile.(Justthattipalonemaybeworthyourputtingusinyourwill!)(You’llbe
impressedbytheefforttheexamdevelopersputintodistractingyoufromtherealproblem.)
(Howdidpeoplemanagetowriteanythingbeforeparentheseswereinvented?)
Asthecodestands,itWILLNOTCOMPILEbecauseit’snotclearwhichversionofdoStuff()should
beused.Inordertomakethecodecompile,youneedtooverridedoStuff()intheclass.Uncommenting
theclass’sdoStuff()methodwouldallowthecodetocompileandwhenrunproducethefollowing
output:
3
CERTIFICATIONOBJECTIVE
LegalReturnTypes(OCAObjectives2.2and6.1)
2.2Differentiatebetweenobjectreferencevariablesandprimitivevariables.
6.1Createmethodswithargumentsandreturnvalues;includingoverloadedmethods.
Thissectioncoverstwoaspectsofreturntypes:whatyoucandeclareasareturntype,andwhatyou
canactuallyreturnasavalue.Whatyoucanandcannotdeclareisprettystraightforward,butitall
dependsonwhetheryou’reoverridinganinheritedmethodorsimplydeclaringanewmethod(which
includesoverloadedmethods).We’lltakejustaquicklookatthedifferencebetweenreturntyperulesfor
overloadedandoverridingmethods,becausewe’vealreadycoveredthatinthischapter.We’llcovera
smallbitofnewground,though,whenwelookatpolymorphicreturntypesandtherulesforwhatisand
isnotlegaltoactuallyreturn.
ReturnTypeDeclarations
Thissectionlooksatwhatyou’reallowedtodeclareasareturntype,whichdependsprimarilyon
whetheryouareoverriding,overloading,ordeclaringanewmethod.
ReturnTypesonOverloadedMethods
Rememberthatmethodoverloadingisnotmuchmorethannamereuse.Theoverloadedmethodisa
completelydifferentmethodfromanyothermethodofthesamename.Soifyouinheritamethodbut
overloaditinasubtype,you’renotsubjecttotherestrictionsofoverriding,whichmeansyoucandeclare
anyreturntypeyoulike.Whatyoucan’tdoischangeonlythereturntype.Tooverloadamethod,
remember,youmustchangetheargumentlist.Thefollowingcodeshowsanoverloadedmethod:
NoticethattheBarversionofthemethodusesadifferentreturntype.That’sperfectlyfine.Aslongas
you’vechangedtheargumentlist,you’reoverloadingthemethod,sothereturntypedoesn’thavetomatch
thatofthesupertypeversion.Whatyou’reNOTallowedtodoisthis:
OverridingandReturnTypesandCovariantReturns
Whenasubtypewantstochangethemethodimplementationofaninheritedmethod(anoverride),the
subtypemustdefineamethodthatmatchestheinheritedversionexactly.Or,sinceJava5,you’reallowed
tochangethereturntypeintheoverridingmethodaslongasthenewreturntypeisasubtypeofthe
declaredreturntypeoftheoverridden(superclass)method.
Let’slookatacovariantreturninaction:
SinceJava5,thiscodecompiles.Ifyouweretoattempttocompilethiscodewitha1.4compilerorwith
thesourceflagasfollows,
javac-source1.4Beta.java
youwouldgetacompilererrorlikethis:
attemptingtouseincompatiblereturntype
Otherrulesapplytooverriding,includingthoseforaccessmodifiersanddeclaredexceptions,but
thoserulesaren’trelevanttothereturntypediscussion.
Fortheexam,besureyouknowthatoverloadedmethodscanchangethereturntype,but
overridingmethodscandosoonlywithintheboundsofcovariantreturns.Justthatknowledge
alonewillhelpyouthroughawiderangeofexamquestions.
ReturningaValue
Youhavetorememberonlysixrulesforreturningavalue:
1.Youcanreturnnullinamethodwithanobjectreferencereturntype.
2.Anarrayisaperfectlylegalreturntype.
3.Inamethodwithaprimitivereturntype,youcanreturnanyvalueorvariablethatcanbe
implicitlyconvertedtothedeclaredreturntype.
4.Inamethodwithaprimitivereturntype,youcanreturnanyvalueorvariablethatcanbe
explicitlycasttothedeclaredreturntype.
5.Youmustnotreturnanythingfromamethodwithavoidreturntype.
6.Inamethodwithanobjectreferencereturntype,youcanreturnanyobjecttypethatcanbe
implicitlycasttothedeclaredreturntype.
Watchformethodsthatdeclareanabstractclassorinterfacereturntype,andknowthatany
objectthatpassestheIS-Atest(inotherwords,wouldtesttrueusingtheinstanceofoperator)
canbereturnedfromthatmethod.Forexample:
Thiscodewillcompile,andthereturnvalueisasubtype.
CERTIFICATIONOBJECTIVE
ConstructorsandInstantiation
(OCAObjectives6.3and7.4)
6.3Createandoverloadconstructors;includingimpactondefaultconstructors(sic)
7.4Usesuperandthistoaccessobjectsandconstructors.
Objectsareconstructed.YouCANNOTmakeanewobjectwithoutinvokingaconstructor.Infact,you
can’tmakeanewobjectwithoutinvokingnotjusttheconstructoroftheobject’sactualclasstype,butalso
theconstructorofeachofitssuperclasses!Constructorsarethecodethatrunswheneveryouusethe
keywordnew.(Okay,tobeabitmoreaccurate,therecanalsobeinitializationblocksthatrunwhenyou
saynew,andwe’regoingtocoverinitblocksandtheirstaticinitializationcounterpartsafterwediscuss
constructors.)We’vegotplentytotalkabouthere—we’lllookathowconstructorsarecoded,whocodes
them,andhowtheyworkatruntime.Sograbyourhardhatandahammer,andlet’sdosomeobject
building.
ConstructorBasics
Everyclass,includingabstractclasses,MUSThaveaconstructor.Burnthatintoyourbrain.Butjust
becauseaclassmusthaveaconstructordoesn’tmeantheprogrammerhastotypeit.Aconstructorlooks
likethis:
classFoo{
Foo(){}//TheconstructorfortheFooclass}
Noticewhat’smissing?There’snoreturntype!Twokeypointstorememberaboutconstructorsare
thattheyhavenoreturntype,andtheirnamesmustexactlymatchtheclassname.Typically,
constructorsareusedtoinitializetheinstancevariablestate,asfollows:
Intheprecedingcodeexample,theFooclassdoesnothaveano-argconstructor.Thatmeansthe
followingwillfailtocompile:
Foof=newFoo();//Won’tcompile,nomatchingconstructor
butthefollowingwillcompile:
Soit’sverycommon(anddesirable)foraclasstohaveano-argconstructor,regardlessofhow
manyotheroverloadedconstructorsareintheclass(yes,constructorscanbeoverloaded).You
can’talwaysmakethatworkforyourclasses;occasionallyyouhaveaclasswhereitmakesno
sensetocreateaninstancewithoutsupplyinginformationtotheconstructor.Ajava.awt.Color
object,forexample,can’tbecreatedbycallingano-argconstructor,becausethatwouldbelike
sayingtotheJVM,“MakemeanewColorobject,andIreallydon’tcarewhatcoloritis…you
pick.”DoyouseriouslywanttheJVMmakingyourstyledecisions?
ConstructorChaining
Weknowthatconstructorsareinvokedatruntimewhenyousaynewonsomeclasstypeasfollows:
Horseh=newHorse();
ButwhatreallyhappenswhenyousaynewHorse()?(AssumeHorseextendsAnimalandAnimal
extendsObject.)
1.TheHorseconstructorisinvoked.Everyconstructorinvokestheconstructorofits
superclasswithan(implicit)calltosuper(),unlesstheconstructorinvokesanoverloaded
constructorofthesameclass(moreonthatinaminute).
2.TheAnimalconstructorisinvoked(AnimalisthesuperclassofHorse).
3.TheObjectconstructorisinvoked(Objectistheultimatesuperclassofallclasses,so
classAnimalextendsObjecteventhoughyoudon’tactuallytype“extendsObject”intothe
Animalclassdeclaration;it’simplicit.)Atthispointwe’reonthetopofthestack.
4.IfclassObjecthadanyinstancevariables,thentheywouldbegiventheirexplicitvalues.
Byexplicitvalues,wemeanvaluesthatareassignedatthetimethevariablesaredeclared,
suchasintx=27,where27istheexplicitvalue(asopposedtothedefaultvalue)ofthe
instancevariable.
5.TheObjectconstructorcompletes.
6.TheAnimalinstancevariablesaregiventheirexplicitvalues(ifany).
7.TheAnimalconstructorcompletes.
8.TheHorseinstancevariablesaregiventheirexplicitvalues(ifany).
9.TheHorseconstructorcompletes.
Figure2-6showshowconstructorsworkonthecallstack.
FIGURE2-6 Constructorsonthecallstack
RulesforConstructors
Thefollowinglistsummarizestherulesyou’llneedtoknowfortheexam(andtounderstandthe
restofthissection).YouMUSTrememberthese,sobesuretostudythemmorethanonce.
Constructorscanuseanyaccessmodifier,includingprivate.(Aprivateconstructor
meansonlycodewithintheclassitselfcaninstantiateanobjectofthattype,soiftheprivate
constructorclasswantstoallowaninstanceoftheclasstobeused,theclassmustprovidea
staticmethodorvariablethatallowsaccesstoaninstancecreatedfromwithintheclass.)
Theconstructornamemustmatchthenameoftheclass.
Constructorsmustnothaveareturntype.
It’slegal(butstupid)tohaveamethodwiththesamenameastheclass,butthatdoesn’t
makeitaconstructor.Ifyouseeareturntype,it’samethodratherthanaconstructor.In
fact,youcouldhavebothamethodandaconstructorwiththesamename—thenameofthe
class—inthesameclass,andthat’snotaproblemforJava.Becarefulnottomistakea
methodforaconstructor—besuretolookforareturntype.
Ifyoudon’ttypeaconstructorintoyourclasscode,adefaultconstructorwillbe
automaticallygeneratedbythecompiler.
ThedefaultconstructorisALWAYSano-argconstructor.
Ifyouwantano-argconstructorandyou’vetypedanyotherconstructor(s)intoyour
classcode,thecompilerwon’tprovidetheno-argconstructor(oranyotherconstructor)for
you.Inotherwords,ifyou’vetypedinaconstructorwitharguments,youwon’thaveano-arg
constructorunlessyoutypeditinyourself!
Everyconstructorhas,asitsfirststatement,eitheracalltoanoverloadedconstructor
(this())oracalltothesuperclassconstructor(super()),althoughrememberthatthiscall
canbeinsertedbythecompiler.
Ifyoudotypeinaconstructor(asopposedtorelyingonthecompiler-generateddefault
constructor),andyoudonottypeinthecalltosuper()oracalltothis(),thecompilerwill
insertano-argcalltosuper()foryouastheveryfirststatementintheconstructor.
Acalltosuper()caneitherbeano-argcallorcanincludeargumentspassedtothe
superconstructor.
Ano-argconstructorisnotnecessarilythedefault(thatis,compiler-supplied)
constructor,althoughthedefaultconstructorisalwaysano-argconstructor.Thedefault
constructoristheonethecompilerprovides!Althoughthedefaultconstructorisalwaysano-
argconstructor,you’refreetoputinyourownno-argconstructor.
Youcannotmakeacalltoaninstancemethodoraccessaninstancevariableuntilafter
thesuperconstructorruns.
Onlystaticvariablesandmethodscanbeaccessedaspartofthecalltosuper()or
this().(Example:super(Animal.NAME)isOK,becauseNAMEisdeclaredasastatic
variable.)
Abstractclasseshaveconstructors,andthoseconstructorsarealwayscalledwhena
concretesubclassisinstantiated.
Interfacesdonothaveconstructors.Interfacesarenotpartofanobject’sinheritance
tree.
Theonlywayaconstructorcanbeinvokedisfromwithinanotherconstructor.Inother
words,youcan’twritecodethatactuallycallsaconstructorasfollows:
DetermineWhetheraDefaultConstructorWillBeCreated
ThefollowingexampleshowsaHorseclasswithtwoconstructors:
Willthecompilerputinadefaultconstructorforthisclass?No!
Howaboutforthefollowingvariationoftheclass?
classHorse{
Horse(Stringname){}
}
Nowwillthecompilerinsertadefaultconstructor?No!
Whataboutthisclass?
classHorse{}
Nowwe’retalking.Thecompilerwillgenerateadefaultconstructorforthisclassbecausetheclass
doesn’thaveanyconstructorsdefined.
Okay,whataboutthisclass?
classHorse{
voidHorse(){}
}
Itmightlooklikethecompilerwon’tcreateaconstructor,sinceoneisalreadyintheHorseclass.
Orisit?TakeanotherlookattheprecedingHorseclass.
What’swrongwiththeHorse()constructor?Itisn’taconstructoratall!It’ssimplyamethod
thathappenstohavethesamenameastheclass.Remember,thereturntypeisadeadgiveaway
thatwe’relookingatamethod,notaconstructor.
Howdoyouknowforsurewhetheradefaultconstructorwillbecreated?Because
youdidn’twriteanyconstructorsinyourclass.
Howdoyouknowwhatthedefaultconstructorwilllooklike?Because...
Thedefaultconstructorhasthesameaccessmodifierastheclass.
Thedefaultconstructorhasnoarguments.
Thedefaultconstructorincludesano-argcalltothesuperconstructor(super()).
Table2-5showswhatthecompilerwill(orwon’t)generateforyourclass.
TABLE2-5 Compiler-GeneratedConstructorCode
Whathappensifthesuperconstructorhasarguments?Constructorscanhave
argumentsjustasmethodscan,andifyoutrytoinvokeamethodthattakes,say,anint,butyou
don’tpassanythingtothemethod,thecompilerwillcomplainasfollows:
Thecompilerwillcomplainthatyoucan’tinvoketakeInt()withoutpassinganint.Ofcourse,
thecompilerenjoystheoccasionalriddle,sothemessageitspitsoutonsomeversionsoftheJVM
(yourmileagemayvary)islessthanobvious:
UseBar.java:7:takeInt(int)inBarcannotbeappliedto()
b.takeInt();
^
Butyougettheidea.Thebottomlineisthattheremustbeamatchforthemethod.Andbymatch,
wemeantheargumenttypesmustbeabletoacceptthevaluesorvariablesyou’repassingandin
theorderyou’repassingthem.Whichbringsusbacktoconstructors(andhereyouwerethinking
we’dnevergetthere),whichworkexactlythesameway.
Soifyoursuperconstructor(thatis,theconstructorofyourimmediatesuperclass/parent)has
arguments,youmusttypeinthecalltosuper(),supplyingtheappropriatearguments.Crucial
point:ifyoursuperclassdoesnothaveano-argconstructor,youmusttypeaconstructorinyour
class(thesubclass)becauseyouneedaplacetoputinthecalltosuper()withtheappropriate
arguments.
Thefollowingisanexampleoftheproblem:
Andonceagainthecompilertreatsuswithstunninglucidity:
Ifyou’relucky(andit’safullmoon),yourcompilermightbealittlemoreexplicit.Butagain,the
problemisthattherejustisn’tamatchforwhatwe’retryingtoinvokewithsuper()—anAnimal
constructorwithnoarguments.
Anotherwaytoputthisisthatifyoursuperclassdoesnothaveano-argconstructor,theninyour
subclassyouwillnotbeabletousethedefaultconstructorsuppliedbythecompiler.It’sthatsimple.
Becausethecompilercanonlyputinacalltoano-argsuper(),youwon’tevenbeabletocompile
somethinglikethis:
Tryingtocompilethiscodegivesusexactlythesameerrorwegotwhenweputaconstructorin
thesubclasswithacalltotheno-argversionofsuper():
Infact,theprecedingClothingandTShirtcodeisimplicitlythesameasthefollowingcode,
wherewe’vesuppliedaconstructorforTShirtthat’sidenticaltothedefaultconstructorsupplied
bythecompiler:
Onelastpointonthewholedefaultconstructorthing(andit’sprobablyveryobvious,butwe
havetosayitorwe’llfeelguiltyforyears),constructorsareneverinherited.Theyaren’tmethods.
Theycan’tbeoverridden(becausetheyaren’tmethods,andonlyinstancemethodscanbe
overridden).Sothetypeofconstructor(s)yoursuperclasshasinnowaydeterminesthetypeof
defaultconstructoryou’llget.Somefolksmistakenlybelievethatthedefaultconstructorsomehow
matchesthesuperconstructor,eitherbytheargumentsthedefaultconstructorwillhave
(remember,thedefaultconstructorisalwaysano-arg)orbytheargumentsusedinthecompiler-
suppliedcalltosuper().
Soalthoughconstructorscan’tbeoverridden,you’vealreadyseenthattheycanbeoverloaded,
andtypicallyare.
OverloadedConstructors
Overloadingaconstructormeanstypinginmultipleversionsoftheconstructor,eachhavinga
differentargumentlist,likethefollowingexamples:
TheprecedingFooclasshastwooverloadedconstructors:onethattakesastring,andonewith
noarguments.Becausethere’snocodeintheno-argversion,it’sactuallyidenticaltothedefault
constructorthecompilersupplies—butremember,sincethere’salreadyaconstructorinthisclass
(theonethattakesastring),thecompilerwon’tsupplyadefaultconstructor.Ifyouwantano-arg
constructortooverloadthewith-argsversionyoualreadyhave,you’regoingtohavetotypeit
yourself,justasintheFooexample.
Overloadingaconstructoristypicallyusedtoprovidealternatewaysforclientstoinstantiate
objectsofyourclass.Forexample,ifaclientknowstheanimalname,theycanpassthattoan
Animalconstructorthattakesastring.Butiftheydon’tknowthename,theclientcancalltheno-
argconstructor,andthatconstructorcansupplyadefaultname.Here’swhatitlookslike:
Runningthecodefourtimesproducesthisoutput:
There’salotgoingonintheprecedingcode.Figure2-7showsthecallstackforconstructor
invocationswhenaconstructorisoverloaded.
FIGURE2-7 Overloadedconstructorsonthecallstack
Takealookatthecallstack,andthenlet’swalkthroughthecodestraightfromthetop.
Line2DeclareaStringinstancevariablename.
Lines3–5ConstructorthattakesaStringandassignsittoinstancevariablename.
Line7Here’swhereitgetsfun.Assumeeveryanimalneedsaname,buttheclient
(callingcode)mightnotalwaysknowwhatthenameshouldbe,sotheAnimalclasswillassign
arandomname.Theno-argconstructorgeneratesanamebyinvokingthemakeRandomName()
method.
Line8Theno-argconstructorinvokesitsownoverloadedconstructorthattakesa
String,ineffectcallingitthesamewayitwouldbecalledifclientcodeweredoinganewto
instantiateanobject,passingitaStringforthename.Theoverloadedinvocationusesthe
keywordthis,butusesitasthoughitwereamethodnamedthis().Soline8issimply
callingtheconstructoronline3,passingitarandomlyselectedStringratherthanaclient-
codechosenname.
Line11NoticethatthemakeRandomName()methodismarkedstatic!That’sbecause
youcannotinvokeaninstance(inotherwords,nonstatic)method(oraccessaninstance
variable)untilafterthesuperconstructorhasrun.Andsincethesuperconstructorwillbe
invokedfromtheconstructoronline3,ratherthanfromtheoneonline7,line8canuseonly
astaticmethodtogeneratethename.Ifwewantedallanimalsnotspecificallynamedbythe
callertohavethesamedefaultname,say,“Fred,”thenline8couldhavereadthis(“Fred”);
ratherthancallingamethodthatreturnsastringwiththerandomlychosenname.
Line12Thisdoesn’thaveanythingtodowithconstructors,butsincewe’reallhereto
learn,itgeneratesarandomintegerbetween0and4.
Line13Weirdsyntax,weknow.We’recreatinganewStringobject(justasingle
Stringinstance),butwewantthestringtobeselectedrandomlyfromalist.Exceptwedon’t
havethelist,soweneedtomakeit.Sointhatonelineofcodewe
1.DeclareaStringvariablename.
2.CreateaStringarray(anonymously—wedon’tassignthearrayitselftoavariable).
3.Retrievethestringatindex[x](xbeingtherandomnumbergeneratedonline12)of
thenewlycreatedStringarray.
4.Assignthestringretrievedfromthearraytothedeclaredinstancevariablename.We
couldhavemadeitmucheasiertoreadifwe’djustwritten
Butwhere’sthefuninthat?Throwinginunusualsyntax(especiallyforcodewholly
unrelatedtotherealquestion)isinthespiritoftheexam.Don’tbestartled!(Okay,be
startled,butthenjustsaytoyourself,“Whoa!”andgetonwithit.)
Line18We’reinvokingtheno-argversionoftheconstructor(causingarandomname
fromthelisttobepassedtotheotherconstructor).
Line20We’reinvokingtheoverloadedconstructorthattakesastringrepresentingthe
name.
Thekeypointtogetfromthiscodeexampleisinline8.Ratherthancallingsuper(),we’re
callingthis(),andthis()alwaysmeansacalltoanotherconstructorinthesameclass.Okay,fine,
butwhathappensafterthecalltothis()?Soonerorlaterthesuper()constructorgetscalled,
right?Yes,indeed.Acalltothis()justmeansyou’redelayingtheinevitable.Someconstructor,
somewhere,mustmakethecalltosuper().
KeyRule:Thefirstlineinaconstructormustbeacalltosuper()oracalltothis().
Noexceptions.Ifyouhaveneitherofthosecallsinyourconstructor,thecompilerwillinserttheno-
argcalltosuper().Inotherwords,ifconstructorA()hasacalltothis(),thecompilerknowsthat
constructorA()willnotbetheonetoinvokesuper().
Theprecedingrulemeansaconstructorcanneverhavebothacalltosuper()andacallto
this().Becauseeachofthosecallsmustbethefirststatementinaconstructor,youcan’tlegally
usebothinthesameconstructor.Thatalsomeansthecompilerwillnotputacalltosuper()inany
constructorthathasacalltothis().
Thoughtquestion:Whatdoyouthinkwillhappenifyoutrytocompilethefollowingcode?
Yourcompilermaynotactuallycatchtheproblem(itvariesdependingonyourcompiler,butmost
won’tcatchtheproblem).Itassumesyouknowwhatyou’redoing.Canyouspottheflaw?Given
thatasuperconstructormustalwaysbecalled,wherewouldthecalltosuper()go?Remember,
thecompilerwon’tputinadefaultconstructorifyou’vealreadygotoneormoreconstructorsin
yourclass.Andwhenthecompilerdoesn’tputinadefaultconstructor,itstillinsertsacallto
super()inanyconstructorthatdoesn’texplicitlyhaveacalltothesuperconstructor—unless,that
is,theconstructoralreadyhasacalltothis().Sointheprecedingcode,wherecansuper()go?
Theonlytwoconstructorsintheclassbothhavecallstothis(),and,infact,you’llgetexactlywhat
you’dgetifyoutypedthefollowingmethodcode:
publicvoidgo(){
doStuff();
}
publicvoiddoStuff(){
go();
}
Nowcanyouseetheproblem?Ofcourseyoucan.Thestackexplodes!Itgetshigherandhigher
andhigheruntilitjustburstsopenandmethodcodegoesspillingout,oozingoutoftheJVMright
ontothefloor.Twooverloadedconstructorsbothcallingthis()aretwoconstructorscallingeach
other—overandoverandover,resultinginthis:
%javaA
Exceptioninthread“main”java.lang.StackOverflowError
Thebenefitofhavingoverloadedconstructorsisthatyouofferflexiblewaystoinstantiate
objectsfromyourclass.Thebenefitofhavingoneconstructorinvokeanotheroverloaded
constructoristoavoidcodeduplication.IntheAnimalexample,therewasn’tanycodeotherthan
settingthename,butimagineifafterline4therewasstillmoreworktobedoneintheconstructor.
Byputtingalltheotherconstructorworkinjustoneconstructor,andthenhavingtheother
constructorsinvokeit,youdon’thavetowriteandmaintainmultipleversionsofthatother
importantconstructorcode.Basically,eachoftheothernot-the-real-oneoverloadedconstructors
willcallanotheroverloadedconstructor,passingitwhateverdataitneeds(datatheclientcode
didn’tsupply).
Constructorsandinstantiationbecomeevenmoreexciting(justwhenyouthoughtitwassafe)
whenyougettoinnerclasses,butweknowyoucanstandtohaveonlysomuchfuninonechapter,
andbesides,youdon’thavetodealwithinnerclassesuntilyoutackletheOCPexam.
CERTIFICATIONOBJECTIVE
InitializationBlocks(OCAObjectives1.2and6.3-ish)
1.2DefinethestructureofaJavaclass
6.3Createandoverloadconstructors;includingimpactondefaultconstructors
We’vetalkedabouttwoplacesinaclasswhereyoucanputcodethatperformsoperations:
methodsandconstructors.InitializationblocksarethethirdplaceinaJavaprogramwhere
operationscanbeperformed.Staticinitializationblocksrunwhentheclassisfirstloaded,and
instanceinitializationblocksrunwheneveraninstanceiscreated(abitsimilartoaconstructor).
Let’slookatanexample:
Asyoucansee,thesyntaxforinitializationblocksisprettyterse.Theydon’thavenames,they
can’ttakearguments,andtheydon’treturnanything.Astaticinitializationblockrunsoncewhen
theclassisfirstloaded.Aninstanceinitializationblockrunsonceeverytimeanewinstanceis
created.Rememberwhenwetalkedabouttheorderinwhichconstructorcodeexecuted?Instance
initblockcoderunsrightafterthecalltosuper()inaconstructor—inotherwords,afterall
superconstructorshaverun.
Youcanhavemanyinitializationblocksinaclass.Itisimportanttonotethatunlikemethodsor
constructors,theorderinwhichinitializationblocksappearinaclassmatters.Whenit’stimefor
initializationblockstorun,ifaclasshasmorethanone,theywillrunintheorderinwhichthey
appearintheclassfile—inotherwords,fromthetopdown.Basedontheruleswejustdiscussed,
canyoudeterminetheoutputofthefollowingprogram?
Tofigurethisout,remembertheserules:
initblocksexecuteintheorderinwhichtheyappear.
Staticinitblocksrunonce,whentheclassisfirstloaded.
Instanceinitblocksruneverytimeaclassinstanceiscreated.
Instanceinitblocksrunaftertheconstructor’scalltosuper().
Withthoserulesinmind,thefollowingoutputshouldmakesense:
Asyoucansee,theinstanceinitblockseachrantwice.Instanceinitblocksareoftenusedasa
placetoputcodethatalltheconstructorsinaclassshouldshare.Thatway,thecodedoesn’thave
tobeduplicatedacrossconstructors.
Finally,ifyoumakeamistakeinyourstaticinitblock,theJVMcanthrowan
ExceptionInInitializerError.Let’slookatanexample:
Itproducessomethinglikethis:
Byconvention,initblocksusuallyappearnearthetopoftheclassfile,somewherearoundthe
constructors.However,thisistheOCAexamwe’retalkingabout.Don’tbesurprisedifyou
findaninitblocktuckedinbetweenacoupleofmethods,lookingforalltheworldlikea
compilererrorwaitingtohappen!
CERTIFICATIONOBJECTIVE
Statics(OCAObjective6.2)
6.2Applythestatickeywordtomethodsandfields.
StaticVariablesandMethods
Thestaticmodifierhassuchaprofoundimpactonthebehaviorofamethodorvariablethatwe’re
treatingitasaconceptentirelyseparatefromtheothermodifiers.Tounderstandthewayastatic
memberworks,we’lllookfirstatareasonforusingone.Imagineyou’vegotautilityclassor
interfacewithamethodthatalwaysrunsthesameway;itssolefunctionistoreturn,say,arandom
number.Itwouldn’tmatterwhichinstanceoftheclassperformedthemethod—itwouldalways
behaveexactlythesameway.Inotherwords,themethod’sbehaviorhasnodependencyonthe
state(instancevariablevalues)ofanobject.Sowhy,then,doyouneedanobjectwhenthemethod
willneverbeinstance-specific?Whynotjustaskthetypeitselftorunthemethod?
Let’simagineanotherscenario:Supposeyouwanttokeeparunningcountofallinstances
instantiatedfromaparticularclass.Wheredoyouactuallykeepthatvariable?Itwon’tworkto
keepitasaninstancevariablewithintheclasswhoseinstancesyou’retracking,becausethecount
willjustbeinitializedbacktoadefaultvaluewitheachnewinstance.Theanswertoboththe
utility-method-always-runs-the-samescenarioandthekeep-a-running-total-of-instancesscenariois
tousethestaticmodifier.Variablesandmethodsmarkedstaticbelongtothetype,ratherthanto
anyparticularinstance.Infact,forclasses,youcanuseastaticmethodorvariablewithouthaving
anyinstancesofthatclassatall.Youneedonlyhavethetypeavailabletobeabletoinvokea
staticmethodoraccessastaticvariable.staticvariables,too,canbeaccessedwithouthaving
aninstanceofaclass.Butifthereareinstances,astaticvariableofaclasswillbesharedbyall
instancesofthatclass;thereisonlyonecopy.
Thefollowingcodedeclaresandusesastaticcountervariable:
Intheprecedingcode,thestaticfrogCountvariableissettozerowhentheFrogclassisfirst
loadedbytheJVM,beforeanyFroginstancesarecreated!(Bytheway,youdon’tactuallyneedto
initializeastaticvariabletozero;staticvariablesgetthesamedefaultvaluesinstancevariables
get.)WheneveraFroginstanceiscreated,theFrogconstructorrunsandincrementsthestatic
frogCountvariable.Whenthiscodeexecutes,threeFroginstancesarecreatedinmain(),andthe
resultis
Frogcountisnow3
NowimaginewhatwouldhappeniffrogCountwereaninstancevariable(inotherwords,
nonstatic):
Whenthiscodeexecutes,itshouldstillcreatethreeFroginstancesinmain(),buttheresultis…a
compilererror!Wecan’tgetthiscodetocompile,letalonerun.
TheJVMdoesn’tknowwhichFrogobject’sfrogCountyou’retryingtoaccess.Theproblemis
thatmain()isitselfastaticmethodandthusisn’trunningagainstanyparticularinstanceofthe
class;insteadit’srunningontheclassitself.Astaticmethodcan’taccessanonstatic(instance)
variablebecausethereisnoinstance!That’snottosaytherearen’tinstancesoftheclassaliveon
theheap,butratherthatevenifthereare,thestaticmethoddoesn’tknowanythingaboutthem.
Thesameappliestoinstancemethods;astaticmethodcan’tdirectlyinvokeanonstaticmethod.
Thinkstatic=class,nonstatic=instance.MakingthemethodcalledbytheJVM(main())astatic
methodmeanstheJVMdoesn’thavetocreateaninstanceofyourclassjusttostartrunningcode.
OneofthemistakesmostoftenmadebynewJavaprogrammersisattemptingtoaccessan
instancevariable(whichmeansnonstaticvariable)fromthestaticmain()method(which
doesn’tknowanythingaboutanyinstances,soitcan’taccessthevariable).Thefollowingcode
isanexampleofillegalaccessofanonstaticvariablefromastaticmethod:
Understandthatthiscodewillnevercompile,becauseyoucan’taccessanonstatic(instance)
variablefromastaticmethod.Justthinkofthecompilersaying,“Hey,Ihavenoideawhich
Fooobject’sxvariableyou’retryingtoprint!”Remember,it’stheclassrunningthemain()
method,notaninstanceoftheclass.
Ofcourse,thetrickypartfortheexamisthatthequestionwon’tlookasobviousasthe
precedingcode.Theproblemyou’rebeingtestedfor—accessinganonstaticvariablefroma
staticmethod—willbeburiedincodethatmightappeartobetestingsomethingelse.For
example,theprecedingcodewouldbemorelikelytoappearas
Sowhileyou’retryingtofollowthelogic,therealissueisthatxandycan’tbeusedwithin
main()becausexandyareinstance,notstatic,variables!Thesameappliesforaccessing
nonstaticmethodsfromastaticmethod.Theruleis,astaticmethodofaclasscan’taccessa
nonstatic(instance)methodorvariableofitsownclass.
AccessingStaticMethodsandVariables
Sinceyoudon’tneedtohaveaninstanceinordertoinvokeastaticmethodoraccessastatic
variable,howdoyouinvokeoruseastaticmember?What’sthesyntax?Weknowthatwitha
regularoldinstancemethod,youusethedotoperatoronareferencetoaninstance:
Intheprecedingcode,weinstantiateaFrog,assignittothereferencevariablef,andthenuse
thatfreferencetoinvokeamethodontheFroginstancewejustcreated.Inotherwords,the
getFrogSize()methodisbeinginvokedonaspecificFrogobjectontheheap.
Butthisapproach(usingareferencetoanobject)isn’tappropriateforaccessingastatic
method,becausetheremightnotbeanyinstancesoftheclassatall!Sothewayweaccessastatic
method(orstaticvariable)istousethedotoperatoronthetypename,asopposedtousingitona
referencetoaninstance,asfollows:
whichproducestheoutput:
fromstatic3
frominstance4
userefvar5
Butjusttomakeitreallyconfusing,theJavalanguagealsoallowsyoutouseanobjectreference
variabletoaccessastaticmember.Didyoucatchthelastlineofmain()?Itincludedthis
invocation:
f.getCount();//Accessastaticusinganinstancevariable
Intheprecedingcode,weinstantiateaFrog,assignthenewFrogobjecttothereference
variablef,andthenusethefreferencetoinvokeastaticmethod!Buteventhoughweareusinga
specificFroginstancetoaccessthestaticmethod,theruleshaven’tchanged.Thisismerelya
syntaxtricktoletyouuseanobjectreferencevariable(butnottheobjectitrefersto)togettoa
staticmethodorvariable,butthestaticmemberisstillunawareoftheparticularinstanceused
toinvokethestaticmember.IntheFrogexample,thecompilerknowsthatthereferencevariable
fisoftypeFrog,andsotheFrogclassstaticmethodisrunwithnoawarenessorconcernforthe
Froginstanceattheotherendofthefreference.Inotherwords,thecompilercaresonlythat
referencevariablefisdeclaredastypeFrog.
Invokingstaticmethodsfrominterfacesisalmostthesameasinvokingstaticmethodsfrom
classes,exceptthe“instancevariablesyntaxtrick”justdiscussedworksonlyforstaticmethodsin
classes.Thefollowingcodedemonstrateshowinterfacestaticmethodscanandcannotbeinvoked:
Let’sreviewthecode:
Line1isalegalinvocationofaninterface’sdefaultmethod.
Line2isanillegalattempttoinvokeaninterface’sstaticmethod.
Line3isTHElegalwaytoinvokeaninterface’sstaticmethod.
Line4isanotherillegalattempttoinvokeaninterface’sstaticmethod.
Figure2-8illustratestheeffectsofthestaticmodifieronmethodsandvariables.
FIGURE2-8 Theeffectsofstaticonmethodsandvariables
Finally,rememberthatstaticmethodscan’tbeoverridden!Thisdoesn’tmeantheycan’tbe
redefinedinasubclass,butredefiningandoverridingaren’tthesamething.Let’slookatan
exampleofaredefined(remember,notoverridden)staticmethod:
Runningthiscodeproducesthisoutput:
aaad
Remember,thesyntaxa[x].doStuff()isjustashortcut(thesyntaxtrick)—thecompileris
goingtosubstitutesomethinglikeAnimal.doStuff()instead.Noticealsothatyoucaninvokea
staticmethodbyusingtheclassname.
Noticethatwedidn’tusetheenhancedforloophere(coveredinChapter5),eventhoughwe
couldhave.ExpecttoseeamixofbothJava1.4andJava5–8codingstylesandpracticesonthe
exam.
CERTIFICATIONSUMMARY
WestartedthechapterbydiscussingtheimportanceofencapsulationingoodOOdesign,andthen
wetalkedabouthowgoodencapsulationisimplemented:withprivateinstancevariablesandpublic
gettersandsetters.
Next,wecoveredtheimportanceofinheritance,sothatyoucangraspoverriding,overloading,
polymorphism,referencecasting,returntypes,andconstructors.
WecoveredIS-AandHAS-A.IS-Aisimplementedusinginheritance,andHAS-Aisimplemented
byusinginstancevariablesthatrefertootherobjects.
Polymorphismwasnext.Althoughareferencevariable’stypecan’tbechanged,itcanbeusedto
refertoanobjectwhosetypeisasubtypeofitsown.Welearnedhowtodeterminewhatmethods
areinvocableforagivenreferencevariable.
Welookedatthedifferencebetweenoverriddenandoverloadedmethods,learningthatan
overriddenmethodoccurswhenasubtypeinheritsamethodfromasupertypeandthen
reimplementsthemethodtoaddmorespecializedbehavior.Welearnedthat,atruntime,theJVM
willinvokethesubtypeversiononaninstanceofasubtypeandthesupertypeversiononaninstance
ofthesupertype.Abstractmethodsmustbe“overridden”(technically,abstractmethodsmustbe
implemented,asopposedtooverridden,sincetherereallyisn’tanythingtooverride).
Wesawthatoverridingmethodsmustdeclarethesameargumentlistandreturntypeortheycan
returnasubtypeofthedeclaredreturntypeofthesupertype’soverriddenmethod),andthatthe
accessmodifiercan’tbemorerestrictive.Theoverridingmethodalsocan’tthrowanynewor
broadercheckedexceptionsthatweren’tdeclaredintheoverriddenmethod.Youalsolearnedthat
theoverriddenmethodcanbeinvokedusingthesyntaxsuper.doSomething();.
Overloadedmethodsletyoureusethesamemethodnameinaclass,butwithdifferent
arguments(and,optionally,adifferentreturntype).Whereasoverridingmethodsmustnotchange
theargumentlist,overloadedmethodsmust.Butunlikeoverridingmethods,overloadedmethods
arefreetovarythereturntype,accessmodifier,anddeclaredexceptionsanywaytheylike.
Welearnedthemechanicsofcasting(mostlydowncasting)referencevariablesandwhenit’s
necessarytodoso.
Implementinginterfacescamenext.Aninterfacedescribesacontractthattheimplementing
classmustfollow.Therulesforimplementinganinterfacearesimilartothoseforextendingan
abstractclass.AsofJava8,interfacescanhaveconcretemethods,whicharelabeleddefault.
Also,rememberthataclasscanimplementmorethanoneinterfaceandthatinterfacescanextend
anotherinterface.
Wealsolookedatmethodreturntypesandsawthatyoucandeclareanyreturntypeyoulike
(assumingyouhaveaccesstoaclassforanobjectreferencereturntype),unlessyou’reoverriding
amethod.Barringacovariantreturn,anoverridingmethodmusthavethesamereturntypeasthe
overriddenmethodofthesuperclass.Wesawthat,althoughoverridingmethodsmustnotchange
thereturntype,overloadedmethodscan(aslongastheyalsochangetheargumentlist).
Finally,youlearnedthatitislegaltoreturnanyvalueorvariablethatcanbeimplicitlyconverted
tothedeclaredreturntype.So,forexample,ashortcanbereturnedwhenthereturntypeis
declaredasanint.And(assumingHorseextendsAnimal),aHorsereferencecanbereturnedwhen
thereturntypeisdeclaredanAnimal.
Wecoveredconstructorsindetail,learningthatifyoudon’tprovideaconstructorforyourclass,
thecompilerwillinsertone.Thecompiler-generatedconstructoriscalledthedefaultconstructor,
anditisalwaysano-argconstructorwithano-argcalltosuper().Thedefaultconstructorwill
neverbegeneratedifevenasingleconstructorexistsinyourclass(regardlessoftheargumentsof
thatconstructor);soifyouneedmorethanoneconstructorinyourclassandyouwantano-arg
constructor,you’llhavetowriteityourself.Wealsosawthatconstructorsarenotinheritedand
thatyoucanbeconfusedbyamethodthathasthesamenameastheclass(whichislegal).The
returntypeisthegiveawaythatamethodisnotaconstructorbecauseconstructorsdonothave
returntypes.
Wesawhowalltheconstructorsinanobject’sinheritancetreewillalwaysbeinvokedwhenthe
objectisinstantiatedusingnew.Wealsosawthatconstructorscanbeoverloaded,whichmeans
definingconstructorswithdifferentargumentlists.Aconstructorcaninvokeanotherconstructorof
thesameclassusingthekeywordthis(),asthoughtheconstructorwereamethodnamedthis().
Wesawthateveryconstructormusthaveeitherthis()orsuper()asthefirststatement(although
thecompilercaninsertitforyou).
Afterconstructors,wediscussedthetwokindsofinitializationblocksandhowandwhentheir
coderuns.
Welookedatstaticmethodsandvariables.staticmembersaretiedtotheclassorinterface,
notaninstance,sothereisonlyonecopyofanystaticmember.Acommonmistakeistoattemptto
referenceaninstancevariablefromastaticmethod.Usetherespectiveclassorinterfacename
withthedotoperatortoaccessstaticmembers.
And,onceagain,youlearnedthattheexamincludestrickyquestionsdesignedlargelytotest
yourabilitytorecognizejusthowtrickythequestionscanbe.
TWO-MINUTEDRILL
Herearesomeofthekeypointsfromeachcertificationobjectiveinthischapter.
Encapsulation,IS-A,HAS-A*(OCAObjective6.5)
Encapsulationhelpshideimplementationbehindaninterface(orAPI).
Encapsulatedcodehastwofeatures:
Instancevariablesarekeptprotected(usuallywiththeprivatemodifier).
Getterandsettermethodsprovideaccesstoinstancevariables.
IS-Areferstoinheritanceorimplementation.
IS-Aisexpressedwiththekeywordextendsorimplements.
IS-A,“inheritsfrom,”and“isasubtypeof”areallequivalentexpressions.
HAS-Ameansaninstanceofoneclass“hasa”referencetoaninstanceofanotherclass
oranotherinstanceofthesameclass.*HAS-AisNOTontheexam,butit’sgoodtoknow.
Inheritance(OCAObjective7.1)
Inheritanceallowsatypetobeasubtypeofasupertypeandtherebyinheritpublicand
protectedvariablesandmethodsofthesupertype.
InheritanceisakeyconceptthatunderliesIS-A,polymorphism,overriding,overloading,
andcasting.
Allclasses(exceptclassObject)aresubclassesoftypeObject,andthereforethey
inheritObject’smethods.
Polymorphism(OCAObjective7.2)
Polymorphismmeans“manyforms.”
Areferencevariableisalwaysofasingle,unchangeabletype,butitcanrefertoa
subtypeobject.
Asingleobjectcanbereferredtobyreferencevariablesofmanydifferenttypes—as
longastheyarethesametypeorasupertypeoftheobject.
Thereferencevariable’stype(nottheobject’stype)determineswhichmethodscanbe
called!
Polymorphicmethodinvocationsapplyonlytooverriddeninstancemethods.
OverridingandOverloading(OCAObjectives6.1and7.2)
Methodscanbeoverriddenoroverloaded;constructorscanbeoverloadedbutnot
overridden.
Withrespecttothemethoditoverrides,theoverridingmethod
Musthavethesameargumentlist
Musthavethesamereturntypeorasubclass(knownasacovariantreturn)
Mustnothaveamorerestrictiveaccessmodifier
Mayhavealessrestrictiveaccessmodifier
Mustnotthrowneworbroadercheckedexceptions
Maythrowfewerornarrowercheckedexceptions,oranyuncheckedexception
finalmethodscannotbeoverridden.
Onlyinheritedmethodsmaybeoverridden,andrememberthatprivatemethodsarenot
inherited.
Asubclassusessuper.overriddenMethodName()tocallthesuperclassversionofan
overriddenmethod.
AsubclassusesMyInterface.super.overriddenMethodName()tocallthesuper
interfaceversiononanoverriddenmethod.
Overloadingmeansreusingamethodnamebutwithdifferentarguments.
Overloadedmethods
Musthavedifferentargumentlists
Mayhavedifferentreturntypes,ifargumentlistsarealsodifferent
Mayhavedifferentaccessmodifiers
Maythrowdifferentexceptions
Methodsfromasupertypecanbeoverloadedinasubtype.
Polymorphismappliestooverriding,nottooverloading.
Objecttype(notthereferencevariable’stype)determineswhichoverriddenmethodis
usedatruntime.
Referencetypedetermineswhichoverloadedmethodwillbeusedatcompiletime.
ReferenceVariableCasting(OCAObjective7.3)
Therearetwotypesofreferencevariablecasting:downcastingandupcasting.
DowncastingIfyouhaveareferencevariablethatreferstoasubtypeobject,you
canassignittoareferencevariableofthesubtype.Youmustmakeanexplicitcasttodo
this,andtheresultisthatyoucanaccessthesubtype’smemberswiththisnewreference
variable.
UpcastingYoucanassignareferencevariabletoasupertypereferencevariable
explicitlyorimplicitly.Thisisaninherentlysafeoperationbecausetheassignment
restrictstheaccesscapabilitiesofthenewvariable.
ImplementinganInterface(OCAObjective7.5)
Whenyouimplementaninterface,youarefulfillingitscontract.
Youimplementaninterfacebyproperlyandconcretelyimplementingalltheabstract
methodsdefinedbytheinterface.
Asingleclasscanimplementmanyinterfaces.
ReturnTypes(OCAObjectives7.2and7.5)
Overloadedmethodscanchangereturntypes;overriddenmethodscannot,exceptinthe
caseofcovariantreturns.
Objectreferencereturntypescanacceptnullasareturnvalue.
Anarrayisalegalreturntype,bothtodeclareandreturnasavalue.
Formethodswithprimitivereturntypes,anyvaluethatcanbeimplicitlyconvertedto
thereturntypecanbereturned.
Nothingcanbereturnedfromavoid,butyoucanreturnnothing.You’reallowedto
simplysayreturninanymethodwithavoidreturntypetobustoutofamethodearly.But
youcan’treturnnothingfromamethodwithanon-voidreturntype.
Methodswithanobjectreferencereturntypecanreturnasubtype.
Methodswithaninterfacereturntypecanreturnanyimplementer.
ConstructorsandInstantiation(OCAObjectives6.3and7.4)
Aconstructorisalwaysinvokedwhenanewobjectiscreated.
Eachsuperclassinanobject’sinheritancetreewillhaveaconstructorcalled.
Everyclass,evenanabstractclass,hasatleastoneconstructor.
Constructorsmusthavethesamenameastheclass.
Constructorsdon’thaveareturntype.Ifyouseecodewithareturntype,it’samethod
withthesamenameastheclass;it’snotaconstructor.
Typicalconstructorexecutionoccursasfollows:
Theconstructorcallsitssuperclassconstructor,whichcallsitssuperclass
constructor,andsoonallthewayuptotheObjectconstructor.
TheObjectconstructorexecutesandthenreturnstothecallingconstructor,which
runstocompletionandthenreturnstoitscallingconstructor,andsoonbackdowntothe
completionoftheconstructoroftheactualinstancebeingcreated.
Constructorscanuseanyaccessmodifier(evenprivate!).
Thecompilerwillcreateadefaultconstructorifyoudon’tcreateanyconstructorsin
yourclass.
Thedefaultconstructorisano-argconstructorwithano-argcalltosuper().
Thefirststatementofeveryconstructormustbeacalleithertothis()(anoverloaded
constructor)ortosuper().
Thecompilerwilladdacalltosuper()unlessyouhavealreadyputinacalltothis()or
super().
Instancemembersareaccessibleonlyafterthesuperconstructorruns.
Abstractclasseshaveconstructorsthatarecalledwhenaconcretesubclassis
instantiated.
Interfacesdonothaveconstructors.
Ifyoursuperclassdoesnothaveano-argconstructor,youmustcreateaconstructorand
insertacalltosuper()withargumentsmatchingthoseofthesuperclassconstructor.
Constructorsareneverinherited;thustheycannotbeoverridden.
Aconstructorcanbedirectlyinvokedonlybyanotherconstructor(usingacallto
super()orthis()).
Regardingissueswithcallstothis():
Theymayappearonlyasthefirststatementinaconstructor.
Theargumentlistdetermineswhichoverloadedconstructoriscalled.
Constructorscancallconstructors,andsoon,butsoonerorlateroneofthembetter
callsuper()orthestackwillexplode.
Callstothis()andsuper()cannotbeinthesameconstructor.Youcanhaveoneor
theother,butneverboth.
InitializationBlocks(OCAObjective1.2and6.3-ish)
Usestaticinitblocks—static{/*codehere*/}—forcodeyouwanttohave
runonce,whentheclassisfirstloaded.Multipleblocksrunfromthetopdown.
Usenormalinitblocks—{/*codehere}—forcodeyouwanttohaverunforevery
newinstance,rightafterallthesuperconstructorshaverun.Again,multipleblocksrunfrom
thetopoftheclassdown.
Statics(OCAObjective6.2)
Usestaticmethodstoimplementbehaviorsthatarenotaffectedbythestateofany
instances.
Usestaticvariablestoholddatathatisclassspecificasopposedtoinstancespecific—
therewillbeonlyonecopyofastaticvariable.
Allstaticmembersbelongtotheclass,nottoanyinstance.
Astaticmethodcan’taccessaninstancevariabledirectly.
Usethedotoperatortoaccessstaticmembers,butrememberthatusingareference
variablewiththedotoperatorisreallyasyntaxtrick,andthecompilerwillsubstitutethe
classnameforthereferencevariable;forinstance:
d.doStuff();
becomes
Dog.doStuff();
Toinvokeaninterface’sstaticmethoduseMyInterface.doStuff()syntax.
staticmethodscan’tbeoverridden,buttheycanberedefined.
SELFTEST
1.Given:
publicabstractinterfaceFrobnicate{publicvoidtwiddle(Strings);}
Whichisacorrectclass?(Chooseallthatapply.)
2.Given:
Whatistheresult?
A.BD
B.DB
C.BDC
D.DBC
E.Compilationfails
3.Given:
Whatistheresult?
A.Clidlet
B.Clidder
C.Clidder
Clidlet
D.Clidlet
Clidder
E.Compilationfails
SpecialNote:Thenextquestioncrudelysimulatesastyleofquestionknownas“drag-and-
drop.”UpthroughtheSCJP6exam,drag-and-dropquestionswereincludedontheexam.Asof
spring2014,OracleDOESNOTincludeanydrag-and-dropquestionsonitsJavaexams,but
justincaseOracle’spolicychanges,weleftafewinthebook.
