OCA OCP Java SE 7 Programmer I II Study Guide (Exams 1Z0 803 804) %5b Sierra Bates

OCA_OCP%20Java%20SE%207%20Programmer%20I%20_%20II%20Study%20Guide%20(Exams%201Z0-803%20_%201Z0-804)%20%5BSierra%20_%20Bates%2020

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OCA/OCP Java SE 7
Programmer I & II
Study Guide
®

(Exams 1Z0-803 & 1Z0-804)

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®

OCA/OCP Java SE 7
Programmer I & II
Study Guide
®

(Exams 1Z0-803 & 1Z0-804)
Kathy Sierra
Bert Bates
McGraw-Hill Education is an independent entity from Oracle Corporation and is not
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CONTRIBUTORS

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, 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 of 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. He is a forum moderator on 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 fourth 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, and now for the OCA 7 and OCP 7 exams. 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 12 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.

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About the SCJP 5 Technical Review Team

Andrew
Burk

Devender

Bill M.

Jeoren

Jef

Jim

Gian

Kristin

Marcelo

Marilyn

Johannes

Mark

Mikalai

Seema

Valentin

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 is
Burk 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

Fred
Marc P.

Mikalai
Christophe

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 handpicked an elite crew of
Marc W.
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
Contributing Authors

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
Tom
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 book and 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 book and other digital
content, 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!)
Mikalai
Roel
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 thank Vijitha Kumara, JavaRanch moderator
and tech reviewer extraordinaire. We had many reviewers help out
Vijitha
Roberto
during the long course of writing this book, but Vijitha was one of the
few who stuck with us from Chapter 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!

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For Andi

For Bob

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CONTENTS AT A GLANCE
Part I
OCA and OCP
1

Declarations and Access Control

..............................

3

2

Object Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83

3

Assignments

4

Operators

5

Working with Strings, Arrays, and ArrayLists

....................

257

6

Flow Control and Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

307

..............................................

165

................................................

223

Part II
OCP
7

Assertions and Java 7 Exceptions

.............................

377

8

String Processing, Data Formatting, Resource Bundles . . . . . . . . . . . . . .

417

9

I/O and NIO

477

10

Advanced OO and Design Patterns

11

Generics and Collections

..............................................
............................

541

....................................

573

12

Inner Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

681

13

Threads

713

14

Concurrency

15

JDBC

A

Serialization

B

Classpaths and JARs Classpaths and JARs . . . . . . . . . . . . . . . . . . . . . . .

B-1

C

About the Download

.......................................

947

....................................................

953

Index

..................................................
..............................................

785

...................................................

841

............................................

A-1

xiii

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CONTENTS

Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments
....................................
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v
xxvii
xxix
xxxi

Part I
OCA and OCP
1

Declarations and Access Control

.................

Java Refresher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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, 6.6, and 7.6) . . . . . . . .
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 7.6) . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaring an Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaring Interface Constants . . . . . . . . . . . . . . . . . . . . . . . . .
Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5,
4.1, 4.2, 6.2, and 6.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Access Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nonaccess Member Modifiers . . . . . . . . . . . . . . . . . . . . . . . . .
Constructor Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variable Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3
4
6
6
7
9
10
11
13
13
15
17
23
24
24
27
28
29
42
49
50

xv

xvi

OCA/OCP Java SE 7 Programmer I & II Study Guide

2

Declare and Use enums (OCA Objective 1.2 and
OCP Objective 2.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaring enums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60
61
68
75
81

Object Orientation

83

.............................

Encapsulation (OCA Objectives 6.1 and 6.7) . . . . . . . . . . . . . . . . . . .
Inheritance and Polymorphism (OCA Objectives 7.1, 7.2, and 7.3) . .
IS-A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HAS-A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Polymorphism (OCA Objectives 7.2 and 7.3) . . . . . . . . . . . . . . . . . . .
Overriding / Overloading (OCA Objectives 6.1, 6.3, 7.2, and 7.3) . . .
Overridden Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overloaded Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Casting (OCA Objectives 7.3 and 7.4) . . . . . . . . . . . . . . . . . . . . . . . . .
Implementing an Interface (OCA Objective 7.6) . . . . . . . . . . . . . . . .
Legal Return Types (OCA Objectives 2.2, 2.5, 6.1, and 6.3) . . . . . . . .
Return Type Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Returning a Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5) . .
Determine Whether a Default Constructor Will Be Created .
Overloaded Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Initialization Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Statics (OCA Objective 6.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Static Variables and Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Assignments

84
88
92
93
96
100
100
106
113
116
122
122
124
126
130
134
138
140
141
149
154
163

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

Stack and Heap—Quick Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Literals, Assignments, and Variables (OCA Objectives 2.1, 2.2, 2.3,
and Upgrade Objective 1.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Literal Values for All Primitive Types . . . . . . . . . . . . . . . . . . .

166
168
168

Contents

Assignment Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercise 3-1: Casting Primitives . . . . . . . . . . . . . . . . . . . . . .
Scope (OCA Objectives 1.1 and 2.5) . . . . . . . . . . . . . . . . . . . . . . . . . .
Variable Initialization (OCA Objective 2.1) . . . . . . . . . . . . . . . . . . . .
Using a Variable or Array Element That Is Uninitialized and
Unassigned . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Local (Stack, Automatic) Primitives and Objects . . . . . . . . . .
Passing Variables into Methods (OCA Objective 6.8) . . . . . . . . . . . . .
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 . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Operators

172
178
182
185
185
188
194
194
195
196
199
199
200
202
207
209
212
220

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

Java Operators (OCA Objectives 3.1, 3.2, and 3.3) . . . . . . . . . . . . . . .
Assignment Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relational Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
instanceof Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Arithmetic Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conditional Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Logical Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

xvii

Working with Strings, Arrays, and ArrayLists

224
224
226
232
235
240
241
247
249
255

. . . . . . . 257

Using String and StringBuilder (OCA Objectives 2.7 and 2.6) . . . . . .
The String Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

258
258

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OCA/OCP Java SE 7 Programmer I & II Study Guide

Important Facts About Strings and Memory . . . . . . . . . . . . . .
Important Methods in the String Class . . . . . . . . . . . . . . . . . .
The StringBuilder Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Important Methods in the StringBuilder Class . . . . . . . . . . . .
Using Arrays (OCA Objectives 4.1 and 4.2) . . . . . . . . . . . . . . . . . . . .
Declaring an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Constructing an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Initializing an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using ArrayList (OCA Objective 4.3) . . . . . . . . . . . . . . . . . . . . . . . . .
When to Use ArrayLists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ArrayList Methods in Action . . . . . . . . . . . . . . . . . . . . . . . . . .
Important Methods in the ArrayList Class . . . . . . . . . . . . . . .
Encapsulation for Reference Variables . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

Flow Control and Exceptions

264
265
269
271
273
274
275
277
289
289
292
293
294
296
298
305

. . . . . . . . . . . . . . . . . . . . . 307

Using if and switch Statements (OCA Objectives 3.4 and 3.5—
also Upgrade Objective 1.1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
if-else Branching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
switch Statements (OCA, OCP, and Upgrade Topic) . . . . . . .
Exercise 6-1: 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 6-2: Creating a Labeled while Loop . . . . . . . . . . . . . . . . . . .
Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4) . . . . . .
Catching an Exception Using try and catch . . . . . . . . . . . . . .
Using finally . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Propagating Uncaught Exceptions . . . . . . . . . . . . . . . . . . . . . .
Exercise 6-3: Propagating and Catching an Exception . . . . . . . . . . .

308
308
313
320
321
321
323
323
330
331
331
333
334
335
336
339
341

Contents

Defining Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exception Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Handling an Entire Class Hierarchy of Exceptions . . . . . . . . .
Exception Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exception Declaration and the Public Interface . . . . . . . . . . .
Rethrowing the Same Exception . . . . . . . . . . . . . . . . . . . . . . .
Exercise 6-4: Creating an 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 . . . . . . . . . .
End of Part I—OCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xix
342
343
344
345
347
353
353
354
355
355
356
357
357
361
364
373

Part II
OCP
7

Assertions and Java 7 Exceptions

. . . . . . . . . . . . . . . . . 377

Working with the Assertion Mechanism (OCP Objective 6.5) . . . . . .
Assertions Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Enabling Assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using Assertions Appropriately . . . . . . . . . . . . . . . . . . . . . . . .
Working with Java 7 Exception Handling
(OCP Objectives 6.2 and 6.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Use the try Statement with multi-catch and finally Clauses . .
Autocloseable Resources with a try-with-resources
Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

378
379
382
386
389
389
396
404
406
414

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OCA/OCP Java SE 7 Programmer I & II Study Guide

8

String Processing, Data Formatting, Resource Bundles . . 417
String, StringBuilder, and StringBuffer (OCP Objective 5.1) . . . . . . .
Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1,
12.4, 12.5, and 12.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Working with Dates, Numbers, and Currencies . . . . . . . . . . .
Parsing, Tokenizing, and Formatting (OCP Objectives 5.1, 5.2,
and 5.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Search Tutorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Locating Data via Pattern Matching . . . . . . . . . . . . . . . . . . . .
Tokenizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting with printf() and format() . . . . . . . . . . . . . . . . . . .
Resource Bundles (OCP Objectives 12.2, 12.3, and 12.5) . . . . . . . . . .
Resource Bundles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Property Resource Bundles . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Java Resource Bundles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Default Locale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Choosing the Right Resource Bundle . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

I/O and NIO

418
418
419
431
432
443
446
451
454
454
456
457
458
459
463
466
474

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

File Navigation and I/O (OCP Objectives 7.1 and 7.2) . . . . . . . . . . . .
Creating Files Using the File Class . . . . . . . . . . . . . . . . . . . . .
Using FileWriter and FileReader . . . . . . . . . . . . . . . . . . . . . . .
Combining I/O Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Working with Files and Directories . . . . . . . . . . . . . . . . . . . . .
The java.io.Console Class . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Files, Path, and Paths (OCP Objectives 8.1 and 8.2) . . . . . . . . . . . . . .
Creating a Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Creating Files and Directories . . . . . . . . . . . . . . . . . . . . . . . . .
Copying, Moving, and Deleting Files . . . . . . . . . . . . . . . . . . .
Retrieving Information about a Path . . . . . . . . . . . . . . . . . . . .
Normalizing a Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resolving a Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relativizing a Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
File and Directory Attributes (OCP Objective 8.3) . . . . . . . . . . . . . . .
Reading and Writing Attributes the Easy Way . . . . . . . . . . . .

478
480
482
484
487
491
493
495
497
498
500
501
503
505
506
506

Contents

Types of Attribute Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . .
Working with BasicFileAttributes . . . . . . . . . . . . . . . . . . . . . .
Working with DosFileAttributes . . . . . . . . . . . . . . . . . . . . . . .
Working with PosixFileAttributes . . . . . . . . . . . . . . . . . . . . . .
Reviewing Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DirectoryStream (OCP Objective 8.4) . . . . . . . . . . . . . . . . . . . . . . . . .
FileVisitor (OCP Objective 8.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PathMatcher (OCP Objective 8.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
WatchService (OCP Objective 8.6) . . . . . . . . . . . . . . . . . . . . . . . . . . .
Serialization (Objective 7.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 Advanced OO and Design Patterns

xxi
508
509
511
512
513
514
515
519
523
526
528
530
538

. . . . . . . . . . . . . . . 541

IS-A and HAS-A (OCP Objectives 3.3 and 3.4) . . . . . . . . . . . . . . . . .
Coupling and Cohesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cohesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Object Composition Principles (OCP Objective 3.4) . . . . . . . . . . . . .
Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Benefits of Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Singleton Design Pattern (OCP Objective 3.5) . . . . . . . . . . . . . . . . . .
What Is a Design Pattern? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DAO Design Pattern (OCP Objective 3.6) . . . . . . . . . . . . . . . . . . . . .
Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Factory Design Pattern (OCP Objective 3.7) . . . . . . . . . . . . . . . . . . . .
Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

542
543
543
544
545
548
549
549
549
550
551
554
555
555
556
559
560
560
560
563
565
567
571

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OCA/OCP Java SE 7 Programmer I & II Study Guide

11 Generics and Collections

. . . . . . . . . . . . . . . . . . . . . . . . 573

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8) . .
The toString() Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overriding equals() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overriding hashCode() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Collections Overview (OCP Objectives 4.5 and 4.6) . . . . . . . . . . . . . .
So What Do You Do with a Collection? . . . . . . . . . . . . . . . . .
Key Interfaces and Classes of the Collections Framework . . . .
List Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Set Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Map Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Queue Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8) . .
ArrayList Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Autoboxing with Collections . . . . . . . . . . . . . . . . . . . . . . . . . .
The Java 7 “Diamond” Syntax . . . . . . . . . . . . . . . . . . . . . . . . .
Sorting Collections and Arrays . . . . . . . . . . . . . . . . . . . . . . . .
Navigating (Searching) TreeSets and TreeMaps . . . . . . . . . . .
Other Navigation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . .
Backed Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the PriorityQueue Class and the Deque Interface . . . . .
Method Overview for Arrays and Collections . . . . . . . . . . . . .
Method Overview for List, Set, Map, and Queue . . . . . . . . . .
Generic Types (OCP Objectives 4.1 and 4.3) . . . . . . . . . . . . . . . . . . . .
The Legacy Way to Do Collections . . . . . . . . . . . . . . . . . . . . .
Generics and Legacy Code . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mixing Generic and Nongeneric Collections . . . . . . . . . . . . .
Polymorphism and Generics . . . . . . . . . . . . . . . . . . . . . . . . . .
Generic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Generic Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

574
575
576
581
588
588
589
593
594
595
596
598
598
600
603
604
620
621
622
625
626
626
629
630
633
633
639
641
652
661
667
678

Contents

12 Inner Classes

xxiii

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681

Nested Classes (OCP Objective 2.4) . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inner Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coding a “Regular” Inner Class . . . . . . . . . . . . . . . . . . . . . . . .
Referencing the Inner or Outer Instance from Within
the Inner Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Method-Local Inner Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What a Method-Local Inner Object Can and Can’t Do . . . . .
Anonymous Inner Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Plain-Old Anonymous Inner Classes, Flavor One . . . . . . . . . .
Plain-Old Anonymous Inner Classes, Flavor Two . . . . . . . . . .
Argument-Defined Anonymous Inner Classes . . . . . . . . . . . .
Static Nested Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instantiating and Using Static Nested Classes . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 Threads

683
683
685
688
690
691
692
693
696
697
699
700
702
704
710

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713

Defining, Instantiating, and Starting Threads (OCP Objective 10.1) . .
Defining a Thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instantiating a Thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Starting a Thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thread States and Transitions (OCP Objective 10.2) . . . . . . . . . . . . .
Thread States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preventing Thread Execution . . . . . . . . . . . . . . . . . . . . . . . . .
Sleeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercise 13-1: Creating a Thread and Putting It to Sleep . .
Thread Priorities and yield( ) . . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronizing Code, Thread Problems (OCP Objectives 10.3
and 10.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronization and Locks . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercise 13-2: Synchronizing a Block of Code . . . . . . . . . . .
Thread Deadlock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thread Interaction (OCP Objectives 10.3 and 10.4) . . . . . . . . . . . . . .
Using notifyAll( ) When Many Threads May Be Waiting . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

714
717
718
720
728
729
731
731
733
734
738
744
747
753
755
760
765
769

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OCA/OCP Java SE 7 Programmer I & II Study Guide

Self Test Answers
Exercise Answers

14 Concurrency

...................................
...................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785

Concurrency with the java.util.concurrent Package . . . . . . . . . . . . . . .
Apply Atomic Variables and Locks (OCP Objective 11.2) . . . . . . . . .
Atomic Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Locks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Use java.util.concurrent Collections (OCP Objective 11.1) and
Use a Deque (OCP Objective 4.5) . . . . . . . . . . . . . . . . . . . . . . . . . .
Copy-on-Write Collections . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concurrent Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blocking Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Use Executors and ThreadPools (OCP Objective 11.3) . . . . . . . . . . . .
Identifying Parallel Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
How Many Threads Can You Run? . . . . . . . . . . . . . . . . . . . . .
CPU-Intensive vs. I/O-Intensive Tasks . . . . . . . . . . . . . . . . . .
Fighting for a Turn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Decoupling Tasks from Threads . . . . . . . . . . . . . . . . . . . . . . . .
Use the Parallel Fork/Join Framework (OCP Objective 11.4) . . . . . . .
Divide and Conquer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ForkJoinPool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ForkJoinTask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 JDBC

781
784

786
786
787
789
797
799
800
801
805
806
807
807
808
809
815
816
817
817
829
832
838

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841

Starting Out: Introduction to Databases and JDBC . . . . . . . . . . . . . . .
Talking to a Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bob’s Books, Our Test Database . . . . . . . . . . . . . . . . . . . . . . . .
Core Interfaces of the JDBC API (OCP Objective 9.1) . . . . . . . . . . . .
Connect to a Database Using DriverManager (OCP Objective 9.2) . .
The DriverManager Class . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The JDBC URL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
JDBC Driver Implementation Versions . . . . . . . . . . . . . . . . . .

842
844
847
851
853
854
858
860

Contents

Submit Queries and Read Results from the Database
(OCP Objective 9.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
All of Bob’s Customers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ResultSets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Updating ResultSets (Not on the Exam!) . . . . . . . . . . . . . . . .
When Things Go Wrong—Exceptions and Warnings . . . . . .
Use PreparedStatement and CallableStatement Objects
(OCP Objective 9.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PreparedStatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CallableStatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Construct and Use RowSet Objects (OCP Objective 9.5) . . . . . . . . . .
Working with RowSets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
JDBC Transactions (OCP Objective 9.4) . . . . . . . . . . . . . . . . . . . . . . .
JDBC Transaction Concepts . . . . . . . . . . . . . . . . . . . . . . . . . .
Starting a Transaction Context in JDBC . . . . . . . . . . . . . . . . .
Rolling Back a Transaction . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using Savepoints with JDBC . . . . . . . . . . . . . . . . . . . . . . . . . .
✓ Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Q&A Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxv

861
861
863
868
889
901
906
908
910
913
914
921
922
922
924
926
932
935
944

Appendix A

Serialization

. . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Serialization (OCP 7 Objective 7.2) . . . . . . . . . . . . . . . . . . . . . . . . . . .
Working with ObjectOutputStream and ObjectInputStream .
Object Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
How Inheritance Affects Serialization . . . . . . . . . . . . . . . . . . .
Serialization Is Not for Statics . . . . . . . . . . . . . . . . . . . . . . . . .
Certification Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix B Classpaths and JARs

. . . . . . . . . . . . . . . . . . . . B-1

Using the javac and java Commands (OCPJP Exam Objectives 7.1, 7.2,
and 7.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compiling with javac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Launching Applications with java . . . . . . . . . . . . . . . . . . . . . .
Searching for Other Classes . . . . . . . . . . . . . . . . . . . . . . . . . . .
JAR Files (Objective 7.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
JAR Files and Searching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using Static Imports (Objective 7.1) . . . . . . . . . . . . . . . . . . . . . . . . . .
Static Imports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Certification Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two-Minute Drill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self Test Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix C About the Download

B-2
B-3
B-5
B-8
B-13
B-13
B-16
B-16
B-18
B-19
B-21
B-29

. . . . . . . . . . . . . . . . . . . 947

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bonus Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windows 8 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . .
McGraw-Hill Education Content Support . . . . . . . . . . . . . . .

Index

A-2
A-2
A-4
A-10
A-14
A-15
A-16
A-17
A-21

948
948
949
950
950
951
951
951
951
952
952
952

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953

ACKNOWLEDGMENTS

K

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 12 years now), LeeAnn Pickrell (and
team), and Jim Kussow. 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 of our friends at Kraftur (and our other horse-related friends) and most

especially to Sherry, Steinar, Stina and the girls, Jec, Lucy, Cait, and Jennifer,
Leslie and David, Annette and Bruce, Kacey, DJ, Gabrielle, and Mary.
Thanks to Pedro and Ely, who can’t believe it can take so long to finish
a book.
■ 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 new, wonderful, and talented team at Oracle: Linda Brown, Julia

Johnson, Peter Fernandez, and Harold Green.
■ The crew at Oracle who worked hard to build these exams: Tom McGinn,

Matt Heimer, Mike Williams, Stuart Marks, Cindy Church, Kenny
Somerville, Raymond Gallardo, Stacy Thurston, Sowmya Kannan, Jim
Holmlund, Mikalai Zaikin, Sharon Zakhour, Lawrence Chow, and Yamuna
Santhakumari.
■ Our old wonderful and talented certification team at Sun Educational

Services, primarily the most persistent get-it-done person we know, Evelyn
Cartagena.
■ Our great friends and gurus, Simon Roberts, Bryan Basham, and

Kathy Collina.

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OCA/OCP Java SE 7 Programmer I & II Study Guide

■ Stu, Steve, Burt, and Eric 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 JavaRanch Trail Boss Paul Wheaton, for running the best Java

community site on the Web, and to all the generous and patient JavaRanch
moderators.
■ To all the past and present Sun Ed 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.

PREFACE

T

his book’s primary objective is to help you prepare for and pass Oracle’s OCA Java SE 7
and OCP Java SE 7 Programmer I & II certification exams.
If you already have an SCJP certification, all of the topics covered in the OCP 7
Upgrade exam are covered here as well. And, if for some reason it’s appropriate for
you to obtain an OCPJP 5 or OCPJP 5 Java certification, the contents of the book
and the bonus material will help you cover all those bases.
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 a regex guru, but if
you study the material and do well on the Self Tests, you’ll have a basic
understanding of regex, and you’ll do well on the exam. After completing this book,
you should feel confident that you have thoroughly reviewed all of the objectives
that Oracle has established for these exams.

In This Book
This book is organized in two parts to optimize your learning of the topics covered
by the OCA 7 exam in Part I and the OCP 7 exam in Part II. Whenever possible,
we’ve organized the chapters to parallel the Oracle objectives, but sometimes we’ll
mix up objectives or partially repeat them in order to present topics in an order
better suited to learning the material.
Serialization was a topic on the old SCJP 5 and SCJP 6 exams, and recently (as of
the summer of 2014), Oracle reintroduced serialization for the OCP 7 exam. Please
see the Appendix A included with this book for in-depth, complete chapter coverage
of serialization, right down to a Self Test. In addition to fully covering the OCA 7
and OCP 7 exams, Appendix B covers OCPJP 5 and OCPJP 6 topics, and eight
chapters that cover important aspects of Oracle’s Java SE 6 Developer exam at
are available for download at McGraw-Hill Professional's Media Center
see Appendix C for details).

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OCA/OCP Java SE 7 Programmer I & II Study Guide

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 Watch notes call attention to information about, and potential
pitfalls in, the exam. Since 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.

✓
Q&A

■ 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.
■ 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
Java SE 7 Programmer I and OCP Java SE 7 Programmer II exams 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.
Appendix A and Appendix B complete the coverage necessary for the OCP 7,
OCPJP 6, and OCPJP 5 certifications. Also an in-depth review of the essential
ingredients for a successful assessment of a project submitted for the Oracle Java SE 6
Developer exam. is available for download at McGraw-Hill's Media Center
(see Appendix C).
Throughout this book and online content, you’ll find support for six exams:
■ OCA Java SE 7 Programmer I
■ OCP Java SE 7 Programmer II
■ Upgrade to Java SE 7 Programmer
■ OCP Java SE 6 Programmer
■ OCP Java SE 5 Programmer
■ Java SE 6 Developer

Finally, the practice exam software with the equivalent of four practice exams
(two 60-question exams for OCA candidates and two 85-question exams for OCP
candidates) is available for download (see Appendix C).

What This Book Is Not
You will not find a beginner’s guide to learning Java in this book. All 1,000+ pages
of this book are dedicated solely to helping you pass the exams. 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.

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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.

Available for Download
For more information about online content, please see the Appendix C.

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.

Introduction

xxxiii

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 OCP 7 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.
The Oracle Java SE 6 Developer Exam (covered in chapters that are available
for download) is unique to the IT certification realm because it actually evaluates
your skill as a developer rather than simply your knowledge of the language or tools.
Becoming a Certified Java Developer is, by definition, a development experience.

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 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.

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OCA/OCP Java SE 7 Programmer I & II Study Guide

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 the majority of its certifications, Oracle evaluates candidates using a computerbased 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 secondguess every answer and talk yourself out of a correct one.
After completing the exam, you will receive an email from Oracle telling you that
your results are available on the Web. As of summer 2014, your results can be found at
certview.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-Choice Questions
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:

Introduction

xxxv

■ Whenever we can, our questions will not 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 that 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, at www.Oracle.com.

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OCA/OCP Java SE 7 Programmer I & II Study Guide

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 that 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-minutesper-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

Introduction

xxxvii

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 the JavaRanch.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.
You’ll find us (the authors) there several times a week, helping those just starting out
on their exam preparation journey. (You won’t actually think of it as anything as
pleasant sounding as a “journey” by the time you’re ready to take the exam.)
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 Your Exam
You can purchase your exam voucher from Oracle or Pearson VUE. Visit Oracle.com
(follow the training/certification links) or visit PearsonVue.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.

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OCA/OCP Java SE 7 Programmer I & II Study Guide

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 pager and telephone in the car, or turn them off. They 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 that the questions you get on the exam will not 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.

Introduction

xxxix

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 that 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

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OCA/OCP Java SE 7 Programmer I & II Study Guide

(although you’ll probably want to skip a review of the drag-and-drop questions!),
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

xli

Objectives Map
The following four tables—one for the OCA Java SE 7 Programmer I Exam, one
for the OCP Java SE 7 Programmer II Exam, one for the Upgrade to Java SE 7
Programmer Exam, and one for the OCP Java Programmer 5 and OCP Java
Programmer 6 exams—describe the objectives and where you will find them in
the book.

Oracle Certified Associate Java SE 7 Programmer (Exam 1Z0-803)
Official Objective

Study Guide Coverage

Java Basics
Define the scope of variables (1.1)

Chapter 3

Define the structure of a Java class (1.2)

Chapter 1

Create executable Java applications with a main method (1.3)

Chapter 1

Import other Java packages to make them accessible in your code (1.4)

Chapter 1

Working with Java Data Types
Declare and initialize variables (2.1)

Chapters 1 and 3

Differentiate between object reference variables and primitive variables (2.2)

Chapter 2

Read or write to object fields (2.3)

Whole book

Explain an object’s lifecycle (creation, “dereference,” and garbage collection) (2.4)

Chapters 2 and 3

Call methods on objects (2.5)

Whole book

Manipulate data using the StringBuilder class and its methods (2.6)

Chapter 5

Create and manipulate Strings (2.7)

Chapter 5

Using Operators and Decision Constructs
Use Java operators (3.1)

Chapter 4

Use parentheses to override operator precedence (3.2)

Chapter 4

Test equality between Strings and other objects using == and equals() (3.3)

Chapter 4

Create if and if/else constructs (3.4)

Chapter 6

Use a switch statement (3.5)

Chapter 6

Creating and Using Arrays
Declare, instantiate, initialize and use a one-dimensional array (4.1)

Chapter 5

Declare, instantiate, initialize and use multi-dimensional array (4.2)

Chapter 5

Declare and use an ArrayList (4.3)

Chapter 5

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OCA/OCP Java SE 7 Programmer I & II Study Guide

OCA Java SE 7 Objectives (cont.)
Official Objective

Study Guide Coverage

Using Loop Constructs
Create and use while loops (5.1)

Chapter 6

Create and use for loops including the enhanced for loop (5.2)

Chapter 6

Create and use do/while loops (5.3)

Chapter 6

Compare loop constructs (5.4)

Chapter 6

Use break and continue (5.5)

Chapter 6

Working with Methods and Encapsulation
Create methods with arguments and return values (6.1)

Chapters 2 and 3

Apply the static keyword to methods and fields (6.2)

Chapter 1

Create an overloaded method (6.3)

Chapter 2

Differentiate between default and user defined constructors (6.4)

Chapter 2

Create and overload constructors (6.5)

Chapter 2

Apply access modifiers (6.6)

Chapter 1

Apply encapsulation principles to a class (6.7)

Chapter 2

Determine the effect upon object references and primitive values when they are passed
into methods that change the values (6.8)

Chapter 3

Working with Inheritance
Implement inheritance (7.1)

Chapter 2

Develop code that demonstrates the use of polymorphism (7.2)

Chapter 2

Differentiate between the type of a reference and the type of an object (7.3)

Chapter 2

Determine when casting is necessary (7.4)

Chapter 2

Use super and this to access objects and constructors (7.5)

Chapter 2

Use abstract classes and interfaces (7.6)

Chapters 1 and 2

Handling Exceptions
Differentiate among checked exceptions, RuntimeExceptions, and Errors (8.1)

Chapter 6

Create a try-catch block and determine how exceptions alter normal program flow (8.2)

Chapter 6

Describe what exceptions are used for in Java (8.3)

Chapter 6

Invoke a method that throws an exception (8.4)

Chapter 6

Recognize common exception classes and categories (8.5)

Chapter 6

Objectives Map

xliii

Oracle Certified Professional Java SE 7 Programmer II
(Exam IZ0-804)
Although the OCP objectives are not specifically listed in Part I of the book, many
of them are covered in those chapters, as detailed here, as material is duplicated
across the two exams.
Official Objective

Study Guide Coverage

Java Class Design
Use access modifiers: private, protected, and public (1.1)

Chapter 1

Override methods (1.2)

Chapter 2

Overload constructors and methods (1.3)

Chapter 2

Use the instanceof operator and casting (1.4)

Chapter 2

Use virtual method invocation (1.5)

Chapters 2 and 10

Override the hashcode, equals, and toString methods from the Object class to improve the
functionality of your class (1.6)

Chapter 11

Use package and import statements (1.7)

Chapter 1

Advanced Class Design
Identify when and how to apply abstract classes (2.1)

Chapter 1

Construct abstract Java classes and subclasses (2.2)

Chapters 1 and 2

Use the static and final keywords (2.3)

Chapter 1

Create top level and nested classes (2.4)

Chapters 1–3, 12

Use enumerated types (2.5)

Chapter 1

Object-Oriented Design Principles
Write code that declares, implements, and/or extends interfaces (3.1)

Chapters 1 and 2

Choose between interface inheritance and class inheritance (3.2)

Chapter 2

Apply cohesion, low-coupling, IS-A, and HAS-A principles (3.3)

Chapters 2 and 10

Apply object composition principles (including HAS-A relationships) (3.4)

Chapters 2 and 10

Design a class using a singleton design pattern (3.5)

Chapter 10

Write code to implement the Data Access Object (DAO) (3.6)

Chapter 10

Design and create objects using a factory and use factories from the API (3.7)

Chapter 10

Generics and Collections
Create a generic class (4.1)

Chapter 11

Use the diamond syntax to create a collection (4.2)

Chapter 11

Analyze the interoperability of collections that use raw and generic types (4.3)

Chapter 11

xliv

OCA/OCP Java SE 7 Programmer I & II Study Guide

OCP Java SE 7 Objectives (cont.)
Official Objective

Study Guide Coverage

Use wrapper classes and autoboxing (4.4)

Chapter 11

Create and use a List, a Set, and a Deque (4.5)

Chapters 11 and 14

Create and use a Map (4.6)

Chapter 11

Use java.util.Comparator and java.lang.Comparable (4.7)

Chapter 11

Sort and search arrays and lists (4.8)

Chapter 11

String Processing
Search, parse, and build strings (including Scanner, StringTokenizer, StringBuilder, String,
and Formatter) (5.1)

Chapter 8

Search, parse, and replace strings by using regular expressions, using expression patterns for
matching limited to . (dot), * (star), + (plus), ?, \d, \D, \s, \S, \w, \W, \b, \B, [], and ().
(5.2)

Chapter 8

Format strings using the formatting parameters %b, %c, %d, %f, and %s in format strings.
(5.3)

Chapter 8

Exceptions and Assertions
Use throw and throws statements (6.1)

Chapters 6 and 7

Develop code that handles multiple Exception types in a single catch block (6.2)

Chapter 7

Develop code that uses try-with-resources statements (including classes that implement the
AutoCloseable interface) (6.3)

Chapter 7

Create custom exceptions (6.4)

Chapters 6 and 7

Test invariants by using assertions (6.5)

Chapter 7

Java I/O Fundamentals
Read and write data from the console (7.1)

Chapter 9

Use streams to read from and write to files by using classes in the java.io package,
including BufferedReader, BufferedWriter, File, FileReader, FileWriter, DataInputStream,
DataOutputStream, ObjectOutputStream, ObjectInputStream, and PrintWriter (7.2)

Chapter 9 and
downloadable content

Java File I/O (NIO.2)
Operate on file and directory paths with the Path class (8.1)

Chapter 9

Check, delete, copy, or move a file or directory with the Files class (8.2)

Chapter 9

Read and change file and directory attributes, focusing on the BasicFileAttributes,
DosFileAttributes, and PosixFileAttributes interfaces (8.3)

Chapter 9

Recursively access a directory tree using the DirectoryStream and FileVisitor interfaces (8.4)

Chapter 9

Find a file with the PathMatcher interface (8.5)

Chapter 9

Watch a directory for changes with the WatchService interface (8.6)

Chapter 9

Objectives Map

Official Objective

Study Guide Coverage

Building Database Applications with JDBC
Describe the interfaces that make up the core of the JDBC API (including the Driver,
Connection, Statement, and ResultSet interfaces and their relationships to provider
implementations) (9.1)

Chapter 15

Identify the components required to connect to a database using the DriverManager class
(including the JDBC URL) (9.2)

Chapter 15

Submit queries and read results from the database (including creating statements; returning
result sets; iterating through the results; and properly closing result sets, statements, and
connections) (9.3)

Chapter 15

Use JDBC transactions (including disabling auto-commit mode, committing and rolling back
transactions, and setting and rolling back to savepoints) (9.4)

Chapter 15

Construct and use RowSet objects using the RowSetProvider class and the RowSetFactory
interface (9.5)

Chapter 15

Create and use PreparedStatement and CallableStatement objects (9.6)

Chapter 15

Threads
Create and use the Thread class and the Runnable interface (10.1)

xlv

Chapter 13

Manage and control thread lifecycle (10.2)

Chapter 13

Synchronize thread access to shared data (10.3)

Chapter 13

Identify code that may not execute correctly in a multithreaded environment (10.4)

Chapter 13

Concurrency
Use collections from the java.util.concurrent package with a focus on the advantages over
and differences from the traditional java.util collections (11.1)

Chapter 14

Use Lock, ReadWriteLock, and ReentrantLock classes in the java.util.concurrent.locks
package to support lock-free thread-safe programming on single variables (11.2)

Chapter 14

Use Executor, ExecutorService, Executors, Callable, and Future to execute tasks using
thread pools (11.3)

Chapter 14

Use the parallel Fork/Join Framework (11.4)

Chapter 14

Localization
Read and set the locale using the Locale object (12.1)

Chapter 8

Build a resource bundle for each locale (12.2)

Chapter 8

Call a resource bundle from an application (12.3)

Chapter 8

Format dates, numbers, and currency values for localization with the NumberFormat and
DateFormat classes (including number format patterns) (12.4)

Chapter 8

Describe the advantages of localizing an application (12.5)

Chapter 8

Define a locale using language and country codes (12.6)

Chapter 8

xlvi

OCA/OCP Java SE 7 Programmer I & II Study Guide

Upgrade to Java SE 7 Programmer (Exam IZ0-805)
Official Objective

Study Guide Coverage

Language Enhancements
Develop code that uses String objects in switch statements (1.1)

Chapter 6

Develop code that uses binary literals and numeric literals with underscores (1.2)

Chapter 3

Develop code that uses try-with-resources statements (including classes that implement
the AutoCloseable interface) (1.3)

Chapter 7

Develop code that handles multiple exception types in a single catch block (1.4)

Chapter 7

Develop code that uses the diamond with generic declarations (1.5)

Chapter 11

Design Patterns
Design a class using a singleton design pattern (2.1)

Chapter 10

Apply object composition principles (including HAS-A relationships) (2.2)

Chapters 2 and 10

Write code to implement the Data Access Object (DAO) (2.3)

Chapter 10

Design and create objects using a factory pattern (2.4)

Chapter 10

Database Applications with JDBC
Describe the interfaces that make up the core of the JDBC API (including the Driver,
Connection, Statement, and ResultSet interfaces and their relationships to provider
implementations) (3.1)

Chapter 15

Identify the components required to connect to a database using the DriverManager class
(including the JDBC URL) (3.2)

Chapter 15

Construct and use RowSet objects using the RowSetProvider class and the RowSetFactory
interface (3.3)

Chapter 15

Use JDBC transactions (including disabling auto-commit mode, committing and rolling back
transactions, and setting and rolling back to savepoints) (3.4)

Chapter 15

Submit queries and read results from the database (including creating statements; returning
result sets; iterating through the results; and properly closing result sets, statements, and
connections) (3.5)

Chapter 15

Create and use PreparedStatement and CallableStatement objects (3.6)

Chapter 15

Concurrency
Identify code that may not execute correctly in a multithreaded environment (4.1)

Chapter 13

Use collections from the java.util.concurrent package with a focus on the advantages over
and differences from the traditional java.util collections (4.2)

Chapter 14

Use Lock, ReadWriteLock, and ReentrantLock classes in the java.util.concurrent.locks
package to support lock-free thread-safe programming on single variables (4.3)

Chapter 14

Use Executor, ExecutorService, Executors, Callable, and Future to execute tasks using
thread pools (4.4)

Chapter 14

Use the parallel Fork/Join Framework (4.5)

Chapter 14

Objectives Map

Official Objective

xlvii

Study Guide Coverage

Localization
Describe the advantages of localizing an application (5.1)

Chapter 8

Define a locale using language and country codes (5.2)

Chapter 8

Read and set the locale by using the Locale object (5.3)

Chapter 8

Build a resource bundle for each locale (5.4)

Chapter 8

Call a resource bundle from an application (5.5)

Chapter 8

Format dates, numbers, and currency values for localization with the NumberFormat and
DateFormat classes (including number format patterns) (5.6)

Chapter 8

Java File I/O (NIO.2)
Operate on file and directory paths with the Path class (6.1)

Chapter 9

Check, delete, copy, or move a file or directory with the Files class (6.2)

Chapter 9

Read and change file and directory attributes, focusing on the BasicFileAttributes,
DosFileAttributes, and PosixFileAttributes interfaces (6.3)

Chapter 9

Recursively access a directory tree using the DirectoryStream and FileVisitor interfaces (6.4)

Chapter 9

Find a file with the PathMatcher interface (6.5)

Chapter 9

Watch a directory for changes with the WatchService interface (6.6)

Chapter 9

Java SE 5 Programmer and OCP Java Programmer 6
Official Objective

Study Guide Coverage

1. Declarations, Initialization and Scoping

Chapters 1–3, 5, and 12

2. Flow Control

Chapters 6 and 7

3. API Contents

Chapters 5, 8, 9, and 11

4. Concurrency

Chapter 13

5. OO Concepts

Chapters 2 and 10

6. Collections/Generics

Chapter 11

7. Fundamentals

Chapters 1–4, Appendix B

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Part I
OCA and OCP

CHAPTERS
1

Declarations and Assets

4

Operators

2

Object Orientation

5

Working with Strings, Arrays, and ArrayLists

3

Assignments

6

Flow Control and Exceptions

This page intentionally left blank

1
Declarations and
Access Control

CERTIFICATION OBJECTIVES

•
•
•
•
•

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

4

Chapter 1:

Declarations and Access Control

W

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 of objects talking to other objects by invoking
each other's methods. Every object is of a certain type, and that type is defined by a
class or an interface. 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 own state 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's state.

■ 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-in keywords. These
keywords must not be used as identifiers. Later in this chapter we'll review the
details of these naming rules, conventions, and the Java keywords.

Java Refresher

5

Inheritance
Central to Java and other OO languages is the concept of inheritance, which allows
code defined in one class 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, a Car superclass could define general methods common to all automobiles,
but a Ferrari subclass could override the accelerate() method that was already
defined in the Car class.

Interfaces
A powerful companion to inheritance is the use of interfaces. Interfaces are like a
100-percent abstract superclass that defines the methods a subclass must support, but
not how they must be supported. In other words, for example, an Animal interface
might declare that all Animal implementation classes have an eat() method, but
the Animal 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 particular Animal type behaves when its eat() method is invoked.

Finding Other Classes
As we'll see later in the book (for you OCP candidates), it's a good idea to make
your classes cohesive. That means that every class should have a focused set of
responsibilities. For instance, if you were creating a zoo simulation program, you'd
want to represent aardvarks with one class and zoo visitors with a different class. In
addition, you might have a Zookeeper class and a PopcornVendor class. The point
is that you don't want a class that has both Aardvark and PopcornVendor behaviors
(more on that in Chapter 10).
Even a simple Java program uses objects from many different classes: some that
you created, and some built by others (such as Oracle's Java API classes). Java
organizes classes into packages and uses import statements to give programmers a
consistent way to manage naming of, and access to, classes they need. The exam
covers a lot of concepts related to packages and class access; we'll explore the details
throughout the book.

6

Chapter 1:

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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.

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, but using 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 that 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 are considered to
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 you do 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!

Identifiers and Keywords (OCA Objectives 1.2 and 2.1)

7

■ 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 of the Java

keywords.
■ Identifiers in Java are case-sensitive; foo and FOO are two different identifiers.

Examples of legal and illegal identifiers follow. First some legal identifiers:
int
int
int
int
int

_a;
$c;
______2_w;
_$;
this_is_a_very_detailed_name_for_an_identifier;

The following are illegal (it's your job to recognize why):
int
int
int
int
int

:b;
-d;
e#;
.f;
7g;

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.
TABLE 1-1

Complete List of Java Keywords (assert added in 1.4, enum added in 1.5)

abstract

boolean

break

byte

case

char

class

const

continue

default

catch
do

double

else

extends

final

finally

float

for

goto

if

implements

import

instanceof

int

interface

long

native

new

package

private

protected

public

return

short

static

strictfp

super

switch

synchronized

this

throw

throws

transient

try

void

volatile

while

assert

enum

8

Chapter 1:

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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
at java.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:
1. class Wombat implements Runnable {
2.
private int i;
3.
public synchronized void run() {
4.
if (i%5 != 0) { i++; }
5.
for(int x=0; x<5; x++, i++)
6.
{ if (x > 1) Thread.yield(); }
7.
System.out.print(i + " ");
8.
}
9.
public static void main(String[] args) {
10.
Wombat n = new Wombat();
11.
for(int x=100; x>0; --x) { new Thread(n).start(); }
12. } }

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 that are
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

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

9

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, 6.6, and 7.6)
1.2

Define the structure of a Java class.

1.3

Create executable Java applications with a main method.

10

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1.4

Import other Java packages to make them accessible in your code.

6.6

Apply access modifiers.

7.6

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, a public 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, the package statement must be the first line

in the source code file, before any import 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.

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

11

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 nonpublic 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 the real 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 for javac:
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 both legal javac 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

12

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and/or files, they should be separated by spaces. (Note: If you're studying for the
OCP 7, in Chapter 7 we'll talk about the assertion mechanism and when you might
use the -source option when compiling a file.)

Launching Applications 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. (Of the two exams, only the OCP 7 will use
[options].) You must specify exactly one class file to execute, and the java
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 -version 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 String
arguments whose values are x and 1." Let's look at the following code:
public class MyClass {
public static void main(String[] args) {
System.out.println(args[0] + " " + args[1]);
}
}

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—uses 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.
Note: Again, for the OCP 7 candidates, in Chapter 7 we'll talk about the
assertion mechanism and when you might use flags such as -ea or -da when
launching an application.

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

13

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% 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 with main(). As far as the compiler
and the JVM are concerned, the only version of main() with superpowers is the
main() with this signature:
public static void main(String[] args)

Other versions of main() 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 as of Java 5 it can be declared using var-args syntax. The following are all legal
declarations for the "special" main():
static public void main(String[] args)
public static void main(String... x)
static public void main(String bang_a_gong[])

For the OCA exam, the only other thing that's important for you to know is that
main() 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:
public class ArrayList {
public static void main(String[] args) {
System.out.println("fake ArrayList class");
}
}

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 is java.util.ArrayList. That's its fully qualified name. The
use of fully qualified names is what helps Java developers make sure that two

14

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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:
public class MyClass {
public static void main(String[] args) {
java.util.ArrayList a =
new java.util.ArrayList();
}
}

(First off, trust us on the  syntax; we'll get to that later.) While 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 using ArrayLists many times.
import statements to the rescue! Instead of the preceding code, our class could
look like this:
import java.util.ArrayList;
public class MyClass {
public static void main(String[] args) {
ArrayList a = new ArrayList();
}
}

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 your import 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:
import java.util.*;
public class MyClass {
public static void main(String[] args) {
ArrayList a = new ArrayList();
TreeSet t = new TreeSet();
}
}

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

15

When the compiler and the JVM see this code, they'll know to look through
java.util for ArrayList and TreeSet. For the exam, the last thing you'll need to
remember about using import statements in your classes is that you're free to mix
and match. It's okay to say this:
ArrayList a = new ArrayList();
java.util.ArrayList a2 = new java.util.ArrayList();

Static Import Statements
Dear Reader, We really struggled with when 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 with Chapter 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 contain static members. (We'll talk more about static class
members later, but since we were 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, 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:
public class TestStatic {
public static void main(String[] args) {
System.out.println(Integer.MAX_VALUE);
System.out.println(Integer.toHexString(42));
}
}

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After static imports:
import static java.lang.System.out;
import static java.lang.Integer.*;
public class TestStaticImport {
public static void main(String[] args)
out.println(MAX_VALUE);
out.println(toHexString(42));
}
}

// 1
// 2
{
// 3
// 4

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 in System.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 say import static; you can't say static 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, since both Integer 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 on static object references, constants (remember

they're static and final), and static methods.

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

17

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:
import java.util.*;

// ok to search the java.util package

In a similar vein, if you want to "search through the java.lang.Integer class for static
members" you can say this:
import static java.lang.Integer.*;

// ok to search the
// java.lang.Integer class

But you can't create broader searches. For instance, you CANNOT use an import to
search through the entire Java API:
import java.*;

// Legal, but this WILL NOT search across packages.

Class Declarations and Modifiers
The class declarations we'll discuss in this section are limited to top-level classes.
Although nested classes (often called inner classes) are included on the OCP
exam, we'll save nested class declarations for Chapter 12. If you're an OCP
candidate, you're going to love that chapter. 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 (including strictfp, 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 or package access) is what you get when you don't use
any of the three access modifiers. In other words, every class, method, and instance

18

Chapter 1:

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variable you declare has an access control, whether you explicitly type one or not.
Although all four access controls (which means all three modifiers) work for most
method and variable declarations, a class can be declared with only public 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 an instance 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.

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

19

Default Access 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 of default 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:
package cert;
class Beverage { }

Now look at the second source file:
package exam.stuff;
import cert.Beverage;
class Tea extends Beverage { }

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 get something like this:
Can't access class cert.Beverage. Class or interface must be public, in same
package, or an accessible member class.
import cert.Beverage;

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.

Public Access
A class declaration with the public 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 a public class. Don't forget, though, that if a public class you're trying to

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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
modifier final. Read on.

Other (Nonaccess) Class Modifiers
You can modify a class declaration using the keyword final, abstract, or
strictfp. These modifiers are in addition to whatever access control is on the class,
so you could, for example, declare a class as both public and final. But you can't
always mix nonaccess modifiers. You're free to use strictfp in combination with
final, for example, but you must never, ever, ever mark a class as both final and
abstract. You'll see why in the next two sections.
You won't need to know how strictfp works, so we're focusing only on
modifying a class as final 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 as strictfp means that any method code in the class will
conform 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 get strictfp 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 class final? After all, doesn't that violate the
whole OO notion of inheritance? You should make a final class only if you need an

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

21

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 using
final 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 new
String 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 of having nonfinal classes is 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 is final, though, you're stuck.
Let's modify our Beverage example by placing the keyword final in the
declaration:
package cert;
public final class Beverage {
public void importantMethod() { }
}

Now let's try to compile the Tea subclass:
package exam.stuff;
import cert.Beverage;
class Tea extends Beverage { }

We get an error—something like this:
Can't subclass final classes: class
cert.Beverage class Tea extends Beverage{
1 error

In practice, you'll almost never make a final class. A final class obliterates a key
benefit of OO—extensibility. So 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
.

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Abstract Classes An abstract 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
class Car that has generic methods common to all vehicles. But you don't want
anyone actually creating a generic, abstract Car 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 following abstract class:
abstract class Car {
private double price;
private String model;
private String year;
public abstract void goFast();
public abstract void goUpHill();
public abstract void impressNeighbors();
// Additional, important, and serious code goes here
}

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:
AnotherClass.java:7: class Car is an abstract
class. It can't be instantiated.
Car x = new Car();
1 error

Notice that the methods marked abstract end 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 an abstract modifier on the class or method. In that case, you could
either mark the method and class abstract or change the semicolon to code (like a
curly brace pair). Remember that if you change a method from abstract to
nonabstract, don't forget to change the semicolon at the end of the method
declaration into a curly brace pair!

Define Classes (OCA Objectives 1.2, 1.3, 1.4, 6.6, and 7.6)

23

We'll look at abstract methods in more detail later in this objective, but always
remember that if even a single method is abstract, the whole class must be
declared abstract. One abstract method spoils the whole bunch. You can,
however, put nonabstract methods in an abstract 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 an
abstract 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 with abstract class types (including interfaces, discussed later in this
chapter) lets you take advantage of polymorphism, and gives you the greatest degree
of flexibility and extensibility. You'll learn more about polymorphism in Chapter 2.
You can't mark a class as both abstract and final. They have nearly opposite
meanings. An abstract class must be subclassed, whereas a final class must not be
subclassed. If you see this combination of abstract and final modifiers used for a
class or method declaration, the code will not compile.

EXERCISE 1-1
Creating an Abstract Superclass and Concrete Subclass
The following exercise will test your knowledge of public, default, final, and
abstract classes. Create an abstract 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 superclass public 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 */}

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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 7.6)
7.6

Use abstract classes and interfaces.

Declaring an Interface
When you create an interface, you're defining a contract for what a class can do,
without saying anything about how the class will do it. An interface is a contract.
You could write an interface Bounceable, for example, that says in effect, "This is
the Bounceable interface. Any class type that implements this interface must agree
to write the code for the bounce() and setBounceFactor() methods."
By defining an interface for Bounceable, any class that wants to be treated as a
Bounceable thing can simply implement the Bounceable 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 a Ball and a Tire to have bounce behavior, but
Ball and Tire don't share any inheritance relationship; Ball extends Toy while
Tire extends only java.lang.Object. But by making both Ball and Tire
implement Bounceable, you're saying that Ball 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.
Think of an interface as a 100-percent abstract class. Like an abstract class,
an interface defines abstract methods that take the following form:
abstract void bounce();

// Ends with a semicolon rather than
// curly braces

Use Interfaces (OCA Objective 7.6)

FIGURE 1-1

The relationship
between
interfaces and
classes

25

interface Bounceable
void bounce( );
void setBounceFactor(int bf);

What you
declare.

interface Bounceable
public abstract void bounce( );
public abstract void setBounceFactor(int bf);

Class Tire implements Bounceable
public void bounce( ){...}
public void setBounceFactor(int bf){ }

What the
compiler
sees.

What the
implementing
class must do.
(All interface
methods must
be implemented,
and must be
marked public.)

But although an abstract class can define both abstract and nonabstract
methods, an interface can have only abstract methods. Another way interfaces
differ from abstract classes is that interfaces have very little flexibility in how the
methods and variables defined in the interface are declared. These rules are strict:
■ All interface methods are implicitly public and abstract. In other words,

you do not need to actually type the public or abstract 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 must not be static.
■ Because interface methods are abstract, they cannot be marked final,
strictfp, or native. (More on these modifiers later in the chapter.)

■ An interface can extend one or more other interfaces.
■ An interface cannot extend anything but another interface.
■ An interface cannot implement another interface or class.

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■ 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:
public interface Bounceable {
public abstract void bounce();
public abstract void setBounceFactor(int bf);
}

Typing in the public and abstract modifiers on the methods is redundant,
though, since all interface methods are implicitly public and abstract. Given that
rule, you can see that the following code is exactly equivalent to the preceding
interface:
public interface Bounceable {
void bounce();
void setBounceFactor(int bf);
}

// No modifiers
// No modifiers

You must remember that all interface methods are public and abstract
regardless of what you see in the interface definition.
Look for interface methods declared with any combination of public, abstract,
or no modifiers. For example, the following five method declarations, if declared
within their own interfaces, are legal and identical!
void bounce();
public void bounce();
abstract void bounce();
public abstract void bounce();
abstract public void bounce();

Use Interfaces (OCA Objective 7.6)

27

The following interface method declarations won't compile:
final void bounce();
static void bounce();
private void bounce();
protected void bounce();

//
//
//
//
//

final and abstract can never be used
together, and abstract is implied
interfaces define instance methods
interface methods are always public
(same as above)

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 be declared as public, static, or final. They must be
public, static, and final, 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):
interface Foo {
int BAR = 42;
void go();
}
class Zap implements Foo {
public void go() {
BAR = 27;
}
}

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 the BAR = 27 assignment will not compile.

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Look for interface definitions that define constants, but without explicitly
using the required modifiers. For example, the following are all identical:
public int x = 1;
int x = 1;
static int x = 1;
final int x = 1;
public static int x = 1;
public final int x = 1;
static final int x = 1
public static final int x = 1;

//
//
//
//
//
//
//
//
//
//

Looks non-static and non-final,
but isn't!
Looks default, non-final,
non-static, but isn't!
Doesn't show final or public
Doesn't show static or public
Doesn't show final
Doesn't show static
Doesn't show public
what you get implicitly

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.

CERTIFICATION OBJECTIVE

Declare Class Members (OCA Objectives 2.1,
2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)
2.1

Declare and initialize variables.

2.2

Differentiate between object reference variables and primitive variables.

2.3

Read or write to object fields.

2.4

Explain an object's lifecycle.

2.5

Call methods on objects.

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

4.1

Declare, instantiate, initialize, and use a one-dimensional array.

4.2

Declare, instantiate, initialize, and use a multidimensional array.

6.2

Apply the static keyword to methods and fields.

6.6

Apply access modifiers.

29

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 a class can use just two of the four access control levels (default or
public), 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 mentioned later.
It's crucial that you know access control inside and out for the exam. There will
be quite a few questions with access control playing 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!

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Chapter 1:

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You need to understand two different access issues:
■ Whether method code in one class can access a member of another class
■ Whether a subclass can inherit 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:
class Zoo {
public String coolMethod() {
return "Wow baby";
}
}
class Moo {
public void useAZoo() {
Zoo z = new Zoo();
// If the preceding line compiles Moo has access
// to the Zoo class
// But... does it have access to the coolMethod()?
System.out.println("A Zoo says, " + z.coolMethod());
// The preceding line works because Moo can access the
// public method
}
}

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 subclass
inherits a member of its superclass. Remember, if a subclass inherits a member, it's
exactly as if the subclass actually declared the member itself. In other words, if a
subclass inherits a member, the subclass has the member. Here's an example:
class Zoo {
public String coolMethod() {
return "Wow baby";
}
}
class Moo extends Zoo {
public void useMyCoolMethod() {
// Does an instance of Moo inherit the coolMethod()?
System.out.println("Moo says, " + this.coolMethod());
// The preceding line works because Moo can inherit the
// public method
// Can an instance of Moo invoke coolMethod() on an
// instance of Zoo?

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

31

Zoo z = new Zoo();
System.out.println("Zoo says, " + z.coolMethod());
// coolMethod() is public, so Moo can invoke it on a Zoo
// reference
}
}

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.
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 A itself 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.

Public Members
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:
package book;
import cert.*; // Import all classes in the cert package
class Goo {
public static void main(String[] args) {
Sludge o = new Sludge();
o.testIt();
}
}

Now look at the second file:
package cert;
public class Sludge {
public void testIt() { System.out.println("sludge"); }
}

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
its testIt() method are marked public.

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Chapter 1:

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FIGURE 1-2

SportsCar

Comparison of
inheritance vs.
dot operator for
member access

goFast( ) { }
doStuff( ){
goFast( );

D

superclass

}

Convertible

R

doThings( ){
SportsCar sc = new SportsCar( );
sc.goFast( );

subclass

}

I

doMore( ){
goFast( );
}

Driver

R

doDriverStuff( ){
SportsCar car = new SportsCar( );
car.goFast( );

R

Convertible con = new Convertible( );
con.goFast( );
}

Three ways to access a method:
D Invoking a method declared in the same class
R Invoking a method using a reference of the class
I Invoking an inherited method

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:
package cert;
public class Roo {
public String doRooThings() {
// imagine the fun code that goes here
return "fun";
}
}

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

33

The Roo class declares the doRooThings() member as public. So if we make
a subclass of Roo, any code in that Roo 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:
package notcert;
// Not the package Roo is in
import cert.Roo;
class Cloo extends Roo {
public void testCloo() {
System.out.println(doRooThings());
}
}

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 call doRooThings() on a Cloo instance, as in the following:
class Toon {
public static void main(String[] args) {
Cloo c = new Cloo();
System.out.println(c.doRooThings()); // No problem; method
// is public
}
}

Private Members
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:
package cert;
public class Roo {
private String doRooThings() {
// imagine the fun code that goes here, but only the Roo
// class knows

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Chapter 1:

Declarations and Access Control

return "fun";
}
}

The doRooThings() method is now private, so no other class can use it. If we
try to invoke the method from any other class, we'll run into trouble:
package notcert;
import cert.Roo;
class UseARoo {
public void testIt() {
Roo r = new Roo(); //So far so good; class Roo is public
System.out.println(r.doRooThings()); // Compiler error!
}
}

If we try to compile UseARoo, 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. A private 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 a private 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.
package cert;
public class Roo {
private String doRooThings() {
// imagine the fun code that goes here, but no other class
// will know
return "fun";
}
}

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

35

The doRooThings() method is now off limits to all subclasses, even those in the
same package as the superclass:
package cert;
// Cloo and Roo are in the same package
class Cloo extends Roo {
// Still OK, superclass Roo is public
public void testCloo() {
System.out.println(doRooThings()); // Compiler error!
}
}

If we try to compile the subclass Cloo, the compiler is delighted to spit out an
error something like this:
%javac Cloo.java
Cloo.java:4: Undefined method: doRooThings()
System.out.println(doRooThings());
1 error

Can a private method be overridden by a subclass? That's an interesting
question, but the answer is technically no. Since 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 in Chapter 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.

Protected and Default Members
The protected and default access control levels are almost identical, but with one
critical difference. A default 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:
package certification;
public class OtherClass {
void testIt() {
// No modifier means method has default
// access
System.out.println("OtherClass");
}
}

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Chapter 1:

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The effect of private access control

FIGURE 1-3

Effects of public
and private access

SportsCar
private
goFast( ){...}
doStuff( ){
D
goFast( );

superclass

}

Convertible

R

doThings( ){
SportsCar sc = new SportsCar( );
sc.goFast( );

subclass

}

I

doMore( ){
goFast( );
}

Driver

R

doDriverStuff( ){
SportsCar car = new SportsCar( );
car.goFast( );

R

Convertible con = new Convertible( );
con.goFast( );
}

Three ways to access a method:
D Invoking a method declared in the same class
R Invoking a method using a reference of the class
I Invoking an inherited method

In another source code file you have the following:
package somethingElse;
import certification.OtherClass;
class AccessClass {
static public void main(String[] args) {
OtherClass o = new OtherClass();
o.testIt();
}
}

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

37

As you can see, the testIt() method in the first file has default (think packagelevel) access. Notice also that class OtherClass is in a different package from the
AccessClass. Will AccessClass 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 that AccessClass 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 it through 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 of default access, think package restriction. No
exceptions. But when you think protected, think package + 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' code), the subclass cannot use the dot operator on the
superclass reference to access the protected member. To a subclass-outside-thepackage, a protected member might as well be default (or even private), when
the subclass is using a reference to the superclass. The subclass can see the
protected member only through inheritance.

38

Chapter 1:

Declarations and Access Control

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.
package certification;
public class Parent {
protected int x = 9; // protected access
}

The preceding code declares the variable x as protected. This makes the
variable accessible to all other classes inside the certification package, as well as
inheritable by any subclasses outside the package.
Now let's create a subclass in a different package, and attempt to use the variable
x (that the subclass inherits):
package other;
// Different package
import certification.Parent;
class Child extends Parent {
public void testIt() {
System.out.println("x is " + x); // No problem; Child
// inherits x
}
}

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'
package) tries to access a protected variable using a Parent class reference:
package other;
import certification.Parent;
class Child extends Parent {
public void testIt() {
System.out.println("x is " + x);

//
//
Parent p = new Parent();
//
//
System.out.println("X in parent is " + p.x); //

}
}

No problem; Child
inherits x
Can we access x using
the p reference?
Compiler error!

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39

The compiler is more than happy to show us the problem:
%javac -d . other/Child.java
other/Child.java:9: x has protected access in certification.Parent
System.out.println("X in parent is " + p.x);
^
1 error

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 a protected
member look like to other classes trying to use the subclass-outside-the-package to
get to the subclass' inherited protected superclass member? For example, using our
previous Parent/Child classes, what happens if some other class—Neighbor, say—in
the same package as the Child (subclass), has a reference to a Child instance and
wants to access the member variable x ? In other words, how does that protected
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 class Neighbor
instantiates a Child object, then even if class Neighbor is in the same package as
class Child, class Neighbor won't have access to the Child's inherited (but
protected) variable x. Figure 1-4 illustrates the effect of protected 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.

Default Details
Let's start with the default behavior of a member in a superclass. We'll modify the
Parent's member x to make it default.
package certification;
public class Parent {
int x = 9; // No access modifier, means default
// (package) access
}

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Chapter 1:

Declarations and Access Control

If goFast( ) is default

FIGURE 1-4

Effects of
protected
access

If goFast( )is protected

Package A
SportsCar

Package A
SportsCar

Package A
SportsCar

Package A
SportsCar

goFast( ){ }

goFast( ){ }

goFast( ){ }

protected goFast( ){ }

D

D

D

D

Convertible

Convertible

R

I

R

I

Driver
R

R
Package B

Package B
Driver
R

R

Convertible
R

Package B
Convertible

I

R

Driver
R

I

Driver

R

R

R

Key:

D

goFast( ){ }
doStuff( ){
goFast( );
}
Where goFast
is Declared in the
same class.

R

doThings( ){
SportsCar sc = new SportsCar( );
sc.goFast( );
}
Invoking goFast( ) using a Reference to the
class in which goFast( ) was declared.

I

doMore( ){
goFast( );
}
Invoking the
goFast( )
method
Inherited from
a superclass.

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 something like this:
Child.java:4: Undefined variable: x
System.out.println("Variable x is " + x);
1 error

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41

The compiler gives the same error as when a member is declared as private. The
subclass Child (in a different package from the superclass Parent) can't see or use
the default superclass member x ! Now, what about default access for two classes in
the same package?
package certification;
public class Parent{
int x = 9; // default access
}

And in the second class you have the following:
package certification;
class Child extends Parent{
static public void main(String[] args) {
Child sc = new Child();
sc.testIt();
}
public void testIt() {
System.out.println("Variable x is " + x); // No problem;
}
}

The preceding source file compiles fine, and the class Child runs and displays the
value of x. Just remember that default members are visible to subclasses only if those
subclasses are in the same package as the superclass.

Local Variables and Access 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:
class Foo {
void doStuff() {
private int x = 7;
this.doMore(x);
}
}

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.

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Chapter 1:

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TABLE 1-2

Determining Access to Class Members

Visibility

Public

Protected

Default

Private

From the same class

Yes

Yes

Yes

Yes

From any class in the same package

Yes

Yes

Yes

No

From a subclass in the same package

Yes

Yes

Yes

No

From a subclass outside the same
package

Yes

Yes, through
inheritance

No

No

From any nonsubclass class outside
the package

Yes

No

No

No

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.
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 how static
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 typical final method declaration looks like this:
class SuperClass{
public final void showSample() {
System.out.println("One thing.");
}
}

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43

It's legal to extend SuperClass, since the class isn't marked final, but we can't
override the final method showSample(), as the following code attempts to do:
class SubClass extends SuperClass{
public void showSample() { // Try to override the final
// superclass method
System.out.println("Another thing.");
}
}

Attempting to compile the preceding code gives us something like this:
%javac FinalTest.java
FinalTest.java:5: The method void showSample() declared in class
SubClass cannot override the final method of the same signature
declared in class SuperClass.
Final methods cannot be overridden.
public void showSample() { }
1 error

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 modifier final:
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 argument must keep
the same value that the parameter had when it was passed into the method.

Abstract Methods
An abstract method is a method that's been declared (as abstract) but not
implemented. In other words, the method contains no functional code. And if you
recall from the earlier section "Abstract Classes," an abstract 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

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Chapter 1:

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example, if you write an abstract class Car with a method goUpHill(), you might
want to force each subtype of Car to define its own goUpHill() 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 single abstract method in a class that is not explicitly
declared abstract! Look at the following illegal class:
public class IllegalClass{
public abstract void doIt();
}

The preceding class will produce the following error if you try to compile it:
IllegalClass.java:1: class IllegalClass must be declared
abstract.
It does not define void doIt() from class IllegalClass.
public class IllegalClass{
1 error

You can, however, have an abstract class with no abstract methods. The
following example will compile fine:
public abstract class LegalClass{
void goodMethod() {
// lots of real implementation code here
}
}

In the preceding example, goodMethod() is not abstract. 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 an abstract class must implement all abstract methods
of the superclass, unless the subclass is also abstract. The rule is this:
The first concrete subclass of an abstract class must implement all abstract
methods of the superclass.

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45

Concrete just means nonabstract, so if you have an abstract 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:
public abstract class Vehicle {
private String type;
public abstract void goUpHill();
public String getType() {
return type;
}
}

// Abstract method
// Non-abstract method

public abstract class Car extends Vehicle {
public abstract void goUpHill(); // Still abstract
public void doCarThings() {
// special car code goes here
}
}
public class Mini extends Car {
public void goUpHill() {
// Mini-specific going uphill code
}
}

So how many methods does class Mini 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 class
Mini—as the first concrete class below Vehicle—must implement the goUpHill()
method. In other words, class Mini can't pass the buck (of abstract method
implementation) to the next class down the inheritance tree, but class Car can,
since Car, like Vehicle, is abstract. Figure 1-5 illustrates the effects of the
abstract modifier on concrete and abstract subclasses.
Look for concrete classes that don't provide method implementations for
abstract methods of the superclass. The following code won't compile:
public abstract class A {
abstract void foo();
}
class B extends A {
void foo(int I) { }
}

46

Chapter 1:

FIGURE 1-5

The effects of
the abstract
modifier on
concrete and
abstract
subclasses

Declarations and Access Control

abstract Car
startEngine( )
abstract goForward( )
abstract reverse( )
stop( )
abstract turn(int whichWay)

SportsCar
startEngine( )//optional
goForward( )//Required
reverse( )//Required
turn(int whichWay)//Required

abstract SUV
enable4wd( )
goForward( )
reverse( )
abstract goOffRoad( )
//turn( )not implemented

Abstract methods must be implemented by a
nonabstract subclass. If the subclass is abstract,
it is not required to implement the abstract
methods, but it is allowed to implement any
or all of the superclass abstract methods. The
AcmeRover class is nonabstract, so it must
implement the abstract method declared in its
superclass, SUV, and it must also implement
turn( ), which was not implemented by SUV.

AcmeRover

enable4wd( )//optional
goOffRoad( )//Required
turn(int whichWay)//Required

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' abstract 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' abstract method. We'll
look at the differences between overloading and overriding in detail in Chapter 2.
A method can never, ever, ever be marked as both abstract and final, or both
abstract 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, an
abstract 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

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47

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 cover static 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. We'll discuss this nearly to death in Chapter 13, 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 typical synchronized
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).

Native Methods
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 that native 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 using strictfp as a class modifier, but even if you don't
declare a class as strictfp, you can still declare an individual method as strictfp.
Remember, strictfp forces floating points (and any floating-point operations) to
adhere to the IEEE 754 standard. With strictfp, you can predict how your floating
points will behave regardless of the underlying platform the JVM is running on. The

48

Chapter 1:

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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 with Variable Argument Lists (var-args)
(For OCP Candidates Only)
As of Java 5, 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're
invoking a method:
doStuff("a", 2);

// invoking doStuff, so "a" & 2 are
// arguments

■ parameters

The things in the method's signature that indicate what the
method must receive when it's invoked:
void doStuff(String s, int a) { }

// we're expecting two
// parameters:
// String and int

We'll cover using var-arg methods more in the next few chapters; for now 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

a var-arg.

It's legal to have other parameters in a method that uses

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49

■ Var-arg limits

The 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:
void doStuff(int... x) { }
void doStuff2(char c, int... x)

{ }

void doStuff3(Animal... animal) { }

//
//
//
//
//

expects from 0 to many ints
as parameters
expects first a char,
then 0 to many ints
0 to many Animals

Illegal:
void doStuff4(int x...) { }
void doStuff5(int... x, char... y) { }
void doStuff6(String... s, byte b) { }

// bad syntax
// too many var-args
// var-arg must be last

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:
class Foo {
protected Foo() { }
protected void Foo() { }
}

// this is Foo's constructor
// this is a badly named, but legal, method

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 or
abstract (because they can't be overridden). Here are some legal and illegal
constructor declarations:
class Foo2 {
// legal constructors
Foo2() { }
private Foo2(byte b) { }
Foo2(int x) { }

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Chapter 1:

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Foo2(int x, int... y) { }
// illegal constructors
void Foo2() { }
Foo() { }
Foo2(short s);
static Foo2(float f) { }
final Foo2(long x) { }
abstract Foo2(char c) { }
Foo2(int... x, int t) { }

//
//
//
//
//
//
//

it's a method, not a constructor
not a method or a constructor
looks like an abstract method
can't be static
can't be final
can't be abstract
bad var-arg syntax

}

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 in
Chapter 2, when we discuss polymorphism.

Declaring Primitives and Primitive Ranges
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:
byte b;
boolean myBooleanPrimitive;
int x, y, z;

// declare three int primitives

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 is byte, short, int, and long, and that doubles are bigger than floats.

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FIGURE 1-6

51

sign bit: 0 = positive
I = negative

The sign bit for
a byte

0 0010011

byte

sign bit

value bits:
byte: 7 bits can represent 27 or
128 different values:
0 thru 127 -or- –128 thru –1

value bits

short: 15 bits can represent
215 or 32768 values:
0 thru 32767 -or- –32768 thru –1

1 111110100000011

short

You will also need to know that the number types (both integer and floatingpoint 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
are signed, 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 in Figure 1-6. The rest of the bits represent the value, using two's
complement notation.
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 zero 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).
TABLE 1-3

Ranges of
Numeric
Primitives

Type

Bits

Bytes

Minimum Range Maximum Range

byte

8

1

–27

27 – 1

short

16

2

–215

215 – 1

31

int

32

4

–2

231 – 1

long

64

8

–263

263 – 1

float

32

4

n/a

n/a

double

64

8

n/a

n/a

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There is not a range of boolean values; a boolean can be only true or false. If
someone asks you for the bit depth of a boolean, 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 in Chapter 3 that because a char is really an integer type, it
can be assigned to any number type large enough to hold 65535 (which means
anything larger than a short; although both chars 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).

Declaring Reference Variables
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. 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
reference variable declarations:
Object o;
Dog myNewDogReferenceVariable;
String s1, s2, s3;

// declare three String vars.

Instance Variables
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:
class Employee {
// define fields (instance variables) for employee instances
private String name;
private String title,
private String manager;
// other code goes here including access methods for private
// fields
}

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

53

The preceding Employee 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 vs. variables.

FIGURE 1-7

Comparison
of modifiers
on variables vs.
methods

Local
Variables

Variables
(non-local)

final

final
public
protected
private
static
transient
volatile

Methods
final
public
protected
private
static
abstract
synchronized
strictfp
native

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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 in Chapter 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, or static, but as we saw earlier, local variables can be
marked final. And as you'll learn in Chapter 3 (but here's a preview), before a
local variable can be used, it must be initialized with a value. For instance:
class TestServer {
public void logIn() {
int count = 10;
}
}

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 of count 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:

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

55

class TestServer {
public void logIn() {
int count = 10;
}
public void doSomething(int i) {
count = i; // Won't compile! Can't access count outside
// method logIn()
}
}

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:
class TestServer {
int count = 9;
// Declare an instance variable named count
public void logIn() {
int count = 10;
// Declare a local variable named count
System.out.println("local variable count is " + count);
}
public void count() {
System.out.println("instance variable count is " + count);
}
public static void main(String[] args) {
new TestServer().logIn();
new TestServer().count();
}
}

The preceding code produces the following output:
local variable count is 10
instance variable count is 9

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:
class Foo {
int size = 27;
public void setSize(int size) {
size = size; // ??? which size equals which size???
}
}

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

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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:
class Foo {
int size = 27;
public void setSize(int size) {
this.size = size; // this.size means the current object's
// instance variable, size. The size
// on the right is the parameter
}
}

Array Declarations
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 in Chapter 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? For OCA candidates, Chapter 5 discusses
ArrayList, and for OCP candidates, Chapter 11 covers Collections 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.

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

57

Declaring an Array of Primitives
int[] key;
int key [];

// Square brackets before name (recommended)
// Square brackets after name (legal but less
// readable)

Declaring an Array of Object References
Thread[] threads; // Recommended
Thread threads []; // Legal but less readable

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, and 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.

In Chapter 5, we'll spend a lot of time discussing arrays, how to initialize and use
them, and how to deal with multidimensional arrays…stay tuned!

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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,
then x 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't ever 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, only final references. We'll explain this in
more detail in Chapter 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 of final.

Transient Variables
If you mark an instance variable as transient, 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.

Volatile Variables
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, as with transient, it can be applied only to instance variables.
Make no mistake: the idea of multiple threads accessing an instance variable is scary
stuff, and very important for any Java programmer to understand. But as you'll see in
Chapter 13, you'll probably use synchronization, rather than the volatile modifier,
to make your data thread-safe.

Declare Class Members (OCA Objectives 2.1, 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 6.2, and 6.6)

FIGURE 1-8

final
class

Effect of final
on variables,
methods, and
classes

59

final class Foo
final class
cannot be
subclassed
class Bar extends Foo

final
method

class Baz
final void go( )

final method
cannot be
overridden by
a subclass

class Bat extends Baz
final void go( )

final
variable

class Roo
final int size = 42;
void changeSize( ){
size = 16;
}

final variable cannot be
assigned a new value, once
the initial method is made
(the initial assignment of a
value must happen before
the constructor completes).

The volatile modifier may also be applied to project managers : )

Static Variables and Methods
The static modifier is used to create variables and methods that will exist
independently of any instances created for the class. All static members exist
before you ever make a new instance of a class, and there will be only one copy of a

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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 given static variable.
We'll cover static members in great detail in the next chapter.
Things you can mark as static:

■ Methods
■ Variables
■ A class nested within another class, but not within a method (more on this in

Chapter 12)
■ Initialization blocks

Things you can't mark as static:
■ 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 (we'll explore this in Chapter 12)
■ Inner class methods and instance variables
■ Local variables

CERTIFICATION OBJECTIVE

Declare and Use enums
(OCA Objective 1.2 and OCP Objective 2.5)
2.5

Use enumerated types.

Note: During the creation of this book, Oracle adjusted some of the objectives for
the OCA and OCP exams. 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.

Declare and Use enums (OCA Objective 1.2 and OCP Objective 2.5)

61

Declaring enums
As of Java 5, 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.)
Using enums can help reduce the bugs in your code. For instance, 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 except BIG, 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 an enum are its constants (that is, BIG, HUGE, and
OVERWHELMING), although in a minute you'll see that there can be a lot more to an
enum. 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 an enum outside a class:
enum CoffeeSize { BIG, HUGE, OVERWHELMING }
class Coffee {
CoffeeSize size;
}
public class CoffeeTest1 {
public static void main(String[] args) {
Coffee drink = new Coffee();
drink.size = CoffeeSize.BIG;
}
}

// this cannot be
// private or protected

// enum outside class

The preceding code can be part of a single file. (Remember, 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 an enum that isn't enclosed in a class can be

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declared with only the public or default modifier, just like a non-inner class. Here's
an example of declaring an enum inside a class:
class Coffee2 {
enum CoffeeSize {BIG, HUGE, OVERWHELMING }
CoffeeSize size;
}
public class CoffeeTest2 {
public static void main(String[] args) {
Coffee2 drink = new Coffee2();
drink.size = Coffee2.CoffeeSize.BIG;
// enclosing class
// name required
}
}

The key points to take away from these examples are that enums can be declared
as their own class or enclosed in another class, and that the syntax for accessing an
enum's members depends on where the enum was declared.
The following is NOT legal:
public class CoffeeTest1 {
public static void main(String[] args) {
enum CoffeeSize { BIG, HUGE, OVERWHELMING } // WRONG! Cannot
// declare enums
// in methods
Coffee drink = new Coffee();
drink.size = CoffeeSize.BIG;
}
}

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 this enum follow):
public class CoffeeTest1 {
enum CoffeeSize { BIG, HUGE, OVERWHELMING }; // <--semicolon
// is optional here
public static void main(String[] args) {
Coffee drink = new Coffee();
drink.size = CoffeeSize.BIG;
}
}

So what gets created when you make an enum? The most important thing to
remember is that enums are not Strings or ints! Each of the enumerated
CoffeeSize types is actually an instance of CoffeeSize. In other words, BIG is of
type CoffeeSize. Think of an enum as a kind of class that looks something (but not
exactly) like this:

Declare and Use enums (OCA Objective 1.2 and OCP Objective 2.5)

63

// conceptual example of how you can think
// about enums
class CoffeeSize {
public static final CoffeeSize BIG =
new CoffeeSize("BIG", 0);
public static final CoffeeSize HUGE =
new CoffeeSize("HUGE", 1);
public static final CoffeeSize OVERWHELMING =
new CoffeeSize("OVERWHELMING", 2);
CoffeeSize(String enumName, int index) {
// stuff here
}
public static void main(String[] args) {
System.out.println(CoffeeSize.BIG);
}
}

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 each enum value knows
its index or position—in other words, the order in which enum 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 an enum by invoking the values() method on any enum type. (Don't worry about
that in this chapter.)

Declaring Constructors, Methods, and Variables in an enum
Because an enum 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 that BIG 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.

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enum CoffeeSize {
// 8, 10 & 16 are passed to the constructor
BIG(8), HUGE(10), OVERWHELMING(16);
CoffeeSize(int ounces) {
// constructor
this.ounces = ounces;
}
private int ounces;
public int getOunces() {
return ounces;
}

// an instance variable

}
class Coffee {
CoffeeSize size;

// each instance of Coffee has an enum

public static void main(String[] args) {
Coffee drink1 = new Coffee();
drink1.size = CoffeeSize.BIG;
Coffee drink2 = new Coffee();
drink2.size = CoffeeSize.OVERWHELMING;
System.out.println(drink1.size.getOunces()); // prints 8
for(CoffeeSize cs: CoffeeSize.values())
System.out.println(cs + " " + cs.getOunces());
}
}

which produces:
8
BIG 8
HUGE 10
OVERWHELMING 16

Note: Every enum 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 about enum constructors are
■ You can NEVER invoke an enum 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
an int, passing the int literal 8 to the constructor. (Behind the scenes, of
course, you can imagine that BIG 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

Declare and Use enums (OCA Objective 1.2 and OCP Objective 2.5)

65

constructor. We discuss constructors in much more detail in Chapter 2.
To initialize a CoffeeSize with both the number of ounces and, say, a lid
type, you'd pass two arguments to the constructor as BIG(8, "A"), which
means you have a constructor in CoffeeSize that takes both an int and
a String.
And, finally, you can define something really strange in an enum that looks like
an anonymous inner class (which we talk about in Chapter 8). 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 want enums 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-maintain if/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:
enum CoffeeSize {
BIG(8),
HUGE(10),
OVERWHELMING(16) {

public String getLidCode() {

// start a code block that defines
// the "body" for this constant
// override the method
// defined in CoffeeSize

return "A";
}
};

// the semicolon is REQUIRED when more code follows

CoffeeSize(int ounces) {
this.ounces = ounces;
}
private int ounces;
public int getOunces() {
return ounces;
}
public String getLidCode() {

return "B";
}
}

// this method is overridden
// by the OVERWHELMING constant
// the default value we want to
// return for CoffeeSize constants

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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 of package and
import statements.
You learned the basic syntax for the java and javac command-line programs.
You learned about when main() has superpowers and when it doesn't.
We covered the basics of import 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 that abstract classes can contain both abstract 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 an
abstract class must provide implementations for all the abstract methods of the
superclass, but that an abstract class does not have to implement the abstract
methods from its superclass. An abstract 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 including static, final, abstract,
synchronized, and so on. You've learned how some modifiers can never be
combined in a declaration, such as mixing abstract with either final or private.
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.

Certification Summary

67

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.
Remember that as of Java 5, 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
that static 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 a feature new as of Java 5: enums. An enum is a much safer and
more flexible way to implement constants than was possible in earlier versions of
Java. 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.

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Declarations and Access Control

TWO-MINUTE DRILL
Remember that in this chapter, when we talk about classes, we're referring to
non-inner classes, or top-level classes. For OCP 7 candidates only, we'll devote all
of Chapter 12 to inner classes. Note on OCA 7 vs. OCP 7 objectives: Part I of this
book is necessary for BOTH OCA 7 and OCP 7 candidates. Since you must now
pass the OCA 7 exam before taking the OCP 7 exam, the references to objectives
in the two-minute drills in the first part of the book are usually for OCA
objectives only.

Identifiers (OCA Objective 2.1)
❑ 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() (OCA Objective 1.3)
❑ You can compile and execute Java programs using the command-line
programs javac and java, respectively. Both programs support a variety of
command-line options.
❑ The only versions of main() methods with special powers are those
versions with method signatures equivalent to public static void
main(String[] args).
❑ main() can be overloaded.

Imports (OCA Objective 1.4)
❑ An import 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 is import static….
❑ You can import API classes and/or custom classes.

Two-Minute Drill

69

Source File Declaration Rules (OCA Objective 1.2)
❑ A source code file can have only one public 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 multiple
imports.
❑ 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 and before the
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 (OCA Objective 6.6)
❑ There are three access modifiers: public, protected, and private.
❑ There are four access levels: public, protected, default, and private.
❑ Classes can have only public or default access.
❑ A class with default access can be seen only by classes within the same
package.
❑ A class with public 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) (OCA Objective 7.6)
❑ Classes can also be modified with final, abstract, or strictfp.
❑ A class cannot be both final and abstract.
❑ A final class cannot be subclassed.

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❑ An abstract class cannot be instantiated.
❑ A single abstract method in a class means the whole class must be
abstract.
❑ An abstract class can have both abstract and nonabstract methods.
❑ The first concrete class to extend an abstract class must implement all of its
abstract methods.

Interface Implementation (OCA Objective 7.6)
❑ 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.
❑ An interface is like a 100-percent abstract class and is implicitly abstract
whether you type the abstract modifier in the declaration or not.
❑ An interface can have only abstract methods, no concrete methods
allowed.
❑ Interface methods are by default public and abstract—explicit declaration
of these modifiers is optional.
❑ Interfaces can have constants, which are always implicitly public,
static, and final.
❑ Interface constant declarations of public, static, and final are optional
in any combination.
❑ Note: This section uses some concepts that we HAVE NOT yet covered.
Don't panic: once you've read through all of Part I 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.

Two-Minute Drill

71

❑ 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.
❑ An abstract 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 (OCA Objective 6.6)
❑ 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 is public, 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 invoking aMethod().
❑ 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.

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❑ 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, the protected member can be
accessed only through inheritance; a subclass outside the package cannot
access a protected 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 a protected member of its superclass.)
❑ A protected member inherited by a subclass from another package is
not accessible to any other class in the subclass package, except for the
subclass' own subclasses.

Local Variables (OCA Objective 2.1)
❑ 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 (OCA Objective 6.6)
❑ 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 an abstract class must
implement all of the abstract class' 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.

Two-Minute Drill

73

❑ abstract methods must be implemented by a subclass, so they must be
inheritable. For that reason:
❑ abstract methods cannot be private.
❑ abstract methods cannot be final.
❑ The native modifier applies only to methods.
❑ The strictfp modifier applies only to classes and methods.

Methods with var-args (OCP Only, OCP Objective 1.3)
❑ As of Java 5, 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 syntax type... 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.

Variable Declarations (OCA Objective 2.1)
❑ Instance variables can
❑ Have any access control
❑ Be marked final or transient
❑ Instance variables can't be abstract, synchronized, native, or strictfp.
❑ 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.

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Array Declarations (OCA Objectives 4.1 and 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 (or
instanceof) test for the declared type of the array. For example, if Horse
extends Animal, then a Horse object can go into an Animal array.

Static Variables and Methods (OCA Objective 6.2)
❑ They are not tied to any particular instance of a class.
❑ No class instances are needed in order to use static members of the class.
❑ 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 Objective 1.2 and OCP Objective 2.5)
❑ An enum specifies a list of constant values assigned to a type.
❑ An enum is NOT a String or an int; an enum constant's type is the enum
type. For example, SUMMER and FALL are of the enum type Season.
❑ An enum can be declared outside or inside a class, but NOT in a method.
❑ An enum 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 syntax
BIG(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 an enum is initialized.
❑ The semicolon at the end of an enum 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.

Self Test

75

SELF TEST
The following questions will help you measure your understanding of the material presented in this
chapter. Read all of 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  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:
class Rocket {
private void blastOff() { System.out.print("bang "); }
}
public class Shuttle extends Rocket {
public static void main(String[] args) {
new Shuttle().go();
}
void go() {
blastOff();
// Rocket.blastOff(); // line A
}
private void blastOff() { System.out.print("sh-bang "); }
}

Which are true? (Choose all that apply.)
A. As the code stands, the output is bang
B. As the code stands, the output is sh-bang
C. As the code stands, compilation fails.
D. If line A is uncommented, the output is bang 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:
import static java.lang.System.*;
class _ {

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static public void main(String[] __A_V_) {
String $ = "";
for(int x=0; ++x < __A_V_.length; )
// for loop
$ += __A_V_[x];
out.println($);
}
}

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:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

enum Animals {
DOG("woof"), CAT("meow"), FISH("burble");
String sound;
Animals(String s) { sound = s; }
}
class TestEnum {
static Animals a;
public static void main(String[] args) {
System.out.println(a.DOG.sound + " " + a.FISH.sound);
}
}

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

Self Test

5. Given two files:
1.
2.
3.
4.
5.
6.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

package pkgA;
public class Foo {
int a = 5;
protected int b = 6;
public int c = 7;
}
package pkgB;
import pkgA.*;
public class Baz {
public static void main(String[] args) {
Foo f = new Foo();
System.out.print(" " + f.a);
System.out.print(" " + f.b);
System.out.println(" " + f.c);
}
}

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:
1. public class Electronic implements Device
{ public void doIt() { } }
2.
3. abstract class Phone1 extends Electronic { }
4.
5. abstract class Phone2 extends Electronic
{ public void doIt(int x) { } }
6.
7. class Phone3 extends Electronic implements Device
{ public void doStuff() { } }
8.
9. interface Device { public void doIt(); }

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A.
B.
C.
D.
E.
F.

Declarations and Access Control

What is the result? (Choose all that apply.)
Compilation succeeds
Compilation fails with an error on line 1
Compilation fails with an error on line 3
Compilation fails with an error on line 5
Compilation fails with an error on line 7
Compilation fails with an error on line 9

7. Given:
4. class Announce {
5.
public static void main(String[] args) {
6.
for(int __x = 0; __x < 3; __x++) ;
7.
int #lb = 7;
8.
long [] x [5];
9.
Boolean []ba[];
10.
}
11. }

A.
B.
C.
D.
E.

What is the result? (Choose all that apply.)
Compilation succeeds
Compilation fails with an error on line 6
Compilation fails with an error on line 7
Compilation fails with an error on line 8
Compilation fails with an error on line 9

8. Given:
3. public class TestDays {
4.
public enum Days { MON, TUE, WED };
5.
public static void main(String[] args) {
6.
for(Days d : Days.values() )
7.
;
8.
Days [] d2 = Days.values();
9.
System.out.println(d2[2]);
10.
}
11. }

What is the result? (Choose all that apply.)
A. TUE
B. WED

Self Test

C.
D.
E.
F.
G.

The output is unpredictable
Compilation fails due to an error on line 4
Compilation fails due to an error on line 6
Compilation fails due to an error on line 8
Compilation fails due to an error on line 9

9. Given:
4.
5.
6.
7.
8.
9.
10.
11.
12.

public class Frodo extends Hobbit
public static void main(String[] args) {
int myGold = 7;
System.out.println(countGold(myGold, 6));
}
}
class Hobbit {
int countGold(int x, int y) { return x + y; }
}

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:
interface Gadget {
void doStuff();
}
abstract class Electronic {
void getPower() { System.out.print("plug in "); }
}
public class Tablet extends Electronic implements Gadget {
void doStuff() { System.out.print("show book "); }
public static void main(String[] args) {
new Tablet().getPower();
new Tablet().doStuff();
}
}

Which are true? (Choose all that apply.)
A. The class Tablet will NOT compile
B. The interface Gadget will NOT compile

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C. The output will be plug in show book
D. The abstract class Electronic will NOT compile
E. The class Tablet CANNOT both extend and implement
11. Given that the Integer class is in the java.lang package, and given:
1. // insert code here
2. class StatTest {
3.
public static void main(String[] args) {
4.
System.out.println(Integer.MAX_VALUE);
5.
}
6. }

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

Self Test Answers

81

SELF TEST 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 7.6)
2. ☑ B and F are correct. Since Rocket.blastOff() is private, it can't be overridden, and it
is invisible to class Shuttle.
☐
✗ A, C, D, and E are incorrect based on the above. (OCA Objective 6.6)
3. ☑ B is correct. This question is using valid (but inappropriate and weird) identifiers, static
imports, main(), and pre-incrementing logic.
☐
✗ A, C, D, E, F, and G are incorrect based on the above. (OCA Objective 1.2 and OCA
Objectives 1.3, 1.4, and 2.1)
4. ☑ A is correct; enums can have constructors and variables.
☐
✗ B, C, D, E, and F are incorrect; these lines all use correct syntax. (OCP Objective 2.5)
5. ☑ D and E are correct. Variable a has default access, so it cannot be accessed from outside the
package. Variable b has protected access in pkgA.
☐
✗ A, B, C, and F are incorrect based on the above information. (OCA Objectives 1.4
and 6.6)
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 7.6)
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. Every enum comes with a static values() method that returns an array of
the enum's values, in the order in which they are declared in the enum.
☐
✗ A, C, D, E, F, and G are incorrect based on the above information. (OCP Objective 2.5)
9. ☑ D is correct. The countGold() method cannot be invoked from a static context.
☐
✗ A, B, C, and E are incorrect based on the above information. (OCA Objectives 2.5
and 6.2)

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10. ☑ A is correct. By default, an interface's methods are public 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 7.6)
11. ☑ C and E are correct syntax for static imports. Line 4 isn't making use of static 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)

2
Object Orientation

CERTIFICATION OBJECTIVES

•
•
•
•
•
•

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

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Object Orientation

B

eing an Oracle Certified Associate (OCA) 7 means you must be at one with the objectoriented 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] 7.) This chapter will prepare you for all of 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 6.7)
6.1

Create methods with arguments and return values.

6.7

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 in your code. (Relax, this is just hypothetical.)
Well, it is a Java program, so you should be able just 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 with public instance
variables, and those other programmers were setting the instance variables directly,
as the following code demonstrates:

Encapsulation (OCA Objectives 6.1 and 6.7)

85

public class BadOO {
public int size;
public int weight;
...
}
public class ExploitBadOO {
public static void main (String [] args) {
BadOO b = new BadOO();
b.size = -5; // Legal but bad!!
}
}

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 for adjusting size (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. By interface, 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 protected (with an access modifier, often private).
■ Make public 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 or get a variable's
value.
■ For these accessor methods, use the most common naming convention of
set and get.

Figure 2-1 illustrates the idea that encapsulation forces callers of our code to go
through methods rather than accessing variables directly.
We call the access methods getters and setters, although some prefer the fancier
terms accessors and mutators. (Personally, we don't like the word "mutate.")
Regardless of what you call them, they're methods that other programmers must go

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Chapter 2:

FIGURE 2-1

Object Orientation

The nature of encapsulation
Class A

Class B

B b = new B();
int x = b.getSize();

getSize()

b.setSize(34);

setSize()

String s = b.getName();

getName()

b.setName("Fred");

setName()

Color c = b.getColor();

getColor()

b.setColor(blue);

setColor()

public

size

name

color

private

Class A cannot access Class B instance variable data
without going through getter and setter methods. Data is
marked private; only the accessor methods are public.

through in order to access your instance variables. They look simple, and you've
probably been using them forever:
public class Box {
// protect the instance variable; only an instance
// of Box can access it
private int size;
// Provide public getters and setters
public int getSize() {
return size;
}
public void setSize(int newSize) {
size = newSize;
}
}

Encapsulation (OCA Objectives 6.1 and 6.7)

87

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: In Chapter 5 we'll be revisiting 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 until Chapter 5 to challenge you with encapsulationthemed 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:
class Foo {
public int left = 9;
public int right = 3;
public void setLeft(int leftNum) {
left = leftNum;
right = leftNum/3;
}
// lots of complex test code here
}

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.

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CERTIFICATION OBJECTIVE

Inheritance and Polymorphism
(OCA Objectives 7.1, 7.2, and 7.3)
7.1

Implement inheritance.

7.2

Develop code that demonstrates the use of polymorphism.

7.3

Differentiate between the type of a reference and the type of an object.

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 in Chapter 5. For now, just remember that instanceof returns true
if the reference variable being tested is of the type being compared to. This code
class Test {
public static void main(String [] args) {
Test t1 = new Test();
Test t2 = new Test();
if (!t1.equals(t2))
System.out.println("they're not equal");
if (t1 instanceof Object)
System.out.println("t1's an Object");
}
}

produces this output:
they're not equal
t1's an Object

Where did that equals method come from? The reference variable t1 is of type
Test, and there's no equals method in the Test class. Or is there? The second if
test asks whether t1 is an instance of class Object, and because it is (more on that
soon), the if test succeeds.
Hold on…how can t1 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 class Object (except of course class Object itself). In other
words, every class you'll ever use or ever write will inherit from class Object. You'll
always have an equals method, a clone method, notify, wait, and others

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89

available to use. Whenever you create a class, you automatically inherit all of class
Object's methods.
Why? Let's look at that equals 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 class Object didn't have an equals
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 been overridden billions of times, but we're getting ahead of ourselves.)
For the exam, you'll need to know that you can create inheritance relationships
in Java by extending a class. 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:
class GameShape {
public void displayShape() {
System.out.println("displaying shape");
}
// more code
}
class PlayerPiece extends GameShape {
public void movePiece() {
System.out.println("moving game piece");
}
// more code
}

public class TestShapes {
public static void main (String[] args) {
PlayerPiece shape = new PlayerPiece();
shape.displayShape();
shape.movePiece();
}
}

outputs:
displaying shape
moving game piece

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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 of GameShape are guaranteed to have the capabilities of the
more generic superclass. You don't want to have to rewrite the displayShape()
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 of GameShape 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 any
subclass of GameShape as a GameShape. 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:
class GameShape {
public void displayShape() {
System.out.println("displaying shape");
}
// more code
}
class PlayerPiece extends GameShape {
public void movePiece() {
System.out.println("moving game piece");
}
// more code
}
class TilePiece extends GameShape {
public void getAdjacent() {

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91

System.out.println("getting adjacent tiles");
}
// more code
}

Now imagine a test class has a method with a declared argument type of
GameShape, which means it can take any kind of GameShape. In other words, any
subclass of GameShape can be passed to a method with an argument of type
GameShape. This code
public class TestShapes {
public static void main (String[] args) {
PlayerPiece player = new PlayerPiece();
TilePiece tile = new TilePiece();
doShapes(player);
doShapes(tile);
}
public static void doShapes(GameShape shape) {
shape.displayShape();
}
}

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) of GameShape.
The method can then invoke any method of GameShape, without any concern for
the actual runtime class type of the object passed to the method. There are
implications, though. The doShapes() 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 that only the
methods of GameShape 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 a
GameShape variable to call, say, the getAdjacent() method even if the object
passed in is of type TilePiece. (We'll see this again when we look at interfaces.)

IS-A and HAS-A Relationships (*OCP Objective 3.3)
Note: As of the Spring of 2014, the OCA 7 exam won't ask you directly about IS-A
and HAS-A relationships. But, understanding IS-A and HAS-A relationships will
help OCA 7 candidates with many of the questions on the exam.

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Given the above, for the OCP exam you need to be able to look at code and
determine whether the code demonstrates an IS-A or HAS-A relationship. The
rules are simple, so this should be one of the few areas where answering the questions
correctly is almost a no-brainer.

IS-A
In OO, the concept of IS-A is based on class 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 keywords extends
(for class inheritance) and implements (for interface implementation).
public class Car {
// Cool Car code goes here
}
public class Subaru extends Car {
// Important Subaru-specific stuff goes here
// Don't forget Subaru inherits accessible Car members which
// can include both methods and variables.
}

A Car is a type of Vehicle, so the inheritance tree might start from the Vehicle
class as follows:
public class Vehicle { ... }
public class Car extends Vehicle { ... }
public class Subaru extends Car { ... }

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.

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93

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-A Bar, even if
Foo 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'
arrow points toward the class from which it extends.