4.Usingthefragmentsbelow,completethefollowingcodesoitcompiles.Notethatyou
maynothavetofillinalloftheslots.
Code:
Fragments:Usethefollowingfragmentszeroormoretimes:
5.Given:
Whatistheresult?
A.preb1b2r3r2hawk
B.preb2b1r2r3hawk
C.preb2b1r2r3hawkr1r4
D.r1r4preb1b2r3r2hawk
E.r1r4preb2b1r2r3hawk
F.prer1r4b1b2r3r2hawk
G.prer1r4b2b1r2r3hawk
H.Theorderofoutputcannotbepredicted
I.Compilationfails
Note:You’llprobablyneverseethismanychoicesontherealexam!
6.Giventhefollowing:
Whichofthefollowing,insertedatline9,willcompile?(Chooseallthatapply.)
A.x2.do2();
B.(Y)x2.do2();
C.((Y)x2).do2();
D.Noneoftheabovestatementswillcompile
7.Given:
Whatistheresult?(Chooseallthatapply.)
A.2willbeincludedintheoutput
B.3willbeincludedintheoutput
C.hiwillbeincludedintheoutput
D.Compilationfails
E.Anexceptionisthrownatruntime
8.Given:
Whatistheresult?(Chooseallthatapply.)
A.howlhowlsniff
B.howlwoofsniff
C.howlhowlfollowedbyanexception
D.howlwooffollowedbyanexception
E.Compilationfailswithanerroratline14
F.Compilationfailswithanerroratline15
9.Given:
Whatistheresult?(Chooseallthatapply.)
A.Anexceptionisthrownatruntime
B.Thecodecompilesandrunswithnooutput
C.Compilationfailswithanerroratline8
D.Compilationfailswithanerroratline9
E.Compilationfailswithanerroratline12
F.Compilationfailswithanerroratline13
10.Given:
Whatistheresult?
A.fafa
B.fala
C.lala
D.Compilationfails
E.Anexceptionisthrownatruntime
11.Given:
Whatistheresult?
A.subsub
B.subsubsub
C.alphasubsub
D.alphasubsubsub
E.Compilationfails
F.Anexceptionisthrownatruntime
12.Given:
Whatistheresult?
A.hhnx
B.hnxh
C.bhhnx
D.bhnxh
E.bnxhhnx
F. bbnxhhnx
G. bnxbhhnx
H.Compilationfails
13.Given:
Whatistheresult?
A.furrybray
B.stripesbray
C.furrygenericnoise
D.stripesgenericnoise
E.Compilationfails
F.Anexceptionisthrownatruntime
14.Given:
Whatistheresult?(Chooseallthatapply.)
A.hopping212
B.Compilationfailsduetoanerroronline2
C.Compilationfailsduetoanerroronline5
D.Compilationfailsduetoanerroronline12
E.Compilationfailsduetoanerroronline13
F.Compilationfailsduetoanerroronline14
G.Compilationfailsduetoanerroronline16
15.Given:
Whatistheresult?
A.1
B.2
C.3
D.Theoutputisunpredictable
E.Compilationfails
F.Anexceptionisthrownatruntime
16.Given:
Whichline(s)ofcode,insertedindependentlyat//INSERTCODEHERE,willallowthe
codetocompile?(Chooseallthatapply.)
A.System.out.println(“class:”+doStuff());
B.System.out.println(“iface:”+super.doStuff());
C.System.out.println(“iface:”+MyInterface.super.doStuff());
D.System.out.println(“iface:”+MyInterface.doStuff());
E.System.out.println(“iface:”+super.MyInterface.doStuff());
F.Noneofthelines,A–Ewillallowthecodetocompile
SELFTESTANSWERS
1. BandEarecorrect.Biscorrectbecauseanabstractclassneednotimplementany
orallofaninterface’smethods.Eiscorrectbecausetheclassimplementstheinterface
methodandadditionallyoverloadsthetwiddle()method.
A,C,andDareincorrect.Aisincorrectbecauseabstractmethodshavenobody.Cis
incorrectbecauseclassesimplementinterfaces;theydon’textendthem.Disincorrect
becauseoverloadingamethodisnotimplementingit.(OCAObjectives7.1and7.5)
2. Eiscorrect.Theimpliedsuper()callinBottom2’sconstructorcannotbesatisfied
becausethereisnono-argconstructorinTop.Adefault,no-argconstructorisgeneratedby
thecompileronlyiftheclasshasnoconstructordefinedexplicitly.
A,B,C,andDareincorrectbasedontheabove.(OCAObjective6.3)
3. Aiscorrect.Althoughafinalmethodcannotbeoverridden,inthiscase,themethod
isprivateand,therefore,hidden.Theeffectisthatanew,accessible,methodflipperis
created.Therefore,nopolymorphismoccursinthisexample,themethodinvokedissimply
thatofthechildclass,andnoerroroccurs.
B,C,D,andEareincorrectbasedonthepreceding.(OCAObjective7.2)
SpecialNote:Thisnextquestioncrudelysimulatesastyleofquestionknownas“drag-and-
drop.”UpthroughtheSCJP6exam,drag-and-dropquestionswereincludedontheexam.As
ofspring2014,OracleDOESNOTincludeanydrag-and-dropquestionsonitsJavaexams,
butjustincaseOracle’spolicychanges,weleftafewinthebook.
4.Hereistheanswer:
Asthereisnodroppabletileforthevariablexandtheparentheses(intheKinder
constructor)arealreadyinplaceandempty,thereisnowaytoconstructacalltothe
superclassconstructorthattakesanargument.Therefore,theonlyremainingpossibilityisto
createacalltotheno-argsuperclassconstructor.Thisisdoneassuper();.Thelinecannot
beleftblank,astheparenthesesarealreadyinplace.Further,sincethesuperclass
constructorcalledistheno-argversion,thisconstructormustbecreated.Itwillnotbe
createdbythecompilerbecauseanotherconstructorisalreadypresent.(OCAObjectives6.3
and7.4)Note:Asyoucansee,manyquestionstestforOCAObjective7.1,we’regoingto
stopmentioningobjective7.1.
5. Discorrect.Staticinitblocksareexecutedatclassloadingtime;instanceinit
blocksrunrightafterthecalltosuper()inaconstructor.Whenmultipleinitblocksofa
singletypeoccurinaclass,theyruninorder,fromthetopdown.
A,B,C,E,F,G,H,andIareincorrectbasedontheabove.Note:You’llprobablyneversee
thismanychoicesontherealexam!(OCAObjective6.3)
6. Ciscorrect.BeforeyoucaninvokeY’sdo2method,youhavetocastx2tobeoftype
Y.
A,B,andDareincorrectbasedonthepreceding.Blookslikeapropercast,butwithoutthe
secondsetofparentheses,thecompilerthinksit’sanincompletestatement.(OCAObjective
7.3)
7. Aiscorrect.It’slegaltooverloadmain().SincenoinstancesofLocomotiveare
created,theconstructordoesnotrunandtheoverloadedversionofmain()doesnotrun.
B,C,D,andEareincorrectbasedonthepreceding.(OCAObjectives1.3and6.3)
8. Fiscorrect.ClassDogdoesn’thaveasniffmethod.
A,B,C,D,andEareincorrectbasedontheaboveinformation.(OCAObjectives7.2and
7.3)
9. Aiscorrect.AClassCastExceptionwillbethrownwhenthecodeattemptsto
downcastaTreetoaRedwood.
B,C,D,E,andFareincorrectbasedontheaboveinformation.(OCAObjective7.3)
10. Biscorrect.Thecodeiscorrect,butpolymorphismdoesn’tapplytostaticmethods.
A,C,D,andEareincorrectbasedontheaboveinformation.(OCAObjectives6.2and7.2)
11. Ciscorrect.Watchout,becauseSubSubAlphaextendsAlpha!Becausethecode
doesn’tattempttomakeaSubAlpha,theprivateconstructorinSubAlphaisokay.
A,B,D,E,andFareincorrectbasedontheaboveinformation.(OCAObjectives6.3and
7.2)
12. Ciscorrect.Rememberthatconstructorscalltheirsuperclassconstructors,which
executefirst,andthatconstructorscanbeoverloaded.
A,B,D,E,F,G,andHareincorrectbasedontheaboveinformation.(OCAObjectives6.3
and7.4)
13. Aiscorrect.Polymorphismisonlyforinstancemethods,notinstancevariables.
B,C,D,E,andFareincorrectbasedontheaboveinformation.(OCAObjective6.3)
14. EandGarecorrect.Neitheroftheselinesofcodeusesthecorrectsyntaxtoinvoke
aninterface’sstaticmethod.
A,B,C,D,andFareincorrectbasedontheaboveinformation.(OCPObjectives6.2and
7.5)
15. Eiscorrect.Thisiskindofatrickquestion;theimplementingmethodmustbe
markedpublic.Ifitwas,alltheothercodeislegal,andtheoutputwouldbe3.Ifyou
understoodallthemultipleinheritancerulesandjustmissedtheaccessmodifier,giveyourself
halfcredit.
A,B,C,D,andFareincorrectbasedontheaboveinformation.(OCPObjective7.5)
16. AandCarecorrect.Ausescorrectsyntaxtoinvoketheclass’smethod,andCuses
thecorrectsyntaxtoinvoketheinterface’soverloadeddefaultmethod.
B,D,E,andFareincorrect.(OCPObjective7.5)
3
Assignments
CERTIFICATIONOBJECTIVES
•UseClassMembers
•UnderstandPrimitiveCasting
•UnderstandVariableScope
•DifferentiateBetweenPrimitiveVariablesandReferenceVariables
•DeterminetheEffectsofPassingVariablesintoMethods
•UnderstandObjectLifecycleandGarbageCollection
Two-MinuteDrill
Q&ASelfTest
StackandHeap—QuickReview
Formostpeople,understandingthebasicsofthestackandtheheapmakesitfareasiertounderstand
topicslikeargumentpassing,polymorphism,threads,exceptions,andgarbagecollection.Inthissection,
we’llsticktoanoverview,butwe’llexpandthesetopicsseveralmoretimesthroughoutthebook.
Forthemostpart,thevariouspieces(methods,variables,andobjects)ofJavaprogramsliveinoneof
twoplacesinmemory:thestackortheheap.Fornow,we’reconcernedaboutonlythreetypesofthings—
instancevariables,localvariables,andobjects:
Instancevariablesandobjectsliveontheheap.
Localvariablesliveonthestack.
Let’stakealookataJavaprogramandhowitsvariouspiecesarecreatedandmapintothestackand
theheap:
Figure3-1showsthestateofthestackandtheheaponcetheprogramreachesline19.Followingare
somekeypoints:
FIGURE3-1 Overviewofthestackandtheheap
Line7—main()isplacedonthestack.
Line9—Referencevariablediscreatedonthestack,butthere’snoDogobjectyet.
Line10—AnewDogobjectiscreatedontheheapandisassignedtothedreferencevariable.
Line11—Acopyofthereferencevariabledispassedtothego()method.
Line13—Thego()methodisplacedonthestack,withthedogparameterasalocalvariable.
Line14—AnewCollarobjectiscreatedontheheapandassignedtoDog’sinstance
variable.
Line17—setName()isaddedtothestack,withthedogNameparameterasitslocalvariable.
Line18—ThenameinstancevariablenowalsoreferstotheStringobject.
NoticethattwodifferentlocalvariablesrefertothesameDogobject.
NoticethatonelocalvariableandoneinstancevariablebothrefertothesameStringAiko.
AfterLine19completes,setName()completesandisremovedfromthestack.Atthispoint
thelocalvariabledogNamedisappears,too,althoughtheStringobjectitreferredtoisstillonthe
heap.
CERTIFICATIONOBJECTIVE
Literals,Assignments,andVariables
(OCAObjectives2.1,2.2,and2.3)
2.1Declareandinitializevariables(includingcastingofprimitivedatatypes).
2.2Differentiatebetweenobjectreferencevariablesandprimitivevariables.
2.3Knowhowtoreadorwritetoobjectfields.
LiteralValuesforAllPrimitiveTypes
Aprimitiveliteralismerelyasourcecoderepresentationoftheprimitivedatatypes—inotherwords,an
integer,floating-pointnumber,boolean,orcharacterthatyoutypeinwhilewritingcode.Thefollowing
areexamplesofprimitiveliterals:
IntegerLiterals
TherearefourwaystorepresentintegernumbersintheJavalanguage:decimal(base10),octal(base8),
hexadecimal(base16),and,asofJava7,binary(base2).Mostexamquestionswithintegerliteralsuse
decimalrepresentations,butthefewthatuseoctal,hexadecimal,orbinaryareworthstudyingfor.Even
thoughtheoddsthatyou’lleveractuallyuseoctalintherealworldareastronomicallytiny,theywere
includedintheexamjustforfun.Beforewelookatthefourwaystorepresentintegernumbers,let’sfirst
discussanewfeatureaddedtoJava7:literalswithunderscores.
NumericLiteralswithUnderscoresAsofJava7,numericliteralscanbedeclaredusing
underscorecharacters(_),ostensiblytoimprovereadability.Let’scompareapre-Java7declarationtoan
easier-to-readJava7declaration:
ThemainruleyouhavetokeeptrackofisthatyouCANNOTusetheunderscoreliteralatthe
beginningorendoftheliteral.Thepotentialgotchahereisthatyou’refreetousetheunderscorein
“weird”places:
Asafinalnote,rememberthatyoucanusetheunderscorecharacterforanyofthenumerictypes
(includingdoublesandfloats),butfordoublesandfloats,youCANNOTaddanunderscorecharacter
directlynexttothedecimalpoint,ornexttotheXorBinhexorbinarynumbers(whicharecomingup
soon).
DecimalLiteralsDecimalintegersneednoexplanation;you’vebeenusingthemsincegradeoneor
earlier.Chancesareyoudon’tkeepyourcheckbookinhex.(Ifyoudo,there’saGeeksAnonymous[GA]
groupreadytohelp.)IntheJavalanguage,theyarerepresentedasis,withnoprefixofanykind,as
follows:
intlength=343;
BinaryLiteralsAlsonewtoJava7istheadditionofbinaryliterals.Binaryliteralscanuseonlythe
digits0and1.Binaryliteralsmuststartwitheither0Bor0b,asshown:
OctalLiteralsOctalintegersuseonlythedigits0to7.InJava,yourepresentanintegerinoctalform
byplacingazeroinfrontofthenumber,asfollows:
Youcanhaveupto21digitsinanoctalnumber,notincludingtheleading0.Ifwerunthepreceding
program,itdisplaysthefollowing:
Octal010=8
HexadecimalLiteralsHexadecimal(hexforshort)numbersareconstructedusing16distinct
symbols.Becauseweneverinventedsingle-digitsymbolsforthenumbers10through15,weuse
alphabeticcharacterstorepresentthesedigits.Countingfrom0through15inhexlookslikethis:
0123456789abcdef
Javawillacceptuppercaseorlowercaselettersfortheextradigits(oneofthefewplacesJavaisnot
casesensitive!).Youareallowedupto16digitsinahexadecimalnumber,notincludingtheprefix0x(or
0X)ortheoptionalsuffixextensionL,whichwillbeexplainedabitlaterinthechapter.Allofthe
followinghexadecimalassignmentsarelegal:
RunningHexTestproducesthefollowingoutput:
x=1y=2147483647z=-559035650
Don’tbemisledbychangesincaseforahexadecimaldigitorthexprecedingit.0XCAFEand0xcafe
arebothlegalandhavethesamevalue.
Allfourintegerliterals(binary,octal,decimal,andhexadecimal)aredefinedasintbydefault,but
theymayalsobespecifiedaslongbyplacingasuffixofLorlafterthenumber:
longjo=110599L;
longso=0xFFFFl;//Notethelowercase‘l’
Floating-pointLiterals
Floating-pointnumbersaredefinedasanumber,adecimalsymbol,andmorenumbersrepresentingthe
fraction.Inthefollowingexample,thenumber11301874.9881024istheliteralvalue:
doubled=11301874.9881024;
Floating-pointliteralsaredefinedasdouble(64bits)bydefault,soifyouwanttoassignafloating-
pointliteraltoavariableoftypefloat(32bits),youmustattachthesuffixForftothenumber.Ifyou
don’tdothis,thecompilerwillcomplainaboutapossiblelossofprecision,becauseyou’retryingtofita
numberintoa(potentially)lessprecise“container.”TheFsuffixgivesyouawaytotellthecompiler,
“Hey,IknowwhatI’mdoing,andI’lltaketherisk,thankyouverymuch.”
YoumayalsooptionallyattachaDordtodoubleliterals,butitisnotnecessarybecausethisisthe
defaultbehavior.
Lookfornumericliteralsthatincludeacomma;here’sanexample:
BooleanLiterals
Booleanliteralsarethesourcecoderepresentationforbooleanvalues.Abooleanvaluecanbedefined
onlyastrueorfalse.AlthoughinC(andsomeotherlanguages),itiscommontousenumbersto
representtrueorfalse,thiswillnotworkinJava.Again,repeatafterme:“JavaisnotC.”
Beonthelookoutforquestionsthatusenumberswherebooleansarerequired.Youmightseeanif
testthatusesanumber,asinthefollowing:
intx=1;if(x){}//Compilererror!
CharacterLiterals
Acharliteralisrepresentedbyasinglecharacterinsinglequotes:
chara=‘a’;
charb=‘@’;
YoucanalsotypeintheUnicodevalueofthecharacter,usingtheUnicodenotationofprefixingthe
valuewith\u,asfollows:
charletterN=‘\u004E’;//Theletter‘N’
Remember,charactersarejust16-bitunsignedintegersunderthehood.Thatmeansyoucanassigna
numberliteral,assumingitwillfitintotheunsigned16-bitrange(0to65535).Forexample,thefollowing
arealllegal:
Andthefollowingarenotlegalandproducecompilererrors:
Youcanalsouseanescapecode(thebackslash)ifyouwanttorepresentacharacterthatcan’tbe
typedinasaliteral,includingthecharactersforlinefeed,newline,horizontaltab,backspace,andquotes:
LiteralValuesforStrings
AstringliteralisasourcecoderepresentationofavalueofaStringobject.Thefollowingisan
exampleoftwowaystorepresentastringliteral:
Althoughstringsarenotprimitives,they’reincludedinthissectionbecausetheycanberepresentedas
literals—inotherwords,theycanbetypeddirectlyintocode.Theonlyothernonprimitivetypethathasa
literalrepresentationisanarray,whichwe’lllookatlaterinthechapter.
Threadt=???//whatliteralvaluecouldpossiblygohere?
AssignmentOperators
Assigningavaluetoavariableseemsstraightforwardenough;yousimplyassignthestuffontherightside
ofthe=tothevariableontheleft.Well,sure,butdon’texpecttobetestedonsomethinglikethis:
x=6;
No,youwon’tbetestedontheno-brainer(technicalterm)assignments.Youwill,however,betested
onthetrickierassignmentsinvolvingcomplexexpressionsandcasting.We’lllookatbothprimitiveand
referencevariableassignments.Butbeforewebegin,let’sbackupandpeekinsideavariable.Whatisa
variable?Howarethevariableanditsvaluerelated?
Variablesarejustbitholderswithadesignatedtype.Youcanhaveanintholder,adoubleholder,a
Buttonholder,andevenaString[]holder.Withinthatholderisabunchofbitsrepresentingthevalue.
Forprimitives,thebitsrepresentanumericvalue(althoughwedon’tknowwhatthatbitpatternlookslike
forboolean,luckily,wedon’tcare).Abytewithavalueof6,forexample,meansthatthebitpatternin
thevariable(thebyteholder)is00000110,representingthe8bits.
Sothevalueofaprimitivevariableisclear,butwhat’sinsideanobjectholder?Ifyousay,
Buttonb=newButton();
what’sinsidetheButtonholderb?IsittheButtonobject?No!Avariablereferringtoanobjectisjust
that—areferencevariable.Areferencevariablebitholdercontainsbitsrepresentingawaytogettothe
object.Wedon’tknowwhattheformatis.Thewayinwhichobjectreferencesarestoredisvirtual-
machinespecific(it’sapointertosomething,wejustdon’tknowwhatthatsomethingreallyis).Allwe
cansayforsureisthatthevariable’svalueisnottheobject,butratheravaluerepresentingaspecific
objectontheheap.Ornull.Ifthereferencevariablehasnotbeenassignedavalueorhasbeenexplicitly
assignedavalueofnull,thevariableholdsbitsrepresenting—youguessedit—null.Youcanread
Buttonb=null;
as“TheButtonvariablebisnotreferringtoanyobject.”
Sonowthatweknowavariableisjustalittleboxo’bits,wecangetonwiththeworkofchanging
thosebits.We’lllookfirstatassigningvaluestoprimitivesandthenfinishwithassignmentstoreference
variables.
PrimitiveAssignments
Theequal(=)signisusedforassigningavaluetoavariable,andit’scleverlynamedtheassignment
operator.Thereareactually12assignmentoperators,butonlythe5mostcommonlyusedassignment
operatorsareontheexam,andtheyarecoveredinChapter4.
Youcanassignaprimitivevariableusingaliteralortheresultofanexpression.
Takealookatthefollowing:
Themostimportantpointtorememberisthataliteralinteger(suchas7)isalwaysimplicitlyanint.
ThinkingbacktoChapter1,you’llrecallthatanintisa32-bitvalue.Nobigdealifyou’reassigninga
valuetoanintoralongvariable,butwhatifyou’reassigningtoabytevariable?Afterall,abyte-
sizedholdercan’tholdasmanybitsasanint-sizedholder.Here’swhereitgetsweird.Thefollowingis
legal,
byteb=27;
butonlybecausethecompilerautomaticallynarrowstheliteralvaluetoabyte.Inotherwords,the
compilerputsinthecast.Theprecedingcodeisidenticaltothefollowing:
byteb=(byte)27;//Explicitlycasttheintliteraltoabyte
Itlooksasthoughthecompilergivesyouabreakandletsyoutakeashortcutwithassignmentsto
integervariablessmallerthananint.(Everythingwe’resayingaboutbyteappliesequallytocharand
short,bothofwhicharesmallerthananint.)We’renotactuallyattheweirdpartyet,bytheway.
Weknowthataliteralintegerisalwaysanint,butmoreimportantly,theresultofanexpression
involvinganythingint-sizedorsmallerisalwaysanint.Inotherwords,addtwobytestogetherand
you’llgetanint—evenifthosetwobytesaretiny.Multiplyanintandashortandyou’llgetanint.
Divideashortbyabyteandyou’llget…anint.Okay,nowwe’reattheweirdpart.Checkthisout:
Thelastlinewon’tcompile!You’llgetanerrorsomethinglikewhatyourgrandfatherusedtoget:
Wetriedtoassignthesumoftwobytestoabytevariable,theresultofwhich(11)wasdefinitely
smallenoughtofitintoabyte,butthecompilerdidn’tcare.Itknewtheruleaboutint-or-smaller
expressionsalwaysresultinginanint.Itwouldhavecompiledifwe’ddonetheexplicitcast:
bytec=(byte)(a+b);
Wewerestrugglingtofindagoodwaytoteachthistopic,andourfriend,co–JavaRanch*
moderatorandrepeattechnicalreviewerMarcPeabody,cameupwiththefollowing.Wethink
hedidagreatjob:It’sperfectlylegaltodeclaremultiplevariablesofthesametypewitha
singlelinebyplacingacommabetweeneachvariable:
inta,b,c;
Youalsohavetheoptiontoinitializeanynumberofthosevariablesrightinplace:
intj,k=1,l,m=3;
Andthesevariablesareeachevaluatedintheorderthatyoureadthem,lefttoright.It’sjustas
ifyouweretodeclareeachoneonaseparateline:
intj;
intk=1;
intl;
intm=3;
Buttheorderisimportant.Thisislegal:
Butthesearenot:
*Iknow,Iknow,butit’llalwaysbeJavaRanchinourhearts.
PrimitiveCasting
Castingletsyouconvertprimitivevaluesfromonetypetoanother.Wementionedprimitivecastinginthe
previoussection,butnowwe’regoingtotakeadeeperlook.(ObjectcastingwascoveredinChapter2.)
Castscanbeimplicitorexplicit.Animplicitcastmeansyoudon’thavetowritecodeforthecast;the
conversionhappensautomatically.Typically,animplicitcasthappenswhenyou’redoingawidening
conversion—inotherwords,puttingasmallerthing(say,abyte)intoabiggercontainer(suchasanint).
Rememberthose“possiblelossofprecision”compilererrorswesawintheassignmentssection?
Thosehappenedwhenwetriedtoputalargerthing(say,along)intoasmallercontainer(suchasa
short).Thelarge-value-into-small-containerconversionisreferredtoasnarrowingandrequiresan
explicitcast,whereyoutellthecompilerthatyou’reawareofthedangerandacceptfullresponsibility.
Firstwe’lllookatanimplicitcast:
inta=100;
longb=a;//Implicitcast,anintvaluealwaysfitsinalong
Anexplicitcastlookslikethis:
floata=100.001f;
intb=(int)a;//explicitcast,theintmightlosesomeoffloat’sinfo!
Integervaluesmaybeassignedtoadoublevariablewithoutexplicitcasting,becauseanyinteger
valuecanfitina64-bitdouble.Thefollowinglinedemonstratesthis:
doubled=100L;//Implicitcast
Intheprecedingstatement,adoubleisinitializedwithalongvalue(asdenotedbytheLafterthe
numericvalue).Nocastisneededinthiscasebecauseadoublecanholdeverypieceofinformationthat
alongcanstore.If,however,wewanttoassignadoublevaluetoanintegertype,we’reattemptinga
narrowingconversionandthecompilerknowsit:
Ifwetrytocompiletheprecedingcode,wegetanerrorsomethinglikethis:
Intheprecedingcode,afloating-pointvalueisbeingassignedtoanintegervariable.Becausean
integerisnotcapableofstoringdecimalplaces,anerroroccurs.Tomakethiswork,we’llcastthe
floating-pointnumbertoanint:
Whenyoucastafloating-pointnumbertoanintegertype,thevaluelosesallthedigitsafterthe
decimal.Theprecedingcodewillproducethefollowingoutput:
intx=3957
Wecanalsocastalargernumbertype,suchasalong,intoasmallernumbertype,suchasabyte.
Lookatthefollowing:
Theprecedingcodewillcompileandrunfine.Butwhathappensifthelongvalueislargerthan127
(thelargestnumberabytecanstore)?Let’smodifythecode:
Thecodecompilesfine,andwhenwerunitwegetthefollowing:
%javaCasting
Thebyteis-126
Wedon’tgetaruntimeerror,evenwhenthevaluebeingnarrowedistoolargeforthetype.Thebitsto
theleftofthelower8just…goaway.Iftheleftmostbit(thesignbit)inthebyte(oranyintegerprimitive)
nowhappenstobea1,theprimitivewillhaveanegativevalue.
EXERCISE3-1
CastingPrimitives
Createafloatnumbertypeofanyvalue,andassignittoashortusingcasting.
1.Declareafloatvariable:floatf=234.56F;
2.Assignthefloattoashort:shorts=(short)f;
AssigningFloating-pointNumbers
Floating-pointnumbershaveaslightlydifferentassignmentbehaviorthanintegertypes.First,youmust
knowthateveryfloating-pointliteralisimplicitlyadouble(64bits),notafloat.Sotheliteral32.3,for
example,isconsideredadouble.Ifyoutrytoassignadoubletoafloat,thecompilerknowsyoudon’t
haveenoughroomina32-bitfloatcontainertoholdtheprecisionofa64-bitdouble,anditletsyou
know.Thefollowingcodelooksgood,butitwon’tcompile:
floatf=32.3;
Youcanseethat32.3shouldfitjustfineintoafloat-sizedvariable,butthecompilerwon’tallowit.In
ordertoassignafloating-pointliteraltoafloatvariable,youmusteithercastthevalueorappendanf
totheendoftheliteral.Thefollowingassignmentswillcompile:
AssigningaLiteralThatIsTooLargefortheVariable
We’llalsogetacompilererrorifwetrytoassignaliteralvaluethatthecompilerknowsistoobigtofit
intothevariable.
bytea=128;//bytecanonlyholdupto127
Theprecedingcodegivesusanerrorsomethinglikethis:
Wecanfixitwithacast:
bytea=(byte)128;
Butthenwhat’stheresult?Whenyounarrowaprimitive,Javasimplytruncatesthehigher-orderbits
thatwon’tfit.Inotherwords,itlosesallthebitstotheleftofthebitsyou’renarrowingto.
Let’stakealookatwhathappensintheprecedingcode.There,128isthebitpattern10000000.It
takesafull8bitstorepresent128.Butbecausetheliteral128isanint,weactuallyget32bits,withthe
128livingintherightmost(lowerorder)8bits.Soaliteral128isactually
00000000000000000000000010000000
Takeourwordforit;thereare32bitsthere.
Tonarrowthe32bitsrepresenting128,Javasimplylopsofftheleftmost(higherorder)24bits.What
remainsisjustthe10000000.Butrememberthatabyteissigned,withtheleftmostbitrepresentingthe
sign(andnotpartofthevalueofthevariable).Soweendupwithanegativenumber(the1thatusedto
represent128nowrepresentsthenegativesignbit).Remember,tofindoutthevalueofanegativenumber
using2’scomplementnotation,youflipallofthebitsandthenadd1.Flippingthe8bitsgivesus
01111111,andadding1tothatgivesus10000000,orbackto128!Andwhenweapplythesignbit,we
endupwith–128.
Youmustuseanexplicitcasttoassign128toabyte,andtheassignmentleavesyouwiththevalue–
128.Acastisnothingmorethanyourwayofsayingtothecompiler,“Trustme.I’maprofessional.Itake
fullresponsibilityforanythingweirdthathappenswhenthosetopbitsarechoppedoff.”
Thatbringsustothecompoundassignmentoperators.Thiswillcompile:
Thecompoundassignmentoperator+=letsyouaddtothevalueofb,withoutputtinginanexplicitcast.
Infact,+=,-=,*=,and/=willallputinanimplicitcast.
AssigningOnePrimitiveVariabletoAnotherPrimitiveVariable
Whenyouassignoneprimitivevariabletoanother,thecontentsoftheright-handvariablearecopied.For
example:
inta=6;
intb=a;
Thiscodecanbereadas,“Assignthebitpatternforthenumber6totheintvariablea.Thencopythebit
patternina,andplacethecopyintovariableb.”
Sobothvariablesnowholdabitpatternfor6,butthetwovariableshavenootherrelationship.We
usedthevariableaonlytocopyitscontents.Atthispoint,aandbhaveidenticalcontents(inother
words,identicalvalues),butifwechangethecontentsofeitheraorb,theothervariablewon’tbe
affected.
Takealookatthefollowingexample:
Theoutputfromthisprogramis
%javaValueTest
a=10
a=10afterchangetob
Noticethevalueofastayedat10.Thekeypointtorememberisthatevenafteryouassignatob,aand
barenotreferringtothesameplaceinmemory.Theaandbvariablesdonotshareasinglevalue;they
haveidenticalcopies.
ReferenceVariableAssignments
Youcanassignanewlycreatedobjecttoanobjectreferencevariableasfollows:
Buttonb=newButton();
Theprecedinglinedoesthreekeythings:
Makesareferencevariablenamedb,oftypeButton
CreatesanewButtonobjectontheheap
AssignsthenewlycreatedButtonobjecttothereferencevariableb
Youcanalsoassignnulltoanobjectreferencevariable,whichsimplymeansthevariableisnot
referringtoanyobject:
Buttonc=null;
TheprecedinglinecreatesspacefortheButtonreferencevariable(thebitholderforareferencevalue),
butitdoesn’tcreateanactualButtonobject.
Aswediscussedinthelastchapter,youcanalsouseareferencevariabletorefertoanyobjectthatis
asubclassofthedeclaredreferencevariabletype,asfollows:
Theruleisthatyoucanassignasubclassofthedeclaredtypebutnotasuperclassofthedeclaredtype.
Remember,aBarobjectisguaranteedtobeabletodoanythingaFoocando,soanyonewithaFoo
referencecaninvokeFoomethodseventhoughtheobjectisactuallyaBar.
Intheprecedingcode,weseethatFoohasamethoddoFooStuff()thatsomeonewithaFooreference
mighttrytoinvoke.IftheobjectreferencedbytheFoovariableisreallyaFoo,noproblem.Butit’salso
noproblemiftheobjectisaBarbecauseBarinheritedthedoFooStuff()method.Youcan’tmakeit
workinreverse,however.IfsomebodyhasaBarreference,they’regoingtoinvokedoBarStuff(),butif
theobjectisaFoo,itwon’tknowhowtorespond.
TheOCA8examcoverswrapperclasses.Wecouldhavediscussedwrapperclassesinthis
chapter,butwefeltitmademoresensetodiscusstheminthecontextofArrayLists(whichwe
willcoverinChapter6).SountilyougettoChapter6,allyou’llneedtoknowaboutwrappers
follows:
Awrapperobjectisanobjectthatholdsthevalueofaprimitive.Everykindofprimitivehasan
associatedwrapperclass:Boolean,Byte,Character,Double,Float,Integer,Long,
andShort.Thefollowingcodecreatestwowrapperobjectsandthenprintstheirvalues:
producesthefollowingoutput:
4257
We’llbedivingmuchmoredeeplyintowrappersinChapter5.
CERTIFICATIONOBJECTIVE
Scope(OCAObjective1.1)
1.1Determinethescopeofvariables.
VariableScope
Onceyou’vedeclaredandinitializedavariable,anaturalquestionis,“Howlongwillthisvariablebe
around?”Thisisaquestionregardingthescopeofvariables.Andnotonlyisscopeanimportantthingto
understandingeneral,italsoplaysabigpartintheexam.Let’sstartbylookingataclassfile:
AswithvariablesinallJavaprograms,thevariablesinthisprogram(s,x,x2,x3,y,andz)allhavea
scope:
sisastaticvariable.
xisaninstancevariable.
yisalocalvariable(sometimescalleda“methodlocal”variable).
zisablockvariable.
x2isaninitblockvariable,aflavoroflocalvariable.
x3isaconstructorvariable,aflavoroflocalvariable.
Forthepurposesofdiscussingthescopeofvariables,wecansaythattherearefourbasicscopes:
1.Staticvariableshavethelongestscope;theyarecreatedwhentheclassisloaded,andthey
surviveaslongastheclassstaysloadedintheJavaVirtualMachine(JVM).
2.Instancevariablesarethenextmostlong-lived;theyarecreatedwhenanewinstanceis
created,andtheyliveuntiltheinstanceisremoved.
3.Localvariablesarenext;theyliveaslongastheirmethodremainsonthestack.Aswe’llsoon
see,however,localvariablescanbealiveandstillbe“outofscope.”
4.Blockvariablesliveonlyaslongasthecodeblockisexecuting.
Scopingerrorscomeinmanysizesandshapes.Onecommonmistakehappenswhenavariableis
shadowedandtwoscopesoverlap.We’lltakeadetailedlookatshadowinginafewpages.Themost
commonreasonforscopingerrorsisanattempttoaccessavariablethatisnotinscope.Let’slookat
threecommonexamplesofthistypeoferror:
Attemptingtoaccessaninstancevariablefromastaticcontext(typicallyfrommain()):
Attemptingtoaccessalocalvariableofthemethodthatinvokedyou.Whenamethod,say
go(),invokesanothermethod,saygo2(),go2()won’thaveaccesstogo()’slocalvariables.
Whilego2()isexecuting,go()’slocalvariablesarestillalive,buttheyareoutofscope.When
go2()completes,itisremovedfromthestack,andgo()resumesexecution.Atthispoint,allof
go()’spreviouslydeclaredvariablesarebackinscope.Forexample:
Attemptingtouseablockvariableafterthecodeblockhascompleted.It’sverycommonto
declareanduseavariablewithinacodeblock,butbecarefulnottotrytousethevariableoncethe
blockhascompleted:
Inthelasttwoexamples,thecompilerwillsaysomethinglikethis:
cannotfindsymbol
Thisisthecompiler’swayofsaying,“Thatvariableyoujusttriedtouse?Well,itmighthavebeenvalid
inthedistantpast(likeonelineofcodeago),butthisisInternettime,baby,Ihavenomemoryofsucha
variable.”
Payextraattentiontocode-blockscopingerrors.Youmightseetheminswitches,try-catches,
for,do,andwhileloops,whichwe’llcoverinlaterchapters.
CERTIFICATIONOBJECTIVE
VariableInitialization
(OCAObjectives2.1,4.1,and4.2)
2.1Declareandinitializevariables(includingcastingofprimitivedatatypes).
4.1Declare,instantiate,initializeanduseaone-dimensionalarray
4.2Declare,instantiate,initializeandusemulti-dimensionalarray(sic)
UsingaVariableorArrayElement
ThatIsUninitializedandUnassigned
Javagivesustheoptionofinitializingadeclaredvariableorleavingituninitialized.Whenweattemptto
usetheuninitializedvariable,wecangetdifferentbehaviordependingonwhattypeofvariableorarray
wearedealingwith(primitivesorobjects).Thebehavioralsodependsonthelevel(scope)atwhichwe
aredeclaringourvariable.Aninstancevariableisdeclaredwithintheclassbutoutsideanymethodor
constructor,whereasalocalvariableisdeclaredwithinamethod(orintheargumentlistofthemethod).
Localvariablesaresometimescalledstack,temporary,automatic,ormethodvariables,buttherules
forthesevariablesarethesameregardlessofwhatyoucallthem.Althoughyoucanleavealocalvariable
uninitialized,thecompilercomplainsifyoutrytousealocalvariablebeforeinitializingitwithavalue,
asweshallsee.
PrimitiveandObjectTypeInstanceVariables
Instancevariables(alsocalledmembervariables)arevariablesdefinedattheclasslevel.Thatmeansthe
variabledeclarationisnotmadewithinamethod,constructor,oranyotherinitializerblock.Instance
variablesareinitializedtoadefaultvalueeachtimeanewinstanceiscreated,althoughtheymaybegiven
anexplicitvalueaftertheobject’ssuperconstructorshavecompleted.Table3-1liststhedefaultvalues
forprimitiveandobjecttypes.
TABLE3-1 DefaultValuesforPrimitivesandReferenceTypes
PrimitiveInstanceVariables
Inthefollowingexample,theintegeryearisdefinedasaclassmemberbecauseitiswithintheinitial
curlybracesoftheclassandnotwithinamethod’scurlybraces:
Whentheprogramisstarted,itgivesthevariableyearavalueofzero,thedefaultvalueforprimitive
numberinstancevariables.
It’sagoodideatoinitializeallyourvariables,evenifyou’reassigningthemwiththedefaultvalue.
Yourcodewillbeeasiertoread;programmerswhohavetomaintainyourcode(afteryouwinthe
lotteryandmovetoTahiti)willbegrateful.
ObjectReferenceInstanceVariables
Whencomparedwithuninitializedprimitivevariables,objectreferencesthataren’tinitializedarea
completelydifferentstory.Let’slookatthefollowingcode:
Thiscodewillcompilefine.Whenwerunit,theoutputis
Thetitleisnull
ThetitlevariablehasnotbeenexplicitlyinitializedwithaStringassignment,sotheinstance
variablevalueisnull.RememberthatnullisnotthesameasanemptyString(“”).Anullvaluemeans
thereferencevariableisnotreferringtoanyobjectontheheap.ThefollowingmodificationtotheBook
coderunsintotrouble:
WhenwetrytoruntheBookclass,theJVMwillproducesomethinglikethis:
Exceptioninthread“main”java.lang.NullPointerException
atBook.main(Book.java:9)
Wegetthiserrorbecausethereferencevariabletitledoesnotpoint(refer)toanobject.Wecancheck
toseewhetheranobjecthasbeeninstantiatedbyusingthekeywordnull,asthefollowingrevisedcode
shows:
Theprecedingcodecheckstomakesuretheobjectreferencedbythevariablesisnotnullbefore
tryingtouseit.Watchoutforscenariosontheexamwhereyoumighthavetotracebackthroughthecode
tofindoutwhetheranobjectreferencewillhaveavalueofnull.Intheprecedingcode,forexample,you
lookattheinstancevariabledeclarationfortitle,seethatthere’snoexplicitinitialization,recognizethat
thetitlevariablewillbegiventhedefaultvalueofnull,andthenrealizethatthevariableswillalso
haveavalueofnull.Remember,thevalueofsisacopyofthevalueoftitle(asreturnedbythe
getTitle()method),soiftitleisanullreference,swillbe,too.
ArrayInstanceVariables
InChapter5we’llbetakingaverydetailedlookatdeclaring,constructing,andinitializingarraysand
multidimensionalarrays.Fornow,we’rejustgoingtolookattheruleforanarrayelement’sdefault
values.
Anarrayisanobject;thus,anarrayinstancevariablethat’sdeclaredbutnotexplicitlyinitializedwill
haveavalueofnull,justasanyotherobjectreferenceinstancevariable.But…ifthearrayisinitialized,
whathappenstotheelementscontainedinthearray?Allarrayelementsaregiventheirdefaultvalues—
thesamedefaultvaluesthatelementsofthattypegetwhenthey’reinstancevariables.Thebottomline:
Arrayelementsarealways,always,alwaysgivendefaultvalues,regardlessofwherethearrayitselfis
instantiated.
Ifweinitializeanarray,objectreferenceelementswillequalnulliftheyarenotinitialized
individuallywithvalues.Ifprimitivesarecontainedinanarray,theywillbegiventheirrespective
defaultvalues.Forexample,inthefollowingcode,thearrayyearwillcontain100integersthatallequal
to0(zero)bydefault:
Whentheprecedingcoderuns,theoutputindicatesthatall100integersinthearrayhaveavalueof0.
Local(Stack,Automatic)PrimitivesandObjects
Localvariablesaredefinedwithinamethod,andtheyincludeamethod’sparameters.
Automaticisjustanothertermforlocalvariable.Itdoesnotmeantheautomaticvariableis
automaticallyassignedavalue!Infact,theoppositeistrue.Anautomaticvariablemustbe
assignedavalueinthecodeorthecompilerwillcomplain.
LocalPrimitives
Inthefollowingtime-travelsimulator,theintegeryearisdefinedasanautomaticvariablebecauseitis
withinthecurlybracesofamethod:
Localvariables,includingprimitives,always,always,alwaysmustbeinitializedbeforeyouattemptto
usethem(thoughnotnecessarilyonthesamelineofcode).Javadoesnotgivelocalvariablesadefault
value;youmustexplicitlyinitializethemwithavalue,asintheprecedingexample.Ifyoutrytousean
uninitializedprimitiveinyourcode,you’llgetacompilererror:
Compilingproducesoutputsomethinglikethis:
Tocorrectourcode,wemustgivetheintegeryearavalue.Inthisupdatedexample,wedeclareitona
separateline,whichisperfectlyvalid:
Noticeintheprecedingexamplewedeclaredanintegercalleddaythatnevergetsinitialized,yetthe
codecompilesandrunsfine.Legally,youcandeclarealocalvariablewithoutinitializingitaslongas
youdon’tusethevariable—but,let’sfaceit,ifyoudeclaredit,youprobablyhadareason(althoughwe
haveheardofprogrammersdeclaringrandomlocalvariablesjustforsport,toseeiftheycanfigureout
howandwhythey’rebeingused).
Thecompilercan’talwaystellwhetheralocalvariablehasbeeninitializedbeforeuse.For
example,ifyouinitializewithinalogicallyconditionalblock(inotherwords,acodeblockthatmay
notrun,suchasanifblockorforloopwithoutaliteralvalueoftrueorfalseinthetest),the
compilerknowsthattheinitializationmightnothappenandcanproduceanerror.Thefollowing
codeupsetsthecompiler:
Thecompilerwillproduceanerrorsomethinglikethis:
TestLocal.java:9:variablexmightnothavebeeninitialized
Becauseofthecompiler-can’t-tell-for-certainproblem,youwillsometimesneedtoinitializeyour
variableoutsidetheconditionalblock,justtomakethecompilerhappy.Youknowwhythat’s
importantifyou’veseenthebumpersticker,“Whenthecompiler’snothappy,ain’tnobodyhappy.”
LocalObjectReferences
Objectsreferences,too,behavedifferentlywhendeclaredwithinamethodratherthanasinstance
variables.Withinstancevariableobjectreferences,youcangetawaywithleavinganobjectreference
uninitialized,aslongasthecodecheckstomakesurethereferenceisn’tnullbeforeusingit.Remember,
tothecompiler,nullisavalue.Youcan’tusethedotoperatoronanullreference,becausethereisno
objectattheotherendofit,butanullreferenceisnotthesameasanuninitializedreference.Locally
declaredreferencescan’tgetawaywithcheckingfornullbeforeuse,unlessyouexplicitlyinitializethe
localvariabletonull.Thecompilerwillcomplainaboutthefollowingcode:
Compilingthecoderesultsinanerrorsimilartothefollowing:
Instancevariablereferencesarealwaysgivenadefaultvalueofnull,untiltheyareexplicitly
initializedtosomethingelse.Butlocalreferencesarenotgivenadefaultvalue;inotherwords,they
aren’tnull.Ifyoudon’tinitializealocalreferencevariable,then,bydefault,itsvalueis—wellthat’sthe
wholepoint:itdoesn’thaveanyvalueatall!Sowe’llmakethissimple:Justsetthedarnthingtonull
explicitlyuntilyou’rereadytoinitializeittosomethingelse.Thefollowinglocalvariablewillcompile
properly:
LocalArrays
Justlikeanyotherobjectreference,arrayreferencesdeclaredwithinamethodmustbeassignedavalue
beforeuse.Thatjustmeansyoumustdeclareandconstructthearray.Youdonot,however,needto
explicitlyinitializetheelementsofanarray.We’vesaiditbefore,butit’simportantenoughtorepeat:
Arrayelementsaregiventheirdefaultvalues(0,false,null,‘\u0000’,andsoon)regardlessof
whetherthearrayisdeclaredasaninstanceorlocalvariable.Thearrayobjectitself,however,willnot
beinitializedifit’sdeclaredlocally.Inotherwords,youmustexplicitlyinitializeanarrayreferenceif
it’sdeclaredandusedwithinamethod,butatthemomentyouconstructanarrayobject,allofitselements
areassignedtheirdefaultvalues.
AssigningOneReferenceVariabletoAnother
Withprimitivevariables,anassignmentofonevariabletoanothermeansthecontents(bitpattern)ofone
variablearecopiedintoanother.Objectreferencevariablesworkexactlythesameway.Thecontentsofa
referencevariableareabitpattern,soifyouassignreferencevariablea1toreferencevariableb1,thebit
patternina1iscopiedandthenewcopyisplacedintob1.(Somepeoplehavecreatedagamearound
countinghowmanytimesweusethewordcopyinthischapter…thiscopyconceptisabiggie!)Ifwe
assignanexistinginstanceofanobjecttoanewreferencevariable,thentworeferencevariableswill
holdthesamebitpattern—abitpatternreferringtoaspecificobjectontheheap.Lookatthefollowing
code:
Intheprecedingexample,aDimensionobjecta1isdeclaredandinitializedwithawidthof5anda
heightof10.Next,Dimensionb1isdeclaredandassignedthevalueofa1.Atthispoint,bothvariables
(a1andb1)holdidenticalvaluesbecausethecontentsofa1werecopiedintob1.Thereisstillonlyone
Dimensionobject—theonethatbotha1andb1referto.Finally,theheightpropertyischangedusingthe
b1reference.Nowthinkforaminute:isthisgoingtochangetheheightpropertyofa1aswell?Let’ssee
whattheoutputwillbe:
Fromthisoutput,wecanconcludethatbothvariablesrefertothesameinstanceoftheDimensionobject.
Whenwemadeachangetob1,theheightpropertywasalsochangedfora1.
OneexceptiontothewayobjectreferencesareassignedisString.InJava,Stringobjectsaregiven
specialtreatment.Foronething,Stringobjectsareimmutable;youcan’tchangethevalueofaString
object(lotsmoreonthisconceptinChapter6).Butitsurelooksasthoughyoucan.Examinethe
followingcode:
BecauseStringsareobjects,youmightthinkStringywillcontainthecharactersJavaBeanafter
thevariablexischanged.Let’sseewhattheoutputis:
Asyoucansee,eventhoughyisareferencevariabletothesameobjectthatxrefersto,whenwe
changex,itdoesn’tchangey!Foranyotherobjecttype,wheretworeferencesrefertothesameobject,if
eitherreferenceisusedtomodifytheobject,bothreferenceswillseethechangebecausethereisstill
onlyasingleobject.ButanytimewemakeanychangesatalltoaString,theVMwillupdatethe
referencevariabletorefertoadifferentobject.Thedifferentobjectmightbeanewobject,oritmight
notbe,butitwilldefinitelybeadifferentobject.Thereasonwecan’tsayforsurewhetheranewobject
iscreatedisbecauseoftheStringconstantpool,whichwe’llcoverinChapter6.
YouneedtounderstandwhathappenswhenyouuseaStringreferencevariabletomodifyastring:
Anewstringiscreated(oramatchingStringisfoundintheStringpool),leavingthe
originalStringobjectuntouched.
ThereferenceusedtomodifytheString(orrather,makeanewStringbymodifyingacopy
oftheoriginal)isthenassignedthebrand-newStringobject.
Sowhenyousay,
youhaven’tchangedtheoriginalStringobjectcreatedonline1.Whenline2completes,bothtands
referencethesameStringobject.Butwhenline3runs,ratherthanmodifyingtheobjectreferredtobyt
ands(whichistheoneandonlyStringobjectuptothispoint),abrandnewStringobjectiscreated.
Andthenit’sabandoned.BecausethenewStringisn’tassignedtoaStringvariable,thenewlycreated
String(whichholdsthestring“FRED”)istoast.SoalthoughtwoStringobjectswerecreatedinthe
precedingcode,onlyoneisactuallyreferenced,andbothtandsrefertoit.ThebehaviorofStringsis
extremelyimportantintheexam,sowe’llcoveritinmuchmoredetailinChapter6.