HAS-A
HAS-A relationships are based on usage, 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:
public class Animal { }
public class Horse extends Animal {
private Halter myHalter;
}

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
FIGURE 2-2
Vehicle

Inheritance tree
for Vehicle,
Car, Subaru
Car extends Vehicle

Subaru extends Car

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HAS-A Halter." In other words, Horse has a reference to a Halter. Horse code
can use that Halter reference to invoke methods on the Halter, and get Halter
behavior without having Halter-related code (methods) in the Horse class itself.
Figure 2-3 illustrates the HAS-A relationship between Horse 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
the Cow 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, the Horse object turns
around and delegates the call to its Halter class by invoking myHalter.tie(rope).
The scenario just described might look like this:
public class Horse extends Animal {
private Halter myHalter = new Halter();
public void tie(LeadRope rope) {
myHalter.tie(rope); // Delegate tie behavior to the
// Halter object
}
}
public class Halter {
public void tie(LeadRope aRope) {
// Do the actual tie work here
}
}

FIGURE 2-3

HAS-A relationship
between Horse and
Halter

Horse
Halter h

Halter

tie(Rope r)

tie(Rope r)

Horse class has a Halter, because Horse
declares an instance variable of type Halter.
When code invokes tie() on a Horse instance,
the Horse invokes tie() on the Horse
object’s Halter instance variable.

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95

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 has to 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 the designer 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

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 from Horse 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 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.

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CERTIFICATION OBJECTIVE

Polymorphism (OCA Objectives 7.2 and 7.3)
7.2

Develop code that demonstrates the use of polymorphism.

7.3

Differentiate between the type of a reference and the type of an object.

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 class Object.
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 any subtype of 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 that implements 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 not all shapes are able to be animated, so what do you
do with class inheritance?
Could we create a class with an animate() method, and have only some of the
GameShape subclasses inherit from that class? If we can, then we could have
PlayerPiece, for example, extend both the GameShape class and Animatable class,
while the TilePiece would extend only GameShape. But no, this won't work! Java
supports only single inheritance! That means a class can have only one immediate
superclass. In other words, if PlayerPiece is a class, there is no way to say
something like this:

Polymorphism (OCA Objective 7.2 and 7.3)

97

class PlayerPiece extends GameShape, Animatable { // NO!
// more code
}

A class cannot extend more than one class: that means one parent per class. A
class can 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 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 (more on cohesion in Chapter 10), 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.
So what else could you do? You already know the answer—create an Animatable
interface, and have only the GameShape subclasses that can be animated implement
that interface. Here's the interface:
public interface Animatable {
public void animate();
}

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And here's the modified PlayerPiece class that implements the interface:
class PlayerPiece extends GameShape implements Animatable {
public void movePiece() {
System.out.println("moving game piece");
}
public void animate() {
System.out.println("animating...");
}
// more code
}

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:
■ An Object (since any object inherits from Object)
■ A GameShape (since PlayerPiece extends GameShape)
■ A PlayerPiece (since that's what it really is)
■ An Animatable (since PlayerPiece implements Animatable)

The following are all legal declarations. Look closely:
PlayerPiece player = new PlayerPiece();
Object o = player;
GameShape shape = player;
Animatable mover = player;

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-A GameShape, so the compiler says, "I see that the declared type is
PlayerPiece, and since PlayerPiece extends GameShape, that means PlayerPiece

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99

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 in PlayerPiece 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's displayShape() 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:
Polymorphic method invocations apply only to instance methods. You can always
refer to an object with a more general reference variable type (a superclass or
interface), but at runtime, the ONLY things that are dynamically selected based
on the actual object (rather than the reference type) are instance methods. Not
static methods. Not variables. Only overridden instance methods are dynamically
invoked based on the real object's type.
Because this definition depends on a clear understanding of overriding and the
distinction between static methods and instance methods, we'll cover those next.

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CERTIFICATION OBJECTIVE

Overriding / Overloading
(OCA Objectives 6.1, 6.3,7.2, and 7.3)
6.1

Create methods with arguments and return values.

6.3

Create an overloaded method.

7.2

Develop code that demonstrates the use of polymorphism.

7.3

Differentiate between the type of a reference and the type of an object.

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 class inherits a method from a superclass, 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 subclass type. The following example demonstrates a Horse subclass of
Animal overriding the Animal version of the eat() method:
public class Animal {
public void eat() {
System.out.println("Generic Animal Eating Generically");
}
}
class Horse extends Animal {
public void eat() {
System.out.println("Horse eating hay, oats, "
+ "and horse treats");
}
}

For abstract methods you inherit from a superclass, you have no choice: You must
implement the method in the subclass unless the subclass is also abstract. Abstract
methods must be implemented by the concrete subclass, but this is a lot like saying

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101

that the concrete subclass overrides the abstract methods of the superclass. So you
could think of abstract methods as methods you're forced to override.
The Animal class creator might have decided that for the purposes of polymorphism,
all Animal subtypes should have an eat() method defined in a unique, specific way.
Polymorphically, when an Animal reference refers not to an Animal instance, but
to an Animal 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 generic eat() method, so you have to come up with your
own eat() method implementation!" A (nonabstract), example of using polymorphism
looks like this:
public class TestAnimals {
public static void main (String [] args) {
Animal a = new Animal();
Animal b = new Horse(); // Animal ref, but a Horse object
a.eat(); // Runs the Animal version of eat()
b.eat(); // Runs the Horse version of eat()
}
}
class Animal {
public void eat() {
System.out.println("Generic Animal Eating Generically");
}
}
class Horse extends Animal {
public void eat() {
System.out.println("Horse eating hay, oats, "
+ "and horse treats");
}
public void buck() { }
}

In the preceding code, the test class uses an Animal reference to invoke a method
on a Horse object. Remember, the compiler will allow only methods in class Animal
to be invoked when using a reference to an Animal. The following would not be
legal given the preceding code:
Animal c = new Horse();
c.buck(); // Can't invoke buck();
// Animal class doesn't have that method

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).

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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 it protected). Think about it: If the Animal class advertises a public
eat() method and someone has an Animal reference (in other words, a reference
declared as type Animal), that someone will assume it's safe to call eat() on the
Animal reference regardless of the actual instance that the Animal reference is
referring to. If a subclass 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 subclass.)
Let's modify the polymorphic example we saw earlier in this section:
public class TestAnimals {
public static void main (String [] args) {
Animal a = new Animal();
Animal b = new Horse(); // Animal ref, but a Horse object
a.eat();
// Runs the Animal version of eat()
b.eat();
// Runs the Horse version of eat()
}
}
class Animal {
public void eat() {
System.out.println("Generic Animal Eating Generically");
}
}
class Horse extends Animal {
private void eat() {
// whoa! - it's private!
System.out.println("Horse eating hay, oats, "
+ "and horse treats");
}
}

If this code compiled (which it doesn't), the following would fail at runtime:
Animal b = new Horse();
b.eat();

// Animal ref, but a Horse
// object, so far so good
// Meltdown 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. An
Animal reference can always refer to a Horse instance, because Horse IS-A(n)
Animal. What makes that superclass reference to a subclass instance possible is that
the subclass is guaranteed to be able to do everything the superclass can do. Whether
the Horse instance overrides the inherited methods of Animal or simply inherits

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103

them, anyone with an Animal reference to a Horse instance is free to call all
accessible Animal methods. For that reason, an overriding method must fulfill the
contract of the superclass.
Note: In Chapter 6 we will explore exception handling in detail. Once you've
studied Chapter 6, you'll appreciate this handy, single 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 subclass.

A subclass within the same package as the instance's superclass can override
any superclass method that is not marked private or final. A subclass in a
different package can override only those nonfinal methods marked public
or protected (since protected methods are inherited by the subclass).
■ The overriding method CAN throw any unchecked (runtime) exception,

regardless of whether the overridden method declares the exception. (More
in Chapter 6.)
■ 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 of FileNotFoundException.
■ The overriding method can throw narrower or fewer exceptions. Just because

an overridden method "takes risks" doesn't mean that the overriding subclass'
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 discuss static methods in more detail.

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■ 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 an eat()
method to Horse, it wouldn't be an override of Animal's eat() method.
public class TestAnimals {
public static void main (String [] args) {
Horse h = new Horse();
h.eat(); // Not legal because Horse didn't inherit eat()
}
}
class Animal {
private void eat() {
System.out.println("Generic Animal Eating Generically");
}
}
class Horse extends Animal { }

Invoking a Superclass Version of an Overridden Method
Often, you'll want to take advantage of some of the code in the superclass version of
a method, yet still override it to provide some additional specific behavior. It's like
saying, "Run the superclass version of the method, and then come back down here
and finish with my subclass additional method code." (Note that there's no requirement
that the superclass version run before the subclass code.) It's easy to do in code using
the keyword super as follows:
public class Animal {
public void eat() { }
public void printYourself() {
// Useful printing code goes here
}
}
class Horse extends Animal {
public void printYourself() {
// Take advantage of Animal code, then add some more
super.printYourself(); // Invoke the superclass
// (Animal) code
// Then do Horse-specific
// print work here
}
}

Note: Using super 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 class' superclass, not the superclass of the
superclass—that is, you can't say super.super.doStuff().

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105

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 6). Let's
take a look at an example:
class Animal {
public void eat() throws Exception {
// throws an Exception
}
}
class Dog2 extends Animal {
public void eat() { /* no Exceptions */}
public static void main(String [] args) {
Animal a = new Dog2();
Dog2 d = new Dog2();
d.eat();
// ok
a.eat();
// compiler error // unreported exception
}
}

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 of Animal:
public class Animal {
public void eat() { }
}

Table 2-1 lists examples of illegal overrides of the Animal eat() method, given
the preceding version of the Animal class.

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Object Orientation

Examples of Illegal Overrides

Illegal Override Code

Problem with the Code

private void eat() { }

Access modifier is more restrictive

public void eat() throws
IOException { }

Declares a checked exception not defined by superclass
version

public void eat(String food) { }

A legal overload, not an override, because the argument list
changed

public String eat() { }

Not an override because of the return type, and not an
overload either because there's no change in the argument list

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 class or in a subclass. 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 an int. So two methods with the same name but in different classes
can still be considered overloaded if the subclass inherits one version of the
method and then declares another overloaded version in its class definition.

Overriding / Overloading (OCA Objectives 6.1, 6.3, 7.2, and 7.3)

107

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:
public class Foo {
public void doStuff(int y, String s) { }
public void moreThings(int x) { }
}
class Bar extends Foo {
public void doStuff(int y, long s) throws IOException { }
}

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.

Legal Overloads
Let's look at a method we want to overload:
public void changeSize(int size, String name, float pattern) { }

The following methods are legal overloads of the changeSize() method:
public void changeSize(int size, String name) { }
private int changeSize(int size, float pattern) { }
public void changeSize(float pattern, String name)
throws IOException { }

Invoking Overloaded Methods
Note for OCP candidates: In Chapter 11 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.

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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
a Foo, then the compiler will complain that it can't find a match. The following are
examples of invoking overloaded methods:
class Adder {
public int addThem(int x, int y) {
return x + y;
}
// Overload the addThem method to add doubles instead of ints
public double addThem(double x, double y) {
return x + y;
}
}
// From another class, invoke the addThem() method
public class TestAdder {
public static void main (String [] args) {
Adder a = new Adder();
int b = 27;
int c = 3;
int result = a.addThem(b,c);
// Which addThem is invoked?
double doubleResult = a.addThem(22.5,9.3); // Which addThem?
}
}

In this TestAdder code, the first call to a.addThem(b,c) passes two ints to the
method, so the first version of addThem()—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 of addThem()—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 an Animal and one takes a Horse (subclass of Animal). If you pass a Horse
object in the method invocation, you'll invoke the overloaded version that takes a
Horse. Or so it looks at first glance:
class Animal { }
class Horse extends Animal { }
class UseAnimals {
public void doStuff(Animal a) {
System.out.println("In the Animal version");
}

Overriding / Overloading (OCA Objectives 6.1, 6.3, 7.2, and 7.3)

109

public void doStuff(Horse h) {
System.out.println("In the Horse version");
}
public static void main (String [] args) {
UseAnimals ua = new UseAnimals();
Animal animalObj = new Animal();
Horse horseObj = new Horse();
ua.doStuff(animalObj);
ua.doStuff(horseObj);
}
}

The output is what you expect:
In the Animal version
In the Horse version

But what if you use an Animal reference to a Horse 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 an Animal, 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 overridden version of the
method to call (in other words, from which class in the inheritance tree) is decided
at runtime based on object type, but which overloaded version of the method to call is
based on the reference type of the argument passed at compile time. If you invoke a
method passing it an Animal reference to a Horse object, the compiler knows only
about the Animal, so it chooses the overloaded version of the method that takes an
Animal. It does not matter that at runtime a Horse is actually being passed.

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Can main() be overloaded?
class DuoMain {
public static void main(String[] args) {
main(1);
}
static void main(int i) {
System.out.println("overloaded main");
}
}

Absolutely! But the only main() 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
an Animal 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 an Animal. So it's true that polymorphism doesn't
determine which overloaded version is called; 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:
public class Animal {
public void eat() {
System.out.println("Generic Animal Eating Generically");
}
}
public class Horse extends Animal {
public void eat() {
System.out.println("Horse eating hay ");
}
public void eat(String s) {
System.out.println("Horse eating " + s);
}
}

Notice that the Horse class has both overloaded and overridden the eat()
method. Table 2-2 shows which version of the three eat() methods will run
depending on how they are invoked.

Overriding / Overloading (OCA Objectives 6.1, 6.3, 7.2, and 7.3)

TABLE 2-2

111

Examples of Legal and Illegal Overrides

Method Invocation Code

Result

Animal a = new Animal();
a.eat();

Generic Animal Eating Generically

Horse h = new Horse();
h.eat();

Horse eating hay

Animal ah = new Horse();
ah.eat();

Horse eating hay

Horse he = new Horse();
he.eat("Apples");

Horse eating Apples
The overloaded eat(String s) method is invoked.

Animal a2 = new Animal();
a2.eat("treats");

Compiler error! Compiler sees that the Animal class doesn't have
an eat() method that takes a String.

Animal ah2 = new Horse();
ah2.eat("Carrots");

Compiler error! Compiler still looks only at the reference and sees
that Animal doesn't have an eat() method that takes a String.
Compiler doesn't care that the actual object might be a Horse at
runtime.

Polymorphism works—the actual object type (Horse), not the
reference type (Animal), is used to determine which eat() is
called.

Don't be fooled by a method that's overloaded but not overridden by a
subclass. It's perfectly legal to do the following:
public class Foo {
void doStuff() { }
}
class Bar extends Foo {
void doStuff(String s) { }
}

The Bar class has two doStuff() methods: the no-arg version it inherits from Foo (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.

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Table 2-3 summarizes the difference between overloaded and overridden methods.

TABLE 1-2

Differences Between Overloaded and Overridden Methods

Overloaded Method

Overridden Method

Argument(s)

Must change.

Must not change.

Return type

Can change.

Can't change except for
covariant returns. (Covered
later this chapter.)

Exceptions

Can change.

Can reduce or eliminate. Must
not throw new or broader
checked exceptions.

Access

Can change.

Must not make more restrictive
(can be less restrictive).

Invocation

Reference type determines which overloaded version
(based on declared argument types) is selected. Happens
at compile time. The actual method that's invoked is still a
virtual method invocation that happens at runtime, but
the compiler will already know the signature of the method
to be invoked. So at runtime, the argument match will
already have been nailed down, just not the class in which
the method lives.

Object type (in other words, the
type of the actual instance on the
heap) determines which method
is selected. Happens at runtime.

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

Overriding

Overloading

Tree

Tree

showLeaves()

setFeatures(String name)

Oak

Oak

showLeaves()

setFeatures(String name, int leafSize)
setFeatures(int leafSize)

Casting (OCA Objectives 7.3 and 7.4)

113

CERTIFICATION OBJECTIVE

Casting (OCA Objectives 7.3 and 7.4)
7.3

Differentiate between the type of a reference and the type of an object.

7.4

Determine when casting is necessary.

You've seen how it's both possible and common to use generic 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 that animal reference variable to invoke
a method that only class Dog 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 of Animals, and
whenever we find a Dog in the array, we want to do a special Dog thing. Let's agree
for now that all of this code is okay, except that we're not sure about the line of code
that invokes the playDead method.
class Animal {
void makeNoise() {System.out.println("generic noise"); }
}
class Dog extends Animal {
void makeNoise() {System.out.println("bark"); }
void playDead() { System.out.println("roll over"); }
}
class CastTest2 {
public static void main(String [] args) {
Animal [] a = {new Animal(), new Dog(), new Animal() };
for(Animal animal : a) {
animal.makeNoise();
if(animal instanceof Dog) {
animal.playDead();
// try to do a Dog behavior?
}
}
}
}

When we try to compile this code, the compiler says something like this:
cannot find symbol

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The compiler is saying, "Hey, class Animal doesn't have a playDead() method."
Let's modify the if code block:
if(animal instanceof Dog) {
Dog d = (Dog) animal;
d.playDead();
}

// casting the ref. var.

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 new Dog 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:
class Animal { }
class Dog extends Animal { }
class DogTest {
public static void main(String [] args) {
Animal animal = new Animal();
Dog d = (Dog) animal;
// compiles but fails later
}
}

It can be maddening! This code compiles! 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 that animal 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 that animal 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 something like this:
inconvertible types

Casting (OCA Objectives 7.3 and 7.4)

115

Unlike downcasting, upcasting (casting up 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
more specific method. Here's an example:
class Animal { }
class Dog extends Animal { }
class DogTest {
public static
Dog d = new
Animal a1 =
Animal a2 =
}
}

void main(String [] args) {
Dog();
d;
// upcast ok with no explicit cast
(Animal) d; // upcast ok with an explicit cast

Both of the previous upcasts will compile and run without exception, because a
Dog IS-A(n) Animal, which means that anything an Animal can do, a Dog can do.
A Dog can do more, of course, but the point is that anyone with an Animal
reference can safely call Animal methods on a Dog instance. The Animal 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 an Animal 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). If Dog implements Pet, and
Pet defines beFriendly(), then a Dog can be implicitly cast to a Pet, but the only
Dog method you can invoke then is beFriendly(), which Dog was forced to
implement because Dog implements the Pet interface.
One more thing…if Dog implements Pet, then if Beagle extends Dog, but Beagle
does not declare that it implements Pet, Beagle is still a Pet! Beagle is a Pet simply
because it extends Dog, 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 from
Dog, including methods that Dog implemented to fulfill its interface contract.
And just one more thing…if Beagle does declare that it implements Pet, just so
that others looking at the Beagle class API can easily see that Beagle IS-A Pet
without having to look at Beagle's superclasses, Beagle still doesn't need to
implement the beFriendly() method if the Dog class (Beagle's superclass) has
already taken care of that. In other words, if Beagle IS-A Dog, and Dog IS-A Pet,
then Beagle IS-A Pet and has already met its Pet obligations for implementing the
beFriendly() method since it inherits the beFriendly() method. The compiler is
smart enough to say, "I know Beagle already IS a Dog, but it's okay to make it more
obvious by adding a cast."

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So don't be fooled by code that shows a concrete class that declares that it
implements an interface but doesn't implement the methods of the interface. Before
you can tell whether the code is legal, you must know what the superclasses of this
implementing class have declared. If any superclass in its inheritance tree has already
provided concrete (that is, nonabstract) method implementations, then regardless of
whether the superclass 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
Animal a = new Dog();
Dog d = (Dog) a;
d.doDogStuff();

can be replaced with this easy-to-read bit of fun:
Animal a = new Dog();
((Dog)a).doDogStuff();

In this case the compiler needs all of those parentheses; otherwise it thinks it's been
handed an incomplete statement.

CERTIFICATION OBJECTIVE

Implementing an Interface (OCA Objective 7.6)
7.6

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 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.

Implementing an Interface (OCA Objective 7.6)

117

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 method in the interface (with a few
exceptions we'll look at in a moment).
Assuming an interface, Bounceable, with two methods, bounce() and
setBounceFactor(), the following class will compile:
public class Ball implements Bounceable {

// Keyword
// 'implements'

public void bounce() { }
public void setBounceFactor(int bf) { }
}

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 will have to include a return statement.) The compiler
will never say to you, "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 an abstract class. To be a legal implementation class, a
nonabstract implementation class must do the following:
■ Provide concrete (nonabstract) implementations for all 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.)

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But wait, there's more! An implementation class can itself be abstract! For
example, the following is legal for a class Ball implementing Bounceable:
abstract class Ball implements Bounceable { }

Notice anything missing? We never provided the implementation methods. And
that's okay. If the implementation class is abstract, it can simply pass the buck to
its first concrete subclass. For example, if class BeachBall extends Ball, and
BeachBall is not abstract, then BeachBall will have to provide all the methods
from Bounceable:
class BeachBall extends Ball {
// Even though we don't say it in the class declaration above,
// BeachBall implements Bounceable, since BeachBall's abstract
// superclass (Ball) implements Bounceable
public void bounce() {
// interesting BeachBall-specific bounce code
}
public void setBounceFactor(int bf) {
// clever BeachBall-specific code for setting
// a bounce factor
}
// if class Ball defined any abstract methods,
// they'll have to be
// implemented here as well.
}

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 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. But
remember that subclassing defines who and what you are, whereas implementing
defines a role you can play or a hat you can wear, d espite 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.

Implementing an Interface (OCA Objective 7.6)

119

2. An interface can itself extend another interface, but it can never implement
anything. The following code is perfectly legal:
public interface Bounceable extends Moveable { }

// ok!

What does that mean? The first concrete (nonabstract) implementation class of
Bounceable must implement all the methods of Bounceable, plus all the methods
of Moveable! 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:
interface Bounceable extends Moveable, Spherical {
void bounce();
void setBounceFactor(int bf);
}
interface Moveable {
void moveIt();
}
interface Spherical {
void doSphericalThing();
}

// ok!

In the next example, Ball is required to implement Bounceable, plus all
methods from the interfaces that Bounceable 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:
class Ball implements Bounceable {
public void bounce() { }
// Implement Bounceable's methods
public void setBounceFactor(int bf) { }
public void moveIt() { }

// Implement Moveable's method

public void doSphericalThing() { }

// Implement Spherical

}

If class Ball fails to implement any of the methods from Bounceable, Moveable,
or Spherical, the compiler will jump up and down wildly, red in the face, until it
does. Unless, that is, class Ball is marked abstract. In that case, Ball could choose

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to implement any, all, or none of the methods from any of the interfaces, thus leaving
the rest of the implementations to a concrete subclass of Ball, as follows:
abstract class Ball implements Bounceable {
public void bounce() { ... }
// Define bounce behavior
public void setBounceFactor(int bf) { ... }
// Don't implement the rest; leave it for a subclass
}
class SoccerBall extends Ball { // class SoccerBall must
// implement the interface
// methods that Ball didn't
public void moveIt() { ... }
public void doSphericalThing() { ... }
// SoccerBall can choose to override the Bounceable methods
// implemented by Ball
public void bounce() { ... }
}

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

interface Bounceable
void bounce( );
void setBounceFactor(int bf);

abstract Ball implements Bounceable
/*no methods of
Bounceable are
implemented
in Ball */
void beSpherical( ){}

class Tire implements Bounceable

class BeachBall extends Ball

public void bounce( ){ }
public void setBounceFactor (int bf){ }

public void bounce( ){ }
public void setBounceFactor (int bf){ }
/*beSpherical is not abstract
so BeachBall is not
required to implement it.*/

Because BeachBall is the first concrete class to implement Bounceable,
it must provide implementations for all methods of Bounceable, except
those defined in the abstract class Ball. Because Ball did not provide
implementations of Bounceable methods, BeachBall was required to
implement all of them.

Implementing an Interface (OCA Objective 7.6)

121

Look for illegal uses of extends and implements.The following shows
examples of legal and illegal class and interface declarations:
class Foo
class Bar
interface
interface
interface

{ }
implements Foo { }
Baz { }
Fi { }
Fee implements Baz { }

//
//
//
//
//
//
interface Zee implements Foo { }
//
//
interface Zoo extends Foo { }
//
//
interface Boo extends Fi { }
//
//
class Toon extends Foo, Button { }
//
//
class Zoom implements Fi, Baz { }
//
//
interface Vroom extends Fi, Baz { }
//
//
class Yow extends Foo implements Fi { }
//
//
class Yow extends Foo implements Fi, Baz { } //
//

OK
No! Can't implement a class
OK
OK
No! an interface can't
implement an interface
No! an interface can't
implement a class
No! an interface can't
extend a class
OK. An interface can extend
an interface
No! a class can't extend
multiple classes
OK. A class can implement
multiple interfaces
OK. An interface can extend
multiple interfaces
OK. A class can do both
(extends must be 1st)
OK. A class can do all three
(extends must be 1st)

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?)

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CERTIFICATION OBJECTIVE

Legal Return Types
(OCA Objectives 2.2, 2.5, 6.1, and 6.3)
2.2

Differentiate between object reference variables and primitive variables.

2.5

Call methods on objects.

6.1

Create methods with arguments and return values.

6.3

Create an overloaded method.

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 subclass, 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:

Legal Return Types (OCA Objectives 2.2, 2.5, 6.1, and 6.3)

123

public class Foo{
void go() { }
}
public class Bar extends Foo {
String go(int x) {
return null;
}
}

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 superclass version.
What you're NOT allowed to do is this:
public class Foo{
void go() { }
}
public class Bar extends Foo {
String go() { // Not legal! Can't change only the return type
return null;
}
}

Overriding and Return Types, and Covariant Returns
When a subclass wants to change the method implementation of an inherited
method (an override), the subclass must define a method that matches the inherited
version exactly. Or, as of 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:
class Alpha {
Alpha doStuff(char c) {
return new Alpha();
}
}
class Beta extends Alpha {
Beta doStuff(char c) {
return new Beta();
}
}

// legal override in Java 1.5

As of Java 5, this code will compile. 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

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you would get a compiler error something like this:
attempting to use incompatible return type

(We'll talk more about compiler flags in Chapter 8.)
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 return null in a method with an object reference return type.
public Button doStuff() {
return null;
}

2. An array is a perfectly legal return type.
public String[] go() {
return new String[] {"Fred", "Barney", "Wilma"};
}

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.
public int foo() {
char c = 'c';
return c; // char is compatible with int
}

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.
public int foo () {
float f = 32.5f;
return (int) f;
}

Legal Return Types (OCA Objectives 2.2, 2.5, 6.1, and 6.3)

125

5. You must not return anything from a method with a void return type.
public void bar() {
return "this is it";
}

// Not legal!!

(Although you can say return;)
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.
public Animal getAnimal() {
return new Horse(); // Assume Horse extends Animal
}
public Object getObject() {
int[] nums = {1,2,3};
return nums;
// Return an int array, which is still an object
}
public interface Chewable { }
public class Gum implements Chewable { }
public class TestChewable {
// Method with an interface return type
public Chewable getChewable() {
return new Gum(); // Return interface implementer
}
}

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:
public abstract class Animal { }
public class Bear extends Animal { }
public class Test {
public Animal go() {
return new Bear(); // OK, Bear "is-a" Animal
}
}

This code will compile, and the return value is a subtype.

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CERTIFICATION OBJECTIVE

Constructors and Instantiation
(OCA Objectives 6.4, 6.5, and 7.5)
6.4

Differentiate between default and user-defined constructors.

6.5

Create and overload constructors.

7.5

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 say new, 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 instance variable state, as
follows:
class Foo {
int size;
String name;
Foo(String name, int size) {
this.name = name;
this.size = size;
}
}

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

127

In the preceding code example, the Foo 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:
Foo f = new Foo("Fred", 43);

// No problem. Arguments match
// the Foo constructor.

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. A java.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 what really happens when you say new Horse()? (Assume Horse extends
Animal and Animal extends Object.)
1. Horse 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. Animal constructor is invoked (Animal is the superclass of Horse).
3. Object constructor is invoked (Object is the ultimate superclass of all
classes, so class Animal extends Object 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. Object instance variables are 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, where 27 is the explicit value (as opposed to the default
value) of the instance variable.

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FIGURE 2-6

Constructors on
the call stack

Object Orientation

4. Object()
3. Animal() calls super()
2. Horse() calls super()
1. main() calls new Horse()

5. Object constructor completes.
6. Animal instance variables are given their explicit values (if any).
7. Animal constructor completes.
8. Horse instance variables are given their explicit values (if any).
9. Horse constructor completes.
Figure 2-6 shows how constructors work 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. (A private

constructor means only code within the class itself can instantiate an object
of that type, so if the private 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.

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

129

■ 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
type 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, because NAME

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:
class Horse {
Horse() { } // constructor
void doStuff() {
Horse(); // calling the constructor - illegal!
}
}

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Determine Whether a Default Constructor Will Be Created
The following example shows a Horse class with two constructors:
class Horse {
Horse() { }
Horse(String name) { }
}

Will the compiler put in a default constructor for the class above? 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 the
Horse class. Or is it? Take another look at the preceding Horse class.
What's wrong with the Horse() 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, and not a constructor.

How do you know for sure whether a default constructor
will be created?
Because you didn't write any constructors in your class.
How do you know what the default constructor will look like?
Because...
■ The default constructor has the same access modifier as the class.
■ The default constructor has no arguments.

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

■ The default constructor includes a no-arg call to the super constructor

(super()).
Table 2-4 shows what the compiler will (or won't) generate for your class.

TABLE 2-4

Compiler-Generated Constructor Code

Class Code (What You Type)

Compiler-Generated Constructor Code (in Bold)

class Foo { }

class Foo {
Foo() {
super();
}
}

class Foo {
Foo() { }
}

class Foo {
Foo() {
super();
}
}

public class Foo { }

public class Foo {
public Foo() {
super();
}
}

class Foo {
Foo(String s) { }
}

class Foo {
Foo(String s) {
super();
}
}

class Foo {
Foo(String s) {
super();
}
}

Nothing; compiler doesn't need to insert anything.

class Foo {
void Foo() { }
}

class Foo {
void Foo() { }
Foo() {
super();
}
}
(void Foo() is a method, not a constructor.)

131

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What happens if the super constructor has arguments?
Constructors can have arguments just as methods can, and if you try to invoke a
method that takes, say, an int, but you don't pass anything to the method, the
compiler will complain as follows:
class Bar {
void takeInt(int x) { }
}
class UseBar {
public static void main (String [] args) {
Bar b = new Bar();
b.takeInt(); // Try to invoke a no-arg takeInt() method
}
}

The compiler will complain that you can't invoke takeInt() without passing an
int. 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 that 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:
class Animal {
Animal(String name) { }
}
class Horse extends Animal {
Horse() {
super(); // Problem!
}
}

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

133

And once again the compiler treats us with stunning lucidity:
Horse.java:7: cannot resolve symbol
symbol : constructor Animal ()
location: class Animal
super(); // Problem!
^

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
with super()—an Animal 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 can only put in a call to a no-arg
super(), you won't even be able to compile something like this:
class Clothing {
Clothing(String s) { }
}
class TShirt extends Clothing { }

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():
Clothing.java:4: cannot resolve symbol
symbol : constructor Clothing ()
location: class Clothing
class TShirt extends Clothing { }
^

In fact, the preceding Clothing and TShirt code is implicitly the same as the
following code, where we've supplied a constructor for TShirt that's identical to the
default constructor supplied by the compiler:
class Clothing {
Clothing(String s) { }
}
class TShirt extends Clothing {
// Constructor identical to compiler-supplied
// default constructor
TShirt() {
super();
// Won't work!
}
// tries to invoke a no-arg Clothing constructor
// but there isn't one
}

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

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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:
class Foo {
Foo() { }
Foo(String s) { }
}

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 the Foo 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:
1. public class Animal {
2.
String name;
3.
Animal(String name) {
4.
this.name = name;
5.
}
6.
7.
Animal() {
8.
this(makeRandomName());
9.
}
10.
11.
static String makeRandomName() {

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

12.
13.

135

int x = (int) (Math.random() * 5);
String name = new String[] {"Fluffy", "Fido",
"Rover", "Spike",
"Gigi"}[x];
return name;

14.
15.
}
16.
17.
public static void main (String [] args) {
18.
Animal a = new Animal();
19.
System.out.println(a.name);
20.
Animal b = new Animal("Zeus");
21.
System.out.println(b.name);
22.
}
23. }

Running the code four times produces this output:
% java Animal
Gigi
Zeus
% java Animal
Fluffy
Zeus
% java Animal
Rover
Zeus
% java Animal
Fluffy
Zeus

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. 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.

FIGURE 2-7

Overloaded
constructors on
the call stack

4. Object()
3. Animal(String s) calls super()
2. Animal() calls this(randomlyChosenNameString)
1. main() calls new Animal()

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■ 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 you'll assign a random name. The no-arg constructor generates a name by
invoking the makeRandomName() 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 keyword this, but uses
it as though it were a method name this(). So line 8 is simply calling the
constructor on line 3, passing it a randomly selected String rather than a
client-code chosen name.

■ Line 11

Notice that the makeRandomName() method is marked static!
That's because you cannot invoke an instance (in other words, nonstatic)
method (or access an instance variable) until after the super constructor has
run. And since the super 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 read this("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
anything).
3. Retrieve the string at index [x] (x being the random number generated
on line 12) of the newly created String 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
String[] nameList = {"Fluffy", "Fido", "Rover", "Spike",
"Gigi"};
String name = nameList[x];

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

137

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(), and this() always means a call to another
constructor in the same class. Okay, fine, but what happens after the call to this()?
Sooner or later the super() 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().

Key Rule: The first line in a constructor must be a call to super()
or a call to this().
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 constructor A() has a call to
this(), the compiler knows that constructor A() will not be the one to invoke
super().
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?
class A {
A() {
this("foo");
}
A(String s) {
this();
}
}

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,

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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 the super constructor—unless,
that is, the constructor already has a call to this(). So in the preceding code, where
can super() 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 the Animal 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, so we're holding the rest of the discussion on
instantiating inner classes until Chapter 12.

Initialization Blocks
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

Constructors and Instantiation (OCA Objectives 6.4, 6.5, and 7.5)

139

Java program where operations can be performed. Initialization blocks run when the
class is first loaded (a static initialization block) or when an instance is created (an
instance initialization block). Let's look at an example:
class SmallInit {
static int x;
int y;
static { x = 7 ; }
{ y = 8; }

// static init block
// instance init block

}

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. A static initialization
block runs once, when the class is first loaded. An instance initialization block runs once
every 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?
class Init {
Init(int x) { System.out.println("1-arg const"); }
Init() { System.out.println("no-arg const"); }
static { System.out.println("1st static init"); }
{ System.out.println("1st instance init"); }
{ System.out.println("2nd instance init"); }
static { System.out.println("2nd static init"); }
public static void main(String [] args) {
new Init();
new Init(7);
}
}

To figure this out, remember these rules:
■ init blocks execute in the order in which they appear.
■ Static init blocks run once, when the class is first loaded.
■ Instance init blocks run every time a class instance is created.
■ Instance init blocks run after the constructor's call to super().

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With those rules in mind, the following output should make sense:
1st static init
2nd static init
1st instance init
2nd instance init
no-arg const
1st instance init
2nd instance init
1-arg const

As you can see, the instance init blocks each ran twice. Instance init 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:
class InitError
static int []
static { x[4]
public static
}

{
x = new int[4];
= 5; }
// bad array index!
void main(String [] args) { }

It produces something like this:
Exception in thread "main" java.lang.ExceptionInInitializerError
Caused by: java.lang.ArrayIndexOutOfBoundsException: 4
at InitError.(InitError.java:3)

By convention, init blocks usually appear near the top of the class file,
somewhere around the constructors. However, these are the OCA and OCP exams 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.

Statics (OCA Objective 6.2)

141

Static Variables and Methods
The static 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 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 class 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-runsthe-same scenario and the keep-a-running-total-of-instances scenario is to use the
static modifier. Variables and methods marked static belong to the class, rather
than to any particular instance. In fact, you can use a static method or variable
without having any instances of that class at all. You need only have the class
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:
class Frog {
static int frogCount = 0;

// Declare and initialize
// static variable

public Frog() {
frogCount += 1;
// Modify the value in the constructor
}
public static void main (String [] args) {
new Frog();
new Frog();
new Frog();
System.out.println("Frog count is now " + frogCount);
}
}

In the preceding code, the static frogCount variable is set to zero when the
Frog 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

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the same default values instance variables get.) Whenever a Frog instance is created,
the Frog constructor runs and increments the static frogCount variable. When
this code executes, three Frog instances are created in main(), and the result is
Frog count is now 3

Now imagine what would happen if frogCount were an instance variable (in
other words, nonstatic):
class Frog {
int frogCount = 0;

// Declare and initialize
// instance variable

public Frog() {
frogCount += 1;
// Modify the value in the constructor
}
public static void main (String [] args) {
new Frog();
new Frog();
new Frog();
System.out.println("Frog count is now " + frogCount);
}
}

When this code executes, it should still create three Frog instances in main(),
but the result is...a compiler error! We can't get this code to compile, let alone run.
Frog.java:11: nonstatic variable frogCount cannot be referenced
from a static context
System.out.println("Frog count is " + frogCount);
^
1 error

The JVM doesn't know which Frog object's frogCount 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. A static
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, the static 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.

Statics (OCA Objective 6.2)

143

One of the mistakes most often made by new Java programmers is
attempting to access an instance variable (which means nonstatic variable) from the
static main() 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:
class Foo {
int x = 3;
public static void main (String [] args) {
System.out.println("x is " + x);
}
}

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
class Foo {
int x = 3;
float y = 4.3f;
public static void main (String [] args) {
for (int z = x; z < ++x; z--, y = y + z)
// complicated looping and branching code
}
}

So while you're trying to follow the logic, the real issue is that x and y can't be used
within main(), because x 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 Static Methods 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

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syntax? We know that with a regular old instance method, you use the dot operator
on a reference to an instance:
class Frog {
int frogSize = 0;
public int getFrogSize() {
return frogSize;
}
public Frog(int s) {
frogSize = s;
}
public static void main (String [] args) {
Frog f = new Frog(25);
System.out.println(f.getFrogSize()); // Access instance
// method using f
}
}

In the preceding code, we instantiate a Frog, assign it to the reference variable f,
and then use that f reference to invoke a method on the Frog 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 (or static variable) is to use the dot operator on
the class name, as opposed to using it on a reference to an instance, as follows:
class Frog {
static int frogCount = 0;
public Frog() {
frogCount += 1;
}

// Declare and initialize
// static variable
// Modify the value in the constructor

}
class TestFrog {
public static void main (String [] args) {
new Frog();
new Frog();
new Frog();
System.out.print("frogCount:"+Frog.frogCount); // Access
// static variable
}
}

Statics (OCA Objective 6.2)

145

But just to make it really confusing, the Java language also allows you to use an
object reference variable to access a static member:
Frog f = new Frog();
int frogs = f.frogCount; // Access static variable
// FrogCount using f

In the preceding code, we instantiate a Frog, assign the new Frog object to the
reference variable f, and then use the f reference to invoke a static method! But
even though we are using a specific Frog instance to access the static 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
the static member is still unaware of the particular instance used to invoke the
static member. In the Frog example, the compiler knows that the reference variable
f is of type Frog, and so the Frog class static method is run with no awareness or
concern for the Frog instance at the other end of the f reference. In other words, the
compiler cares only that reference variable f is declared as type Frog. Figure 2-8
illustrates the effects of the static modifier on methods and variables.
FIGURE 2-8

The effects of
static on
methods and
variables

class Foo
int size = 42;
static void doMore( ){
int x = size;
}

static method cannot
access an instance
(non-static) variable

class Bar
void go(){}
static void doMore( ){
go( );
}

static method cannot
access a non-static
method

class Baz
static int count;
static void woo( ){ }
static void doMore( ){
woo( );
int x = count;
}

static method
can access a static
method or variable

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Finally, remember that static methods 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 take a look at an example of a redefined (remember, not overridden)
static method:
class Animal {
static void doStuff() {
System.out.print("a ");
}
}
class Dog extends Animal {
static void doStuff() {

// it's a redefinition,
// not an override

System.out.print("d ");
}
public static void main(String [] args) {
Animal [] a = {new Animal(), new Dog(), new Animal()};
for(int x = 0; x < a.length; x++) {
a[x].doStuff();
// invoke the static method
}
Dog.doStuff();
// invoke using the class name
}
}

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 like Animal.doStuff() instead.
Notice also that you can invoke a static method by using the class name.
Notice that we didn't use the Java 5 enhanced for loop here (covered in Chapter 6),
even though we could have. Expect to see a mix of both Java 1.4 and Java 5–7 coding
styles and practices on the exam.

Certification Summary

147

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 subclass inherits a method from a
superclass, and then reimplements the method to add more specialized behavior. We
learned that, at runtime, the JVM will invoke the subclass version on an instance of
a subclass and the superclass version on an instance of the superclass. 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, as of Java 5, they can return a subtype of the declared return type of the
superclass 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. 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

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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 an int. And (assuming Horse extends
Animal), a Horse reference can be returned when the return type is declared an
Animal.
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, since
constructors do not have return types.
We saw how all of 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 keyword
this(), as though the constructor were a method named this(). We saw that
every constructor must have either this() or super() 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 at static methods and variables. static members are tied to the
class, 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 class name with the dot operator to access static 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

✓

149

TWO-MINUTE DRILL
Here are some of the key points from each certification objective in this chapter.

Encapsulation, IS-A, HAS-A (OCA Objective 6.7)
❑ Encapsulation helps hide implementation behind an interface (or API).
❑ Encapsulated code has two features:
❑ Instance variables are kept protected (usually with the private modifier).
❑ Getter and setter methods provide access to instance variables.
❑ IS-A refers to inheritance or implementation.
❑ IS-A is expressed with the keyword extends or implements.
❑ 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.

Inheritance (OCA Objectives 7.1 and 7.3)
❑ Inheritance allows a class to be a subclass of a superclass and thereby inherit
public and protected variables and methods of the superclass.
❑ Inheritance is a key concept that underlies IS-A, polymorphism, overriding,
overloading, and casting.
❑ All classes (except class Object) are subclasses of type Object, and therefore
they inherit Object's methods.

Polymorphism (OCA Objectives 7.2 and 7.3)
❑ 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 overridden instance methods.

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Overriding and Overloading (OCA Objective 6.3)
❑ 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, except that, as of Java 5, the return type
can be a subclass, and this is 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 uses super.overriddenMethodName() to call the superclass
version of 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 superclass can be overloaded in a subclass.
❑ 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.

Two-Minute Drill

151

Reference Variable Casting (OCA Objectives 7.3 and 7.4)
❑ 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 (OCA Objective 7.6)
❑ When you implement an interface, you are fulfilling its contract.
❑ You implement an interface by properly and concretely implementing all of
the methods defined by the interface.
❑ A single class can implement many interfaces.

Return Types (OCA Objectives 6.1 and 6.3)
❑ Overloaded methods can change return types; overridden methods cannot,
except in the case of covariant returns.
❑ Object reference return types can accept null 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.

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Constructors and Instantiation (OCA Objectives 6.5 and 7.5)
❑ 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 the Object constructor.
❑ The Object 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 (even private!).
❑ 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 the super 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() or this()).

Two-Minute Drill

153

❑ 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() and super() cannot be in the same constructor. You can
have one or the other, but never both.

Initialization Blocks (OCA Objective 6.5-ish)
❑ Use static 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 normal init 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 Objective 6.2)
❑ Use static methods to implement behaviors that are not affected by the
state of any instances.
❑ Use static variables to hold data that is class specific as opposed to instance
specific—there will be only one copy of a static variable.
❑ All static members belong to the class, not to any instance.
❑ A static method can't access an instance variable directly.
❑ Use the dot operator to access static 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();

❑ static methods can't be overridden, but they can be redefined.

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SELF TEST
1. Given:
public abstract interface Frobnicate { public void twiddle(String s); }

Which is a correct class? (Choose all that apply.)
A. public abstract class Frob implements Frobnicate {
public abstract void twiddle(String s) { }
}

B. public abstract class Frob implements Frobnicate { }
C. public class Frob extends Frobnicate {
public void twiddle(Integer i) { }
}

D. public class Frob implements Frobnicate {
public void twiddle(Integer i) { }
}

E. public class Frob implements Frobnicate {
public void twiddle(String i) { }
public void twiddle(Integer s) { }
}

2. Given:
class Top {
public Top(String s) { System.out.print("B"); }
}
public class Bottom2 extends Top {
public Bottom2(String s) { System.out.print("D"); }
public static void main(String [] args) {
new Bottom2("C");
System.out.println(" ");
}
}

What is the result?
A. BD
B. DB
C. BDC
D. DBC
E. Compilation fails

Self Test

155

3. Given:
class Clidder {
private final void flipper() { System.out.println("Clidder"); }
}
public class Clidlet extends Clidder {
public final void flipper() { System.out.println("Clidlet");
public static void main(String [] args) {
new Clidlet().flipper();
}
}

}

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-anddrop." Up through the SCJP 6 exam, drag-and-drop questions were included on the exam. As of
the Spring of 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 all of the slots.
Code:
class AgedP {
__________ __________ __________ __________ __________
public AgedP(int x) {
__________ __________ __________ __________ __________
}
}
public class Kinder extends AgedP {
__________ __________ __________ _________ ________ __________
public Kinder(int x) {
__________ __________ __________ __________ __________ ();
}
}

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Fragments: Use the following fragments zero or more times:
AgedP
(

super

this
{

}

;

5. Given:
class Bird {
{ System.out.print("b1 "); }
public Bird() { System.out.print("b2 "); }
}
class Raptor extends Bird {
static { System.out.print("r1 "); }
public Raptor() { System.out.print("r2 "); }
{ System.out.print("r3 "); }
static { System.out.print("r4 "); }
}
class Hawk extends Raptor {
public static void main(String[] args) {
System.out.print("pre ");
new Hawk();
System.out.println("hawk ");
}
}

What is the result?
A. pre b1 b2 r3 r2 hawk
B. pre b2 b1 r2 r3 hawk
C. pre b2 b1 r2 r3 hawk r1 r4
D. r1 r4 pre b1 b2 r3 r2 hawk
E. r1 r4 pre b2 b1 r2 r3 hawk
F. pre r1 r4 b1 b2 r3 r2 hawk
G. pre r1 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!

Self Test

6. Given the following:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

class X { void do1() { } }
class Y extends X { void do2() { } }
class Chrome {
public static void main(String [] args) {
X x1 = new X();
X x2 = new Y();
Y y1 = new Y();
// insert code here
} }

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:
public class Locomotive {
Locomotive() { main("hi"); }
public static void main(String[] args) {
System.out.print("2 ");
}
public static void main(String args) {
System.out.print("3 " + args);
}
}

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

157

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8. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.

class Dog {
public void bark() { System.out.print("woof "); }
}
class Hound extends Dog {
public void sniff() { System.out.print("sniff "); }
public void bark() { System.out.print("howl "); }
}
public class DogShow {
public static void main(String[] args) { new DogShow().go(); }
void go() {
new Hound().bark();
((Dog) new Hound()).bark();
((Dog) new Hound()).sniff();
}
}

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:
3. public class Redwood extends Tree {
4.
public static void main(String[] args) {
5.
new Redwood().go();
6.
}
7.
void go() {
8.
go2(new Tree(), new Redwood());
9.
go2((Redwood) new Tree(), new Redwood());
10.
}
11.
void go2(Tree t1, Redwood r1) {
12.
Redwood r2 = (Redwood)t1;
13.
Tree t2 = (Tree)r1;
14.
}
15. }
16. class Tree { }

Self Test

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:
3. public class Tenor extends Singer {
4.
public static String sing() { return "fa"; }
5.
public static void main(String[] args) {
6.
Tenor t = new Tenor();
7.
Singer s = new Tenor();
8.
System.out.println(t.sing() + " " + s.sing());
9.
}
10. }
11. class Singer { public static String sing() { return "la"; } }

What is the result?
A. fa fa
B. fa la
C. la la
D. Compilation fails
E. An exception is thrown at runtime
11. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

class Alpha {
static String s = " ";
protected Alpha() { s += "alpha "; }
}
class SubAlpha extends Alpha {
private SubAlpha() { s += "sub "; }
}
public class SubSubAlpha extends Alpha {
private SubSubAlpha() { s += "subsub "; }
public static void main(String[] args) {
new SubSubAlpha();
System.out.println(s);
}
}

159

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What is the result?
A.
B.
C.
D.
E.
F.

subsub
sub subsub
alpha subsub
alpha sub subsub

Compilation fails
An exception is thrown at runtime

12. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.

class Building {
Building() { System.out.print("b "); }
Building(String name) {
this();
System.out.print("bn " + name);
}
}
public class House extends Building {
House() { System.out.print("h "); }
House(String name) {
this();
System.out.print("hn " + name);
}
public static void main(String[] args) { new House("x "); }
}

What is the result?
A. h hn x
B. hn x h
C. b h hn x
D. b hn x h
E. bn x h hn x
F. b bn x h hn x
G. bn x b h hn x
H. Compilation fails

Self Test

161

13. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.

class Mammal {
String name = "furry ";
String makeNoise() { return "generic noise"; }
}
class Zebra extends Mammal {
String name = "stripes ";
String makeNoise() { return "bray"; }
}
public class ZooKeeper {
public static void main(String[] args) { new ZooKeeper().go(); }
void go() {
Mammal m = new Zebra();
System.out.println(m.name + m.makeNoise());
}
}

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. (OCP Only) Given:
You're designing a new online board game in which Floozels are a type of Jammers, Jammers
can have Quizels, Quizels are a type of Klakker, and Floozels can have several Floozets.
Which of the following fragments represent this design? (Choose all that apply.)
A. import java.util.*;
interface Klakker { }
class Jammer { Set q; }
class Quizel implements Klakker { }
public class Floozel extends Jammer { List f; }
interface Floozet { }

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B. import java.util.*;
class Klakker { Set q; }
class Quizel extends Klakker { }
class Jammer { List f; }
class Floozet extends Floozel { }
public class Floozel { Set k; }

C. import java.util.*;
class Floozet { }
class Quizel implements Klakker { }
class Jammer { List q; }
interface Klakker { }
class Floozel extends Jammer { List f; }

D. import java.util.*;
interface
interface
interface
interface
interface

Jammer extends Quizel { }
Klakker { }
Quizel extends Klakker { }
Floozel extends Jammer, Floozet { }
Floozet { }

Self Test Answers

163

SELF TEST ANSWERS
1. ☑ B and E are correct. B is correct because an abstract 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 the twiddle() method.
☐
✗ A, C, and D are incorrect. A is incorrect because abstract 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 7.6)
2. ☑ E is correct. The implied super() call in Bottom2's constructor cannot be satisfied
because there is no no-arg constructor in Top. 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 Objectives 6.5 and 7.5)
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 Objectives 7.1 and 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 the
Spring of 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:
class AgedP {
AgedP() {}
public AgedP(int x) {
}
}
public class Kinder extends AgedP {
public Kinder(int x) {
super();
}
}

As there is no droppable tile for the variable x and the parentheses (in the Kinder 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 6.5, 7.1, and 7.5)
Note: As you can see, many questions test for OCA Objective 7.1.

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5. ☑ D is correct. Static init blocks are executed at class loading time; instance init blocks
run right after the call to super() in a constructor. When multiple init 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 Objectives 6.5 and 7.5)
6. ☑ C is correct. Before you can invoke Y's do2 method, you have to cast x2 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 7.4)
7. ☑ A is correct. It's legal to overload main(). Since no instances of Locomotive 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 7.4)
9. ☑ A is correct. A ClassCastException 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 7.4)
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, because SubSubAlpha extends Alpha! Since the code doesn't
attempt to make a SubAlpha, the private constructor in SubAlpha is okay.
☐
✗ A, B, D, E, and F are incorrect based on the above information. (OCA Objectives 6.5 and 7.5)
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
6.5 and 7.5)
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 Objectives 6.2 and
7.2)
14. ☑ A and C are correct. The phrase "type of" indicates an IS-A relationship (extends or
implements), and the word "have" of course indicates a HAS-A relationship (usually instance
variables).
☐
✗ B and D are incorrect based on the above information. (OCP Objective 3.3)

3
Assignments

CERTIFICATION OBJECTIVES

•
•
•
•

•

Determine the Effects of Passing Variables
into Methods

Understand Variable Scope

•

Differentiate Between Primitive Variables
and Reference Variables

✓

Understand Object Lifecycle and Garbage
Collection

Use Class Members
Understand Primitive Casting

Two-Minute Drill

Q&A Self Test

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Assignments

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:
1. class Collar { }
2.
3. class Dog {
4.
Collar c;
// instance variable
5.
String name;
// instance variable
6.
7.
public static void main(String [] args) {
8.
9.
Dog d;
// local variable: d
10.
d = new Dog();
11.
d.go(d);
12.
}
13.
void go(Dog dog) {
// local variable: dog
14.
c = new Collar();
15.
dog.setName("Aiko");
16.
}
17.
void setName(String dogName) {
// local var: dogName
18.
name = dogName;
19.
// do more stuff
20.
}
21. }

Figure 3-1 shows the state of the stack and the heap once the program reaches
line 19. Following are some key points:
■ 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.

Stack and Heap—Quick Review

FIGURE 3-1

167

The Heap

Overview of the
stack and the
heap

String object
"Aiko"

Instance
variables:
setName()

- name
- c

dogName

go()

dog

main()

d

method

local
variables

Dog object

The Stack

Collar object

■ Line 10—A new Dog object is created 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 the dog parameter as

a local variable.
■ Line 14—A new Collar 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.

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CERTIFICATION OBJECTIVE

Literals, Assignments, and Variables
(OCA Objectives 2.1, 2.2, 2.3, and
Upgrade Objective 1.2)
2.1

Declare and initialize variables.

2.2

Differentiate between object references and primitive variables.

2.3

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:
'b'
42
false
2546789.343

//
//
//
//

char literal
int literal
boolean literal
double literal

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.

Numeric Literals with Underscores (Upgrade Exam Topic 1.2) 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:

Literals, Assignments, and Variables (OCA Objectives 2.1, 2.2, 2.3, and Upgrade Objective 1.2)

int pre7 = 1000000;
int with7 = 1_000_000;

169

// pre Java 7 – we hope it's a million
// much clearer!

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:
int i1 = _1_000_000;
int i2 = 10_0000_0;

// illegal, can't begin with an "_"
// legal, but confusing

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.

Decimal Literals 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;

Binary Literals (Upgrade Exam Topic 1.2) 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 or 0b, as shown:
int b1 = 0B101010;
int b2 = 0b00011;

// set b1 to binary 101010 (decimal 42)
// set b2 to binary 11 (decimal 3)

Octal Literals 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:
class Octal {
public static void main(String [] args) {
int six = 06;
// Equal to decimal 6
int seven = 07;
// Equal to decimal 7
int eight = 010; // Equal to decimal 8
int nine = 011;
// Equal to decimal 9
System.out.println("Octal 010 = " + eight);
}
}

You can have up to 21 digits in an octal number, not including the leading zero. If
we run the preceding program, it displays the following:
Octal 010 = 8

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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 prefix 0x (or 0X) or the optional suffix extension L, which
will be explained a bit later in the chapter. All of the following hexadecimal
assignments are legal:
class HexTest {
public static void main (String [] args) {
int x = 0X0001;
int y = 0x7fffffff;
int z = 0xDeadCafe;
System.out.println("x = " + x + " y = " + y + " z = " + z);
}
}

Running HexTest 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 legal and 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 of L or l
after the number:
long jo = 110599L;
long so = 0xFFFFl;

// Note the lowercase 'l'

Floating-point Literals
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 as double (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 suffix F or f to the number. If you don't do this, the compiler will complain

Literals, Assignments, and Variables (OCA Objectives 2.1, 2.2, 2.3, and Upgrade Objective 1.2)

171

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."
float f = 23.467890;
float g = 49837849.029847F;

// Compiler error, possible loss
// of precision
// OK; has the suffix "F"

You may also optionally attach a D or d to double literals, but it is not necessary
because this is the default behavior.
double d = 110599.995011D; // Optional, not required
double g = 987.897;
// No 'D' suffix, but OK because the
// literal is a double by default

Look for numeric literals that include a comma; here's an example:
int x = 25,343;

// Won't compile because of the comma

Boolean Literals
Boolean literals are the source code representation for boolean values. A boolean
value can be defined only as true or false. Although in C (and some other
languages) it is common to use numbers to represent true or false, this will not
work in Java. Again, repeat after me: "Java is not C++."
boolean t = true;
boolean f = 0;

// Legal
// Compiler error!

Be on the lookout for questions that use numbers where booleans are required.
You might see an if test that uses a number, as in the following:
int x = 1;

if (x) {

} // Compiler error!

Character Literals
A char 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'

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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:
char a = 0x892;
char b = 982;
char c = (char)70000;
char d = (char) -98;

//
//
//
//
//

hexadecimal
int literal
The cast is
out of char
Ridiculous,

literal
required; 70000 is
range
but legal

And the following are not legal and produce compiler errors:
char e = -29;
char f = 70000;

// Possible loss of precision; needs a cast
// Possible loss of precision; needs a cast

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:
char c = '\"';
char d = '\n';
char tab = '\t';

// A double quote
// A newline
// A tab

Literal Values 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:
String s = "Bill Joy";
System.out.println("Bill" + " Joy");

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;

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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 an int 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). A byte 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 holder b? 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 a way 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 is not 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 of null, the variable
holds bits representing—you guessed it—null. You can read
Button b = null;

as "The Button variable b 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.

Primitive Assignments
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 in
Chapter 4.
You can assign a primitive variable using a literal or the result of an expression.

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Take a look at the following:
int x = 7;
// literal assignment
int y = x + 2; // assignment with an expression
// (including a literal)
int z = x * y; // assignment with an expression

The most important point to remember is that a literal integer (such as 7) is
always implicitly an int. Thinking back to Chapter 1, you'll recall that an int is a
32-bit value. No big deal if you're assigning a value to an int or a long variable, but
what if you're assigning to a byte variable? After all, a byte-sized holder can't hold
as many bits as an int-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 the cast. 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 an int. (Everything we're saying about
byte applies equally to char and short, both of which are smaller than an int.)
We're not actually at the weird part yet, by the way.
We know that a literal integer is always an int, but more importantly, the result
of an expression involving anything int-sized or smaller is always an int. In other
words, add two bytes together and you'll get an int—even if those two bytes are
tiny. Multiply an int and a short and you'll get an int. Divide a short by a byte
and you'll get…an int. Okay, now we're at the weird part. Check this out:
byte a = 3;
// No problem, 3 fits in a byte
byte b = 8;
// No problem, 8 fits in a byte
byte c = a + b; // Should be no problem, sum of the two bytes
// fits in a byte

The last line won't compile! You'll get an error something like this:
TestBytes.java:5: possible loss of precision
found
: int
required: byte
byte c = a + b;
^

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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 about int-or-smaller expressions always resulting in an int. It would
have compiled if we'd done the explicit 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
int
int
int

j;
k=1;
l;
m=3;

But the order is important.This is legal:
int j, k=1, l, m=k+3; // legal: k is initialized before m uses it

But these are not:
int j, k=m+3, l, m=1; // illegal: m is not initialized before k uses it
int x, y=x+1, z;
// illegal: x is not initialized before y uses it

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Primitive Casting
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 in Chapter 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 an int). 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 casts looks like this:
float a = 100.001f;
int b = (int)a; // Explicit cast, the float could lose info

Integer values may be assigned to a double variable without explicit casting, because
any integer value can fit in a 64-bit double. 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 a double
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:
class Casting {
public static void main(String [] args) {
int x = 3957.229; // illegal
}
}

If we try to compile the preceding code, we get an error something like this:
%javac Casting.java
Casting.java:3: Incompatible type for declaration. Explicit cast
needed to convert double to int.
int x = 3957.229; // illegal
1 error

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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 an int:
class Casting {
public static void main(String [] args) {
int x = (int)3957.229; // legal cast
System.out.println("int x = " + x);
}
}

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 a byte. Look at the following:
class Casting {
public static void main(String [] args) {
long l = 56L;
byte b = (byte)l;
System.out.println("The byte is " + b);
}
}

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:
class Casting {
public static void main(String [] args) {
long l = 130L;
byte b = (byte)l;
System.out.println("The byte is " + b);
}
}

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.

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EXERCISE 3-1
Casting Primitives
Create a float number type of any value, and assign it to a short using casting.
1. Declare a float variable: float f = 234.56F;
2. Assign the float to a short: short s = (short)f;

Assigning Floating-point Numbers
Floating-point numbers have 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 literal 32.3, for example, is considered a double. If
you try to assign a double 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 that 32.3 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 an f to the end of the literal.
The following assignments will compile:
float f = (float) 32.3;
float g = 32.3f;
float h = 32.3F;

Assigning a Literal That Is Too Large for 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:
TestBytes.java:5: possible loss of precision
found
: int
required: byte
byte a = 128;

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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 an int, 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:
byte b = 3;
b += 7;

// No problem - adds 7 to b (result is 10)

and it is equivalent to this:
byte b = 3;
b = (byte) (b + 7);

// Won't compile without the
// cast, since b + 7 results in an int

The compound assignment operator += lets you add to the value of b, without
putting in an explicit cast. In fact, +=, -=, *=, and /= will all put in an implicit cast.

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Assigning One Primitive Variable to Another Primitive Variable
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 in a, 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 a only to copy its contents. At this point,
a and b have identical contents (in other words, identical values), but if we change
the contents of either a or b, the other variable won't be affected.
Take a look at the following example:
class ValueTest {
public static void main (String [] args) {
int a = 10; // Assign a value to a
System.out.println("a = " + a);
int b = a;
b = 30;
System.out.println("a = " + a + " after change to b");
}
}

The output from this program is
%java ValueTest
a = 10
a = 10 after change to b

Notice the value of a stayed at 10. The key point to remember is that even after
you assign a 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 Variable Assignments
You can assign a newly created object to an object reference variable as follows:
Button b = new Button();

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The preceding line does three key things:
■ Makes a reference variable named b, of type Button
■ Creates a new Button object on the heap
■ Assigns the newly created Button object to the reference variable b

You can also assign null 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:
public class Foo {
public void doFooStuff() { }
}
public class Bar extends Foo {
public void doBarStuff() { }
}
class Test {
public static void main (String [] args) {
Foo reallyABar = new Bar(); // Legal because Bar is a
// subclass of Foo
Bar reallyAFoo = new Foo(); // Illegal! Foo is not a
// subclass of Bar
}
}

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 that Foo has a method doFooStuff() that
someone with a Foo reference might try to invoke. If the object referenced by the
Foo variable is really a Foo, no problem. But it's also no problem if the object is a
Bar, since 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.

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You might see questions on the exam that use "wrapper" objects like so:
Long x = new Long(42);
Short s = new Short("57");

// create an instance of Long with value 42
// create an instance of Short with value 57

The OCA 7 exam touches on wrappers very lightly, so for now 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. Printing the value of the wrappers above,
System.out.println(x + " " + s);

produces the following output:
42 57

We'll be diving much more deeply into wrappers in Chapter 11.

CERTIFICATION OBJECTIVE

Scope (OCA Objectives 1.1 and 2.5)
1.1

Determine the scope of variables.

2.5

Call methods on objects.

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:
class Layout {
static int s = 343;
int x;
{ x = 7; int x2 = 5; }
Layout() { x += 8; int x3 = 6;}

//
//
//
//
//

class
static variable
instance variable
initialization block
constructor

Scope (OCA Objectives 1.1 and 2.5)

void doStuff() {
int y = 0;
for(int z = 0; z < 4; z++) {
y += z + x;
}
}

183

// method
// local variable
// 'for' code block

}

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 an init 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:
■ 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).
■ 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.
■ 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."
■ 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

from main()):
class ScopeErrors {
int x = 5;

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public static void main(String[] args) {
x++;
// won't compile, x is an 'instance' variable
}
}

■ Attempting to access a local variable from a nested method.

When a method, say go(), invokes another method, say go2(), go2() won't
have access to go()'s local variables. While go2() is executing, go()'s local
variables are still alive, 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:
class ScopeErrors {
public static void main(String [] args) {
ScopeErrors s = new ScopeErrors();
s.go();
}
void go() {
int y = 5;
go2();
y++;
// once go2() completes, y is back in scope
}
void go2() {
y++;
// won't compile, y is local to go()
}
}

■ 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:
void go3() {
for(int z = 0; z < 5; z++) {
boolean test = false;
if(z == 3) {
test = true;
break;
}
}
System.out.print(test);
// 'test' is an ex-variable,
// it has ceased to be...
}

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."

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185

Pay extra attention to code block scoping errors. You might see them in
switches, try-catches, for, do, and while loops.

CERTIFICATION OBJECTIVE

Variable Initialization (OCA Objective 2.1)
2.1

Declare and initialize variables.

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.

Primitive and 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.

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TABLE 3-1

Default Values for
Primitives and
Reference Types

Assignments

Variable Type

Default Value

Object reference

null (not referencing any object)

byte, short, int, long

0

float, double

0.0

boolean

false

char

'\u0000'

Primitive Instance Variables
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:
public class BirthDate {
int year;
// Instance variable
public static void main(String [] args) {
BirthDate bd = new BirthDate();
bd.showYear();
}
public void showYear() {
System.out.println("The year is " + year);
}
}

When the program is started, it gives the variable year 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 Reference Instance 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:
public class Book {
private String title;
// instance reference variable
public String getTitle() {
return title;
}
public static void main(String [] args) {
Book b = new Book();
System.out.println("The title is " + b.getTitle());
}
}

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187

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 that null is not the same as an
empty String (""). A null value means the reference variable is not referring to any
object on the heap. The following modification to the Book code runs into trouble:
public class Book {
private String title;
public String getTitle() {
return title;
}
public static void main(String
Book b = new Book();
String s = b.getTitle();
String t = s.toLowerCase();
}
}

// instance reference variable

[] args) {
// Compiles and runs
// Runtime Exception!

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 variable title 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:
public class Book {
private String title;
// instance reference variable
public String getTitle() {
return title;
}
public static void main(String [] args) {
Book b = new Book();
String s = b.getTitle();
// Compiles and runs
if (s != null) {
String t = s.toLowerCase();
}
}
}

The preceding code checks to make sure the object referenced by the variable s is
not null 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 of null. 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 of null, and then

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realize that the variable s will also have a value of null. Remember, the value of s is
a copy of the value of title (as returned by the getTitle() method), so if title
is a null reference, s will be, too.

Array Instance Variables
In Chapter 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 of null, 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 declared or 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
array year will contain 100 integers that all equal zero by default:
public class BirthDays {
static int [] year = new int[100];
public static void main(String [] args) {
for(int i=0;i<100;i++)
System.out.println("year[" + i + "] = " + year[i]);
}
}

When the preceding code runs, the output indicates that all 100 integers in the array
have a value of zero.

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.

Variable Initialization (OCA Objective 2.1)

189

Local Primitives
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:
public class TimeTravel {
public static void main(String [] args) {
int year = 2050;
System.out.println("The year is " + year);
}
}

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:
public class TimeTravel {
public static void main(String [] args) {
int year; // Local variable (declared but not initialized)
System.out.println("The year is " + year); // Compiler error
}
}

Compiling produces output something like this:
%javac TimeTravel.java
TimeTravel.java:4: Variable year may not have been initialized.
System.out.println("The year is " + year);
1 error

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:
public class TimeTravel {
public static void main(String [] args) {
int year;
// Declared but not initialized
int day;
// Declared but not initialized
System.out.println("You step into the portal.");
year = 2050;
// Initialize (assign an explicit value)
System.out.println("Welcome to the year " + year);
}
}

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).

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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:
public class TestLocal {
public static void main(String [] args) {
int x;
if (args[0] != null) { // assume you know this is true
x = 7;
// compiler can't tell that this
// statement will run
}
int y = x;
// the compiler will choke here
}
}

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."

Local Object 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't null 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 an uninitialized
reference. Locally declared references can't get away with checking for null before
use, unless you explicitly initialize the local variable to null. The compiler will
complain about the following code:
import java.util.Date;
public class TimeTravel {
public static void main(String [] args) {
Date date;
if (date == null)
System.out.println("date is null");
}
}

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191

Compiling the code results in an error similar to the following:
%javac TimeTravel.java
TimeTravel.java:5: Variable date may not have been initialized.
if (date == null)
1 error

Instance variable references are always given a default value of null, until they
are explicitly initialized to something else. But local references are not given a
default value; in other words, they aren't null. 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:
Date date = null; // Explicitly set the local reference
// variable to null

Local Arrays
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.

Assigning One Reference Variable to Another
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 in a1 is copied and the new copy 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

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pattern—a bit pattern referring to a specific object on the heap. Look at the
following code:
import java.awt.Dimension;
class ReferenceTest {
public static void main (String [] args) {
Dimension a1 = new Dimension(5,10);
System.out.println("a1.height = " + a1.height);
Dimension b1 = a1;
b1.height = 30;
System.out.println("a1.height = " + a1.height +
" after change to b");
}
}

In the preceding example, a Dimension object a1 is declared and initialized with
a width of 5 and a height of 10. Next, Dimension b1 is declared and assigned the
value of a1. At this point, both variables (a1 and b1) hold identical values, because
the contents of a1 were copied into b1. There is still only one Dimension object—
the one that both a1 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 of a1 as well? Let's see what the output will be:
%java ReferenceTest
a.height = 10
a.height = 30 after change to b

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
in Chapter 5). But it sure looks as though you can. Examine the following code:
class StringTest {
public static void main(String [] args) {
String x = "Java"; // Assign a value to x
String y = x;
// Now y and x refer to the same
// String object
System.out.println("y string = " + y);
x = x + " Bean";
// Now modify the object using
// the x reference
System.out.println("y string = " + y);
}
}

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You might think String y will contain the characters Java Bean after the
variable x is changed, because Strings are objects. Let's see what the output is:
%java StringTest
y string = Java
y string = Java

As you can see, even though y is a reference variable to the same object that x
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 a String, 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 in Chapter 5.
You need to understand what happens when you use a String reference variable
to modify a string:
■ A new string is created (or a matching String is found in the String pool),

leaving the original String object untouched.
■ The reference used to modify the String (or rather, make a new String

by modifying a copy of the original) is then assigned the brand new String
object.
So when you say,
1. String s = "Fred";
2. String t = s;
//
//
3. t.toUpperCase(); //
//

Now t and s refer to the same
String object
Invoke a String method that changes
the String

you haven't changed the original String object created on line 1. When line 2
completes, both t and s reference the same String object. But when line 3 runs,
rather than modifying the object referred to by t and s (which is the one and only
String object up to this point), a brand new String object is created. And then it's
abandoned. Because the new String 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 both
t and s refer to it. The behavior of Strings is extremely important in the exam, so
we'll cover it in much more detail in Chapter 5.

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CERTIFICATION OBJECTIVE

Passing Variables into Methods
(OCA Objective 6.8)
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, and 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 a copy 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:
1. import java.awt.Dimension;
2. class ReferenceTest {
3.
public static void main (String [] args) {
4.
Dimension d = new Dimension(5,10);
5.
ReferenceTest rt = new ReferenceTest();
6.
System.out.println("Before modify() d.height = "
+ d.height);
7.
rt.modify(d);
8.
System.out.println("After modify() d.height = "
+ d.height);
9.
}

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195

10.
void modify(Dimension dim) {
11.
dim.height = dim.height + 1;
12.
System.out.println("dim = " + dim.height);
13.
}
14. }

When we run this class, we can see that the modify() method was indeed able to
modify the original (and only) Dimension object created on line 4.
C:\Java Projects\Reference>java ReferenceTest
Before modify() d.height = 10
dim = 11
After modify() d.height = 11

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 variables d 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-ofthe-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 an int
variable with the value of 3, you're passing a copy of the bits representing 3. 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

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reassign the caller's original reference variable and make it refer to a different object
or null. For example, in the following code fragment,
void bar() {
Foo f = new Foo();
doStuff(f);
}
void doStuff(Foo g) {
g.setName("Boo");
g = new Foo();
}

reassigning g does not reassign f! At the end of the bar() method, two Foo objects
have been created: one referenced by the local variable f and one referenced by the
local (argument) variable g. Because the doStuff() method has a copy of the
reference variable, it has a way to get to the original Foo 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, but doStuff() can't change the actual contents (bit pattern) of f. In other
words, doStuff() can change the state of the object that f 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:
class ReferenceTest {
public static void main (String [] args) {
int a = 1;
ReferenceTest rt = new ReferenceTest();
System.out.println("Before modify() a = " + a);
rt.modify(a);
System.out.println("After modify() a = " + a);
}
void modify(int number) {
number = number + 1;
System.out.println("number = " + number);
}
}

In this simple program, the variable a is passed to a method called modify(),
which increments the variable by 1. The resulting output looks like this:
Before modify() a = 1
number = 2
After modify() a = 1

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197

Notice that a did not change after it was passed to the method. Remember, it was
a copy of a 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
The Shadowy World of Variables
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:
class Foo {
static int size = 7;
static void changeIt(int size) {
size = size + 200;
System.out.println("size in changeIt is " + size);
}
public static void main (String [] args) {
Foo f = new Foo();
System.out.println("size = " + size);
changeIt(size);
System.out.println("size after changeIt is " + size);
}
}

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.

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FROM THE CLASSROOM
Running class Foo prints this:
%java Foo
size = 7
size in changeIt is 207
size after changeIt is 7

Things become more interesting when the shadowed variable is an object reference, rather
than a primitive:
class Bar {
int barNum = 28;
}
class Foo {
Bar myBar = new Bar();
void changeIt(Bar myBar) {
myBar.barNum = 99;
System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
myBar = new Bar();
myBar.barNum = 420;
System.out.println("myBar.barNum in changeIt is now " + myBar.barNum);
}
public static void main (String [] args) {
Foo f = new Foo();
System.out.println("f.myBar.barNum is " + f.myBar.barNum);
f.changeIt(f.myBar);
System.out.println("f.myBar.barNum after changeIt is "
+ f.myBar.barNum);
}
}

The preceding code prints out this:
f.myBar.barNum is 28
myBar.barNum in changeIt is 99
myBar.barNum in changeIt is now 420
f.myBar.barNum after changeIt is 99

You can see that the shadowing variable (the local parameter myBar in changeIt()) can
still affect the myBar instance variable, because the myBar parameter receives a reference to
the same Bar object. But when the local myBar is reassigned a new Bar object, which we then
modify by changing its barNum value, Foo's original myBar instance variable is untouched.

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199

CERTIFICATION OBJECTIVE

Garbage Collection (OCA Objective 2.4)
2.4

Explain an object's lifecycle.

As of Spring 2014, the official exam objectives don't use the phrases "garbage
collection" or "memory management." These two concepts are implied when the
objective uses the phrase "object's lifecycle."

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.

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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 ft. 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 of garbage collection revolves around making sure that 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 the Garbage Collector Run?
The garbage collector is under the control of the JVM; 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.

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201

How Does the Garbage Collector 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? To answer this question fully, we have to
jump ahead a little bit and talk about threads. (See Chapter 13 for the real scoop on
threads.)
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, as you'll learn in excruciating detail in Chapter 13, there are
many really cool reasons to launch additional threads from your initial thread. 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 that an 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.

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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:
1. public class GarbageTruck {
2.
public static void main(String [] args) {
3.
StringBuffer sb = new StringBuffer("hello");
4.
System.out.println(sb);
5.
// The StringBuffer object is not eligible for collection
6.
sb = null;
7.
// Now the StringBuffer object is eligible for collection
8.
}
9. }

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 variable sb 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 Reference Variable
We can also decouple a reference variable from an object by setting the reference
variable to refer to another object. Examine the following code:
class GarbageTruck {
public static void main(String [] args) {
StringBuffer s1 = new StringBuffer("hello");
StringBuffer s2 = new StringBuffer("goodbye");
System.out.println(s1);
// At this point the StringBuffer "hello" is not eligible
s1 = s2; // Redirects s1 to refer to the "goodbye" object
// Now the StringBuffer "hello" is eligible for collection
}
}

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203

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:
import java.util.Date;
public class GarbageFactory {
public static void main(String [] args) {
Date d = getDate();
doComplicatedStuff();
System.out.println("d = " + d);
}
public static Date getDate() {
Date d2 = new Date();
StringBuffer now = new StringBuffer(d2.toString());
System.out.println(now);
return d2;
}
}

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.

Isolating a Reference
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
can usually discover any such islands of objects and remove them. As you can

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imagine, such islands can become quite large, theoretically containing hundreds of
objects. Examine the following code:
public class Island {
Island i;
public static void main(String [] args) {
Island i2 = new Island();
Island i3 = new Island();
Island i4 = new Island();
i2.i = i3;
i3.i = i4;
i4.i = i2;

// i2 refers to i3
// i3 refers to i4
// i4 refers to i2

i2 = null;
i3 = null;
i4 = null;
// do complicated, memory intensive stuff
}
}

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. Study Figure 3-2 to reinforce the concepts of objects without
references and islands of isolation.

Forcing Garbage Collection (OCP 5 Candidates Only)
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: As of the Java 6 exam, the topic of using System.gc() has been removed
from the exam. The 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. We are leaving this section in the book in case you're studying for the
OCP 5 exam.
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

Garbage Collection (OCA Objective 2.4)

205

Island objects eligible for garbage collection

FIGURE 3-2

public class Island (
Island n;
public static void main(String [] args)
Island i2 = new Island();
Island i3 = new Island();
Island i4 = new Island();
i2.n = i3;
i3.n = i4;
i4.n = i2;
i2 = null;
i3 = null;
i2
i4 = null;
doComplexStuff();
i3
}
}

{

i2.n

i3.n
i4.n

i4

Three island Objects

The heap
Lost Object
public class Lost {
public static void main(String
Lost x = new Lost ();
x = null;
doComplexStuff();

Indicated an
active reference

Indicates a
deleted reference

[]

args)

{

x

}
}

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. The Runtime 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();

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Theoretically, after calling System.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 (see Chapter 13) 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 on System.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:
1. import java.util.Date;
2. public class CheckGC {
3.
public static void main(String [] args) {
4.
Runtime rt = Runtime.getRuntime();
5.
System.out.println("Total JVM memory: "
+ rt.totalMemory());
6.
System.out.println("Before Memory = "
+ rt.freeMemory());
7.
Date d = null;
8.
for(int i = 0;i<10000;i++) {
9.
d = new Date();
10.
d = null;
11.
}
12.
System.out.println("After Memory = "
+ rt.freeMemory());
13.
rt.gc();
// an alternate to System.gc()
14.
System.out.println("After GC Memory = "
+ rt.freeMemory());
15.
}
16. }

Now, let's run the program and check the results:
Total JVM memory: 1048568
Before Memory = 703008
After Memory = 458048
After GC Memory = 818272

Garbage Collection (OCA Objective 2.4)

207

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 to 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 when gc() is
called may be different for different JVMs, so 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 an OutOfMemoryException.

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.

Cleaning Up Before Garbage Collection—the finalize() 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 class Object. 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 overridden finalize() 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.

Tricky Little finalize() Gotchas
There are a couple of concepts concerning finalize() that you need to remember:
■ For any given object, finalize() will be called only once (at most) by the

garbage collector.
■ Calling finalize() can actually result in saving an object from deletion.

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Let's look into 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, effectively ineligible-izing the object for
garbage collection. If at some point later on 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 run finalize() 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

✓

209

TWO-MINUTE DRILL
Here are some of the key points from this chapter.

Stack and Heap
❑ Local variables (method variables) live on the stack.
❑ Objects and their instance variables live on the heap.

Literals and Primitive Casting (OCA Objective 2.1)
❑ Integer literals can be binary, decimal, octal (such as 013), or hexadecimal
(such as 0x3d).
❑ Literals for longs end in L or l.
❑ Float literals end in F or f, and double literals end in a digit or D or d.
❑ The boolean literals are true and false.
❑ Literals for chars are a single character inside single quotes: 'd'.

Scope (OCA Objective 1.1)
❑ 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 a for or an if) live until the block
completes.

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Basic Assignments (OCA Objectives 2.1, 2.2, and 2.3)
❑ Literal integers are implicitly ints.
❑ Integer expressions always result in an int-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 as Button b = new Button();, the
JVM does three things:
❑ Makes a reference variable named b, of type Button.
❑ Creates a new Button object.
❑ Assigns the Button object to the reference variable b.

Using a Variable or Array Element That Is Uninitialized and
Unassigned (OCA Objectives 4.1 and 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 of null.
❑ 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.

Two-Minute Drill

211

Passing Variables into Methods (OCA Objective 6.8)
❑ 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 (OCA Objective 2.4)
❑ 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 with System.gc(); (for OCP 5 candidates only).
❑ The Class object 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 within finalize().

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SELF TEST
1. Given:
class CardBoard {
Short story = 200;
CardBoard go(CardBoard cb) {
cb = null;
return cb;
}
public static void main(String[] args) {
CardBoard c1 = new CardBoard();
CardBoard c2 = new CardBoard();
CardBoard c3 = c1.go(c2);
c1 = null;
// do Stuff
} }

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:
public class Fishing {
byte b1 = 4;
int i1 = 123456;
long L1 = (long)i1;
short s2 = (short)i1;
byte b2 = (byte)i1;
int i2 = (int)123.456;
byte b3 = b1 + 7;
}

//
//
//
//
//

line
line
line
line
line

A
B
C
D
E

Which lines WILL NOT compile? (Choose all that apply.)
A. Line A
B. Line B
C. Line C
D. Line D
E. Line E

Self Test

3. Given:
public class Literally {
public static void main(String[] args) {
int i1 = 1_000;
// line A
int i2 = 10_00;
// line B
int i3 = _10_000;
// line C
int i4 = 0b101010;
// line D
int i5 = 0B10_1010; // line E
int i6 = 0x2_a;
// line F
}
}

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:
class Mixer {
Mixer() { }
Mixer(Mixer m) { m1 = m; }
Mixer m1;
public static void main(String[] args) {
Mixer m2 = new Mixer();
Mixer m3 = new Mixer(m2); m3.go();
Mixer m4 = m3.m1;
m4.go();
Mixer m5 = m2.m1;
m5.go();
}
void go() { System.out.print("hi "); }
}

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

213

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5. Given:
class Fizz {
int x = 5;
public static void main(String[] args) {
final Fizz f1 = new Fizz();
Fizz f2 = new Fizz();
Fizz f3 = FizzSwitch(f1,f2);
System.out.println((f1 == f3) + " " + (f1.x == f3.x));
}
static Fizz FizzSwitch(Fizz x, Fizz y) {
final Fizz z = x;
z.x = 6;
return z;
} }

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:
public class Mirror {
int size = 7;
public static void main(String[] args) {
Mirror m1 = new Mirror();
Mirror m2 = m1;
int i1 = 10;
int i2 = i1;
go(m2, i2);
System.out.println(m1.size + " " + i1);
}
static void go(Mirror m, int i) {
m.size = 8;
i = 12;
}
}

Self Test

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:
public class Wind {
int id;
Wind(int i) { id = i; }
public static void main(String[] args) {
new Wind(3).go();
// commented line
}
void go() {
Wind w1 = new Wind(1);
Wind w2 = new Wind(2);
System.out.println(w1.id + " " + w2.id);
}
}

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 objects are eligible for garbage collection
E. One object is eligible for garbage collection
F. Two objects are eligible for garbage collection
G. Three objects are eligible for garbage collection

215

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8. Given:
3. public class Ouch {
4.
static int ouch = 7;
5.
public static void main(String[] args) {
6.
new Ouch().go(ouch);
7.
System.out.print(" " + ouch);
8.
}
9.
void go(int ouch) {
10.
ouch++;
11.
for(int ouch = 3; ouch < 6; ouch++)
12.
;
13.
System.out.print(" " + ouch);
14.
}
15. }

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:
public class Happy {
int id;
Happy(int i) { id = i; }
public static void main(String[] args) {
Happy h1 = new Happy(1);
Happy h2 = h1.go(h1);
System.out.println(h2.id);
}
Happy go(Happy h) {
Happy h3 = h;
h3.id = 2;
h1.id = 3;
return h1;
}
}

Self Test

What is the result?
A. 1
B. 2
C. 3
D. Compilation fails
E. An exception is thrown at runtime
10. Given:
public class Network {
Network(int x, Network n) {
id = x;
p = this;
if(n != null) p = n;
}
int id;
Network p;
public static void main(String[] args) {
Network n1 = new Network(1, null);
n1.go(n1);
}
void go(Network n1) {
Network n2 = new Network(2, n1);
Network n3 = new Network(3, n2);
System.out.println(n3.p.p.id);
}
}

What is the result?
A. 1
B. 2
C. 3
D. null
E. Compilation fails

217

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11. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.

class Beta { }
class Alpha {
static Beta b1;
Beta b2;
}
public class Tester {
public static void main(String[] args) {
Beta b1 = new Beta();
Beta b2 = new Beta();
Alpha a1 = new Alpha();
Alpha a2 = new Alpha();
a1.b1 = b1;
a1.b2 = b1;
a2.b2 = b2;
a1 = null; b1 = null; b2 = null;
// do stuff
}
}

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:
public class Telescope {
static int magnify = 2;
public static void main(String[] args) {
go();
}
static void go() {
int magnify = 3;
zoomIn();
}
static void zoomIn() {
magnify *= 5;
zoomMore(magnify);
System.out.println(magnify);
}
static void zoomMore(int magnify) {
magnify *= 7;
}
}

Self Test

What is the result?
A. 2
B. 10
C. 15
D. 30
E. 70
F. 105
G. Compilation fails
13. Given:
3. public class Dark {
4.
int x = 3;
5.
public static void main(String[] args) {
6.
new Dark().go1();
7.
}
8.
void go1() {
9.
int x;
10.
go2(++x);
11.
}
12.
void go2(int y) {
13.
int x = ++y;
14.
System.out.println(x);
15.
}
16. }

What is the result?
A. 2
B. 3
C. 4
D. 5
E. Compilation fails
F. An exception is thrown at runtime

219

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SELF TEST 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 than ints, 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 G are incorrect. A and B are legal numeric literals. D and E are examples
of valid binary literals, which are also new to Java 7, and G is a valid hexadecimal literal that
uses an underscore. (OCA Objective 2.1 and Upgrade Objective 1.2)
4. ☑ F is correct. The m2 object's m1 instance variable is never initialized, so when m5 tries to use
it a NullPointerException is thrown.
☐
✗ A, B, C, D, and E are incorrect based on the above. (OCA Objectives 2.1, 2.3, and 2.5)
5. ☑

A is correct. The references f1, z, and f3 all refer to the same instance of Fizz. The
final modifier assures that a reference variable cannot be referred to a different object, but
final 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 single Mirror instance, but the int i is a
new int variable, a detached copy of i2.
☐
✗ 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 of id for w1 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
name ouch 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, 2.1, and 2.5)
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. Three Network 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 that id."
☐
✗ B, C, D, and E are incorrect based on the above. (OCA Objectives, 2.2, 2.3, and 6.4)

Self Test Answers

221

11. ☑ B is correct. It should be clear that there is still a reference to the object referred to by a2,
and that there is still a reference to the object referred to by a2.b2. What might be less clear is
that you can still access the other Beta object through the static variable a2.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 in zoomMore() stays in zoomMore(). 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 6.8)
13. ☑ E is correct. In go1() the local variable x is not initialized.
☐
✗ A, B, C, D, and F are incorrect based on the above. (OCA Objectives 2.1, 2.3, and 2.5)

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4
Operators

CERTIFICATION OBJECTIVES

•
•

Using Java Operators

•

Test Equality Between Strings and Other
Objects Using == and equals( )

Use Parentheses to Override Operator
Precedence

✓

Two-Minute Drill

Q&A Self Test

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I

f you've got variables, you're going to modify them. 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. For 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.

3.2

Use parentheses to override operator precedence.

3.3

Test equality between strings and other objects using == and equals().

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
threads question, only to blow it on a pre-increment statement.

Assignment Operators
We covered most of the functionality of the equal (=) assignment operator in
Chapter 3. To summarize:

Java Operators (OCA Objectives 3.1, 3.2, and 3.3)

225

■ 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 with Strings (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 Assignment Operators
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;

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Now, with compound operators:
y -= 6;
x += 2 * 5;

The last two assignments give the same result as the first two.

Earlier versions of the exam put big emphasis on operator precedence
(such as, What’s the result of x = y++ + ++x/z;). Other than having a very basic
knowledge of precedence (such as * and / are higher precedence than + and -), you
won’t need to study operator precedence. But you do need to know that when using a
compound operator, the expression on the right side of the = will always be evaluated
first. For example, you might expect
x *= 2 + 5;

to be evaluated like this,
x = (x * 2) + 5;

// incorrect precedence

because multiplication has higher precedence than addition. Instead, however, the
expression on the right is always placed inside parentheses. It is evaluated like this:
x = x * (2 + 5);

Relational Operators
The exam covers six relational operators (<, <=, >, >=, ==, and !=). Relational
operators always result in a boolean (true or false) value. This boolean value is
most often used in an if test, as follows:
int x = 8;
if (x < 9) {
// do something
}

But the resulting value can also be assigned directly to a boolean primitive:
class CompareTest {
public static void main(String [] args) {
boolean b = 100 > 99;
System.out.println("The value of b is " + b);
}
}

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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:
class GuessAnimal {
public static void main(String[] args) {
String animal = "unknown";
int weight = 700;
char sex = 'm';
double colorWaveLength = 1.630;
if (weight >= 500) { animal = "elephant"; }
if (colorWaveLength > 1.621) { animal = "gray " + animal; }
if (sex <= 'f') { animal = "female " + animal; }
System.out.println("The animal is a " + animal);
}
}

In the preceding code, we are using a comparison between characters. It's also
legal to compare a character primitive with any number (though 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 (including char), two
boolean values, or two object reference variables. You can't compare incompatible

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types, however. What would it mean to ask if a boolean is equal to a char? Or if a
Button 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.

Equality for Primitives
Most programmers are familiar with comparing primitive values. The following code
shows some equality tests on primitive variables:
class ComparePrimitives {
public static void main(String[] args) {
System.out.println("char 'a' == 'a'? " + ('a' == 'a'));
System.out.println("char 'a' == 'b'? " + ('a' == 'b'));
System.out.println("5 != 6? " + (5 != 6));
System.out.println("5.0 == 5L? " + (5.0 == 5L));
System.out.println("true == false? " + (true == false));
}
}

This program produces the following output:
char 'a' == 'a'? true
char 'a' == 'b'? false
5 != 6? true
5.0 == 5L? true
true == false? false

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.

Equality for Reference Variables
As you saw earlier, two reference variables can refer to the same object, as the
following code snippet demonstrates:
JButton a = new JButton("Exit");
JButton b = a;

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Don't mistake = for == in a boolean expression.The following is legal:
11. boolean b = false;
12. if (b = true) { System.out.println("b is true");
13. } else { System.out.println("b is false"); }

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 get b 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, since the if test can be done
only on boolean expressions.Thus, this does not compile:
7. int x = 1;
8. if (x = 0) { }

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:
8. if (x == 0) { }

After running this code, both variable a and variable b will refer to the same
object (a JButton with the label Exit). 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:
import javax.swing.JButton;
class CompareReference {
public static void main(String[] args) {
JButton a = new JButton("Exit");
JButton b = new JButton("Exit");
JButton c = a;
System.out.println("Is reference a == b? " + (a == b));
System.out.println("Is reference a == c? " + (a == c));
}
}

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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 that a refers to. When this program runs, the
following output is produced:
Is reference a == b? false
Is reference a == c? true

This shows us that a 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 in Chapter 11, when we look at the equals()
method (as opposed to the equals operator we're looking at here).

Equality for 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 class Object 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 class Object, and
creating your own definition of "meaningfully equivalent" for instances of your class.
(There's lots more about overriding equals() in Chapter 11.)
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 class Object
■ What equals() means in class String

The equals() Method in Class Object The equals() method in class
Object 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.
The equals() Method in Class String The equals() method in class
String 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. For String's equals() method, values ARE case
sensitive.

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Let's take a look at how the equals() method works in action (notice that the
Budgie class did NOT override Object.equals()):
class Budgie {
public static
Budgie b1 =
Budgie b2 =
Budgie b3 =

void main(String[] args) {
new Budgie();
new Budgie();
b1;

String s1 = "Bob";
String s2 = "Bob";
String s3 = "bob";

// lower case "b"

System.out.println(b1.equals(b2));
System.out.println(b1.equals(b3));
System.out.println(s1.equals(s2));
System.out.println(s1.equals(s3));

//
//
//
//

false, different objects
true, same objects
true, same values
false, values are case sensitive

}
}

which produces the output:
false
true
true
false

As we mentioned earlier, when we get to Chapter 11, we'll take a deep dive into
overriding equals()—and its companion hashCode()—but for the OCA, this is
all you need to know.

Equality for enums (OCP Only)
Once you've declared an enum, it's not expandable. At runtime, there's no way to
make additional enum constants. Of course, you can have as many variables as you'd
like, refer to a given enum 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 same enum
constant. You can use either the == operator or the equals() method to determine
whether two variables are referring to the same enum constant:
class EnumEqual {
enum Color {RED, BLUE}
// ; is optional
public static void main(String[] args) {
Color c1 = Color.RED; Color c2 = Color.RED;
if(c1 == c2) { System.out.println("=="); }
if(c1.equals(c2)) { System.out.println("dot equals"); }
} }

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(We know } } is ugly; we're prepping you.) This produces the output:
==
dot equals

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,
public static void main(String[] args) {
String s = new String("foo");
if (s instanceof String) {
System.out.print("s is a String");
}
}

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 return true if the object being
compared is assignment compatible with the type on the right.
The following example demonstrates a common use for instanceof: testing an
object to see if it's an instance of one of its subtypes, before attempting a downcast:
class A { }
class B extends A {
public static void main (String [] args) {
A myA = new B();
m2(myA);
}
public static void m2(A a) {
if (a instanceof B)
((B)a).doBstuff();
// downcasting an A reference
// to a B reference
}
public static void doBstuff() {
System.out.println("'a' refers to a B");
}
}

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233

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:
B b = new B();
if (b instanceof Object) {
System.out.print("b is definitely an Object");
}

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,
interface Foo { }
class A implements Foo { }
class B extends A { }
...
A a = new A();
B b = new B();

the following are true:
a instanceof Foo
b instanceof A
b instanceof Foo

// implemented indirectly

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.

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In addition, it is legal to test whether the null reference is an instance of a class.
This will always result in false, of course. This example,
class InstanceTest {
public static void main(String [] args) {
String a = null;
boolean b = null instanceof String;
boolean c = a instanceof String;
System.out.println(b + " " + c);
}
}

prints this:
false false

instanceof Compiler Error
You can't use the instanceof operator to test across two different class hierarchies.
For instance, the following will NOT compile:
class Cat { }
class Dog {
public static void main(String [] args) {
Dog d = new Dog();
System.out.println(d instanceof Cat);
}
}

Compilation fails—there's no way d could ever refer to a Cat or a subtype of Cat.

Remember that arrays are objects, even if the array is an array of
primitives. Watch for questions that look something like this:
int [] nums = new int[3];
if (nums instanceof Object) { } // result is true

An array is always an instance of Object. Any array.

Table 4-1 summarizes the use of the instanceof operator given the following:
interface Face { }
class Bar implements Face{ }
class Foo extends Bar { }

Java Operators (OCA Objectives 3.1, 3.2, and 3.3)

TABLE 4-1

Operands and
Results Using
instanceof

Operator

First Operand
(Reference Being Tested)

instanceof Operand
(Type We’re Comparing)
the Reference Against)

Result

null

Any class or interface type

false

Foo instance

Foo, Bar, Face, Object

true

Bar instance

Bar, Face, Object

true

Bar instance

Foo

false

Foo [ ]

Foo, Bar, Face

false

Foo [ ]

Object

true

Foo [ 1 ]

Foo, Bar, Face, Object

true

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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:
int x = 5 * 3;
int y = x - 4;
System.out.println("x - 4 is " +

y);

// Prints 11

The Remainder (%) 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:
class MathTest {
public static void main (String [] args) {
int x = 15;
int y = x % 4;
System.out.println("The result of 15 % 4 is the "
+ "remainder of 15 divided by 4. The remainder is " + y);
}
}

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Running class MathTest 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.

String Concatenation 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 animal = "Gray " + "elephant";

String concatenation gets interesting when you combine numbers with Strings.
Check out the following:
String a = "String";
int b = 3;
int c = 7;
System.out.println(a + b + c);

Will the + operator act as a plus sign when adding the int variables b 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

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237

"Start with the value String, and concatenate the character 3 (the value of b)
to it, to produce a new string String3, and then concatenate the character 7
(the value of c) 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 perform String concatenation. The
previous code can be read as
"Add the values of b and c together, and then take the sum and convert it to
a String and concatenate it with the String from variable a."
The rule to remember is this:
If either operand is a String, the + operator becomes a String concatenation
operator. If both operands are numbers, the + operator is the addition 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, then 7 is concatenated to the returned
String. But if foo() returns a number, then the + operator is used to add 7 to the
return value of foo().
Finally, you need to know that it's legal to mush together the compound additive
operator (+=) and Strings, like so:
String s = "123";
s += "45";
s += 67;
System.out.println(s);

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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:
int b = 2;
System.out.println("" + b + 3);

It prints this:
23

But if the print statement changes to this:
System.out.println(b + 3);

The printed result becomes
5

Increment and Decrement Operators
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:

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239

1. class MathTest {
2.
static int players = 0;
3.
public static void main (String [] args) {
4.
System.out.println("players online: " + players++);
5.
System.out.println("The value of players is "
+ players);
6.
System.out.println("The value of players is now "
+ ++players);
7.
}
8. }

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 only after the value of players is used
in the expression. When we run this program, it outputs the following:
%java MathTest
players online: 0
The value of players is 1
The value of players is now 2

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 means after. Line 5 doesn't increment players; 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 is 2.
Expect to see questions mixing the increment and decrement operators with
other operators, as in the following example:
int x = 2; int y = 3;
if ((y == x++) | (x < ++y)) {
System.out.println("x = " + x + " y = " + y);
}

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 is false, because the
increment on x doesn't happen until after the == test is made. Next, we increment x,
so now x is 3. Then we check to see if x is less than y, but we increment y before
comparing it with x! So the second logical test is (3 < 4). The result is true, so the
print statement runs.