CERTIFICATIONOBJECTIVE
PassingVariablesintoMethods(OCAObjective6.6)
6.8Determinetheeffectuponobjectreferencesandprimitivevalueswhentheyarepassedinto
methodsthatchangethevalues.
Methodscanbedeclaredtotakeprimitivesand/orobjectreferences.Youneedtoknowhow(orif)the
caller’svariablecanbeaffectedbythecalledmethod.Thedifferencebetweenobjectreferenceand
primitivevariables,whenpassedintomethods,ishugeandimportant.Tounderstandthissection,you’ll
needtobecomfortablewiththeinformationcoveredinthe“Literals,Assignments,andVariables”section
intheearlypartofthischapter.
PassingObjectReferenceVariables
Whenyoupassanobjectvariableintoamethod,youmustkeepinmindthatyou’repassingtheobject
reference,nottheactualobjectitself.Rememberthatareferencevariableholdsbitsthatrepresent(tothe
underlyingVM)awaytogettoaspecificobjectinmemory(ontheheap).Moreimportantly,youmust
rememberthatyouaren’tevenpassingtheactualreferencevariable,butratheracopyofthereference
variable.Acopyofavariablemeansyougetacopyofthebitsinthatvariable,sowhenyoupassa
referencevariable,you’repassingacopyofthebitsrepresentinghowtogettoaspecificobject.Inother
words,boththecallerandthecalledmethodwillnowhaveidenticalcopiesofthereference;thus,both
willrefertothesameexact(notacopy)objectontheheap.
Forthisexample,we’llusetheDimensionclassfromthejava.awtpackage:
Whenwerunthisclass,wecanseethemodify()methodwas,indeed,abletomodifytheoriginal
(andonly)Dimensionobjectcreatedonline4.
NoticewhentheDimensionobjectonline4ispassedtothemodify()method,anychangestothe
objectthatoccurinsidethemethodarebeingmadetotheobjectwhosereferencewaspassed.Inthe
precedingexample,referencevariablesdanddimbothpointtothesameobject.
DoesJavaUsePass-By-ValueSemantics?
IfJavapassesobjectsbypassingthereferencevariableinstead,doesthatmeanJavausespass-by-
referenceforobjects?Notexactly,althoughyou’lloftenhearandreadthatitdoes.Javaisactuallypass-
by-valueforallvariablesrunningwithinasingleVM.Pass-by-valuemeanspass-by-variable-value.And
thatmeanspass-by-copy-of-the-variable!(There’sthatwordcopyagain!)
Itmakesnodifferenceifyou’repassingprimitiveorreferencevariables;youarealwayspassinga
copyofthebitsinthevariable.Soforaprimitivevariable,you’repassingacopyofthebitsrepresenting
thevalue.Forexample,ifyoupassanintvariablewiththevalueof3,you’repassingacopyofthebits
representing3.Thecalledmethodthengetsitsowncopyofthevaluetodowithitwhatitlikes.
Andifyou’repassinganobjectreferencevariable,you’repassingacopyofthebitsrepresentingthe
referencetoanobject.Thecalledmethodthengetsitsowncopyofthereferencevariabletodowithit
whatitlikes.Butbecausetwoidenticalreferencevariablesrefertotheexactsameobject,ifthecalled
methodmodifiestheobject(byinvokingsettermethods,forexample),thecallerwillseethattheobject
thecaller’soriginalvariablereferstohasalsobeenchanged.Inthenextsection,we’lllookathowthe
picturechangeswhenwe’retalkingaboutprimitives.
Thebottomlineonpass-by-value:Thecalledmethodcan’tchangethecaller’svariable,althoughfor
objectreferencevariables,thecalledmethodcanchangetheobjectthevariablereferredto.What’sthe
differencebetweenchangingthevariableandchangingtheobject?Forobjectreferences,itmeansthe
calledmethodcan’treassignthecaller’soriginalreferencevariableandmakeitrefertoadifferentobject
ornull.Forexample,inthefollowingcodefragment,
reassigninggdoesnotreassignf!Attheendofthebar()method,twoFooobjectshavebeencreated:
onereferencedbythelocalvariablefandonereferencedbythelocal(argument)variableg.Becausethe
doStuff()methodhasacopyofthereferencevariable,ithasawaytogettotheoriginalFooobject,for
instancetocallthesetName()method.ButthedoStuff()methoddoesnothaveawaytogettothef
referencevariable.SodoStuff()canchangevalueswithintheobjectfrefersto,butdoStuff()can’t
changetheactualcontents(bitpattern)off.Inotherwords,doStuff()canchangethestateoftheobject
thatfrefersto,butitcan’tmakefrefertoadifferentobject!
PassingPrimitiveVariables
Let’slookatwhathappenswhenaprimitivevariableispassedtoamethod:
Inthissimpleprogram,thevariableaispassedtoamethodcalledmodify(),whichincrementsthe
variableby1.Theresultingoutputlookslikethis:
Noticethatadidnotchangeafteritwaspassedtothemethod.Remember,itwasacopyofathatwas
passedtothemethod.Whenaprimitivevariableispassedtoamethod,itispassedbyvalue,which
meanspass-by-copy-of-the-bits-in-the-variable.
FROMTHECLASSROOM
TheShadowyWorldofVariables
Justwhenyouthinkyou’vegotitallfiguredout,youseeapieceofcodewithvariablesnotbehaving
thewayyouthinktheyshould.Youmighthavestumbledintocodewithashadowedvariable.Youcan
shadowavariableinseveralways.We’lllookatonewaythatmighttripyouup:hidingastatic
variablebyshadowingitwithalocalvariable.
Shadowinginvolvesreusingavariablenamethat’salreadybeendeclaredsomewhereelse.
Theeffectofshadowingistohidethepreviouslydeclaredvariableinsuchawaythatitmaylookas
thoughyou’reusingthehiddenvariable,butyou’reactuallyusingtheshadowingvariable.Youmight
findreasonstoshadowavariableintentionally,buttypicallyithappensbyaccidentandcauseshard-
to-findbugs.Ontheexam,youcanexpecttoseequestionswhereshadowingplaysarole.
Youcanshadowavariablebydeclaringalocalvariableofthesamename,eitherdirectlyoras
partofanargument:
TheprecedingcodeappearstochangethestaticsizevariableinthechangeIt()method,but
becausechangeIt()hasaparameternamedsize,thelocalsizevariableismodifiedwhilethestatic
sizevariableisuntouched.
RunningclassFooprintsthis:
Thingsbecomemoreinterestingwhentheshadowedvariableisanobjectreference,ratherthana
primitive:
Theprecedingcodeprintsoutthis:
Youcanseethattheshadowingvariable(thelocalparametermyBarinchangeIt())canstill
affectthemyBarinstancevariable,becausethemyBarparameterreceivesareferencetothesameBar
object.ButwhenthelocalmyBarisreassignedanewBarobject,whichwethenmodifybychanging
itsbarNumvalue,Foo’soriginalmyBarinstancevariableisuntouched.
CERTIFICATIONOBJECTIVE
GarbageCollection(OCAObjective2.4)
2.4Explainanobject’slifecycle(creation,“dereferencebyreassignment,”andgarbagecollection)
Thephrasegarbagecollectionseemstocomeandgofromtheexamobjectives.AsoftheOCA8
exam,it’sback,andwe’rehappy.Garbagecollectionisawell-knownideaandauniversalphrasein
computerscience.
OverviewofMemoryManagementandGarbageCollection
Thisisthesectionyou’vebeenwaitingfor!It’sfinallytimetodigintothewonderfulworldofmemory
managementandgarbagecollection.
Memorymanagementisacrucialelementinmanytypesofapplications.Consideraprogramthatreads
inlargeamountsofdata,sayfromsomewhereelseonanetwork,andthenwritesthatdataintoadatabase
onaharddrive.Atypicaldesignwouldbetoreadthedataintosomesortofcollectioninmemory,
performsomeoperationsonthedata,andthenwritethedataintothedatabase.Afterthedataiswritten
intothedatabase,thecollectionthatstoredthedatatemporarilymustbeemptiedofolddataordeleted
andre-createdbeforeprocessingthenextbatch.Thisoperationmightbeperformedthousandsoftimes,
andinlanguageslikeCorC++thatdonotofferautomaticgarbagecollection,asmallflawinthelogic
thatmanuallyemptiesordeletesthecollectiondatastructurescanallowsmallamountsofmemorytobe
improperlyreclaimedorlost.Forever.Thesesmalllossesarecalledmemoryleaks,andovermany
thousandsofiterationstheycanmakeenoughmemoryinaccessiblethatprogramswilleventuallycrash.
Creatingcodethatperformsmanualmemorymanagementcleanlyandthoroughlyisanontrivialand
complextask,andwhileestimatesvary,itisarguablethatmanualmemorymanagementcandoublethe
developmenteffortforacomplexprogram.
Java’sgarbagecollectorprovidesanautomaticsolutiontomemorymanagement.Inmostcasesitfrees
youfromhavingtoaddanymemorymanagementlogictoyourapplication.Thedownsidetoautomatic
garbagecollectionisthatyoucan’tcompletelycontrolwhenitrunsandwhenitdoesn’t.
OverviewofJava’sGarbageCollector
Let’slookatwhatwemeanwhenwetalkaboutgarbagecollectioninthelandofJava.Fromthe30,000-
footlevel,garbagecollectionisthephraseusedtodescribeautomaticmemorymanagementinJava.
Wheneverasoftwareprogramexecutes(inJava,C,C++,Lisp,Ruby,andsoon),itusesmemoryin
severaldifferentways.We’renotgoingtogetintoComputerScience101here,butit’stypicalformemory
tobeusedtocreateastack,aheap,inJava’scaseconstantpoolsandmethodareas.Theheapisthat
partofmemorywhereJavaobjectslive,andit’stheoneandonlypartofmemorythatisinanyway
involvedinthegarbagecollectionprocess.
Aheapisaheapisaheap.Fortheexam,it’simportantthatyouknowthatyoucancallittheheap,you
cancallitthegarbagecollectibleheap,oryoucancallitJohnson,butthereisoneandonlyoneheap.
Soallgarbagecollectionrevolvesaroundmakingsuretheheaphasasmuchfreespaceaspossible.
Forthepurposeoftheexam,whatthisboilsdowntoisdeletinganyobjectsthatarenolongerreachable
bytheJavaprogramrunning.We’lltalkmoreaboutwhat“reachable”meansinaminute,butlet’sdrill
thispointin.Whenthegarbagecollectorruns,itspurposeistofindanddeleteobjectsthatcannotbe
reached.IfyouthinkofaJavaprogramasbeinginaconstantcycleofcreatingtheobjectsitneeds(which
occupyspaceontheheap)andthendiscardingthemwhenthey’renolongerneeded,creatingnewobjects,
discardingthem,andsoon,themissingpieceofthepuzzleisthegarbagecollector.Whenitruns,itlooks
forthosediscardedobjectsanddeletesthemfrommemory,sothatthecycleofusingmemoryand
releasingitcancontinue.Ah,thegreatcircleoflife.
WhenDoestheGarbageCollectorRun?
ThegarbagecollectorisunderthecontroloftheJVM;theJVMdecideswhentorunthegarbage
collector.FromwithinyourJavaprogram,youcanasktheJVMtorunthegarbagecollector;butthereare
noguarantees,underanycircumstances,thattheJVMwillcomply.Lefttoitsowndevices,theJVMwill
typicallyrunthegarbagecollectorwhenitsensesthatmemoryisrunninglow.Experienceindicatesthat
whenyourJavaprogrammakesarequestforgarbagecollection,theJVMwillusuallygrantyourrequest
inshortorder,buttherearenoguarantees.Justwhenyouthinkyoucancountonit,theJVMwilldecideto
ignoreyourrequest.
HowDoestheGarbageCollectorWork?
Youjustcan’tbesure.Youmighthearthatthegarbagecollectorusesamarkandsweepalgorithm,andfor
anygivenJavaimplementationthatmightbetrue,buttheJavaspecificationdoesn’tguaranteeany
particularimplementation.Youmighthearthatthegarbagecollectorusesreferencecounting;onceagain,
maybeyes,maybeno.Theimportantconceptforyoutounderstandfortheexamis:Whendoesanobject
becomeeligibleforgarbagecollection?
Inanutshell,everyJavaprogramhasfromonetomanythreads.Eachthreadhasitsownlittle
executionstack.Normally,you(theprogrammer)causeatleastonethreadtoruninaJavaprogram,the
onewiththemain()methodatthebottomofthestack.However,therearemanyreallycoolreasonsto
launchadditionalthreadsfromyourinitialthread(whichyou’llgetintoifyoupreparefortheOCP8
exam).Inadditiontohavingitsownlittleexecutionstack,eachthreadhasitsownlifecycle.Fornow,all
youneedtoknowisthatthreadscanbealiveordead.
Withthisbackgroundinformation,wecannowsaywithstunningclarityandresolvethatanobjectis
eligibleforgarbagecollectionwhennolivethreadcanaccessit.(Note:Duetothevagariesofthe
Stringconstantpool,theexamfocusesitsgarbagecollectionquestionsonnon-Stringobjects,andso
ourgarbagecollectiondiscussionsapplytoonlynon-Stringobjectstoo.)
Basedonthatdefinition,thegarbagecollectorperformssomemagical,unknownoperations;andwhen
itdiscoversanobjectthatcan’tbereachedbyanylivethread,itwillconsiderthatobjectaseligiblefor
deletion,anditmightevendeleteitatsomepoint.(Youguessedit:italsomightneverdeleteit.)Whenwe
talkaboutreachinganobject,we’rereallytalkingabouthavingareachablereferencevariablethatrefers
totheobjectinquestion.IfourJavaprogramhasareferencevariablethatreferstoanobjectandthat
referencevariableisavailabletoalivethread,thenthatobjectisconsideredreachable.We’lltalkmore
abouthowobjectscanbecomeunreachableinthefollowingsection.
CanaJavaapplicationrunoutofmemory?Yes.Thegarbagecollectionsystemattemptstoremove
objectsfrommemorywhentheyarenotused.However,ifyoumaintaintoomanyliveobjects(objects
referencedfromotherliveobjects),thesystemcanrunoutofmemory.Garbagecollectioncannotensure
thatthereisenoughmemory,onlythatthememorythatisavailablewillbemanagedasefficientlyas
possible.
WritingCodeThatExplicitlyMakesObjectsEligibleforCollection
Intheprecedingsection,youlearnedthetheoriesbehindJavagarbagecollection.Inthissection,weshow
howtomakeobjectseligibleforgarbagecollectionusingactualcode.Wealsodiscusshowtoattemptto
forcegarbagecollectionifitisnecessaryandhowyoucanperformadditionalcleanuponobjectsbefore
theyareremovedfrommemory.
NullingaReference
Aswediscussedearlier,anobjectbecomeseligibleforgarbagecollectionwhentherearenomore
reachablereferencestoit.Obviously,iftherearenoreachablereferences,itdoesn’tmatterwhathappens
totheobject.Forourpurposesitisjustfloatinginspace,unused,inaccessible,andnolongerneeded.
Thefirstwaytoremoveareferencetoanobjectistosetthereferencevariablethatreferstotheobject
tonull.Examinethefollowingcode:
TheStringBufferobjectwiththevaluehelloisassignedtothereferencevariablesbinthethird
line.Tomaketheobjecteligible(forgarbagecollection),wesetthereferencevariablesbtonull,which
removesthesinglereferencethatexistedtotheStringBufferobject.Onceline6hasrun,ourhappy
littlehelloStringBufferobjectisdoomed,eligibleforgarbagecollection.
ReassigningaReferenceVariable
Wecanalsodecoupleareferencevariablefromanobjectbysettingthereferencevariabletoreferto
anotherobject.Examinethefollowingcode:
Objectsthatarecreatedinamethodalsoneedtobeconsidered.Whenamethodisinvoked,anylocal
variablescreatedexistonlyforthedurationofthemethod.Oncethemethodhasreturned,theobjects
createdinthemethodareeligibleforgarbagecollection.Thereisanobviousexception,however.Ifan
objectisreturnedfromthemethod,itsreferencemightbeassignedtoareferencevariableinthemethod
thatcalledit;hence,itwillnotbeeligibleforcollection.Examinethefollowingcode:
Intheprecedingexample,wecreatedamethodcalledgetDate()thatreturnsaDateobject.This
methodcreatestwoobjects:aDateandaStringBuffercontainingthedateinformation.Sincethe
methodreturnsareferencetotheDateobjectandthisreferenceisassignedtoalocalvariable,itwillnot
beeligibleforcollectionevenafterthegetDate()methodhascompleted.TheStringBufferobject,
though,willbeeligible,eventhoughwedidn’texplicitlysetthenowvariabletonull.
IsolatingaReference
Thereisanotherwayinwhichobjectscanbecomeeligibleforgarbagecollection,eveniftheystillhave
validreferences!Wecallthisscenario“islandsofisolation.”
Asimpleexampleisaclassthathasaninstancevariablethatisareferencevariabletoanother
instanceofthesameclass.Nowimaginethattwosuchinstancesexistandthattheyrefertoeachother.If
allotherreferencestothesetwoobjectsareremoved,theneventhougheachobjectstillhasavalid
reference,therewillbenowayforanylivethreadtoaccesseitherobject.Whenthegarbagecollector
runs,itcanusuallydiscoveranysuchislandsofobjectsandremovethem.Asyoucanimagine,such
islandscanbecomequitelarge,theoreticallycontaininghundredsofobjects.Examinethefollowingcode:
Whenthecodereaches//docomplicated,thethreeIslandobjects(previouslyknownasi2,i3,
andi4)haveinstancevariablessothattheyrefertoeachother,buttheirlinkstotheoutsideworld(i2,
i3,andi4)havebeennulled.Thesethreeobjectsareeligibleforgarbagecollection.
Thiscoverseverythingyouwillneedtoknowaboutmakingobjectseligibleforgarbagecollection.
StudyFigure3-2toreinforcetheconceptsofobjectswithoutreferencesandislandsofisolation.
FIGURE3-2 Islandobjectseligibleforgarbagecollection
ForcingGarbageCollection
Thefirstthingthatweshouldmentionhereisthat,contrarytothissection’stitle,garbagecollection
cannotbeforced.However,JavaprovidessomemethodsthatallowyoutorequestthattheJVMperform
garbagecollection.
Note:TheJavagarbagecollectorhasevolvedtosuchanadvancedstatethatit’srecommendedthatyou
neverinvokeSystem.gc()inyourcode—leaveittotheJVM.
Inreality,itispossibleonlytosuggesttotheJVMthatitperformgarbagecollection.However,there
arenoguaranteestheJVMwillactuallyremovealloftheunusedobjectsfrommemory(evenifgarbage
collectionisrun).Itisessentialthatyouunderstandthisconceptfortheexam.
ThegarbagecollectionroutinesthatJavaprovidesaremembersoftheRuntimeclass.TheRuntime
classisaspecialclassthathasasingleobject(aSingleton)foreachmainprogram.TheRuntime
objectprovidesamechanismforcommunicatingdirectlywiththevirtualmachine.TogettheRuntime
instance,youcanusethemethodRuntime.getRuntime(),whichreturnstheSingleton.Onceyouhave
theSingleton,youcaninvokethegarbagecollectorusingthegc()method.Alternatively,youcancall
thesamemethodontheSystemclass,whichhasstaticmethodsthatcandotheworkofobtainingthe
Singletonforyou.Thesimplestwaytoaskforgarbagecollection(remember—justarequest)is
System.gc();
Theoretically,aftercallingSystem.gc(),youwillhaveasmuchfreememoryaspossible.Wesay
“theoretically”becausethisroutinedoesnotalwaysworkthatway.First,yourJVMmaynothave
implementedthisroutine;thelanguagespecificationallowsthisroutinetodonothingatall.Second,
anotherthreadmightgrablotsofmemoryrightafteryourunthegarbagecollector.
ThisisnottosaythatSystem.gc()isauselessmethod—it’smuchbetterthannothing.Youjustcan’t
relyonSystem.gc()tofreeupenoughmemorysothatyoudon’thavetoworryaboutrunningoutof
memory.TheCertificationExamisinterestedinguaranteedbehavior,notprobablebehavior.
Nowthatyouaresomewhatfamiliarwithhowthisworks,let’sdoalittleexperimenttoseetheeffects
ofgarbagecollection.ThefollowingprogramletsusknowhowmuchtotalmemorytheJVMhas
availabletoitandhowmuchfreememoryithas.Itthencreates10,000Dateobjects.Afterthis,ittellsus
howmuchmemoryisleftandthencallsthegarbagecollector(which,ifitdecidestorun,shouldhaltthe
programuntilallunusedobjectsareremoved).Thefinalfreememoryresultshouldindicatewhetherit
hasrun.Let’slookattheprogram:
Asyoucansee,theJVMactuallydiddecidetogarbagecollect(thatis,delete)theeligibleobjects.In
theprecedingexample,wesuggestedthattheJVMperformgarbagecollectionwith458,048bytesof
memoryremaining,andithonoredourrequest.Thisprogramhasonlyoneuserthreadrunning,sothere
wasnothingelsegoingonwhenwecalledrt.gc().Keepinmindthatthebehaviorwhengc()iscalled
maybedifferentfordifferentJVMs;hence,thereisnoguaranteethattheunusedobjectswillberemoved
frommemory.Abouttheonlythingyoucanguaranteeisthatifyouarerunningverylowonmemory,the
garbagecollectorwillrunbeforeitthrowsanOutOfMemoryException.
EXERCISE3-2
GarbageCollectionExperiment
TrychangingtheCheckGCprogrambyputtinglines13and14insidealoop.Youmightseethatnotall
memoryisreleasedonanygivenrunoftheGC.
CleaningUpBeforeGarbageCollection—thefinalize()Method
Javaprovidesamechanismthatletsyourunsomecodejustbeforeyourobjectisdeletedbythegarbage
collector.Thiscodeislocatedinamethodnamedfinalize()thatallclassesinheritfromclassObject.
Onthesurface,thissoundslikeagreatidea;maybeyourobjectopenedupsomeresources,andyou’dlike
toclosethembeforeyourobjectisdeleted.Theproblemisthat,asyoumayhavegatheredbynow,you
cannevercountonthegarbagecollectortodeleteanobject.So,anycodethatyouputintoyourclass’s
overriddenfinalize()methodisnotguaranteedtorun.Becausethefinalize()methodforanygiven
objectmightrun,butyoucan’tcountonit,don’tputanyessentialcodeintoyourfinalize()method.In
fact,werecommendthat,ingeneral,youdon’toverridefinalize()atall.
TrickyLittlefinalize()Gotchas
Thereareacoupleofconceptsconcerningfinalize()thatyouneedtoremember:
Foranygivenobject,finalize()willbecalledonlyonce(atmost)bythegarbage
collector.
Callingfinalize()canactuallyresultinsavinganobjectfromdeletion.
Let’sexaminethesestatementsalittlefurther.Firstofall,rememberthatanycodeyoucanputintoa
normalmethodyoucanputintofinalize().Forexample,inthefinalize()methodyoucouldwrite
codethatpassesareferencetotheobjectinquestionbacktoanotherobject,effectivelyineligible-izing
theobjectforgarbagecollection.Ifatsomelaterpointthissameobjectbecomeseligibleforgarbage
collectionagain,thegarbagecollectorcanstillprocesstheobjectanddeleteit.Thegarbagecollector,
however,willrememberthat,forthisobject,finalize()alreadyran,anditwillnotrunfinalize()
again.
CERTIFICATIONSUMMARY
Thischaptercoveredawiderangeoftopics.Don’tworryifyouhavetoreviewsomeofthesetopicsas
yougetintolaterchapters.Thischapterincludesalotoffoundationalstuffthatwillcomeintoplaylater.
Westartedthechapterbyreviewingthestackandtheheap;rememberthatlocalvariablesliveonthe
stackandinstancevariableslivewiththeirobjectsontheheap.
WereviewedlegalliteralsforprimitivesandStrings,andthenwediscussedthebasicsofassigning
valuestoprimitivesandreferencevariablesandtherulesforcastingprimitives.
Nextwediscussedtheconceptofscope,or“Howlongwillthisvariablelive?”Rememberthefour
basicscopesinorderoflesseninglifespan:static,instance,local,andblock.
Wecoveredtheimplicationsofusinguninitializedvariablesandtheimportanceofthefactthatlocal
variablesMUSTbeassignedavalueexplicitly.Wetalkedaboutsomeofthetrickyaspectsofassigning
onereferencevariabletoanotherandsomeofthefinerpointsofpassingvariablesintomethods,including
adiscussionof“shadowing.”
Finally,wedoveintogarbagecollection,Java’sautomaticmemorymanagementfeature.Welearned
thattheheapiswhereobjectsliveandwhereallthecoolgarbagecollectionactivitytakesplace.We
learnedthatintheend,theJVMwillperformgarbagecollectionwheneveritwantsto.You(the
programmer)canrequestagarbagecollectionrun,butyoucan’tforceit.Wetalkedaboutgarbage
collectiononlyapplyingtoobjectsthatareeligible,andthateligiblemeans“inaccessiblefromanylive
thread.”Finally,wediscussedtherarelyusefulfinalize()methodandwhatyou’llhavetoknowabout
itfortheexam.Allinall,thiswasonefascinatingchapter.
TWO-MINUTEDRILL
Herearesomeofthekeypointsfromthischapter.
StackandHeap
Localvariables(methodvariables)liveonthestack.
Objectsandtheirinstancevariablesliveontheheap.
LiteralsandPrimitiveCasting(OCAObjective2.1)
Integerliteralscanbebinary,decimal,octal(suchas013),orhexadecimal(suchas0x3d).
LiteralsforlongsendinLorl.(Forthesakeofreadability,werecommend“L“.)
FloatliteralsendinForf,anddoubleliteralsendinadigitorDord.
Thebooleanliteralsaretrueandfalse.
Literalsforcharsareasinglecharacterinsidesinglequotes:‘d’.
Scope(OCAObjective1.1)
Scopereferstothelifetimeofavariable.
Therearefourbasicscopes:
Staticvariableslivebasicallyaslongastheirclasslives.
Instancevariablesliveaslongastheirobjectlives.
Localvariablesliveaslongastheirmethodisonthestack;however,iftheirmethod
invokesanothermethod,theyaretemporarilyunavailable.
Blockvariables(forexample,inafororanif)liveuntiltheblockcompletes.
BasicAssignments(OCAObjectives2.1,2.2,and2.3)
Literalintegersareimplicitlyints.
Integerexpressionsalwaysresultinanint-sizedresult,neversmaller.
Floating-pointnumbersareimplicitlydoubles(64bits).
Narrowingaprimitivetruncatesthehighorderbits.
Compoundassignments(suchas+=)performanautomaticcast.
Areferencevariableholdsthebitsthatareusedtorefertoanobject.
Referencevariablescanrefertosubclassesofthedeclaredtypebutnottosuperclasses.
Whenyoucreateanewobject,suchasButtonb=newButton();,theJVMdoesthree
things:
Makesareferencevariablenamedb,oftypeButton.
CreatesanewButtonobject.
AssignstheButtonobjecttothereferencevariableb.
UsingaVariableorArrayElementThatIsUninitializedandUnassigned(OCA
Objectives4.1and4.2)
Whenanarrayofobjectsisinstantiated,objectswithinthearrayarenotinstantiated
automatically,butallthereferencesgetthedefaultvalueofnull.
Whenanarrayofprimitivesisinstantiated,elementsgetdefaultvalues.
Instancevariablesarealwaysinitializedwithadefaultvalue.
Local/automatic/methodvariablesarenevergivenadefaultvalue.Ifyouattempttouseone
beforeinitializingit,you’llgetacompilererror.
PassingVariablesintoMethods(OCAObjective6.6)
Methodscantakeprimitivesand/orobjectreferencesasarguments.
Methodargumentsarealwayscopies.
Methodargumentsareneveractualobjects(theycanbereferencestoobjects).
Aprimitiveargumentisanunattachedcopyoftheoriginalprimitive.
Areferenceargumentisanothercopyofareferencetotheoriginalobject.
Shadowingoccurswhentwovariableswithdifferentscopessharethesamename.Thisleads
tohard-to-findbugsandhard-to-answerexamquestions.
GarbageCollection(OCAObjective2.4)
InJava,garbagecollection(GC)providesautomatedmemorymanagement.
ThepurposeofGCistodeleteobjectsthatcan’tbereached.
OnlytheJVMdecideswhentoruntheGC;youcanonlysuggestit.
Youcan’tknowtheGCalgorithmforsure.
Objectsmustbeconsideredeligiblebeforetheycanbegarbagecollected.
Anobjectiseligiblewhennolivethreadcanreachit.
Toreachanobject,youmusthavealive,reachablereferencetothatobject.
Javaapplicationscanrunoutofmemory.
Islandsofobjectscanbegarbagecollected,eventhoughtheyrefertoeachother.
RequestgarbagecollectionwithSystem.gc();.
TheObjectclasshasafinalize()method.
Thefinalize()methodisguaranteedtorunonceandonlyoncebeforethegarbagecollector
deletesanobject.
Thegarbagecollectormakesnoguarantees;finalize()mayneverrun.
Youcanineligible-izeanobjectforGCfromwithinfinalize().
SELFTEST
1.Given:
When//doStuffisreached,howmanyobjectsareeligibleforgarbagecollection?
A.0
B.1
C.2
D.Compilationfails
E.Itisnotpossibletoknow
F.Anexceptionisthrownatruntime
2.Given:
WhichlinesWILLNOTcompile?(Chooseallthatapply.)
A.LineA
B.LineB
C.LineC
D.LineD
E.LineE
3.Given:
WhichlinesWILLNOTcompile?(Chooseallthatapply.)
A.LineA
B.LineB
C.LineC
D.LineD
E.LineE
F.LineF
4.Given:
Whatistheresult?
A.hi
B.hihi
C.hihihi
D.Compilationfails
E.hi,followedbyanexception
F.hihi,followedbyanexception
5.Given:
Whatistheresult?
A.truetrue
B.falsetrue
C.truefalse
D.falsefalse
E.Compilationfails
F.Anexceptionisthrownatruntime
6.Given:
Whatistheresult?
A.710
B.810
C.712
D.812
E.Compilationfails
F.Anexceptionisthrownatruntime
7.Given:
Whenexecutionreachesthecommentedline,whicharetrue?(Chooseallthatapply.)
A.Theoutputcontains1
B.Theoutputcontains2
C.Theoutputcontains3
D.ZeroWindobjectsareeligibleforgarbagecollection
E.OneWindobjectiseligibleforgarbagecollection
F.TwoWindobjectsareeligibleforgarbagecollection
G.ThreeWindobjectsareeligibleforgarbagecollection
8.Given:
Whatistheresult?
A.57
B.58
C.87
D.88
E.Compilationfails
F.Anexceptionisthrownatruntime
9.Given:
Whatistheresult?
A.1
B.2
C.3
D.Compilationfails
E.Anexceptionisthrownatruntime
10.Given:
Whatistheresult?
A.1
B.2
C.3
D.null
E.Compilationfails
11.Given:
Whenline16isreached,howmanyobjectswillbeeligibleforgarbagecollection?
A.0
B.1
C.2
D.3
E.4
F.5
12.Given:
Whatistheresult?
A.2
B.10
C.15
D.30
E.70
F.105
G.Compilationfails
13.Given:
Whatistheresult?
A.2
B.3
C.4
D.5
E.Compilationfails
F.Anexceptionisthrownatruntime
SELFTESTANSWERS
1. Ciscorrect.OnlyoneCardBoardobject(c1)iseligible,butithasanassociatedShort
wrapperobjectthatisalsoeligible.
A,B,D,E,andFareincorrectbasedontheabove.(OCAObjective2.4)
2. Eiscorrect;compilationoflineEfails.Whenamathematicaloperationisperformedon
anyprimitivessmallerthanints,theresultisautomaticallycasttoaninteger.
A,B,C,andDarealllegalprimitivecasts.(OCAObjective2.1)
3. Ciscorrect;lineCwillNOTcompile.AsofJava7,underscorescanbeincludedin
numericliterals,butnotatthebeginningortheend.
A,B,D,E,andFareincorrect.AandBarelegalnumericliterals.DandEareexamplesof
validbinaryliterals,whichwerenewtoJava7,andFisavalidhexadecimalliteralthatusesan
underscore.(OCAObjective2.1)
4. Fiscorrect.Them2object’sm1instancevariableisneverinitialized,sowhenm5triesto
useit,aNullPointerExceptionisthrown.
A,B,C,D,andEareincorrectbasedontheabove.(OCAObjectives2.1and2.3)
5. Aiscorrect.Thereferencesf1,z,andf3allrefertothesameinstanceofFizz.Thefinal
modifierassuresthatareferencevariablecannotbereferredtoadifferentobject,butfinal
doesn’tkeeptheobject’sstatefromchanging.
B,C,D,E,andFareincorrectbasedontheabove.(OCAObjective2.2)
6. Biscorrect.Inthego()method,mreferstothesingleMirrorinstance,buttheintiisa
newintvariable,adetachedcopyofi2.
A,C,D,E,andFareincorrectbasedontheabove.(OCAObjectives2.2and2.3)
7. A,B,andGarecorrect.Theconstructorsetsthevalueofidforw1andw2.Whenthe
commentedlineisreached,noneofthethreeWindobjectscanbeaccessed,sotheyareeligibleto
begarbagecollected.
C,D,E,andFareincorrectbasedontheabove.(OCAObjectives1.1,2.3,and2.4)
8. Eiscorrect.Theparameterdeclaredonline9isvalid(althoughugly),butthevariable
nameouchcannotbedeclaredagainonline11inthesamescopeasthedeclarationonline9.
A,B,C,D,andFareincorrectbasedontheabove.(OCAObjectives1.1and2.1)
9. Discorrect.Insidethego()method,h1isoutofscope.
A,B,C,andEareincorrectbasedontheabove.(OCAObjectives1.1and6.1)
10. Aiscorrect.ThreeNetworkobjectsarecreated.Then2objecthasareferencetothen1
object,andthen3objecthasareferencetothen2object.TheS.O.P.canbereadas,“Usethen3
object’sNetworkreference(thefirstp),tofindthatobject’sreference(n2),andusethatobject’s
reference(thesecondp)tofindthatobject’s(n1’s)id,andprintthatid.”
B,C,D,andEareincorrectbasedontheabove.(OCAObjectives,2.2,2.3,and6.4)
11. Biscorrect.Itshouldbeclearthatthereisstillareferencetotheobjectreferredtobya2,
andthatthereisstillareferencetotheobjectreferredtobya2.b2.Whatmightbelessclearisthat
youcanstillaccesstheotherBetaobjectthroughthestaticvariablea2.b1—becauseit’sstatic.
A,C,D,E,andFareincorrectbasedontheabove.(OCAObjective2.4)
12. Biscorrect.IntheTelescopeclass,therearethreedifferentvariablesnamedmagnify.
Thego()method’sversionandthezoomMore()method’sversionarenotusedinthezoomIn()
method.ThezoomIn()methodmultipliestheclassvariable*5.Theresult(10)issentto
zoomMore(),butwhathappensinzoomMore()staysinzoomMore().TheS.O.P.printsthevalueof
zoomIn()’smagnify.
A,C,D,E,F,andGareincorrectbasedontheabove.(OCAObjectives1.1and6.6)
13. Eiscorrect.Ingo1()thelocalvariablexisnotinitialized.
A,B,C,D,andFareincorrectbasedontheabove.(OCAObjectives2.1and2.3)
I
4
Operators
CERTIFICATIONOBJECTIVES
•UsingJavaOperators
•UseParenthesestoOverrideOperatorPrecedence
•TestEqualityBetweenStringsandOtherObjectsUsing==andequals()
Two-MinuteDrill
Q&ASelfTest
fyou’vegotvariables,you’regoingtomodifythem.(Unlessyou’reoneofthosenew-fangled“FP”
programmers.)You’llincrementthem,addthemtogether,andcompareonetoanother(inabouta
dozendifferentways).Inthischapter,you’lllearnhowtodoallthatinJava.Asanaddedbonus,
you’lllearnhowtodothingsthatyou’llprobablyneveruseintherealworld,butthatwillalmost
certainlybeontheexam.
CERTIFICATIONOBJECTIVE
JavaOperators(OCAObjectives3.1,3.2,and3.3)
3.1UseJavaoperators;includingparenthesestooverrideoperatorprecedence.
3.2TestequalitybetweenStringsandotherobjectsusing==andequals().
3.3Createifandif/elseandternaryconstructs
Javaoperatorsproducenewvaluesfromoneormoreoperands.(Justsowe’reallclear,rememberthat
operandsarethethingsontherightorleftsideoftheoperator.)Theresultofmostoperationsiseithera
booleanornumericvalue.BecauseyouknowbynowthatJavaisnotC++,youwon’tbesurprisedthat
Javaoperatorsaren’ttypicallyoverloaded.Thereare,however,afewexceptionaloperatorsthatcome
overloaded:
The+operatorcanbeusedtoaddtwonumericprimitivestogetherortoperforma
concatenationoperationifeitheroperandisaString.
The&,|,and^operatorscanallbeusedintwodifferentways,althoughonthisversionofthe
exam,theirbit-twiddlingcapabilitieswon’tbetested.
Stayawake.Operatorsareoftenthesectionoftheexamwherecandidatesseetheirlowestscores.
Additionally,operatorsandassignmentsareapartofmanyquestionsdealingwithothertopics—itwould
beashametonailareallytrickylambdasquestiononlytoblowitonapre-incrementstatement.
AssignmentOperators
Wecoveredmostofthefunctionalityoftheequal(=)assignmentoperatorinChapter3.Tosummarize:
Whenassigningavaluetoaprimitive,sizematters.Besureyouknowwhenimplicitcasting
willoccur,whenexplicitcastingisnecessary,andwhentruncationmightoccur.
Rememberthatareferencevariableisn’tanobject;it’sawaytogettoanobject.(Weknow
allyouC++programmersarejustdyingforustosay,“it’sapointer,”butwe’renotgoingto.)
Whenassigningavaluetoareferencevariable,typematters.Remembertherulesfor
supertypes,subtypes,andarrays.
Nextwe’llcoverafewmoredetailsabouttheassignmentoperatorsthatareontheexam,andwhenwe
gettothenextchapter,we’lltakealookathowtheassignmentoperator=workswithStrings(whichare
immutable).
Don’tspendtimepreparingfortopicsthatarenolongerontheexam!Thefollowingtopics
haveNOTbeenontheexamsinceJava1.4:
Bit-shiftingoperators
Bitwiseoperators
Two’scomplement
Divide-by-zerostuff
It’snotthatthesearen’timportanttopics;it’sjustthatthey’renotontheexamanymore,and
we’rereallyfocusedontheexam.(Note:Thereasonwebringthisupatallisbecauseyou
mightencountermockexamquestionsonthesetopics—youcanignorethosequestions!)
CompoundAssignmentOperators
Thereareactually11orsocompoundassignmentoperators,butonlythe4mostcommonlyused(+=,-=,
*=,and/=)areontheexam.Thecompoundassignmentoperatorsletlazytypistsshaveafewkeystrokes
offtheirworkload.
Hereareseveralexampleassignments,firstwithoutusingacompoundoperator:
y=y-6;
x=x+2*5;
Now,withcompoundoperators:
y-=6;
x+=2*5;
Thelasttwoassignmentsgivethesameresultasthefirsttwo.
RelationalOperators
Theexamcoverssixrelationaloperators(<,<=,>,>=,==,and!=).Relationaloperatorsalwaysresultin
aboolean(trueorfalse)value.Thisbooleanvalueismostoftenusedinaniftest,asfollows:
Buttheresultingvaluecanalsobeassigneddirectlytoabooleanprimitive:
Javahasfourrelationaloperatorsthatcanbeusedtocompareanycombinationofintegers,floating-
pointnumbers,orcharacters:
>Greaterthan
>=Greaterthanorequalto
<Lessthan
<=Lessthanorequalto
Let’slookatsomelegalcomparisons:
Intheprecedingcode,weareusingacomparisonbetweencharacters.It’salsolegaltocomparea
characterprimitivewithanynumber(althoughitisn’tgreatprogrammingstyle).Runningthepreceding
classwilloutputthefollowing:
Theanimalisagrayelephant
Wementionedthatcharacterscanbeusedincomparisonoperators.Whencomparingacharacterwith
acharacteroracharacterwithanumber,JavawillusetheUnicodevalueofthecharacterasthe
numericalvalueforcomparison.
“Equality”Operators
Javaalsohastworelationaloperators(sometimescalled“equalityoperators”)thatcomparetwosimilar
“things”andreturnaboolean(trueorfalse)thatrepresentswhat’strueaboutthetwo“things”being
equal.Theseoperatorsare
==Equal(alsoknownasequalto)
!=Notequal(alsoknownasnotequalto)
Eachindividualcomparisoncaninvolvetwonumbers(includingchar),twobooleanvalues,ortwo
objectreferencevariables.Youcan’tcompareincompatibletypes,however.Whatwoulditmeantoaskif
abooleanisequaltoachar?OrifaButtonisequaltoaStringarray?(Thisisnonsense,whichis
whyyoucan’tdoit.)Therearefourdifferenttypesofthingsthatcanbetested:
Numbers
Characters
Booleanprimitives
Objectreferencevariables
Sowhatdoes==lookat?Thevalueinthevariable—inotherwords,thebitpattern.
EqualityforPrimitives
Mostprogrammersarefamiliarwithcomparingprimitivevalues.Thefollowingcodeshowssome
equalitytestsonprimitivevariables:
Thisprogramproducesthefollowingoutput:
Asyoucansee,ifafloating-pointnumberiscomparedwithanintegerandthevaluesarethesame,the
==operatorusuallyreturnstrueasexpected.
EqualityforReferenceVariables
Asyousawearlier,tworeferencevariablescanrefertothesameobject,asthefollowingcodesnippet
demonstrates:
Don’tmistake=for==inabooleanexpression.Thefollowingislegal:
Lookcarefully!Youmightbetemptedtothinktheoutputisbisfalse,butlookatthe
booleantestinline12.Thebooleanvariablebisnotbeingcomparedtotrue;it’sbeingsetto
true.Oncebissettotrue,theprintlnexecutesandwegetbistrue.Theresultofany
assignmentexpressionisthevalueofthevariablefollowingtheassignment.Thissubstitution
of=for==worksonlywithbooleanvariablesbecausetheiftestcanbedoneonlyonboolean
expressions.Thus,thisdoesnotcompile:
Becausexisaninteger(andnotaboolean),theresultof(x=0)is0(theresultofthe
assignment).Primitiveintscannotbeusedwhereabooleanvalueisexpected,sothecodein
line8won’tworkunlessit’schangedfromanassignment(=)toanequalitytest(==)as
follows:
Afterrunningthiscode,bothvariableaandvariablebwillrefertothesameobject(aJButtonwith
thelabelExit).Referencevariablescanbetestedtoseeiftheyrefertothesameobjectbyusingthe==
operator.Remember,the==operatorislookingatthebitsinthevariable,soforreferencevariables,this
meansthatifthebitsinbothreferencevariablesareidentical,thenbothrefertothesameobject.Lookat
thefollowingcode:
Thiscodecreatesthreereferencevariables.Thefirsttwo,aandb,areseparateJButtonobjectsthat
happentohavethesamelabel.Thethirdreferencevariable,c,isinitializedtorefertothesameobject
thatarefersto.Whenthisprogramruns,thefollowingoutputisproduced:
ThisshowsusthataandcreferencethesameinstanceofaJButton.The==operatorwillnottest
whethertwoobjectsare“meaningfullyequivalent,”aconceptwe’llcoverinmuchmoredetailinChapter
6,whenwelookattheequals()method(asopposedtotheequalsoperatorwe’relookingathere).
EqualityforStringsandjava.lang.Object.equals()
Wejustused==todeterminewhethertworeferencevariablesrefertothesameobject.Becauseobjects
aresocentraltoJava,everyclassinJavainheritsamethodfromclassObjectthatteststoseeiftwo
objectsoftheclassare“equal.”Notsurprisingly,thismethodiscalledequals().Inthiscaseofthe
equals()method,thephrase“meaningfullyequivalent”shouldbeusedinsteadoftheword“equal.”So
theequals()methodisusedtodetermineiftwoobjectsofthesameclassare“meaningfullyequivalent.”
Forclassesthatyoucreate,youhavetheoptionofoverridingtheequals()methodthatyourclass
inheritedfromclassObjectandcreatingyourowndefinitionof“meaningfullyequivalent”forinstances
ofyourclass.
Intermsofunderstandingtheequals()methodfortheOCAexam,youneedtounderstandtwoaspects
oftheequals()method:
Whatequals()meansinclassObject
Whatequals()meansinclassString
Theequals()MethodinClassObjectTheequals()methodinclassObjectworksthesame
waythatthe==operatorworks.Iftworeferencespointtothesameobject,theequals()methodwill
returntrue.Iftworeferencespointtodifferentobjects,eveniftheyhavethesamevalues,themethodwill
returnfalse.
Theequals()MethodinClassStringTheequals()methodinclassStringhasbeen
overridden.Whentheequals()methodisusedtocomparetwostrings,itwillreturntrueifthestrings
havethesamevalue,anditwillreturnfalseifthestringshavedifferentvalues.ForString’sequals()
method,valuesAREcasesensitive.
Let’stakealookathowtheequals()methodworksinaction(noticethattheBudgieclassdidNOT
overrideObject.equals()):
whichproducestheoutput:
false
true
true
false
Equalityforenums
Onceyou’vedeclaredanenum,it’snotexpandable.Atruntime,there’snowaytomakeadditionalenum
constants.Ofcourse,youcanhaveasmanyvariablesasyou’dlikerefertoagivenenumconstant,soit’s
importanttobeabletocomparetwoenumreferencevariablestoseeifthey’re“equal”—thatis,dothey
refertothesameenumconstant?Youcanuseeitherthe==operatorortheequals()methodtodetermine
whethertwovariablesarereferringtothesameenumconstant:
(Weknow}}isugly;we’repreppingyou.)Thisproducestheoutput:
instanceofComparison
Theinstanceofoperatorisusedforobjectreferencevariablesonly,andyoucanuseittocheckwhether
anobjectisofaparticulartype.By“type,”wemeanclassorinterfacetype—inotherwords,whetherthe
objectreferredtobythevariableontheleftsideoftheoperatorpassestheIS-Atestfortheclassor
interfacetypeontherightside.(Chapter2coveredIS-Arelationshipsindetail.)Thefollowingsimple
example,
printsthis:
sisaString
Eveniftheobjectbeingtestedisnotanactualinstantiationoftheclasstypeontherightsideofthe
operator,instanceofwillstillreturntrueiftheobjectbeingcomparedisassignmentcompatiblewith
thetypeontheright.
Thefollowingexampledemonstratesacommonuseforinstanceof:testinganobjecttoseeifit’san
instanceofoneofitssubtypesbeforeattemptingadowncast:
Thecodecompilesandproducesthisoutput:
‘a’referstoaB
Inexampleslikethis,theuseoftheinstanceofoperatorprotectstheprogramfromattemptingan
illegaldowncast.
Youcantestanobjectreferenceagainstitsownclasstypeoranyofitssuperclasses.Thismeansthat
anyobjectreferencewillevaluatetotrueifyouusetheinstanceofoperatoragainsttypeObject,as
follows:
Thisprints
bisdefinitelyanObject
Lookforinstanceofquestionsthattestwhetheranobjectisaninstanceofaninterfacewhen
theobject’sclassimplementstheinterfaceindirectly.Anindirectimplementationoccurswhen
oneofanobject’ssuperclassesimplementsaninterface,buttheactualclassoftheinstance
doesnot.Inthisexample,
thefollowingaretrue:
Anobjectissaidtobeofaparticularinterfacetype(meaningitwillpasstheinstanceoftest)
ifanyoftheobject’ssuperclassesimplementtheinterface.
Inaddition,itislegaltotestwhetherthenullreferenceisaninstanceofaclass.Thiswillalways
resultinfalse,ofcourse.Thisexample,
printsthis:
falsefalse
instanceofCompilerError
Youcan’tusetheinstanceofoperatortotestacrosstwodifferentclasshierarchies.Forinstance,the
followingwillNOTcompile:
Compilationfails—there’snowaydcouldeverrefertoaCatorasubtypeofCat.
Rememberthatarraysareobjects,evenifthearrayisanarrayofprimitives.Watchfor
questionsthatlooksomethinglikethis:
AnarrayisalwaysaninstanceofObject.Anyarray.
Table4-1summarizestheuseoftheinstanceofoperatorgiventhefollowing:
TABLE4-1 OperandsandResultsUsinginstanceofOperator
ArithmeticOperators
We’resureyou’refamiliarwiththebasicarithmeticoperators:
+addition
–subtraction
*multiplication
/division
Thesecanbeusedinthestandardway:
TheRemainder(%)Operator(a.k.a.theModulusOperator)
Oneoperatoryoumightnotbeasfamiliarwithistheremainderoperator:%.Theremainderoperator
dividestheleftoperandbytherightoperand,andtheresultistheremainder,asthefollowingcode
demonstrates:
RunningclassMathTestprintsthefollowing:
Theresultof15%4istheremainderof15dividedby4.Theremainderis3
(Remember:Expressionsareevaluatedfromlefttorightbydefault.Youcanchangethissequence,or
precedence,byaddingparentheses.Alsorememberthatthe*,/,and%operatorshaveahigher
precedencethanthe+and-operators.)
Whenworkingwithints,theremainderoperator(a.k.a.themodulusoperator)andthe
divisionoperatorrelatetoeachotherinaninterestingway:
Themodulusoperatorthrowsouteverythingbuttheremainder.
Thedivisionoperatorthrowsouttheremainder.
StringConcatenationOperator
Theplussigncanalsobeusedtoconcatenatetwostringstogether,aswesawearlier(andaswe’ll
definitelyseeagain):
StringconcatenationgetsinterestingwhenyoucombinenumberswithStrings.Checkoutthe
following:
Willthe+operatoractasaplussignwhenaddingtheintvariablesbandc?Orwillthe+operator
treat3and7ascharactersandconcatenatethemindividually?WilltheresultbeString10orString37?