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As with String 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 on for 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:
final int x = 5;
int y = x++;

It produces this error:
Test.java:4: cannot assign a value to final variable x
int y = x++;
^

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 an if statement, except instead of
executing a block of code if the test is true, 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:

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241

class Salary {
public static void main(String [] args) {
int numOfPets = 3;
String status = (numOfPets<4) ? "Pet limit not exceeded"
: "too many pets";
System.out.println("This pet status is " + status);
}
}

You can read the preceding code as "Set numOfPets equal to 3".
Next we're going to assign a String to the status variable. If numOfPets is less
than 4, 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 is true,
and the second value is assigned if the conditional test is false. You can even nest
conditional operators into one statement:
class AssignmentOps {
public static void main(String [] args) {
int sizeOfYard = 10;
int numOfPets = 3;
String status = (numOfPets<4)?"Pet count OK"
:(sizeOfYard > 8)? "Pet limit on the edge"
:"too many pets";
System.out.println("Pet status is " + status);
}
}

Don't expect many questions using conditional operators, but remember that
conditional operators are sometimes confused with assertion statements, so be
certain you can tell the difference. Chapter 7 covers assertions in detail.

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.

Bitwise Operators (For OCJP 5 Candidates Only!)
Okay, this is going to be confusing. Of the six logical operators listed above, 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 or
Java 7 exam.

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Here are several legal statements that use bitwise operators:
byte b1 = 6 & 8;
byte b2 = 7 | 9;
byte b3 = 5 ^ 4;
System.out.println(b1 + " " + b2 + " " + b3);

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 is
this:
BITWISE OPERATORS ARE NOT ON THE Java 6 or Java 7 EXAM!

Short-Circuit Logical Operators
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 little boolean expressions together to form bigger boolean
expressions. The && and || operators evaluate only boolean 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 already knows
that the complete expression can't possibly be true.

Java Operators (OCA Objectives 3.1, 3.2, and 3.3)

class Logical {
public static void main(String [] args) {
boolean b1 = false, b2 = false;
boolean b3 = (b1 == true) && (b2 = true);
System.out.println(b3 + " " + b2);
}
}

243

// will b2 be set to true?

When we run the preceding code, the assignment (b2 = true) never runs
because of the short-circuit operator, so the output is
%java Logical
false false

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 is true, 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
or false. Pay close attention to the following example; you'll see quite a few
questions like this on the exam:
1. class TestOR {
2.
public static void main(String[] args) {
3.
if ((isItSmall(3)) || (isItSmall(7))) {
4.
System.out.println("Result is true");
5.
}
6.
if ((isItSmall(6)) || (isItSmall(9))) {
7.
System.out.println("Result is true");
8.
}
9.
}
10.
11.
public static boolean isItSmall(int i) {
12.
if (i < 5) {
13.
System.out.println("i < 5");
14.
return true;
15.
} else {
16.
System.out.println("i >= 5");
17.
return false;
18.
}
19.
}
20. }

What is the result?
% java TestOR
i < 5
Result is true
i >= 5
i >= 5

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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.

Logical Operators (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

Java Operators (OCA Objectives 3.1, 3.2, and 3.3)

245

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, since the result will vary depending on whether the
second operand in the expression is evaluated. Consider this,
int z = 5;
if(++z > 5 || ++z > 6) z++;

// z = 7 after this code

versus this:
int z = 5;
if(++z > 5 | ++z > 6) z++;

// z = 8 after this code

Logical Operators ^ and !
The last two logical operators on the exam are
■ ^

Exclusive-OR (XOR)

■ !

Boolean invert

The ^ (exclusive-OR) operator evaluates only boolean 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 be true. 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"); }

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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:
boolean t = true;
boolean f = false;
System.out.println("! " + (t & !f) + " " + f);

It produces this output:
! true false

In the preceding example, notice that the & test succeeded (printing true) and
that the value of the boolean variable f did not change, so it printed false.

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 perform String 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, threeoperand 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.
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

✓

247

TWO-MINUTE DRILL
Here are some of the key points from each section in this chapter.

Relational Operators (OCA Objectives 3.1 and 3.3)
❑ Relational operators always result in a boolean value (true or false).
❑ 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.

instanceof Operator (OCA Objective 3.1)
❑ 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 (or null) 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.

Arithmetic Operators (OCA Objectives 3.1 and 3.2)
❑ 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 –.

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String Concatenation 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 Objectives 3.1 and 3.2)
❑ Prefix operators (for example, ++x and --x) run before the value is used in
the expression.
❑ Postfix operators (for example, x++ and 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 (OCA Objective 3.1)
❑ Returns one of two values based on whether its boolean expression is true
or false.
❑ Returns the value after the ? if the expression is true.
❑ Returns the value after the : if the expression is false.

Logical Operators (OCA Objective 3.1)
❑ The exam covers six "logical" operators: &, |, ^, !, &&, and ||.
❑ Logical operators work with two expressions (except for !) that must resolve
to boolean values.
❑ The && and & operators return true only if both operands are true.
❑ The || and | operators return true if either or both operands are true.
❑ The && and || operators are known as short-circuit operators.
❑ The && operator does not evaluate the right operand if the left operand
is false.
❑ 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.

Self Test

SELF TEST
1. Given:
class Hexy {
public static void main(String[] args) {
int i = 42;
String s = (i<40)?"life":(i>50)?"universe":"everything";
System.out.println(s);
}
}

What is the result?
A. null
B. life
C. universe
D. everything
E. Compilation fails
F. An exception is thrown at runtime
2. Given:
public class Dog {
String name;
Dog(String s) { name = s; }
public static void main(String[] args) {
Dog d1 = new Dog("Boi");
Dog d2 = new Dog("Tyri");
System.out.print((d1 == d2) + " ");
Dog d3 = new Dog("Boi");
d2 = d1;
System.out.print((d1 == d2) + " ");
System.out.print((d1 == d3) + " ");
}
}

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

249

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3. Given:
class Fork {
public static void main(String[] args) {
if(args.length == 1 | args[1].equals("test")) {
System.out.println("test case");
} else {
System.out.println("production " + args[0]);
}
}
}

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:
class Feline {
public static void main(String[] args) {
long x = 42L;
long y = 44L;
System.out.print(" " + 7 + 2 + " ");
System.out.print(foo() + x + 5 + " ");
System.out.println(x + y + foo());
}
static String foo() { return "foo"; }
}

What is the result?
A. 9 foo47 86foo
B. 9 foo47 4244foo
C. 9 foo425 86foo
D. 9 foo425 4244foo
E. 72 foo47 86foo

Self Test

F.
G.
H.
I.

72 foo47 4244foo
72 foo425 86foo
72 foo425 4244foo

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 output 33. Note that you must use each
fragment exactly once.
CODE:
class Incr {
public static void main(String[] args) {
Integer x = 7;
int y = 2;
x
___
___
___

___
___
___
___

___;
___;
___;
___;

System.out.println(x);
}
}

FRAGMENTS:
y

y

y

y

x

x

y

-= *= *= *=

251

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6. Given:
public class Cowboys {
public static void main(String[] args) {
int x = 12;
int a = 5;
int b = 7;
System.out.println(x/a + " " + x/b);
}
}

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. (OCP Only) Given:
3. public class McGee {
4.
public static void main(String[] args) {
5.
Days d1 = Days.TH;
6.
Days d2 = Days.M;
7.
for(Days d: Days.values()) {
8.
if(d.equals(Days.F)) break;
9.
d2 = d;
10.
}
11.
System.out.println((d1 == d2)?"same old" : "newly new");
12.
}
13.
enum Days {M, T, W, TH, F, SA, SU};
14. }

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

Self Test

253

8. Given:
4. public class SpecialOps {
5.
public static void main(String[] args) {
6.
String s = "";
7.
boolean b1 = true;
8.
boolean b2 = false;
9.
if((b2 = false) | (21%5) > 2) s += "x";
10.
if(b1 || (b2 == true))
s += "y";
11.
if(b2 == true)
s += "z";
12.
System.out.println(s);
13.
}
14. }

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:
3. public class Spock {
4.
public static void main(String[] args) {
5.
int mask = 0;
6.
int count = 0;
7.
if( ((5<7) || (++count < 10)) | mask++ < 10 )
8.
if( (6 > 8) ^ false)
9.
if( !(mask > 1) && ++count > 1)
10.
System.out.println(mask + " " + count);
11.
}
12. }

mask = mask + 1;
mask = mask + 10;
mask = mask + 100;

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

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10. Given:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.

interface Vessel { }
interface Toy { }
class Boat implements Vessel { }
class Speedboat extends Boat implements Toy { }
public class Tree {
public static void main(String[] args) {
String s = "0";
Boat b = new Boat();
Boat b2 = new Speedboat();
Speedboat s2 = new Speedboat();
if((b instanceof Vessel) && (b2 instanceof Toy)) s += "1";
if((s2 instanceof Vessel) && (s2 instanceof Toy)) s += "2";
System.out.println(s);
}
}

What is the result?
A. 0
B. 01
C. 02
D. 012
E. Compilation fails
F. An exception is thrown at runtime

Self Test Answers

255

SELF TEST ANSWERS
1. ☑ D is correct. This is a ternary nested in a ternary. Both of the ternary expressions
are false.
☐
✗ A, B, C, E, and F are incorrect based on the above. (OCA Objective 3.1)
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 3.3)
3. ☑ E is correct. Because the short circuit (||) is not used, both operands are evaluated.
Since args[1] is past the args array bounds, an ArrayIndexOutOfBoundsException is
thrown.
☐
✗ A, B, C, and D are incorrect based on the above. (OCA Objectives 3.1 and 3.3)
4. ☑ G is correct. Concatenation runs from left to right, and if either operand is a String,
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:
class Incr {
public static void main(String[] args) {
Integer x = 7;
int y = 2;
x
y
y
x

*=
*=
*=
-=

x;
y;
y;
y;

System.out.println(x);
}
}

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 dividing ints, 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 of this syntax is correct. The for-each iterates through the enum using
the values() method to return an array. An enum can be compared using either equals()
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 and 3.3)

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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 test b2's value. Line
10 sets b2 to true, and it doesn’t test its value; however, the short-circuit operator keeps the
expression b2 = true from being executed.
☐
✗ A, B, D, and E are incorrect based on the above. (OCA Objectives 3.1, 3.2, and 3.3)
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 && keeps count from being incremented.
☐
✗ A, B, D, E, and G are incorrect based on the above. (OCA Objectives 3.1 and 3.2)
10. ☑ D is correct. First, remember that instanceof 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 say Speedboat s3 =
(Speedboat)b2;.
☐
✗ A, B, C, E, and F are incorrect based on the above. (OCA Objectives 3.1 and 3.2)

5
Working with
Strings, Arrays,
and ArrayLists
CERTIFICATION OBJECTIVES

•
•

Create and Manipulate Strings

•

Declare, Instantiate, Initialize, and Use a
One-Dimensional Array

•

Declare, Instantiate, Initialize, and Use a
Multidimensional Array

Manipulate Data Using the StringBuilder
Class and Its Methods

•
•
✓

Declare and Use an ArrayList
Use Encapsulation for Reference Variables
Two-Minute Drill

Q&A Self Test

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CERTIFICATION OBJECTIVE

Using String and StringBuilder
(OCA Objectives 2.7 and 2.6)
2.7

Create and manipulate strings.

2.6

Manipulate data using the StringBuilder class and its methods.

Everything you needed to know about strings in the older OCJP exams, you'll
need to know for the OCA 7 and OCP 7 exams. 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, but StringBuilder is
faster because its methods aren't synchronized.) Both classes, StringBuilder and
StringBuffer, give you String-like objects that handle some of the String class's
shortcomings (such as immutability).

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 a String object seems to be changing? Let's find out.

Strings Are Immutable Objects
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 the new keyword, as follows:
String s = new String();

This line of code creates a new object of class String and assigns it to the reference
variable s.

Using String and StringBuilder (OCA Objectives 2.7 and 2.6)

259

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 that the String 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 new String object, with a
value of "abcdef", and assign it to a reference variable s. Now let's say that you
want a second reference to the String object referred to by s:
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:
s = s.concat(" more stuff");

// the concat() method 'appends'
// a literal to the end

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 string s (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 by s, so it created a new String 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

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itself a new String object. But we have references only to "abcdef" (referenced by
s2) and "abcdef more stuff" (referenced by s).
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 5-1 shows what happens on the heap when you reassign a reference
variable. Note that the dashed line indicates a deleted reference.
To review our first example:
String s = "abcdef";
String s2 = s;

//
//
//
//

//
//
//
//

create a new String object, with
value "abcdef", refer s to it
create a 2nd reference variable
referring to the same String

create a new String object, with value "abcdef more stuff",
refer s to it. (Change s's reference from the old String
to the new String.) (Remember s2 is still referring to
the original "abcdef" String.)

s = s.concat(" more stuff");

Let's look at another example:
String x = "Java";
x.concat(" Rules!");
System.out.println("x = " + x);

// the output is "x = Java"

The first line is straightforward: Create a new String object, give it the value
"Java", and refer x 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 original
String with the value "Java". Figure 5-2 shows creating a String without
assigning a reference to it.
Let's expand this current example. We started with
String x = "Java";
x.concat(" Rules!");
System.out.println("x = " + x);

// the output is: x = Java

Now let's add
x.toUpperCase();
System.out.println("x = " + x);

// the output is still:
// x = Java

Using String and StringBuilder (OCA Objectives 2.7 and 2.6)

FIGURE 5-1
Step 1:

261

The heap

String s = "abc";

String

objects and
their reference
variables

Step 2:

s

"abc"

String reference
variable

String object

The heap

String s2 = s;
s2

"abc"

String reference
variable

String object

s
String reference
variable

Step 3:

The heap

s = s.concat ("def");
s2

"abc"

String reference
variable

String object

s

"abcdef"
String object

String reference
variable

(We actually did just create a new String object with the value "JAVA", but it
was lost, and x still refers to the original, unchanged string "Java".) How about
adding this:
x.replace('a', 'X');
System.out.println("x = " + x);

// the output is still:
// x = Java

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FIGURE 5-2
Step 1:

A String
object is
abandoned
upon creation.

Step 2:

The heap

String x = "Java";
x

"Java"

String reference
variable

String object

The heap

x.concat (" Rules!");
"Java"
x
String object

String reference
variable

"Java Rules!"
String object

Notice that no reference
variable is created to access
the "Java Rules!" String.

Can you determine what happened? The JVM created yet another new String
object, with the value "JXvX", (replacing the a's with X's), but once again this new
String was lost, leaving x to refer to the original unchanged and unchangeable
String object, with the value "Java". In all of these cases, we called various string
methods to create a new String by altering an existing String, but we never
assigned the newly created String to a reference variable.
But we can put a small spin on the previous example:
String x = "Java";
x = x.concat(" Rules!");
System.out.println("x = " + x);

//
//
//
//

Now
new
the
x =

we're assigning the
String to x
output will be:
Java Rules!

This time, when the JVM runs the second line, a new String object is created with
the value "Java Rules!", and x is set to reference it. But wait…there's more—now
the original String object, "Java", has been lost, and no one is referring to it. So in

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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 5-3 for a graphic
depiction of this sad story. The dashed line indicates a deleted reference.
Let's take this example a little further:
String x = "Java";
x = x.concat(" Rules!");
System.out.println("x = " + x);

// the output is:
// x = Java Rules!

x.toLowerCase();

// no assignment, create a
// new, abandoned String

System.out.println("x = " + x);

// no assignment, the output
// is still: x = Java Rules!

x = x.toLowerCase();

//
//
//
//

System.out.println("x = " + x);

create a new String,
assigned to x
the assignment causes the
output: x = java rules!

FIGURE 5-3
Step 1:

An old String
object being
abandoned

Step 2:

The heap

String x = "Java";
x

"Java"

String reference
variable

String object

x = x.concat (" Rules!");

The heap
"Java"

x
String reference
variable

String object
"Java Rules!"
String object

Notice in step 2 that there is no
valid reference to the "Java" String;
that object has been "abandoned,"
and a new object created.

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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 the String 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 how String objects work in Java.
We'll finish this section by presenting an example of the kind of devilish String
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.)
String s1 = "spring ";
String s2 = s1 + "summer ";
s1.concat("fall ");
s2.concat(s1);
s1 += "winter ";
System.out.println(s1 + " " + s2);

What is the output? For extra credit, how many String objects and how many
reference variables were created prior to the println statement?
Answer: The result of this code fragment is spring winter spring 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 handles String 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 of String 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 identical
String already exists. If a match is found, the reference to the new literal is directed
to the existing String, and no new String literal object is created. (The existing
String simply has an additional reference.) Now you can start to see why making

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String objects immutable is such a good idea. If several reference variables refer to
the same String 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.

Creating New 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 a String
might be created, and let's further assume that no other String objects exist in the
pool. In this simple case, "abc" will go in the pool and s will refer to it:
String s = "abc";

//
//

creates one String object and one
reference variable

In the next case, because we used the new keyword, Java will create a new String
object in normal (nonpool) memory and s will refer to it. In addition, the literal
"abc" will be placed in the pool:
String s = new String("abc");

// creates two objects,
// and one reference variable

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 that 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()
■ length()
■ replace()

Returns the number of characters in a string
Replaces occurrences of a character with a new character

■ substring()

Returns a part of a string

■ toLowerCase()

lowercase

Determines the equality of two strings, ignoring case

Returns a string, with uppercase characters converted to

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■ 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.

public char charAt(int index) This method returns the character located at
the String's specified index. Remember, String indexes are zero-based—here's an
example:
String x = "airplane";
System.out.println( x.charAt(2) );

//

output is 'r'

public String concat(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:
String x = "taxi";
System.out.println( x.concat(" cab") ); // output is "taxi cab"

The overloaded + and += operators perform functions similar to the concat()
method—here's an example:
String x = "library";
System.out.println( x + " card");
String x = "Atlantic";
x+= " ocean";
System.out.println( x );

// output is "library card"

// output is "Atlantic ocean"

In the preceding "Atlantic ocean" example, notice that the value of x really
did change! Remember that 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 string x was referring to, "Atlantic", is abandoned.

public boolean equalsIgnoreCase(String s) This method returns a boolean
value (true or false) depending on whether the value of the String in the
argument is the same as the value of the String 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:

Using String and StringBuilder (OCA Objectives 2.7 and 2.6)

String x = "Exit";
System.out.println( x.equalsIgnoreCase("EXIT"));
System.out.println( x.equalsIgnoreCase("tixe"));

267

// is "true"
// is "false"

public int length() This method returns the length of the String used to
invoke the method—here's an example:
String x = "01234567";
System.out.println( x.length() );

// returns "8"

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:
String x = "test";
System.out.println( x.length );

// compiler error

and
String[] x = new String[3];
System.out.println( x.length() );

// compiler error

public String replace(char old, char new) This method returns a String
whose value is that of the String used to invoke the method, 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:
String x = "oxoxoxox";
System.out.println( x.replace('x', 'X') );

// output is "oXoXoXoX"

public String substring(int begin) and public String substring(int begin,
int end) 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 original String. If
the call has two arguments, the substring returned will end with the character
located in the nth position of the original String where n is the second argument.

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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 original String's 7 position,
which is index 6 (ouch). Let's look at some examples:
String x = "0123456789";
System.out.println( x.substring(5) );
System.out.println( x.substring(5, 8));

//
//
//
//

as if by magic, the value of each
char is the same as its index!
output is "56789"
output is "567"

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).

public String toLowerCase() Converts all characters of a String to
lowercase—here's an example:
String x = "A New Moon";
System.out.println( x.toLowerCase() );

// output is "a new moon"

public String toString() 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:
String x = "big surprise";
System.out.println( x.toString() );

// output? [reader's exercise :-) ]

public String toUpperCase() Converts all characters of a String to
uppercase—here's an example:
String x = "A New Moon";
System.out.println( x.toUpperCase() );

// output is "A NEW MOON"

public String trim() 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:
String x = " hi ";
System.out.println( x + "t" );
System.out.println( x.trim() + "t");

// output is " hi
// output is "hit"

t"

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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 with
String 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 for StringBuilders 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 to StringBuffer
The StringBuilder class was added in Java 5. It has exactly the same API as the
StringBuffer class, except StringBuilder is not thread-safe. In other words, its
methods are not synchronized. (More about thread safety in Chapter 13.) Oracle
recommends that you use StringBuilder instead of StringBuffer whenever
possible, because StringBuilder will run faster (and perhaps jump higher). So
apart from synchronization, anything we say about StringBuilder's methods holds
true for StringBuffer's methods, and vice versa. That said, for the OCA 7 and
OCP 7 exams, StringBuffer is not tested.

Using StringBuilder
(and This Is the Last Time We'll Say This: StringBuffer)
In the previous section, you saw how the exam might test your understanding of
String immutability with code fragments like this:
String x = "abc";
x.concat("def");
System.out.println("x = " + x);

//

output is "x = abc"

Because no new assignment was made, the new String object created with the
concat() method was abandoned instantly. You also saw examples like this:
String x = "abc";
x = x.concat("def");
System.out.println("x = " + x);

// output is "x = abcdef"

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We got a nice new String 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:
StringBuilder sb = new StringBuilder("abc");
sb.append("def");
System.out.println("sb = " + sb);
// output is "sb = abcdef"

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:
StringBuilder sb = new StringBuilder("abc");
sb.append("def").reverse().insert(3, "---");
System.out.println( sb );
// output is

"fed---cba"

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 single StringBuilder 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:
1. new StringBuilder();
2. new StringBuilder("ab");
3. new StringBuilder(x);

// default cap. = 16 chars
// cap. = 16 + arg's length
// capacity = x (an integer)

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.

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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.

public StringBuilder append(String s) 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. This method will take many different
arguments, including boolean, char, double, float, int, long, and others, but
the most likely use on the exam will be a String argument—for example,
StringBuilder sb = new StringBuilder("set ");
sb.append("point");
System.out.println(sb);
// output is "set point"
StringBuilder sb2 = new StringBuilder("pi = ");
sb2.append(3.14159f);
System.out.println(sb2);
// output is "pi = 3.14159"

public StringBuilder delete(int start, int end) This method modifies the
value of the StringBuilder 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:
StringBuilder sb = new StringBuilder("0123456789");
System.out.println(sb.delete(4,6));
// output is "01236789"

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 uses
String objects:
StringBuilder sb = new StringBuilder("abc");
sb.append("def");
System.out.println( sb );

In this case, the output will be: "abcdef"

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public StringBuilder insert(int offset, String s) This method updates the
value of the StringBuilder object that invoked the method call. The String
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:
StringBuilder sb = new StringBuilder("01234567");
sb.insert(4, "---");
System.out.println( sb );
//
output is

"0123---4567"

public StringBuilder reverse() This method updates the value of the
StringBuilder 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:
StringBuilder s = new StringBuilder("A man a plan a canal Panama");
sb.reverse();
System.out.println(sb); // output: "amanaP lanac a nalp a nam A"

public String toString() This method returns the value of the StringBuilder
object that invoked the method call as a String:
StringBuilder sb = new StringBuilder("test string");
System.out.println( sb.toString() ); // output is "test 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:

Using Arrays (OCA Objectives 4.1 and 4.2)

1.
2.
3.

273

Determine what the leftmost method call will return (let's call it x).
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.
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,

String x = "abc";
String y = x.concat("def").toUpperCase().replace('C','x');
System.out.println("y = " + y); // result is "y = ABxDEF"

//chained methods

Let's look at what happened.The literal def was concatenated to abc, creating a temporary,
intermediate String (soon to be lost), with the value abcdef.The toUpperCase() method
was called on this String, which created a new (soon to be lost) temporary String 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

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.

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Arrays are efficient, but most of the time you'll want to use one of the
Collection types from java.util (including HashMap, ArrayList, 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? Chapter 11 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:
int[] key;
int key [];

// brackets before name (recommended)
// brackets after name (legal but less readable)
// spaces between the name and [] legal, but bad

Declaring an array of object references:
Thread[] threads;
Thread threads[];

// Recommended
// Legal but less readable

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 and 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 [];

// recommended
// yucky, but legal

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

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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.

Constructing One-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:
int[] testScores;
// Declares the array of ints
testScores = new int[4]; // constructs an array and assigns it
// to the testScores variable

The preceding code puts one new object on the heap—an array object holding four
elements—with each element containing an int 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
each int element to zero. Thanks." (The compiler appreciates good manners.)
Figure 5-4 shows the testScores array on the heap, after construction.
FIGURE 5-4

A onedimensional array
on the heap

testScores
The heap
int[ ]array
reference
variable

0

0

0

0

0

1

2

3
Values

int[ ]array object
Indices

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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:
Thread[] threads = new Thread[5];

// no Thread objects created!
// one Thread array created

Remember that, despite how the code appears, the Thread constructor is not being
invoked. We're not creating a Thread instance, but rather a single Thread array
object. After the preceding statement, there are still no actual Thread 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 by threads 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.

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277

In addition to being constructed with new, arrays can also 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.

Constructing Multidimensional 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 another int array. The second dimension holds
the actual int 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, since
the JVM needs to know only the size of the object assigned to the variable myArray.
Figure 5-5 shows how a two-dimensional int array works 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 or not those references are actually pointing (oops,
this is Java, we mean referring) to real String objects or are simply null. 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];

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FIGURE 5-5
The heap

A two-dimensional
array on the heap

int[ ]array object

int[ ]array object

myArray[0]
myArray[1]
null

2-D int[ ][ ]array object

myArray
int[ ][ ] (2-D array)
reference variable

Picture demonstrates the result of the following code:
int[ ][ ] myArray = new int[3][ ];
myArray[0] = new int[2];
myArray[0][0] = 6;
myArray[0][1] = 7;
myArray[1] = new int[3];
myArray[1][0] = 9;
myArray[1][1] = 8;
myArray[1][2] = 5;

We have one array object on the heap, with three null references of type
Animal, but we don't have any Animal objects. The next step is to create some
Animal objects and assign them to index positions in the array referenced by pets:
pets[0] = new Animal();
pets[1] = new Animal();
pets[2] = new Animal();

This code puts three new Animal objects on the heap and assigns them to the three
index positions (elements) in the pets array.

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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 of the exam questions list both runtime exception and compiler error as
possible answers:
int[] x = new int[5];
x[4] = 2;
// OK, the last element is at index 4
x[5] = 3;
// Runtime exception. There is no element at index 5!
int[] z = new int[2];
int y = -3;
z[y] = 4;
// Runtime exception. y is a negative number

These 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:
int[][] scores = new int[3][];
// Declare and create an array (scores) holding three references
// to int arrays
scores[0] = new int[4];
// the first element in the scores array is an int array
// of four int elements
scores[1] = new int[6];
// the second element is an int array of six int elements
scores[2] = new int[1];
// the third element is an int array of one int element

Initializing Elements in a Loop
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 the length.

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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:
Dog[] myDogs = new Dog[6]; // creates an array of 6 Dog references
for(int x = 0; x < myDogs.length; x++) {
myDogs[x] = new Dog(); // assign a new Dog to index position x
}

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:
1.
2.

int x = 9;
int[] dots = {6,x,8};

Line 2 in the preceding code does four things:
■ Declares an int array reference variable named dots.
■ Creates an int 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:
int[] dots;
dots = new int[3];
int x = 9;
dots[0] = 6;
dots[1] = x;
dots[2] = 8;

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.

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With object references rather than primitives, it works exactly the same way:
Dog puppy = new Dog("Frodo");
Dog[] myDogs = {puppy, new Dog("Clover"), new Dog("Aiko")};

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 new
Dog 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 5-6 shows the result.

FIGURE 5-6
The heap

Declaring,
constructing,
and initializing an
array of objects

Dog object

Dog object

puppy
Dog reference
variable

Clover

Frodo

Dog object
Aiko

myDogs
Dog[ ]array
reference variable

0

1

2

Dog[ ]array object

Picture demonstrates the result of the following code:
Dog puppy = new Dog ("Frodo");
Dog[ ] myDogs = {puppy, new Dog("Clover"), new Dog("Aiko")};

Four objects are created:
1 Dog object referenced by puppy and by myDogs[0]
1 Dog[ ] array referenced by myDogs
2 Dog object referenced by myDogs[1]and myDogs[2]

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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 the scores
array is a reference variable to an int 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
an int array), and three int arrays (each element in the three int arrays is an int
value). Finally, the three int arrays are initialized with the actual int 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:
scores[0]
scores[1]
scores[2]
scores[0][1]
scores[2][1]

//
//
//
//
//

an array of 4
an array of 2
an array of 2
the int value
the int value

ints
ints
ints
2
4

Figure 5-7 shows the result of declaring, constructing, and initializing a twodimensional array in one statement.

Constructing and 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:
int[] testScores;
testScores = new int[] {4,7,2};

The preceding code creates a new int 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:

Using Arrays (OCA Objectives 4.1 and 4.2)

public class JIT {
void takesAnArray(int[] someArray) {
// use the array parameter
}
public static void main (String [] args) {
JIT j = new JIT();
j.takesAnArray(new int[] {7,7,8,2,5});
// pass an array
}
}

FIGURE 5-7

Declaring,
constructing, and
initializing a twodimensional array

The heap

Cat object
Cat object

Cat object

Cat object

Legolas

Zeus

Bilbo

Fluffy

Bert

0

1

0

1

Cat[ ]array
object

Cat[ ][ ] array
reference variable

Cat object

0

2
Cat[ ]array
object

1

2-D Cat[ ][ ] array object

Picture demonstrates the result of the following code:
Cat[ ][ ] myCats = { { new Cat("Fluffy"), new Cat("Zeus") } ,
{ new Cat("Bilbo"), new Cat("Legolas"), new Cat("Bert") } }
Eight objects are created:
1 2-D Cat[ ][ ] array object
2 Cat[ ] array object
5 Cat object

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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:
new Object[3] {null, new Object(), new Object()};
// not legal; size must not be specified

Legal Array 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, an int array can hold any value that can fit
into a 32-bit int variable. Thus, the following code is legal:
int[] weightList = new int[5];
byte b = 4;
char c = 'c';
short s = 7;
weightList[0] = b; // OK, byte is smaller than int
weightList[1] = c; // OK, char is smaller than int
weightList[2] = s; // OK, short is smaller than int

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, if Subaru is a subclass of Car, you can put
both Subaru objects and Car objects into an array of type Car as follows:

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285

class Car {}
class Subaru extends Car {}
class Ferrari extends Car {}
...
Car [] myCars = {new Subaru(), new Car(), new Ferrari()};

It helps to remember that the elements in a Car array are nothing more than Car
reference variables. So anything that can be assigned to a Car reference variable can
be legally assigned to a Car 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:
interface Sporty {
void beSporty();
}
class Ferrari extends Car implements Sporty {
public void beSporty() {
// implement cool sporty method in a Ferrari-specific way
}
}
class RacingFlats extends AthleticShoe implements Sporty {
public void beSporty() {
// implement cool sporty method in a RacingFlat-specific way
}
}
class GolfClub { }
class TestSportyThings {
public static void main (String [] args) {
Sporty[] sportyThings = new Sporty [3];
sportyThings[0] = new Ferrari();
// OK, Ferrari
// implements Sporty
sportyThings[1] = new RacingFlats(); // OK, RacingFlats
// implements Sporty
sportyThings[2] = new GolfClub();
// NOT ok..
// Not OK; GolfClub does not implement Sporty
// I don't care what anyone says
}
}

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.

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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 an
int array, the reference variable you declared can be reassigned to any int array (of
any size), but the variable cannot be reassigned to anything that is not an int array,
including an int value. Remember, all arrays are objects, so an int array reference
cannot refer to an int primitive. The following code demonstrates legal and illegal
assignments for primitive arrays:
int[] splats;
int[] dats = new int[4];
char[] letters = new char[5];
splats = dats;
// OK, dats refers to an int array
splats = letters; // NOT OK, letters refers to a char array

It's tempting to assume that because a variable of type byte, short, or char can
be explicitly promoted and assigned to an int, an array of any of those types could
be assigned to an int 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:
Car[] cars;
Honda[] cuteCars =
cars = cuteCars;
Beer[] beers = new
cars = beers;

new Honda[5];
// OK because Honda is a type of Car
Beer [99];
// NOT OK, Beer is not a type of Car

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, if Box implements
Foldable, the following is legal:

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287

Foldable[] foldingThings;
Box[] boxThings = new Box[3];
foldingThings = boxThings;
// OK, Box implements Foldable, so Box IS-A Foldable

You cannot reverse the legal assignments. A Car 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 of int arrays cannot be assigned to a regular int
array reference, as follows:
int[] blots;
int[][] squeegees = new int[3][];
blots = squeegees;
// NOT OK, squeegees is a
// two-d array of int arrays
int[] blocks = new int[6];
blots = blocks;
// OK, blocks is an int array

Pay particular attention to array assignments using different dimensions. You
might, for example, be asked if it's legal to assign an int array to the first element in
an array of int arrays, as follows:
int[][] books = new int[3][];
int[] numbers = new int[6];
int aNumber = 7;
books[0] = aNumber;
// NO, expecting an int array not an int
books[0] = numbers;
// OK, numbers is an int array

Figure 5-8 shows an example of legal and illegal assignments for references to
an array.

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Working with Strings, Arrays, and ArrayLists

Legal and illegal array assignments

moreCats
The heap

Cat[ ]array
reference variable
Array reference variable can
ONLY refer to a 1-D Cat array

null

0

1

Cat[ ]array
object
Cat object

Cat object
Fluffy

Cat object
Zeus

Legolas

Cat object

Bilbo

Bert

D

B
A
myCats

Cat object

Cat[ ]array
object

Cat[ ][ ]2-D array
reference variable
2-D reference variable can
ONLY refer to a 2-D Cat array

0

1

0

0

1

C

1 2

Element in a 1-D Cat array can
ONLY refer to a Cat object

Cat[ ]array
object

2-D Cat[ ][ ]array object
Element in a 2-D Cat array can ONLY
refer to a 1-D Cat array

Illegal Array Reference Assignments

KEY

A myCats = myCats[0];
// Can't assign a 1-D array to a 2-D array reference
B myCats = myCats[0][0];
// Can't assign a nonarray object to a 2-D array reference

Legal

C myCats[1] = myCats[1][2];
// Can't assign a nonarray object to a 1-D array reference
D myCats[0][1] = moreCats;
// Can't assign an array object to a nonarray reference
// myCats[0][1] can only refer to a Cat object

Illegal

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CERTIFICATION OBJECTIVE

Using ArrayList (OCA Objective 4.3)
4.3

Declare and use an ArrayList.

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 as Lists,
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 the structures listed above, which, along
with the Collection API, we'll discuss in Chapter 11.)

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 of these 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.

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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 an ArrayList:
import java.util.*;
public class Cities {
public static void main(String[] args) {
List c = new ArrayList();
c.add("Oslo");
c.add("Paris");
c.add("Rome");
int index = c.indexOf("Paris");
System.out.println(c + " " + index);
c.add(index, "London");
System.out.println(c);

// ArrayList lives in .util

// create an ArrayList, c
// add original cities

// find Paris' index
// add London before Paris
// show the contents of c

}
}

The output will be something like this:
[Oslo, Paris, Rome] 1
[Oslo, London, Paris, Rome]

By reviewing the code, we can learn some important facts about ArrayLists:
■ The ArrayList class is in the java.util package.
■ Similar to arrays, when you build an ArrayList you have to declare what

kind of objects it can contain. In this case, we're building an ArrayList of
String 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 the ArrayList 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 on toString() in
a minute.)
■ Like arrays, indexes for ArrayLists are zero-based.
■ We didn't declare how big the ArrayList 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.

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291

As promised, we need to look at the following line of code more closely:
List c = new ArrayList();

First off, we see that this is a polymorphic declaration. As we said earlier,
ArrayList implements the List interface (also in java.util). If you plan to take the

OCP 7 exam after you've aced the OCA 7, we'll be talking a lot more about why we
might want to do a polymorphic declaration in the OCP part of the book. For now,
imagine that someday you might want to create a List of your ArrayLists.
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 particular ArrayList you want only Strings to be allowed. What
this means is that if the compiler can tell that you're trying to add a "not-a-String"
object to this ArrayList, 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 class Object. 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 from Object, 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

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the coast from Rome! You've got to add that side trip to your itinerary. The question
is, can an ArrayList have duplicate entries? Is it legal to say this:
c.add("Rome");
c.add("Naples");
c.add("Rome");

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 7.

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 that the int is being autoboxed (converted) into an Integer
object and then added to the ArrayList. We'll talk more about autoboxing in the OCP
part of the book.

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:
import java.util.*;
public class TweakLists {
public static void main(String[] args) {
List myList = new ArrayList();
myList.add("z");
myList.add("x");
myList.add(1, "y");
myList.add(0, "w");
System.out.println(myList);
myList.clear();
myList.add("b");
myList.add("a");

// zero based
// "
"
// [w, z, y, x]
// remove everything

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myList.add("c");
System.out.println(myList);
// [b, a, c]
System.out.println(myList.contains("a") + " " + myList.contains("x"));
System.out.println("get 1: " + myList.get(1));
System.out.println("index of c: " + myList.indexOf("c"));
myList.remove(1);
// remove "a"
System.out.println("size: " + myList.size() + " contents: " + myList);
}
}

which should produce something like this:
[w, z, y, x]
[b, a, c]
true false
get 1: a
index of c: 2
size: 2 contents: [b, c]

A couple of quick notes about this code: First off, notice that contains() returns
a boolean. This makes contains() great to use in "if" tests. Second, notice that
ArrayList has a size() 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 at index is now at
index + 1)

■ clear()

Removes all the elements from the ArrayList

■ boolean contains(element)
■ Object get(index)

Returns whether the element is in the list

Returns the Object located at index

■ int indexOf(Object)

Returns the (int) location of the element, or -1 if

the Object is not found
■ remove(index)

Removes the element at that index and shifts later
elements toward the beginning one space

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■ 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 7 exam tests only for very basic knowledge of
ArrayLists. If you go on to take the OCP 7 exam, you'll learn a lot more
about ArrayLists and other common, collections-oriented classes.

Encapsulation for Reference Variables
In Chapter 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 non-primitive 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:
class Special {
private StringBuilder s = new StringBuilder("bob");
StringBuilder getName() { return s; }
void printName() { System.out.println(s); }

// our special data
// verify our special
// data

}
public class TestSpecial {
public static void main(String[] args) {
Special sp = new Special();
StringBuilder s2 = sp.getName();
s2.append("fred");
sp.printName();
}
}

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….

Certification Summary

295

Okay—just to verify your answer—when we invoke getName(), 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-only StringBuilder object we ever built. So, at the
point that getName() 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 an ArrayList, 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, you DO NOT have encapsulation.

CERTIFICATION SUMMARY
The most important thing to remember about Strings is that String objects are
immutable, but references to Strings are not! You can make a new String by using
an existing String as a starting point, but if you don't assign a reference variable to
the new String it will be lost to your program—you will have no way to access your
new String. 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, except StringBuilder's methods aren't thread-safe.
Because StringBuilder's methods are not thread-safe, they tend to run faster than
StringBuffer methods, so choose StringBuilder whenever threading is not an
issue. Both StringBuffer 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.
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.
Finally, we discussed the basics of ArrayLists. 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.

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TWO-MINUTE DRILL
Here are some of the key points from the certification objectives in this chapter.

Using String and StringBuilder (OCA Objectives 2.6 and 2.7)
❑ String objects are immutable, and String reference variables are not.
❑ If you create a new String without assigning it, it will be lost to your program.
❑ If you redirect a String reference to a new String, the old String can be lost.
❑ String methods use zero-based indexes, except for the second argument of
substring().
❑ The String class is final—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 an attribute 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().

Using Arrays (OCA Objectives 4.1 and 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 (using new) 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.

Two-Minute Drill

297

❑ Arrays are indexed beginning with zero.
❑ An ArrayIndexOutOfBoundsException 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, a byte variable can go in an int array.
❑ An array of objects can hold any object that passes the IS-A (or
instanceof) test for the declared type of the array. For example, if Horse
extends Animal, then a Horse object can go into an Animal 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 extends Car).

Using ArrayList (OCA Objective 4.3)
❑ ArrayLists allow you to resize your list and make insertions and deletions to
your list far more easily than arrays.
❑ For the OCA 7 exam, the only ArrayList declarations you need to know are
of this form:
ArrayList myList = new ArrayList();
List myList2 = new ArrayList(); // polymorphic

❑ ArrayLists can hold only objects, not primitives, but remember that
autoboxing can make it look like you're adding primitives to an ArrayList
when in fact you're adding a wrapper version of a primitive.
❑ An ArrayList'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 7 exam.
❑ ArrayList methods to remember: add(element), add(index, element),
clear(), contains(), get(index), indexOf(), remove(index),
remove(object), and size().

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SELF TEST
1. Given:
public class Mutant {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder("abc");
String s = "abc";
sb.reverse().append("d");
s.toUpperCase().concat("d");
System.out.println("." + sb + ". ." + s + ".");
}
}

Which two substrings will be included in the result? (Choose two.)
A. .abc.
B. .ABCd.
C. .ABCD.
D. .cbad.
E. .dcba.
2. Given:
public class Hilltop {
public static void main(String[] args) {
String[] horses = new String[5];
horses[4] = null;
for(int i = 0; i < horses.length; i++) {
if(i < args.length)
horses[i] = args[i];
System.out.print(horses[i].toUpperCase() + " ");
}
}
}

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

Self Test

C.
D.
E.
F.

An exception is thrown with no other output
EYRA VAFI DRAUMUR KARA, and then a NullPointerException
EYRA VAFI DRAUMUR KARA, and then an ArrayIndexOutOfBoundsException
Compilation fails

3. Given:
public class Actors {
public static void main(String[] args) {
char[] ca = {0x4e, \u004e, 78};
System.out.println((ca[0] == ca[1]) + " " + (ca[0] == ca[2]));
}
}

What is the result?
A. true true
B. true false
C. false true
D. false false
E. Compilation fails
4. Given:
1. class Dims {
2.
public static void main(String[] args) {
3.
int[][] a = {{1,2}, {3,4}};
4.
int[] b = (int[]) a[1];
5.
Object o1 = a;
6.
int[][] a2 = (int[][]) o1;
7.
int[] b2 = (int[]) o1;
8.
System.out.println(b[1]);
9. } }

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

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5. Given:
import java.util.*;
public class Sequence {
public static void main(String[] args) {
ArrayList myList = new ArrayList();
myList.add("apple");
myList.add("carrot");
myList.add("banana");
myList.add(1, "plum");
System.out.print(myList);
}
}

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:
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

class Dozens {
int[] dz = {1,2,3,4,5,6,7,8,9,10,11,12};
}
public class Eggs {
public static void main(String[] args) {
Dozens [] da = new Dozens[3];
da[0] = new Dozens();
Dozens d = new Dozens();
da[1] = d;
d = null;
da[1] = null;
// do stuff
}
}

Which two are true about the objects created within main(), and which are eligible for garbage
collection when line 14 is reached?

Self Test

A.
B.
C.
D.
E.
F.
G.

Three objects were created
Four objects were created
Five objects were created
Zero objects are eligible for GC
One object is eligible for GC
Two objects are eligible for GC
Three objects are eligible for GC

7. Given:
public class Tailor {
public static void main(String[] args) {
byte[][] ba = {{1,2,3,4}, {1,2,3}};
System.out.println(ba[1].length + " " + ba.length);
}
}

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:
3. public class Theory {
4.
public static void main(String[] args) {
5.
String s1 = "abc";
6.
String s2 = s1;
7.
s1 += "d";
8.
System.out.println(s1 + " " + s2 + " " + (s1==s2));
9.
10.
StringBuilder sb1 = new StringBuilder("abc");
11.
StringBuilder sb2 = sb1;
12.
sb1.append("d");
13.
System.out.println(sb1 + " " + sb2 + " " + (sb1==sb2));
14.
}
15. }

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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 is abcd abc false
E. The second line of output is abcd abc false
F. The second line of output is abcd abcd true
G. The second line of output is abcd abcd false
9. Given:
public class Mounds {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
String s = new String();
for(int i = 0; i < 1000; i++) {
s = " " + i;
sb.append(s);
}
// done with loop
}
}

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:
3. class Box {
4.
int size;
5.
Box(int s) { size = s; }
6. }
7. public class Laser {
8.
public static void main(String[] args) {
9.
Box b1 = new Box(5);
10.
Box[] ba = go(b1, new Box(6));
11.
ba[0] = b1;

Self Test

12.
for(Box b : ba) System.out.print(b.size + " ");
13.
}
14.
static Box[] go(Box b1, Box b2) {
15.
b1.size = 4;
16.
Box[] ma = {b2, b1};
17.
return ma;
18.
}
19. }

What is the result?
A. 4 4
B. 5 4
C. 6 4
D. 4 5
E. 5 5
F. Compilation fails
11. Given:
public class Hedges {
public static void main(String[] args) {
String s = "JAVA";
s = s + "rocks";
s = s.substring(4,8);
s.toUpperCase();
System.out.println(s);
}
}

What is the result?
A. JAVA
B. JAVAROCKS
C. rocks
D. rock
E. ROCKS
F. ROCK
G. Compilation fails

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12. Given:
1. import java.util.*;
2. class Fortress {
3.
private String name;
4.
private ArrayList list;
5.
Fortress() { list = new ArrayList(); }
6.
7.
String getName() { return name; }
8.
void addToList(int x) { list.add(x); }
9.
ArrayList getList() { return list; }
10. }

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

Self Test Answers

305

SELF TEST 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 2.6 and 2.7)

2. ☑ D is correct. The horses array's first four elements contain Strings, 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 2.7 and 4.1)
3. ☑ E is correct. The Unicode declaration must be enclosed in single quotes: '\u004e'. If this
were done, the answer would be A, but knowing 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. A ClassCastException is thrown at line 7 because o1 refers to an int[][],
not an int[]. If line 7 were removed, the output would be 4.
☐
✗ A, B, D, E, F, and G are incorrect based on the above. (OCA Objectives 4.2 and 7.4)
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 last add() 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 4.3)
6. ☑ C and F are correct. da refers to an object of type "Dozens array" and each Dozens 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 of ba is 2 because
it contains two, 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. Although String objects are immutable, references to Strings
are mutable. The code s1 += "d"; creates a new String object. StringBuilder objects
are mutable, so the append() is changing the single StringBuilder object to which both
StringBuilder references refer.
☐
✗ A, B, C, E, and G are incorrect based on the above. (OCA Objectives 2.6 and 2.7)
9. ☑ B is correct. StringBuilders are mutable, so all of the append() invocations are acting
upon the same StringBuilder object over and over. Strings, however, are immutable, so
every String concatenation operation results in a new String 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 2.6 and 2.7)

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10. ☑ A is correct. Although main()'s b1 is a different reference variable than go()'s b1, they
refer to the same Box object.
☐
✗ B, C, D, E, and F are incorrect based on the above. (OCA Objectives 4.1, 6.1, and 6.8)
11. ☑ D is correct. The substring() 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 new String object that is instantly lost. The toUpperCase() invocation
does NOT affect the String referred to by s.
☐
✗ A, B, C, E, F, and G are incorrect based on the above. (OCA Objectives 2.6 and 2.7)
12. ☑ F is correct. When encapsulating a mutable object like an ArrayList, 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 6.7)

6
Flow Control
and Exceptions

CERTIFICATION OBJECTIVES

•
•
•
•

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

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C

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 the switch statement are types of conditional/decision
controls that allow your program to behave differently at a "fork in the road,"
depending on the result of a logical test. Java also provides three different looping
constructs—for, while, and do—so you can execute the same code over and over
again depending on some condition being true. Exceptions give you a clean, simple
way to organize code that deals with problems that might crop up at runtime.
With these tools, you can build a robust program that can handle any logical
situation with grace. Expect to see a wide range of questions on the exam that
include flow control as part of the question code, even on questions that aren't
testing your knowledge of flow control.