Okay,you’vehadlongenoughtothinkaboutit.
TheintvaluesweresimplytreatedascharactersandgluedontotherightsideoftheString,giving
theresult:
String37
Sowecouldreadthepreviouscodeas
“StartwiththevalueString,andconcatenatethecharacter3(thevalueofb)toit,toproduceanew
stringString3,andthenconcatenatethecharacter7(thevalueofc)tothat,toproduceanewstring
String37.Thenprintitout.”
However,ifyouputparenthesesaroundthetwointvariables,asfollows,
System.out.println(a+(b+c));
you’llgetthis:
String10
Usingparenthesescausesthe(b+c)toevaluatefirst,sotherightmost+operatorfunctionsasthe
additionoperator,giventhatbothoperandsareintvalues.Thekeypointhereisthatwithinthe
parentheses,theleft-handoperandisnotaString.Ifitwere,thenthe+operatorwouldperformString
concatenation.Thepreviouscodecanbereadas
“Addthevaluesofbandctogether,andthentakethesumandconvertittoaStringandconcatenateitwiththeStringfrom
variablea.”
Theruletorememberisthis:
IfeitheroperandisaString,the+operatorbecomesaStringconcatenationoperator.Ifbothoperandsarenumbers,the+
operatoristheadditionoperator.
You’llfindthatsometimesyoumighthavetroubledecidingwhether,say,theleft-handoperatorisa
Stringornot.Ontheexam,don’texpectitalwaystobeobvious.(Actually,nowthatwethinkaboutit,
don’texpectitevertobeobvious.)Lookatthefollowingcode:
System.out.println(x.foo()+7);
Youcan’tknowhowthe+operatorisbeinguseduntilyoufindoutwhatthefoo()methodreturns!If
foo()returnsaString,then7isconcatenatedtothereturnedString.Butiffoo()returnsanumber,
thenthe+operatorisusedtoadd7tothereturnvalueoffoo().
Finally,youneedtoknowthatit’slegaltomushtogetherthecompoundadditiveoperator(+=)and
Strings,likeso:
Sincebothtimesthe+=operatorwasusedandtheleftoperandwasaString,bothoperationswere
concatenations,resultingin
1234567
Ifyoudon’tunderstandhowStringconcatenationworks,especiallywithinaprintstatement,
youcouldactuallyfailtheexamevenifyouknowtherestoftheanswerstothequestions!
Becausesomanyquestionsask"Whatistheresult?",youneedtoknownotonlytheresultof
thecoderunningbutalsohowthatresultisprinted.Althoughatleastafewquestionswill
directlytestyourStringknowledge,Stringconcatenationshowsupinotherquestionson
virtuallyeveryobjective.Experiment!Forexample,youmightseealinesuchasthis:
Itprintsthis:
23
Butiftheprintstatementchangestothis:
System.out.println(b+3);
Theprintedresultbecomes
5
IncrementandDecrementOperators
Javahastwooperatorsthatwillincrementordecrementavariablebyexactlyone.Theseoperatorsare
eithertwoplussigns(++)ortwominussigns(--):
++Increment(prefixandpostfix)
--Decrement(prefixandpostfix)
Theoperatorisplacedeitherbefore(prefix)orafter(postfix)avariabletochangeitsvalue.Whether
theoperatorcomesbeforeoraftertheoperandcanchangetheoutcomeofanexpression.Examinethe
following:
Noticethatinthefourthlineoftheprogramtheincrementoperatorisafterthevariableplayers.That
meanswe’reusingthepostfixincrementoperator,whichcausesplayerstobeincrementedbyonebut
onlyafterthevalueofplayersisusedintheexpression.Whenwerunthisprogram,itoutputsthe
following:
Noticethatwhenthevariableiswrittentothescreen,atfirstitsaysthevalueis0.Becauseweusedthe
postfixincrementoperator,theincrementdoesn’thappenuntilaftertheplayersvariableisusedinthe
printstatement.Getit?The“post”inpostfixmeansafter.Line5doesn’tincrementplayers;itjust
outputsitsvaluetothescreen,sothenewlyincrementedvaluedisplayedis1.Line6appliestheprefix
incrementoperatortoplayers,whichmeanstheincrementhappensbeforethevalueofthevariableis
used,sotheoutputis2.
Expecttoseequestionsmixingtheincrementanddecrementoperatorswithotheroperators,asinthe
followingexample:
Theprecedingcodeprintsthis:
x=3y=4
Youcanreadthecodeasfollows:“If3isequalto2OR3<4”
Thefirstexpressioncomparesxandy,andtheresultisfalse,becausetheincrementonxdoesn’t
happenuntilafterthe==testismade.Next,weincrementx,sonowxis3.Thenwechecktoseeifxis
lessthany,butweincrementybeforecomparingitwithx!Sothesecondlogicaltestis(3<4).The
resultistrue,sotheprintstatementruns.
AswithStringconcatenation,theincrementanddecrementoperatorsareusedthroughouttheexam,
evenonquestionsthataren’ttryingtotestyourknowledgeofhowthoseoperatorswork.Youmightsee
theminquestionsonforloops,exceptions,oreventhreads.Beready.
Lookoutforquestionsthatusetheincrementordecrementoperatorsonafinalvariable.
Becausefinalvariablescan’tbechanged,theincrementanddecrementoperatorscan’tbeused
withthem,andanyattempttodosowillresultinacompilererror.Thefollowingcodewon’t
compile:
Itproducesthiserror:
Youcanexpectaviolationlikethistobeburieddeepinacomplexpieceofcode.Ifyouspotit,
youknowthecodewon’tcompile,andyoucanmoveonwithoutworkingthroughtherestof
thecode.
Thisquestionmightseemtobetestingyouonsomecomplexarithmeticoperatortrivia,
when,infact,it’stestingyouonyourknowledgeofthefinalmodifier.
ConditionalOperator
Theconditionaloperatorisaternaryoperator(ithasthreeoperands)andisusedtoevaluateboolean
expressions—muchlikeanifstatement,exceptinsteadofexecutingablockofcodeifthetestistrue,a
conditionaloperatorwillassignavaluetoavariable.Inotherwords,thegoaloftheconditionaloperator
istodecidewhichoftwovaluestoassigntoavariable.Thisoperatorisconstructedusinga?(question
mark)anda:(colon).Theparenthesesareoptional.Hereisitsstructure:
x=(booleanexpression)?valuetoassigniftrue:valuetoassigniffalse
Let’stakealookataconditionaloperatorincode:
Youcanreadtheprecedingcodeas“SetnumOfPetsequalto3“.
Nextwe’regoingtoassignaStringtothestatusvariable.IfnumOfPetsislessthan4,assign“Pet
limitnotexceeded”tothestatusvariable;otherwise,assign“toomanypets”tothestatus
variable.
Aconditionaloperatorstartswithabooleanoperation,followedbytwopossiblevaluesforthe
variabletotheleftoftheassignment(=)operator.Thefirstvalue(theonetotheleftofthecolon)is
assignediftheconditional(boolean)testistrue,andthesecondvalueisassignediftheconditionaltest
isfalse.Youcanevennestconditionaloperatorsintoonestatement:
Don’texpectmanyquestionsusingconditionaloperators,butyoumightgetone.
LogicalOperators
Theexamobjectivesspecifysix“logical”operators(&,|,^,!,&&,and||).SomeOracledocumentation
usesotherterminologyfortheseoperators,butforourpurposesandintheexamobjectives,thesesixare
thelogicaloperators.
BitwiseOperators(NotanExamTopic!)
Okay,thisisgoingtobeconfusing.Ofthesixlogicaloperatorsjustlisted,threeofthem(&,|,and^)can
alsobeusedas“bitwise”operators.Bitwiseoperatorswereincludedinpreviousversionsoftheexam,
butthey’reNOTontheJava6,Java7,orJava8exam.Webringthemupherejustsoyouhaveamore
completepictureofthelogicaloperators.
Hereareseverallegalstatementsthatusebitwiseoperators:
Bitwiseoperatorscomparetwovariablesbitbybitandreturnavariablewhosebitshavebeenset
basedonwhetherthetwovariablesbeingcomparedhadrespectivebitsthatwereeitherboth“on”(&),
oneortheother“on”(|),orexactlyone“on”(^).Bytheway,whenweruntheprecedingcode,weget
0151
Havingsaidallthisaboutbitwiseoperators,thekeythingtoremember
BITWISEOPERATORSARENOTONTHEJava6,Java7,orJava8EXAM!
Short-CircuitLogicalOperators
Fivelogicaloperatorsontheexamareusedtoevaluatestatementsthatcontainmorethanoneboolean
expression.Themostcommonlyusedofthefivearethetwoshort-circuitlogicaloperators:
&&Short-circuitAND
||Short-circuitOR
Theyareusedtolinklittlebooleanexpressionstogethertoformbiggerbooleanexpressions.The&&
and||operatorsevaluateonlybooleanvalues.ForanAND(&&)expressiontobetrue,bothoperands
mustbetrue.Forexample:
if((2<3)&&(3<4)){}
Theprecedingexpressionevaluatestotruebecausebothoperandone(2<3)andoperandtwo(3<
4)evaluatetotrue.
Theshort-circuitfeatureofthe&&operatorissonamedbecauseitdoesn’twasteitstimeon
pointlessevaluations.Ashort-circuit&&evaluatestheleftsideoftheoperationfirst(operandone),andif
itresolvestofalse,the&&operatordoesn’tbotherlookingattherightsideoftheexpression(operand
two)sincethe&&operatoralreadyknowsthatthecompleteexpressioncan’tpossiblybetrue.
Whenweruntheprecedingcode,theassignment(b2=true)neverrunsbecauseoftheshort-circuit
operator,sotheoutputis
The||operatorissimilartothe&&operator,exceptthatitevaluatestotrueifEITHERofthe
operandsistrue.IfthefirstoperandinanORoperationistrue,theresultwillbetrue,sotheshort-
circuit||doesn’twastetimelookingattherightsideoftheequation.Ifthefirstoperandisfalse,
however,theshort-circuit||hastoevaluatethesecondoperandtoseeiftheresultoftheORoperation
willbetrueorfalse.Paycloseattentiontothefollowingexample;you’llseequiteafewquestionslike
thisontheexam:
Whatistheresult?
Here’swhathappenedwhenthemain()methodran:
1.Whenwehitline3,thefirstoperandinthe||expression(inotherwords,theleftsideofthe
||operation)isevaluated.
2.TheisItSmall(3)methodisinvoked,prints“i<5”,andreturnstrue.
3.Becausethefirstoperandinthe||expressiononline3istrue,the||operatordoesn’t
botherevaluatingthesecondoperand.Soweneverseethe“i>=5”thatwouldhaveprintedhad
thesecondoperandbeenevaluated(whichwouldhaveinvokedisItSmall(7)).
4.Line6isevaluated,beginningwiththefirstoperandinthe||expression.
5.TheisItSmall(6)methodiscalled,prints“i>=5”,andreturnsfalse.
6.Becausethefirstoperandinthe||expressiononline6isfalse,the||operatorcan’tskip
thesecondoperand;there’sstillachancetheexpressioncanbetrue,ifthesecondoperand
evaluatestotrue.
7.TheisItSmall(9)methodisinvokedandprints“i>=5”.
8.TheisItSmall(9)methodreturnsfalse,sotheexpressiononline6isfalse,andthusline
7neverexecutes.
The||and&&operatorsworkonlywithbooleanoperands.Theexammaytrytofoolyouby
usingintegerswiththeseoperators:
if(5&&6){}
Itlooksasthoughwe’retryingtodoabitwiseANDonthebitsrepresentingtheintegers5and
6,butthecodewon’tevencompile.
LogicalOperators(notShort-Circuit)
Therearetwonon-short-circuitlogicaloperators:
&Non-short-circuitAND
|Non-short-circuitOR
Theseoperatorsareusedinlogicalexpressionsjustlikethe&&and||operatorsareused,butbecause
theyaren’ttheshort-circuitoperators,theyevaluatebothsidesoftheexpression—always!They’re
inefficient.Forexample,evenifthefirstoperand(leftside)inan&expressionisfalse,thesecond
operandwillstillbeevaluated—eventhoughit’snowimpossiblefortheresulttobetrue!Andthe|is
justasinefficient:ifthefirstoperandistrue,theJavaVirtualMachine(JVM)stillplowsaheadand
evaluatesthesecondoperandevenwhenitknowstheexpressionwillbetrueregardless.
You’llfindalotofquestionsontheexamthatuseboththeshort-circuitandnon-short-circuitlogical
operators.You’llhavetoknowexactlywhichoperandsareevaluatedandwhicharenot,becausethe
resultwillvarydependingonwhetherthesecondoperandintheexpressionisevaluated.Considerthis,
versusthis:
LogicalOperators^and!
Thelasttwologicaloperatorsontheexamare
^Exclusive-OR(XOR)
!Booleaninvert
The^(exclusive-OR)operatorevaluatesonlybooleanvalues.The^operatorisrelatedtothenon-
short-circuitoperatorswejustreviewed,inthatitalwaysevaluatesboththeleftandrightoperandsinan
expression.Foranexclusive-OR(^)expressiontobetrue,EXACTLYoneoperandmustbetrue.This
example,
System.out.println(“xor”+((2<3)^(4>3)));
producesthisoutput:
xorfalse
TheprecedingexpressionevaluatestofalsebecauseBOTHoperandone(2<3)andoperandtwo
(4>3)evaluatetotrue.
The!(booleaninvert)operatorreturnstheoppositeofaboolean’scurrentvalue.Thefollowing
statement,
if(!(7==5)){System.out.println(“notequal”);}
canberead“Ifit’snottruethat7==5,”andthestatementproducesthisoutput:
notequal
Here’sanotherexampleusingbooleans:
Itproducesthisoutput:
!truefalse
Intheprecedingexample,noticethatthe&testsucceeded(printingtrue)andthatthevalueofthe
booleanvariablefdidnotchange,soitprintedfalse.
OperatorPrecedence
TheOCA8examhasreintroducedthetopicofoperatorprecedence.Asyouprobablyalreadyknowbut
willdefinitelyseedemonstratedinthissection,whenseveraloperatorsareusedincombination,the
orderinwhichtheyareevaluatedcanaltertheresultoftheexpression.
OperatorPrecedenceRant
Allowustorantforaminutehere.MemorizingoperatorprecedencewasontheoldSCJP1.2examabout
15yearsago.StartingwiththeSCJP1.4exam,andforalltheexamsuntiltheOCA8,operatorprecedence
hasnotbeenontheexam.Foraglorious15years,candidatesdidn’thavetodothisbitofmemorization.
Sadly,thistopicsnuckitswaybackintotheexamforOCA8.Whydowecaresomuchaboutthis?Takea
lookatthiscode:
System.out.println(true&false==false|true);
Whatresultwouldyouexpect?Imagineamorerealisticversion,evaluatingsomebooleans:
System.out.println(b1&b2==b3|b4);
Whatwouldyouguesstheprogrammer’sintentionwashere?Therearetwolikelyscenarios:
Scenario1:(b1&b2)==(b3|b4)Ifthiswastheprogrammer’sintention,thenhejustcreateda
bug.
Scenario2:b1&(b2==b3)|b4Ifthiswastheprogrammer’sintention,thenthecodewillwork
asintended,buthisbossandfellowworkerswillwanttostranglehim.
Thisisalong-windedwaytosaythatwhenyou’rewritingcode,youshouldn’trelyoneveryone’s
memoryofoperatorprecedence.Youshouldjustuseparentheseslikecivilizedpeopledo.
TheActualBeginningoftheOperatorPrecedenceSection
Table4-2liststhemostcommonlyusedoperatorsandtheirrelativeprecedence,startingatthetopwith
thehighestprecedenceoperatorsandendingatthebottomwiththelowest.(Note,notallofJava’s
operatorsareinthistable!)
TABLE4-2 PrecedenceHierarchyofCommonOperators(fromHighesttoLowest)
JavaRanch(weknow,weknow,“Coderanch”)moderatorFritzWalravensharedthistipwithus.We
likeit,andwe’repassingitalongtoyou:forthetableabove,youmightmakeupawordlike
“UMARELSA,”orasentenceusingthosefirstletters,tohelpyouremembertheprecedencerules!
TherearethreeimportantgeneralrulesfordetermininghowJavawillevaluateexpressionswith
operators:
Whentwooperatorsofthesameprecedenceareinthesameexpression,Javaevaluatesthe
expressionfromlefttoright.
Whenpartsofanexpressionareplacedinparentheses,thosepartsareevaluatedfirst.
Whenparenthesesarenested,theinnermostparenthesesareevaluatedfirst.
Agoodwaytoburntheseprecedencerulesintoyourbrainisto—asalways—writesometestcode
andplayaroundwithit.We’veaddedanexampleofsometestcodethatdemonstratesseveralofthe
precedencehierarchyruleslistedhere.Asyoucansee,weoftencomparedparentheses-freeexpressions
withtheirparentheses-richcounterpartstoprovetherules:
Andtorepeat,theoutputis:
We’resosorrythatyouneedtomemorizethisstuff,butifyoumasterwhat’sinthisshortsection,you
shouldbeabletohandlewhateverweirdprecedence-relatedquestionstheexamthrowsatyou.
CERTIFICATIONSUMMARY
Ifyou’vestudiedthischapterdiligently,youshouldhaveafirmgrasponJavaoperators,andyoushould
understandwhatequalitymeanswhentestedwiththe==operator.Let’sreviewthehighlightsofwhat
you’velearnedinthischapter.
Thelogicaloperators(&&,||,&,|,and^)canbeusedonlytoevaluatetwobooleanexpressions.The
differencebetween&&and&isthatthe&&operatorwon’tbothertestingtherightoperandiftheleft
evaluatestofalse,becausetheresultofthe&&expressioncanneverbetrue.Thedifferencebetween||
and|isthatthe||operatorwon’tbothertestingtherightoperandiftheleftevaluatestotruebecausethe
resultisalreadyknowntobetrueatthatpoint.
The==operatorcanbeusedtocomparevaluesofprimitives,butitcanalsobeusedtodetermine
whethertworeferencevariablesrefertothesameobject.
Theinstanceofoperatorisusedtodeterminewhethertheobjectreferredtobyareferencevariable
passestheIS-Atestforaspecifiedtype.
The+operatorisoverloadedtoperformStringconcatenationtasksandcanalsoconcatenate
Stringsandprimitives,butbecareful—concatenationcanbetricky.
Theconditionaloperator(a.k.a.the“ternaryoperator”)hasanunusual,three-operandsyntax—don’t
mistakeitforacomplexassertstatement.
The++and--operatorswillbeusedthroughouttheexam,andyoumustpayattentiontowhetherthey
areprefixedorpostfixedtothevariablebeingupdated.
Eventhoughyoushoulduseparenthesesinreallife,fortheexamyoushouldmemorizeTable4-2so
youcandeterminehowcodethatdoesn’tuseparenthesesforcomplexexpressionswillbeevaluated,
basedonJava’soperator-precedencehierarchy.
Bepreparedforalotofexamquestionsinvolvingthetopicsfromthischapter.Evenwithinquestions
testingyourknowledgeofanotherobjective,thecodewillfrequentlyuseoperators,assignments,object
andprimitivepassing,andsoon.
TWO-MINUTEDRILL
Herearesomeofthekeypointsfromeachsectioninthischapter.
RelationalOperators(OCAObjectives3.1and3.2)
Relationaloperatorsalwaysresultinabooleanvalue(trueorfalse).
Therearesixrelationaloperators:>,>=,<,<=,==,and!=.Thelasttwo(==and!=)are
sometimesreferredtoasequalityoperators.
Whencomparingcharacters,JavausestheUnicodevalueofthecharacterasthenumerical
value.
Equalityoperators
Therearetwoequalityoperators:==and!=.
Fourtypesofthingscanbetested:numbers,characters,booleans,andreference
variables.
Whencomparingreferencevariables,==returnstrueonlyifbothreferencesrefertothe
sameobject.
instanceofOperator(OCAObjective3.1)
instanceofisforreferencevariablesonly;itcheckswhethertheobjectisofaparticular
type.
Theinstanceofoperatorcanbeusedonlytotestobjects(ornull)againstclasstypesthat
areinthesameclasshierarchy.
Forinterfaces,anobjectpassestheinstanceoftestifanyofitssuperclassesimplementthe
interfaceontherightsideoftheinstanceofoperator.
ArithmeticOperators(OCAObjective3.1)
Thefourprimarymathoperatorsareadd(+),subtract(–),multiply(*),anddivide(/).
Theremainder(a.k.a.modulus)operator(%)returnstheremainderofadivision.
Expressionsareevaluatedfromlefttoright,unlessyouaddparentheses,orunlesssome
operatorsintheexpressionhavehigherprecedencethanothers.
The*,/,and%operatorshavehigherprecedencethan+and–.
StringConcatenationOperator(OCAObjective3.1)
IfeitheroperandisaString,the+operatorconcatenatestheoperands.
Ifbothoperandsarenumeric,the+operatoraddstheoperands.
Increment/DecrementOperators(OCAObjective3.1)
Prefixoperators(e.g.--x)runbeforethevalueisusedintheexpression.
Postfixoperators(e.g.,x++)runafterthevalueisusedintheexpression.
Inanyexpression,bothoperandsarefullyevaluatedbeforetheoperatorisapplied.
Variablesmarkedfinalcannotbeincrementedordecremented.
Ternary(Conditional)Operator(OCAObjective3.3)
Returnsoneoftwovaluesbasedonthestateofitsbooleanexpression.
Returnsthevalueafterthe?iftheexpressionistrue.
Returnsthevalueafterthe:iftheexpressionisfalse.
LogicalOperators(OCAObjective3.1)
Theexamcoverssix“logical”operators:&,|,^,!,&&,and||.
Workwithtwoexpressions(exceptfor!)thatmustresolvetobooleanvalues.
The&&and&operatorsreturntrueonlyifbothoperandsaretrue.
The||and|operatorsreturntrueifeitherorbothoperandsaretrue.
The&&and||operatorsareknownasshort-circuitoperators.
The&&operatordoesnotevaluatetherightoperandiftheleftoperandisfalse.
The||doesnotevaluatetherightoperandiftheleftoperandistrue.
The&and|operatorsalwaysevaluatebothoperands.
The^operator(calledthe“logicalXOR”)returnstrueifexactlyoneoperandistrue.
The!operator(calledthe“inversion”operator)returnstheoppositevalueoftheboolean
operanditprecedes.
ParenthesesandOperatorPrecedence(OCAObjective3.1)
Inreallife,useparenthesestoclarifyyourcode,andforceJavatoevaluateexpressionsas
intended.
Fortheexam,memorizeTable4-2todeterminehowparentheses-freecodewillbeevaluated.
SELFTEST
1.Given:
Whatistheresult?
A.null
B.life
C.universe
D.everything
E.Compilationfails
F.Anexceptionisthrownatruntime
2.Given:
Whatistheresult?
A.truetruetrue
B.truetruefalse
C.falsetruefalse
D.falsetruetrue
E.falsefalsefalse
F.Anexceptionwillbethrownatruntime
3.Given:
Andthecommand-lineinvocation:
javaForklive2
Whatistheresult?
A.testcase
B.productionlive2
C.testcaselive2
D.Compilationfails
E.Anexceptionisthrownatruntime
4.Given:
Whatistheresult?
A.9foo4786foo
B.9foo474244foo
C.9foo42586foo
D.9foo4254244foo
E.72foo4786foo
F.72foo474244foo
G.72foo42586foo
H.72foo4254244foo
I.Compilationfails
5.Note:Here’sanotherold-styledrag-and-dropquestion…justincase.
Placethefragmentsintothecodetoproducetheoutput33.Notethatyoumustuseeachfragment
exactlyonce.
FRAGMENTS:
6.Given:
Whatistheresult?(Chooseallthatapply.)
A.21
B.22
C.31
D.32
E.Anexceptionisthrownatruntime
7.Given:
Whatistheresult?
A.sameold
B.newlynew
C.Compilationfailsduetomultipleerrors
D.Compilationfailsdueonlytoanerroronline7
E.Compilationfailsdueonlytoanerroronline8
F.Compilationfailsdueonlytoanerroronline11
G.Compilationfailsdueonlytoanerroronline13
8.Given:
Whicharetrue?(Chooseallthatapply.)
A.Compilationfails
B.xwillbeincludedintheoutput
C.ywillbeincludedintheoutput
D.zwillbeincludedintheoutput
E.Anexceptionisthrownatruntime
9.Given:
Whichtwoaretrueaboutthevalueofmaskandthevalueofcountatline10?(Choosetwo.)
A.maskis0
B.maskis1
C.maskis2
D.maskis10
E.maskisgreaterthan10
F.countis0
G.countisgreaterthan0
10.Given:
Whatistheresult?
A.0
B.01
C.02
D.012
E.Compilationfails
F.Anexceptionisthrownatruntime
11.Given:
Whatistheresult?(Thisisatrickyone.Ifyouwantahint,gotakeanotherlookattheoperator
precedencerantinthechapter.)
A.truetrue
B.falsetrue
C.truefalse
D.falsefalse
E.Compilationfails
F.Anexceptionisthrownatruntime
SELFTESTANSWERS
1. Discorrect.Thisisaternarynestedinaternary.Bothternaryexpressionsarefalse.
A,B,C,E,andFareincorrectbasedontheabove.(OCAObjective3.1and3.3)
2. Ciscorrect.The==operatortestsforreferencevariableequality,notobjectequality.
A,B,D,E,andFareincorrectbasedontheabove.(OCAObjectives3.1and3.2)
3. Eiscorrect.Becausetheshort-circuit(||)isnotused,bothoperandsareevaluated.Since
args[1]ispasttheargsarraybounds,anArrayIndexOutOfBoundsExceptionisthrown.
A,B,C,andDareincorrectbasedontheabove.(OCAObjectives3.1and3.2)
4. Giscorrect.Concatenationrunsfromlefttoright,andifeitheroperandisaString,the
operandsareconcatenated.Ifbothoperandsarenumbers,theyareaddedtogether.
A,B,C,D,E,F,H,andIareincorrectbasedontheabove.(OCAObjective3.1)
5.Answer:
Yeah,weknowit’skindofpuzzle-y,butyoumightencountersomethinglikeitontherealexam
ifOraclereinstatesthistypeofquestion.(OCAObjective3.1)
6. Aiscorrect.Whendividingints,remaindersarealwaysroundeddown.
B,C,D,andEareincorrectbasedontheabove.(OCAObjective3.1)
7. Aiscorrect.Allthissyntaxiscorrect.Thefor-eachiteratesthroughtheenumusingthe
values()methodtoreturnanarray.Anenumcanbecomparedusingeitherequals()or==.An
enumcanbeusedinaternaryoperator’sbooleantest.
B,C,D,E,F,andGareincorrectbasedontheabove.(OCAObjectives3.1,3.2,and3.3)
8. Ciscorrect.Line9usesthemodulusoperator,whichreturnstheremainderofthedivision,
whichinthiscaseis1.Also,line9setsb2tofalse,anditdoesn’ttestb2’svalue.Line10would
setb2totrue;however,theshort-circuitoperatorkeepstheexpressionb2=truefrombeing
executed.
A,B,D,andEareincorrectbasedontheabove.(OCAObjectives3.1,and3.2)
9. CandFarecorrect.Atline7the||keepscountfrombeingincremented,butthe|allows
masktobeincremented.Atline8the^returnstrueonlyifexactlyoneoperandistrue.Atline9
maskis2andthe&&keepscountfrombeingincremented.
A,B,D,E,andGareincorrectbasedontheabove.(OCAObjective3.1)
10. Discorrect.First,rememberthatinstanceofcanlookupthroughmultiplelevelsofan
inheritancetree.Alsorememberthatinstanceofiscommonlyusedbeforeattemptingadowncast;
sointhiscase,afterline15,itwouldbepossibletosaySpeedboats3=(Speedboat)b2;.
A,B,C,E,andFareincorrectbasedontheabove.(OCAObjective3.1)
11. Aiscorrect.We’reprettysureyouwon’tencounteranythingashorribleasthisonthereal
exam.Butifyougotthisonecorrect,patyourselfontheback!Thewaytotackleaproblemlikethis
istoevaluatetheexpressioninstages.Inthiscaseyoumightsolveitlikeso:
Stage1:resolveanyuseofunaryoperators
Stage2:resolveanyuseofmultiplication-relatedoperators
Stage3:handleadditionandsubtraction
Stage4:handleanyrelationshipoperators
Stage5:dealwiththeequalityoperators
Stage6:dealwiththelogicaloperators
Stage7:dotheshort-circuitoperators
Stage8:finally,dotheassignmentoperators
B,C,D,E,andFareincorrectbasedontheabove.(OCAObjective3.1)
C
5
FlowControlandExceptions
CERTIFICATIONOBJECTIVES
•UseifandswitchStatements
•Developfor,do,andwhileLoops
•UsebreakandcontinueStatements
•Usetry,catch,andfinallyStatements
•StatetheEffectsofExceptions
•RecognizeCommonExceptions
Two-MinuteDrill
Q&ASelfTest
anyouimaginetryingtowritecodeusingalanguagethatdidn’tgiveyouawaytoexecute
statementsconditionally?Flowcontrolisakeypartofmostanyusefulprogramming
language,andJavaoffersseveralwaystoaccomplishit.Somestatements,suchasif
statementsandforloops,arecommontomostlanguages.ButJavaalsothrowsinacoupleof
flowcontrolfeaturesyoumightnothaveusedbefore—exceptionsandassertions.(We’lldiscuss
assertionsinthenextchapter.)
Theifstatementandtheswitchstatementaretypesofconditional/decisioncontrolsthatallowyourprogramtobehave
differentlyata“forkintheroad,”dependingontheresultofalogicaltest.Javaalsoprovidesthreedifferentloopingconstructs
—for,while,anddo—soyoucanexecutethesamecodeoverandoveragaindependingonsomeconditionbeingtrue.
Exceptionsgiveyouaclean,simplewaytoorganizecodethatdealswithproblemsthatmightcropupatruntime.
Withthesetools,youcanbuildarobustprogramthatcanhandleanylogicalsituationwithgrace.Expecttoseeawiderange
ofquestionsontheexamthatincludeflowcontrolaspartofthequestioncode,evenonquestionsthataren’ttestingyour
knowledgeofflowcontrol.
CERTIFICATIONOBJECTIVE
UsingifandswitchStatements(OCAObjectives3.3and
3.4)
3.3Createifandif-elseandternaryconstructs.
3.5Useaswitchstatement.
Theifandswitchstatementsarecommonlyreferredtoasdecisionstatements.Whenyouuse
decisionstatementsinyourprogram,you’reaskingtheprogramtoevaluateagivenexpressionto
determinewhichcourseofactiontotake.We’lllookattheifstatementfirst.
if-elseBranching
Thebasicformatofanifstatementisasfollows:
Theexpressioninparenthesesmustevaluateto(aboolean)trueorfalse.Typically,you’retesting
somethingtoseeifit’strueandthenrunningacodeblock(oneormorestatements)ifitistrueand
(optionally)anotherblockofcodeifitisn’t.Thefollowingcodedemonstratesalegalif-elsestatement:
Theelseblockisoptional,soyoucanalsousethefollowing:
Theprecedingcodewillassign2toyifthetestsucceeds(meaningxreallyisgreaterthan3),butthe
othertwolineswillexecuteregardless.Eventhecurlybracesareoptionalifyouhaveonlyonestatement
toexecutewithinthebodyoftheconditionalblock.Thefollowingcodeexampleislegal(althoughnot
recommendedforreadability):
Mostdevelopersconsideritgoodpracticetoencloseblockswithincurlybraces,evenifthere’sonly
onestatementintheblock.Becarefulwithcodelikethepreceding,becauseyoumightthinkitshouldread
as
“Ifxisgreaterthan3,thensetyto2,ztoz+8,andatoy+x.”
Butthelasttwolinesaregoingtoexecutenomatterwhat!Theyaren’tpartoftheconditionalflow.You
mightfinditevenmoremisleadingifthecodewereindentedasfollows:
Youmighthaveaneedtonestif-elsestatements(although,again,it’snotrecommendedforreadability,
sonestediftestsshouldbekepttoaminimum).Youcansetupanif-elsestatementtotestformultiple
conditions.Thefollowingexampleusestwoconditions,soifthefirsttestfails,wewanttoperforma
secondtestbeforedecidingwhattodo:
Thisbringsuptheotherif-elseconstruct,theif,elseif,else.Theprecedingcodecould(and
should)berewrittenlikethis:
Thereareacoupleofrulesforusingelseandelseif:
Youcanhavezerooroneelseforagivenif,anditmustcomeafteranyelseifs.
Youcanhavezerotomanyelseifsforagivenif,andtheymustcomebeforethe(optional)
else.
Onceanelseifsucceeds,noneoftheremainingelseifsnortheelsewillbetested.
Thefollowingexampleshowscodethatishorriblyformattedfortherealworld.Asyou’veprobably
guessed,it’sfairlylikelythatyou’llencounterformattinglikethisontheexam.Inanycase,thecode
demonstratestheuseofmultipleelseifs:
Itproducesthisoutput:
<4
(Noticethateventhoughthesecondelseifistrue,itisneverreached.)
Sometimesyoucanhaveaproblemfiguringoutwhichifyourelseshouldpairwith,asfollows:
Weintentionallyleftouttheindentinginthispieceofcodesoitdoesn’tgivecluesastowhichif
statementtheelsebelongsto.Didyoufigureitout?Javalawdecreesthatanelseclausebelongstothe
innermostifstatementtowhichitmightpossiblybelong(inotherwords,theclosestprecedingifthat
doesn’thaveanelse).Inthecaseoftheprecedingexample,theelsebelongstothesecondifstatement
inthelisting.Withproperindenting,itwouldlooklikethis:
Followingourcodingconventionsbyusingcurlybraces,itwouldbeeveneasier
toread:
Don’tgetyourhopesupabouttheexamquestionsbeingallniceandindentedproperly.Someexamtakers
evenhaveasloganforthewayquestionsarepresentedontheexam:Anythingthatcanbemademore
confusingwillbe.
Bepreparedforquestionsthatnotonlyfailtoindentnicelybutalsointentionallyindentinamisleading
way.Paycloseattentionformisdirectionlikethefollowing:
Ofcourse,theprecedingcodeisexactlythesameastheprevioustwoexamples,exceptforthewayit
looks.
LegalExpressionsforifStatements
Theexpressioninanifstatementmustbeabooleanexpression.Anyexpressionthatresolvestoa
booleanisfine,andsomeoftheexpressionscanbecomplex.AssumedoStuff()returnstrue,
whichprints
true
Youcanreadtheprecedingcodeas,“Ifboth(x>3)and(y<2)aretrue,oriftheresultofdoStuff()
istrue,thenprinttrue.”So,basically,ifjustdoStuff()aloneistrue,we’llstillgettrue.If
doStuff()isfalse,though,thenboth(x>3)and(y<2)willhavetobetrueinordertoprinttrue.
Theprecedingcodeisevenmorecomplexifyouleaveoffonesetofparenthesesasfollows:
Thisnowprints…nothing!Becausetheprecedingcode(withonelesssetofparentheses)evaluatesas
thoughyouweresaying,“If(x>3)istrue,andeither(y<2)ortheresultofdoStuff()istrue,then
printtrue.Soif(x>3)isnottrue,nopointinlookingattherestoftheexpression.”Becauseofthe
short-circuit&&,theexpressionisevaluatedasthoughtherewereparenthesesaround(y<2)|
doStuff().Inotherwords,itisevaluatedasasingleexpressionbeforethe&&andasingleexpression
afterthe&&.
Rememberthattheonlylegalexpressioninaniftestisaboolean.Insomelanguages,0==falseand
1==true.NotsoinJava!Thefollowingcodeshowsifstatementsthatmightlooktemptingbutare
illegal,followedbylegalsubstitutions:
Onecommonmistakeprogrammersmake(andthatcanbedifficulttospot)isassigninga
booleanvariablewhenyoumeanttotestabooleanvariable.Lookoutforcodelikethe
following:
Youmightthinkoneofthreethings:
1.Thecodecompilesandrunsfine,andtheiftestfailsbecausebooisfalse.
2.Thecodewon’tcompilebecauseyou’reusinganassignment(=)ratherthanan
equalitytest(==).
3.Thecodecompilesandrunsfine,andtheiftestsucceedsbecausebooisSETto
true(ratherthanTESTEDfortrue)intheifargument!
Well,number3iscorrect–pointless,butcorrect.Giventhattheresultofanyassignmentisthe
valueofthevariableaftertheassignment,theexpression(boo=true)hasaresultoftrue.
Hence,theiftestsucceeds.Buttheonlyvariablesthatcanbeassigned(ratherthantested
againstsomethingelse)areabooleanoraBoolean;allotherassignmentswillresultin
somethingnon-boolean,sothey’renotlegal,asinthefollowing:
Becauseiftestsrequirebooleanexpressions,youneedtobereallysolidonbothlogical
operatorsandiftestsyntaxandsemantics.
switchStatements
You’veseenhowifandelse-ifstatementscanbeusedtosupportbothsimpleandcomplexdecision
logic.Inmanycases,theswitchstatementprovidesacleanerwaytohandlecomplexdecisionlogic.
Let’scomparethefollowingif-elseifstatementtotheequivalentlyperformingswitchstatement:
Nowlet’sseethesamefunctionalityrepresentedinaswitchconstruct:
Note:Thereasonthisswitchstatementemulatestheifisbecauseofthebreakstatementsthatwere
placedinsideoftheswitch.Ingeneral,breakstatementsareoptional,andasyouwillseeinafew
pages,theirinclusionorexclusioncauseshugechangesinhowaswitchstatementwillexecute.
LegalExpressionsforswitchandcase
Thegeneralformoftheswitchstatementis
Aswitch’sexpressionmustevaluatetoachar,byte,short,int,anenum(asofJava5),anda
String(asofJava7).Thatmeansifyou’renotusinganenumoraString,onlyvariablesandvalues
thatcanbeautomaticallypromoted(inotherwords,implicitlycast)toanintareacceptable.Youwon’t
beabletocompileifyouuseanythingelse,includingtheremainingnumerictypesoflong,float,and
double.
Acaseconstantmustevaluatetothesametypethattheswitchexpressioncanuse,withoneadditional
—andbig—constraint:thecaseconstantmustbeacompile-timeconstant!Sincethecaseargumenthasto
beresolvedatcompiletime,youcanuseonlyaconstantorfinalvariablethatisimmediatelyinitialized
withaliteralvalue.Itisnotenoughtobefinal;itmustbeacompile-timeconstant.Here’sanexample:
Also,theswitchcanonlycheckforequality.Thismeanstheotherrelationaloperatorssuchasgreater
thanarerenderedunusableinacase.Thefollowingisanexampleofavalidexpressionusingamethod
invocationinaswitchstatement.Notethatforthiscodetobelegal,themethodbeinginvokedonthe
objectreferencemustreturnavaluecompatiblewithanint.
Oneotherruleyoumightnotexpectinvolvesthequestion,“WhathappensifIswitchonavariable
smallerthananint?”Lookatthefollowingswitch:
Thiscodewon’tcompile.Althoughtheswitchargumentislegal—abyteisimplicitlycasttoanint—
thesecondcaseargument(128)istoolargeforabyte,andthecompilerknowsit!Attemptingtocompile
theprecedingexamplegivesyouanerrorsomethinglikethis:
It’salsoillegaltohavemorethanonecaselabelusingthesamevalue.Forexample,thefollowing
blockofcodewon’tcompilebecauseitusestwocaseswiththesamevalueof80:
Itislegaltoleveragethepowerofboxinginaswitchexpression.Forinstance,thefollowingislegal:
Lookforanyviolationoftherulesforswitchandcasearguments.Forexample,youmight
findillegalexampleslikethefollowingsnippets:
Inthefirstexample,thecaseomitsthecolon.Thesecondexampleomitsthekeywordcase.
AnIntrotoString“equality”
Aswe’vebeendiscussing,theoperationofswitchstatementsdependsontheexpression“matching”or
being“equal”tooneofthecases.We’vetalkedabouthowweknowwhenprimitivesareequal,butwhat
doesitmeanforobjectstobeequal?Thisisanotheroneofthosesurprisinglytrickytopics,andforthose
ofyouwhointendtotaketheOCPexam,you’llspendalotoftimestudying“objectequality.”Foryou
OCAcandidates,allyouhavetoknowisthatforaswitchstatement,twoStringswillbeconsidered
“equal”iftheyhavethesamecase-sensitivesequenceofcharacters.Forexample,inthefollowingpartial
switchstatement,theexpressionwouldmatchthecase:
ButthefollowingwouldNOTmatch:
BreakandFall-ThroughinswitchBlocks
We’refinallyreadytodiscussthebreakstatementandoffermoredetailsaboutflowcontrolwithina
switchstatement.Themostimportantthingtorememberabouttheflowofexecutionthroughaswitch
statementisthis:
caseconstantsareevaluatedfromthetopdown,andthefirstcaseconstantthatmatchestheswitch’sexpressionistheexecution
entrypoint.
Inotherwords,onceacaseconstantismatched,theJavaVirtualMachine(JVM)willexecutethe
associatedcodeblockandALLsubsequentcodeblocks(barringabreakstatement),too!Thefollowing
exampleusesaStringinacasestatement:
Inthisexamplecase“green”:matched,sotheJVMexecutedthatcodeblockandallsubsequentcode
blockstoproducetheoutput:
greenbluedone
Again,whentheprogramencountersthekeywordbreakduringtheexecutionofaswitchstatement,
executionwillimmediatelymoveoutoftheswitchblocktothenextstatementaftertheswitch.Ifbreak
isomitted,theprogramjustkeepsexecutingtheremainingcaseblocksuntileitherabreakisfoundorthe
switchstatementends.Examinethefollowingcode:
Thecodewillprintthefollowing:
Thiscombinationoccursbecausethecodedidn’thitabreakstatement;executionjustkeptdropping
downthrougheachcaseuntiltheend.Thisdroppingdownisactuallycalled“fall-through,”becauseof
thewayexecutionfallsfromonecasetothenext.Remember,thematchingcaseissimplyyourentry
pointintotheswitchblock!Inotherwords,youmustnotthinkofitas,“Findthematchingcase,execute
justthatcode,andgetout.”That’snothowitworks.Ifyoudowantthat“justthematchingcode”
behavior,you’llinsertabreakintoeachcaseasfollows:
Runningtheprecedingcode,nowthatwe’veaddedthebreakstatements,prints:
Andthat’sit.Weenteredintotheswitchblockatcase1.Becauseitmatchedtheswitch()argument,
wegottheprintlnstatementandthenhitthebreakandjumpedtotheendoftheswitch.
Aninterestingexampleofthisfall-throughlogicisshowninthefollowingcode:
Thisswitchstatementwillprintxisanevennumberornothing,dependingonwhetherthenumberis
betweenoneandtenandisoddoreven.Forexample,ifxis4,executionwillbeginatcase4,butthen
falldownthrough6,8,and10,whereitprintsandthenbreaks.Thebreakatcase10,bytheway,isnot
needed;we’realreadyattheendoftheswitchanyway.
Note:Becausefall-throughislessthanintuitive,Oraclerecommendsthatyouaddacommentsuchas
//fallthroughwhenyouusefall-throughlogic.
TheDefaultCase
Whatif,usingtheprecedingcode,youwantedtoprintxisanoddnumberifnoneofthecases(the
evennumbers)matched?Youcouldn’tputitaftertheswitchstatement,orevenasthelastcaseinthe
switch,becauseinbothofthosesituationsitwouldalwaysprintxisanoddnumber.Togetthis
behavior,you’dusethedefaultkeyword.(Bytheway,ifyou’vewonderedwhythereisadefault
keywordeventhoughwedon’tuseamodifierfordefaultaccesscontrol,nowyou’llseethatthedefault
keywordisusedforacompletelydifferentpurpose.)Theonlychangeweneedtomakeistoaddthe
defaultcasetotheprecedingcode:
Thedefaultcasedoesn’thavetocomeattheendoftheswitch.Lookforitinstrangeplaces
suchasthefollowing:
Runningtheprecedingcodeprintsthis:
Andifwemodifyitsotheonlymatchisthedefaultcase,likethis,
thenrunningtheprecedingcodeprintsthis:
Theruletorememberisthatdefaultworksjustlikeanyothercaseforfall-through!
EXERCISE5-1
Creatingaswitch-caseStatement
Trycreatingaswitchstatementusingacharvalueasthecase.Includeadefaultbehaviorifnoneofthe
charvaluesmatch.
Makesureacharvariableisdeclaredbeforetheswitchstatement.
Eachcasestatementshouldbefollowedbyabreak.
Thedefaultcasecanbelocatedattheend,middle,ortop.
CERTIFICATIONOBJECTIVE
CreatingLoopsConstructs(OCAObjectives5.1,5.2,5.3,
5.4,and5.5)
5.1Createandusewhileloops.
5.2Createanduseforloopsincludingtheenhancedforloop.
5.3Createandusedo/whileloops.
5.4Compareloopconstructs.
5.5Usebreakandcontinue.
Javaloopscomeinthreeflavors:while,do,andfor(andasofJava5,theforloophastwo
variations).Allthreeletyourepeatablockofcodeaslongassomeconditionistrueorforaspecific
numberofiterations.You’reprobablyfamiliarwithloopsfromotherlanguages,soevenifyou’re
somewhatnewtoJava,thesewon’tbeaproblemtolearn.
UsingwhileLoops
Thewhileloopisgoodwhenyoudon’tknowhowmanytimesablockorstatementshouldrepeatbutyou
wanttocontinueloopingaslongassomeconditionistrue.Awhilestatementlookslikethis:
Orthis:
Inthiscase,asinallloops,theexpression(test)mustevaluatetoabooleanresult.Thebodyofthe
whileloopwillexecuteonlyiftheexpression(sometimescalledthe“condition”)resultsinavalueof
true.Onceinsidetheloop,theloopbodywillrepeatuntiltheconditionisnolongermetbecauseit
evaluatestofalse.Inthepreviousexample,programcontrolwillentertheloopbodybecausexisequal
to2.However,xisincrementedintheloop,sowhentheconditionischeckedagainitwillevaluateto
falseandexittheloop.
Anyvariablesusedintheexpressionofawhileloopmustbedeclaredbeforetheexpressionis
evaluated.Inotherwords,youcan’tsaythis:
while(intx=2){}//notlegal
Thenagain,whywouldyou?Insteadoftestingthevariable,you’dbedeclaringandinitializingit,soit
wouldalwayshavetheexactsamevalue.Notmuchofatestcondition!
Thekeypointtorememberaboutawhileloopisthatitmightnoteverrun.Ifthetestexpressionis
falsethefirsttimethewhileexpressionischecked,theloopbodywillbeskippedandtheprogramwill
beginexecutingatthefirststatementafterthewhileloop.Lookatthefollowingexample:
Runningthiscodeproduces
pasttheloop
Becausetheexpression(x>8)evaluatestofalse,noneofthecodewithinthewhileloopever
executes.
UsingdoLoops
Thedoloopissimilartothewhileloop,excepttheexpressionisnotevaluateduntilafterthedoloop’s
codeisexecuted.Therefore,thecodeinadoloopisguaranteedtoexecuteatleastonce.Thefollowing
showsadoloopinaction:
TheSystem.out.println()statementwillprintonce,eventhoughtheexpressionevaluatestofalse.
Remember,thedoloopwillalwaysrunthecodeintheloopbodyatleastonce.Besuretonotetheuseof
thesemicolonattheendofthewhileexpression.
Aswithiftests,lookforwhileloops(andthewhiletestinadoloop)withanexpressionthat
doesnotresolvetoaboolean.Takealookatthefollowingexamplesoflegalandillegalwhile
expressions:
UsingforLoops
AsofJava5,theforlooptookonasecondstructure.We’llcalltheoldstyleofforloopthe“basicfor
loop,”andwe’llcallthenewstyleofforloopthe“enhancedforloop”(it’salsosometimescalledthe
for-each).Dependingonwhatdocumentationyouuse,you’llseebothterms,alongwithfor-in.The
termsfor-in,for-each,and“enhancedfor”allrefertothesameJavaconstruct.
Thebasicforloopismoreflexiblethantheenhancedforloop,buttheenhancedforloopwas
designedtomakeiteratingthrougharraysandcollectionseasiertocode.
TheBasicforLoop
Theforloopisespeciallyusefulforflowcontrolwhenyoualreadyknowhowmanytimesyouneedto
executethestatementsintheloop’sblock.Theforloopdeclarationhasthreemainpartsbesidesthebody
oftheloop:
Declarationandinitializationofvariables
Thebooleanexpression(conditionaltest)
Theiterationexpression
Thethreefordeclarationpartsareseparatedbysemicolons.Thefollowingtwoexamplesdemonstrate
theforloop.Thefirstexampleshowsthepartsofaforloopinapseudocodeform,andthesecond
showsatypicalexampleofaforloop:
TheBasicforLoop:DeclarationandInitialization
Thefirstpartoftheforstatementletsyoudeclareandinitializezero,one,ormultiplevariablesofthe
sametypeinsidetheparenthesesaftertheforkeyword.Ifyoudeclaremorethanonevariableofthesame
type,you’llneedtoseparatethemwithcommasasfollows:
for(intx=10,y=3;y>3;y++){}
Thedeclarationandinitializationhappenbeforeanythingelseinaforloop.Andwhereastheothertwo
parts—thebooleantestandtheiterationexpression—willrunwitheachiterationoftheloop,the
declarationandinitializationhappenjustonce,attheverybeginning.Youalsomustknowthatthescope
ofvariablesdeclaredintheforloopendswiththeforloop!Thefollowingdemonstratesthis:
Ifyoutrytocompilethis,you’llgetsomethinglikethis:
BasicforLoop:Conditional(boolean)Expression
Thenextsectionthatexecutesistheconditionalexpression,which(likeallotherconditionaltests)must
evaluatetoabooleanvalue.Youcanhaveonlyonelogicalexpression,butitcanbeverycomplex.Look
outforcodethatuseslogicalexpressionslikethis:
for(intx=0;((((x<10)&&(y-->2))|x==3));x++){}
Theprecedingcodeislegal,butthefollowingisnot:
Thecompilerwillletyouknowtheproblem:
Theruletorememberisthis:Youcanhaveonlyonetestexpression.
Inotherwords,youcan’tusemultipletestsseparatedbycommas,eventhough,theothertwopartsofa
forstatementcanhavemultipleparts.
BasicforLoop:IterationExpression
Aftereachexecutionofthebodyoftheforloop,theiterationexpressionisexecuted.Thisiswhereyou
gettosaywhatyouwanttohappenwitheachiterationoftheloop.Rememberthatitalwayshappensafter
theloopbodyruns!Lookatthefollowing:
Thisloopexecutesjustonce.Thefirsttimeintotheloop,xissetto0,thenxistestedtoseeifit’sless
than1(whichitis),andthenthebodyoftheloopexecutes.Afterthebodyoftheloopruns,theiteration
expressionruns,incrementingxby1.Next,theconditionaltestischecked,andsincetheresultisnow
false,executionjumpstobelowtheforloopandcontinues.
Keepinmindthatbarringaforcedexit,evaluatingtheiterationexpressionandthenevaluating
theconditionalexpressionarealwaysthelasttwothingsthathappeninaforloop!
Examplesofforcedexitsincludeabreak,areturn,aSystem.exit(),andanexception,whichwill
allcausealooptoterminateabruptly,withoutrunningtheiterationexpression.Lookatthefollowing
code:
Runningthiscodeproduces
inforloop
Thestatementprintsonlyoncebecauseareturncausesexecutiontoleavenotjustthecurrentiteration
ofaloop,buttheentiremethod.Sotheiterationexpressionneverrunsinthatcase.Table5-1liststhe
causesandresultsofabruptlooptermination.
TABLE5-1 CausesofEarlyLoopTermination
BasicforLoop:forLoopIssues
Noneofthethreesectionsofthefordeclarationarerequired!Thefollowingexampleisperfectlylegal
(althoughnotnecessarilygoodpractice):
Inthisexample,allthedeclarationpartsareleftout,sotheforloopwillactlikeanendlessloop.
Fortheexam,it’simportanttoknowthatwiththeabsenceoftheinitializationandincrementsections,
theloopwillactlikeawhileloop.Thefollowingexampledemonstrateshowthisisaccomplished:
Thenextexampledemonstratesaforloopwithmultiplevariablesinplay.Acommaseparatesthe
variables,andtheymustbeofthesametype.Rememberthatthevariablesdeclaredintheforstatement
arealllocaltotheforloopandcan’tbeusedoutsidethescopeoftheloop.
Variablescopeplaysalargeroleintheexam.Youneedtoknowthatavariabledeclaredinthe
forloopcan’tbeusedbeyondtheforloop.Butavariableonlyinitializedintheforstatement
(butdeclaredearlier)canbeusedbeyondtheloop.Forexample,thefollowingislegal:
Butthisisnot:
for(intx=3;x<20;x++){}System.out.println(x);
Thelastthingtonoteisthatallthreesectionsoftheforloopareindependentofeachother.Thethree
expressionsintheforstatementdon’tneedtooperateonthesamevariables,althoughtheytypicallydo.
Buteventheiteratorexpression,whichmanymistakenlycallthe“incrementexpression,”doesn’tneedto
incrementorsetanything;youcanputinvirtuallyanyarbitrarycodestatementsthatyouwanttohappen
witheachiterationoftheloop.Lookatthefollowing:
Theprecedingcodeprints
iterate
iterate
ManyquestionsintheJava8examslist"Compilationfails"and"Anexceptionoccursat
runtime"aspossibleanswers,makingthemmoredifficultbecauseyoucan’tsimplywork
throughthebehaviorofthecode.Youmustfirstmakesurethecodeisn’tviolatingany
fundamentalrulesthatwillleadtoacompilererrorandthenlookforpossibleexceptions.
Onlyafteryou’vesatisfiedthosetwoshouldyoudigintothelogicandflowofthecodeinthe
question.
TheEnhancedforLoop(forArrays)
Theenhancedforloop,newasofJava5,isaspecializedforloopthatsimplifiesloopingthroughan
arrayoracollection.Inthischapterwe’regoingtofocusonusingtheenhancedfortoloopthrough
arrays.InChapter6we’llrevisittheenhancedfor,whenwediscusstheArrayListcollectionclass—
wheretheenhancedforreallycomesintoitsown.
Insteadofhavingthreecomponents,theenhancedforhastwo.Let’sloopthroughanarraythebasic
(old)wayandthenusingtheenhancedfor:
Thisproducesthefollowingoutput:
12341234
Moreformally,let’sdescribetheenhancedforasfollows:
for(declaration:expression)
Thetwopiecesoftheforstatementare
declarationThenewlydeclaredblockvariableofatypecompatiblewiththeelementsof
thearrayyouareaccessing.Thisvariablewillbeavailablewithintheforblock,anditsvalue
willbethesameasthecurrentarrayelement.
expressionThismustevaluatetothearrayyouwanttoloopthrough.Thiscouldbeanarray
variableoramethodcallthatreturnsanarray.Thearraycanbeanytype:primitives,objects,or
evenarraysofarrays.
Usingtheprecedingdefinitions,let’slookatsomelegalandillegalenhancedfordeclarations:
Theenhancedforloopassumesthat,barringanearlyexitfromtheloop,you’llalwaysloopthrough
everyelementofthearray.Thefollowingdiscussionsofbreakandcontinueapplytoboththebasicand
enhancedforloops.
Usingbreakandcontinue
Thebreakandcontinuekeywordsareusedtostopeithertheentireloop(break)orjustthecurrent
iteration(continue).Typically,ifyou’reusingbreakorcontinue,you’lldoaniftestwithintheloop,
andifsomeconditionbecomestrue(orfalsedependingontheprogram),youwanttogetout
immediately.Thedifferencebetweenthemiswhetherornotyoucontinuewithanewiterationorjumpto
thefirststatementbelowtheloopandcontinuefromthere.
Remember,continuestatementsmustbeinsidealoop;otherwise,you’llgetacompilererror.
breakstatementsmustbeusedinsideeitheralooporaswitchstatement.
Thebreakstatementcausestheprogramtostopexecutionoftheinnermostloopandstartprocessing
thenextlineofcodeaftertheblock.
Thecontinuestatementcausesonlythecurrentiterationoftheinnermostlooptoceaseandthenext
iterationofthesamelooptostartiftheconditionoftheloopismet.Whenusingacontinuestatement
withaforloop,youneedtoconsidertheeffectsthatcontinuehasontheloopiteration.Examinethe
followingcode:
Thequestionis,isthisanendlessloop?Theanswerisno.Whenthecontinuestatementishit,the
iterationexpressionstillruns!Itrunsjustasthoughthecurrentiterationended“inthenaturalway.”Soin
theprecedingexample,iwillstillincrementbeforethecondition(i<10)ischeckedagain.
Mostofthetime,acontinueisusedwithinaniftestasfollows:
UnlabeledStatements
Boththebreakstatementandthecontinuestatementcanbeunlabeledorlabeled.Althoughit’sfarmore
commontousebreakandcontinueunlabeled,theexamexpectsyoutoknowhowlabeledbreakand
continuestatementswork.Asstatedbefore,abreakstatement(unlabeled)willexitoutoftheinnermost
loopingconstructandproceedwiththenextlineofcodebeyondtheloopblock.Thefollowingexample
demonstratesabreakstatement:
Inthepreviousexample,thebreakstatementisunlabeled.Thefollowingisanexampleofan
unlabeledcontinuestatement:
Inthisexample,afileisbeingreadonefieldatatime.Whenanerrorisencountered,theprogram
movestothenextfieldinthefileandusesthecontinuestatementtogobackintotheloop(ifitisnotat
theendofthefile)andkeepsreadingthevariousfields.Ifthebreakcommandwereusedinstead,the
codewouldstopreadingthefileoncetheerroroccurredandmoveontothenextlineofcodeafterthe
loop.Thecontinuestatementgivesyouawaytosay,“Thisparticulariterationoftheloopneedstostop,
butnotthewholeloopitself.Ijustdon’twanttherestofthecodeinthisiterationtofinish,sodothe
iterationexpressionandthenstartoverwiththetest,anddon’tworryaboutwhatwasbelowthe
continuestatement.”
LabeledStatements
AlthoughmanystatementsinaJavaprogramcanbelabeled,it’smostcommontouselabelswithloop
statementslikefororwhile,inconjunctionwithbreakandcontinuestatements.Alabelstatementmust
beplacedjustbeforethestatementbeinglabeled,anditconsistsofavalididentifierthatendswitha
colon(:).
Youneedtounderstandthedifferencebetweenlabeledandunlabeledbreakandcontinue.The
labeledvarietiesareneededonlyinsituationswhereyouhaveanestedloop,andtheyneedtoindicate
whichofthenestedloopsyouwanttobreakfrom,orfromwhichofthenestedloopsyouwanttocontinue
withthenextiteration.Abreakstatementwillexitoutofthelabeledloop,asopposedtotheinnermost
loop,ifthebreakkeywordiscombinedwithalabel.
Here’sanexampleofwhatalabellookslike:
ThelabelmustadheretotherulesforavalidvariablenameandshouldadheretotheJavanaming
convention.Thesyntaxfortheuseofalabelnameinconjunctionwithabreakstatementisthebreak
keyword,thenthelabelname,followedbyasemicolon.Amorecompleteexampleoftheuseofalabeled
breakstatementisasfollows:
Runningthiscodeproduces
Hello
Good-Bye
Inthisexample,thewordHellowillbeprintedonetime.Then,thelabeledbreakstatementwillbe
executed,andtheflowwillexitoutofthelooplabeledouter.ThenextlineofcodewillthenprintGood-
Bye.
Let’sseewhatwillhappenifthecontinuestatementisusedinsteadofthebreakstatement.The
followingcodeexampleissimilartotheprecedingone,withtheexceptionofsubstitutingcontinuefor
break:
Runningthiscodeproduces
Hello
Hello
Hello
Hello
Hello
Good-Bye
Inthisexample,Hellowillbeprintedfivetimes.Afterthecontinuestatementisexecuted,theflow
continueswiththenextiterationoftheloopidentifiedwiththelabel.Finally,whentheconditioninthe
outerloopevaluatestofalse,thisloopwillfinishandGood-Byewillbeprinted.
EXERCISE5-2
CreatingaLabeledwhileLoop
Trycreatingalabeledwhileloop.Makethelabelouterandprovideaconditiontocheckwhethera
variableageislessthanorequalto21.Withintheloop,incrementageby1.Everytimetheprogramgoes
throughtheloop,checkwhetherageis16.Ifitis,printthemessage“getyourdriver’slicense”and
continuetotheouterloop.Ifnot,print“Anotheryear.”
Theouterlabelshouldappearjustbeforethewhileloopbegins.
Makesureageisdeclaredoutsideofthewhileloop.
Labeledcontinueandbreakstatementsmustbeinsidetheloopthathasthesamelabelname;
otherwise,thecodewillnotcompile.
CERTIFICATIONOBJECTIVE
HandlingExceptions(OCAObjectives8.1,8.2,8.3,8.4,and
8.5)
8.1Differentiateamongcheckedexceptions,uncheckedexceptions,anderrors.
8.2Createatry-catchblockanddeterminehowexceptionsalternormalprogramflow.
8.3DescribetheadvantagesofExceptionhandling.
8.4Createandinvokeamethodthatthrowsanexception.
8.5Recognizecommonexceptionclasses(suchasNullPointerException,ArithmeticException,
ArrayIndexOutOfBoundsException,ClassCastException)(sic)
Anoldmaximinsoftwaredevelopmentsaysthat80percentoftheworkisused20percentofthetime.
The80percentreferstotheeffortrequiredtocheckandhandleerrors.Inmanylanguages,writing
programcodethatchecksforanddealswitherrorsistediousandbloatstheapplicationsourceinto
confusingspaghetti.Still,errordetectionandhandlingmaybethemostimportantingredientofanyrobust
application.HerearesomeofthebenefitsofJava’sexception-handlingfeatures:
Itarmsdeveloperswithanelegantmechanismforhandlingerrorsthatproducesefficientand
organizederror-handlingcode.
Itallowsdeveloperstodetecterrorseasilywithoutwritingspecialcodetotestreturnvalues.
Itletsuskeepexception-handlingcodecleanlyseparatedfromexception-generatingcode.
Italsoletsususethesameexception-handlingcodetodealwitharangeofpossible
exceptions.
Java7addedseveralnewexception-handlingcapabilitiestothelanguage.Forourpurposes,Oracle
splitthevariousexception-handlingtopicsintotwomainparts:
1.TheOCAexamcoverstheJava6versionofexceptionhandling.
2.TheOCPexamaddsthenewexceptionfeaturesaddedinJava7.
InordertomirrorOracle’sOCA8objectivesversustheOCP8objectives,thischapterwillgiveyou
onlythebasicsofexceptionhandling—butplentytohandletheOCA8exam.
CatchinganExceptionUsingtryandcatch
Beforewebegin,let’sintroducesometerminology.Thetermexceptionmeans“exceptionalcondition”
andisanoccurrencethataltersthenormalprogramflow.Abunchofthingscanleadtoexceptions,
includinghardwarefailures,resourceexhaustion,andgoodoldbugs.Whenanexceptionaleventoccursin
Java,anexceptionissaidtobe“thrown.”Thecodethat’sresponsiblefordoingsomethingaboutthe
exceptioniscalledan“exceptionhandler,”andit“catches”thethrownexception.
Exceptionhandlingworksbytransferringtheexecutionofaprogramtoanappropriateexception
handlerwhenanexceptionoccurs.Forexample,ifyoucallamethodthatopensafilebutthefilecannot
beopened,executionofthatmethodwillstop,andcodethatyouwrotetodealwiththissituationwillbe
run.Therefore,weneedawaytotelltheJVMwhatcodetoexecutewhenacertainexceptionhappens.To
dothis,weusethetryandcatchkeywords.Thetryisusedtodefineablockofcodeinwhich
exceptionsmayoccur.Thisblockofcodeiscalleda“guardedregion”(whichreallymeans“riskycode
goeshere”).Oneormorecatchclausesmatchaspecificexception(orgroupofexceptions—moreon
thatlater)toablockofcodethathandlesit.Here’showitlooksinpseudocode:
Inthispseudocodeexample,lines2through5constitutetheguardedregionthatisgovernedbythetry
clause.Line7isanexceptionhandlerforanexceptionoftypeMyFirstException.Line12isan
exceptionhandlerforanexceptionoftypeMySecondException.Noticethatthecatchblocks
immediatelyfollowthetryblock.Thisisarequirement;ifyouhaveoneormorecatchblocks,theymust
immediatelyfollowthetryblock.Additionally,thecatchblocksmustallfolloweachother,withoutany
otherstatementsorblocksinbetween.Also,theorderinwhichthecatchblocksappearmatters,aswe’ll
seealittlelater.
Executionoftheguardedregionstartsatline2.Iftheprogramexecutesallthewaypastline5withno
exceptionsbeingthrown,executionwilltransfertoline15andcontinuedownward.However,ifatany
timeinlines2through5(thetryblock)anexceptionoftypeMyFirstExceptionisthrown,execution
willimmediatelytransfertoline7.Lines8through10willthenbeexecutedsothattheentirecatch
blockruns,andthenexecutionwilltransfertoline15andcontinue.
Notethatifanexceptionoccurredon,say,line3ofthetryblock,theremaininglinesinthetryblock
(4and5)wouldneverbeexecuted.Oncecontroljumpstothecatchblock,itneverreturnstocomplete
thebalanceofthetryblock.Thisisexactlywhatyouwant,though.Imaginethatyourcodelooks
somethinglikethispseudocode:
Thispseudocodedemonstrateshowyoutypicallyworkwithexceptions.Codethat’sdependentonarisky
operation(aspopulatingatablewithfiledataisdependentongettingthefilefromthenetwork)is
groupedintoatryblockinsuchawaythatif,say,thefirstoperationfails,youwon’tcontinuetryingto
runothercodethat’salsoguaranteedtofail.Inthepseudocodeexample,youwon’tbeabletoreadfrom
thefileifyoucan’tgetthefileoffthenetworkinthefirstplace.
Oneofthebenefitsofusingexceptionhandlingisthatcodetohandleanyparticularexception
thatmayoccurinthegovernedregionneedstobewrittenonlyonce.Returningtoourearliercode
example,theremaybethreedifferentplacesinourtryblockthatcangenerateaMyFirstException,but
whereveritoccursitwillbehandledbythesamecatchblock(online7).We’lldiscussmorebenefitsof
exceptionhandlingneartheendofthischapter.
Usingfinally
Althoughtryandcatchprovideaterrificmechanismfortrappingandhandlingexceptions,weareleft
withtheproblemofhowtocleanupafterourselvesifanexceptionoccurs.Becauseexecutiontransfers
outofthetryblockassoonasanexceptionisthrown,wecan’tputourcleanupcodeatthebottomofthe
tryblockandexpectittobeexecutedifanexceptionoccurs.Almostasbadanideawouldbeplacing
ourcleanupcodeineachofthecatchblocks—let’sseewhy.
Exceptionhandlersareapoorplacetocleanupafterthecodeinthetryblockbecauseeachhandler
thenrequiresitsowncopyofthecleanupcode.If,forexample,youallocatedanetworksocketoropened
afilesomewhereintheguardedregion,eachexceptionhandlerwouldhavetoclosethefileorreleasethe
socket.Thatwouldmakeittooeasytoforgettodocleanupandalsoleadtoalotofredundantcode.To
addressthisproblem,Javaoffersthefinallyblock.
Afinallyblockenclosescodethatisalwaysexecutedatsomepointafterthetryblock,whetheran
exceptionwasthrownornot.Evenifthereisareturnstatementinthetryblock,thefinallyblock
executesrightafterthereturnstatementisencounteredandbeforethereturnexecutes!
Thisistherightplacetocloseyourfiles,releaseyournetworksockets,andperformanyothercleanup
yourcoderequires.Ifthetryblockexecuteswithnoexceptions,thefinallyblockisexecuted
immediatelyafterthetryblockcompletes.Iftherewasanexceptionthrown,thefinallyblockexecutes
immediatelyafterthepropercatchblockcompletes.Let’slookatanotherpseudocodeexample:
Asbefore,executionstartsatthefirstlineofthetryblock,line2.Iftherearenoexceptionsthrownin
thetryblock,executiontransferstoline11,thefirstlineofthefinallyblock.Ontheotherhand,ifa
MySecondExceptionisthrownwhilethecodeinthetryblockisexecuting,executiontransferstothe
firstlineofthatexceptionhandler,line8inthecatchclause.Afterallthecodeinthecatchclauseis
executed,theprogrammovestoline11,thefirstlineofthefinallyclause.Repeatafterme:finally
alwaysruns!Okay,we’llhavetorefinethatalittle,butfornow,startburningintheideathatfinally
alwaysruns.Ifanexceptionisthrown,finallyruns.Ifanexceptionisnotthrown,finallyruns.Ifthe
exceptioniscaught,finallyruns.Iftheexceptionisnotcaught,finallyruns.Laterwe’lllookatthe
fewscenariosinwhichfinallymightnotrunorcomplete.
Remember,finallyclausesarenotrequired.Ifyoudon’twriteone,yourcodewillcompileandrun
justfine.Infact,ifyouhavenoresourcestocleanupafteryourtryblockcompletes,youprobablydon’t
needafinallyclause.Also,becausethecompilerdoesn’tevenrequirecatchclauses,sometimesyou’ll
runacrosscodethathasatryblockimmediatelyfollowedbyafinallyblock.Suchcodeisusefulwhen
theexceptionisgoingtobepassedbacktothecallingmethod,asexplainedinthenextsection.Usinga
finallyblockallowsthecleanupcodetoexecuteevenwhenthereisn’tacatchclause.
Thefollowinglegalcodedemonstratesatrywithafinallybutnocatch:
Thefollowinglegalcodedemonstratesatry,catch,andfinally:
ThefollowingILLEGALcodedemonstratesatrywithoutacatchorfinally:
ThefollowingILLEGALcodedemonstratesamisplacedcatchblock:
Itisillegaltouseatryclausewithouteitheracatchclauseorafinallyclause.Atryclause
byitselfwillresultinacompilererror.Anycatchclausesmustimmediatelyfollowthetry
block.Anyfinallyclausemustimmediatelyfollowthelastcatchclause(oritmust
immediatelyfollowthetryblockifthereisnocatch).Itislegaltoomiteitherthecatch
clauseorthefinallyclause,butnotboth.
PropagatingUncaughtExceptions
Whyaren’tcatchclausesrequired?Whathappenstoanexceptionthat’sthrowninatryblockwhen
thereisnocatchclausewaitingforit?Actually,there’snorequirementthatyoucodeacatchclausefor
everypossibleexceptionthatcouldbethrownfromthecorrespondingtryblock.Infact,it’sdoubtfulthat
youcouldaccomplishsuchafeat!Ifamethoddoesn’tprovideacatchclauseforaparticularexception,
thatmethodissaidtobe“ducking”theexception(or“passingthebuck”).
Sowhathappenstoaduckedexception?Beforewediscussthat,weneedtobrieflyreviewtheconcept
ofthecallstack.Mostlanguageshavetheconceptofamethodstackoracallstack.Simplyput,thecall
stackisthechainofmethodsthatyourprogramexecutestogettothecurrentmethod.Ifyourprogram
startsinmethodmain()andmain()callsmethoda(),whichcallsmethodb(),whichinturncalls
methodc(),thecallstackconsistsofthefollowing:
c
b
a
main
Wewillrepresentthestackasgrowingupward(althoughitcanalsobevisualizedasgrowing
downward).Asyoucansee,thelastmethodcalledisatthetopofthestack,whilethefirstcallingmethod
isatthebottom.Themethodattheverytopofthestacktracewouldbethemethodyouwerecurrently
executing.Ifwemovebackdownthecallstack,we’removingfromthecurrentmethodtothepreviously
calledmethod.Figure5-1illustratesawaytothinkabouthowthecallstackinJavaworks.
FIGURE5-1 TheJavamethodcallstack
Nowlet’sexaminewhathappenstoduckedexceptions.Imagineabuilding,say,fivestorieshigh,and
ateachfloorthereisadeckorbalcony.Nowimaginethatoneachdeck,onepersonisstandingholdinga
baseballmitt.Exceptionsarelikeballsdroppedfrompersontoperson,startingfromtheroof.An
exceptionisfirstthrownfromthetopofthestack(inotherwords,thepersonontheroof);andifitisn’t
caughtbythesamepersonwhothrewit(thepersonontheroof),itdropsdownthecallstacktothe
previousmethod,whichisthepersonstandingonthedeckonefloordown.Ifnotcaughttherebythe
persononefloordown,theexception/ballagaindropsdowntothepreviousmethod(persononthenext
floordown),andsoon,untilitiscaughtoruntilitreachestheverybottomofthecallstack.Thisiscalled
“exceptionpropagation.”
Ifanexceptionreachesthebottomofthecallstack,it’slikereachingthebottomofaverylongdrop;
theballexplodes,andsodoesyourprogram.Anexceptionthat’snevercaughtwillcauseyourapplication
tostoprunning.Adescription(ifoneisavailable)oftheexceptionwillbedisplayed,andthecallstack
willbe“dumped.”Thishelpsyoudebugyourapplicationbytellingyouwhatexceptionwasthrown,from
whatmethoditwasthrown,andwhatthestacklookedlikeatthetime.
Youcankeepthrowinganexceptiondownthroughthemethodsonthestack.Butwhat
happenswhenyougettothemain()methodatthebottom?Youcanthrowtheexceptionoutof
main()aswell.ThisresultsintheJVMhalting,andthestacktracewillbeprintedtothe
output.Thefollowingcodethrowsanexception:
Itprintsoutastacktracesomethinglikethis:
EXERCISE5-3
PropagatingandCatchinganException
Inthisexercise,you’regoingtocreatetwomethodsthatdealwithexceptions.Oneofthemethodsisthe
main()method,whichwillcallanothermethod.Ifanexceptionisthrownintheothermethod,main()
mustdealwithit.Afinallystatementwillbeincludedtoindicatethattheprogramhascompleted.The
methodthatmain()willcallwillbenamedreverse,anditwillreversetheorderofthecharactersina
String.IftheStringcontainsnocharacters,reversewillpropagateanexceptionuptothemain()
method.
1.CreateaclasscalledPropagateandamain()method,whichwillremainemptyfornow.
2.Createamethodcalledreverse.IttakesanargumentofaStringandreturnsaString.
3.Inreverse,checkwhethertheStringhasalengthof0byusingtheString.length()
method.Ifthelengthis0,thereversemethodwillthrowanexception.
4.NowincludethecodetoreversetheorderoftheString.Becausethisisn’tthemaintopicof
thischapter,thereversalcodehasbeenprovided,butfeelfreetotryitonyourown.
5.Nowinthemain()methodyouwillattempttocallthismethodanddealwithanypotential
exceptions.Additionally,youwillincludeafinallystatementthatdisplayswhenmain()has
finished.
DefiningExceptions
Wehavebeendiscussingexceptionsasaconcept.Weknowthattheyarethrownwhenaproblemofsome
typehappens,andweknowwhateffecttheyhaveontheflowofourprogram.Inthissection,wewill
developtheconceptsfurtheranduseexceptionsinfunctionalJavacode.
Earlierwesaidthatanexceptionisanoccurrencethataltersthenormalprogramflow.Butbecause
thisisJava,anythingthat’snotaprimitivemustbe…anobject.Exceptionsarenodifferent.Every
exceptionisaninstanceofaclassthathasclassExceptioninitsinheritancehierarchy.Inotherwords,
exceptionsarealwayssomesubclassofjava.lang.Exception.
Whenanexceptionisthrown,anobjectofaparticularExceptionsubtypeisinstantiatedandhanded
totheexceptionhandlerasanargumenttothecatchclause.Anactualcatchclauselookslikethis:
Inthisexample,eisaninstanceoftheArrayIndexOutOfBoundsExceptionclass.Aswithanyother
object,youcancallitsmethods.
ExceptionHierarchy
AllexceptionclassesaresubtypesofclassException.ThisclassderivesfromtheclassThrowable
(whichderivesfromtheclassObject).Figure5-2showsthehierarchyfortheexceptionclasses.
FIGURE5-2 Exceptionclasshierarchy
Asyoucansee,therearetwosubclassesthatderivefromThrowable:ExceptionandError.Classes
thatderivefromErrorrepresentunusualsituationsthatarenotcausedbyprogramerrorsandindicate
thingsthatwouldnotnormallyhappenduringprogramexecution,suchastheJVMrunningoutofmemory.
Generally,yourapplicationwon’tbeabletorecoverfromanError,soyou’renotrequiredtohandle
them.Ifyourcodedoesnothandlethem(anditusuallywon’t),itwillstillcompilewithnotrouble.
Althoughoftenthoughtofasexceptionalconditions,Errorsaretechnicallynotexceptionsbecausethey
donotderivefromclassException.
Ingeneral,anexceptionrepresentssomethingthathappensnotasaresultofaprogrammingerror,but
ratherbecausesomeresourceisnotavailableorsomeotherconditionrequiredforcorrectexecutionis
notpresent.Forexample,ifyourapplicationissupposedtocommunicatewithanotherapplicationor
computerthatisnotanswering,thisisanexceptionthatisnotcausedbyabug.Figure5-2alsoshowsa
subtypeofExceptioncalledRuntimeException.Theseexceptionsareaspecialcasebecausethey
sometimesdoindicateprogramerrors.Theycanalsorepresentrare,difficult-to-handleexceptional
conditions.Runtimeexceptionsarediscussedingreaterdetaillaterinthischapter.
Javaprovidesmanyexceptionclasses,mostofwhichhavequitedescriptivenames.Therearetwo
waystogetinformationaboutanexception.Thefirstisfromthetypeoftheexceptionitself.Thenextis
frominformationthatyoucangetfromtheexceptionobject.ClassThrowable(atthetopofthe
inheritancetreeforexceptions)providesitsdescendantswithsomemethodsthatareusefulinexception
handlers.OneoftheseisprintStackTrace().Asyouwouldexpect,ifyoucallanexceptionobject’s
printStackTrace()method,asintheearlierexample,astacktracefromwheretheexceptionoccurred
willbeprinted.
Wediscussedthatacallstackbuildsupwardwiththemostrecentlycalledmethodatthetop.Youwill
noticethattheprintStackTrace()methodprintsthemostrecentlyenteredmethodfirstandcontinues
down,printingthenameofeachmethodasitworksitswaydownthecallstack(thisiscalled“unwinding
thestack”)fromthetop.
Fortheexam,youdon’tneedtoknowanyofthemethodscontainedintheThrowableclasses,
includingExceptionandError.YouareexpectedtoknowthatException,Error,
RuntimeException,andThrowabletypescanallbethrownusingthethrowkeywordandcan
allbecaught(althoughyourarelywillcatchanythingotherthanExceptionsubtypes).
HandlinganEntireClassHierarchyofExceptions
We’vediscussedthatthecatchkeywordallowsyoutospecifyaparticulartypeofexceptiontocatch.
Youcanactuallycatchmorethanonetypeofexceptioninasinglecatchclause.Iftheexceptionclassthat
youspecifyinthecatchclausehasnosubclasses,thenonlythespecifiedclassofexceptionwillbe
caught.However,iftheclassspecifiedinthecatchclausedoeshavesubclasses,anyexceptionobject
thatsubclassesthespecifiedclasswillbecaughtaswell.
Forexample,classIndexOutOfBoundsExceptionhastwosubclasses,
ArrayIndexOutOfBoundsExceptionandStringIndexOutOfBoundsException.Youmaywanttowrite
oneexceptionhandlerthatdealswithexceptionsproducedbyeithertypeofboundaryerror,butyoumight
notbeconcernedwithwhichexceptionyouactuallyhave.Inthiscase,youcouldwriteacatchclause
likethefollowing:
IfanycodeinthetryblockthrowsArrayIndexOutOfBoundsExceptionor
StringIndexOutOfBoundsException,theexceptionwillbecaughtandhandled.Thiscanbe
convenient,butitshouldbeusedsparingly.Byspecifyinganexceptionclass’ssuperclassinyourcatch
clause,you’rediscardingvaluableinformationabouttheexception.Youcan,ofcourse,findoutexactly
whatexceptionclassyouhave,butifyou’regoingtodothat,you’rebetteroffwritingaseparatecatch
clauseforeachexceptiontypeofinterest.
Resistthetemptationtowriteasinglecatchallexceptionhandlersuchasthefollowing:
Thiscodewillcatcheveryexceptiongenerated.Ofcourse,nosingleexceptionhandlercanproperly
handleeveryexception,andprogramminginthiswaydefeatsthedesignobjective.Exception
handlersthattrapmanyerrorsatoncewillprobablyreducethereliabilityofyourprogram,because
it’slikelythatanexceptionwillbecaughtthatthehandlerdoesnotknowhowtohandle.
ExceptionMatching
Ifyouhaveanexceptionhierarchycomposedofasuperclassexceptionandanumberofsubtypes,and
you’reinterestedinhandlingoneofthesubtypesinaspecialwaybutwanttohandlealltheresttogether,
youneedwriteonlytwocatchclauses.
Whenanexceptionisthrown,Javawilltrytofind(bylookingattheavailablecatchclausesfromthe
topdown)acatchclausefortheexceptiontype.Ifitdoesn’tfindone,itwillsearchforahandlerfora
supertypeoftheexception.Ifitdoesnotfindacatchclausethatmatchesasupertypefortheexception,
thentheexceptionispropagateddownthecallstack.Thisprocessiscalled“exceptionmatching.”Let’s
lookatanexample.
Thisshortprogramattemptstoopenafileandtoreadsomedatafromit.Openingandreadingfilescan
generatemanyexceptions,mostofwhicharesometypeofIOException.Imaginethatinthisprogram
we’reinterestedinknowingonlywhethertheexactexceptionisaFileNotFoundException.Otherwise,
wedon’tcareexactlywhattheproblemis.
FileNotFoundExceptionisasubclassofIOException.Therefore,wecouldhandleitinthecatch
clausethatcatchesallsubtypesofIOException,butthenwewouldhavetotesttheexceptionto
determinewhetheritwasaFileNotFoundException.Instead,wecodedaspecialexceptionhandlerfor
theFileNotFoundExceptionandaseparateexceptionhandlerforallotherIOExceptionsubtypes.
IfthiscodegeneratesaFileNotFoundException,itwillbehandledbythecatchclausethatbegins
atline10.IfitgeneratesanotherIOException—perhapsEOFException,whichisasubclassof
IOException—itwillbehandledbythecatchclausethatbeginsatline15.Ifsomeotherexceptionis
generated,suchasaruntimeexceptionofsometype,neithercatchclausewillbeexecutedandthe
exceptionwillbepropagateddownthecallstack.
NoticethatthecatchclausefortheFileNotFoundExceptionwasplacedabovethehandlerforthe
IOException.Thisisreallyimportant!Ifwedoittheoppositeway,theprogramwillnotcompile.The
handlersforthemostspecificexceptionsmustalwaysbeplacedabovethoseformoregeneralexceptions.
Thefollowingwillnotcompile:
You’llgetacompilererrorsomethinglikethis:
Ifyouthinkbacktothepeoplewithbaseballmitts(inthesection“PropagatingUncaughtExceptions”),
imaginethatthemostgeneralmittsarethelargestandcanthuscatchmanykindsofballs.An
IOExceptionmittislargeenoughandflexibleenoughtocatchanytypeofIOException.Soiftheperson
onthefifthfloor(say,Fred)hasabigol’IOExceptionmitt,hecan’thelpbutcatcha
FileNotFoundExceptionballwithit.Andiftheguy(say,Jimmy)onthesecondfloorisholdinga
FileNotFoundExceptionmitt,thatFileNotFoundExceptionballwillnevergettohimbecauseitwill
alwaysbestoppedbyFredonthefifthfloor,standingtherewithhisbig-enough-for-any-IOException
mitt.
Sowhatdoyoudowithexceptionsthataresiblingsintheclasshierarchy?IfoneExceptionclassis
notasubtypeorsupertypeoftheother,thentheorderinwhichthecatchclausesareplaceddoesn’t
matter.
ExceptionDeclarationandthePublicInterface
So,howdoweknowthatsomemethodthrowsanexceptionthatwehavetocatch?Justasamethodmust
specifywhattypeandhowmanyargumentsitacceptsandwhatisreturned,theexceptionsthatamethod
canthrowmustbedeclared(unlesstheexceptionsaresubclassesofRuntimeException).Thelistof
thrownexceptionsispartofamethod’spublicinterface.Thethrowskeywordisusedasfollowstolist
theexceptionsthatamethodcanthrow:
Thismethodhasavoidreturntype,acceptsnoarguments,anddeclaresthatitcanthrowoneoftwotypes
ofexceptions:eithertypeMyException1ortypeMyException2.(Justbecausethemethoddeclaresthatit
throwsanexceptiondoesn’tmeanitalwayswill.Itjusttellstheworldthatitmight.)
Supposeyourmethoddoesn’tdirectlythrowanexceptionbutcallsamethodthatdoes.Youcanchoose
nottohandletheexceptionyourselfandinsteadjustdeclareit,asthoughitwereyourmethodthatactually
throwstheexception.Ifyoudodeclaretheexceptionthatyourmethodmightgetfromanothermethodand
youdon’tprovideatry/catchforit,thenthemethodwillpropagatebacktothemethodthatcalledyour
methodandwilleitherbecaughtthereorcontinueontobehandledbyamethodfurtherdownthestack.
Anymethodthatmightthrowanexception(unlessit’sasubclassofRuntimeException)mustdeclare
theexception.Thatincludesmethodsthataren’tactuallythrowingitdirectly,butare“ducking”andletting
theexceptionpassdowntothenextmethodinthestack.Ifyou“duck”anexception,itisjustasifyou
weretheoneactuallythrowingtheexception.RuntimeExceptionsubclassesareexempt,sothecompiler
won’tchecktoseeifyou’vedeclaredthem.Butallnon-RuntimeExceptionsareconsidered“checked”
exceptionsbecausethecompilercheckstobecertainyou’veacknowledgedthat“badthingscouldhappen
here.”
Rememberthis:
Eachmethodmusteitherhandleallcheckedexceptionsbysupplyingacatchclauseorlisteachunhandledcheckedexceptionasa
thrownexception.
ThisruleisreferredtoasJava’s“handleordeclare”requirement(sometimescalled“catchordeclare”).
Lookforcodethatinvokesamethoddeclaringanexception,wherethecallingmethoddoesn’t
handleordeclarethecheckedexception.Thefollowingcode(whichusesthethrowkeywordto
throwanexceptionmanually–moreonthisnext)hastwobigproblemsthatthecompilerwill
prevent:
First,thedoMore()methodthrowsacheckedexceptionbutdoesnotdeclareit!Butsuppose
wefixthedoMore()methodasfollows:
voiddoMore()throwsIOException{…}
ThedoStuff()methodisstillintroublebecauseit,too,mustdeclaretheIOException,
unlessithandlesitbyprovidingatry/catch,withacatchclausethatcantakean
IOException.
Again,someexceptionsareexemptfromthisrule.AnobjectoftypeRuntimeExceptionmaybe
thrownfromanymethodwithoutbeingspecifiedaspartofthemethod’spublicinterface(andahandler
neednotbepresent).AndevenifamethoddoesdeclareaRuntimeException,thecallingmethodis
undernoobligationtohandleordeclareit.RuntimeException,Error,andalltheirsubtypesare
uncheckedexceptions,anduncheckedexceptionsdonothavetobespecifiedorhandled.Hereisan
example:
Let’slookatmyMethod1().BecauseEOFExceptionsubclassesIOExceptionandIOException
subclassesException,itisacheckedexceptionandmustbedeclaredasanexceptionthatmaybethrown
bythismethod.Butwherewilltheexceptionactuallycomefrom?Thepublicinterfaceformethod
myMethod2()calledheredeclaresthatanexceptionofthistypecanbethrown.Whetherthatmethod
actuallythrowstheexceptionitselforcallsanothermethodthatthrowsitisunimportanttous;wesimply
knowthatweeitherhavetocatchtheexceptionordeclarethatwethrewit.ThemethodmyMethod1()
doesnotcatchtheexception,soitdeclaresthatitthrowsit.Nowlet’slookatanotherlegalexample,
myMethod3():
Accordingtothecomment,thismethodcanthrowaNullPointerException.Because
RuntimeExceptionisthesuperclassofNullPointerException,itisanuncheckedexceptionandneed
notbedeclared.WecanseethatmyMethod3()doesnotdeclareanyexceptions.
Runtimeexceptionsarereferredtoasuncheckedexceptions.Allotherexceptionsarechecked
exceptions,andtheydon’tderivefromjava.lang.RuntimeException.Acheckedexceptionmustbe
caughtsomewhereinyourcode.Ifyouinvokeamethodthatthrowsacheckedexceptionbutyoudon’t
catchthecheckedexceptionsomewhere,yourcodewillnotcompile.That’swhythey’recalledchecked
exceptions:thecompilercheckstomakesurethey’rehandledordeclared.Anumberofthemethodsinthe
JavaAPIthrowcheckedexceptions,soyouwilloftenwriteexceptionhandlerstocopewithexceptions
generatedbymethodsyoudidn’twrite.
Youcanalsothrowanexceptionyourself,andthatexceptioncanbeeitheranexistingexceptionfrom
theJavaAPIoroneofyourown.Tocreateyourownexception,yousimplysubclassException(orone
ofitssubclasses)asfollows:
classMyExceptionextendsException{}
Andifyouthrowtheexception,thecompilerwillguaranteethatyoudeclareitasfollows:
Theprecedingcodeupsetsthecompiler:
WhenanobjectofasubtypeofExceptionisthrown,itmustbehandledordeclared.These
objectsarecalled"checkedexceptions"andincludeallexceptionsexceptthosethatare
subtypesofRuntimeException,whichareuncheckedexceptions.Bereadytospotmethods
thatdon’tfollowthe"handleordeclare"rule,suchasthis:
Youneedtorecognizethatthiscodewon’tcompile.Ifyoutry,you’llgetthis:
NoticethatsomeMethod()failseithertohandleordeclaretheexceptionthatcanbethrownby
doStuff().Inthenextpages,we’lldiscussseveralwaystodealwiththissortofsituation.
YouneedtoknowhowanErrorcompareswithcheckedanduncheckedexceptions.Objectsoftype
ErrorarenotExceptionobjects,althoughtheydorepresentexceptionalconditions.BothException
andErrorshareacommonsuperclass,Throwable;thus,bothcanbethrownusingthethrowkeyword.
WhenanErrororasubclassofError(likeStackOverflowError)isthrown,it’sunchecked.Youare
notrequiredtocatchErrorobjectsorErrorsubtypes.YoucanalsothrowanErroryourself(although,
otherthanAssertionError,youprobablywon’teverwantto),andyoucancatchone,butagain,you
probablywon’t.What,forexample,wouldyouactuallydoifyougotanOutOfMemoryError?It’snotlike
youcantellthegarbagecollectortorun;youcanbettheJVMfoughtdesperatelytosaveitself(and
reclaimedallthememoryitcould)bythetimeyougottheerror.Inotherwords,don’texpecttheJVMat
thatpointtosay,“Runthegarbagecollector?Oh,thankssomuchfortellingme.Thatjustneveroccurred
tome.Sure,I’llgetrightonit.”Evenbetter,whatwouldyoudoifaVirtualMachineErrorarose?Your
programistoastbythetimeyou’dcatchtheerror,sothere’sreallynopointintryingtocatchoneofthese
babies.Justremember,though,thatyoucan!Thefollowingcompilesjustfine:
IfwewerethrowingacheckedexceptionratherthanError,thenthedoStuff()methodwouldneedto
declaretheexception.Butremember,sinceErrorisnotasubtypeofException,itdoesn’tneedtobe
declared.You’refreetodeclareitifyoulike,butthecompilerjustdoesn’tcareonewayoranotherwhen
orhowtheErroristhrownorbywhom.
BecauseJavahascheckedexceptions,it’scommonlysaidthatJavaforcesdeveloperstohandle
exceptions.Yes,Javaforcesustowriteexceptionhandlersforeachexceptionthatcanoccurduring
normaloperation,butit’suptoustomaketheexceptionhandlersactuallydosomethinguseful.We
knowsoftwaremanagerswhomeltdownwhentheyseeaprogrammerwritesomethinglikethis:
Noticeanythingmissing?Don’t“eat”theexceptionbycatchingitwithoutactuallyhandlingit.You
won’tevenbeabletotellthattheexceptionoccurredbecauseyou’llneverseethestacktrace.
RethrowingtheSameException
Justasyoucanthrowanewexceptionfromacatchclause,youcanalsothrowthesameexceptionyou
justcaught.Here’sacatchclausethatdoesthis:
Allothercatchclausesassociatedwiththesametryareignored;ifafinallyblockexists,itruns,
andtheexceptionisthrownbacktothecallingmethod(thenextmethoddownthecallstack).Ifyouthrow
acheckedexceptionfromacatchclause,youmustalsodeclarethatexception!Inotherwords,youmust
handleanddeclare,asopposedtohandleordeclare.Thefollowingexampleisillegal:
Intheprecedingcode,thedoStuff()methodisclearlyabletothrowacheckedexception—inthiscase
anIOException—sothecompilersays,“Well,that’sjustpeachythatyouhaveatry/catchinthere,but
it’snotgoodenough.IfyoumightrethrowtheIOExceptionyoucatch,thenyoumustdeclareit(inthe
methodsignature)!”
EXERCISE5-4
CreatinganException
Inthisexercise,weattempttocreateacustomexception.Wewon’tputinanynewmethods(itwillhave
onlythoseinheritedfromException);andbecauseitextendsException,thecompilerconsidersita
checkedexception.Thegoaloftheprogramistodeterminewhetheracommand-lineargument
representingaparticularfood(asastring)isconsideredbadorokay.
1.Let’sfirstcreateourexception.WewillcallitBadFoodException.Thisexceptionwillbe
thrownwhenabadfoodisencountered.
2.CreateanenclosingclasscalledMyExceptionandamain()method,whichwillremain
emptyfornow.
3.CreateamethodcalledcheckFood().IttakesaStringargumentandthrowsourexceptionif
itdoesn’tlikethefooditwasgiven.Otherwise,ittellsusitlikesthefood.Youcanaddanyfoods
youaren’tparticularlyfondoftothelist.
4.Nowinthemain()method,you’llgetthecommand-lineargumentoutoftheStringarrayand
thenpassthatStringontothecheckFood()method.Becauseit’sacheckedexception,the
checkFood()methodmustdeclareit,andthemain()methodmusthandleit(usingatry/catch).
Donothavemain()declaretheexception,becauseifmain()duckstheexception,whoelseisback
theretocatchit?(Actually,main()canlegallydeclareexceptions,butdon’tdothatinthis
exercise.)
Asniftyasexceptionhandlingis,it’sstilluptothedevelopertomakeproperuseofit.Exception
handlingmakesorganizingcodeandsignalingproblemseasy,buttheexceptionhandlersstillhavetobe
written.You’llfindthateventhemostcomplexsituationscanbehandled,andyourcodewillbereusable,
readable,andmaintainable.
CERTIFICATIONOBJECTIVE
CommonExceptionsandErrors(OCAObjective8.5)
8.5Recognizecommonexceptionclasses(suchasNullPointerException,ArithmeticException,
ArrayIndexOutOfBoundsException,ClassCastException)(sic)
Theintentionofthisobjectiveistomakesurethatyouarefamiliarwithsomeofthemostcommon
exceptionsanderrorsyou’llencounterasaJavaprogrammer.
Thequestionsfromthissectionarelikelytobealongthelinesof,"Here’ssomecodethatjust
didsomethingbad,whichexceptionwillbethrown?"Throughouttheexam,questionswill
presentsomecodeandaskyoutodeterminewhetherthecodewillrunorwhetheranexception
willbethrown.Sincethesequestionsaresocommon,understandingthecausesforthese
exceptionsiscriticaltoyoursuccess.
Thisisanotheroneofthoseobjectivesthatwillturnupallthroughtherealexam(does“Anexception
isthrownatruntime”ringabell?),somakesurethissectiongetsalotofyourattention.
WhereExceptionsComeFrom
Jumpbackapageandtakealookatthelastsentence.It’simportantthatyouunderstandwhatcauses
exceptionsanderrorsandwheretheycomefrom.Forthepurposesofexampreparation,let’sdefinetwo
broadcategoriesofexceptionsanderrors:
JVMexceptionsThoseexceptionsorerrorsthatareeitherexclusivelyormostlogically
thrownbytheJVM
ProgrammaticexceptionsThoseexceptionsthatarethrownexplicitlybyapplicationand/or
APIprogrammers
JVM-ThrownExceptions
Let’sstartwithaverycommonexception,theNullPointerException.Aswesawinearlierchapters,
thisexceptionoccurswhenyouattempttoaccessanobjectusingareferencevariablewithacurrentvalue
ofnull.There’snowaythatthecompilercanhopetofindtheseproblemsbeforeruntime.Takealookat
thefollowing:
Surely,thecompilercanfindtheproblemwiththattinylittleprogram!Nope,you’reonyourown.The
codewillcompilejustfine,andtheJVMwillthrowaNullPointerExceptionwhenittriestoinvoke
thelength()method.
Earlierinthischapterwediscussedthecallstack.Asyourecall,weusedtheconventionthatmain()
wouldbeatthebottomofthecallstack,andthatasmain()invokesanothermethod,andthatmethod
invokesanother,andsoon,thestackgrowsupward.Ofcourse,thestackresidesinmemory,andevenif
yourOSgivesyouagigabyteofRAMforyourprogram,it’sstillafiniteamount.It’spossibletogrowthe
stacksolargethattheOSrunsoutofspacetostorethecallstack.Whenthishappens,youget(waitfor
it…)aStackOverflowError.Themostcommonwayforthistooccuristocreatearecursivemethod.A
recursivemethodinvokesitselfinthemethodbody.Althoughthatmaysoundweird,it’saverycommon
andusefultechniqueforsuchthingsassearchingandsortingalgorithms.Takealookatthiscode:
Asyoucansee,ifyouevermakethemistakeofinvokingthego()method,yourprogramwillfallintoa
blackhole—go()invokinggo()invokinggo(),until,nomatterhowmuchmemoryyouhave,you’llgeta
StackOverflowError.Again,onlytheJVMknowswhenthismomentoccurs,andtheJVMwillbethe
sourceofthiserror.
ProgrammaticallyThrownExceptions
Nowlet’slookatprogrammaticallythrownexceptions.Rememberwedefinedprogrammaticallyas
meaningsomethinglikethis:
Createdbyanapplicationand/orAPIdeveloper
Forinstance,manyclassesintheJavaAPIhavemethodsthattakeStringargumentsandconvertthese
Stringsintonumericprimitives.Agoodexampleoftheseclassesistheso-called“wrapperclasses”that
wewillstudyinChapter6.Eventhoughwehaven’ttalkedmuchaboutwrapperclassesyet,thefollowing
exampleshouldmakesense.
Atsomepointlongago,someprogrammerwrotethejava.lang.Integerclassandcreatedmethods
likeparseInt()andvalueOf().Thatprogrammerwiselydecidedthatifoneofthesemethodswas
passedaStringthatcouldnotbeconvertedintoanumber,themethodshouldthrowa
NumberFormatException.Thepartiallyimplementedcodemightlooksomethinglikethis:
OtherexamplesofprogrammaticexceptionsincludeanAssertionError(okay,it’snotanexception,
butitISthrownprogrammatically)andthrowinganIllegalArgumentException.Infact,ourmythical
APIdevelopercouldhaveusedIllegalArgumentExceptionforherparseInt()method.Butitturns
outthatNumberFormatExceptionextendsIllegalArgumentExceptionandisalittlemoreprecise,so
inthiscase,usingNumberFormatExceptionsupportsthenotionwediscussedearlier:thatwhenyou
haveanexceptionhierarchy,youshouldusethemostpreciseexceptionthatyoucan.