CERTIFICATION OBJECTIVE

Using if and switch Statements (OCA Objectives
3.4 and 3.5—also Upgrade Objective 1.1)
3.4

Create if and if-else 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 an if statement is as follows:

Using if and switch Statements (OCA Objectives 3.4 and 3.5—also Upgrade Objective 1.1)

309

if (booleanExpression) {
System.out.println("Inside if statement");
}

The expression in parentheses must evaluate to (a boolean) true or false.
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:
if (x > 3) {
System.out.println("x is greater than 3");
} else {
System.out.println("x is not greater than 3");
}

The else block is optional, so you can also use the following:
if (x > 3) {
y = 2;
}
z += 8;
a = y + x;

The preceding code will assign 2 to y if the test succeeds (meaning x 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):
if (x > 3)
y = 2;
z += 8;
a = y + x;

// bad practice, but seen on the exam

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
"If x 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:
if (x > 3)
y = 2;
z += 8;
a = y + x;

You might have a need to nest if-else statements (although, again, it's not
recommended for readability, so nested if tests should be kept to a minimum). You
can set up an if-else statement to test for multiple conditions. The following

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example uses two conditions so that if the first test fails, we want to perform a
second test before deciding what to do:
if (price < 300) {
buyProduct();
} else {
if (price < 400) {
getApproval();
}
else {
dontBuyProduct();
}
}

This brings up the other if-else construct, the if, else if, else. The preceding
code could (and should) be rewritten like this:
if (price < 300) {
buyProduct();
} else if (price < 400) {
getApproval();
} else {
dontBuyProduct();
}

There are a couple of rules for using else and else if:
■ You can have zero or one else for a given if, and it must come after any
else ifs.

■ You can have zero to many else ifs for a given if and they must come

before the (optional) else.
■ Once an else if succeeds, none of the remaining else 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 multiple else ifs:
int x = 1;
if ( x == 3 ) { }
else if (x < 4) {System.out.println("<4"); }
else if (x < 2) {System.out.println("<2"); }
else { System.out.println("else"); }

It produces this output:
<4

(Notice that even though the second else if is true, it is never reached.)

Using if and switch Statements (OCA Objectives 3.4 and 3.5—also Upgrade Objective 1.1)

311

Sometimes you can have a problem figuring out which if your else should pair
with, as follows:
if (exam.done())
if (exam.getScore() < 0.61)
System.out.println("Try again.");
// Which if does this belong to?
else System.out.println("Java master!");

We intentionally left out the indenting in this piece of code so it doesn't give clues
as to which if statement the else belongs to. Did you figure it out? Java law decrees
that an else clause belongs to the innermost if statement to which it might
possibly belong (in other words, the closest preceding if that doesn't have an else).
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:
if (exam.done())
if (exam.getScore() < 0.61)
System.out.println("Try again.");
// Which if does this belong to?
else
System.out.println("Java master!");

Following our coding conventions by using curly braces, it would be even easier
to read:
if (exam.done()) {
if (exam.getScore() < 0.61) {
System.out.println("Try again.");
// Which if does this belong to?
} else {
System.out.println("Java master!");
}
}

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 intentionally
indent in a misleading way. Pay close attention for misdirection like the following:
if (exam.done())
if (exam.getScore() < 0.61)
System.out.println("Try again.");
else
System.out.println("Java master!"); // Hmmmmm… now where does
// it belong?

Of course, the preceding code is exactly the same as the previous two examples,
except for the way it looks.

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Legal Expressions for if Statements
The expression in an if statement must be a boolean expression. Any expression
that resolves to a boolean is fine, and some of the expressions can be complex.
Assume doStuff() returns true,
int y = 5;
int x = 2;
if (((x > 3) && (y < 2)) | doStuff()) {
System.out.println("true");
}

which prints
true

You can read the preceding code as, "If both (x > 3) and (y < 2) are true, or if the
result of doStuff() is true, then print true." So, basically, if just doStuff() alone
is true, we'll still get true. If doStuff() is false, though, then both (x > 3) and
(y < 2) will have to be true in order to print true. The preceding code is even
more complex if you leave off one set of parentheses as follows:
int y = 5;
int x = 2;
if ((x > 3) && (y < 2) | doStuff()) {
System.out.println("true");
}

This now prints…nothing! Because the preceding code (with one less set of
parentheses) evaluates as though you were saying, "If (x > 3) is true, and either (y
< 2) or the result of doStuff() is true, then print true. So if (x > 3) is not true,
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 an if 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:
int trueInt = 1;
int falseInt = 0;
if (trueInt)
if (trueInt == true)
if (1)
if (falseInt == false)
if (trueInt == 1)
if (falseInt == 0)

//
//
//
//
//
//

illegal
illegal
illegal
illegal
legal
legal

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313

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:
boolean boo = false;
if (boo = true) { }

You might think one of three things:
1.
2.

The code compiles and runs fine, and the if test fails because boo is false.
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 because boo is SET to
true (rather than TESTED for true) 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 of true. 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:
int x = 3;
if (x = 5) { }

// Won't compile because x is not a boolean!

Because if tests require boolean expressions, you need to be really solid on both logical
operators and if test syntax and semantics.

switch Statements (OCA, OCP, and Upgrade Topic)
You've seen how if 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 following if-else if statement
to the equivalently performing switch statement:
int x = 3;
if(x == 1) {
System.out.println("x equals 1");
}
else if(x == 2) {
System.out.println("x equals 2");
}
else {
System.out.println("No idea what x is");
}

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Now let's see the same functionality represented in a switch construct:
int x = 3;
switch (x) {
case 1:
System.out.println("x equals 1");
break;
case 2:
System.out.println("x equals 2");
break;
default:
System.out.println("No idea what x is");
}

Note: The reason this switch 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 a switch statement will execute.

Legal Expressions for switch and case
The general form of the switch statement is
switch (expression) {
case constant1: code block
case constant2: code block
default: code block
}

A switch's expression must evaluate to a char, byte, short, int, an enum (as of
Java 5), and a String (as of Java 7). That means if you're not using an enum or a
String, only variables and values that can be automatically promoted (in other
words, implicitly cast) to an int are acceptable. You won't be able to compile if you
use anything else, including the remaining numeric types of long, float, and
double.
Note: For OCA candidates, enums are not covered on your exam, and you won't
encounter any questions related to switch statements that use enums.
A case 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 or final variable that is immediately initialized with a
literal value. It is not enough to be final; it must be a compile time constant. Here's
an example:

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315

final int a = 1;
final int b;
b = 2;
int x = 0;
switch (x) {
case a:
// ok
case b:
// compiler error

Also, the switch can only check for equality. This means that 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 an int.
String s = "xyz";
switch (s.length()) {
case 1:
System.out.println("length is one");
break;
case 2:
System.out.println("length is two");
break;
case 3:
System.out.println("length is three");
break;
default:
System.out.println("no match");
}

One other rule you might not expect involves the question, "What happens if I
switch on a variable smaller than an int?" Look at the following switch:
byte g = 2;
switch(g) {
case 23:
case 128:
}

This code won't compile. Although the switch argument is legal—a byte is
implicitly cast to an int—the second case argument (128) is too large for a byte,
and the compiler knows it! Attempting to compile the preceding example gives you
an error something like this:
Test.java:6: possible loss of precision
found
: int
required: byte
case 128:
^

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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:
int temp = 90;
switch(temp) {
case 80 : System.out.println("80");
case 80 : System.out.println("80");
// won't compile!
case 90 : System.out.println("90");
default : System.out.println("default");
}

It is legal to leverage the power of boxing in a switch expression. For instance,
the following is legal:
switch(new Integer(4)) {
case 4: System.out.println("boxing is OK");
}

Look for any violation of the rules for switch and case arguments. For
example, you might find illegal examples like the following snippets:
switch(x) {
case 0 {
y = 7;
}
}
switch(x) {
0: { }
1: { }
}

In the first example, the case uses a curly brace and omits the colon.The second example
omits the keyword case.

An Intro to String "equality"
As we've been discussing, the operation of switch 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

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317

intend to take the OCP exam, we'll spend a lot of time discussing "object equality"
in Part II. For you OCA candidates, all you have to know is that for a switch
statement, two Strings will be considered "equal" if they have the same casesensitive sequence of characters. For example, in the following partial switch
statement, the expression would match the case:
String s = "Monday";
switch(s) {
case "Monday":
// matches!

But the following would NOT match:
String s = "MONDAY";
switch(s) {
case "Monday":
// Strings are case-sensitive, DOES 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 from the top down, and the first case constant that
matches the switch's expression is the execution 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 a
break statement) too! The following example uses a String in a case statement:
class SwitchString {
public static void main(String [] args) {
String s = "green";
switch(s) {
case "red": System.out.print("red ");
case "green": System.out.print("green ");
case "blue": System.out.print("blue ");
default: System.out.println("done");
}
}
}

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 the switch block to

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the next statement after the switch. If break is omitted, the program just keeps
executing the remaining case blocks until either a break is found or the switch
statement ends. Examine the following code:
int x = 1;
switch(x) {
case 1: System.out.println("x is one");
case 2: System.out.println("x is two");
case 3: System.out.println("x is three");
}
System.out.println("out of the switch");

The code will print the following:
x is one
x is two
x is three
out of the switch

This combination occurs because the code didn't hit a break statement;
execution just kept dropping down through each case 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 matching case is simply your entry point into the
switch block! In other words, you must not think of it as, "Find the matching case,
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 a break into each case as follows:
int x = 1;
switch(x) {
case 1: {
System.out.println("x is one"); break;
}
case 2: {
System.out.println("x is two"); break;
}
case 3: {
System.out.println("x is two"); break;
}
}
System.out.println("out of the switch");

Running the preceding code, now that we've added the break statements, will print this:
x is one
out of the switch

And that's it. We entered into the switch block at case 1. Because it matched the
switch() argument, we got the println statement and then hit the break and
jumped to the end of the switch.

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An interesting example of this fall-through logic is shown in the following code:
int x = someNumberBetweenOneAndTen;
switch (x) {
case 2:
case 4:
case 6:
case 8:
case 10: {
System.out.println("x is an even number");
}
}

break;

This switch statement will print x is an even number or nothing, depending on
whether the number is between one and ten and is odd or even. For example, if x is
4, execution will begin at case 4, but then fall down through 6, 8, and 10, where it
prints and then breaks. The break at case 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.

The Default Case
What if, using the preceding code, you wanted to print x 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 last case 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:
int x = someNumberBetweenOneAndTen;
switch (x) {
case 2:
case 4:
case 6:
case 8:
case 10: {
System.out.println("x is an even number");
break;
}
default: System.out.println("x is an odd number");
}

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The default case doesn't have to come at the end of the switch. Look
for it in strange places such as the following:
int x = 2;
switch (x) {
case 2: System.out.println("2");
default: System.out.println("default");
case 3: System.out.println("3");
case 4: System.out.println("4");
}

Running the preceding code prints this:
2
default
3
4

And if we modify it so that the only match is the default case, like this,
int x = 7;
switch (x) {
case 2: System.out.println("2");
default: System.out.println("default");
case 3: System.out.println("3");
case 4: System.out.println("4");
}

then running the preceding code prints this:
default
3
4

The rule to remember is that default works just like any other case for fall-through!

EXERCISE 6-1
Creating a switch-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.

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321

■ Make sure a char variable is declared before the switch statement.
■ Each case 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. A
while statement looks like this:
while (expression) {
// do stuff
}

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Or this:
int x = 2;
while(x == 2) {
System.out.println(x);
++x;
}

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 a while 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 statement after
the while loop. Look at the following example:
int x = 8;
while (x > 8) {
System.out.println("in the loop");
x = 10;
}
System.out.println("past the loop");

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.

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323

Using do Loops
The do loop is similar to the while loop, except that 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 a do loop in action:
do {
System.out.println("Inside loop");
} while(false);

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 illegal while expressions:
int x = 1;
while (x) { }
while (x = 5) { }
while (x == 5) { }
while (true) { }

//
//
//
//
//

Won't compile; x is not a boolean
Won't compile; resolves to 5
(as the result of assignment)
Legal, equality test
Legal

Using for Loops
As of Java 5, the for loop took on a second structure. We'll call the old style of for
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 with for-in. The terms for-in, for-each,
and "enhanced for" all refer to the same Java construct.
The basic for 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.

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The Basic for 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:
for (/*Initialization*/ ; /*Condition*/ ;
/* loop body */
}

/* Iteration */) {

for (int i = 0; i<10; i++) {
System.out.println("i is " + i);
}

The Basic for 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 happens before anything else in a for 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 happens 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:
for (int x = 1; x < 2; x++) {
System.out.println(x); // Legal
}
System.out.println(x);
// Not Legal! x is now out of scope
// and can't be accessed.

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325

If you try to compile this, you'll get something like this:
Test.java:19: cannot resolve symbol
symbol : variable x
location: class Test
System.out.println(x);
^

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:
for (int x = 0; (x > 5), (y < 2); x++) { } // too many
// expressions

The compiler will let you know the problem:
TestLong.java:20: ';' expected
for (int x = 0; (x > 5), (y < 2); x++) { }
^

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: Iteration Expression
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:
for (int x = 0; x < 1; x++) {
// body code that doesn't change the value of x
}

This loop executes just once. The first time into the loop, x is set to 0, then x 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, incrementing x by 1.

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Next, the conditional test is checked, and since the result is now false, execution
jumps to below the for loop and continues on.
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:
static boolean doStuff() {
for (int x = 0; x < 3; x++) {
System.out.println("in for loop");
return true;
}
return true;
}

Running this code produces
in for loop

The statement prints only once, because a return 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 6-1 lists the causes and results of abrupt loop termination.

Basic for Loop: for Loop Issues
None of the three sections of the for declaration are required! The following
example is perfectly legal (although not necessarily good practice):
for( ; ; ) {
System.out.println("Inside an endless loop");
}

In this example, all the declaration parts are left out, so the for loop will act like an
endless loop.
TABLE 6-1

Causes of Early
Loop Termination

Code in Loop

What Happens

break

Execution jumps immediately to the first statement after the
for loop.

return

Execution jumps immediately back to the calling method.

System.exit()

All program execution stops; the VM shuts down.

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327

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:
int i = 0;
for (;i<10;) {
i++;
// do some other work
}

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.
for (int i = 0,j = 0; (i<10) && (j<10); i++, j++) {
System.out.println("i is " + i + " j is " +j);
}

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:
int x = 3;
for (x = 12; x < 20; x++) { }
System.out.println(x);

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 the for 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

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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:
int b = 3;
for (int a = 1;
b = b - a;
}

b != 1; System.out.println("iterate")) {

The preceding code prints
iterate
iterate

Many questions in the Java 7 exams list "Compilation fails" and "An
exception occurs at runtime" as possible answers.This makes 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.

The Enhanced 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. In Chapter 11 we'll revisit the
enhanced for as we discuss collections—where the enhanced for really comes into
its own.
Instead of having three components, the enhanced for has two. Let's loop
through an array the basic (old) way, and then using the enhanced for:
int [] a = {1,2,3,4};
for(int x = 0; x < a.length; x++)
System.out.print(a[x]);
for(int n : a)
System.out.print(n);

This produces the following output:
12341234

// basic for loop
// enhanced for loop

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329

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 the for 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:
int x;
long x2;
long [] la = {7L,
int [][] twoDee =
String [] sNums =
Animal [] animals

8L, 9L};
{{1,2,3}, {4,5,6}, {7,8,9}};
{"one", "two", "three"};
= {new Dog(), new Cat()};

// legal 'for' declarations
for(long y : la ) ;
//
for(int[] n : twoDee) ;
//
for(int n2 : twoDee[2]) ; //
for(String s : sNums) ;
//
for(Object o : sNums) ;
//
//
for(Animal a : animals) ; //
//

loop thru an array of longs
loop thru the array of arrays
loop thru the 3rd sub-array
loop thru the array of Strings
set an Object reference to
each String
set an Animal reference to each
element

// ILLEGAL 'for' declarations
for(x2 : la) ;
//
for(int x2 : twoDee) ;
//
for(int x3 : la) ;
//
for(Dog d : animals) ;
//

x2 is already declared
can't stuff an array into an int
can't stuff a long into an int
you might get a Cat!

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 of break
and continue apply to both the basic and enhanced for loops.

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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 using break or
continue, you'll do an if test within the loop, and if some condition becomes true
(or false 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 a continue statement with a for loop, you need to consider the
effects that continue has on the loop iteration. Examine the following code:
for (int i = 0; i < 10; i++) {
System.out.println("Inside loop");
continue;
}

The question is, is this an endless loop? The answer is no. When the continue
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 an if test as follows:
for (int i = 0; i < 10; i++) {
System.out.println("Inside loop");
if (foo.doStuff() == 5) {
continue;
}
// more loop code, that won't be reached when the above if
// test is true
}

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331

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, a break 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:
boolean problem = true;
while (true) {
if (problem) {
System.out.println("There was a problem");
break;
}
}
// next line of code

In the previous example, the break statement is unlabeled. The following is an
example of an unlabeled continue statement:
while (!EOF) {
// read a field from a file
if (wrongField) {
continue;
}

// move to the next field in the file
// otherwise do other stuff with the field

}

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 with break and

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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. A break 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:
foo:
for (int x = 3; x < 20; x++) {
while(y > 7) {
y--;
}
}

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:
boolean isTrue = true;
outer:
for(int i=0; i<5; i++) {
while (isTrue) {
System.out.println("Hello");
break outer;
}
// end of inner while loop
System.out.println("Outer loop."); // Won't print
}
// end of outer for loop
System.out.println("Good-Bye");

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 print out Good-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 substituting continue for break:

Creating Loops Constructs (OCA Objectives 5.1, 5.2, 5.3, 5.4, and 5.5)

333

outer:
for (int i=0; i<5; i++) {
for (int j=0; j<5; j++) {
System.out.println("Hello");
continue outer;
}
// end of inner loop
System.out.println("outer"); // Never prints
}
System.out.println("Good-Bye");

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 6-2
Creating a Labeled while Loop
Try creating a labeled while loop. Make the label outer and provide a condition to
check whether a variable age is less than or equal to 21. Within the loop, increment
age by 1. Every time the program goes through the loop, check whether age 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 sure age 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.

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CERTIFICATION OBJECTIVE

Handling Exceptions
(OCA Objectives 8.1, 8.2, 8.3, and 8.4)
8.1

Differentiate among checked exceptions, RuntimeExceptions, and errors.

8.2

Create a try-catch block and determine how exceptions alter normal program flow.

8.3

Describe what exceptions are used for in Java.

8.4

Invoke a method that throws an exception.

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. Java arms developers with an elegant mechanism for handling
errors that produces efficient and organized error-handling code: exception handling.
Exception handling allows developers to detect errors easily without writing
special code to test return values. Even better, 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 objectives, we split exception handling into two
chapters. This chapter will give you the basics—plenty to handle the OCA exam.
Chapter 7 (which also marks the beginning of the OCP part of the book) will pick
up where we left off by discussing the new Java 7 exception handling features.

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

335

Catching an Exception Using try and catch
Before we begin, let's introduce some terminology. The term "exception" 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. The try 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 more catch
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:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

try {
// This is the first line of the "guarded region"
// that is governed by the try keyword.
// Put code here that might cause some kind of exception.
// We may have many code lines here or just one.
}
catch(MyFirstException) {
// Put code here that handles this exception.
// This is the next line of the exception handler.
// This is the last line of the exception handler.
}
catch(MySecondException) {
// Put code here that handles this exception
}
// Some other unguarded (normal, non-risky) code begins here

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 more catch blocks, they must
immediately follow the try block. Additionally, the catch blocks must all follow

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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
entire catch block runs, and then execution will transfer to line 15 and continue.
Note that if an exception occurred on, say, line 3 of the try block, the rest of the
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:
try {
getTheFileFromOverNetwork
readFromTheFileAndPopulateTable
}
catch(CantGetFileFromNetwork) {
displayNetworkErrorMessage
}

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
our try 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
Although try 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

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

337

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 the try 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.
A finally 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
proper catch block completes. Let's look at another pseudocode example:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:

try {
// This is the first line of the "guarded region".
}
catch(MyFirstException) {
// Put code here that handles this exception
}
catch(MySecondException) {
// Put code here that handles this exception
}
finally {
// Put code here to release any resource we
// allocated in the try clause
}
// More code here

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 the try 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 that finally always runs. If an exception is thrown,
finally runs. If an exception is not thrown, finally runs. If the exception is

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caught, finally runs. If the exception is not caught, finally runs. Later we'll look
at the few scenarios in which finally 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 your
try 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:
try {
// do stuff
} finally {
// clean up
}

The following legal code demonstrates a try, catch, and finally:
try {
// do stuff
} catch (SomeException ex) {
// do exception handling
} finally {
// clean up
}

The following ILLEGAL code demonstrates a try without a catch or finally:
try {
// do stuff
}
// need a catch or finally here
System.out.println("out of try block");

The following ILLEGAL code demonstrates a misplaced catch block:
try {
// do stuff
}
// can't have code between try/catch
System.out.println("out of try block");
catch(Exception ex) { }

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

339

It is illegal to use a try clause without either a catch clause or a finally
clause. A try clause by itself will result in a compiler error. Any catch clauses must
immediately follow the try block. Any finally clause must immediately follow the last
catch 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 a catch clause for every possible exception that could be
thrown from the corresponding try block. In fact, it's doubtful that you could
accomplish such a feat! If a method doesn't provide a catch 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 6-1 illustrates a way to think about how the call stack in Java works.
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

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FIGURE 6-1

The Java method
call stack

Flow Control and Exceptions

1) The call stack while method3() is running.
4
3
2
1

method3()
method2()
method1()
main()

method2 invokes method3
method1 invokes method2
main invokes method1
main begins

The order in which methods are put on the call stack

2) The call stack after method3() completes
Execution returns to method2()
1
2
3

method2()
method1()
main()

method2() will complete
method1() will complete
main() will complete and the JVM will exit

The order in which methods complete

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:

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

341

class TestEx {
public static void main (String [] args) {
doStuff();
}
static void doStuff() {
doMoreStuff();
}
static void doMoreStuff() {
int x = 5/0; // Can't divide by zero!
// ArithmeticException is thrown here
}
}

It prints out a stack trace something like this:
%java TestEx
Exception in thread "main" java.lang.ArithmeticException: / by zero
at TestEx.doMoreStuff(TestEx.java:10)
at TestEx.doStuff(TestEx.java:7)
at TestEx.main(TestEx.java:3)

EXERCISE 6-3
Propagating and Catching an 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. A finally
statement will be included to indicate that the program has completed. The method
that main() 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. In reverse, 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.

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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.
String reverseStr = "";
for(int i=s.length()-1;i>=0;--i) {
reverseStr += s.charAt(i);
}
return reverseStr;

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 when main() 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 exception to this rule. Every exception is an instance of a class
that has class Exception in its inheritance hierarchy. In other words, exceptions are
always some subclass of java.lang.Exception.
When an exception is thrown, an object of a particular Exception subtype is
instantiated and handed to the exception handler as an argument to the catch
clause. An actual catch clause looks like this:
try {
// some code here
}
catch (ArrayIndexOutOfBoundsException e) {
e.printStackTrace();
}

In this example, e is an instance of the ArrayIndexOutOfBoundsException class.
As with any other object, you can call its methods.

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

343

Exception Hierarchy
All exception classes are subtypes of class Exception. This class derives from the
class Throwable (which derives from the class Object). Figure 6-2 shows the
hierarchy for the exception classes.
As you can see, there are two subclasses that derive from Throwable: Exception
and Error. Classes that derive from Error 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 an Error, 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 class Exception.
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 6-2 also
shows a subtype of Exception 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.
FIGURE 6-2

Object

Exception class
hierarchy
Throwable

Error

Exception

RuntimeException

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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 that
Exception, 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 the catch clause has
no subclasses, then only the specified class of exception will be caught. However, if
the class specified in the catch clause does have subclasses, any exception object
that subclasses the specified class will be caught as well.
For example, class IndexOutOfBoundsException 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:

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

345

try {
// Some code here that can throw a boundary exception
}
catch (IndexOutOfBoundsException e) {
e.printStackTrace();
}

If any code in the try block throws ArrayIndexOutOfBoundsException 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 your catch 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 separate catch clause for
each exception type of interest.
Resist the temptation to write a single catchall exception handler such as the
following:
try {
// some code
}
catch (Exception e) {
e.printStackTrace();
}

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 available
catch clauses from the top down) a catch 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.

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1: import java.io.*;
2: public class ReadData {
3:
public static void main(String args[]) {
4:
try {
5:
RandomAccessFile raf =
6:
new RandomAccessFile("myfile.txt", "r");
7:
byte b[] = new byte[1000];
8:
raf.readFully(b, 0, 1000);
9:
}
10:
catch(FileNotFoundException e) {
11:
System.err.println("File not found");
12:
System.err.println(e.getMessage());
13:
e.printStackTrace();
14:
}
15:
catch(IOException e) {
16:
System.err.println("IO Error");
17:
System.err.println(e.toString());
18:
e.printStackTrace();
19:
}
20:
}
21: }

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 of
IOException. 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 of IOException. Therefore, we could handle
it in the catch clause that catches all subtypes of IOException, but then we would
have to test the exception to determine whether it was a FileNotFoundException.
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 another IOException—perhaps
EOFException, which is a subclass of IOException—it will be handled by the
catch 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:

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

347

try {
// do risky IO things
} catch (IOException e) {
// handle general IOExceptions
} catch (FileNotFoundException ex) {
// handle just FileNotFoundException
}

You'll get a compiler error something like this:
TestEx.java:15: exception java.io.FileNotFoundException has
already been caught
} catch (FileNotFoundException ex) {
^

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 different kinds of balls. An IOException mitt is large enough
and flexible enough to catch any type of IOException. 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, that FileNotFoundException
ball will never get to him, since 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
the catch 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 be declared (unless
the exceptions are subclasses of RuntimeException). 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:
void myFunction() throws MyException1, MyException2 {
// code for the method here
}

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.

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(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 a try/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 of
RuntimeException) 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:
Each method must either handle all checked exceptions by supplying a catch
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:
void doStuff() {
doMore();
}
void doMore() {
throw new IOException();
}

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349

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 a RuntimeException, the calling method is under no
obligation to handle or declare it. RuntimeException, Error, and all of their
subtypes are unchecked exceptions, and unchecked exceptions do not have to be
specified or handled. Here is an example:
import java.io.*;
class Test {
public int myMethod1() throws EOFException {
return myMethod2();
}
public int myMethod2() throws EOFException {
// code that actually could throw the exception goes here
return 1;
}
}

Let's look at myMethod1(). Because EOFException subclasses IOException, 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():
public void myMethod3() {
// code that could throw a NullPointerException goes here
}

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According to the comment, this method can throw a NullPointerException.
Because RuntimeException is the superclass of NullPointerException, it is an
unchecked exception and need not be declared. We can see that myMethod3() does
not declare any exceptions.
Runtime exceptions are referred to as unchecked exceptions. All other exceptions
are checked exceptions, and they don't derive from java.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 that 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:
class TestEx {
void doStuff() {
throw new MyException();
}
}

// Throw a checked exception

The preceding code upsets the compiler:
TestEx.java:6: unreported exception MyException; must be caught or
declared to be thrown
throw new MyException();
^

You need to know how an Error compares with checked and unchecked
exceptions. Objects of type Error are not Exception objects, although they do
represent exceptional conditions. Both Exception and Error share a common
superclass, Throwable; thus both can be thrown using the throw keyword. When an
Error or a subclass of Error (like RuntimeException) is thrown, it's unchecked.
You are not required to catch Error objects or Error subtypes. You can also throw

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

351

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 of RuntimeException, which are unchecked exceptions.
Be ready to spot methods that don't follow the "handle or declare" rule, such as this:
class MyException extends Exception {
void someMethod () {
doStuff();
}
void doStuff() throws MyException {
try {
throw new MyException();
}
catch(MyException me) {
throw me;
}
}
}

You need to recognize that this code won't compile. If you try, you'll get this:
MyException.java:3: unreported exception MyException;
must be caught or declared to be thrown
doStuff();
^

Notice that someMethod() fails either to handle or declare the exception that can be
thrown by doStuff().

an Error yourself (although, other than AssertionError, 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 an OutOfMemoryError? 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

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one of these babies. Just remember, though, that you can! The following compiles
just fine:
class TestEx {
public static void main (String [] args) {
badMethod();
}
static void badMethod() { // No need to declare an Error
doStuff();
}
static void doStuff() {
// No need to declare an Error
try {
throw new Error();
}
catch(Error me) {
throw me;
// We catch it, but then rethrow it
}
}
}

If we were throwing a checked exception rather than Error, then the doStuff()
method would need to declare the exception. But remember, since Error is not a
subtype of Exception, 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:
try {
callBadMethod();
} catch (Exception ex) { }

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.

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)

353

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 a catch clause that does this:
catch(IOException e) {
// Do things, then if you decide you can't handle it…
throw e;
}

All other catch 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 a catch
clause, you must also declare that exception! In other words, you must handle and
declare, as opposed to handle or declare. The following example is illegal:
public void doStuff() {
try {
// risky IO things
} catch(IOException ex) {
// can't handle it
throw ex; // Can't throw it unless you declare it
}
}

In the preceding code, the doStuff() method is clearly able to throw a checked
exception—in this case an IOException—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 6-4
Creating an 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 from Exception), and because it extends
Exception, 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.

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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 that String 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 if main() 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 our 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 and categories.

Exception handling is another area that the exam creation team decided to
expand for the OCJP 5, OCJP 6, and both Java 7 exams. 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.

Common Exceptions and Errors (OCA Objective 8.5)

355

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, the NullPointerException. As we saw
in earlier chapters, this exception occurs when you attempt to access an object using
a reference variable with a current value of null. There's no way that the compiler
can hope to find these problems before runtime. Take a look at the following:
class NPE {
static String s;
public static void main(String [] args) {
System.out.println(s.length());
}
}

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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 a
NullPointerException when it tries to invoke the length() method.
Earlier in this chapter we discussed the call stack. As you recall, we used the
convention that main() 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:
void go() {
go();
}

// recursion gone bad

As you can see, if you ever make the mistake of invoking the go() method, your
program will fall into a black hole—go() invoking go() invoking go(), 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:
Created by an application and/or API developer.
For instance, many classes in the Java API have methods that take String
arguments and convert these Strings into numeric primitives. A good example of
these classes are the so-called "wrapper classes" that OCP candidates will study in
Chapter 8. Even though we haven't talked 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 a String that could not be

Common Exceptions and Errors (OCA Objective 8.5)

357

converted into a number, the method should throw a NumberFormatException.
The partially implemented code might look something like this:
int parseInt(String s) throws NumberFormatException {
boolean parseSuccess = false;
int result = 0;
// do complicated parsing
if (!parseSuccess)
// if the parsing failed
throw new NumberFormatException();
return result;
}

Other examples of programmatic exceptions include an AssertionError (okay,
it's not an exception, but it IS thrown programmatically), and throwing an
IllegalArgumentException. In fact, our mythical API developer could have used
IllegalArgumentException for her parseInt() method. But it turns out that
NumberFormatException extends IllegalArgumentException and is a little
more precise, so in this case, using NumberFormatException 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 Objective 8.5 does not list specific exceptions and errors; it says "recognize
common exceptions…." Table 6-2 summarizes the ten exceptions and errors that are
a part of the SCJP 6 exam; it will cover OCA Objective 8.5, too.

End of Part I—OCA
Barring our standard end-of-chapter stuff, such as mock exam questions, you've
reached the end of the OCA part of the book. If you've studied these six chapters
carefully, and then taken and reviewed the end-of-chapter mock exams and the
OCA master exams and done well on them, we're confident that you're a little bit
over-prepared for the official Oracle OCA exam. (Not "way" over-prepared—just a
little.) Good luck, and we hope to see you back here for Part II, Chapter 7, in which
we'll explore the exception handling features added in Java 7.

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Flow Control and Exceptions

Descriptions and Sources of Common Exceptions

Exception

Description

ArrayIndexOutOfBoundsException

Thrown when attempting to access an
By the JVM
array with an invalid index value (either
negative or beyond the length of the
array).

(Chapter 5)

Typically Thrown

Thrown when attempting to cast a
reference variable to a type that fails the
IS-A test.

By the JVM

IllegalArgumentException

Thrown when a method receives an
argument formatted differently than the
method expects.

Programmatically

IllegalStateException

Thrown when the state of the
environment doesn't match the
operation being attempted—for
example, using a scanner that's been
closed.

Programmatically

NullPointerException

Thrown when attempting to invoke a
method on, or access a property from, a
reference variable whose current value
is null.

By the JVM

ClassCastException

(Chapter 2)

(Chapter 3)

NumberFormatException

(this chapter)

Thrown when a method that converts a Programmatically
String to a number receives a String
that it cannot convert.

AssertionError

Thrown when an assert statement's
boolean test returns false.

Programmatically

ExceptionInInitializerError

Thrown when attempting to initialize a
static variable or an initialization block.

By the JVM

Typically thrown when a method
recurses too deeply. (Each invocation is
added to the stack.)

By the JVM

Thrown when the JVM can't find a
class it needs, because of a commandline error, a classpath issue, or a missing
.class file.

By the JVM

(Chapter 2)
StackOverflowError

(this chapter)
NoClassDefFoundError

Certification Summary

359

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 about if and switch
statements. The if statement evaluates one or more expressions to a boolean result.
If the result is true, the program will execute the code in the block that is
encompassed by the if. If an else 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 the if statement will execute.
You also learned that the switch statement can be used to replace multiple
if-else statements. The switch statement can evaluate integer primitive types that
can be implicitly cast to an int (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 corresponding case statement. If a match is found, execution will begin
at the matching case and continue on from there, executing code in all the remaining
case 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 the for
loop and the while loop, the expression will have 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 the switch statement. When there is a match
between the switch expression and the case constant, the code following the case
constant will be performed. To stop execution, a break is needed.

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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 that the main code doesn't become cluttered by error-checking code.
Another elegant feature allows you to handle similar errors with a single errorhandling block, without code duplication. Also, the error handling can be deferred
to methods further back on the call stack.
You learned that Java's try 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. All catch 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
a try but no catch). Use finally blocks to release system resources and to perform
any cleanup required by the code in the try block. A finally block is not required,
but if there is one, it must immediately follow the last catch. (If there is no catch
block, the finally block must immediately follow the try block.) It's guaranteed to
be called except when the try or catch issues a System.exit().
An exception object is an instance of class Exception 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 an Error, 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

✓

361

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.

Writing Code Using if and switch Statements
(OCA Objectives 3.4 and 3.5)
❑ The only legal expression in an if 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 for boolean
equality (==) tests:
boolean x = false;
if (x = true) { } // an assignment, so x will always be true!

❑ Curly braces are optional for if 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 final variable, or a constant
expression, including an enum or a String. You cannot have a case that
includes a non-final variable or a range of values.
❑ If the condition in a switch statement matches a case constant, execution
will run through all code in the switch following the matching case
statement until a break statement or the end of the switch statement is
encountered. In other words, the matching case is just the entry point into
the case block, but unless there's a break statement, the matching case is
not the only case 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 preceding case matches, the default block will be entered, and 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.

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Writing Code Using Loops (OCA Objectives 5.1, 5.2, 5.3, and 5.4)
❑ A basic for statement has three parts: declaration and/or initialization,
boolean evaluation, and the iteration expression.
❑ If a variable is incremented or evaluated within a basic for loop, it must be
declared before the loop or within the for loop declaration.
❑ A variable declared (not just initialized) within the basic for 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 basic for loop declaration; each initialization must be separated by a
comma.
❑ 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.
❑ You cannot use a number (old C-style language construct) or anything that
does not evaluate to a boolean value as a condition for an if statement or
looping construct. You can't, for example, say if(x), unless x is a boolean
variable.
❑ The do loop will enter the body of the loop at least once, even if the test
condition is not met.

Using break and continue (OCA Objective 5.5)
❑ An unlabeled break statement will cause the current iteration of the
innermost looping construct 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
similar action to occur on the labeled loop, not the innermost loop.

Two-Minute Drill

363

Handling Exceptions (OCA Objectives 8.1, 8.2, 8.3, and 8.4)
❑ Exceptions come in two flavors: checked and unchecked.
❑ Checked exceptions include all subtypes of Exception, 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 using
throws, 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.
❑ If you use an optional finally block, it will always be invoked, regardless
of whether an exception in the corresponding try is thrown or not, and
regardless of whether a thrown exception is caught or not.
❑ 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 create your own exceptions, normally by extending Exception
or one of its subtypes. Your exception will then be considered a checked
exception (unless you are extending from RuntimeException), and the
compiler will enforce the handle or declare rule for that exception.
❑ All catch blocks must be ordered from most specific to most general. If you
have a catch clause for both IOException and Exception, you must put
the catch for IOException first in your code. Otherwise, the IOException
would be caught by catch(Exception e), because a catch argument can
catch the specified exception or any of its subtypes! The compiler will stop
you from defining catch clauses that can never be reached.
❑ Some exceptions are created by programmers, and some by the JVM.

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SELF TEST
1. (Also an Upgrade topic) Given:
public class Flipper {
public static void main(String[] args) {
String o = "-";
switch("FRED".toLowerCase().substring(1,3)) {
case "yellow":
o += "y";
case "red":
o += "r";
case "green":
o += "g";
}
System.out.println(o);
}
}

What is the result?
A. B. -r
C. -rg
D. Compilation fails
E. An exception is thrown at runtime
2. Given:
class Plane {
static String s = "-";
public static void main(String[] args) {
new Plane().s1();
System.out.println(s);
}
void s1() {
try { s2(); }
catch (Exception e) { s += "c"; }
}
void s2() throws Exception {
s3(); s += "2";
s3(); s += "2b";
}
void s3() throws Exception {
throw new Exception();
}
}

Self Test

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:
public class Flip2 {
public static void main(String[] args) {
String o = "-";
String[] sa = new String[4];
for(int i = 0; i < args.length; i++)
sa[i] = args[i];
for(String n: sa) {
switch(n.toLowerCase()) {
case "yellow": o += "y";
case "red":
o += "r";
case "green": o += "g";
}
}
System.out.print(o);
}
}

And given the command-line invocation:
Java Flip2 RED Green YeLLow

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Which are true? (Choose all that apply.)
A. The string rgy will appear somewhere in the output
B. The string rgg will appear somewhere in the output
C. The string gyr will appear somewhere in the output
D. Compilation fails
E. An exception is thrown at runtime
5. Given:
1. class Loopy {
2.
public static void main(String[] args) {
3.
int[] x = {7,6,5,4,3,2,1};
4.
// insert code here
5.
System.out.print(y + " ");
6.
}
7.
}
8. }

Which, inserted independently at line 4, compiles? (Choose all that apply.)
A. for(int y : x) {
B. for(x : int y) {
C. int y = 0; for(y : x) {
D. for(int y=0, z=0; z 5; x++)
10.
if(x > 10000000) x = 10;
11.
break; }
12.
case 1: { int y = 7 * i; break; }
13.
case 2: { Infinity inf = new Beyond();
14.
Beyond b = (Beyond)inf; }
15.
}
16.
}
17. }

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. A ClassCastException might be thrown
C. A StackOverflowError might be thrown
D. A NullPointerException might be thrown
E. An IllegalStateException might be thrown
F. The program might hang without ever completing
G. The program will always complete without exception

Self Test

10. Given:
3. public class Circles {
4.
public static void main(String[] args) {
5.
int[] ia = {1,3,5,7,9};
6.
for(int x : ia) {
7.
for(int j = 0; j < 3; j++) {
8.
if(x > 4 && x < 8) continue;
9.
System.out.print(" " + x);
10.
if(j == 1) break;
11.
continue;
12.
}
13.
continue;
14.
}
15.
}
16. }

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:
3. public class OverAndOver {
4.
static String s = "";
5.
public static void main(String[] args) {
6.
try {
7.
s += "1";
8.
throw new Exception();
9.
} catch (Exception e) { s += "2";
10.
} finally { s += "3"; doStuff(); s += "4";
11.
}
12.
System.out.println(s);
13.
}
14.
static void doStuff() { int x = 0; int y = 7/x; }
15. }

What is the result?
A. 12
B. 13
C. 123
D. 1234

369

370
E.
F.
G.
H.

Chapter 6:

Flow Control and Exceptions

Compilation fails
123 followed by an exception
1234 followed by an exception
An exception is thrown with no other output

12. Given:
3. public class Wind {
4.
public static void main(String[] args) {
5.
foreach:
6.
for(int j=0; j<5; j++) {
7.
for(int k=0; k< 3; k++) {
8.
System.out.print(" " + j);
9.
if(j==3 && k==1) break foreach;
10.
if(j==0 || j==2) break;
11.
}
12.
}
13.
}
14. }

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:
3. public class Gotcha {
4.
public static void main(String[] args) {
5.
// insert code here
6.
7.
}
8.
void go() {
9.
go();
10.
}
11. }

And given the following three code fragments:
I.

new Gotcha().go();

II.

try { new Gotcha().go(); }
catch (Error e) { System.out.println("ouch"); }

III. try { new Gotcha().go(); }
catch (Exception e) { System.out.println("ouch"); }

Self Test

371

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:
String s = "bob";
String[] sa = {"a", "bob"};
final String s2 = "bob";
StringBuilder sb = new StringBuilder("bob");
// switch(sa[1]) {
// switch("b" + "ob") {
// switch(sb.toString()) {

// line 1
// line 2
// line 3

// case "ann":
// case s:
// case s2:
}

// line 4
// line 5
// line 6

;
;
;

And given that the numbered lines will all be tested by un-commenting one switch statement
and one case 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:
1. public class Frisbee {
2.
// insert code here
3.
int x = 0;
4.
System.out.println(7/x);
5.
}
6. }

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And given the following four code fragments:
I.
II.
III.
IV.

public
public
public
public

static
static
static
static

void
void
void
void

main(String[]
main(String[]
main(String[]
main(String[]

args)
args)
args)
args)

{
throws Exception {
throws IOException {
throws RuntimeException {

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:
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.

class MyException extends Exception { }
class Tire {
void doStuff() { }
}
public class Retread extends Tire {
public static void main(String[] args) {
new Retread().doStuff();
}
// insert code here
System.out.println(7/0);
}
}

And given the following four code fragments:
I.
II.
III.
IV.

void
void
void
void

doStuff()
doStuff()
doStuff()
doStuff()

{
throws MyException {
throws RuntimeException {
throws ArithmeticException {

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 of 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

Self Test Answers

373

SELF TEST ANSWERS
1. ☑ A is correct. As of Java 7 the code is legal, but the substring() method's second
argument is exclusive. If the invocation had been substring(1,4), the output would have
been –rg. Note: We hope you won't have too many exam questions that focus on API trivia
like this one. If you knew the switch was legal, give yourself "almost full credit."
☐
✗ B, C, D, and E are incorrect based on the above. (OCA Objectives 2.7 and 3.5, and
Upgrade Objective 1.1)
2. ☑ B is correct. Once s3() throws the exception to s2(), s2() throws it to s1(), and no
more of s2()'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 a NumberFormatException, and
IllegalArgumentException is its superclass (that is, a broader exception).
☐
✗ A, B, E, and F are not in NumberFormatException'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 through sa[], a NullPointerException is
thrown.
☐
✗ A, B, C, and D are incorrect based on the above. (OCA Objectives 3.5, 5.2, and 8.5, and
Upgrade Objective 1.1)
5. ☑ A, D, and F are correct. A is an example of the enhanced for loop. D and F are examples
of the basic for 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 nesting try/catch blocks. As is normal, when an
exception is thrown, the code in the catch block runs, and then the code in the finally
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 3.5)

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Chapter 6:

Flow Control and Exceptions

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 will not cause an
exception.
☐
✗ A, B, C, E, and G are incorrect based on the above. (OCA Objectives 3.5 and 8.4)
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 of String s is 123 at the time that the divide-by-zero
exception is thrown, but finally() is not 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 Objective 8.2)
12. ☑

C is correct. A break 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 a StackOverflowError. Since Exception is not a superclass of Error, catching an
Exception will not help handle an Error, 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. A switch'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 3.5 and Upgrade
Objective 1.1)
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 an IOException, 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)

Part II
OCP

CHAPTERS
7

Assertions and Java 7 Exceptions

11

Generics and Collections

8

String Processing, Data Formatting,
Resource Bundles

12

Inner Classes

13

Threads

9

I/O and NIO

14

Concurrency

15

JDBC

10

Advanced OO and Design Patterns

This page intentionally left blank

7
Assertions and
Java 7 Exceptions

CERTIFICATION OBJECTIVES

•
•

Test Invariants by Using Assertions

•

Develop Code That Uses try-withresources Statements (Including
Using Classes That Implement the
AutoCloseable Interface)

Develop Code That Handles Multiple
Exception Types in a Single catch Block

✓

Two-Minute Drill

Q&A Self Test

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Assertions and Java 7 Exceptions

I

f you are coming back after having sat the OCA, congratulations! You are now ready to
progress to the OCP. The assertion mechanism, added to the language with version 1.4,
gives you a way to do testing and debugging checks on conditions you expect to smoke out
while developing, when you don't necessarily need or want the runtime overhead associated with
exception handling.
If you do need or want exception handling, you'll be learning about two new
features added to exception handling in Java 7. Multi-catch gives you a way of
dealing with two or more exception types at once. try-with-resources lets you close
your resources very easily.

CERTIFICATION OBJECTIVE

Working with the Assertion Mechanism
(OCP Objective 6.5)
6.5

Test invariants by using assertions.

You know you're not supposed to make assumptions, but you can't help it when
you're writing code. You put them in comments:
if (x > 2) {
// do something
} else if (x < 2) {
// do something
} else {
// x must be 2
// do something else
}

You write print statements with them:
while (true) {
if (x > 2) {
break;
}
System.out.print("If we got here " +
"something went horribly wrong");
}

Working with the Assertion Mechanism (OCP Objective 6.5)

379

Added to the Java language beginning with version 1.4, assertions let you test
your assumptions during development, without the expense (in both your time and
program overhead) of writing exception handlers for exceptions that you assume will
never happen once the program is out of development and fully deployed.
Starting with exam 310-035 (version 1.4 of the Sun Certified Java Programmer
exam) and continuing through to the current exam 1Z0-804 (OCPJP 7), you're
expected to know the basics of how assertions work, including how to enable them,
how to use them, and how not to use them.

Assertions Overview
Suppose you assume that a number passed into a method (say, methodA()) will
never be negative. While testing and debugging, you want to validate your
assumption, but you don't want to have to strip out print statements, runtime
exception handlers, or if/else tests when you're done with development. But leaving
any of those in is, at the least, a performance hit. Assertions to the rescue! Check
out the following code:
private void methodA(int num) {
if (num >= 0) {
useNum(num + x);
} else { // num < 0 (this should never happen!)
System.out.println("Yikes! num is a negative number! "
+ num);
}
}

Because you're so certain of your assumption, you don't want to take the time (or
program performance hit) to write exception-handling code. And at runtime, you
don't want the if/else either because if you do reach the else condition, it means
your earlier logic (whatever was running prior to this method being called) is flawed.
Assertions let you test your assumptions during development, but the assertion
code basically evaporates when the program is deployed, leaving behind no overhead
or debugging code to track down and remove. Let's rewrite methodA() to validate
that the argument was not negative:
private void methodA(int num) {
assert (num>=0);
// throws an AssertionError
// if this test isn't true
useNum(num + x);
}

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Not only do assertions let your code stay cleaner and tighter, but because
assertions are inactive unless specifically "turned on" (enabled), the code will run as
though it were written like this:
private void methodA(int num) {
useNum(num + x); // we've tested this;
// we now know we're good here
}

Assertions work quite simply. You always assert that something is true. If it is, no
problem. Code keeps running. But if your assertion turns out to be wrong (false),
then a stop-the-world AssertionError is thrown (which you should never, ever
handle!) right then and there, so you can fix whatever logic flaw led to the problem.
Assertions come in two flavors: really simple and simple, as follows:
Really simple:
private void doStuff() {
assert (y > x);
// more code assuming y is greater than x
}

Simple:
private void doStuff() {
assert (y > x): "y is " + y + " x is " + x;
// more code assuming y is greater than x
}

The difference between the two is that the simple version adds a second
expression separated from the first (boolean expression) by a colon—this
expression's string value is added to the stack trace. Both versions throw an
immediate AssertionError, but the simple version gives you a little more
debugging help, while the really simple version tells you only that your assumption
was false.
Assertions are typically enabled when an application is being tested and
debugged, but disabled when the application is deployed.The assertions are
still in the code, although ignored by the JVM, so if you do have a deployed
application that starts misbehaving, you can always choose to enable
assertions in the field for additional testing.