Ofcourse,aswediscussedearlier,youcanalsomakeupyourveryownspecialcustomexceptions
andthrowthemwheneveryouwantto.Thesehomemadeexceptionsalsofallintothecategoryof
“programmaticallythrownexceptions.”
ASummaryoftheExam’sExceptionsandErrors
OCA8Objective8.5listsafewspecificexceptionsanderrors;itsays“Recognizecommonexception
classes(suchas….”).Table5-2summarizesthetenexceptionsanderrorsthataremostlikelyapartofthe
OCA8exam.
TABLE5-2 DescriptionsandSourcesofCommonExceptions
CERTIFICATIONSUMMARY
Thischaptercoveredalotofground,allofwhichinvolvedwaysofcontrollingyourprogramflowbased
onaconditionaltest.First,youlearnedaboutifandswitchstatements.Theifstatementevaluatesone
ormoreexpressionstoabooleanresult.Iftheresultistrue,theprogramwillexecutethecodeinthe
blockthatisencompassedbytheif.Ifanelsestatementisusedandtheifexpressionevaluatesto
false,thenthecodefollowingtheelsewillbeperformed.Ifnoelseblockisdefined,thennoneofthe
codeassociatedwiththeifstatementwillexecute.
Youalsolearnedthattheswitchstatementcanbeusedtoreplacemultiple
if-elsestatements.Theswitchstatementcanevaluateintegerprimitivetypesthatcanbeimplicitlycast
toanint(thosetypesarebyte,short,int,andchar);oritcanevaluateenums;andasofJava7,itcan
evaluateStrings.Atruntime,theJVMwilltrytofindamatchbetweentheexpressionintheswitch
statementandaconstantinacorrespondingcasestatement.Ifamatchisfound,executionwillbeginat
thematchingcaseandcontinueonfromthere,executingcodeinalltheremainingcasestatementsuntila
breakstatementisfoundortheendoftheswitchstatementoccurs.Ifthereisnomatch,thenthedefault
casewillexecute,ifthereisone.
You’velearnedaboutthethreeloopingconstructsavailableintheJavalanguage.Theseconstructsare
theforloop(includingthebasicforandtheenhancedfor,whichwasnewtoJava5),thewhileloop,
andthedoloop.Ingeneral,theforloopisusedwhenyouknowhowmanytimesyouneedtogothrough
theloop.Thewhileloopisusedwhenyoudonotknowhowmanytimesyouwanttogothrough,whereas
thedoloopisusedwhenyouneedtogothroughatleastonce.Intheforloopandthewhileloop,the
expressionhastoevaluatetotruetogetinsidetheblockandwillcheckaftereveryiterationoftheloop.
Thedoloopdoesnotchecktheconditionuntilafterithasgonethroughthelooponce.Themajorbenefit
oftheforloopistheabilitytoinitializeoneormorevariablesandincrementordecrementthose
variablesintheforloopdefinition.
Thebreakandcontinuestatementscanbeusedineitheralabeledorunlabeledfashion.When
unlabeled,thebreakstatementwillforcetheprogramtostopprocessingtheinnermostloopingconstruct
andstartwiththelineofcodefollowingtheloop.Usinganunlabeledcontinuecommandwillcausethe
programtostopexecutionofthecurrentiterationoftheinnermostloopandproceedwiththenext
iteration.Whenabreakoracontinuestatementisusedinalabeledmanner,itwillperforminthesame
way,withoneexception:thestatementwillnotapplytotheinnermostloop;instead,itwillapplytothe
loopwiththelabel.Thebreakstatementisusedmostofteninconjunctionwiththeswitchstatement.
Whenthereisamatchbetweentheswitchexpressionandthecaseconstant,thecodefollowingthecase
constantwillbeperformed.Tostopexecution,abreakisneeded.
You’veseenhowJavaprovidesanelegantmechanisminexceptionhandling.Exceptionhandling
allowsyoutoisolateyourerror-correctioncodeintoseparateblockssothemaincodedoesn’tbecome
clutteredbyerror-checkingcode.Anotherelegantfeatureallowsyoutohandlesimilarerrorswitha
singleerror-handlingblock,withoutcodeduplication.Also,theerrorhandlingcanbedeferredto
methodsfurtherbackonthecallstack.
YoulearnedthatJava’strykeywordisusedtospecifyaguardedregion—ablockofcodeinwhich
problemsmightbedetected.Anexceptionhandleristhecodethatisexecutedwhenanexceptionoccurs.
ThehandlerisdefinedbyusingJava’scatchkeyword.Allcatchclausesmustimmediatelyfollowthe
relatedtryblock.
Javaalsoprovidesthefinallykeyword.Thisisusedtodefineablockofcodethatisalways
executed,eitherimmediatelyafteracatchclausecompletesorimmediatelyaftertheassociatedtry
blockinthecasethatnoexceptionwasthrown(ortherewasatrybutnocatch).Usefinallyblocksto
releasesystemresourcesandtoperformanycleanuprequiredbythecodeinthetryblock.Afinally
blockisnotrequired,butifthereisone,itmustimmediatelyfollowthelastcatch.(Ifthereisnocatch
block,thefinallyblockmustimmediatelyfollowthetryblock.)It’sguaranteedtobecalledexcept
whenthetryorcatchissuesaSystem.exit().
AnexceptionobjectisaninstanceofclassExceptionoroneofitssubclasses.Thecatchclause
takes,asaparameter,aninstanceofanobjectofatypederivedfromtheExceptionclass.Javarequires
thateachmethodeithercatchesanycheckedexceptionitcanthroworelsedeclaresthatitthrowsthe
exception.Theexceptiondeclarationispartofthemethod’ssignature.Todeclarethatanexceptionmay
bethrown,thethrowskeywordisusedinamethoddefinition,alongwithalistofallcheckedexceptions
thatmightbethrown.
RuntimeexceptionsareoftypeRuntimeException(oroneofitssubclasses).Theseexceptionsarea
specialcasebecausetheydonotneedtobehandledordeclared,andthusareknownas“unchecked”
exceptions.Errorsareoftypejava.lang.Errororitssubclasses,andlikeruntimeexceptions,theydo
notneedtobehandledordeclared.Checkedexceptionsincludeanyexceptiontypesthatarenotoftype
RuntimeExceptionorError.Ifyourcodefailseithertohandleacheckedexceptionordeclarethatitis
thrown,yourcodewon’tcompile.ButwithuncheckedexceptionsorobjectsoftypeError,itdoesn’t
mattertothecompilerwhetheryoudeclarethemorhandlethem,donothingaboutthem,ordosome
combinationofdeclaringandhandling.Inotherwords,you’refreetodeclarethemandhandlethem,but
thecompilerwon’tcareonewayortheother.It’snotgoodpracticetohandleanError,though,because
youcanrarelyrecoverfromone.
Finally,rememberthatexceptionscanbegeneratedbytheJVMorbyaprogrammer.
TWO-MINUTEDRILL
Herearesomeofthekeypointsfromeachcertificationobjectiveinthischapter.Youmightwanttoloop
throughthemseveraltimes.
WritingCodeUsingifandswitchStatements(OCAObjectives3.3and3.4)
Theonlylegalexpressioninanifstatementisabooleanexpression—inotherwords,an
expressionthatresolvestoabooleanoraBooleanreference.
Watchoutforbooleanassignments(=)thatcanbemistakenforbooleanequality(==)tests:
booleanx=false;
if(x=true){}//anassignment,soxwillalwaysbetrue!
Curlybracesareoptionalforifblocksthathaveonlyoneconditionalstatement.Butwatch
outformisleadingindentations.
switchstatementscanevaluateonlytoenumsorthebyte,short,int,char,and,asofJava
7,Stringdatatypes.Youcan’tsaythis:
longs=30;
switch(s){}
Thecaseconstantmustbealiteraloracompile-timeconstant,includinganenumora
String.Youcannothaveacasethatincludesanonfinalvariableorarangeofvalues.
Iftheconditioninaswitchstatementmatchesacaseconstant,executionwillrunthroughall
codeintheswitchfollowingthematchingcasestatementuntilabreakstatementortheendofthe
switchstatementisencountered.Inotherwords,thematchingcaseisjusttheentrypointintothe
caseblock,butunlessthere’sabreakstatement,thematchingcaseisnottheonlycasecodethat
runs.
Thedefaultkeywordshouldbeusedinaswitchstatementifyouwanttorunsomecode
whennoneofthecasevaluesmatchtheconditionalvalue.
Thedefaultblockcanbelocatedanywhereintheswitchblock,soifnoprecedingcase
matches,thedefaultblockwillbeentered;ifthedefaultdoesnotcontainabreak,thencode
willcontinuetoexecute(fall-through)totheendoftheswitchoruntilthebreakstatementis
encountered.
WritingCodeUsingLoops(OCAObjectives5.1,5.2,5.3,and5.4)
Abasicforstatementhasthreeparts:declarationand/orinitialization,booleanevaluation,
andtheiterationexpression.
Ifavariableisincrementedorevaluatedwithinabasicforloop,itmustbedeclaredbefore
thelooporwithintheforloopdeclaration.
Avariabledeclared(notjustinitialized)withinthebasicforloopdeclarationcannotbe
accessedoutsidetheforloop—inotherwords,codebelowtheforloopwon’tbeabletousethe
variable.
Youcaninitializemorethanonevariableofthesametypeinthefirstpartofthebasicfor
loopdeclaration;eachinitializationmustbecommaseparated.
Anenhancedforstatement(newasofJava5)hastwoparts:thedeclarationandthe
expression.Itisusedonlytoloopthrougharraysorcollections.
Withanenhancedfor,theexpressionisthearrayorcollectionthroughwhichyouwantto
loop.
Withanenhancedfor,thedeclarationistheblockvariable,whosetypeiscompatiblewith
theelementsofthearrayorcollection,andthatvariablecontainsthevalueoftheelementforthe
giveniteration.
UnlikewithC,youcannotuseanumberoranythingthatdoesnotevaluatetoabooleanvalue
asaconditionforanifstatementorloopingconstruct.Youcan’t,forexample,sayif(x),unlessx
isabooleanvariable.
Thedoloopwillalwaysenterthebodyoftheloopatleastonce.
Usingbreakandcontinue(OCAObjective5.5)
Anunlabeledbreakstatementwillcausethecurrentiterationoftheinnermostlooptostop
andthelineofcodefollowingthelooptorun.
Anunlabeledcontinuestatementwillcausethecurrentiterationoftheinnermostloopto
stop,theconditionofthatlooptobechecked,andiftheconditionismet,thelooptorunagain.
Ifthebreakstatementorthecontinuestatementislabeled,itwillcause
asimilaractiontooccuronthelabeledloop,nottheinnermostloop.
HandlingExceptions(OCAObjectives8.1,8.2,8.3,8.4,and8.5)
SomeofthebenefitsofJava’sexception-handlingfeaturesincludeorganizederror-handling
code,easyerrordetection,keepingexception-handlingcodeseparatefromothercode,andthe
abilitytoreuseexception-handlingcodeforarangeofissues.
Exceptionscomeintwoflavors:checkedandunchecked.
CheckedexceptionsincludeallsubtypesofException,excludingclassesthatextend
RuntimeException.
Checkedexceptionsaresubjecttothehandleordeclarerule;anymethodthatmightthrowa
checkedexception(includingmethodsthatinvokemethodsthatcanthrowacheckedexception)
musteitherdeclaretheexceptionusingthrowsorhandletheexceptionwithanappropriate
try/catch.
SubtypesofErrororRuntimeExceptionareunchecked,sothecompilerdoesn’tenforcethe
handleordeclarerule.You’refreetohandlethemortodeclarethem,butthecompilerdoesn’tcare
onewayortheother.
Afinallyblockwillalwaysbeinvoked,regardlessofwhetheranexceptionisthrownor
caughtinitstry/catch.
Theonlyexceptiontothefinally-will-always-be-calledruleisthatafinallywillnotbe
invokediftheJVMshutsdown.Thatcouldhappenifcodefromthetryorcatchblockscalls
System.exit().
Justbecausefinallyisinvokeddoesnotmeanitwillcomplete.Codeinthefinallyblock
coulditselfraiseanexceptionorissueaSystem.exit().
Uncaughtexceptionspropagatebackthroughthecallstack,startingfromthemethodwherethe
exceptionisthrownandendingwitheitherthefirstmethodthathasacorrespondingcatchforthat
exceptiontypeoraJVMshutdown(whichhappensiftheexceptiongetstomain()andmain()is
“ducking”theexceptionbydeclaringit).
YoucanalmostalwayscreateyourownexceptionsbyextendingExceptionoroneofits
checkedexceptionsubtypes.Suchanexceptionwillthenbeconsideredacheckedexceptionbythe
compiler.(Inotherwords,it’sraretoextendRuntimeException.)
Allcatchblocksmustbeorderedfrommostspecifictomostgeneral.Ifyouhaveacatch
clauseforbothIOExceptionandException,youmustputthecatchforIOExceptionfirstin
yourcode.Otherwise,theIOExceptionwouldbecaughtbycatch(Exceptione),becausea
catchargumentcancatchthespecifiedexceptionoranyofitssubtypes!
SomeexceptionsarecreatedbyprogrammersandsomebytheJVM.
SELFTEST
1.GiventhattoLowerCase()isanaptlynamedStringmethodthatreturnsaString,andgiven
thecode:
Whatistheresult?
A.-
B.-r
C.-rg
D.Compilationfails
E.Anexceptionisthrownatruntime
2.Given:
Whatistheresult?
A.-
B.-c
C.-c2
D.-2c
E.-c22b
F.-2c2b
G.-2c2bc
H.Compilationfails
3.Given:
try{intx=Integer.parseInt(“two”);}
Whichcouldbeusedtocreateanappropriatecatchblock?(Chooseallthatapply.)
A.ClassCastException
B.IllegalStateException
C.NumberFormatException
D.IllegalArgumentException
E.ExceptionInInitializerError
F.ArrayIndexOutOfBoundsException
4.Given:
Andgiventhecommand-lineinvocation:
JavaFlip2REDGreenYeLLow
Whicharetrue?(Chooseallthatapply.)
A.Thestringrgywillappearsomewhereintheoutput
B.Thestringrggwillappearsomewhereintheoutput
C.Thestringgyrwillappearsomewhereintheoutput
D.Compilationfails
E.Anexceptionisthrownatruntime
5.Given:
Which,insertedindependentlyatline4,compiles?(Chooseallthatapply.)
6.Given:
Whatistheresult?
A.-icof
B.-mfof
C.-mcmf
D.-icmfof
E.-icmcmfof
F.-icmcofmf
G.Compilationfails
7.Given:
Whatistheresult?(Chooseallthatapply.)
A.Compilationsucceeds
B.Compilationfailsduetoanerroronline8
C.Compilationfailsduetoanerroronline10
D.Compilationfailsduetoanerroronline12
E.Compilationfailsduetoanerroronline14
8.Given:
Whatistheresult?
A.910d
B.8910d
C.91010d
D.91010d13
E.891010d13
F.891091010d13
G.Compilationfails
9.Given:
Andgiventhatline7willassignthevalue0,1,or2tosw,whicharetrue?(Chooseallthat
apply.)
A.Compilationfails
B.AClassCastExceptionmightbethrown
C.AStackOverflowErrormightbethrown
D.ANullPointerExceptionmightbethrown
E.AnIllegalStateExceptionmightbethrown
F.Theprogrammighthangwithoutevercompleting
G.Theprogramwillalwayscompletewithoutexception
10.Given:
Whatistheresult?
A.139
B.5577
C.13399
D.113399
E.111333999
F.Compilationfails
11.Given:
Whatistheresult?
A.12
B.13
C.123
D.1234
E.Compilationfails
F.123followedbyanexception
G.1234followedbyanexception
H.Anexceptionisthrownwithnootheroutput
12.Given:
Whatistheresult?
A.0123
B.11133
C.0111233
D.11133444
E.0111233444
F.Compilationfails
13.Given:
Andgiventhefollowingthreecodefragments:
WhenfragmentsI–IIIareadded,independently,atline5,whicharetrue?(Chooseallthatapply.)
A.Somewillnotcompile
B.Theywillallcompile
C.Allwillcompletenormally
D.Nonewillcompletenormally
E.Onlyonewillcompletenormally
F.Twoofthemwillcompletenormally
14.Giventhecodesnippet:
Andgiventhatthenumberedlineswillallbetestedbyuncommentingoneswitchstatementand
onecasestatementtogether,whichline(s)willFAILtocompile?(Chooseallthatapply.)
A.line1
B.line2
C.line3
D.line4
E.line5
F.line6
G.Allsixlinesofcodewillcompile
15.GiventhatIOExceptionisinthejava.iopackageandgiven:
Andgiventhefollowingfourcodefragments:
Ifthefourfragmentsareinsertedindependentlyatline2,whicharetrue?(Chooseallthatapply.)
A.Allfourwillcompileandexecutewithoutexception
B.Allfourwillcompileandexecuteandthrowanexception
C.Some,butnotall,willcompileandexecutewithoutexception
D.Some,butnotall,willcompileandexecuteandthrowanexception
E.WhenconsideringfragmentsII,III,andIV,ofthosethatwillcompile,addingatry/catch
blockaroundline4willcausecompilationtofail
16.Given:
Andgiventhefollowingfourcodefragments:
WhenfragmentsI–IVareadded,independently,atline10,whicharetrue?(Chooseallthat
apply.)
A.Nonewillcompile
B.Theywillallcompile
C.Some,butnotall,willcompile
D.Allthosethatcompilewillthrowanexceptionatruntime
E.Noneofthosethatcompilewillthrowanexceptionatruntime
F.Onlysomeofthosethatcompilewillthrowanexceptionatruntime
SELFTESTANSWERS
1. Ciscorrect.AsofJava7it’slegaltoswitchonaString,andrememberthatswitchesuse
“entrypoint”logic.
A,B,D,andEareincorrectbasedontheabove.(OCAObjective3.4)
2. Biscorrect.Onces3()throwstheexceptiontos2(),s2()throwsittos1(),andnomore
ofs2()’scodewillbeexecuted.
A,C,D,E,F,G,andHareincorrectbasedontheabove.(OCAObjectives8.2and8.4)
3. CandDarecorrect.Integer.parseIntcanthrowaNumberFormatException,and
IllegalArgumentExceptionisitssuperclass(thatis,abroaderexception).
A,B,E,andFarenotinNumberFormatException’sclasshierarchy.(OCAObjective8.5)
4. Eiscorrect.AsofJava7thesyntaxislegal.Thesa[]arrayreceivesonlythreearguments
fromthecommandline,soonthelastiterationthroughsa[],aNullPointerExceptionisthrown.
A,B,C,andDareincorrectbasedontheabove.(OCAObjectives1.3,5.2,and8.5)
5. A,D,andFarecorrect.Aisanexampleoftheenhancedforloop.DandFareexamples
ofthebasicforloop.
B,C,andEareincorrect.Bisincorrectbecauseitsoperandsareswapped.Cisincorrect
becausetheenhancedformustdeclareitsfirstoperand.Eisincorrectsyntaxtodeclaretwo
variablesinaforstatement.(OCAObjective5.2)
6. Eiscorrect.Thereisnoproblemnestingtry/catchblocks.Asisnormal,whenan
exceptionisthrown,thecodeinthecatchblockruns,andthenthecodeinthefinallyblockruns.
A,B,C,D,andFareincorrectbasedontheabove.(OCAObjectives8.2and8.4)
7. Ciscorrect.Anoverridingmethodcannotthrowabroaderexceptionthanthemethodit’s
overriding.ClassCC4’smethodisanoverload,notanoverride.
A,B,D,andEareincorrectbasedontheabove.(OCAObjectives8.2and8.4)
8. Discorrect.Didyoucatchthestaticinitializerblock?Rememberthatswitchesworkon
“fall-through”logicandthatfall-throughlogicalsoappliestothedefaultcase,whichisusedwhen
noothercasematches.
A,B,C,E,F,andGareincorrectbasedontheabove.(OCAObjective3.4)
9. DandFarecorrect.Becauseiwasnotinitialized,case1willthrowa
NullPointerException.Case0willinitiateanendlessloop,notastackoverflow.Case2’s
downcastwillnotcauseanexception.
A,B,C,E,andGareincorrectbasedontheabove.(OCAObjectives3.4and8.5)
10. Discorrect.Thebasicruleforunlabeledcontinuestatementsisthatthecurrentiteration
stopsearlyandexecutionjumpstothenextiteration.Thelasttwocontinuestatementsare
redundant!
A,B,C,E,andFareincorrectbasedontheabove.(OCAObjectives5.2and5.5)
11. Hiscorrect.It’struethatthevalueofStringsis123atthetimethatthedivide-by-zero
exceptionisthrown,butfinally()isnotguaranteedtocomplete,andinthiscasefinally()
nevercompletes,sotheSystem.out.println(S.O.P)neverexecutes.
A,B,C,D,E,F,andGareincorrectbasedontheabove.(OCAObjectives8.2and8.5)
12. Ciscorrect.Abreakbreaksoutofthecurrentinnermostloopandcarrieson.Alabeled
breakbreaksoutofandterminatesthelabeledloops.
A,B,D,E,andFareincorrectbasedontheabove.(OCAObjectives5.2and5.5)
13. BandEarecorrect.Firstoff,go()isabadlydesignedrecursivemethod,guaranteedto
causeaStackOverflowError.SinceExceptionisnotasuperclassofError,catchingan
ExceptionwillnothelphandleanError,sofragmentIIIwillnotcompletenormally.Only
fragmentIIwillcatchtheError.
A,C,D,andFareincorrectbasedontheabove.(OCAObjectives8.1,8.2,and8.4)
14. Eiscorrect.Aswitch’scasesmustbecompile-timeconstantsorenumvalues.
A,B,C,D,F,andGareincorrectbasedontheabove.(OCAObjective3.4)
15. Discorrect.Thisiskindofsneaky,butrememberthatwe’retryingtotoughenyouupfor
therealexam.Ifyou’regoingtothrowanIOException,youhavetoimportthejava.iopackageor
declaretheexceptionwithafullyqualifiedname.
A,B,C,andEareincorrect.A,B,andCareincorrectbasedontheabove.Eisincorrect
becauseit’sokaybothtohandleanddeclareanexception.(OCAObjectives8.2and8.5)
16. CandDarecorrect.Anoverridingmethodcannotthrowcheckedexceptionsthatare
broaderthanthosethrownbytheoverriddenmethod.However,anoverridingmethodcanthrow
RuntimeExceptionsnotthrownbytheoverriddenmethod.
A,B,E,andFareincorrectbasedontheabove.(OCAObjective8.1)
T
6
Strings,Arrays,ArrayLists,Dates,andLambdas
CERTIFICATIONOBJECTIVES
•CreateandManipulateStrings
•ManipulateDataUsingtheStringBuilderClassandItsMethods
•CreateandUseCalendarData
•Declare,Instantiate,Initialize,andUseaOne-DimensionalArray
•Declare,Instantiate,Initialize,andUseaMultidimensionalArray
•DeclareandUseanArrayList
•UseWrapperClasses
•UseEncapsulationforReferenceVariables
•UseSimpleLambdaExpressions
Two-MinuteDrill
Q&ASelfTest
hischapterfocusesontheexamobjectivesrelatedtosearching,formatting,andparsingstrings;
creatingandusingcalendar-relatedobjects;creatingandusingarraysandArrayLists;and
usingsimplelambdaexpressions.Manyofthesetopicscouldfillanentirebook.Fortunately,
youwon’thavetobecomeatotalgurutodowellontheexam.Theexamteamintendedto
includejustthebasicaspectsofthesetechnologies,andinthischapter,wecovermorethanyou’llneedto
getthroughtherelatedobjectivesontheexam.
CERTIFICATIONOBJECTIVE
UsingStringandStringBuilder(OCAObjectives9.2and
9.1)
9.2CreatingandmanipulatingStrings.
9.1ManipulatedatausingtheStringBuilderclassanditsmethods.
EverythingyouneededtoknowaboutstringsintheolderOCJPexamsyou’llneedtoknowforthe
OCA8exam.CloselyrelatedtotheStringclassaretheStringBuilderclassandthealmostidentical
StringBufferclass.(Fortheexam,theonlythingyouneedtoknowabouttheStringBufferclassis
thatithasexactlythesamemethodsastheStringBuilderclass,butStringBuilderisfasterbecause
itsmethodsaren’tsynchronized.)Bothclasses,StringBuilderandStringBuffer,giveyouString-
likeobjectsandwaystomanipulatethem,withtheimportantdifferencebeingthattheseobjectsare
mutable.
TheStringClass
ThissectioncoverstheStringclass,andthekeyconceptforyoutounderstandisthatonceaString
objectiscreated,itcanneverbechanged.So,then,whatishappeningwhenaStringobjectseemstobe
changing?Let’sfindout.
StringsAreImmutableObjects
We’llstartwithalittlebackgroundinformationaboutstrings.Youmaynotneedthisforthetest,buta
littlecontextwillhelp.Handling"strings"ofcharactersisafundamentalaspectofmostprogramming
languages.InJava,eachcharacterinastringisa16-bitUnicodecharacter.BecauseUnicodecharacters
are16bits(nottheskimpy7or8bitsthatASCIIprovides),arich,internationalsetofcharactersiseasily
representedinUnicode.
InJava,stringsareobjects.Aswithotherobjects,youcancreateaninstanceofastringwiththenew
keyword,asfollows:
Strings=newString();
ThislineofcodecreatesanewobjectofclassStringandassignsittothereferencevariables.
Sofar,Stringobjectsseemjustlikeotherobjects.Now,let’sgivethestringavalue:
s="abcdef";
(Asyou’llfindoutshortly,thesetwolinesofcodearen’tquitewhattheyseem,sostaytuned.)
ItturnsouttheStringclasshasaboutazillionconstructors,soyoucanuseamoreefficientshortcut:
Strings=newString("abcdef");
Andthisisevenmoreconcise:
Strings="abcdef";
Therearesomesubtledifferencesbetweentheseoptionsthatwe’lldiscusslater,butwhattheyhavein
commonisthattheyallcreateanewStringobject,withavalueof"abcdef",andassignittoa
referencevariables.Nowlet’ssayyouwantasecondreferencetotheStringobjectreferredtobys:
Strings2=s;//refers2tothesameStringass
Sofarsogood.Stringobjectsseemtobebehavingjustlikeotherobjects,sowhat’sallthefuss
about?Immutability!(Whattheheckisimmutability?)OnceyouhaveassignedaStringavalue,that
valuecanneverchange—it’simmutable,frozensolid,won’tbudge,fini,done.(We’lltalkaboutwhy
later;don’tletusforget.)ThegoodnewsisthatalthoughtheStringobjectisimmutable,itsreference
variableisnot,sotocontinuewithourpreviousexample,considerthis:
Now,waitjustaminute,didn’twejustsaythatStringobjectswereimmutable?Sowhat’sallthis
"appendingtotheendofthestring"talk?Excellentquestion:let’slookatwhatreallyhappened.
TheJavaVirtualMachine(JVM)tookthevalueofstrings(whichwas"abcdef")andtacked"more
stuff"ontotheend,givingusthevalue"abcdefmorestuff".Sincestringsareimmutable,theJVM
couldn’tstuffthisnewvalueintotheoldStringreferencedbys,soitcreatedanewStringobject,gave
itthevalue"abcdefmorestuff",andmadesrefertoit.Atthispointinourexample,wehavetwo
Stringobjects:thefirstonewecreated,withthevalue"abcdef",andthesecondonewiththevalue
"abcdefmorestuff".TechnicallytherearenowthreeStringobjects,becausetheliteralargumentto
concat,"morestuff",isitselfanewStringobject.Butwehavereferencesonlyto"abcdef"
(referencedbys2)and"abcdefmorestuff"(referencedbys).
Whatifwedidn’thavetheforesightorlucktocreateasecondreferencevariableforthe"abcdef"
stringbeforewecalleds=s.concat("morestuff");?Inthatcase,theoriginal,unchangedstring
containing"abcdef"wouldstillexistinmemory,butitwouldbeconsidered"lost."Nocodeinour
programhasanywaytoreferenceit—itislosttous.Note,however,thattheoriginal"abcdef"string
didn’tchange(itcan’t,remember;it’simmutable);onlythereferencevariableswaschangedsothatit
wouldrefertoadifferentstring.
Figure6-1showswhathappensontheheapwhenyoureassignareferencevariable.Notethatthe
dashedlineindicatesadeletedreference.
FIGURE6-1 Stringobjectsandtheirreferencevariables
Toreviewourfirstexample:
Let’slookatanotherexample:
Thefirstlineisstraightforward:CreateanewStringobject,giveitthevalue“Java”,andreferxtoit.
NexttheJVMcreatesasecondStringobjectwiththevalue“JavaRules!”butnothingreferstoit.The
secondStringobjectisinstantlylost;youcan’tgettoit.Thereferencevariablexstillreferstothe
originalStringwiththevalue“Java”.Figure6-2showscreatingaStringwithoutassigninga
referencetoit.
FIGURE6-2 AStringobjectisabandoneduponcreation.
Let’sexpandthiscurrentexample.Westartedwith
Nowlet’sadd
(WeactuallydidjustcreateanewStringobjectwiththevalue"JAVA",butitwaslost,andxstill
referstotheoriginalunchangedstring"Java".)Howaboutaddingthis:
Canyoudeterminewhathappened?TheJVMcreatedyetanothernewStringobject,withthevalue
"JXvX",(replacingthea’swithX’s),butonceagainthisnewStringwaslost,leavingxtorefertothe
originalunchangedandunchangeableStringobject,withthevalue"Java".Inallthesecases,wecalled
variousstringmethodstocreateanewStringbyalteringanexistingString,butweneverassignedthe
newlycreatedStringtoareferencevariable.
Butwecanputasmallspinonthepreviousexample:
Thistime,whentheJVMrunsthesecondline,anewStringobjectiscreatedwiththevalue"Java
Rules!",andxissettoreferenceit.Butwait…there’smore—nowtheoriginalStringobject,"Java",
hasbeenlost,andnooneisreferringtoit.Soinbothexamples,wecreatedtwoStringobjectsandonly
onereferencevariable,sooneofthetwoStringobjectswasleftoutinthecold.(SeeFigure6-3fora
graphicdepictionofthissadstory.)
FIGURE6-3 AnoldStringobjectbeingabandoned.Thedashedlineindicatesadeletedreference.
Let’stakethisexamplealittlefurther:
TheprecedingdiscussioncontainsthekeystounderstandingJavastringimmutability.Ifyoureally,
reallygettheexamplesanddiagrams,backwardandforward,youshouldget80percentoftheString
questionsontheexamcorrect.
Wewillcovermoredetailsaboutstringsnext,butmakenomistake—intermsofbangforyourbuck,
whatwe’vealreadycoveredisbyfarthemostimportantpartofunderstandinghowStringobjectswork
inJava.
We’llfinishthissectionbypresentinganexampleofthekindofdevilishStringquestionyoumight
expecttoseeontheexam.Takethetimetoworkitoutonpaper.(Hint:trytokeeptrackofhowmany
objectsandreferencevariablesthereare,andwhichonesrefertowhich.)
Whatistheoutput?Forextracredit,howmanyStringobjectsandhowmanyreferencevariables
werecreatedpriortotheprintlnstatement?
Answer:Theresultofthiscodefragmentisspringwinterspringsummer.Therearetworeference
variables:s1ands2.AtotalofeightStringobjectswerecreatedasfollows:"spring","summer"
(lost),"springsummer","fall"(lost),"springfall"(lost),"springsummerspring"(lost),
"winter"(lost),"springwinter"(atthispoint"spring"islost).OnlytwooftheeightString
objectsarenotlostinthisprocess.
ImportantFactsAboutStringsandMemory
Inthissection,we’lldiscusshowJavahandlesStringobjectsinmemoryandsomeofthereasons
behindthesebehaviors.
Oneofthekeygoalsofanygoodprogramminglanguageistomakeefficientuseofmemory.Asan
applicationgrows,it’sverycommonforstringliteralstooccupylargeamountsofaprogram’smemory,
andthereisoftenalotofredundancywithintheuniverseofStringliteralsforaprogram.TomakeJava
morememoryefficient,theJVMsetsasideaspecialareaofmemorycalledtheStringconstantpool.
WhenthecompilerencountersaStringliteral,itchecksthepooltoseeifanidenticalStringalready
exists.Ifamatchisfound,thereferencetothenewliteralisdirectedtotheexistingString,andnonew
Stringliteralobjectiscreated.(TheexistingStringsimplyhasanadditionalreference.)Nowyoucan
starttoseewhymakingStringobjectsimmutableissuchagoodidea.Ifseveralreferencevariables
refertothesameStringwithoutevenknowingit,itwouldbeverybadifanyofthemcouldchangethe
String’svalue.
Youmightsay,"Wellthat’sallwellandgood,butwhatifsomeoneoverridestheStringclass
functionality;couldn’tthatcauseproblemsinthepool?"That’soneofthemainreasonsthattheString
classismarkedfinal.NobodycanoverridethebehaviorsofanyoftheStringmethods,soyoucanrest
assuredthattheStringobjectsyouarecountingontobeimmutablewill,infact,beimmutable.
CreatingNewStrings
Earlierwepromisedtotalkmoreaboutthesubtledifferencesbetweenthevariousmethodsofcreatinga
String.Let’slookatacoupleofexamplesofhowaStringmightbecreated,andlet’sfurtherassume
thatnootherStringobjectsexistinthepool.Inthissimplecase,"abc"willgointhepool,andswill
refertoit:
Inthenextcase,becauseweusedthenewkeyword,JavawillcreateanewStringobjectinnormal
(nonpool)memory,andswillrefertoit.Inaddition,theliteral"abc"willbeplacedinthepool:
ImportantMethodsintheStringClass
ThefollowingmethodsaresomeofthemorecommonlyusedmethodsintheStringclass,andtheyare
alsotheonesyou’remostlikelytoencounterontheexam.
charAt()Returnsthecharacterlocatedatthespecifiedindex
concat()Appendsonestringtotheendofanother(+alsoworks)
equalsIgnoreCase()Determinestheequalityoftwostrings,ignoringcase
length()Returnsthenumberofcharactersinastring
replace()Replacesoccurrencesofacharacterwithanewcharacter
substring()Returnsapartofastring
toLowerCase()Returnsastring,withuppercasecharactersconvertedtolowercase
toString()Returnsthevalueofastring
toUpperCase()Returnsastring,withlowercasecharactersconvertedtouppercase
trim()Removeswhitespacefrombothendsofastring
Let’slookatthesemethodsinmoredetail.
publiccharcharAt(intindex)
ThismethodreturnsthecharacterlocatedattheString’sspecifiedindex.Remember,Stringindexes
arezero-based—here’sanexample:
publicStringconcat(Strings)
ThismethodreturnsastringwiththevalueoftheStringpassedintothemethodappendedtotheendof
theStringusedtoinvokethemethod—here’sanexample:
Theoverloaded+and+=operatorsperformfunctionssimilartotheconcat()method—here’san
example:
Inthepreceding"Atlanticocean"example,noticethatthevalueofxreallydidchange!Remember
the+=operatorisanassignmentoperator,soline2isreallycreatinganewstring,"Atlanticocean",
andassigningittothexvariable.Afterline2executes,theoriginalstringxwasreferringto,
"Atlantic",isabandoned.
publicbooleanequalsIgnoreCase(Strings)
Thismethodreturnsabooleanvalue(trueorfalse)dependingonwhetherthevalueoftheStringin
theargumentisthesameasthevalueoftheStringusedtoinvokethemethod.Thismethodwillreturn
trueevenwhencharactersintheStringobjectsbeingcomparedhavedifferingcases—here’san
example:
publicintlength()
ThismethodreturnsthelengthoftheStringusedtoinvokethemethod—here’sanexample:
Arrayshaveanattribute(notamethod)calledlength.Youmayencounterquestionsinthe
examthatattempttousethelength()methodonanarrayorthatattempttousethelength
attributeonaString.Bothcausecompilererrors–considerthese,forexample:
and
publicStringreplace(charold,charnew)
ThismethodreturnsaStringwhosevalueisthatoftheStringusedtoinvokethemethod,butupdated
sothatanyoccurrenceofthecharinthefirstargumentisreplacedbythecharinthesecondargument—
here’sanexample:
publicStringsubstring(intbegin)andpublicStringsubstring(intbegin,intend)
Thesubstring()methodisusedtoreturnapart(orsubstring)oftheStringusedtoinvokethemethod.
Thefirstargumentrepresentsthestartinglocation(zero-based)ofthesubstring.Ifthecallhasonlyone
argument,thesubstringreturnedwillincludethecharactersattheendoftheoriginalString.Ifthecall
hastwoarguments,thesubstringreturnedwillendwiththecharacterlocatedinthenthpositionofthe
originalStringwherenisthesecondargument.Unfortunately,theendingargumentisnotzero-based,so
ifthesecondargumentis7,thelastcharacterinthereturnedStringwillbeintheoriginalString’s7
position,whichisindex6(ouch).Let’slookatsomeexamples:
Thefirstexampleshouldbeeasy:startatindex5andreturntherestoftheString.Thesecondexample
shouldbereadasfollows:startatindex5andreturnthecharactersuptoandincludingthe8thposition
(index7).
publicStringtoLowerCase()
ConvertsallcharactersofaStringtolowercase—here’sanexample:
publicStringtoString()
ThismethodreturnsthevalueoftheStringusedtoinvokethemethod.What?Whywouldyouneedsuch
aseemingly"donothing"method?AllobjectsinJavamusthaveatoString()method,whichtypically
returnsaStringthatinsomemeaningfulwaydescribestheobjectinquestion.InthecaseofaString
object,what’samoremeaningfulwaythantheString’svalue?Forthesakeofconsistency,here’san
example:
publicStringtoUpperCase()
ConvertsallcharactersofaStringtouppercase–here’sanexample:
publicStringtrim()
ThismethodreturnsaStringwhosevalueistheStringusedtoinvokethemethod,butwithanyleading
ortrailingwhitespaceremoved—here’sanexample:
TheStringBuilderClass
Thejava.lang.StringBuilderclassshouldbeusedwhenyouhavetomakealotofmodificationsto
stringsofcharacters.Asdiscussedintheprevioussection,Stringobjectsareimmutable,soifyou
choosetodoalotofmanipulationswithStringobjects,youwillendupwithalotofabandonedString
objectsintheStringpool.(EveninthesedaysofgigabytesofRAM,it’snotagoodideatowaste
preciousmemoryondiscardedStringpoolobjects.)Ontheotherhand,objectsoftypeStringBuilder
canbemodifiedoverandoveragainwithoutleavingbehindagreateffluenceofdiscardedString
objects.
AcommonuseforStringBuildersisfileI/Owhenlarge,ever-changingstreamsofinputarebeing
handledbytheprogram.Inthesecases,largeblocksofcharactersarehandledasunits,and
StringBuilderobjectsaretheidealwaytohandleablockofdata,passiton,andthenreusethe
samememorytohandlethenextblockofdata.
PreferStringBuildertoStringBuffer
TheStringBuilderclasswasaddedinJava5.IthasexactlythesameAPIastheStringBufferclass,
exceptStringBuilderisnotthread-safe.Inotherwords,itsmethodsarenotsynchronized.Oracle
recommendsthatyouuseStringBuilderinsteadofStringBufferwheneverpossible,because
StringBuilderwillrunfaster(andperhapsjumphigher).Soapartfromsynchronization,anythingwe
sayaboutStringBuilder’smethodsholdstrueforStringBuffer’smethods,andviceversa.Thatsaid,
fortheOCA8exam,StringBufferisnottested.
UsingStringBuilder(andThisIstheLastTimeWe’llSayThis:StringBuffer)
Intheprevioussection,yousawhowtheexammighttestyourunderstandingofStringimmutabilitywith
codefragmentslikethis:
Becausenonewassignmentwasmade,thenewStringobjectcreatedwiththeconcat()methodwas
abandonedinstantly.Youalsosawexampleslikethis:
WegotanicenewStringoutofthedeal,butthedownsideisthattheoldString"abc"hasbeenlostin
theStringpool,thuswastingmemory.IfwewereusingaStringBuilderinsteadofaString,thecode
wouldlooklikethis:
AlloftheStringBuildermethodswewilldiscussoperateonthevalueoftheStringBuilderobject
invokingthemethod.Soacalltosb.append("def");isactuallyappending"def"toitself
(StringBuildersb).Infact,thesemethodcallscanbechainedtoeachother—here’sanexample:
Noticethatineachoftheprevioustwoexamples,therewasasinglecalltonew,soineachexample
weweren’tcreatinganyextraobjects.EachexampleneededonlyasingleStringBuilderobjectto
execute.
Sofarwe’veseenStringBuildersbeingbuiltwithanargumentspecifyinganinitialvalue.
StringBuilderscanalsobebuiltempty,andtheycanalsobeconstructedwithaspecificsize
or,moreformally,a"capacity."Fortheexam,therearethreewaystocreateanew
StringBuilder:
ThetwomostcommonwaystoworkwithStringBuildersisviaanappend()methodoran
insert()method.IntermsofaStringBuilder’scapacity,therearethreerulestokeepin
mindwhenappendingandinserting:
Ifanappend()growsaStringBuilderpastitscapacity,thecapacityisupdated
automatically.
Ifaninsert()startswithinaStringBuilder’scapacitybutendsafterthecurrent
capacity,thecapacityisupdatedautomatically.
Ifaninsert()attemptstostartatanindexaftertheStringBuilder’scurrentlength,
anexceptionwillbethrown.
ImportantMethodsintheStringBuilderClass
TheStringBuilderclasshasazillionmethods.Followingarethemethodsyou’remostlikelytousein
therealworldand,happily,theonesyou’remostlikelytofindontheexam.
publicStringBuilderappend(Strings)
Asyou’veseenearlier,thismethodwillupdatethevalueoftheobjectthatinvokedthemethod,whether
ornotthereturnedvalueisassignedtoavariable.Versionsofthisheavilyoverloadedmethodwilltake
manydifferentarguments,includingboolean,char,double,float,int,long,andothers,buttheone
mostlikelyusedontheexamwillbeaStringargument—forexample,
publicStringBuilderdelete(intstart,intend)
ThismethodmodifiesthevalueoftheStringBuilderobjectusedtoinvokeit.Thestartingindexofthe
substringtoberemovedisdefinedbythefirstargument(whichiszero-based),andtheendingindexofthe
substringtoberemovedisdefinedbythesecondargument(butitisone-based)!Studythefollowing
examplecarefully:
TheexamwillprobablytestyourknowledgeofthedifferencebetweenStringand
StringBuilderobjects.BecauseStringBuilderobjectsarechangeable,thefollowingcode
fragmentwillbehavedifferentlythanasimilarcodefragmentthatusesStringobjects:
Inthiscase,theoutputwillbe:"abcdef"
publicStringBuilderinsert(intoffset,Strings)
ThismethodupdatesthevalueoftheStringBuilderobjectthatinvokedthemethodcall.TheString
passedintothesecondargumentisinsertedintotheStringBuilderstartingattheoffsetlocation
representedbythefirstargument(theoffsetiszero-based).Again,othertypesofdatacanbepassedin
throughthesecondargument(boolean,char,double,float,int,long,andsoon),buttheStringargument
istheoneyou’remostlikelytosee:
publicStringBuilderreverse()
ThismethodupdatesthevalueoftheStringBuilderobjectthatinvokedthemethodcall.Wheninvoked,
thecharactersintheStringBuilderarereversed—thefirstcharacterbecomingthelast,thesecond
becomingthesecondtothelast,andsoon:
publicStringtoString()
ThismethodreturnsthevalueoftheStringBuilderobjectthatinvokedthemethodcallasaString:
That’sitforStringBuilders.Ifyoutakeonlyonethingawayfromthissection,it’sthatunlikeString
objects,StringBuilderobjectscanbechanged.
Manyoftheexamquestionscoveringthischapter’stopicsuseatrickybitofJavasyntax
knownas“chainedmethods.”Astatementwithchainedmethodshasthisgeneralform:
result=method1().method2().method3();
Intheory,anynumberofmethodscanbechainedinthisfashion,althoughtypicallyyouwon’t
seemorethanthree.Here’showtodecipherthese"handyJavashortcuts"whenyouencounter
them:
1.Determinewhattheleftmostmethodcallwillreturn(let’scallitx).
2.Usexastheobjectinvokingthesecond(fromtheleft)method.Ifthereareonlytwo
chainedmethods,theresultofthesecondmethodcallistheexpression’sresult.
3.Ifthereisathirdmethod,theresultofthesecondmethodcallisusedtoinvokethe
thirdmethod,whoseresultistheexpression’sresult–forexample,
Let’slookatwhathappened.Theliteraldefwasconcatenatedtoabc,creatingatemporary,
intermediateString(soontobelost),withthevalueabcdef.ThetoUpperCase()methodwas
calledonthisString,whichcreatedanew(soontobelost)temporaryStringwiththevalue
ABCDEF.Thereplace()methodwasthencalledonthissecondStringobject,whichcreateda
finalStringwiththevalueABxDEFandreferredytoit.
CERTIFICATIONOBJECTIVE
WorkingwithCalendarData(OCAObjective9.3)
9.3Createandmanipulatecalendardatausingthefollowingclasses:java.time.LocalDateTime,
java.time.LocalDate,java.time.LocalTime,java.time.format.DateTimeFormatter,java.time.Period
Java8introducedalargecollection(argh)ofnewpackagesrelatedtoworkingwithcalendars,dates,
andtimes.TheOCA8creatorschosetoincludeknowledgeofasubsetofthesepackagesandclassesas
anexamobjective.Ifyouunderstandtheclassesincludedintheexamobjective,you’llhaveagood
introductiontotheentirecalendar/date/timetopic.Asweworkthroughthissection,we’llusethephrase
"calendarobject,"whichweusetorefertoobjectsofoneoftheseveraltypesofcalendar-relatedclasses
we’recovering.So"calendarobject"isamade-upumbrellaterm.Here’sasummaryofthefivecalendar-
relatedclasseswe’llstudy,plusaninterfacethatloomslarge:
java.time.LocalDateTimeThisclassisusedtocreateimmutableobjects,eachofwhich
representsaspecificdateandtime.Additionally,thisclassprovidesmethodsthatcanmanipulate
thevaluesofthedate/timeobjectscreatedandassignthemtonewimmutableobjects.
LocalDateTimeobjectscontainBOTHinformationaboutdays,months,andyears,ANDabout
hours,minutes,seconds,andfractionsofseconds.
java.time.LocalDateThisclassisusedtocreateimmutableobjects,eachofwhich
representsaspecificdate.Additionally,thisclassprovidesmethodsthatcanmanipulatethevalues
ofthedateobjectscreatedandassignthemtonewimmutableobjects.LocalDateobjectsare
accurateonlytodays.Hours,minutes,andsecondsarenotpartofaLocalDateobject.
java.time.LocalTimeThisclassisusedtocreateimmutableobjects,eachofwhich
representsaspecifictime.Additionally,thisclassprovidesmethodsthatcanmanipulatethevalues
ofthetimeobjectscreatedandassignthemtonewimmutableobjects.LocalTimeobjectsrefer
onlytohours,minutes,seconds,andfractionsofseconds.Days,months,andyearsarenotapartof
LocalTimeobjects.
java.time.format.DateTimeFormatterThisclassisusedbytheclassesjustdescribedto
formatdate/timeobjectsforoutputandtoparseinputstringsandconvertthemtodate/timeobjects.
DateTimeFormatterobjectsarealsoimmutable.
java.time.PeriodThisclassisusedtocreateimmutableobjectsthatrepresentaperiodof
time,forexample,"oneyear,twomonths,andthreedays."Thisclassworksinyears,months,and
days.Ifyouwanttorepresentchunksoftimeinincrementsfinerthanaday(e.g.,hoursand
minutes),youcanusethejava.time.Durationclass,butDurationisnotontheexam.
java.time.temporal.TemporalAmountThisinterfaceisimplementedbythePeriodclass.
WhenyouusePeriodobjectstomanipulate(seethefollowingsection),calendarobjects,you’ll
oftenusemethodsthattakeobjectsthatimplementTemporalAmount.Ingeneral,asyouusethe
JavaAPImoreandmore,it’sagoodideatolearnwhichclassesimplementwhichinterfaces;this
isakeywaytolearnhowtheclassesincomplexpackagesinteractwitheachother.
Immutability
Thereareacoupleofrecurringthemesinthepreviousdefinitions.First,noticethatmostofthecalendar-
relatedobjectsyou’llcreateareimmutable.JustlikeStringobjects!Sowhenwesaywe’regoingto
"manipulate"acalendarobject,whatwe"really"meanisthatwe’llinvokeamethodonacalendar
object,andwe’llreturnanewcalendarobjectthatrepresentstheresultofmanipulatingthevalueofthe
originalcalendarobject.Buttheoriginalcalendarobject’svalueisnot,andcannot,bechanged.Justlike
Strings!Let’sseeanexample:
whichproduces:
2017-01-31
2017-01-31
2017-02-28
Noticethatinvokingtheplusmethodondate1doesn’tchangeitsvalue,butassigningtheresultofthe
plusmethodtodate2capturesanewvalue.Expectexamquestionsthattestyourunderstandingofthe
immutabilityofcalendarobjects.
FactoryClasses
Thenextthingtonoticeinthepreviouscodelistingisthatweneverusedthekeywordnewinthecode.
Wedidn’tdirectlyinvokeaconstructor.NoneofthefiveclasseslistedinOCA8objective9.3have
publicconstructors.Instead,foralltheseclasses,youinvokeapublicstaticmethodintheclassto
createanewobject.AsyougofurtherintoyourstudiesofOOdesign,you’llcomeacrossthephrases
"factorypattern,""factorymethods,"and"factoryclasses."Usually,whenaclasshasnopublic
constructorsandprovidesatleastonepublicstaticmethodthatcancreatenewinstancesoftheclass,
thatclassiscalledafactoryclass,andanymethodthatisinvokedtogetanewinstanceoftheclassis
calledafactorymethod.Therearemanygoodreasonstocreatefactoryclasses,mostofwhichare
beyondthescopeofthisbook,butoneofthemwewilldiscussnow.IfweusetheLocalDateclassasan
example,wefindthefollowingstaticmethodsthatcreateandreturnanewinstance:
Sowehavewhat,abouttendifferentwaystocreateanewLocalDateobject?Byusingmethodswith
differentnames(insteadofusingoverloadedconstructors),themethodnamesthemselvesmakethecode
morereadable.It’sclearerwhatvariationofLocalDatewe’remaking.Asyouusemoreandmore
classesfromtheJavaAPI,you’lldiscoverthattheAPIcreatorsusefactoryclassesalot.
Wheneveryouseeanexamquestionrelatingtodatesortimes,beonthelookoutforthenew
keyword.Thisisyourtipoffthatthecodewon’tcompile:
LocalDateTimed1=newLocalDateTime();//won’tcompile
Remembertheexam’sdateandtimeclassesusefactorymethodstocreatenewobjects.
UsingandManipulatingDatesandTimes
Nowthatweknowhowtocreatenewcalendar-relatedobjects,let’sturntousingandmanipulatingthem.
(Andyouknowwhatwemeanwhenwesay"manipulate.")Thefollowingcodedemonstratessome
commonusesandpowerfulfeaturesofthenewJava8calendar-relatedclasses:
Invokingtheprogramwitharelevantbirthday:
java01201934
producestheoutput(whenrunonJanuary13,2017):
There’salotgoingonhere,solet’sdoawalk-through.First,wewantuserstoentertheirbirthdayin
theformofmmddyyyy.WeuseaDateTimeFormatterobjecttoparsetheuser’sfirstargumentandverify
thatit’savaliddateoftheformwe’rehopingfor.UsuallyintheJavaAPI,parse()methodscanthrow
exceptions,sowehavetodoourparsinginatry/catchblock.
Next,weprintouttheverifieddateandshowoffabitbyprintingoutwhatdayoftheweekthatdate
occurredon.Thiscalculationwouldbequitetrickytodobyhand!
Next,wecreateaPeriodobjectthatrepresentstheamountoftimebetweentheuser’sbirthdayand
today,andweusevariousgetX()methodstolistthedetailsofthePeriodobject.
Aftermakingsurewe’renotdealingwithatimelord,wethenusetheverypowerfuluntil()method
and"day"astheunitoftimetodeterminehowmanydaystheuserhasbeenalive.Wecheatedabithere
andusedtheChronoUnitenumfromthejava.time.temporalpackage.(EventhoughChronoUnitisn’t
ontheexam,wethinkifyoudoalotofcalendarcalculations,you’llendupusingthisenumalot.)
Next,weadd30,000daystotheuser’sbirthdaysowecancalculateonwhichdateouruserwillhave
livedfor30,000days.It’sashortjumptoseeinghowthesesortsofcalendarcalculationswillbevery
powerfulforschedulingapplications,projectmanagementapplications,travelplanning,andsoon.
Finally,weuseacommonfactorymethod,of(),tocreateanotherdateobject(representingtoday’s
date),andweusethatinconjunctionwiththeverypowerfulbetween()methodtoseehowlongit’sbeen
sinceJanuary1,2000,Y2K.
FormattingDatesandTimes
Nowlet’sturntoformattingdatesandtimesusingtheDateTimeFormatterclass,soyourcalendar
objectswilllookallshinywhenyouwanttoincludetheminyourprogram’soutput.Fortheexam,you
shouldknowthefollowingtwo-stepprocessforcreatingStringsthatrepresentwell-formattedcalendar
objects:
1.UseformattersandpatternsfromtheHUGElistsprovidedintheDateTimeFormatterclass
tocreateaDataTimeFormatterobject.
2.IntheLocalDate,LocalDateTime,andLocalTimeclasses,usetheformat()methodwith
theDateTimeFormatterobjectastheargumenttocreateawell-formedString—orusethe
DateTimeFormatter.format()methodwithacalendarargumenttocreateawell-formedString.
Let’slookatafewexamples:
which,whenweranthiscode,producedthefollowing(youroutputwillvary):
Jan14,2017
SatJan14,2017AD
14:17:951429958PM
Someofthepatterncodesweusedareself-evident(e.g.MMMddyyyy),andsomearefairlyarbitrary
like"E"fordayofweekor"k"formilitaryhours.Allofthecodescanbefoundinthe
DateTimeFormatterAPI.
That’senoughaboutcalendars;ontoarrays!
CERTIFICATIONOBJECTIVE
UsingArrays(OCAObjectives4.1and4.2)
4.1Declare,instantiate,initialize,anduseaone-dimensionalarray.
4.2Declare,instantiate,initialize,anduseamulti-dimensionalarray.
ArraysareobjectsinJavathatstoremultiplevariablesofthesametype.Arrayscanholdeither
primitivesorobjectreferences,butthearrayitselfwillalwaysbeanobjectontheheap,evenifthearray
isdeclaredtoholdprimitiveelements.Inotherwords,thereisnosuchthingasaprimitivearray,butyou
canmakeanarrayofprimitives.Forthisobjective,youneedtoknowthreethings:
Howtomakeanarrayreferencevariable(declare)
Howtomakeanarrayobject(construct)
Howtopopulatethearraywithelements(initialize)
Thereareseveraldifferentwaystodoeachofthese,andyouneedtoknowaboutallofthemfortheexam.
Arraysareefficient,butmostofthetimeyou’llwanttouseoneoftheCollectiontypesfrom
java.util(includingHashMap,ArrayList,andTreeSet).Collectionclassesoffermoreflexible
waystoaccessanobject(forinsertion,deletion,andsoon),andunlikearrays,theycanexpandor
contractdynamicallyasyouaddorremoveelements(they’rereallymanagedarrays,sincetheyuse
arraysbehindthescenes).There’saCollectiontypeforawiderangeofneeds.Doyouneedafast
sort?Agroupofobjectswithnoduplicates?Awaytoaccessaname/valuepair?Alinkedlist?The
OCP8examcoverscollectionsinmoredetail.
DeclaringanArray
Arraysaredeclaredbystatingthetypeofelementthearraywillhold,whichcanbeanobjectora
primitive,followedbysquarebracketstotheleftorrightoftheidentifier.
Declaringanarrayofprimitives:
Declaringanarrayofobjectreferences:
Whendeclaringanarrayreference,youshouldalwaysputthearraybracketsimmediatelyafterthe
declaredtyperatherthanaftertheidentifier(variablename).Thatway,anyonereadingthecodecan
easilytellthat,forexample,keyisareferencetoanintarrayobjectandnotanintprimitive.
Wecanalsodeclaremultidimensionalarrays,whichare,infact,arraysofarrays.Thiscanbedonein
thefollowingmanner:
Thefirstexampleisathree-dimensionalarray(anarrayofarraysofarrays)andthesecondisatwo-
dimensionalarray.Noticeinthesecondexamplewehaveonesquarebracketbeforethevariablename
andoneafter.Thisisperfectlylegaltothecompiler,provingonceagainthatjustbecauseit’slegal
doesn’tmeanit’sright.
Itisneverlegaltoincludethesizeofthearrayinyourdeclaration.Yes,weknowyoucandothatin
someotherlanguages,whichiswhyyoumightseeaquestionortwointheexamthatincludecodesimilar
tothefollowing:
int[5]scores;//willNOTcompile
Theprecedingcodewon’tmakeitpastthecompiler.Remember,theJVMdoesn’tallocatespaceuntilyou
actuallyinstantiatethearrayobject.That’swhensizematters.
ConstructinganArray
Constructinganarraymeanscreatingthearrayobjectontheheap(whereallobjectslive)—thatis,doing
anewonthearraytype.Tocreateanarrayobject,Javamustknowhowmuchspacetoallocateonthe
heap,soyoumustspecifythesizeofthearrayatcreationtime.Thesizeofthearrayisthenumberof
elementsthearraywillhold.
ConstructingOne-DimensionalArrays
Themoststraightforwardwaytoconstructanarrayistousethekeywordnewfollowedbythearraytype,
withabracketspecifyinghowmanyelementsofthattypethearraywillhold.Thefollowingisanexample
ofconstructinganarrayoftypeint:
Theprecedingcodeputsonenewobjectontheheap—anarrayobjectholdingfourelements—witheach
elementcontaininganintwithadefaultvalueof0.Thinkofthiscodeassayingtothecompiler,"Create
anarrayobjectthatwillholdfourints,andassignittothereferencevariablenamedtestScores.Also,
goaheadandseteachintelementtozero.Thanks."(Thecompilerappreciatesgoodmanners.)
Figure6-4showsthetestScoresarrayontheheap,afterconstruction.
FIGURE6-4 Aone-dimensionalarrayontheheap
Youcanalsodeclareandconstructanarrayinonestatement,asfollows:
int[]testScores=newint[4];
Thissinglestatementproducesthesameresultasthetwopreviousstatements.
Arraysofobjecttypescanbeconstructedinthesameway:
Rememberthat,despitehowthecodeappears,theThreadconstructorisnotbeinginvoked.We’renot
creatingaThreadinstance,butratherasingleThreadarrayobject.Aftertheprecedingstatement,there
arestillnoactualThreadobjects!
Thinkcarefullyabouthowmanyobjectsareontheheapafteracodestatementorblock
executes.Theexamwillexpectyoutoknow,forexample,thattheprecedingcodeproducesjust
oneobject(thearrayassignedtothereferencevariablenamedthreads).Thesingleobject
referencedbythreadsholdsfiveThreadreferencevariables,butnoThreadobjectshavebeen
createdorassignedtothosereferences.
Remember,arraysmustalwaysbegivenasizeatthetimetheyareconstructed.TheJVMneedsthesize
toallocatetheappropriatespaceontheheapforthenewarrayobject.Itisneverlegal,forexample,todo
thefollowing:
int[]carList=newint[];//Willnotcompile;needsasize
Sodon’tdoit,andifyouseeitonthetest,runscreamingtowardthenearestanswermarked"Compilation
fails."
Youmayseethewords"construct,""create,"and"instantiate"usedinterchangeably.Theyall
mean,"Anobjectisbuiltontheheap."Thisalsoimpliesthattheobject’sconstructorrunsas
aresultoftheconstruct/create/instantiatecode.Youcansaywithcertainty,forexample,that
anycodethatusesthekeywordnewwill(ifitrunssuccessfully)causetheclassconstructorand
allsuperclassconstructorstorun.
Inadditiontobeingconstructedwithnew,arrayscanbecreatedusingakindofsyntaxshorthandthat
createsthearraywhilesimultaneouslyinitializingthearrayelementstovaluessuppliedincode(as
opposedtodefaultvalues).We’lllookatthatinthenextsection.Fornow,understandthatbecauseof
thesesyntaxshortcuts,objectscanstillbecreatedevenwithoutyoueverusingorseeingthekeywordnew.
ConstructingMultidimensionalArrays
Multidimensionalarrays,remember,aresimplyarraysofarrays.Soatwo-dimensionalarrayoftypeint
isreallyanobjectoftypeintarray(int[]),witheachelementinthatarrayholdingareferenceto
anotherintarray.Theseconddimensionholdstheactualintprimitives.
Thefollowingcodedeclaresandconstructsatwo-dimensionalarrayoftypeint:
int[][]myArray=newint[3][];
Noticethatonlythefirstbracketsaregivenasize.That’sacceptableinJavabecausetheJVMneedsto
knowonlythesizeoftheobjectassignedtothevariablemyArray.
Figure6-5showshowatwo-dimensionalintarrayworksontheheap.
FIGURE6-5 Atwo-dimensionalarrayontheheap
InitializinganArray
Initializinganarraymeansputtingthingsintoit.The"things"inthearrayarethearray’selements,and
they’reeitherprimitivevalues(2,x,false,andsoon)orobjectsreferredtobythereferencevariablesin
thearray.Ifyouhaveanarrayofobjects(asopposedtoprimitives),thearraydoesn’tactuallyholdthe
objects—justasanyothernonprimitivevariableneveractuallyholdstheobject—butinsteadholdsa
referencetotheobject.Butwetalkaboutarraysas,forexample,"anarrayoffivestrings,"eventhough
whatwereallymeanis"anarrayoffivereferencestoStringobjects."Thenthebigquestionbecomes
whetherthosereferencesareactuallypointing(oops,thisisJava,wemeanreferring)torealString
objectsoraresimplynull.Remember,areferencethathasnothadanobjectassignedtoitisanull
reference.Andifyouactuallytrytousethatnullreferenceby,say,applyingthedotoperatorto
invokeamethodonit,you’llgettheinfamousNullPointerException.
Theindividualelementsinthearraycanbeaccessedwithanindexnumber.Theindexnumberalways
beginswithzero(0),soforanarrayoftenobjects,theindexnumberswillrunfrom0through9.Suppose
wecreateanarrayofthreeAnimalsasfollows:
Animal[]pets=newAnimal[3];
Wehaveonearrayobjectontheheap,withthreenullreferencesoftypeAnimal,butwedon’thave
anyAnimalobjects.ThenextstepistocreatesomeAnimalobjectsandassignthemtoindexpositionsin
thearrayreferencedbypets:
ThiscodeputsthreenewAnimalobjectsontheheapandassignsthemtothethreeindexpositions
(elements)inthepetsarray.
Lookforcodethattriestoaccessanout-of-rangearrayindex.Forexample,ifanarrayhas
threeelements,tryingtoaccesstheelement[3]willraisean
ArrayIndexOutOfBoundsException,becauseinanarrayofthreeelements,thelegalindex
valuesare0,1,and2.Youalsomightseeanattempttouseanegativenumberasanarray
index.Thefollowingareexamplesoflegalandillegalarrayaccessattempts.Besureto
recognizethatthesecauseruntimeexceptionsandnotcompilererrors!
Nearlyalltheexamquestionslistbothruntimeexceptionandcompilererroraspossible
answers:
numberThesecanbehardtospotinacomplexloop,butthat’swhereyou’remostlikelytosee
arrayindexproblemsinexamquestions.
Atwo-dimensionalarray(anarrayofarrays)canbeinitializedasfollows:
InitializingElementsinaLoop
Arrayobjectshaveasinglepublicvariable,length,thatgivesyouthenumberofelementsinthearray.
Thelastindexvalue,then,isalwaysonelessthanthelength.Forexample,ifthelengthofanarrayis
4,theindexvaluesarefrom0through3.Often,you’llseearrayelementsinitializedinaloop,asfollows:
Thelengthvariabletellsushowmanyelementsthearrayholds,butitdoesnottelluswhetherthose
elementshavebeeninitialized.
Declaring,Constructing,andInitializingonOneLine
Youcanusetwodifferentarray-specificsyntaxshortcutsbothtoinitialize(putexplicitvaluesintoan
array’selements)andconstruct(instantiatethearrayobjectitself)inasinglestatement.Thefirstisused
todeclare,create,andinitializeinonestatement,asfollows:
Line2intheprecedingcodedoesfourthings:
Declaresanintarrayreferencevariablenameddots.
Createsanintarraywithalengthofthree(threeelements).
Populatesthearray’selementswiththevalues6,9,and8.
Assignsthenewarrayobjecttothereferencevariabledots.
Thesize(lengthofthearray)isdeterminedbythenumberofcomma-separateditemsbetweenthecurly
braces.Thecodeisfunctionallyequivalenttothefollowinglongercode:
Thisbegsthequestion,"Whywouldanyoneusethelongerway?"Onereasoncomestomind.You
mightnotknow—atthetimeyoucreatethearray—thevaluesthatwillbeassignedtothearray’selements.
Withobjectreferencesratherthanprimitives,itworksexactlythesameway:
TheprecedingcodecreatesoneDogarray,referencedbythevariablemyDogs,withalengthofthree
elements.ItassignsapreviouslycreatedDogobject(assignedtothereferencevariablepuppy)tothefirst
elementinthearray.ItalsocreatestwonewDogobjects(CloverandAiko)andaddsthemtothelasttwo
DogreferencevariableelementsinthemyDogsarray.Thisarrayshortcutalone(combinedwiththe
stimulatingprose)isworththepriceofthisbook.Figure6-6showstheresult.
FIGURE6-6 Declaring,constructing,andinitializinganarrayofobjects
Youcanalsousetheshortcutsyntaxwithmultidimensionalarrays,asfollows:
int[][]scores={{5,2,4,7},{9,2},{3,4}};
Thiscodecreatesatotaloffourobjectsontheheap.First,anarrayofintarraysisconstructed(the
objectthatwillbeassignedtothescoresreferencevariable).Thescoresarrayhasalengthofthree,
derivedfromthenumberofcomma-separateditemsbetweentheoutercurlybraces.Eachofthethree
elementsinthescoresarrayisareferencevariabletoanintarray,sothethreeintarraysare
constructedandassignedtothethreeelementsinthescoresarray.
Thesizeofeachofthethreeintarraysisderivedfromthenumberofitemswithinthecorresponding
innercurlybraces.Forexample,thefirstarrayhasalengthoffour,thesecondarrayhasalengthoftwo,
andthethirdarrayhasalengthoftwo.Sofar,wehavefourobjects:onearrayofintarrays(each
elementisareferencetoanintarray),andthreeintarrays(eachelementinthethreeintarraysisan
intvalue).Finally,thethreeintarraysareinitializedwiththeactualintvalueswithintheinnercurly
braces.Thus,thefirstintarraycontainsthevalues5,2,4,7.Thefollowingcodeshowsthevaluesof
someoftheelementsinthistwo-dimensionalarray:
Figure6-7showstheresultofdeclaring,constructing,andinitializingatwo-dimensionalarrayinone
statement.
FIGURE6-7 Declaring,constructing,andinitializingatwo-dimensionalarray
ConstructingandInitializinganAnonymousArray
Thesecondshortcutiscalled"anonymousarraycreation"andcanbeusedtoconstructandinitializean
arrayandthenassignthearraytoapreviouslydeclaredarrayreferencevariable:
Theprecedingcodecreatesanewintarraywiththreeelements;initializesthethreeelementswiththe
values4,7,and2;andthenassignsthenewarraytothepreviouslydeclaredintarrayreferencevariable
testScores.Wecallthisanonymousarraycreationbecausewiththissyntax,youdon’tevenneedto
assignthenewarraytoanything.Maybeyou’rewondering,"Whatgoodisanarrayifyoudon’tassignitto
areferencevariable?"Youcanuseittocreateajust-in-timearraytouse,forexample,asanargumenttoa
methodthattakesanarrayparameter.
Thefollowingcodedemonstratesajust-in-timearrayargument:
Rememberthatyoudonotspecifyasizewhenusinganonymousarraycreationsyntax.The
sizeisderivedfromthenumberofitems(comma-separated)betweenthecurlybraces.Payvery
closeattentiontothearraysyntaxusedinexamquestions(andtherewillbealotofthem).You
mightseesyntaxsuchasthis:
LegalArrayElementAssignments
Whatcanyouputinaparticulararray?Fortheexam,youneedtoknowthatarrayscanhaveonlyone
declaredtype(int[],Dog[],String[],andsoon),butthatdoesn’tnecessarilymeanthatonlyobjects
orprimitivesofthedeclaredtypecanbeassignedtothearrayelements.Andwhataboutthearray
referenceitself?Whatkindofarrayobjectcanbeassignedtoaparticulararrayreference?Fortheexam,
you’llneedtoknowtheanswerstoallofthesequestions.And,asifbymagic,we’reactuallycovering
thoseverysametopicsinthefollowingsections.Payattention.
ArraysofPrimitivesPrimitivearrayscanacceptanyvaluethatcanbepromotedimplicitlytothe
declaredtypeofthearray.Forexample,anintarraycanholdanyvaluethatcanfitintoa32-bitint
variable.Thus,thefollowingcodeislegal:
ArraysofObjectReferencesIfthedeclaredarraytypeisaclass,youcanputobjectsofanysubclassof
thedeclaredtypeintothearray.Forexample,ifSubaruisasubclassofCar,youcanputbothSubaru
objectsandCarobjectsintoanarrayoftypeCarasfollows:
IthelpstorememberthattheelementsinaCararrayarenothingmorethanCarreferencevariables.So
anythingthatcanbeassignedtoaCarreferencevariablecanbelegallyassignedtoaCararrayelement.
Ifthearrayisdeclaredasaninterfacetype,thearrayelementscanrefertoanyinstanceofanyclass
thatimplementsthedeclaredinterface.Thefollowingcodedemonstratestheuseofaninterfaceasan
arraytype:
Thebottomlineisthis:anyobjectthatpassestheIS-Atestforthedeclaredarraytypecanbeassigned
toanelementofthatarray.
ArrayReferenceAssignmentsforOne-DimensionalArraysFortheexam,youneedtorecognizelegal
andillegalassignmentsforarrayreferencevariables.We’renottalkingaboutreferencesinthearray(in
otherwords,arrayelements),butratherreferencestothearrayobject.Forexample,ifyoudeclareanint
array,thereferencevariableyoudeclaredcanbereassignedtoanyintarray(ofanysize),butthe
variablecannotbereassignedtoanythingthatisnotanintarray,includinganintvalue.Remember,all
arraysareobjects,soanintarrayreferencecannotrefertoanintprimitive.Thefollowingcode
demonstrateslegalandillegalassignmentsforprimitivearrays:
It’stemptingtoassumethatbecauseavariableoftypebyte,short,orcharcanbeexplicitly
promotedandassignedtoanint,anarrayofanyofthosetypescouldbeassignedtoanintarray.You
can’tdothatinJava,butitwouldbejustlikethosecruel,heartless(butotherwiseattractive)exam
developerstoputtrickyarrayassignmentquestionsintheexam.
Arraysthatholdobjectreferences,asopposedtoprimitives,aren’tasrestrictive.Justasyoucanputa
HondaobjectinaCararray(becauseHondaextendsCar),youcanassignanarrayoftypeHondatoaCar
arrayreferencevariableasfollows:
ApplytheIS-Atesttohelpsortthelegalfromtheillegal.HondaIS-ACar,soaHondaarraycanbe
assignedtoaCararray.BeerIS-ACarisnottrue;BeerdoesnotextendCar(plusitdoesn’tmakesense,
unlessyou’vealreadyhadtoomuchofit).
Therulesforarrayassignmentapplytointerfacesaswellasclasses.Anarraydeclaredasaninterface
typecanreferenceanarrayofanytypethatimplementstheinterface.Remember,anyobjectfromaclass
implementingaparticularinterfacewillpasstheIS-A(instanceof)testforthatinterface.Forexample,
ifBoximplementsFoldable,thefollowingislegal:
Youcannotreversethelegalassignments.ACararraycannotbeassignedtoaHondaarray.A
CarisnotnecessarilyaHonda,soifyou’vedeclaredaHondaarray,itmightblowupifyou
assignedaCararraytotheHondareferencevariable.Thinkaboutit:aCararraycouldholda
referencetoaFerrari,sosomeonewhothinkstheyhaveanarrayofHondascouldsuddenly
findthemselveswithaFerrari.RememberthattheIS-Atestcanbecheckedincodeusingthe
instanceofoperator.
ArrayReferenceAssignmentsforMultidimensionalArraysWhenyouassignanarraytoapreviously
declaredarrayreference,thearrayyou’reassigningmustbeinthesamedimensionasthereferenceyou’re
assigningitto.Forexample,atwo-dimensionalarrayofintarrayscannotbeassignedtoaregularint
arrayreference,asfollows:
Payparticularattentiontoarrayassignmentsusingdifferentdimensions.Youmight,forexample,be
askedifit’slegaltoassignanintarraytothefirstelementinanarrayofintarrays,asfollows:
Figure6-8showsanexampleoflegalandillegalassignmentsforreferencestoanarray.
FIGURE6-8 Legalandillegalarrayassignments
CERTIFICATIONOBJECTIVE
UsingArrayListsandWrappers(OCAObjectives9.4and
2.5)
9.3DeclareanduseanArrayListofagiventype.
2.5DevelopcodethatuseswrapperclassessuchasBoolean,Double,andInteger.
Datastructuresareapartofalmosteveryapplicationyou’lleverworkon.TheJavaAPIprovides
anextensiverangeofclassesthatsupportcommondatastructuressuchasLists,Sets,Maps,and
Queues.ForthepurposeoftheOCAexam,youshouldrememberthattheclassesthatsupportthese
commondatastructuresareapartofwhatisknownas"TheCollectionAPI"(oneofitsmanyaliases).
(TheOCPexamcoversthemostcommonimplementationsofallthesestructures.)
WhentoUseArrayLists
We’vealreadytalkedaboutarrays.Arraysseemusefulandprettydarnedflexible.Sowhydoweneed
morefunctionalitythanarraysprovide?Considerthesetwosituations:
Youneedtobeabletoincreaseanddecreasethesizeofyourlistofthings.
Theorderofthingsinyourlistisimportantandmightchange.
Bothsituationscanbehandledwitharrays,butit’snoteasy….
SupposeyouwanttoplanavacationtoEurope.Youhaveseveraldestinationsinmind(Paris,Oslo,
Rome),butyou’renotyetsureinwhatorderyouwanttovisitthesecities,andasyourplanning
progresses,youmightwanttoaddorsubtractcitiesfromyourlist.Let’ssayyourfirstideaistotravel
fromnorthtosouth,soyourlistlookslikethis:
Oslo,Paris,Rome.
Ifwewereusinganarray,wecouldstartwiththis:
String[]cities={"Oslo","Paris","Rome"};
ButnowimaginethatyourememberthatyouREALLYwanttogotoLondon,too!You’vegottwo
problems:
Yourcitiesarrayisalreadyfull.
Ifyou’regoingfromnorthtosouth,youneedtoinsertLondonbeforeParis.
Ofcourse,youcanfigureoutawaytodothis.Maybeyoucreateasecondarray,andyoucopycities
fromonearraytotheother,andatthecorrectmomentyouaddLondontothesecondarray.Doable,but
difficult.
Nowlet’sseehowyoucoulddothesamethingwithanArrayList:
Theoutputwillbesomethinglikethis:
Byreviewingthecode,wecanlearnsomeimportantfactsaboutArrayLists:
TheArrayListclassisinthejava.utilpackage.
Similartoarrays,whenyoubuildanArrayList,youhavetodeclarewhatkindofobjectsit
cancontain.Inthiscase,we’rebuildinganArrayListofStringobjects.(We’lllookatthelineof
codethatcreatestheArrayListinalotmoredetailinaminute.)
ArrayListimplementstheListinterface.
WeworkwiththeArrayListthroughmethods.Inthiscaseweusedacoupleofversionsof
add();weusedindexOf();and,indirectly,weusedtoString()todisplaytheArrayList’s
contents.(MoreontoString()inaminute.)
Likearrays,indexesforArrayListsarezero-based.
Wedidn’tdeclarehowbigtheArrayListwaswhenwebuiltit.
WewereabletoaddanewelementtotheArrayListonthefly.
Wewereabletoaddthenewelementinthemiddleofthelist.
TheArrayListmaintaineditsorder.
Aspromised,weneedtolookatthefollowinglineofcodemoreclosely:
List<String>c=newArrayList<String>();
Firstoff,weseethatthisisapolymorphicdeclaration.Aswesaidearlier,ArrayListimplementsthe
Listinterface(alsoinjava.util).IfyouplantotaketheOCP8examafteryou’veacedtheOCA8,
you’lllearnalotmoreaboutwhywemightwanttodoapolymorphicdeclaration.Fornow,imaginethat
somedayyoumightwanttocreateaListofyourArrayLists.
Next,wehavethisweird-lookingsyntaxwiththe<and>characters.Thissyntaxwasaddedtothe
languageinJava5,andithastodowith"generics."Genericsaren’treallyincludedintheOCAexam,so
wedon’twanttospendalotoftimeonthemhere,butwhat’simportanttoknowisthatthisishowyoutell
thecompilerandtheJVMthatforthisparticularArrayListyouwantonlyStringstobeallowed.What
thismeansisifthecompilercantellthatyou’retryingtoadda"not-a-String"objecttothisArrayList,
yourcodewon’tcompile.Thisisagoodthing!
Alsoaspromised,let’slookatTHISlineofcode:
System.out.println(c);
RememberthatallclassesultimatelyinheritfromclassObject.ClassObjectcontainsamethodcalled
toString().Again,toString()isn’t"officially"ontheOCAexam(ofcourse,itISintheOCPexam!),
butyouneedtounderstanditabitfornow.Whenyoupassanobjectreferencetoeither
System.out.print()orSystem.out.println(),you’retellingthemtoinvokethatobject’s
toString()method.(Wheneveryoumakeanewclass,youcanoptionallyoverridethetoString()
methodyourclassinheritedfromObjecttoshowusefulinformationaboutyourclass’sobjects.)TheAPI
developerswereniceenoughtooverrideArrayList’stoString()methodforyoutoshowthecontents
oftheArrayList,asyousawintheprogram’soutput.Hooray!
ArrayListsandDuplicates
Asyou’replanningyourtriptoEurope,yourealizethathalfwaythroughyourstayinRome,there’sgoing
tobeafantasticmusicfestivalinNaples!NaplesisjustdownthecoastfromRome!You’vegottoadd
thatsidetriptoyouritinerary.Thequestionis,cananArrayListhaveduplicateentries?Isitlegaltosay
this:
Andtheshortansweris:Yes,ArrayListscanhaveduplicates.Nowifyoustopandthinkaboutit,the
notionof"duplicateJavaobjects"isactuallyabittricky.Relax,becauseyouwon’thavetogetintothat
trickinessuntilyoustudyfortheOCP8.
Technicallyspeaking,ArrayListsholdonlyobjectreferences,notactualobjectsandnot
primitives.Ifyouseecodelikethis,
myArrayList.add(7);
what’sreallyhappeningistheintisbeingautoboxed(converted)intoanIntegerobjectand
thenaddedtotheArrayList.We’lltalkmoreaboutautoboxinginafewpages.
ArrayListMethodsinAction
Let’slookatanotherpieceofcodethatshowsoffmostoftheArrayListmethodsyouneedtoknowfor
theexam:
whichshouldproducesomethinglikethis:
Acoupleofquicknotesaboutthiscode:Firstoff,noticethatcontains()returnsaboolean.This
makescontains()greattousein"if"tests.Second,noticethatArrayListhasasize()method.It’s
importanttorememberthatarrayshavealengthattributeandArrayListshaveasize()method.
ImportantMethodsintheArrayListClass
ThefollowingmethodsaresomeofthemorecommonlyusedmethodsintheArrayListclassandalso
thosethatyou’remostlikelytoencounterontheexam:
add(element)AddsthiselementtotheendoftheArrayList
add(index,element)Addsthiselementattheindexpointandshiftstheremainingelements
back(forexample,whatwasatindexisnowatindex+1)
clear()RemovesalltheelementsfromtheArrayList
booleancontains(element)Returnswhethertheelementisinthelist
Objectget(index)ReturnstheObjectlocatedatindex
intindexOf(Object)Returnsthe(int)locationoftheelementor-1iftheObjectisnot
found
remove(index)Removestheelementatthatindexandshiftslaterelementstowardthe
beginningonespace
remove(Object)RemovesthefirstoccurrenceoftheObjectandshiftslaterelements
towardthebeginningonespace
intsize()ReturnsthenumberofelementsintheArrayList
Tosummarize,theOCA8examtestsonlyforverybasicknowledgeofArrayLists.Ifyougoonto
taketheOCP8exam,you’lllearnalotmoreaboutArrayListsandothercommoncollections-oriented
classes.
AutoboxingwithArrayLists
Ingeneral,collectionslikeArrayListcanholdobjectsbutnotprimitives.PriortoJava5,acommonuse
fortheso-calledwrapperclasses(e.g.,Integer,Float,Boolean,andsoon)wastoprovideawayto
getprimitivesintoandoutofcollections.PriortoJava5,youhadto"wrap"aprimitivemanuallybefore
youcouldputitintoacollection.AsofJava5,primitivesstillhavetobewrappedbeforetheycanbe
addedtoArrayLists,butautoboxingtakescareofitforyou.
Inthepreviousexample,wecreateaninstanceofclassIntegerwithavalueof42.We’vecreatedan
entireobjectto"wraparound"aprimitivevalue.AsofJava5,wecansay:
Inthislastexample,wearestilladdinganIntegerobjecttomyInts(notanintprimitive);it’sjustthat
autoboxinghandlesthewrappingforus.Therearesomesneakyimplicationswhenweneedtouse
wrapperobjects;let’stakeacloserlook…
Intheold,pre–Java5days,ifyouwantedtomakeawrapper,unwrapit,useit,andthenrewrapit,you
mightdosomethinglikethis:
Nowyoucansay:
Bothexamplesproducethefollowingoutput:
y=568
Andyes,youreadthatcorrectly.Thecodeappearstobeusingthepostincrementoperatoronanobject
referencevariable!Butit’ssimplyaconvenience.Behindthescenes,thecompilerdoestheunboxingand
reassignmentforyou.Earlier,wementionedthatwrapperobjectsareimmutable…thisexampleappears
tocontradictthatstatement.Itsurelookslikey’svaluechangedfrom567to568.Whatactuallyhappened,
however,isthatasecondwrapperobjectwascreatedanditsvaluewassetto568.Ifonlywecould
accessthatfirstwrapperobject,wecouldproveit….
Let’strythis:
whichproducestheoutput:
So,underthecovers,whenthecompilergottotheliney++;ithadtosubstitutesomethinglikethis:
Justaswesuspected,there’sgottabeacalltonewintheresomewhere.
AllthewrapperclassesexceptCharacterprovidetwoconstructors:onetakesaprimitiveof
thetypebeingconstructed,andtheothertakesaStringrepresentationofthetypebeing
constructed.Forexample,
arebothvalidwaystoconstructanewIntegerobject(that"wraps"thevalue42).
Boxing,==,andequals()
Wejustused==todoalittleexplorationofwrappers.Let’stakeamorethoroughlookathowwrappers
workwith==,!=,andequals().TheAPIdevelopersdecidedthatforallthewrapperclasses,two
objectsareequaliftheyareofthesametypeandhavethesamevalue.Itshouldn’tbesurprisingthat
producestheoutput
It’sjusttwowrapperobjectshappentohavethesamevalue.Becausetheyhavethesameintvalue,
theequals()methodconsidersthemtobe"meaningfullyequivalent"and,therefore,returnstrue.How
aboutthisone?
Thisexampleproducestheoutput:
Yikes!Theequals()methodseemstobeworking,butwhathappenedwith==and!=?Whyis!=
tellingusthati1andi2aredifferentobjects,when==issayingthati3andi4arethesameobject?In
ordertosavememory,twoinstancesofthefollowingwrapperobjects(createdthroughboxing)will
alwaysbe==whentheirprimitivevaluesarethesame:
Boolean
Byte
Characterfrom\u0000to\u007f(7fis127indecimal)
ShortandIntegerfrom–128to127
When==isusedtocompareaprimitivetoawrapper,thewrapperwillbeunwrappedandthe
comparisonwillbeprimitivetoprimitive.
WhereBoxingCanBeUsed
Aswediscussedearlier,it’scommontousewrappersinconjunctionwithcollections.Anytimeyouwant
yourcollectiontoholdobjectsandprimitives,you’llwanttousewrapperstomakethoseprimitives
collection-compatible.Thegeneralruleisthatboxingandunboxingworkwhereveryoucannormallyuse
aprimitiveorawrappedobject.Thefollowingcodedemonstratessomelegalwaystouseboxing:
Remember,wrapperreferencevariablescanbenull.Thatmeansyouhavetowatchoutfor
codethatappearstobedoingsafeprimitiveoperationsbutthatcouldthrowa
NullPointerException:
Thiscodecompilesfine,buttheJVMthrowsaNullPointerExceptionwhenitattemptsto
invokedoStuff(x)becausexdoesn’trefertoanIntegerobject,sothere’snovaluetounbox.
TheJava7"Diamond"Syntax
Earlierinthebook,wediscussedseveralsmalladditions/improvementstothelanguagethatwereadded
underthename"ProjectCoin."ThelastProjectCoinimprovementwe’lldiscussisthe"diamondsyntax."
We’vealreadyseenseveralexamplesofdeclaringtype-safeArrayListslikethis:
Noticethatthetypeparametersareduplicatedinthesedeclarations.AsofJava7,thesedeclarations
couldbesimplifiedto:
NoticethatinthesimplerJava7declarations,therightsideofthedeclarationincludedthetwocharacters
"<>,"whichtogethermakeadiamondshape—doh!
Youcannotswapthese;forexample,thefollowingdeclarationisNOTlegal:
Forthepurposesoftheexam,that’sallyou’llneedtoknowaboutthediamondoperator.Forthe
remainderofthebook,we’llusethepre-diamondsyntaxandtheJava7diamondsyntaxsomewhat
randomly—justliketherealworld!
CERTIFICATIONOBJECTIVE
AdvancedEncapsulation(OCAObjective6.5)
6.5Applyencapsulationprinciplestoaclass.
EncapsulationforReferenceVariables
InChapter2webeganourdiscussionoftheobject-orientedconceptofencapsulation.Atthatpoint,we
limitedourdiscussiontoprotectingaclass’sprimitivefieldsand(immutable)Stringfields.Nowthat
you’velearnedmoreaboutwhatitmeansto"pass-by-copy"andwe’velookedatnonprimitivewaysof
handlingdatasuchasarrays,StringBuilders,andArrayLists,it’stimetotakeacloserlookat
encapsulation.
Let’ssaywehavesomespecialdatawhosevaluewe’resavinginaStringBuilder.We’rehappyto
sharethevaluewithotherprogrammers,butwedon’twantthemtochangethevalue:
Whenwerunthecode,wegetthis:
bobfred
Uhoh!Itlookslikewepracticedgoodencapsulationtechniquesbymakingourfieldprivateand
providinga"getter"method,butbasedontheoutput,it’sclearthatwedidn’tdoaverygoodjobof
protectingthedataintheSpecialclass.Canyoufigureoutwhy?Takeaminute….
Okay—justtoverifyyouranswer—whenweinvokegetName(),wedo,infact,returnacopy,justlike
Javaalwaysdoes.Butwe’renotreturningacopyoftheStringBuilderobject;we’rereturningacopy
ofthereferencevariablethatpointsto(Iknow)theoneandonlyStringBuilderobjectweeverbuilt.So
atthepointthatgetName()returns,wehaveoneStringBuilderobjectandtworeferencevariables
pointingtoit(sands2).
ForthepurposeoftheOCAexam,thekeypointisthis:Whenencapsulatingamutableobjectlikea
StringBuilder,oranarray,oranArrayList,ifyouwanttoletoutsideclasseshaveacopyofthe
object,youmustactuallycopytheobjectandreturnareferencevariabletotheobjectthatisacopy.Ifall
youdoisreturnacopyoftheoriginalobject’sreferencevariable,youDONOThaveencapsulation.
CERTIFICATIONOBJECTIVE
UsingSimpleLambdas(OCAObjective9.5)
9.5WriteasimpleLambdaexpressionthatconsumesaLambdaPredicateexpression.
Java8isprobablybestknownastheversionofJavathatfinallyaddedlambdasandstreams.These
twonewfeatures(lambdasandstreams)giveprogrammerstoolstotacklesomecommonandcomplex
problemswitheasier-to-read,moreconcise,and,inmanycases,faster-runningcode.Thecreatorsofthe
OCA8examfeltthat,ingeneral,lambdasandstreamsaretopicsmoreappropriatefortheOCP8exam,
buttheywantedOCA8candidatestogetanintroduction,perhapstowhettheirappetite…
Inthissection,we’regoingtodoareallybasicintroductiontolambdas.Wesuspectthisdiscussion
willraisesomequestionsinyourmind,andwe’resorryforthat,butwe’regoingtorestrictourselvesto
justtheintroductionthattheexamcreatorshadinmind.
Ontherealexam,youshouldexpecttoseemanyquestionsthattestformorethanone
objective.Inthefollowingpages,aswediscusslambdas,we’llbeleaningheavilyon
ArrayListsandwrapperclasses.You’llseethatwecombineArrayLists,wrappers,AND
lambdasintomanyofourcodelistings,andwealsousethiscombinationinthemockexam
questionsweprovide.Therealexam(andreal-lifeprogramming)willdothesame.
Supposeyou’recreatinganapplicationforaveterinaryhospital.Wewanttofocusonthatpartofthe
applicationthatallowsthevetstogetsummaryinformationaboutallthedogsthattheyworkwith.Here’s
ourDogclass:
Nowlet’swritesometestcodetocreatesomesampleDogs,putthemintoanArrayListaswego,and
thenrunsome"queries"againsttheArrayList.
Firsthere’sthesummarypseudocode:
Here’stheactualtestcode:
whichproducesthefollowing(predictablewehope)output:
You’reprobablywayaheadofushere,butnoticehowsimilartheminAge()andmaxWeight()
methodsare.AnditshouldbeeasytoimagineothersimilarmethodswithnameslikemaxAge()or
namesStartingWithC().Sothelineofcodewewanttofocusonis:
Whatif—justsayin’—wecouldcreateasingleDog-queryingmethod(insteadofthemanywe’vebeen
contemplating)andpassitthequeryexpressionwewantedittouse?Itwouldsortoflooklikethis:
Nowwe’rethinkingaboutpassingcodeasanargument?Lambdasletusdojustthat!Let’slookatour
dogQuerier()methodalittlemoreclosely.Firstoff,thecodewe’regoingtopassinisgoingtobeused
astheexpressioninanifstatement.Whatdoweknowaboutifexpressions?Right!Theyhaveto
resolvetoabooleanvalue.Sowhenwedeclareourmethod,thesecondargument(theonethat’sgoingto
holdthepassed-in-code)hastobedeclaredasaboolean.ThefolkswhobroughtusJava8andlambdas
andstreamsprovidedabunchofnewinterfacesintheAPI,andoneofthemostusefuloftheseisthe
java.util.function.Predicateinterface.ThePredicateinterfacehassomeofthosenew-fangled
staticanddefaultinterfacemethods,wediscussedearlierinthebook,and,mostimportantlyforus,it
hasonenonconcretemethodcalledtest()thatreturns—youguessedit—aboolean.
Here’sthemultipurposedogQuerier()method:
Sofarthislookslikegood-oldJava;it’swhenweinvokedogQuerier()thatthesyntaxgets
interesting:
Whenwesay[c]d->d.getAge()<9[/c]—THATisthelambdaexpression.Thedrepresentsthe
argument,andthenthecodemustreturnaboolean.Let’sputallofthistogetherinanewversionof
TestDogs:
whichproducesthefollowingoutput(thelasttwolinesgeneratedusinglambdas!):
Let’sstepbacknowandcoversomesyntaxrules.ThefollowingrulesareforthepurposesoftheOCA
8examonly!IfyoudecidetoearnyourOCP8,you’lldoamuchdeeperdiveintolambdas,andthereare
lotsof"cansofworms"you’llhavetoopen,whichwe’repurposelygoingtoavoid.Sowhatfollowsisan
OCA8appropriatesimplification.
ThebasicsyntaxforaPredicatelambdahasthreeparts:
Othertypesoflambdastakezeroormoreparameters,butfortheOCA8,we’refocusedexclusivelyon
thePredicate,whichmusttakeexactlyoneparameter.Herearesomedetailedsyntaxrulesfor
Predicatelambdas:
Theparametercanbejustavariablename,oritcanbethetypefollowedbyavariablename
allinparentheses.
ThebodyMUST(onewayoranother)returnaboolean.
Thebodycanbeasingleexpression,whichcannothaveareturnstatement.
Thebodycanbeacodeblocksurroundedbycurlybraces,containingoneormorevalid
statements,eachendingwithasemicolon,andtheblockmustendwithareturnstatement.
FollowingisacodelistingthatshowsexamplesoflegalandthenillegalexamplesofPredicate
lambdas:
Thiscodeismostlyaboutvalidandinvalidsyntax,butlet’slookalittlemorecloselyatthego()method.
Thetestismainlyconcernedwiththecodetobepassedtoamethod,butit’susefultolook(butnotTOO
closely)atamethodthatreceiveslambdacode.InboththeDogscodeandthecodedirectlyabove,the
receivingmethodtookaPredicate.Insidethereceivingmethods,wecreatedanobjectofthetypewe’re
workingwith,whichwepasstothePredicate.test()method.Thereceivingmethodexpectsthe
test()methodtoreturnaboolean.
Wehavetoadmitthatlambdasareabittrickytolearn.Again,weexpectwe’veleftyouwithsome
unansweredquestions,butwethinkOracledidareasonablejobofslicingoutapieceofthelambda
puzzletostartwith.Ifyouunderstandthebitswe’vecovered,youshouldbeabletohandlethelambda-
relatedquestionsOraclethrowsyou.
CERTIFICATIONSUMMARY
ThemostimportantthingtorememberaboutStringsisthatStringobjectsareimmutable,but
referencestoStringsarenot!YoucanmakeanewStringbyusinganexistingStringasastarting
point,butifyoudon’tassignareferencevariabletothenewString,itwillbelosttoyourprogram—you
willhavenowaytoaccessyournewString.ReviewtheimportantmethodsintheStringclass.
TheStringBuilderclasswasaddedinJava5.Ithasexactlythesamemethodsastheold
StringBufferclass,exceptStringBuilder’smethodsaren’tthread-safe.BecauseStringBuilder’s
methodsarenotthread-safe,theytendtorunfasterthanStringBuffermethods,sochoose
StringBuilderwheneverthreadingisnotanissue.BothStringBufferandStringBuilderobjects
canhavetheirvaluechangedoverandoverwithoutyourhavingtocreatenewobjects.Ifyou’redoinga
lotofstringmanipulation,theseobjectswillbemoreefficientthanimmutableStringobjects,whichare,
moreorless,"useonce,remaininmemoryforever."Remember,thesemethodsALWAYSchangethe
invokingobject’svalue,evenwithnoexplicitassignment.
NextwediscussedkeyclassesandinterfacesinthenewJava8calendarandtime-relatedpackages.
SimilartoStrings,allofthecalendarclasseswestudiedcreateimmutableobjects.Inaddition,these
classesusefactorymethodsexclusivelytocreatenewobjects.Thekeywordnewcannotbeusedwith
theseclasses.Welookedatsomeofthepowerfulfeaturesoftheseclasses,likecalculatingtheamountof
timebetweentwodifferentdatesortimes.ThenwetookalookathowtheDateTimeFormatterclassis
usedtoparseStringsintocalendarobjectsandhowitisusedtobeautifycalendarobjects.
Thenexttopicwasarrays.Wetalkedaboutdeclaring,constructing,andinitializingone-dimensional
andmultidimensionalarrays.Wetalkedaboutanonymousarraysandthefactthatarraysofobjectsare
actuallyarraysofreferencestoobjects.
Next,wediscussedthebasicsofArrayLists.ArrayListsarelikearrayswithsuperpowersthat
allowthemtogrowandshrinkdynamicallyandtomakeiteasyforyoutoinsertanddeleteelementsat
locationsofyourchoosingwithinthelist.WediscussedtheideathatArrayListscannotholdprimitives,
andthatifyouwanttomakeanArrayListfilledwithagiventypeofprimitivevalues,youuse
"wrapper"classestoturnaprimitivevalueintoanobjectthatrepresentsthatvalue.Thenwediscussed
howwithautoboxing,turningprimitivesintowrapperobjects,andviceversa,isdoneautomatically.
Finally,wediscussedaspecificsubsetofthetopicoflambdas,usingthePredicateinterface.The
basicideaoflambdasisthatyoucanpassabitofcodefromonemethodtoanother.ThePredicate
interfaceisoneofmany"functionalinterfaces"providedintheJava8API.Afunctionalinterfaceisone
thathasonlyonemethodtobeimplemented.InthecaseofthePredicateinterface,thismethodiscalled
test(),andittakesasingleargumentandreturnsaboolean.Towrapupourdiscussionoflambdas,we
coveredsomeofthetrickysyntaxrulesyouneedtoknowtowritevalidlambdas.
TWO-MINUTEDRILL
Herearesomeofthekeypointsfromthecertificationobjectivesinthischapter.
UsingStringandStringBuilder(OCAObjectives9.2and9.1)
Stringobjectsareimmutable,andStringreferencevariablesarenot.
IfyoucreateanewStringwithoutassigningit,itwillbelosttoyourprogram.
IfyouredirectaStringreferencetoanewString,theoldStringcanbelost.
Stringmethodsusezero-basedindexes,exceptforthesecondargumentofsubstring().
TheStringclassisfinal—itcannotbeextended.
WhentheJVMfindsaStringliteral,itisaddedtotheStringliteralpool.
Stringshaveamethodcalledlength()—arrayshaveanattributenamedlength.
StringBuilderobjectsaremutable—theycanchangewithoutcreatinganewobject.
StringBuildermethodsactontheinvokingobject,andobjectscanchangewithoutan
explicitassignmentinthestatement.
Rememberthatchainedmethodsareevaluatedfromlefttoright.
Stringmethodstoremember:charAt(),concat(),equalsIgnoreCase(),length(),
replace(),substring(),toLowerCase(),toString(),toUpperCase(),andtrim().
StringBuildermethodstoremember:append(),delete(),insert(),reverse(),and
toString().
ManipulatingCalendarData(OCAObjective9.3)
Ontheexamalltheobjectscreatedusingthecalendarclassesareimmutable,buttheir
referencevariablesarenot.
Ifyoucreateanewcalendarobjectwithoutassigningit,itwillbelosttoyourprogram.
Ifyouredirectacalendarreferencetoanewcalendarobject,theoldcalendarobjectcanbe
lost.
Alloftheobjectscreatedusingtheexam’scalendarclassesmustbecreatedusingfactory
methods(e.g.,from(),now(),of(),parse());thekeywordnewisnotallowed.
Theuntil()andbetween()methodsperformcomplexcalculationsthatdeterminethe
amountoftimebetweenthevaluesoftwocalendarobjects.
TheDateTimeFormatterclassusestheparse()methodtoparseinputStringsintovalid
calendarobjects.
TheDateTimeFormatterclassusestheformat()methodtoformatcalendarobjectsinto
beautifullyformedStrings.
UsingArrays(OCAObjectives4.1and4.2)
Arrayscanholdprimitivesorobjects,butthearrayitselfisalwaysanobject.
Whenyoudeclareanarray,thebracketscanbetotheleftorrightofthename.
Itisneverlegaltoincludethesizeofanarrayinthedeclaration.
Youmustincludethesizeofanarraywhenyouconstructit(usingnew)unlessyouarecreating
ananonymousarray.