Working with the Assertion Mechanism (OCP Objective 6.5)

381

Assertion Expression Rules
Assertions can have either one or two expressions, depending on whether you're
using the "simple" or the "really simple." The first expression must always result in a
boolean value! Follow the same rules you use for if and while tests. The whole
point is to assert aTest, which means you're asserting that aTest is true. If it is
true, no problem. If it's not true, however, then your assumption was wrong and
you get an AssertionError.
The second expression, used only with the simple version of an assert
statement, can be anything that results in a value. Remember, the second expression
is used to generate a String message that displays in the stack trace to give you a
little more debugging information. It works much like System.out.println() in
that you can pass it a primitive or an object, and it will convert it into a String
representation. It must resolve to a value!
The following code lists legal and illegal expressions for both parts of an assert
statement. Remember, expression2 is used only with the simple assert statement,
whereas the second expression exists solely to give you a little more debugging
detail:
void noReturn() { }
int aReturn() { return 1; }
void go() {
int x = 1;
boolean b = true;

}

// the following
assert(x == 1);
assert(b);
assert true;
assert(x == 1) :
assert(x == 1) :
assert(x == 1) :

six are legal assert statements

// the following
assert(x = 1);
assert(x);
assert 0;
assert(x == 1) :
assert(x == 1) :
assert(x == 1) :

six are ILLEGAL assert statements
// none of these are booleans

x;
aReturn();
new ValidAssert();

;
// none of these return a value
noReturn();
ValidAssert va;

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If you see the word "expression" in a question about assertions and the
question doesn't specify whether it means expression1 (the boolean test) or expression2
(the value to print in the stack trace), always assume the word "expression" refers to
expression1, the boolean test. For example, consider the following question:
Exam Question: An assert expression must result in a boolean value, true or false?
Assume that the word "expression" refers to expression1 of an assert, so the question
statement is correct. If the statement were referring to expression2, however, the
statement would not be correct since expression2 can have a result of any value, not just
a boolean.

Enabling Assertions
If you want to use assertions, you have to think first about how to compile with
assertions in your code and then about how to run with assertions enabled. Both
require version 1.4 or greater, and that brings us to the first issue: how to compile
with assertions in your code.

Identifier vs. Keyword
Prior to version 1.4, you might very well have written code like this:
int assert = getInitialValue();
if (assert == getActualResult()) {
// do something
}

Notice that in the preceding code, assert is used as an identifier. That's not a
problem prior to 1.4. But you cannot use a keyword/reserved word as an identifier,
and beginning with version 1.4, assert is a keyword. The bottom line is this:
You can use assert as a keyword or as an identifier, but not both.
If, for some reason, you're using a Java 1.4 compiler and you're using assert
as a keyword (in other words, you're actually trying to assert something in
your code), then you must explicitly enable assertion-awareness at compile
time, as follows:
javac -source 1.4 com/geeksanonymous/TestClass.java

You can read that as "compile the class TestClass, in the directory com/
geeksanonymous, and do it in the 1.4 way, where assert is a keyword."

Working with the Assertion Mechanism (OCP Objective 6.5)

383

Use Version 7 of java and javac
As far as the exam is concerned, you'll ALWAYS be using version 7 of the Java
compiler (javac) and version 7 of the Java application launcher (java). You might
see questions about older versions of source code, but those questions will always be
in the context of compiling and launching old code with the current versions of
javac and java.

Compiling Assertion-Aware Code
The Java 7 compiler will use the assert keyword by default. Unless you tell it
otherwise, the compiler will generate an error message if it finds the word assert
used as an identifier. However, you can tell the compiler that you're giving it an old
piece of code to compile and that it should pretend to be an old compiler! Let's say
you've got to make a quick fix to an old piece of 1.3 code that uses assert as an
identifier. At the command line, you can type
javac -source 1.3 OldCode.java

The compiler will issue warnings when it discovers the word assert used as an
identifier, but the code will compile and execute. Suppose you tell the compiler that
your code is version 1.4 or later; for instance:
javac -source 1.4 NotQuiteSoOldCode.java

In this case, the compiler will issue errors when it discovers the word assert used as
an identifier.
If you want to tell the compiler to use Java 7 rules, you can do one of three things:
omit the -source option, which is the default, or add one of two source options:
-source 1.7 or -source 7

If you want to use assert as an identifier in your code, you MUST compile using
the -source 1.3 option. Table 7-1 summarizes how the Java 7 compiler will react
to assert as either an identifier or a keyword.

384

Chapter 7:

TABLE 7-1

Using Various
Java Versions
to Compile
Code That Uses
assert as an
Identifier or a
Keyword

Assertions and Java 7 Exceptions

Command Line

If assert Is
an Identifier

If assert Is
a Keyword

javac -source 1.3 TestAsserts.java

Code compiles
with warnings

Compilation fails

javac -source 1.4 TestAsserts.java

Compilation fails

Code compiles

javac -source 1.5 TestAsserts.java
javac -source 5 TestAsserts.java

Compilation fails

Code compiles

javac -source 1.6 TestAsserts.java
javac -source 6 TestAsserts.java

Compilation fails

Code compiles

javac -source 1.7 TestAsserts.java
javac -source 7 TestAsserts.java

Compilation fails

Code compiles

javac TestAsserts.java

Compilation fails

Code compiles

Running with Assertions
Here's where it gets cool. Once you've written your assertion-aware code (in other
words, code that uses assert as a keyword, to actually perform assertions at runtime),
you can choose to enable or disable your assertions at runtime! Remember, assertions
are disabled by default.

Enabling Assertions at Runtime
You enable assertions at runtime with
java -ea com.geeksanonymous.TestClass

or
java -enableassertions com.geeksanonymous.TestClass

The preceding command-line switches tell the JVM to run with assertions enabled.

Disabling Assertions at Runtime
You must also know the command-line switches for disabling assertions:
java -da com.geeksanonymous.TestClass

or
java -disableassertions com.geeksanonymous.TestClass

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Because assertions are disabled by default, using the disable switches might seem
unnecessary. Indeed, using the switches the way we do in the preceding example just
gives you the default behavior (in other words, you get the same result, regardless of
whether you use the disabling switches). But… you can also selectively enable and
disable assertions in such a way that they're enabled for some classes and/or packages
and disabled for others while a particular program is running.

Selective Enabling and Disabling
The command-line switches for assertions can be used in various ways:
■ With no arguments (as in the preceding examples)

Enables or disables

assertions in all classes, except for the system classes.
■ With a package name

Enables or disables assertions in the package
specified and in any packages below this package in the same directory
hierarchy (more on that in a moment).

■ With a class name

Enables or disables assertions in the class specified.

You can combine switches to, say, disable assertions in a single class but keep
them enabled for all others as follows:
java -ea

-da:com.geeksanonymous.Foo

The preceding command line tells the JVM to enable assertions in general, but
disable them in the class com.geeksanonymous.Foo. You can do the same
selectivity for a package as follows:
java -ea -da:com.geeksanonymous...

The preceding command line tells the JVM to enable assertions in general, but
disable them in the package com.geeksanonymous and all of its subpackages! You
may not be familiar with the term subpackages, since there wasn't much use of that
term prior to assertions. A subpackage is any package in a subdirectory of the named
package. For example, look at the following directory tree:
com
|_geeksanonymous
|_Foo.class
|_twelvesteps
|_StepOne.class
|_StepTwo.class

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This tree lists three directories:
com
geeksanonymous
twelvesteps
and three classes:
com.geeksanonymous.Foo
com.geeksanonymous.twelvesteps.StepOne
com.geeksanonymous.twelvesteps.StepTwo

The subpackage of com.geeksanonymous is the twelvesteps package.
Remember that in Java, the com.geeksanonymous.twelvesteps package is treated
as a completely distinct package that has no relationship with the packages above it
(in this example, the com.geeksanonymous package), except they just happen to
share a couple of directories. Table 7-2 lists examples of command-line switches for
enabling and disabling assertions.

Using Assertions Appropriately
Not all legal uses of assertions are considered appropriate. As with so much of Java,
you can abuse the intended use of assertions, despite the best efforts of Oracle’s Java
engineers to discourage you from doing so. For example, you’re never supposed to
handle an assertion failure. That means you shouldn’t catch it with a catch clause
and attempt to recover. Legally, however, AssertionError is a subclass of
Throwable, so it can be caught. But just don’t do it! If you’re going to try to recover
from something, it should be an exception. To discourage you from trying to
substitute an assertion for an exception, the AssertionError doesn’t provide access
to the object that generated it. All you get is the String message.
So who gets to decide what’s appropriate? Oracle. The exam uses Oracle’s
“official” assertion documentation to define appropriate and inappropriate uses.

Don’t Use Assertions to Validate Arguments to a public Method
The following is an inappropriate use of assertions:
public void doStuff(int x) {
assert (x > 0);
// do things with x
}

// inappropriate !

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TABLE 7-2

Assertion
Command-Line
Switches

387

Command-Line Example

What It Means

java -ea
java -enableassertions

Enable assertions.

java -da
java -disableassertions

Disable assertions (the default behavior).

java -ea:com.foo.Bar

Enable assertions in class com.foo.Bar.

java -ea:com.foo...

Enable assertions in package com.foo and any of its
subpackages.

java -ea -dsa

Enable assertions in general, but disable assertions in
system classes.

java -ea -da:com.foo...

Enable assertions in general, but disable assertions in
package com.foo and any of its subpackages.

If you see the word "appropriate" on the exam, do not mistake that for
"legal." "Appropriate" always refers to the way in which something is supposed to be
used, according to either the developers of the mechanism or best practices officially
embraced by Oracle. If you see the word "correct" in the context of assertions, as in,
"Line 3 is a correct use of assertions," you should also assume that correct is referring to
how assertions SHOULD be used rather than how they legally COULD be used.

A public method might be called from code that you don't control (or from
code you have never seen). Because public methods are part of your interface to
the outside world, you're supposed to guarantee that any constraints on the
arguments will be enforced by the method itself. But since assertions aren't
guaranteed to actually run (they're typically disabled in a deployed application), the
enforcement won't happen if assertions aren't enabled. You don't want publicly
accessible code that works only conditionally, depending on whether assertions are
enabled.
If you need to validate public method arguments, you'll probably use exceptions
to throw, say, an IllegalArgumentException if the values passed to the public
method are invalid.

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Do Use Assertions to Validate Arguments to a private Method
If you write a private method, you almost certainly wrote (or control) any code
that calls it. When you assume that the logic in code calling your private method
is correct, you can test that assumption with an assertion as follows:
private void doMore(int x) {
assert (x > 0);
// do things with x
}

The only difference that matters between the preceding example and the one
before it is the access modifier. So, do enforce constraints on private methods'
arguments, but do not enforce constraints on public methods. You're certainly free
to compile assertion code with an inappropriate validation of public arguments, but
for the exam (and real life), you need to know that you shouldn't do it.

Don't Use Assertions to Validate Command-Line Arguments
This is really just a special case of the "Do not use assertions to validate arguments to
a public method" rule. If your program requires command-line arguments, you'll
probably use the exception mechanism to enforce them.

Do Use Assertions, Even in public Methods, to Check for Cases
That You Know Are Never, Ever Supposed to Happen
This can include code blocks that should never be reached, including the default of
a switch statement as follows:
switch(x) {
case 1: y = 3; break;
case 2: y = 9; break;
case 3: y = 27; break;
default: assert false; // we're never supposed to get here!
}

If you assume that a particular code block won't be reached, as in the preceding
example where you assert that x must be 1, 2, or 3, then you can use assert false
to cause an AssertionError to be thrown immediately if you ever do reach that
code. So in the switch example, we're not performing a boolean test—we've already
asserted that we should never be there, so just getting to that point is an automatic
failure of our assertion/assumption.

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Don't Use assert Expressions That Can Cause Side Effects!
The following would be a very bad idea:
public void doStuff() {
assert (modifyThings());
// continues on
}
public boolean modifyThings() {
y = x++;
return true;
}

The rule is that an assert expression should leave the program in the same state
it was in before the expression! Think about it. assert expressions aren't
guaranteed to always run, so you don't want your code to behave differently
depending on whether assertions are enabled. Assertions must not cause any side
effects. If assertions are enabled, the only change to the way your program runs is
that an AssertionError can be thrown if one of your assertions (think assumptions)
turns out to be false.
Using assertions that cause side effects can cause some of the most
maddening and hard-to-find bugs known to man! When a hot-tempered QA
analyst is screaming at you that your code doesn't work, trotting out the old
"well, it works on MY machine" excuse won't get you very far.

CERTIFICATION OBJECTIVE

Working with Java 7 Exception Handling
(OCP Objectives 6.2 and 6.3)
6.2

Develop code that handles multiple exception types in a single catch block.

6.3 Develop code that uses try-with-resources statements (including using classes that
implement the AutoCloseable interface).

Use the try Statement with multi-catch and finally Clauses
Sometimes we want to handle different types of exceptions the same way. Especially
when all we can do is log the exception and declare defeat. But we don't want to

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repeat code. So what to do? In the previous chapter's section "Handling an Entire
Class Hierarchy of Exceptions," we've already seen that having a single catch-all
exception handler is a bad idea. Prior to Java 7, the best we could do was:
try {
// access the database and write to a file
} catch (SQLException e) {
handleErrorCase(e);
} catch (IOException e) {
handleErrorCase(e);
}

You may be thinking that it is only one line of duplicate code. But what happens
when you are catching six different exception types? That's a lot of duplication.
Luckily, Java 7 made this nice and easy with a feature called multi-catch:
try {
// access the database and write to a file
} catch (SQLException | IOException e) {
handleErrorCase(e);
}

No more duplication. This is great. As you might imagine, multi-catch is short for
"multiple catch." You just list out the types you want the multi-catch to handle
separated by pipe (|) characters. This is easy to remember because | is the "or"
operator in Java. Which means the catch can be read as "SQLException or
IOException e."

You can't use the variable name multiple times in a multi-catch.The
following won't compile:
catch(Exception1 e1 | Exception2 e2)

It makes sense that this example doesn't compile. After all, the code in the exception
handler needs to know which variable name to refer to.
catch(Exception1 e | Exception2 e)

This one is tempting. When we declare variables, we normally put the variable name right
after the type.Try to think of it as a list of types. We are declaring variable e to be caught
and it must be one of Exception1 or Exception2 types.

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With multi-catch, order doesn't matter. The following two snippets are
equivalent to each other:
catch(SQLException | IOException e)
catch(IOException | SQLException e)

// these two statements are equivalent

Just like with exception matching in a regular catch block, you can't just throw
any two exceptions together. With multi-catch, you have to make sure a given
exception can only match one type. The following will not compile:
catch(FileNotFoundException | IOException e)
catch(IOException | FileNotFoundException e)

You'll get a compiler error that looks something like:
The exception FileNotFoundException is already caught by the
alternative IOException

Since FileNotFoundException is a subclass of IOException, we could have just
written that in the first place! There was no need to use multi-catch. The simplified
and working version simply says:
catch(IOException e)

Remember, multi-catch is only for exceptions in different inheritance hierarchies.
To make sure this is clear, what do you think happens with the following code:
catch(IOException | Exception e)

That's right. It won't compile because IOException is a subclass of Exception.
Which means it is redundant and the compiler won't accept it.
To summarize, we use multi-catch when we want to reuse an exception handler.
We can list as many types as we want so long as none of them have a superclass/
subclass relationship with each other.

Multi-catch and catch Parameter Assignment
There is one tricky thing with multi-catch. And we know the exam creators like
tricky things!
The following LEGAL code demonstrates assigning a new value to the single
catch parameter:
try {
// access the database and write to a file
} catch (IOException e) {
e = new IOException();
}

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Don't assign a new value to the catch parameter. It isn't good practice and
creates confusing, hard-to-maintain code. But it is legal Java code to assign a
new value to the catch block's parameter when there is only one type listed,
and it will compile.
The following ILLEGAL code demonstrates trying to assign a value to the final
multi-catch parameter:
try {
// access the database and write to a file
} catch (SQLException | IOException e) {
e = new IOException();
}

At least you get a clear compiler error if you try to do this. The compiler tells you:
The parameter e of a multi-catch block cannot be assigned

Since multi-catch uses multiple types, there isn't a clearly defined type for the
variable that you can set. Java solves this by making the catch parameter final
when that happens. And then the code doesn't compile because you can't assign to
a final variable.

Rethrowing Exceptions
Sometimes, we want to do something with the thrown exceptions before we rethrow
them:
public void couldThrowAnException() throws IOException, SQLException {}
public void rethrow() throws SQLException, IOException {
try {
couldThrowAnException();
} catch (SQLException | IOException e) {
log(e);
throw e;
}
}

This is a common pattern called "handle and declare." We want to do something
with the exception—log it. We also want to acknowledge we couldn't completely
handle it, so we declare it and let the caller deal with it. (As an aside, many
programmers believe that logging an exception and rethrowing it is a bad practice,
but you never know—you might see this kind of code on the exam.)

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You may have noticed that couldThrowAnException() doesn't actually throw
an exception. The compiler doesn't know this. The method signature is key to the
compiler. It can't assume that no exception gets thrown, as a subclass could override
the method and throw an exception.
There is a bit of duplicate code here. We have the list of exception types thrown
by the methods we call typed twice. Multi-catch was introduced to avoid having
duplicate code, yet here we are with duplicate code.
Lucky for us, Java 7 helps us out here as well with a new feature. This example is
a nicer way of writing the previous code:
1. public void rethrow() throws SQLException, IOException {
2. try {
3.
couldThrowAnException();
4. } catch (Exception e) {
// watch out: this isn't really
5.
// catching all exception subclasses
6.
log(e);
7.
throw e;
// note: won't compile in Java 6
8. }
9. }

Notice the multi-catch is gone and replaced with catch(Exception e). It's not
bad practice here, though, because we aren't really catching all exceptions. The
compiler is treating Exception as "any exceptions that the called methods happen
to throw." (You'll see this idea of code shorthand again with the diamond operator
when you get to generics.)
This is very different from Java 6 code that catches Exception. In Java 6, we'd
need the rethrow() method signature to be throws Exception in order to make
this code compile.
In Java 7, } catch (Exception e) { doesn't really catch ANY Exception
subclass. The code may say that, but the compiler is translating for you. The
compiler says, "Well, I know it can't be just any exception because the throws clause
won't let me. I'll pretend the developer meant to only catch SQLException and
IOException. After all, if any others show up, I'll just fail compilation on throw e;
—just like I used to in Java 6." Tricky, isn't it?
At the risk of being too repetitive, remember that catch (Exception e)
doesn't necessarily catch all Exception subclasses. In Java 7, it means catch all
Exception subclasses that would allow the method to compile.
Got that? Now why on earth would Oracle do this to us? It sounds more
complicated than it used to be! Turns out they were trying to solve another problem
at the same time they were changing this stuff. Suppose the API developer of

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couldThrowAnException() decided the method will never throw a SQLException
and removes SQLException from the signature to reflect that.
Imagine we were using the Java 6 style of having one catch block per exception
or even the multi-catch style of:
} catch (SQLException | IOException e) {

Our code would stop compiling with an error like:
Unreachable catch block for SQLException

It is reasonable for code to stop compiling if we add exceptions to a method. But
we don't want our code to break if a method's implementation gets LESS brittle.
And that's the advantage of using:
} catch (Exception e) {

Java infers what we mean here and doesn't say a peep when the API we are calling
removes an exception.

Don't go changing your API signatures on a whim. Most code was written
before Java 7 and will break if you change signatures.Your callers won't thank you when their
code suddenly fails compilation because they tried to use your new, shiny, "cleaner" API.

You've probably noticed by now that Oracle values backward compatibility and
doesn't change the behavior or "compiler worthiness" of code from older versions of
Java. That still stands. In Java 6, we can't write catch (Exception e) and merely
throw specific exceptions. If we tried, it would still complain about:
Unhandled exception type Exception.

Backward compatibility only needs to work for code that compiles! It's OK for the
compiler to get less strict over time.
To make sure you understand what is going on here, think about what happens in
this example:
public class A extends Exception{}
public class B extends Exception{}
public void rain() throws A, B {}

Table 7.3 summarizes handling changes to the exception-related parts of method
signatures in Java 6 and Java 7.

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TABLE 7-3

395

Exceptions and Signatures

Java 6 style:
public void ahhh() throws A, B {
try {
rain();
} catch (A e) {
throw e;
} catch (B e) {
throw e;
}
}

Java 7 with duplication:
public void ahhh() throws A, B {
try {
rain();
} catch (A | B e) {
throw e;
}
}

Java 7 without duplication:
public void ahhh() throws A, B {
try {
rain();
} catch (Exception e) {
throw e;
}
}

What happens if rain()
adds a new checked
exception?

What happens if rain()
removes a checked
exception from the
signature?

Add another catch block to
handle the new exception.

Remove a catch block to
avoid compiler error about
unreachable code.

Add another exception to the Remove an expression from
multi-catch block to handle the multi-catch block to
the new exception.
avoid compiler error about
unreachable code.

Add another exception to the No code changes needed.
method signature to handle
the new exception that can
be thrown.

There is one more trick. If you assign a value to the catch parameter, the code no
longer compiles:
public void rethrow() throws SQLException, IOException {
try {
couldThrowAnException();
} catch (Exception e) {
e = new IOException();
throw e;
}
}

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As with multi-catch, you shouldn't be assigning a new value to the catch parameter
in real life anyway. The difference between this and multi-catch is where the
compiler error occurs. For multi-catch, the compiler error occurs on the line where
we attempt to assign a new value to the parameter, whereas here, the compiler error
occurs on the line where we throw e. It is different because code written prior to
Java 7 still needs to compile. Since the multi-catch syntax is brand new, there is no
legacy code to worry about.

Autocloseable Resources with a try-with-resources Statement
When we learned about using finally in Chapter 6, we saw that the finally
block is a good place for closing files and assorted other resources. The examples
made this clean-up code in the finally block look nice and short by writing
// clean up. Unfortunately, real-world clean-up code is easy to get wrong. And
when correct, it is verbose. Let's look at the code to close our one resource when
closing a file:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:

Reader reader = null;
try {
// read from file
} catch(IOException e) {
log(); throw e;
} finally {
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
// ignore exceptions on closing file
}
}
}

That's a lot of code just to close a single file! But it's all necessary. First, we need
to check if the reader is null on line 7. It is possible the try block threw an
exception before creating the reader, or while trying to create the reader if the file
we are trying to read doesn't exist. It isn't until line 9 that we get to the one line in
the whole finally block that does what we care about—closing the file. Lines 8
and 10 show a bit more housekeeping. We can get an IOException on attempting
to close the file. While we could try to handle that exception, there isn't much we
can do, thus making it common to just ignore the exception. This gives us nine lines
of code (lines 6–14) just to close a file.

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Developers typically write a helper class to close resources or they use the opensource, Apache Commons helper to get this mess down to three lines:
6:
7:
8:

} finally {
HelperClass.close(reader);
}

Which is still three lines too many.
Lucky for us, Java 7 introduced a new feature called Automatic Resource
Management using "try-with-resources" to get rid of even these three lines. The
following code is equivalent to the previous example:
1:
2:
3:
4:

try (Reader reader =
new BufferedReader(new FileReader(file))) {
// read from file
} catch (IOException e) { log(); throw e;}

// note the new syntax

No finally left at all! We don't even mention closing the reader. Automatic
Resource Management takes care of it for us. Let's take a look at what happens here.
We start out by declaring the reader inside the try declaration. The parentheses
are new. Think of them as a for loop in which we declare a loop index variable
that is scoped to just the loop. Here, the reader is scoped to just the try block.
Not the catch block; just the try block.
The actual try block does the same thing as before. It reads from the file. Or, at
least, it comments that it would read from the file. The catch block also does the
same thing as before. And just like in our traditional try statement, catch is
optional.
Remembering back to the section "Using finally" in Chapter 6, we learned that
a try must have catch or finally. Time to learn something new about that rule.
We remember this is ILLEGAL code because it demonstrates a try without a
catch or finally:
1: try {
2:
// do stuff
3: } // need a catch or finally here

The following LEGAL code demonstrates a try-with-resources with no catch or
finally:
1:
2:
3:
4:

try (Reader reader =
new BufferedReader(new FileReader(file))) {
// do stuff
}

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What's the difference? The legal example does have a finally block; you just
don't see it. The try-with-resources statement is logically calling a finally block
to close the reader. And just to make this even trickier, you can add your own
finally block to try-with-resources as well. Both will get called. We'll take a look
at how this works shortly.
Since the syntax is inspired from the for loop, we get to use a semicolon when
declaring multiple resources in the try. For example:
try (MyResource mr = MyResource.createResource();
MyThingy mt = mr.createThingy()) {
// do stuff
}

// first resource
// second resource

There is something new here. Our declaration calls methods. Remember that the
try-with-resources is just Java code. It is just restricted to only be declarations. This

means if you want to do anything more than one statement long, you'll need to put
it into a method.
To review, Table 7-4 lists the big differences that are new for try-with-resources.

AutoCloseable and Closeable
Because Java is a statically typed language, it doesn't let you declare just any type in
a try-with-resources statement. The following code will not compile:
try (String s = "hi") {}

You'll get a compiler error that looks something like:
The resource type String does not implement java.lang.AutoCloseable

AutoCloseable only has one method to implement. Let's take a look at the
simplest code we can write using this interface:
public class MyResource implements AutoCloseable {
public void close() {
// take care of closing the resource
}
}

There's also an interface called Closeable, which is similar to AutoCloseable
but with some key differences. Why are there two similar interfaces, you may
wonder? The Closeable interface was introduced in Java 5. When try-withresources was invented in Java 7, the language designers wanted to change some

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try-catch-finally

try-with-resources

Resource declared

Before try keyword

In parentheses within try declaration

Resource initialized

In try block

In parentheses within try declaration

TABLE 7-4

Comparing
Traditional
try Statement
to try-withresources

399

Resource closed

In finally block

Nowhere—happens automatically

Required keywords

try

try

One of catch or finally

things but needed backward compatibility with all existing code. So they created a
superinterface with the rules they wanted.
One thing the language designers wanted to do was make the signature more
generic. Closeable allows implementors to throw only an IOException or a
RuntimeException. AutoCloseable allows any Exception at all to be thrown.
Look at some examples:
// ok because AutoCloseable allows throwing any Exception
class A implements AutoCloseable { public void close() throws Exception{}}
// ok because subclasses or implementing methods can throw
// a subclass of Exception or none at all
class B implements AutoCloseable { public void close() {}}
class C implements AutoCloseable { public void close() throws IOException {}}
// ILLEGAL – Closeable only allows IOExceptions or subclasses
class D implements Closeable { public void close() throws Exception{}}
// ok because Closeable allows throwing IOException
class E implements Closeable { public void close() throws IOException{}}

In your code, Oracle recommends throwing the narrowest Exception subclass that
will compile. However, they do limit Closeable to IOException, and you must use
AutoCloseable for anything more.
The next difference is even trickier. What happens if we call the close()
multiple times? It depends. For classes that implement AutoCloseable, the
implementation is required to be idempotent. Which means you can call close()
all day and nothing will happen the second time and beyond. It will not attempt to
close the resource again and it will not blow up. For classes that implement
Closeable, there is no such guarantee.

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If you look at the JavaDoc, you'll notice many classes implement both
AutoCloseable and Closeable. These classes use the stricter signature rules and
are idempotent. They still need to implement Closeable for backward compatibility,
but added AutoCloseable for the new contract.
To review, Table 7-5 shows the differences between AutoCloseable and
Closeable. Remember the exam creators like to ask about "similar but not quite

the same" things!

A Complex try-with-resources Example The following example is as
complicated as try-with-resources gets:
1: class One implements AutoCloseable {
2:
public void close() {
3:
System.out.println("Close - One");
4:
} }
5: class Two implements AutoCloseable {
6:
public void close() {
7:
System.out.println("Close - Two");
8:
} }
9: class TryWithResources {
10:
public static void main(String[] args) {
11:
try (One one = new One(); Two two = new Two()) {
12:
System.out.println("Try");
13:
throw new RuntimeException();
14:
} catch (Exception e) {
15:
System.out.println("Catch");
16:
} finally {
17:
System.out.println("Finally");
18:
} } }

Running the preceding code will print:
Try
Close – Two
Close – One
Catch
Finally

TABLE 7-5

Extends

Comparing
AutoCloseable close method throws
Must be idempotent (can call more than once
and Closeable

without side effects)

AutoCloseable

Closeable

None

AutoCloseable

Exception

IOException

Yes

No, but encouraged

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401

It's actually more logical than it looks at first glance. We first enter the try block
on line 11, and Java creates our two resources. Line 12 prints Try. When we throw
an exception on line 13, the first interesting thing happens. The try block "ends"
and Automatic Resource Management automatically cleans up the resources before
moving on to the catch or finally. The resources get cleaned up, "backwards"
printing Close – Two and then Close – One. The close() method gets called in
the reverse order in which resources are declared to allow for the fact that resources
might depend on each other. Then we are back to the regular try block order,
printing Catch and Finally on lines 15 and 17.
If you only remember two things from this example, remember that try-withresources is part of the try block, and resources are cleaned up in the reverse order
they were created.

Suppressed Exceptions
We're almost done with exceptions. There's only one more wrinkle to cover in Java 7
exception handling. Now that we have an extra step of closing resources in the try,
it is possible for multiple exceptions to get thrown. Each close() method can throw
an exception in addition to the try block itself.
1: public class Suppressed {
2:
public static void main(String[] args) {
3:
try (One one = new One()) {
4:
throw new Exception("Try");
5:
} catch (Exception e) {
6:
System.err.println(e.getMessage());
7:
for (Throwable t : e.getSuppressed()) {
8.
System.err.println("suppressed:" + t);
9. } } } }
class One implements AutoCloseable {
public void close() throws IOException {
throw new IOException("Closing");
} }

We know that after the exception in the try block gets thrown on line 4, the
try-with-resources still calls close() on line 3 and the catch block on line 5
catches one of the exceptions. Running the code prints:
Try
suppressed:java.io.IOException: Closing

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This tells us the exception we thought we were throwing still gets treated as most
important. Java also adds any exceptions thrown by the close() methods to a
suppressed array in that main exception. The catch block or caller can deal with
any or all of these. If we remove line 4, the code just prints Closing.
In other words, the exception thrown in close() doesn't always get suppressed.
It becomes the main exception if there isn't already one existing. As one more
example, think about what the following prints:
class Bad implements AutoCloseable {
String name;
Bad(String n) { name = n; }
public void close() throws IOException {
throw new IOException("Closing - " + name);
} }
public class Suppressed {
public static void main(String[] args) {
try (Bad b1 = new Bad("1"); Bad b2 = new Bad("2")) {
// do stuff
} catch (Exception e) {
System.err.println(e.getMessage());
for (Throwable t : e.getSuppressed()) {
System.err.println("suppressed:" + t);
} } } }

The answer is:
Closing - 2
suppressed:java.io.IOException: Closing – 1

Up until try-with-resources calls close(), everything is going just dandy. When
Automatic Resource Management calls b2.close(), we get our first exception.
This becomes the main exception. Then, Automatic Resource Management calls
b1.close() and throws another exception. Since there was already an exception
thrown, this second exception gets added as a second exception.
If the catch or finally block throws an exception, no suppressions happen. The
last exception thrown gets sent to the caller rather than the one from the try—just
like before try-with-resources was created.

Certification Summary

403

CERTIFICATION SUMMARY
Assertions, added to the language in version 1.4, are a useful debugging tool. You
learned how you can use them for testing by enabling them, but keep them disabled
when the application is deployed. If you have older Java code that uses the word
assert as an identifier, then you won't be able to use assertions, and you must
recompile your older code using the -source 1.3 flag. Remember that for Java 7,
assertions are compiled as a keyword by default, but must be enabled explicitly at
runtime.
You learned how assert statements always include a boolean expression, and if
the expression is true, the code continues on, but if the expression is false, an
AssertionError is thrown. If you use the two-expression assert statement, then the
second expression is evaluated, converted to a String representation, and inserted
into the stack trace to give you a little more debugging info. Finally, you saw why
assertions should not be used to enforce arguments to public methods, and why
assert expressions must not contain side effects!
Exception handling was enhanced in version 7, making exceptions easier to use.
First you learned that you can specify multiple exception types to share a catch
block using the new multi-catch syntax. The major benefit is in reducing code
duplication by having multiple exception types share the same exception handler.
The variable name is listed only once, even though multiple types are listed. You
can't assign a new exception to that variable in the catch block. Then you saw
the "handle and declare" pattern where the exception types in the multi-catch are
listed in the method signature and Java translates "catch Exception e" into that
exception type list.
Next, you learned about the try-with-resources syntax where Java will take care
of calling close() for you. The objects are scoped to the try block. Java treats
them as a finally block and closes these resources for you in the opposite order to
which they were opened. If you have your own finally block, it is executed after
try-with-resources closes the objects. You also learned the difference between
AutoCloseable and Closeable. Closable was introduced in Java 5, allowing only
IOException (and RuntimeException) to be thrown. AutoCloseable was added
in Java 7, allowing any type of Exception.

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✓

Assertions and Java 7 Exceptions

TWO-MINUTE DRILL
Here are some of the key points from the certification objectives in this chapter.

Test Invariants Using Assertions (OCP Objective 6.5)
❑ Assertions give you a way to test your assumptions during development and
debugging.
❑ Assertions are typically enabled during testing but disabled during
deployment.
❑ You can use assert as a keyword (as of version 1.4) or an identifier, but
not both together. To compile older code that uses assert as an identifier
(for example, a method name), use the -source 1.3 command-line flag to
javac.
❑ Assertions are disabled at runtime by default. To enable them, use a
command-line flag: -ea or -enableassertions.
❑ Selectively disable assertions by using the -da or -disableassertions flag.
❑ If you enable or disable assertions using the flag without any arguments,
you're enabling or disabling assertions in general. You can combine enabling
and disabling switches to have assertions enabled for some classes and/or
packages, but not others.
❑ You can enable and disable assertions on a class-by-class basis, using the
following syntax:
java -ea -da:MyClass TestClass

❑ You can enable and disable assertions on a package-by-package basis, and any
package you specify also includes any subpackages (packages further down the
directory hierarchy).
❑ Do not use assertions to validate arguments to public methods.
❑ Do not use assert expressions that cause side effects. Assertions aren't
guaranteed to always run, and you don't want behavior that changes
depending on whether assertions are enabled.
❑ Do use assertions—even in public methods—to validate that a particular
code block will never be reached. You can use assert false; for code that
should never be reached so that an assertion error is thrown immediately if
the assert statement is executed.

Two-Minute Drill

405

Use the try Statement with Multi-catch and finally Clauses
(OCP Objective 6.2)
❑ If two catch blocks have the same exception handler code, you can merge
them with multi-catch using catch (Exception1 | Exception2 e).
❑ The types in a multi-catch list must not extend one another.
❑ When using multi-catch, the catch block parameter is final and cannot
have a new value assigned in the catch block.
❑ If you catch a general exception as shorthand for specific subclass exceptions
and rethrow the caught exception, you can still list the specific subclasses in
the method signature. The compiler will treat it as if you had listed them out
in the catch.

Autocloseable Resources with a try-with-resources Statement
(OCP Objective 6.3)
❑ try-with-resources automatically calls close() on any resources declared in
the try as try(Resource r = new Foo()).
❑ A try must have at least a catch or finally unless it is a try-with-resources.
For try-with-resources, it can have neither, one, or both of the keywords.
❑ AutoCloseable's close() method throws Exception and must be
idempotent. Closeable's close() throws IOException and is not required
to be idempotent.
❑ try-with-resources are closed in reverse order of creation and before going on
to catch or finally.
❑ If more than one exception is thrown in a try-with-resources block, it gets
added as a suppressed exception.
❑ The type used in a try-with-resources statement must implement
AutoCloseable.

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SELF TEST
The following questions will help you measure your understanding of the material presented in this
chapter. Read all of the choices carefully, as there may be more than one correct answer. Choose all
correct answers for each question. Stay focused.
1. Given two files:
1. class One {
2.
public static void main(String[] args) {
3.
int assert = 0;
4.
}
5. }
1. class Two {
2.
public static void main(String[] args) {
3.
assert(false);
4.
}
5. }

And the four command-line invocations:
javac
javac
javac
javac

-source
-source
-source
-source

1.3
1.4
1.3
1.4

One.java
One.java
Two.java
Two.java

What is the result? (Choose all that apply.)
A. Only one compilation will succeed
B. Exactly two compilations will succeed
C. Exactly three compilations will succeed
D. All four compilations will succeed
E. No compiler warnings will be produced
F. At least one compiler warning will be produced
2. Which are true? (Choose all that apply.)
A. It is appropriate to use assertions to validate arguments to methods marked public
B. It is appropriate to catch and handle assertion errors
C. It is NOT appropriate to use assertions to validate command-line arguments
D. It is appropriate to use assertions to generate alerts when you reach code that should not
be reachable
E. It is NOT appropriate for assertions to change a program's state

Self Test

3. Given:
3. public class Clumsy {
4.
public static void main(String[] args) {
5.
int j = 7;
6.
assert(++j > 7);
7.
assert(++j > 8): "hi";
8.
assert(j > 10): j=12;
9.
assert(j==12): doStuff();
10.
assert(j==12): new Clumsy();
11.
}
12.
static void doStuff() { }
13. }

Which are true? (Choose all that apply.)
A. Compilation succeeds
B. Compilation fails due to an error on line 6
C. Compilation fails due to an error on line 7
D. Compilation fails due to an error on line 8
E. Compilation fails due to an error on line 9
F. Compilation fails due to an error on line 10
4. Given:
class AllGoesWrong {
public static void main(String[] args) {
AllGoesWrong a = new AllGoesWrong();
try {
a.blowUp();
System.out.println("a");
} catch (IOException e | SQLException e) {
System.out.println("c");
} finally {
System.out.println("d");
}
}
void blowUp() throws IOException, SQLException {
throw new SQLException();
}
}

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What is the result?
A. ad
B. acd
C. cd
D. d
E. Compilation fails
F. An exception is thrown at runtime
5. Given:
class BadIO {
public static void main(String[] args) {
BadIO a = new BadIO();
try {
a.fileBlowUp();
a.databaseBlowUp();
System.out.println("a");
} // insert code here
System.out.println("b");
} catch (Exception e) {
System.out.println("c");
} }
void databaseBlowUp() throws SQLException {
throw new SQLException();
}
void fileBlowUp() throws IOException {
throw new IOException();
}}

Which inserted independently at // insert code here will compile and produce the output: b?
(Choose all that apply.)
A. catch(Exception e) {
B. catch(FileNotFoundException e) {
C. catch(IOException e) {
D. catch(IOException | SQLException e) {
E. catch(IOException e | SQLException e) {
F. catch(SQLException e) {
G. catch(SQLException | IOException e) {
H. catch(SQLException e | IOException e) {

Self Test

409

6. Given:
class Train {
class RanOutOfTrack extends Exception { }
class AnotherTrainComing extends Exception { }
public static void main(String[] args) throws RanOutOfTrack,
AnotherTrainComing {
Train a = new Train();
try {
a.drive();
System.out.println("honk! honk!");
} // insert code here
System.out.println("error driving");
throw e;
}
}
void drive() throws RanOutOfTrack, AnotherTrainComing {
throw new RanOutOfTrack();
} }

Which inserted independently at // insert code here will compile and produce the output
error driving before throwing an exception? (Choose all that apply.)
A. catch(AnotherTrainComing e) {
B. catch(AnotherTrainComing | RanOutOfTrack e) {
C. catch(AnotherTrainComing e | RanOutOfTrack e) {
D. catch(Exception e) {
E. catch(IllegalArgumentException e) {
F. catch(RanOutOfTrack e) {
G. None of the above—code fails to compile for another reason
7. Given:
class Conductor {
static String s = "-";
class Whistle implements AutoCloseable
public void toot() {
s += "t";
public void close() {
s += "c";
}
public static void main(String[] args)
new Conductor().run();
System.out.println(s);
}

{
}
}
{

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public void run() {
try (Whistle w = new Whistle()) {
w.toot();
s += "1";
throw new Exception();
} catch (Exception e) { s += "2";
} finally { s += "3"; } } }

What is the result?
A. -t123t
B. -t12c3
C. -t123
D. -t1c3
E. -t1c23
F. None of the above; main() throws an exception
G. Compilation fails
8. Given:
public class MultipleResources {
class Lamb implements AutoCloseable {
public void close() throws Exception {
System.out.print("l");
} }
class Goat implements AutoCloseable {
public void close() throws Exception {
System.out.print("g");
} }
public static void main(String[] args) throws Exception {
new MultipleResources().run();
}
public void run() throws Exception {
try (Lamb l = new Lamb();
System.out.print("t");
Goat g = new Goat();) {
System.out.print("2");
} finally {
System.out.print("f");
} } }

Self Test

What is the result?
A. 2glf
B. 2lgf
C. tglf
D. t2lgf
E. t2lgf
F. None of the above; main() throws an exception
G. Compilation fails
9. Given:
1: public class Animals {
2:
class Lamb {
3:
public void close() throws Exception { }
4:
}
5:
public static void main(String[] args) throws Exception {
6:
new Animals().run();
7:
}
8:
9:
public void run() throws Exception {
10:
try (Lamb l = new Lamb();) {
11:
}
12:
}
13: }

And the following possible changes:
C1. Replace line 2 with class Lamb implements AutoCloseable {
C2. Replace line 2 with class Lamb implements Closeable {
C3. Replace line 11 with } finally {}
What change(s) allow the code to compile? (Choose all that apply.)
A. Just C1 is sufficient
B. Just C2 is sufficient
C. Just C3 is sufficient
D. Both C1 and C3
E. Both C2 and C3
F. The code compiles without any changes

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10. Given:
public class Animals {
class Lamb implements Closeable {
public void close() {
throw new RuntimeException("a");
} }
public static void main(String[] args)
new Animals().run();
}
public void run() {
try (Lamb l = new Lamb();) {
throw new IOException();
} catch(Exception e) {
throw new RuntimeException("c");
} } }

{

Which exceptions will the code throw?
A. IOException with suppressed RuntimeException a
B. IOException with suppressed RuntimeException c
C. RuntimeException a with no suppressed exception
D. RuntimeException c with no suppressed exception
E. RuntimeException a with suppressed RuntimeException c
F. RuntimeException c with suppressed RuntimeException a
G. Compilation fails
11. Given:
public class Animals {
class Lamb implements AutoCloseable {
public void close() {
throw new RuntimeException("a");
} }
public static void main(String[] args) throws IOException {
new Animals().run();
}
public void run() throws IOException {
try (Lamb l = new Lamb();) {
throw new IOException();
} catch(Exception e) {
throw e;
} } }

Self Test

Which exceptions will the code throw?
A. IOException with suppressed RuntimeException a
B. IOException with suppressed RuntimeException c
C. RuntimeException a with no suppressed exception
D. RuntimeException c with no suppressed exception
E. RuntimeException a with suppressed RuntimeException c
F. RuntimeException c with suppressed RuntimeException a
G. Compilation fails
12. Given:
public class Concert {
static class PowerOutage extends Exception {}
static class Thunderstorm extends Exception {}
public static void main(String[] args) {
try {
new Concert().listen();
System.out.println("a");
} catch(PowerOutage | Thunderstorm e) {
e = new PowerOutage();
System.out.println("b");
} finally { System.out.println("c"); }
}
public void listen() throws PowerOutage, Thunderstorm{ }
}

What will this code print?
A. a
B. ab
C. ac
D. abc
E. bc
F. Compilation fails

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SELF TEST ANSWERS
1. ☑

B and F are correct. class One will compile (and issue a warning) using the 1.3 flag, and
class Two will compile using the 1.4 flag.
☐
✗ A, C, D, and E are incorrect based on the above. (OCP Objective 6.5)

2. ☑ C, D, and E are correct statements.
☐
✗ A is incorrect. It is acceptable to use assertions to test the arguments of private methods.
B is incorrect. While assertion errors can be caught, Oracle discourages you from doing so.
(OCP Objective 6.5)
3. ☑ E is correct. When an assert statement has two expressions, the second expression must
return a value. The only two-expression assert statement that doesn't return a value is on line 9.
☐
✗ A, B, C, D, and F are incorrect based on the above. (OCP Objective 6.5)
4. ☑ E is correct. catch (IOException e | SQLException e) doesn't compile. While
multiple exception types can be specified in the multi-catch, only one variable name is
allowed. The correct syntax is catch (IOException | SQLException e). Other than this,
the code is valid. Note that it is legal for blowUp() to have IOException in its signature even
though that Exception can't be thrown.
☐
✗ A, B, C, D, and F are incorrect based on the above. If the catch block's syntax error were
corrected, the code would output cd. The multi-catch would catch the SQLException from
blowUp() since it is one of the exception types listed. And, of course, the finally block runs
at the end of the try/catch. (OCP Objective 6.2)
5. ☑ C, D, and G are correct. Since order doesn't matter, both D and G show correct use of the
multi-catch block. And C catches the IOException from fileBlowUp() directly. Note that
databaseBlowUp() is never called at runtime. However, if you remove the call, the compiler
won't let you catch the SQLException since it would be impossible to be thrown.
☐
✗ A is incorrect because it will not compile. Since there is already a catch block for
Exception, adding another will make the compiler think there is unreachable code.
B is incorrect because it will print c rather than b. Since FileNotFoundException is a
subclass of IOException, the thrown IOException will not match the catch block for
FileNotFoundException. E and H are incorrect because they are invalid syntax for multicatch. The catch parameter e can only appear once. F is incorrect because it will print c
rather than b. Since the IOException thrown by fileBlowUp() is never caught, the thrown
exception will match the catch block for Exception. (OCP Objective 6.2)
6. ☑ B, D, and F are correct. B uses multi-catch to identify both exceptions drive() may
throw. D still compiles since it uses the new enhanced exception typing to recognize that
Exception may only refer to AnotherTrainComing and RanOutOfTrack. F is the simple case
that catches a single exception. Since main throws AnotherTrainComing, the catch block
doesn't need to handle it.

Self Test Answers

415

☐
✗ A and E are incorrect because the catch block will not handle RanOutOfTrack when
drive() throws it. The main method will still throw the exception, but the println() will
not run. C is incorrect because it is invalid syntax for multi-catch. The catch parameter e can
only appear once. G is incorrect because of the above. (OCP Objective 6.2)
7. ☑ E is correct. After the exception is thrown, Automatic Resource Management calls
close() before completing the try block. From that point, catch and finally execute in the
normal order.
☐
✗ F is incorrect because the catch block catches the exception and does not rethrow it.
A, B, C, D, and G are incorrect because of the above. (OCP Objective 6.3)
8. ☑ G is correct. System.out.println cannot be in the declaration clause of a try-withresources block because it does not declare a variable. If the println was removed, the answer
would be A because resources are closed in the opposite order they are created.
☐
✗ A, B, C, D, E, and F are incorrect because of the above. (OCP Objective 6.3)
9. ☑

A and D are correct. If the code is left with no changes, it will not compile because

try-with-resources requires Lamb to implement AutoCloseable or a subinterface. If C2
is implemented, the code will not compile because close() throws Exception instead of
IOException. Unlike the traditional try, try-with-resources does not require catch or
finally to present. So the code works equally well with or without C3.

☐
✗ B, C, E, and F are incorrect because of the above. (OCP Objective 6.3)
10. ☑ D is correct. While the exception caught by the catch block matches choice A, it is
ignored by the catch block. The catch block just throws RuntimeException c without any
suppressed exceptions.
☐
✗ A, B, C, E, F, and G are incorrect because of the above. (OCP Objective 6.3)
11. ☑ A is correct. After the try block throws an IOException, Automatic Resource Management
calls close() to clean up the resources. Since an exception was already thrown in the try
block, RuntimeException a gets added to it as a suppressed exception. The catch block
merely rethrows the caught exception. The code does compile even though the catch block
catches an Exception and the method merely throws an IOException. In Java 7, the compiler
is able to pick up on this.
☐
✗ B, C, D, E, F, and G are incorrect because of the above. (OCP Objective 6.3)
12. ☑ F is correct. The exception variable in a catch block may not be reassigned when using
multi-catch. It CAN be reassigned if we are only catching one exception.
☐
✗ C would have been correct if e = new PowerOutage(); were removed. A, B, D, and E
are incorrect because of the above. (OCP Objectives 6.2 and 6.4)

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8
String Processing,
Data Formatting,
Resource Bundles
CERTIFICATION OBJECTIVES

•
•

Search, Parse, and Build Strings (Including
Scanner, StringTokenizer, StringBuilder,
String, and Formatter)
Search, Parse, and Replace Strings by Using
Regular Expressions, Using Expression
Patterns for Matching Limited to . (dot),
* (star), + (plus), ?, \d, \D, \s, \S, \w, \W, \b, \B,
[ ], and ( )

•

Format Strings Using the Formatting
Parameters %b, %c, %d, %f, and %s in
Format Strings

•

Read and Set the Locale Using the Locale
Object

•
•
•

Build a Resource Bundle for Each Locale

•

Describe the Advantages of Localizing an
Application

•

Define a Locale Using Language and
Country Codes

✓

Call a Resource Bundle from an Application
Format Dates, Numbers, and Currency
Values for Localization with the
NumberFormat and DateFormat Classes
(Including Number Format Patterns)

Two-Minute Drill

Q&A Self Test

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T

his chapter focuses on the exam objectives related to searching, formatting, and parsing
strings; formatting dates, numbers, and currency values; and using resource bundles for
localization and internationalization tasks. Many of these topics could fill an entire book.
Fortunately, you won't have to become a total regex guru to do well on the exam. The intention of
the exam team was to include just the basic aspects of these technologies, and in this chapter, we
cover more than you'll need to get through the related objectives on the exam.