Elementsinanarrayofobjectsarenotautomaticallycreated,althoughprimitivearray
elementsaregivendefaultvalues.
You’llgetaNullPointerExceptionifyoutrytouseanarrayelementinanobjectarrayif
thatelementdoesnotrefertoarealobject.
Arraysareindexedbeginningwithzero.
AnArrayIndexOutOfBoundsExceptionoccursifyouuseabadindexvalue.
Arrayshavealengthattributewhosevalueisthenumberofarrayelements.
Thelastindexyoucanaccessisalwaysonelessthanthelengthofthearray.
Multidimensionalarraysarejustarraysofarrays.
Thedimensionsinamultidimensionalarraycanhavedifferentlengths.
Anarrayofprimitivescanacceptanyvaluethatcanbepromotedimplicitlytothearray’s
declaredtype—forexample,abytevariablecangoinanintarray.
AnarrayofobjectscanholdanyobjectthatpassestheIS-A(orinstanceof)testforthe
declaredtypeofthearray.Forexample,ifHorseextendsAnimal,thenaHorseobjectcangointo
anAnimalarray.
Ifyouassignanarraytoapreviouslydeclaredarrayreference,thearrayyou’reassigning
mustbethesamedimensionasthereferenceyou’reassigningitto.
Youcanassignanarrayofonetypetoapreviouslydeclaredarrayreferenceofoneofits
supertypes.Forexample,aHondaarraycanbeassignedtoanarraydeclaredastypeCar(assuming
HondaextendsCar).
UsingArrayList(OCAObjective9.4)
ArrayListsallowyoutoresizeyourlistandmakeinsertionsanddeletionstoyourlistfar
moreeasilythanarrays.
ArrayListsareorderedbydefault.Whenyouusetheadd()methodwithnoindexargument,
thenewentrywillbeappendedtotheendoftheArrayList.
FortheOCA8exam,theonlyArrayListdeclarationsyouneedtoknowareofthisform:
ArrayListscanholdonlyobjects,notprimitives,butrememberthatautoboxingcanmakeit
looklikeyou’readdingprimitivestoanArrayListwhen,infact,you’readdingawrapperobject
versionofaprimitive.
AnArrayList’sindexstartsat0.
ArrayListscanhaveduplicateentries.Note:Determiningwhethertwoobjectsare
duplicatesistrickierthanitseemsanddoesn’tcomeupuntiltheOCP8exam.
ArrayListmethodstoremember:add(element),add(index,element),clear(),
contains(object),get(index),indexOf(object),remove(index),remove(object),and
size().
EncapsulatingReferenceVariables(OCAObjective6.5)
IfyouwanttoencapsulatemutableobjectslikeStringBuildersorarraysorArrayLists,
youcannotreturnareferencetotheseobjects;youmustfirstmakeacopyoftheobjectandreturna
referencetothecopy.
Anyclassthathasamethodthatreturnsareferencetoamutableobjectisbreaking
encapsulation.
UsingPredicateLambdaExpressions(OCAObjective9.5)
Lambdasallowyoutopassbitsofcodefromonemethodtoanother.Andthereceiving
methodcanrunwhatevercomplyingcodeitissent.
WhiletherearemanytypesoflambdasthatJava8supports,forthisexam,theonlylambda
typeyouneedtoknowisthePredicate.
ThePredicateinterfacehasasinglemethodtoimplementthat’scalledtest(),andittakes
oneargumentandreturnsaboolean.
AsthePredicate.test()methodreturnsaboolean,itcanbeplaced(mostly?)wherevera
booleanexpressioncango,e.g.,inif,while,do,andternarystatements.
Predicatelambdaexpressionshavethreeparts:asingleargument,anarrow(->),andan
expressionorcodeblock.
APredicatelambdaexpression’sargumentcanbejustavariableoratypeandvariable
togetherinparentheses,e.g.,(MyClassm).
APredicatelambdaexpression’sbodycanbeanexpressionthatresolvestoaboolean,OR
itcanbeablockofstatements(surroundedbycurlybraces)thatendswithaboolean-returning
returnstatement.
SELFTEST
1.Given:
Whichtwosubstringswillbeincludedintheresult?(Choosetwo.)
2.Given:
And,ifthecodecompiles,thecommandline:
javaHilltopeyravafidraumurkara
Whatistheresult?
A.EYRAVAFIDRAUMURKARA
B.EYRAVAFIDRAUMURKARAnull
C.Anexceptionisthrownwithnootheroutput
D.EYRAVAFIDRAUMURKARA,andthenaNullPointerException
E.EYRAVAFIDRAUMURKARA,andthenanArrayIndexOutOfBoundsException
F.Compilationfails
3.Given:
Whatistheresult?
A.truetrue
B.truefalse
C.falsetrue
D.falsefalse
E.Compilationfails
4.Given:
Whatistheresult?(Chooseallthatapply.)
A.2
B.4
C.Anexceptionisthrownatruntime
D.Compilationfailsduetoanerroronline4
E.Compilationfailsduetoanerroronline5
F.Compilationfailsduetoanerroronline6
G.Compilationfailsduetoanerroronline7
5.Given:
Whatistheresult?
A.[apple,banana,carrot,plum]
B.[apple,plum,carrot,banana]
C.[apple,plum,banana,carrot]
D.[plum,banana,carrot,apple]
E.[plum,apple,carrot,banana]
F.[banana,plum,carrot,apple]
G.Compilationfails
6.Given:
Whichtwoaretrueabouttheobjectscreatedwithinmain(),andwhichareeligibleforgarbage
collectionwhenline14isreached?
A.Threeobjectswerecreated
B.Fourobjectswerecreated
C.Fiveobjectswerecreated
D.ZeroobjectsareeligibleforGC
E.OneobjectiseligibleforGC
F.TwoobjectsareeligibleforGC
G.ThreeobjectsareeligibleforGC
7.Given:
Whatistheresult?
A.24
B.27
C.32
D.37
E.42
F.47
G.Compilationfails
8.Given:
Whicharetrue?(Chooseallthatapply.)
A.Compilationfails
B.Thefirstlineofoutputisabcabctrue
C.Thefirstlineofoutputisabcabcfalse
D.Thefirstlineofoutputisabcdabcfalse
E.Thesecondlineofoutputisabcdabcfalse
F.Thesecondlineofoutputisabcdabcdtrue
G.Thesecondlineofoutputisabcdabcdfalse
9.Given:
IfthegarbagecollectordoesNOTrunwhilethiscodeisexecuting,approximatelyhowmany
objectswillexistinmemorywhentheloopisdone?
A.Lessthan10
B.About1000
C.About2000
D.About3000
E.About4000
10.Given:
Whatistheresult?
A.44
B.54
C.64
D.45
E.55
F.Compilationfails
11.Given:
Whatistheresult?
A.JAVA
B.JAVAROCKS
C.rocks
D.rock
E.ROCKS
F.ROCK
G.Compilationfails
12.Given:
Whichlinesofcode(ifany)breakencapsulation?(Chooseallthatapply.)
A.Line3
B.Line4
C.Line5
D.Line7
E.Line8
F.Line9
G.Theclassisalreadywellencapsulated
13.Given:
Whatistheresult?
A.truetruefalse
B.CompilationfailsdueonlytoanerroratlineA
C.CompilationfailsdueonlytoanerroratlineB
D.CompilationfailsdueonlytoanerroratlineC
E.CompilationfailsdueonlytoerrorsatlinesAandB
F.CompilationfailsdueonlytoerrorsatlinesAandC
G.CompilationfailsdueonlytoerrorsatlinesA,B,andC
H.Compilationfailsforreasonsnotlisted
14.Given:
Whatistheresult?
A.2018-01-152018-01-15
B.2018-01-152018-01-16
C.Jan15,2018Jan15,2018
D.Jan15,2018Jan16,2018
E.Compilationfails
F.Anexceptionisthrownatruntime
15.Given:
Whatistheresult?
A.[5,42,113,7]
B.Compilationfailsdueonlytoanerroronline5
C.Compilationfailsdueonlytoanerroronline8
D.Compilationfailsdueonlytoerrorsonlines5and8
E.Compilationfailsdueonlytoerrorsonlines7and8
F.Compilationfailsdueonlytoerrorsonlines5,7,and8
G.Compilationfailsdueonlytoerrorsonlines5,7,8,and9
16.Giventhatadder()returnsanint,whicharevalidPredicatelambdas?(Chooseallthatapply.)
17.Given:
Whatistheresult?
A.[knead,oilpan,roll,turnonoven,bake]
B.[knead,oilpan,turnonoven,roll,bake]
C.[knead,oilpan,turnonoven,roll,turnonoven,bake]
D.Theoutputisunpredictable
E.Compilationfails
F.Anexceptionisthrownatruntime
18.Given:
Whicharetrue?(Chooseallthatapply.)
A.Theoutputis:2018-08-162018-08-172018-08-18
B.Theoutputis:2018-08-162018-08-182018-08-17
C.Theoutputis:2018-08-162018-08-172018-08-17
D.AtlineX,zeroLocalDateobjectsareeligibleforgarbagecollection
E.AtlineX,oneLocalDateobjectiseligibleforgarbagecollection
F.AtlineX,twoLocalDateobjectsareeligibleforgarbagecollection
G.Compilationfails
19.GiventhatereferstoanobjectthatimplementsPredicate,whichcouldbevalidcodesnippetsor
statements?(Chooseallthatapply.)
A.if(e.test(m))
B.switch(e.test(m))
C.while(e.test(m))
D.e.test(m)?"yes":"no";
E.do{}while(e.test(m));
F.System.out.print(e.test(m));
G.booleanb=e.test(m));
SELFTESTANSWERS
1. AandDarecorrect.TheStringoperationsareworkingonanew(lost)Stringnot
Strings.TheStringBuilderoperationsworkfromlefttoright.
B,C,andEareincorrectbasedontheabove.(OCAObjectives9.2and9.1)
2. Discorrect.Thehorsesarray’sfirstfourelementscontainStrings,butthefifthisnull,
sothetoUpperCase()invocationforthefifthelementthrowsaNullPointerException.
A,B,C,E,andFareincorrectbasedontheabove.(OCAObjectives4.1and1.3)
3. Eiscorrect.TheUnicodedeclarationmustbeenclosedinsinglequotes:’\u004e’.Ifthis
weredone,theanswerwouldbeA,butthatequalityisn’tontheOCAexam.
A,B,C,andDareincorrectbasedontheabove.(OCAObjectives2.1and4.1)
4. Ciscorrect.AClassCastExceptionisthrownatline7becauseo1referstoanint[][],
notanint[].Ifline7wereremoved,theoutputwouldbe4.
A,B,D,E,F,andGareincorrectbasedontheabove.(OCAObjective4.2)
5. Biscorrect.ArrayListelementsareautomaticallyinsertedintheorderofentry;theyare
notautomaticallysorted.ArrayListsusezero-basedindexes,andthelastadd()insertsanew
elementandshiftstheremainingelementsback.
A,C,D,E,F,andGareincorrectbasedontheabove.(OCAObjective9.4)
6. CandFarecorrect.dareferstoanobjectoftype"Dozensarray,"andeachDozensobject
thatiscreatedcomeswithitsown"intarray"object.Whenline14isreached,onlythesecond
Dozensobject(andits"intarray"object)arenotreachable.
A,B,D,E,andGareincorrectbasedontheabove.(OCAObjectives4.1and2.4)
7. Ciscorrect.Atwo-dimensionalarrayisan"arrayofarrays."Thelengthofbais2because
itcontains2one-dimensionalarrays.Arrayindexesarezero-based,soba[1]referstoba’ssecond
array.
A,B,D,E,F,andGareincorrectbasedontheabove.(OCAObjective4.2)
8. DandFarecorrect.AlthoughStringobjectsareimmutable,referencestoStringsare
mutable.Thecodes1+="d";createsanewStringobject.StringBuilderobjectsaremutable,
sotheappend()ischangingthesingleStringBuilderobjecttowhichbothStringBuilder
referencesrefer.
A,B,C,E,andGareincorrectbasedontheabove.(OCAObjectives9.2and9.1)
9. Biscorrect.StringBuildersaremutable,soalloftheappend()invocationsareacting
onthesameStringBuilderobjectoverandover.Strings,however,areimmutable,soevery
StringconcatenationoperationresultsinanewStringobject.Also,thestring""iscreated
onceandreusedineveryloopiteration.
A,C,D,andEareincorrectbasedontheabove.(OCAObjectives9.2and9.1)
10. Aiscorrect.Althoughmain()’sb1isadifferentreferencevariablethango()’sb1,they
refertothesameBoxobject.
B,C,D,E,andFareincorrectbasedontheabove.(OCAObjectives4.1,6.1,and6.6)
11. Discorrect.Thesubstring()invocationusesazero-basedindexandthesecond
argumentisexclusive,sothecharacteratindex8isNOTincluded.ThetoUpperCase()invocation
makesanewStringobjectthatisinstantlylost.ThetoUpperCase()invocationdoesNOTaffect
theStringreferredtobys.
A,B,C,E,F,andGareincorrectbasedontheabove.(OCAObjective9.2)
12. Fiscorrect.WhenencapsulatingamutableobjectlikeanArrayList,yourgettermust
returnareferencetoacopyoftheobject,notjustthereferencetotheoriginalobject.
A,B,C,D,E,andGareincorrectbasedontheabove.(OCAObjective6.5)
13. Fiscorrect.Predicatelambdastakeexactlyoneparameter;therestofthecodeiscorrect.
A,B,C,D,E,G,andHareincorrectbasedontheabove.(OCAObjective9.5)
14. Discorrect.InvokingtheplusDays()methodcreatesanewobject,andbothLocalDate
andDateTimeFormatterhaveformat()methods.
A,B,C,E,andFareincorrectbasedontheabove.(OCAObjective9.3)
15. Ciscorrect.TheonlyerrorinthiscodeisattemptingtoaddaStringtoanArrayListof
Integerwrapperobjects.Line7usesautoboxing,andlines6and9demonstrateusingawrapper
class’stwoconstructors.
A,B,D,E,F,andGareincorrectbasedontheabove.(OCAObjectives2.5and9.4)
16. B,EandFusecorrectsyntax.
A,C,D,andGareincorrect.Apassestwoparameters.C,areturn,mustbeinacodeblock,
andcodeblocksmustbeincurlybraces.D,ablock,musthaveareturnstatement.G,theresult,is
notaboolean.(OCAObjective9.5)
17. Ciscorrect.ArrayListscanhaveduplicateentries.
A,B,D,E,andFareincorrectbasedontheabove.(OCAObjective9.4)
18. BandEarecorrect.AtotaloffourLocalDateobjectsarecreated,buttheonecreated
usingtheof()methodisabandonedonthenextlineofcodewhenitsreferencevariableisassigned
tothenewLocalDateobjectcreatedviathefirstplusDays()invocation.Thereferencevariables
areswappedabit,whichaccountsforthedatesnotprintinginchronologicalorder.
A,C,D,F,andGareincorrectbasedontheabove.(OCAObjectives2.4and9.3)
19. A,C,D,E,F,andGarecorrect;theyallrequireaboolean.
Bisincorrect.Aswitchdoesn’ttakeaboolean.(OCAObjective9.5)
A
AbouttheDownload
Thise-bookcomeswithfreedownloadableOraclePressPracticeExamSoftware,whichcanbe
downloadedusingthelinksprovidedinthisappendix.ThissoftwareiseasytoinstallonanyMacor
WindowscomputerandmustbeinstalledtoaccessthePracticeExamfeature.
SystemRequirements
Windows
•2.33GHzorfasterx86-compatibleprocessor,orIntelAtom™1.6GHzorfasterprocessorfor
netbookclassdevices
•Microsoft®WindowsServer2008,Windows7,Windows8.1ClassicorWindows10
•512MBofRAM(1GBrecommended)
MacOS
•Intel®Core™Duo1.83GHzorfasterprocessor
•MacOSXv10.7andabove
•512MBofRAM(1GBrecommended)
DownloadingfromMcGraw-HillProfessional’sMedia
Center
Todownloadtheglossary,additionalcontent,andOraclePressPracticeExamSoftware,visitMcGraw-
HillProfessional’sMediaCenterbyclickingthelinkbelowandenteringthise-book’sISBNandyoure-
mailaddress.Youwillthenreceiveane-mailmessagewithadownloadlinkfortheadditionalcontent.
http://mhprofessional.com/mediacenter
Thise-book’sISBNis1260011380.
Onceyou’vereceivedthee-mailmessagefromMcGraw-HillProfessional’sMediaCenter,clickthe
linkincludedtodownloadthepracticeexams.Ifyoudonotreceivethee-mail,besuretocheckyour
spamfolder.
InstallingthePracticeExamSoftware
FollowtheinstructionsbelowforWindowsorMacOS.
Windows
Step1OpentheInstallerforPC.zipfile.Youwillneedtounzipthefileandextractorcopyandpastethe
contentstoyourharddrive.
Step2LocatetheInstaller.exefileanddoubleclickthefile.Afterafewmoments,theinstallerwillopen.
Step3Followtheonscreeninstructionstoinstalltheapplication.
MacOS
Step1OpentheInstallerforMac.zipfile.Youwillneedtounzipthefileandextractorcopyandpastethe
contentstoyourharddrive.
Step2Afterafewmoments,thecontentsofthe.zipfilewillbedisplayed.
Step3DoubleclickonInstallertobegininstallation.
Step4Followtheonscreeninstructionstoinstalltheapplication.
NOTE:Ifyougetanerrorwhileinstallingthesoftwarepleaseensureyouranti-virusorinternetsecurity
programsaredisabledandtryinstallingthesoftwareagain.Youmayenabletheantivirusorinternet
securityprogramagainafterinstallationiscomplete.
RunningthePracticeExamSoftware
Followtheinstructionsbelowafteryouhavecompletedthesoftwareinstallation.
Windows
Afterinstalling,youcanstarttheapplicationusingeitherofthetwomethodsbelow:
1.Double-clicktheOraclePressJavaSE8Practiceicononyourdesktop,or
2.GototheStartmenuandclickProgramsorAllPrograms.ClickOraclePressJavaSE8
Practicetostarttheapplication.
MacOS
OpentheOraclePressJavaSE8PracticefolderinsideyourMac’sapplicationfolderanddouble-click
theOraclePressJavaSE8Practiceicontoruntheapplication.
PracticeExamSoftwareFeatures
ThePracticeExamSoftwareprovidesyouwithasimulationoftheactualexam.Thesoftwarealso
featuresacustommodethatcanbeusedtogeneratequizzesbyexamobjectivedomain.Quizmodeisthe
defaultmode.Tolaunchanexamsimulation,selectoneoftheOCAexambuttonsatthetopofthescreen,
orchecktheExamModecheckboxatthebottomofthescreenandselecttheOCAexaminthecustom
window.
Thenumberofquestions,typesofquestions,andthetimeallowedontheexamsimulationareintended
tobearepresentationoftheliveexam.Thecustomexammodeincludeshintsandreferences,andin-depth
answerexplanationsareprovidedthroughtheFeedbackfeature.
Whenyoulaunchthesoftware,adigitalclockdisplaywillappearintheupper-rightcornerofthe
questionwindow.TheclockwillcontinuetocountunlessyouchoosetoendtheexambyselectingGrade
TheExam.
RemovingInstallation
ThePracticeExamSoftwareisinstalledonyourharddrive.Forbestresultsforremovalofprograms
usingaWindowsPCusetheControlPanel|UninstallAProgramoptionandthenchooseOraclePress
JavaSE8Practicetouninstall.
ForbestresultsforremovalofprogramsusingaMacgototheOraclePressJavaSE8Practicefolder
insideyourapplicationsfolderanddragthe“OraclePressJavaSE8Practice”icontothetrash.
Help
AhelpfileisprovidedthroughtheHelpbuttononthemainpageinthetop-rightcorner.Areadmefileis
alsoincludedintheBonusContentfolder.
TechnicalSupport
TechnicalSupportinformationisprovidedinthefollowingsectionsbyfeature.
Windows8Troubleshooting
ThefollowingknownerrorsonWindows8havebeenreported.Pleaseseebelowforinformationon
troubleshootingtheseknownissues.
Ifyougetanerrorwhileinstallingthesoftware,suchas“Theapplicationcouldnotbeinstalled
becausetheinstallerfileisdamaged.Tryobtainingthenewinstallerfromtheapplicationauthor,”you
mayneedtodisableyouranti-virusorInternetsecurityprogramsandtryinstallingthesoftwareagain.
YoumayenabletheantivirusorInternetsecurityprogramagainafterinstallationiscomplete.
Formoreinformationonhowtodisableanti-virusprogramsinWindows,pleasevisitthewebsiteof
thesoftwareproviderofyouranti-virusprogram.Forexample,ifyouuseNortonorMacAfeeproducts,
youmayneedtovisittheNortonortheMacAfeewebsiteandsearchfor“howtodisableantivirusin
Windows8.”Anti-virusprogramsaredifferentfromfirewalltechnology,sobesuretodisabletheanti-
virusprogram,andbesuretore-enabletoprogramafteryouhaveinstalledthepracticeexamsoftware.
WhileWindowsdoesn’tincludedefaultantivirussoftware,Windowscanoftendetectantivirus
softwareinstalledbyyouorthemanufacturerofyourcomputerandtypicallydisplaysthestatusofany
suchsoftwareintheActionCenter,whichislocatedintheControlPanelunderSystemandSecurity
(selectReviewYourComputer’sStatus).Window’shelpfeaturecanalsoprovidemoreinformationon
howtodetectyouranti-virussoftware.Iftheanti-virussoftwareison,checktheHelpfeaturethatcame
withthatsoftwareforinformationonhowtodisableit.
Windowswillnotdetectallanti-virussoftware.Ifyouranti-virussoftwareisn’tdisplayedinthe
ActionCenteryoucantrytypingthenameofthesoftwareorthepublisherintheStartMenu’ssearchfield.
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customersoutsidetheUnitedStates,e-mailinternational_cs@mheducation.com.
INDEX
A
abandonedstrings,344–346
abstractclasses,19
constructors,133,136
creating,23
implementation,123
vs.interfaces,25
overview,21–22
abstractmethods
inheritance,93
overview,45–48
subclassimplementation,106
accessandaccessmodifiers,30–33
classes,17–21
constructors,135
encapsulation,89
keypoints,71
levels,54
localvariables,42–43
overloadedmethods,111
overriddenmethods,107–108
private,34–36
protectedanddefaultmembers,36–42
public,33–34
staticmethodsandvariables,151–154
add()methodinArrayList,383
addition
compoundassignment,236
operator,244
precedence,257
ampersands(&)
bitwiseoperators,251–252
logicaloperators,252–255
precedence,257
ANDexpressions,252–255
anglebrackets(<>)ingenericcode,381
anonymousarrays,372–373
append()method,353–354
appendingstrings,341–342,353–354
applications,launching,11–12
@argfilesoptions,11
arguments
final,44–45
overloadedmethods,111,113
overriddenmethods,108
vs.parameters,49–50
superconstructors,137
variableargumentlists,49–50
arithmeticoperators,244–249
basic,244–245
incrementanddecrement,248–250
keypoints,260–261
remainder,245–246
stringconcatenation,246–248
ArithmeticExceptionclass,322
ArrayIndexOutOfBoundsExceptionclass,306
description,322
out-of-rangearrayindexes,369
superclass,308–309
ArrayListclass,13–14
autoboxing,384–388
basics,380–381
diamondsyntax,388–389
duplicates,383
keypoints,401
methods,382–384
uses,379–381
arrays,363
constructing,364–367,370–372
declaring,57–59,363–364,370–372
defaultelementvalues,198–199,202,221
elementassignments,374–378
enhancedforloops,292–293
indexes,12,368
initializing,368–374
instanceofcomparisons,244
keypoints,76,399–400
lengthattribute,350
referenceassignments,376–378
returning,131
ASCIIset,53
AssertionErrorclass,316,321
assignments
arrayelements,374–378
compilererrors,188–189
floating-pointnumbers,188
keypoints,220–221
objectcompatibility,242
operators,182–183,235–236,257
primitives,183–185,190
referencevariables,190–191,202–203,376–378
asterisks(*)
compoundassignmentoperators,236
importstatements,14,16
multiplication,244
precedence,257
autoboxingwithArrayLists,384–388
automaticlocalvariables,55–57
automaticvariables.Seelocalvariables
B
backslashes(\)forescapedcharacters,182
base2(binary)integers,178
base8(octal)integers,178–179
base10(decimal)integers,178
base16(hexadecimal)literals,178,180
basicforloops,288
conditionalexpressions,289–290
declarationandinitialization,288–289
iterationexpressions,289–290
loopissues,290–292
behaviors,description,2
between()method,361
binary(base2)integers,178
binaryliterals,179
bitwiseoperators,251–252
blocks
initialization,145–147
variables,193
booleantypeandvalues
bitdepth,53
defaultvalues,196
doloops,287
inforloops,288–289
ifstatements,276–277
invertoperator,255–256
literals,181
relationaloperators,236
whileloops,286
braces({}).Seecurlybraces({})
branching
if-else,273–277
switchstatements.Seeswitchstatementsbreakstatement
inforloops,290
keypoints,326
loopconstructs,294–296
switchstatements,278,281–283
bugs.Seeexceptions
bytes
caseconstants,278–279
defaultvalues,196
andint,184
ranges,53
C
calendardata
classes,357–358
factoryclasses,359
formatting,362
immutability,358–359
keypoints,398–399
usingandmanipulating,360–361
callstackexceptions,303–305
CamelCase,9
capacityofstrings,354
carets(^)
bitwiseoperators,251–252
exclusive-ORoperators,255
casesensitivity
identifiers,7
stringcomparisons,348–349
casestatements,278–280
casts,184
assignment,235
explicit,185–186
implicit,186
keypoints,159,220
overview,118–121
precision,188–189
primitives,185–188
catchclause,299–300
chainedmethods,356
chainingconstructors,134
charactersandchartype
bitdepth,53
caseconstants,278–279
comparisons,237
defaultvalues,196
literals,181–182
charAt()method,348–349
checkedexceptions
handling,314–315
interfaceimplementation,123
overloadedmethods,111
overriddenmethods,108,110
ChronoUnitenum,361
.classfiles,12
ClassCastExceptionclass
description,322
downcasts,119
classes
access,17–21
compiling,11
constructors.Seeconstructorsdeclaring,17–18
defining,9–10
description,2
extending,18,102
final,20–21,59–61
importstatements,13–14
interfaceimplementation,123
launchingapplicationswithjava,11–12
main()method,12–13
memberdeclarations,28
names,8–9,151
overriding,3
sourcefiledeclarationrules,10
wrapper,320
cleaningupgarbagecollection,218
clear()method,383
codeconventions,7–9
collections,58
colons(:)
conditionaloperators,250–251
labels,296
commas(,)
inforloops,288–289
variables,185
commentsinsourcecodefiles,10
comparisons
instanceof,242–244
relationaloperators,236–241
strings,348–349
compilerandcompiling
casts,119
interfaceimplementation,122–123
javac,11
overloadedmethods,114
compilererrors
assignments,188–189
instanceof,244
compoundassignments
withcasts,189
operators,236
strings,247
concat()method,348–349
concatenatingstrings,246–248,261,348–349
concreteclasses
abstractmethodsimplementedby,106
creating,23
subclasses,46–47
concretemethods,102
conditionaloperators
keypoints,261
overview,250–251
conditions
doloops,287
inforloops,288–290
if-elsebranching,273–277
switchstatements.Seeswitchstatementswhileloops,286
constantspecificclassbody,65–66
constants
case,278–279
enum,62
interface,26–27
names,9
Stringclass,347
constructingarrays,364–367,370–372
constructors,132
basics,133
calendardata,359
chaining,134
declarations,50–51
default,135–137
enums,65–66
inheritance,93,140
keypoints,75,160–161
overloaded,140–145
rules,135–136
strings,341
super()andthis()calls,144
contains()method,383
continuestatement
keypoints,326
loopconstructs,294–295
controls,17
conventions
code,7–9
identifiers,6
names,2
conversions
returntype,131
types.Seecasts
costreduction,object-orienteddesignfor,100
counting
instances,148
references,212
covariantreturns,129–130
cross-platformexecution,5
curlybraces({})
abstractmethods,46
arrays,370,372,374
classmembers,196,199
ifexpressions,273,275
instancevariables,196
lambdas,395
methods,46
optional,273
D
Dsuffix,181
dashes(-)
compoundassignmentoperators,236
decrementoperators,248–249
precedence,257
subtraction,244
datedata
usingandmanipulating,360–361
formatting,362
DateTimeFormatterclass,358,361–362
DeadlyDiamondofDeath,102
decimal(base10)integers,178
decimalliteralswithunderscores,179
declarations
arrays,57–59,363–364,370–372
basicforloops,288–289
classmembers,28
classes,17–18
constructors,50–51
enhancedforloops,293
enumelements,65–67
enums,62–64
exceptions,312–317
interfaceconstants,26–27
interfaces,23–26
polymorphic,381
referencevariables,53–54
returntypes,129–132
sourcefilerules,10
variables,51–61,75–76
decrementoperators
keypoints,261
workingwith,248–250
defaultaccess
description,17
overview,18–19
andprotected,30,36–42
defaultcaseinswitchstatements,283–284
Defaultprotection,30
defaults
constructors,135–137
interfacemethods,28
primitiveandobjecttypeinstancevariables,195–198
defining
classes,9–10
exceptions,306–307
delete()method,354–355
deletingstrings,354–355
diamondsyntax,388–389
distributedcomputing,5
division
compoundassignment,236
operator,244
precedence,257
doloops,287
dots(.)
access,31–32
classnames,151
instancereferences,151
variableargumentlists,50
doubletype
casts,186
defaultvalues,196
floating-pointliterals,180
ranges,53
underscores,179
downcasts,119
duckingexceptions,303,312
duplicatesinArrayLists,383
E
elementsinarrays.Seearrays
ellipses(...)invariableargumentlists,50
elsestatements,273–277
encapsulation
benefits,100
description,4
keypoints,157
overview,88–91
referencevariables,389–390,401
entrypointsinswitchstatements,281
enums,62
caseconstants,278–279
constants,62
declaring,62–64
declaringelements,65–67
equalitytests,241
keypoints,76–77
EOFExceptionclass,311,314
equalsigns(=)
assignment,182,235
boxing,386–387
compoundassignmentoperators,236
equalitytests,237–241
relationaloperators,236–237
equalityandequalityoperators
enums,241
objects,386–387
precedence,257
primitives,238
references,238–240
strings,240–241,281
equals()method,240–241,386–387
equalsIgnoreCase()method,348–349
Errorclass,307
Exceptionclass,306–307
ExceptionInInitializerErrorclass
description,322
initblocks,147
exceptions,298–299
creating,317–318
declarations,312–317
defining,306–307
hierarchy,307–309
interfaceimplementation,123
JVMthrown,319–320
keypoints,327
listof,321–322
matching,309–310
overriddenmethods,108,110
programmaticallythrown,320–321
propagating,303–306
rethrowing,317
sources,319
tryandcatch,299–303
exclamationpoints(!)
booleaninvertoperator,255–256
precedence,257
relationaloperators,236–237
exclusive-OR(XOR)operator,255
executionentrypointsinswitchstatements,281
exit()method
loops,290
tryandcatch,324
explicitcasts,185–186
explicitvaluesinconstructors,134
expressions
enhancedforloops,293
ifstatements,276–277
extendedASCIIset,53
extending
classes,18,102
inheritancein,94
interfaces,124
extendskeyword
illegaluses,127
IS-Arelationships,97
F
Fsuffix,181
factoryclasses,359
fall-throughinswitchstatements,281–283
falsevalue,181
featuresandbenefits,3–5
FileNotFoundExceptionclass,311–312
finalarguments,44–45
finalclasses,20–21,59–61
finalconstants,26–27
finalmethods
nonaccessmembermodifiers,43–45
overriding,108
finalmodifiers
incrementanddecrementoperators,250
variables,42,59
finalize()method,218–219
finallyclauses,301–303
flexibilityfromobjectorientation,88
floattypeandfloating-pointnumbers
assigning,188
casts,187
classes,20
comparisons,237
defaultvalues,196
literals,180–181
ranges,53
underscores,179
flowcontrol,272
breakandcontinuestatements,294–295
doloops,287
forloops,287–293
if-elsebranching,273–277
labeledstatements,295–297
switchstatements.Seeswitchstatementsunlabeledstatements,295
whileloops,285–286
forloops
basic,288–292
enhanced,292–293
forcedexitsfromloops,290
forcinggarbagecollection,215–218
format()method,362
formattingcalendardata,362
fractions,180
fullyqualifiednames,13
G
garbagecollection
cleaningupbefore,218
forcing,215–218
keypoints,222
objectseligiblefor,213–215
overview,210
garbagecollector
operation,212
overview,210
running,210
gc()method,215–216
generics,381
get()method,383
getRuntime()method,216
gettersencapsulation,89
greaterthansigns(>)
precedence,257
relationaloperators,236–237
guardedregions,299
H
HAS-Arelationships
keypoints,157
overview,96–100
heap
garbagecollection,210
keypoints,220
overview,176–177
hexadecimal(base16)literals,178,180
hidingimplementationdetails,89
hierarchyofexceptions,307–309
I
IDE(integrateddevelopmentenvironment),11
identifiers,2
keypoints,70
legal,6–7
if-elsebranching
keypoints,325–326
overview,273–277
IllegalArgumentExceptionclass
description,322
programmaticallythrownexceptions,321
IllegalStateExceptionclass,322
immutability
calendardata,358–359
strings,340–347
implementationdetails,hiding,89
implementinginterfaces
keypoints,159
overview,122–128
implementskeyword
illegaluses,127
IS-Arelationships,97
implicitcasts
assignment,235
primitives,186
importstatements
keypoints,70
overview,13–14
sourcecodefiles,10
static,14–16
incrementoperators
keypoints,261
workingwith,248–250
indexes
ArrayLists,380
arrays,12,368
out-of-range,369
string,348–349
zero-based,12
indexOf()method,ArrayLists,383
IndexOutOfBoundsExceptionclass,308
indirectinterfaceimplementation,243
inheritance
accessmodifiers,31–33,38–39
constructors,140
evolution,93–95
HAS-Arelationships,98–100
IS-Arelationships,96–97
keypoints,157
multiple,102,126–128
overview,3,92–93
inheritedmethods,overriding,109
initialization
arrays,202,368–374
basicforloops,288–289
loopelements,370
objectreferences,201–202
primitives,199–201
variables,56,185
initializationblocks,145–147
inheritance,93
keypoints,161
innerclasses,17
insert()method,354–355
insertingstringelements,355
instancemethods
inheritance,93
overriding,108
polymorphic,104
instancevariables,54–55
constructors,134
defaultvalues,195–198
description,2,193
onheap,176–177
inheritance,93
instanceofoperator
keypoints,260
objecttests,242–243
instances
counting,148
initializationblocks,146
referencesto,151
instantiation,160–161
Integerclass,15–16,321
integersandintdatatype
andbyte,184
caseconstants,278–279
casts,186–187
comparisons,237
defaultvalues,196
literals,178
ranges,53
remainderoperator,246
integrateddevelopmentenvironment(IDE),11
interfaces,2,89
vs.abstractclasses,25
constants,26–27
constructors,136
declaring,23–26
extending,124
implementing,122–128,159
indirectimplementation,243
keypoints,72–73
methods,28–29
names,8–9
overview,3
invertoperator,255–256
invoking
overloadedmethods,112–115
polymorphicmethods,103
IOExceptionclass
checkedexceptions,314
files,311–312
IS-Arelationships
keypoints,157
overview,96–97
polymorphism,101
returntypes,132
ISOLatin-1characters,53
isolationofreferences,214–215
iterationinbasicforloops,289–290
J
javacommand,11–12
java.io.IOExceptionclass
checkedexceptions,314
files,311–312
java.lang.ClassCastExceptionclass
description,322
downcasts,119
java.lang.Exceptionclass,306–307
java.lang.Integerclass,15–16,321
java.lang.Objectclass,92
java.lang.Object.equals()method,240–241
java.lang.Runtimeclass,216
java.lang.RuntimeExceptionclass,308,312–314
java.lang.StringBuilderclass,340
keypoints,398
methods,354–356
overview,352
vs.StringBuffer,352–353
java.time.DateTimeFormatterclass,358,361–362
java.time.LocalDateclass,357,362
java.time.LocalDateTimeclass,357,362
java.time.LocalTimeclass,357,362
java.time.Periodclass,358,361
java.time.temporalpackage,361
java.time.temporal.TemporalAmountinterface,358
java.util.ArrayListclass.SeeArrayListclass
java.util.function.Predicateinterface,393–395
java.util.Listinterface,381
JavaVirtualMachine(JVM),5,11
javac,compilingwith,11
K
keywords,2,7
L
Lsuffix,180
labeledstatements,295–297
lambdas,390–396,401
launchingapplications,11–12
leaks,memory,4
legalidentifiers,6–7
length()method,350
lengthofarraysandstrings,350
lessthansigns(<)
precedence,257
relationaloperators,236–237
libraries,4
listsandListinterface,381
ArrayLists.SeeArrayListclassimplementations,381
literals
binary,179
boolean,181
character,181–182
floating-point,180–181
hexadecimal,180
integer,178
keypoints,220
octal,179
primitiveassignments,183–184
strings,182,347
localarrays,202
localobjectreferences,201–202
localprimitives,199–201
localvariables,198–199
accessmodifiers,42–43
description,193
keypoints,74
onstack,55,176–177
workingwith,55–57
LocalDateclass,357,362
LocalDateTimeclass,357,362
LocalTimeclass,357,362
logicaloperators,251
bitwise,251–252
keypoints,261–262
non-short-circuit,254–256
precedence,257
short-circuit,252–254,276–277
longtype
defaultvalues,196
ranges,53
loopconstructs,285
breakandcontinue,294–295
do,287
elementinitialization,370
for,287–293
keypoints,326
while,285–286
lowercasecharactersinstrings,351
M
main()method,12–13
exceptions,303–305
overloaded,115
maintainability,objectorientationfor,88
markandsweepalgorithm,212
matchingexceptions,309–310
MAX_VALUEconstant,16
meaningfullyequivalentobjects,240
membermodifiers,nonaccess,43–50
members
access.Seeaccessandaccessmodifiersdeclaring,28
keypoints,74–75
memory
garbagecollection.Seegarbagecollectionstrings,347
memoryleaks,210
memorymanagement,4
methods
abstract,45–48
accessmodifiers.SeeaccessandaccessmodifiersArrayLists,382–384
chained,356
description,2
enums,65–66
factory,359
final,43–45,59
instance,104,108
interfaceimplementation,122–123
interfaces,28–29
names,9
native,49
overloaded,111–117
overridden,105–111
recursive,320
stacks,303–305
static,61,148–150
strictfp,49
String,348–352
StringBuilder,354–356
synchronized,48–49
variableargumentlists,49–50
minussigns(–)
compoundassignmentoperators,236
decrementoperators,248–249
precedence,257
subtraction,244
modifiers.Seeaccessandaccessmodifiers
modulusoperator
overview,245–246
precedence,257
multidimensionalarrays
constructing,366
declaring,58–59,364
referenceassignments,377–378
multipleinheritance,102,126–128
multiplication
compoundassignment,236
operator,244
precedence,257
multithreadedprogramming,5
mutatorsinencapsulation,89
N
names
classesandinterfaces,8–9
constants,9
constructors,51,133,135
conventions,2
dotoperator,151
fullyqualified,13
labels,296
methods,9
shadowvariables,208–209
variables,9
narrowingconversions,186
nativemethods,49
negativenumbers
fromcasts,189
representing,53
nestedclasses,17
nestedif-elsestatements,273
newkeyword
arrays,364
calendardata,359
no-argconstructors,135–136
NoClassDefFoundErrorclass,322
nonaccessmembermodifiers,19–20,43
abstractmethods,45–48
finalarguments,44–45
finalmethods,43–45
keypoints,71–72
methodswithvariableargumentlists,49–50
nativemethods,49
strictfpmethods,49
synchronizedmethods,48–49
notequaloperator(!=),237
nullvalues
referencevariables,183,191
returning,130
wrappervariables,388
nullingreferences,213
NullPointerExceptionclass,314
arrays,368
description,322
referencevariables,319–320
wrappervariables,388
NumberFormatExceptionclass
description,322
stringconversions,321
numbers
primitives.Seeprimitives
withstringconcatenation,246–247
withunderscores,178–179
O
Objectclass,92
objectorientation(OO),87–88
benefits,100
casting,118–121
constructors.Seeconstructorsdescription,4
encapsulation,88–91
inheritance,92–100
initializationblocks,145–147
interfaceimplementation,122–128
overloadedmethods,111–117
overriddenmethods,105–111
polymorphism,101–105
returntypes,129–132
statics,148–154
objectsandobjectreferences
arrays,58,363–364,374–375
defaultvalues,196–198
description,2,183
equality,386–387
garbagecollection,213–219
onheap,176–177
initializing,201–202
instanceofcomparisons,242–244
overloadedmethods,113–114
passing,205–207
stringsas,341
octal(base8)integers,178–179
of()method,361
one-dimensionalarrays
constructing,364–366
referenceassignments,376–377
operands,234
operators,233–234
arithmetic,244–249
assignment,235–236
conditional,250–251
incrementanddecrement,248–250
instanceof,242–244
logical,251–256
precedence,256–258,262
relational,236–241
ORexpressions,253–255
orderofinstanceinitializationblocks,146
out-of-rangearrayindexes,369
out-of-scopevariables,194
OutOfMemoryExceptionclass,218
overloadedconstructors,140–145
overloadedmethods,111
invoking,112–115
keypoints,158
legal,112,116
main(),13,115
vs.overridden,112,117
polymorphism,115
returntypes,129
overriddenmethods,105–109
illegal,110–111
invokingsuperclass,109–110
vs.overloaded,112,117
polymorphism,115
returntypes,129–130
static,154
overriding
classes,3
keypoints,158
privatemethods,36
P
package-centriclanguages,17
package-levelaccess,18–19
packagestatementinsourcecodefiles,10
packages
access,17
classesin,14
parameters
vs.arguments,49–50
lambdas,395
parentheses()
arguments,44,50
conditionaloperator,250
inforloops,288
ifexpressions,273,276–277
operatorprecedence,246–247,258,262
stringconcatenation,247
parseInt()method,321
passing
keypoints,221
objectreferencevariables,205–207
pass-by-value,206–207
primitivevariables,207–208
Peabody,Marc,185
percentsigns(%)
precedence,257
remainderoperator,245–246
Periodclass,358,361
pipe(|)characters
bitwiseoperators,251–252
logicalORoperator,253–255
plussigns(+)
addition,244
compoundassignmentoperators,236
incrementoperators,248–249
precedence,257
stringconcatenation,246–248
polymorphism
abstractclasses,22
declarations,381
inheritance,95
keypoints,157
overloadedandoverriddenmethods,115
overview,101–105
poolsforStringconstants,347
postfixincrementoperators,248–249
precedenceofoperators,245,247,256–258,262
precision
casts,186,188–189
floating-pointliterals,181
floating-pointnumbers,188
Predicateinterface,393–395
prefixincrementoperators,248–249
primitives
arrays,58,363–364,374
assignments,183–185,190
casting,185–188,220
comparisons,238
declarations,51–53
defaultvalues,195–196
final,59
initializing,199–201
literals,178–182
passing,207–208
returning,131
wrapperclasses,192
printStackTrace()method,308
privatemodifiers,30
overriding,36
overview,34–36
programmaticallythrownexceptions,320–321
ProjectCoin,diamondsyntax,388–389
propagatinguncaughtexceptions,303–306
protectedmodifiers,30,36–42
publicaccess,19
publicinterfaceforexceptions,312–317
publicmodifiers,30
constants,26–27
encapsulation,89
overview,33–34
publicstaticvoidmain()method,12–13
Q
questionmarks(?)forconditionaloperators,250–251
R
ranges,numbers,53
reachableobjects,210
reassigningreferencevariables,213–214
recursivemethods,320
redefinedstaticmethods,154
referencecounting,212
references,101
arrays,368
assigning,190–191,202–203
casting,118–121,159
declaring,53–54
description,183
encapsulation,389–390,401
equality,238–240
instances,151
isolating,214–215
multidimensionalarrays,377–378
nulling,213
one-dimensionalarrays,376–377
overloadedmethods,113–114
passing,205–207
reassigning,213–214
returning,130
strings,342–347
regions,guarded,299
relationaloperators,236–237
equality,237–241
keypoints,260
precedence,257
remainderoperator,245–246
remove()method,384
removingArrayListelements,384
replace()method,350
replacingstringelements,350
rethrowingexceptions,317
returnstatement
forloops,290
lambdas,395
returntype
constructors,51,133,135
declarations,129–132
keypoints,159
overloadedmethods,111,129
overriddenmethods,108,129–130
returningvalues,130–131
reuse
inheritancefor,94
names,208–209
reverse()method,355
reversingstrings,355
rules
constructors,135–136
sourcefile,10
Runtimeclass,216
runtimeexceptionsforoverriddenmethods,108
RuntimeExceptionclass,308,312–314
S
sandbox,5
scope
inforloops,291
keypoints,220
variables,192–195
security,5
semicolons(;)
abstractmethods,22,45
enums,64
inforloops,288
labels,296
lambdas,395
nativemethods,49
whileloops,287
serialization,5
settersencapsulation,89
shadowedvariables,56,193,208–209
short-circuitlogicaloperators
ifstatements,276–277
overview,252–254
precedence,257
shorttype
caseconstants,278–279
defaultvalues,196
ranges,53
signaturesofmethods,117,123
signednumbers,53
size
ArrayLists,383–384
arrays,59,364,366,370,372,374
assignmentissues,235
numbers,53
size()method,383–384
slashes(/)
compoundassignmentoperators,236
division,244
precedence,257
sourcecodefiledeclarationrules,10,71
squarebrackets([])
arrayelements,58
arrays,363
stack
exceptions,320
keypoints,220
localvariables,55
methods,303–305
overview,176–177
StackOverflowErrorclass
description,322
recursivemethods,320
states,2
staticconstants,26–27
staticimports,14–16
staticinitializationblocks,146
staticinterfacemethods,28–29
staticvariablesandmethods,14–15,61
constructors,136
description,193
inheritance,93
keypoints,76,161
overriding,109,154
overview,148–150
streams,390
strictfpmodifiers
classes,19–20
methods,49
Stringclass,340–347
constantpool,347
keypoints,398
methods,348–352
objectreferences,203–204
StringBufferclass,340,352–353
StringBuilderclass,340
keypoints,398
methods,354–356
overview,352
vs.StringBuffer,352–353
StringIndexOutOfBoundsExceptionclass,308–309
strings,340
appending,341–342,353–354
caseconstants,278–279
comparing,348–349
concatenating,246–248,261,348–349
creating,348
deleting,354–355
equality,240–241,281
immutability,340–347
insertingelementsinto,355
keypoints,398
length,350
literals,182,347
lowercase,351
memory,347
methods,348–352
replacingelementsin,350
reversing,355
substrings,350–351
trimming,352
uppercase,351
strongtyping,5
subclasses
concrete,46–47
inheritance,3
substring()method,350–351
subtraction
compoundassignment,236
operator,244
precedence,257
subtypesforreferencevariables,101
super()callsforconstructors,144
superconstructorarguments,137
superclasses,3
constructors,135
overriddenmethods,109–110
switchstatements,278
breakandfall-through,281–283
defaultcase,283–284
exercise,285
keypoints,325–326
legalexpressions,278–280
stringequality,281
synchronizationmethods,48–49
System.exit()method
loops,290
tryandcatch,324
System.gc()method,215–216
T
TemporalAmountinterface,358
ternaryoperator
conditional,250–251
keypoints,261
test()method,393,396
this()callsforconstructors,135,144
thiskeyword,57
threadsingarbagecollector,212
three-dimensionalarrays,59,364
Throwableclass,307
thrownexceptions
description,299
JVM,319–320
programmatically,320–321
timedata
usingandmanipulating,360–361
formatting,362
toLowerCase()method,351
toString()method
ArrayLists,381
String,351
StringBuilder,356
toUpperCase()method,351
transientvariables,60–61
trim()method,352
truevalue,181
truncatingfromcasts,189
tryandcatchfeature
finally,301–303
overview,299–300
two-dimensionalarrays,59,364
two’scomplementnotationandcasts,189
types
arraydeclarations,363
assignments,235
casting.Seecasts
return.Seereturntype
variables,183
U
\uprefix,181
UML(UnifiedModelingLanguage),100
unaryoperators,precedence,257
unassignedvariables
keypoints,221
workingwith,195–199
uncaughtexceptions,303–306
uncheckedexceptions
description,314–315
overriddenmethods,108
underscores(_)innumericliterals,178–179
Unicodecharacters
chartype,53
identifiers,6
literals,181
strings,341
UnifiedModelingLanguage(UML),100
uninitializedvariables
keypoints,221
workingwith,195–199
unlabeledstatements,295
until()method,361
unwindingthestack,308
upcasting,120
uppercaseforstrings,351
V
values()method,66
valuesofvariables,182
var-args
keypoints,75
methods,49–50
variableargumentlists,49–50
variables
access.Seeaccessandaccessmodifiersassignments.Seeassignmentsdeclarations,51–53,75–
76
description,2
enums,65–66
final,59
inforloops,291
heapandstack,176–177
initializing,185
instance,54–55
local.Seelocalvariables
names,9
primitives,51–53
scope,192–195
shadow,208–209
static,61,148–150
transient,60–61
uninitializedandunassigned,195–199,221
values,182
volatile,61
verticalbars(|)
bitwiseoperators,251–252
logicalORoperator,253–255
precedence,257
visibility,access,18,43
voidreturntype,131
volatilevariables,61
W
Walraven,Fritz,258
whileloops
labeled,297
workingwith,285–286
whitespace,trimmingfromstrings,352
wideningconversions,186
wildcardsinimportstatements,14,16
wrapperclasses
primitives,192
strings,320
wrappers,ArrayLists,384–388
X
XOR(exclusive-OR)operator,255
Z
zero-basedindexes,12
0xprefix,179

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