CERTIFICATION OBJECTIVE

String, StringBuilder, and StringBuffer
(OCP Objective 5.1)
5.1 Search, parse, and build strings (including Scanner, StringTokenizer, StringBuilder,
String, and Formatter).
The OCA 7 exam covers the basics of building and using Strings and
StringBuilders. While most of the OCP 7 String and StringBuilder questions
will focus on searching and parsing, you might also get more basic questions, similar
to those found on the OCA 7 exam. We recommend that you refresh your String
and StringBuilder knowledge (the stuff we covered in Chapter 5), before taking
the OCP 7 exam.
We're going to start this chapter with date and number formatting and such, and
we'll return to parsing and tokenizing later in the chapter.

CERTIFICATION OBJECTIVE

Dates, Numbers, Currencies, and Locales
(OCP Objectives 12.1, 12.4, 12.5, and 12.6)
12.1

Read and set the locale using the Locale object.

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

419

12.4 Format dates, numbers, and currency values for localization with the
NumberFormat and DateFormat classes (including number format patterns).
12.5

Describe the advantages of localizing an application.

12.6

Define a locale using language and country codes.

The Java API provides an extensive (perhaps a little too extensive) set of classes
to help you work with dates, numbers, and currency. The exam will test your
knowledge of the basic classes and methods you'll use to work with dates and such.
When you've finished this section, you should have a solid foundation in tasks such
as creating new Date and DateFormat objects, converting Strings to Dates and
back again, performing Calendaring functions, printing properly formatted currency
values, and doing all of this for locations around the globe. In fact, a large part of
why this section was added to the exam was to test whether you can do some basic
internationalization (often shortened to "i18n").
Note: In this section, we'll introduce the Locale class. Later in the chapter, we'll
be discussing resource bundles, and you'll learn more about Locale then.

Working with Dates, Numbers, and Currencies
If you want to work with dates from around the world (and who doesn't?), you'll
need to be familiar with at least four classes from the java.text and java.util
packages. In fact, we'll admit it right up front: You might encounter questions on the
exam that use classes that aren't specifically mentioned in the Oracle objective.
Here are the five date-related classes you'll need to understand:
■ java.util.Date

Most of this class's methods have been deprecated, but
you can use this class to bridge between the Calendar and DateFormat class.
An instance of Date represents a mutable date and time, to a millisecond.

■ java.util.Calendar

This class provides a huge variety of methods that
help you convert and manipulate dates and times. For instance, if you want
to add a month to a given date or find out what day of the week January 1,
3000, falls on, the methods in the Calendar class will save your bacon.

■ java.text.DateFormat

This class is used to format dates, not only
providing various styles such as "01/01/70" or "January 1, 1970," but also dates
for numerous locales around the world.

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■ java.text.NumberFormat

This class is used to format numbers and
currencies for locales around the world.

■ java.util.Locale

This class is used in conjunction with DateFormat
and NumberFormat to format dates, numbers, and currency for specific
locales. With the help of the Locale class, you'll be able to convert a date
like "10/10/2005" to "Segunda-feira, 10 de Outubro de 2005" in no time. If
you want to manipulate dates without producing formatted output, you can
use the Locale class directly with the Calendar class.

Orchestrating Date- and Number-Related Classes
When you work with dates and numbers, you'll often use several classes together. It's
important to understand how the classes we described earlier relate to each other
and when to use which classes in combination. For instance, you'll need to know
that if you want to do date formatting for a specific locale, you need to create your
Locale object before your DateFormat object, because you'll need your Locale
object as an argument to your DateFormat factory method. Table 8-1 provides a
quick overview of common date- and number-related use cases and solutions using
these classes. Table 8-1 will undoubtedly bring up specific questions about individual
classes, and we will dive into specifics for each class next. Once you've gone through
the class-level discussions, you should find that Table 8-1 provides a good summary.

The Date Class
The Date class has a checkered past. Its API design didn’t do a good job of handling
internationalization and localization situations. In its current state, most of its
methods have been deprecated, and for most purposes, you’ll want to use the
Calendar class instead of the Date class. The Date class is on the exam for several
reasons: You might find it used in legacy code; it’s really easy if all you want is a
quick and dirty way to get the current date and time; it’s good when you want a
universal time that is not affected by time zones; and finally, you’ll use it as a
temporary bridge to format a Calendar object using the DateFormat class.
As we mentioned briefly earlier, an instance of the Date class represents a single
date and time. Internally, the date and time are stored as a primitive long. Specifically,
the long holds the number of milliseconds (you know, 1000 of these per second)
between the date being represented and January 1, 1970.

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

TABLE 8-1

Common Use
Cases When
Working with
Dates and
Numbers

421

Use Case

Steps

Get the current date
and time.

1. Create a Date: Date d = new Date();
2. Get its value: String s = d.toString();

Get an object that
lets you perform date
and time calculations
in your locale.

1. Create a Calendar:

Get an object that
lets you perform date
and time calculations
in a different locale.

1. Create a Locale:
Locale loc = new Locale(language); or
Locale loc = Locale(language, country);new
2. Create a Calendar for that locale:
Calendar c = Calendar.getInstance(loc);
3. Use c.add(...) and c.roll(...) to perform date and time

Calendar c = Calendar.getInstance();
2. Use c.add(...) and c.roll(...) to perform date and time

manipulations.

manipulations.
Get an object that
lets you perform
date and time
calculations, and
then format it for
output in different
locales with different
date styles.

1. Create a Calendar:
Calendar c = Calendar.getInstance();
2. Create a Locale for each location:
Locale loc = new Locale(...);
3. Convert your Calendar to a Date:
Date d = c.getTime();
4. Create a DateFormat for each Locale:
DateFormat df = DateFormat.getDateInstance
(style, loc);
5. Use the format() method to create formatted dates:
String s = df.format(d);

Get an object
that lets you
format numbers or
currencies across
many different
locales.

1. Create a Locale for each location:
Locale loc = new Locale(...);
2. Create a NumberFormat:
NumberFormat nf = NumberFormat.getInstance(loc);
-or- NumberFormat nf = NumberFormat.
getCurrencyInstance(loc);
3. Use the format() method to create formatted output:
String s = nf.format(someNumber);

Have you ever tried to grasp how big really big numbers are? Let's use the Date class
to find out how long it took for a trillion milliseconds to pass, starting at January 1, 1970:
import java.util.*;
class TestDates {
public static void main(String[] args) {
Date d1 = new Date(1_000_000_000_000L);
// a trillion, Java 7 style
System.out.println("1st date " + d1.toString());
}
}

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On our JVM, which has a U.S. locale, the output is
1st date Sat Sep 08 19:46:40 MDT 2001

Okay, for future reference, remember that there are a trillion milliseconds for every
31 and 2/3 years.
Although most of Date's methods have been deprecated, it's still acceptable to
use the getTime and setTime methods, although, as we'll soon see, it's a bit painful.
Let's add an hour to our Date instance, d1, from the previous example:
import java.util.*;
class TestDates {
public static void main(String[] args) {
Date d1 = new Date(1_000_000_000_000L);
// a trillion!
System.out.println("1st date " + d1.toString());
d1.setTime(d1.getTime() + 3_600_000);
// 3_600_000 millis / hour
System.out.println("new time " + d1.toString());
}
}

which produces (again, on our JVM):
1st date Sat Sep 08 19:46:40 MDT 2001
new time Sat Sep 08 20:46:40 MDT 2001

Notice that both setTime() and getTime() used the handy millisecond scale…
if you want to manipulate dates using the Date class, that's your only choice. While
that wasn't too painful, imagine how much fun it would be to add, say, a year to a
given date.
We'll revisit the Date class later on, but for now, the only other thing you need to
know is that if you want to create an instance of Date to represent "now," you use
Date's no-argument constructor:
Date now = new Date();

(We're guessing that if you call now.getTime(), you'll get a number somewhere
between one trillion and two trillion.)

The Calendar Class
We've just seen that manipulating dates using the Date class is tricky. The Calendar
class is designed to make date manipulation easy! (Well, easier.) While the Calendar
class has about a million fields and methods, once you get the hang of a few of them,
the rest tend to work in a similar fashion.
When you first try to use the Calendar class, you might notice that it's an
abstract class. You can't say

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

Calendar c = new Calendar();

423

// illegal, Calendar is abstract

In order to create a Calendar instance, you have to use one of the overloaded
getInstance() static factory methods:
Calendar cal = Calendar.getInstance();

When you get a Calendar reference like cal, from earlier, your Calendar reference
variable is actually referring to an instance of a concrete subclass of Calendar.
You can't know for sure what subclass you'll get (java.util.GregorianCalendar
is what you'll almost certainly get), but it won't matter to you. You'll be using
Calendar's API. (As Java continues to spread around the world, in order to maintain
cohesion, you might find additional, locale-specific subclasses of Calendar.)
Okay, so now we've got an instance of Calendar, let's go back to our earlier
example and find out what day of the week our trillionth millisecond falls on, and
then let's add a month to that date:
import java.util.*;
class Dates2 {
public static void main(String[] args) {
Date d1 = new Date(1_000_000_000_000L);
System.out.println("1st date " + d1.toString());
Calendar c = Calendar.getInstance();
c.setTime(d1);

// #1

if(Calendar.SUNDAY == c.getFirstDayOfWeek())
// #2
System.out.println("Sunday is the first day of the week");
System.out.println("trillionth milli day of week is "
+ c.get(Calendar.DAY_OF_WEEK));
// #3
c.add(Calendar.MONTH, 1);
// #4
Date d2 = c.getTime();
// #5
System.out.println("new date " + d2.toString() );
}
}

This produces something like
1st date Sat Sep 08 19:46:40 MDT 2001
Sunday is the first day of the week
trillionth milli day of week is 7
new date Mon Oct 08 19:46:40 MDT 2001

Let's take a look at this program, focusing on the five highlighted lines:
1. We assign the Date d1 to the Calendar instance c.

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2. We use Calendar's SUNDAY field to determine whether, for our JVM, SUNDAY
is considered to be the first day of the week. (In some locales, MONDAY is the
first day of the week.) The Calendar class provides similar fields for days of
the week, months, the day of the month, the day of the year, and so on.
3. We use the DAY_OF_WEEK field to find out the day of the week that the trillionth millisecond falls on.
4. So far, we've used "setter" and "getter" methods that should be intuitive to
figure out. Now we're going to use Calendar's add() method. This very powerful method lets you add or subtract units of time appropriate for whichever
Calendar field you specify. For instance:
c.add(Calendar.HOUR, -4);
c.add(Calendar.YEAR, 2);
c.add(Calendar.DAY_OF_WEEK, -2);

//
//
//
//
//

subtract 4 hours from c's
value
add 2 years to c's value
subtract two days from
c's value

5. Convert c's value back to an instance of Date.
The other Calendar method you should know for the exam is the roll() method.
The roll() method acts like the add() method, except that when a part of a Date
gets incremented or decremented, larger parts of the Date will not get incremented
or decremented. Hmmm… for instance:
// assume c is October 8, 2001
c.roll(Calendar.MONTH, 9);
// notice the year in the output
Date d4 = c.getTime();
System.out.println("new date " + d4.toString() );

The output would be something like this:
new date Fri Jul 08 19:46:40 MDT 2001

Notice that the year did not change, even though we added nine months to an
October date. In a similar fashion, invoking roll() with HOUR won't change the
date, the month, or the year.
For the exam, you won't have to memorize the Calendar class's fields. If you need
them to help answer a question, they will be provided as part of the question.

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

425

The DateFormat Class
Having learned how to create dates and manipulate them, let's find out how to
format them. So that we're all on the same page, here's an example of how a date
can be formatted in different ways:
import java.text.*;
import java.util.*;
class Dates3 {
public static void main(String[] args) {
Date d1 = new Date(1_000_000_000_000L);
// project Coin at work!
DateFormat[] dfa = new DateFormat[6];
dfa[0] = DateFormat.getInstance();
dfa[1] = DateFormat.getDateInstance();
dfa[2] = DateFormat.getDateInstance(DateFormat.SHORT);
dfa[3] = DateFormat.getDateInstance(DateFormat.MEDIUM);
dfa[4] = DateFormat.getDateInstance(DateFormat.LONG);
dfa[5] = DateFormat.getDateInstance(DateFormat.FULL);
for(DateFormat df : dfa)
System.out.println(df.format(d1));
}
}

which on our JVM produces
9/8/01 7:46 PM
Sep 8, 2001
9/8/01
Sep 8, 2001
September 8, 2001
Saturday, September 8, 2001

Examining this code, we see a couple of things right away. First off, it looks like
DateFormat is another abstract class, so we can't use new to create instances of
DateFormat. In this case, we used two factory methods: getInstance() and
getDateInstance(). Notice that getDateInstance() is overloaded; when we
discuss locales, we'll look at the other version of getDateInstance() that you'll

need to understand for the exam.
Next, we used static fields from the DateFormat class to customize our various
instances of DateFormat. Each of these static fields represents a formatting style. In
this case, it looks like the no-arg version of getDateInstance() gives us the same
style as the MEDIUM version of the method, but that's not a hard-and-fast rule. (More
on this when we discuss locales.) Finally, we used the format() method to create
strings representing the properly formatted versions of the Date we're working with.
The last method you should be familiar with is the parse() method. The
parse() method takes a string formatted in the style of the DateFormat instance

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being used and converts the string into a Date object. As you might imagine, this is
a risky operation because the parse() method could easily receive a badly formatted
string. Because of this, parse() can throw a ParseException. The following code
creates a Date instance, uses DateFormat.format() to convert it into a string, and
then uses DateFormat.parse() to change it back into a Date:
Date d1 = new Date(1000000000000L);
System.out.println("d1 = " + d1.toString());
DateFormat df = DateFormat.getDateInstance(
DateFormat.SHORT);
String s = df.format(d1);
System.out.println(s);
try {
Date d2 = df.parse(s);
System.out.println("parsed = " + d2.toString());
} catch (ParseException pe) {
System.out.println("parse exc"); }

which on our JVM produces
d1 = Sat Sep 08 19:46:40 MDT 2001
9/8/01
parsed = Sat Sep 08 00:00:00 MDT 2001

Note: If we'd wanted to retain the time along with the date, we could have used
the getDateTimeInstance()method, but it's not on the exam.
The API for DateFormat.parse() explains that, by default, the parse()
method is lenient when parsing dates. Our experience is that parse() isn't
very lenient about the formatting of strings it will successfully parse into
dates; take care when you use this method!

The Locale Class
Earlier, we said that a big part of why this objective exists is to test your ability to do
some basic internationalization tasks. Your wait is over; the Locale class is your
ticket to worldwide domination. Both the DateFormat class and the NumberFormat
class (which we'll cover next) can use an instance of Locale to customize formatted
output to be specific to a locale. You might ask how Java defines a locale. The API
says a locale is "a specific geographical, political, or cultural region." The two
Locale constructors you'll need to understand for the exam are
Locale(String language)
Locale(String language, String country)

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

427

The language argument represents an ISO 639 Language code, so, for instance, if
you want to format your dates or numbers in Walloon (the language sometimes used
in southern Belgium), you'd use "wa" as your language string. There are over 500
ISO Language codes, including one for Klingon ("tlh"), although, unfortunately,
Java doesn't yet support the Klingon locale. We thought about telling you that you'd
have to memorize all these codes for the exam… but we didn't want to cause any
heart attacks. So rest assured, you won't have to memorize any ISO Language codes
or ISO Country codes (of which there are about 240) for the exam.
Let's get back to how you might use these codes. If you want to represent basic
Italian in your application, all you need is the language code. If, on the other hand,
you want to represent the Italian used in Switzerland, you'd want to indicate that
the country is Switzerland (yes, the country code for Switzerland is "CH"), but that
the language is Italian:
Locale locIT = new Locale("it");
Locale locCH = new Locale("it", "CH");

// Italian
// Switzerland

Using these two locales on a date could give us output like this:
sabato 1 ottobre 2005
sabato, 1. ottobre 2005

Now let's put this all together in some code that creates a Calendar object, sets
its date, and then converts it to a Date. After that, we'll take that Date object and
print it out using locales from around the world:
Calendar c = Calendar.getInstance();
c.set(2010, 11, 14);

// December 14, 2010
// (month is 0-based)

Date d2 = c.getTime();
Locale
Locale
Locale
Locale
Locale

locIT
locPT
locBR
locIN
locJA

=
=
=
=
=

new
new
new
new
new

Locale("it", "IT");
Locale("pt");
Locale("pt", "BR");
Locale("hi", "IN");
Locale("ja");

//
//
//
//
//

Italy
Portugal
Brazil
India
Japan

DateFormat dfUS = DateFormat.getInstance();
System.out.println("US
" + dfUS.format(d2));
DateFormat dfUSfull = DateFormat.getDateInstance(
DateFormat.FULL);
System.out.println("US full " + dfUSfull.format(d2));
DateFormat dfIT = DateFormat.getDateInstance(
DateFormat.FULL, locIT);
System.out.println("Italy
" + dfIT.format(d2));

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DateFormat dfPT = DateFormat.getDateInstance(
DateFormat.FULL, locPT);
System.out.println("Portugal " + dfPT.format(d2));
DateFormat dfBR = DateFormat.getDateInstance(
DateFormat.FULL, locBR);
System.out.println("Brazil
" + dfBR.format(d2));
DateFormat dfIN = DateFormat.getDateInstance(
DateFormat.FULL, locIN);
System.out.println("India
" + dfIN.format(d2));
DateFormat dfJA = DateFormat.getDateInstance(
DateFormat.FULL, locJA);
System.out.println("Japan
" + dfJA.format(d2));

This, on our JVM, produces
US
US full
Italy
Portugal
Brazil
India
Japan

12/14/10 3:32 PM
Sunday, December 14, 2010
domenica 14 dicembre 2010
Domingo, 14 de Dezembro de 2010
Domingo, 14 de Dezembro de 2010
??????, ?? ??????, ????
2010?12?14?

Oops! Our machine isn't configured to support locales for India or Japan, but you
can see how a single Date object can be formatted to work for many locales.

Remember that both DateFormat and NumberFormat objects can have
their locales set only at the time of instantiation. Watch for code that attempts to change
the locale of an existing instance—no such methods exist!
There are a couple more methods in Locale (getDisplayCountry() and
getDisplayLanguage()) that you'll have to know for the exam. These methods let
you create strings that represent a given locale's country and language in terms of
both the default locale and any other locale:
Locale locBR = new Locale("pt", "BR");
Locale locDK = new Locale("da", "DK");
Locale locIT = new Locale("it", "IT");

// Brazil
// Denmark
// Italy

System.out.println("def " + locBR.getDisplayCountry());

Dates, Numbers, Currencies, and Locales (OCP Objectives 12.1, 12.4, 12.5, and 12.6)

429

System.out.println("loc " + locBR.getDisplayCountry(locBR));
System.out.println("def " + locDK.getDisplayLanguage());
System.out.println("loc " + locDK.getDisplayLanguage(locDK));
System.out.println("D>I " + locDK.getDisplayLanguage(locIT));

This, on our JVM, produces
def
loc
def
loc
D>I

Brazil
Brasil
Danish
dansk
danese

Given that our JVM's locale (the default for us) is US, the default for the country
Brazil is Brazil, and the default for the Danish language is Danish. In Brazil, the
country is called Brasil, and in Denmark, the language is called dansk. Finally, just
for fun, we discovered that in Italy, the Danish language is called danese.

The NumberFormat Class
We'll wrap up this objective by discussing the NumberFormat class. Like the
DateFormat class, NumberFormat is abstract, so you'll typically use some version of
either getInstance() or getCurrencyInstance() to create a NumberFormat
object. Not surprisingly, you use this class to format numbers or currency values:
float f1 = 123.4567f;
Locale locFR = new Locale("fr");
// France
NumberFormat[] nfa = new NumberFormat[4];
nfa[0]
nfa[1]
nfa[2]
nfa[3]

=
=
=
=

NumberFormat.getInstance();
NumberFormat.getInstance(locFR);
NumberFormat.getCurrencyInstance();
NumberFormat.getCurrencyInstance(locFR);

for(NumberFormat nf : nfa)
System.out.println(nf.format(f1));

This, on our JVM, produces
123.457
123,457
$123.46
123,46 ?

Don't be worried if, like us, you're not set up to display the symbols for francs,
pounds, rupees, yen, baht, or drachmas. You won't be expected to know the symbols
used for currency: If you need one, it will be specified in the question. You might

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encounter methods other than the format() method on the exam. Here's a little
code that uses getMaximumFractionDigits(), setMaximumFractionDigits(),
parse(), and setParseIntegerOnly():
float f1 = 123.45678f;
NumberFormat nf = NumberFormat.getInstance();
System.out.print(nf.getMaximumFractionDigits() + " ");
System.out.print(nf.format(f1) + " ");
nf.setMaximumFractionDigits(5);
System.out.println(nf.format(f1) + "

");

try {
System.out.println(nf.parse("1234.567"));
nf.setParseIntegerOnly(true);
System.out.println(nf.parse("1234.567"));
} catch (ParseException pe) {
System.out.println("parse exc");
}

This, on our JVM, produces
3 123.457
1234.567
1234

123.45678

Notice that in this case, the initial number of fractional digits for the default
NumberFormat is three, and that the format() method rounds f1's value—it
doesn't truncate it. After changing nf's fractional digits, the entire value of f1 is
displayed. Next, notice that the parse() method must run in a try/catch block and
that the setParseIntegerOnly() method takes a boolean and, in this case, causes
subsequent calls to parse() to return only the integer part of strings formatted as

floating-point numbers.
As we've seen, several of the classes covered in this objective are abstract. In
addition, for all of these classes, key functionality for every instance is established at
the time of creation. Table 8-2 summarizes the constructors or methods used to
create instances of all the classes we've discussed in this section.

Parsing, Tokenizing, and Formatting (OCP Objectives 5.1, 5.2, and 5.3)

TABLE 8-2

Instance Creation
for Key java
.text and
java.util
Classes

431

Class

Key Instance Creation Options

util.Date

new Date();
new Date(long millisecondsSince010170);

util.Calendar

Calendar.getInstance();
Calendar.getInstance(Locale);

util.Locale

Locale.getDefault();
new Locale(String language);
new Locale(String language, String country);

text.DateFormat

DateFormat.getInstance();
DateFormat.getDateInstance();
DateFormat.getDateInstance(style);
DateFormat.getDateInstance(style, Locale);

text.NumberFormat

NumberFormat.getInstance()
NumberFormat.getInstance(Locale)
NumberFormat.getNumberInstance()
NumberFormat.getNumberInstance(Locale)
NumberFormat.getCurrencyInstance()
NumberFormat.getCurrencyInstance(Locale)

CERTIFICATION OBJECTIVE

Parsing,Tokenizing, and Formatting
(OCP Objectives 5.1, 5.2, and 5.3)
5.1 Search, parse, and build strings (including Scanner, StringTokenizer, StringBuilder,
String, and Formatter).
5.2 Search, parse, and replace strings by using regular expressions, using expression
patterns for matching limited to . (dot), * (star), + (plus), ?, \d, \D, \s, \S, \w, \W,
\b, \B, [], and ().
5.3 Format strings using the formatting parameters %b, %c, %d, %f, and %s in
format strings.
We're going to start with yet another disclaimer: This small section isn't going to
morph you from regex newbie to regex guru. In this section, we'll cover three basic ideas:
■ Finding stuff

You've got big heaps of text to look through. Maybe you're
doing some screen scraping; maybe you're reading from a file. In any case,

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you need easy ways to find textual needles in textual haystacks. We'll use
the java.util.regex.Pattern, java.util.regex.Matcher, and java
.util.Scanner classes to help us find stuff.
■ Tokenizing stuff

You've got a delimited file that you want to get useful
data out of. You want to transform a piece of a text file that looks like
"1500.00,343.77,123.4" into some individual float variables. We'll show you the
basics of using the String.split() method and the java.util.Scanner
class to tokenize your data.

■ Formatting stuff

You've got a report to create and you need to take a float
variable with a value of 32500.000f and transform it into a string with a value
of "$32,500.00". We'll introduce you to the java.util.Formatter class and
to the printf() and format() methods.

A Search Tutorial
Whether you're looking for stuff or tokenizing stuff, a lot of the concepts are the
same, so let's start with some basics. No matter what language you're using, sooner or
later you'll probably be faced with the need to search through large amounts of
textual data, looking for some specific stuff.
Regular expressions (regex for short) are a kind of language within a language,
designed to help programmers with these searching tasks. Every language that provides
regex capabilities uses one or more regex engines. Regex engines search through textual
data using instructions that are coded into expressions. A regex expression is like a very
short program or script. When you invoke a regex engine, you'll pass it the chunk of
textual data you want it to process (in Java, this is usually a string or a stream), and you
pass it the expression you want it to use to search through the data.
It's fair to think of regex as a language, and we will refer to it that way throughout
this section. The regex language is used to create expressions, and as we work
through this section, whenever we talk about expressions or expression syntax, we're
talking about syntax for the regex "language." Oh, one more disclaimer… we know
that you regex mavens out there can come up with better expressions than what
we're about to present. Keep in mind that for the most part, we're creating these
expressions using only a portion of the total regex instruction set, thanks.

Simple Searches
For our first example, we'd like to search through the following source String
abaaaba

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433

for all occurrences (or matches) of the expression
ab

In all of these discussions, we'll assume that our data sources use zero-based
indexes, so if we display an index under our source String, we get
source: abaaaba
index: 0123456

We can see that we have two occurrences of the expression ab: one starting at
position 0 and the second starting at position 4. If we sent the previous source data
and expression to a regex engine, it would reply by telling us that it found matches
at positions 0 and 4. Below is a program (which we'll explain in a few pages) that
you can use to perform as many regex experiments as you want to get the feel for
how regex works. We'll use this program to show you some of the basics that are
covered in the exam:
import java.util.regex.*;
class RegTest {
public static void main(String [] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
System.out.println("\nsource: " + args[1]);
System.out.println(" index: 01234567890123456\n");
System.out.println("expression: " + m.pattern());
System.out.print("match positions: ");
while(m.find()) {
System.out.print(m.start() + " ");
}
System.out.println("");
}
}

// string to search
// the index
// the search expression
// matches positions

So this invocation:
java RegTest "ab" "abaaaba"

produces
source: abaaaba
index: 01234567890123456
expression: ab
match positions: 0 4

In a few pages, we're going to show you a lot more regex code, but first we want to
go over some more regex syntax. Once you understand a little more regex, the code

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samples will make a lot more sense. Here's a more complicated example of a source
and an expression:
source: abababa
index: 0123456
expression: aba

How many occurrences do we get in this case? Well, there is clearly an occurrence
starting at position 0 and another starting at position 4. But how about starting at
position 2? In general in the world of regex, the aba string that starts at position 2
will not be considered a valid occurrence. The first general regex search rule is
In general, a regex search runs from left to right, and once a source's character
has been used in a match, it cannot be reused.
So in our previous example, the first match used positions 0, 1, and 2 to match
the expression. (Another common term for this is that the first three characters of
the source were consumed.) Because the character in position 2 was consumed in the
first match, it couldn't be used again. So the engine moved on and didn't find
another occurrence of aba until it reached position 4. This is the typical way that a
regex matching engine works. However, in a few pages, we'll look at an exception to
the first rule we stated earlier.
So we've matched a couple of exact strings, but what would we do if we wanted to
find something a little more dynamic? For instance, what if we wanted to find all of
the occurrences of hex numbers or phone numbers or ZIP codes?

Searches Using Metacharacters
As luck would have it, regex has a powerful mechanism for dealing with the cases we
described earlier. At the heart of this mechanism is the idea of a metacharacter. As an
easy example, let's say that we want to search through some source data looking for
all occurrences of numeric digits. In regex, the following expression is used to look
for numeric digits:
\d

If we change the previous program to apply the expression \d to the following
source string, we'd see:
java RegTest "\\d" "a12c3e456f"
source: a12c3e456f
index: 01234567890123456
expression: \d
match positions: 1 2 4 6 7 8

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regex will tell us that it found digits at positions 1, 2, 4, 6, 7, and 8. (If you want to
try this at home, you'll need to "escape" the compile method's \d argument by
making it "\\d"; more on this a little later.)
Regex provides a rich set of metacharacters that you can find described in the
API documentation for java.util.regex.Pattern. We won't discuss them all
here, but we will describe the ones you'll need for the exam:
■ \d

A digit (0–9)

\D A non-digit (anything BUT 0–9)

■ \s
\S

A whitespace character (e.g. space, \t, \n, \f, \r)
A non-whitespace character

■ \w

A word character (letters (a–z and A–Z), digits, or the "_" [underscore])
\W A non-word character (everything else)

■ \b A word "boundary" (ends of the string and between \w and not
\w—more soon)
\B A non-word "boundary" (between two \w's or two not \w's)

So, for example, given
source: "a 1 56 _Z"
index:
012345678
pattern: \w

regex will return positions 0, 2, 4, 5, 7, and 8. The only characters in this source that
don't match the definition of a word character are the whitespaces. (Note: In this
example, we enclosed the source data in quotes to clearly indicate that there was no
whitespace at either end.)

Character Matching The first six ( \d, \D, \s, \S, \w, \W), are fairly
straightforward. Regex returns the positions where occurrences of those types of
characters (or their opposites occur). Here's an example of an "opposites" match:
java RegTest "\\S" "w1w w$ &#w1"
source: w1w w$ &#w1
index: 01234567890123456
expression: \S
match positions: 0 1 2 4 5 7 8 9 10

Here you can see that regex matched on everything BUT whitespace.

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Boundary Matching The last two ( \b and \B) are a bit different. In these
cases, regex is looking for a specific relationship between two adjacent characters.
When it finds a match, it returns the position of the second character. Also note
that the ends of the strings are considered to be "non-word" characters. Let's look at
a few examples:
java RegTest "\\b" "w2w w$ &#w2"
source: w2w w$ &#w2
index: 01234567890123456
expression: \b
match positions: 0 3 4 5 9 11

First, let's recall that "word characters" are A–Z, a–z, and 0–9. It's not too tricky
to understand the matches at positions 3, 4, 5, and 9. Regex is telling us that
characters 2 and 3 are a boundary between a word character and a non-word
character. Remembering that order doesn't matter, it's easy to see that positions 4, 5,
and 9 are similar "boundaries" between the two classes of characters—the character
specified and the one preceding it.
But the matches on positions 0 and 11 are a bit confusing. For the sake of the
exam, just imagine that for \b and \B, there is a hidden, non-word character at each
end of the string that you can see. Let's look at an example of using \b and then \B
against the same string:
source: #ab de#
index: 01234567890123456
expression: \b
match positions: 1 3 4 6

In this case, the matches should be intuitive; they mark the second character in a
pair of characters that represent a boundary (word versus non-word). But here:
source: #ab de#
index: 01234567890123456
expression: \B
match positions: 0 2 5 7

in this case, assuming invisible, non-word characters at each end of the string, we see
places where there are NOT word boundaries (i.e., where two-word characters abut
or where two non-word characters abut).

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Searches Using Ranges
You can also specify sets of characters to search for using square brackets and ranges
of characters to search for using square brackets and a dash:
■ [abc]

Searches only for a's, b's, or c's

■ [a-f]

Searches only for a, b, c, d, e, or f characters

In addition, you can search across several ranges at once. The following
expression is looking for occurrences of the letters a-f or A-F; it's NOT looking for
an fA combination:
■ [a-fA-F] Searches for the first six letters of the alphabet, both cases.
So, for instance,
source: "cafeBABE"
index:
01234567
pattern: [a-cA-C]

returns positions 0, 1, 4, 5, 6.
In addition to the capabilities described for the exam, you can apply the
following attributes to sets and ranges within square brackets: "^" to negate
the characters specified, nested brackets to create a union of sets, and "&&"
to specify the intersection of sets. While these constructs are not on the exam,
they are quite useful, and good examples can be found in the API for the
java.util.regex.Pattern class.

Searches Using Quantifiers
Let's say that we want to create a regex pattern to search for hexadecimal literals. As
a first step, let's solve the problem for one-digit hexadecimal numbers:
0[xX][0-9a-fA-F]

The preceding expression could be stated:
Find a set of characters in which the first character is a "0", the second character
is either an "x" or an "X", and the third character is a digit from "0" to "9", a letter
from "a" to "f", or an uppercase letter from "A" to "F".
Using the preceding expression and the following data:
source: 12 0x 0x12 0Xf 0xg
index: 012345678901234567

regex would return 6 and 11. (Note: 0x and 0xg are not valid hex numbers.)

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As a second step, let's think about an easier problem. What if we just wanted
regex to find occurrences of integers? Integers can be one or more digits long, so it
would be great if we could say "one or more" in an expression. There is a set of regex
constructs called quantifiers that let us specify concepts such as "one or more." In
fact, the quantifier that represents "one or more" is the "+" character. We'll see the
others shortly.
The other issue this raises is that when we're searching for something whose
length is variable, getting only a starting position as a return value is of limited use.
So, in addition to returning starting positions, another bit of information that a
regex engine can return is the entire match, or group, that it finds. We're going to
change the way we talk about what regex returns by specifying each return on its
own line, remembering that now for each return we're going to get back the starting
position AND then the group. Here's the revised code:
import java.util.regex.*;
class GroupTest {
public static void main(String [] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
System.out.println("\nsource: " + args[1]);
System.out.println(" index: 01234567890123456\n");
System.out.println("pattern: " + m.pattern());
while(m.find()) {
System.out.println(m.start() + " " + m.group());
}
System.out.println("");
}
}

So, if we invoke GroupTest like this:
java GroupTest "\d+" "1 a12 234b"

you can read this expression as saying: "Find one or more digits in a row." This
expression produces this regex output:
source: 1 a12 234b
index: 01234567890123456
pattern: \d+
0 1
3 12
6 234

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You can read this as "At position 0, there's an integer with a value of 1; then at
position 3, there's an integer with a value of 12; then at position 6, there's an integer
with a value of 234." Returning now to our hexadecimal problem, the last thing we
need to know is how to specify the use of a quantifier for only part of an expression.
In this case, we must have exactly one occurrence of 0x or 0X, but we can have from
one to many occurrences of the hex "digits" that follow. The following expression
adds parentheses to limit the "+" quantifier to only the hex digits:
0[xX]([0-9a-fA-F])+

The parentheses and "+" augment the previous find-the-hex expression by saying
in effect: "Once we've found our 0x or 0X, you can find from one to many
occurrences of hex digits." Notice that we put the "+" quantifier at the end of the
expression. It's useful to think of quantifiers as always quantifying the part of the
expression that precedes them.
The other two quantifiers we're going to look at are
■ *

Zero or more occurrences

■ ?

Zero or one occurrence

Let's say you have a text file containing a comma-delimited list of all the
filenames in a directory that contains several very important projects. (BTW, this
isn't how we'd arrange our directories. :)) You want to create a list of all the files
whose names start with proj1. You might discover .txt files, .java files, .pdf files—
who knows? What kind of regex expression could we create to find these various
proj1 files? First, let's take a look at what a part of this text might look like:
..."proj3.txt,proj1sched.pdf,proj1,proj2,proj1.java"...

To solve this problem, we're going to use the regex ^ (carat) operator, which we
mentioned earlier. The regex ^ operator isn't on the exam, but it will help us create
a fairly clean solution to our problem. The ^ is the negation symbol in regex. For
instance, if you want to find anything but a's, b's, or c's in a file, you could say
[^abc]

So, armed with the ^ operator and the * (zero or more) quantifier, we can create the
following:
proj1([^,])*

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If we apply this expression to just the portion of the text file we listed earlier,
regex returns
10 proj1sched.pdf
25 proj1
37 proj1.java

The key part of this expression is the "give me zero or more characters that aren't a
comma."
The last quantifier example we'll look at is the ? (zero or one) quantifier. Let's say
that our job this time is to search a text file and find anything that might be a local
seven-digit phone number. We're going to say, arbitrarily, that if we find seven digits
in a row, or three digits followed by a dash, or a space followed by four digits, that we
have a candidate. Here are examples of "valid" phone numbers:
1234567
123 4567
123-4567

The key to creating this expression is to see that we need "zero or one instance of
either a space or a dash" in the middle of our digits:
\d\d\d([-\s])?\d\d\d\d

The Predefined Dot
In addition to the \s, \d, and \w metacharacters that we discussed, you have to
understand the "." (dot) metacharacter. When you see this character in a regex
expression, it means "any character can serve here." For instance, the following
source and pattern:
source: "ac abc a c"
pattern: a.c

will produce the output
3 abc
7 a c

The "." was able to match both the "b" and the " " in the source data.

Greedy Quantifiers
When you use the *, +, and ? quantifiers, you can fine-tune them a bit to produce
behavior that's known as "greedy," "reluctant," or "possessive." Although you need to

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understand only the greedy quantifier for the exam, we're also going to discuss the
reluctant quantifier to serve as a basis for comparison. First, the syntax:
■ ? is greedy, ?? is reluctant, for zero or once
■ *is greedy, *? is reluctant, for zero or more
■ + is greedy, +? is reluctant, for one or more

What happens when we have the following source and pattern:
source: yyxxxyxx
pattern: .*xx

First off, we're doing something a bit different here by looking for characters that
prefix the static (xx) portion of the expression. We think we're saying something
like: "Find sets of characters that end with xx". Before we tell what happens, we at
least want you to consider that there are two plausible results… can you find them?
Remember we said earlier that in general, regex engines worked from left to right
and consumed characters as they went. So, working from left to right, we might
predict that the engine would search the first four characters (0–3), find xx starting
in position 2, and have its first match. Then it would proceed and find the second
xx starting in position 6. This would lead us to a result like this:
0 yyxx
4 xyxx

A plausible second argument is that since we asked for a set of characters that
ends with xx, we might get a result like this:
0 yyxxxyxx

The way to think about this is to consider the name greedy. In order for the
second answer to be correct, the regex engine would have to look (greedily) at the
entire source data before it could determine that there was an xx at the end. So, in
fact, the second result is the correct result because in the original example we used
the greedy quantifier *. The result that finds two different sets can be generated by
using the reluctant quantifier *?. Let's review:
source: yyxxxyxx
pattern: .*xx

is using the greedy quantifier * and produces
0 yyxxxyxx

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If we change the pattern to
source: yyxxxyxx
pattern: .*?xx

we're now using the reluctant qualifier *?, and we get the following:
0 yyxx
4 xyxx

The greedy quantifier does, in fact, read the entire source data and then it works
backward (from the right) until it finds the rightmost match. At that point, it
includes everything from earlier in the source data, up to and including the data that
is part of the rightmost match.
There are a lot more aspects to regex quantifiers than we've discussed here,
but we've covered more than enough for the exam. Oracle has several tutorials
that will help you learn more about quantifiers and turn you into the go-to
person at your job.

When Metacharacters and Strings Collide
So far, we've been talking about regex from a theoretical perspective. Before we can
put regex to work, we have to discuss one more gotcha. When it's time to implement
regex in our code, it will be quite common that our source data and/or our
expressions will be stored in strings. The problem is that metacharacters and strings
don't mix too well. For instance, let's say we just want to do a simple regex pattern
that looks for digits. We might try something like
String pattern = "\d";

// compiler error!

This line of code won't compile! The compiler sees the \ and thinks, "Okay, here
comes an escape sequence; maybe it'll be a new line!" But no, next comes the d and
the compiler says, "I've never heard of the \d escape sequence." The way to satisfy
the compiler is to add another backslash in front of the \d:
String pattern = "\\d";

// a compilable metacharacter

The first backslash tells the compiler that whatever comes next should be taken
literally, not as an escape sequence. How about the dot (.) metacharacter? If we
want a dot in our expression to be used as a metacharacter, no problem, but what if
we're reading some source data that happens to use dots as delimiters? Here's another
way to look at our options:

Parsing, Tokenizing, and Formatting (OCP Objectives 5.1, 5.2, and 5.3)

String p = ".";
String p = "\.";
String p = "\\.";

//
//
//
//
//

443

regex sees this as the "." metacharacter
the compiler sees this as an illegal
Java escape sequence
the compiler is happy, and regex sees a
dot, not a metacharacter

A similar problem can occur when you hand metacharacters to a Java program via
command-line arguments. If we want to pass the \d metacharacter into our Java
program, our JVM does the right thing if we say
% java DoRegex "\d"

But your JVM might not. If you have problems running the following examples,
you might try adding a backslash (i.e., \\d) to your command-line metacharacters.
Don't worry—you won't see any command-line metacharacters on the exam!
The Java language defines several escape sequences, including
\n = linefeed (which you might see on the exam)
\b = backspace
\t = tab

And others, which you can find in the Java Language Specification. Other
than perhaps seeing a \n inside a string, you won't have to worry about Java's
escape sequences on the exam.
At this point, we've learned enough of the regex language to start using it in our
Java programs. We'll start by looking at using regex expressions to find stuff, and
then we'll move to the closely related topic of tokenizing stuff.

Locating Data via Pattern Matching
Over the last few pages, we've used a few small Java programs to explore some regex
basics. Now we're going to take a more detailed look at the two classes we've been
using: java.util.regex.Pattarn and java.util.regex.Matcher. Once you
know a little regex, using the java.util.regex.Pattern (Pattern) and java.
util.regex.Matcher (Matcher) classes is pretty straightforward. The Pattern
class is used to hold a representation of a regex expression so that it can be used and
reused by instances of the Matcher class. The Matcher class is used to invoke the
regex engine, with the intention of performing match operations. The following
program shows Pattern and Matcher in action, and, as we've seen, it's not a bad

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way for you to do your own regex experiments. Note, once you've read about the
Console class in Chapter 9, you might want to modify the following class by adding
some functionality from the Console class. That way, you'll get some practice with
the Console class, and it'll be easier to run multiple regex experiments.
import java.util.regex.*;
class Regex {
public static void main(String [] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
System.out.println("Pattern is " + m.pattern());
while(m.find()) {
System.out.println(m.start() + " " + m.group());
}
}
}

As with our earlier programs, this program uses the first command-line argument
(args[0]) to represent the regex expression you want to use, and it uses the second
argument (args[1]) to represent the source data you want to search. Here's a test run:
% java Regex "\d\w" "ab4 56_7ab"

produces the output
Pattern is \d\w
4 56
7 7a

(Remember, if you want this expression to be represented in a string, you'd use
\\d\\w.) Because you'll often have special characters or whitespace as part of your

arguments, you'll probably want to get in the habit of always enclosing your
argument in quotes. Let's take a look at this code in more detail. First off, notice that
we aren't using new to create a Pattern; if you check the API, you'll find no
constructors are listed. You'll use the overloaded, static compile() method (which
takes String expression) to create an instance of Pattern. For the exam, all
you'll need to know to create a Matcher is to use the Pattern.matcher() method
(which takes String sourceData).
The important method in this program is the find() method. This is the method
that actually cranks up the regex engine and does some searching. The find()
method returns true if it gets a match and remembers the start position of the
match. If find() returns true, you can call the start() method to get the starting
position of the match, and you can call the group() method to get the string that
represents the actual bit of source data that was matched.

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To provide the most flexibility, Matcher.find(), when coupled with the
greedy quantifiers ? or *, allows for (somewhat unintuitively) the idea of a zero-length
match. As an experiment, modify the previous Regex class and add an invocation of
m.end() to the System.out.print (S.O.P.) in the while loop. With that modification in
place, the invocation
java Regex "a?" "aba"

should produce something very similar to this:
Pattern is a?
0 1 a
1 1
2 3 a
3 3

The lines of output 1 1 and 3 3 are examples of zero-length matches. Zero-length
matches can occur in several places:
■
■
■
■

After the last character of source data (the 3 3 example)
In between characters after a match has been found (the 1 1 example)
At the beginning of source data (try java Regex "a?" "baba")
At the beginning of zero-length source data

A common reason to use regex is to perform search-and-replace operations.
Although replace operations are not on the exam, you should know that the
Matcher class provides several methods that perform search-and-replace
operations. See the appendReplacement(), appendTail(), and replaceAll()
methods in the Matcher API for more details.
The Matcher class allows you to look at subsets of your source data by using a
concept called regions. In real life, regions can greatly improve performance, but you
won't need to know anything about them for the exam.

Searching Using the Scanner Class Although the java.util.Scanner
class is primarily intended for tokenizing data (which we'll cover next), it can also
be used to find stuff, just like the Pattern and Matcher classes. While Scanner

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doesn't provide location information or search-and-replace functionality, you can
use it to apply regex expressions to source data to tell you how many instances of an
expression exist in a given piece of source data. The following program uses the first
command-line argument as a regex expression and then asks for input using System
.in. It outputs a message every time a match is found:
import java.util.*;
class ScanIn {
public static void main(String[] args) {
System.out.print("input: ");
System.out.flush();
try {
Scanner s = new Scanner(System.in);
String token;
do {
token = s.findInLine(args[0]);
System.out.println("found " + token);
} while (token != null);
} catch (Exception e) { System.out.println("scan exc"); }
}
}

The invocation and input
java ScanIn "\d\d"
input: 1b2c335f456

produce the following:
found 33
found 45
found null

Tokenizing
Tokenizing is the process of taking big pieces of source data, breaking them into little
pieces, and storing the little pieces in variables. Probably the most common
tokenizing situation is reading a delimited file in order to get the contents of the file
moved into useful places, like objects, arrays, or collections. We'll look at two classes
in the API that provide tokenizing capabilities: String (using the split()
method) and Scanner, which has many methods that are useful for tokenizing.

Tokens and Delimiters
When we talk about tokenizing, we're talking about data that starts out composed
of two things: tokens and delimiters. Tokens are the actual pieces of data, and

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delimiters are the expressions that separate the tokens from each other. When most
people think of delimiters, they think of single characters, like commas or backslashes
or maybe a single whitespace. These are indeed very common delimiters, but strictly
speaking, delimiters can be much more dynamic. In fact, as we hinted at a few
sentences ago, delimiters can be anything that qualifies as a regex expression. Let's
take a single piece of source data and tokenize it using a couple of different delimiters:
source: "ab,cd5b,6x,z4"

If we say that our delimiter is a comma, then our four tokens would be
ab
cd5b
6x
z4

If we use the same source but declare our delimiter to be \d, we get three tokens:
ab,cd
b,
x,z

In general, when we tokenize source data, the delimiters themselves are discarded
and all that we are left with are the tokens. So in the second example, we defined
digits to be delimiters, so the 5, 6, and 4 do not appear in the final tokens.

Tokenizing with String.split()
The String class's split() method takes a regex expression as its argument and
returns a String array populated with the tokens produced by the split (or
tokenizing) process. This is a handy way to tokenize relatively small pieces of data.
The following program uses args[0] to hold a source string, and args[1] to hold
the regex pattern to use as a delimiter:
import java.util.*;
class SplitTest {
public static void main(String[] args) {
String[] tokens = args[0].split(args[1]);
System.out.println("count " + tokens.length);
for(String s : tokens)
System.out.println(">" + s + "<");
}
}

Everything happens all at once when the split() method is invoked. The source
string is split into pieces, and the pieces are all loaded into the tokens String

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array. All the code after that is just there to verify what the split operation generated.
The following invocation
% java SplitTest "ab5 ccc 45 @" "\d"

produces
count 4
>ab<
> ccc <
><
> @<

(Note: Remember that to represent "\" in a string, you may need to use the
escape sequence "\\". Because of this, and depending on your OS, your second
argument might have to be "\\d" or even "\\\\d".)
We put the tokens inside > < characters to show whitespace. Notice that every
digit was used as a delimiter and that contiguous digits created an empty token.
One drawback to using the String.split() method is that often you'll want to
look at tokens as they are produced, and possibly quit a tokenization operation early
when you've created the tokens you need. For instance, you might be searching a
large file for a phone number. If the phone number occurs early in the file, you'd like
to quit the tokenization process as soon as you've got your number. The Scanner
class provides a rich API for doing just such on-the-fly tokenization operations.

Because System.out.println() is so heavily used on the exam, you
might see examples of escape sequences tucked in with questions on most any topic,
including regex. Remember that if you need to create a string that contains a double
quote (") or a backslash (\), you need to add an escape character first:
System.out.println("\" \\");

This prints
" \

But what if you need to search for periods (.) in your source data? If you just put a period
in the regex expression, you get the "any character" behavior. But what if you try "\."?
Now the Java compiler thinks you're trying to create an escape sequence that doesn't
exist.The correct syntax is
String s = "ab.cde.fg";
String[] tokens = s.split("\\.");

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Tokenizing with Scanner
The java.util.Scanner class is the Cadillac of tokenizing. When you need to do
some serious tokenizing, look no further than Scanner—this beauty has it all. In
addition to the basic tokenizing capabilities provided by String.split(), the
Scanner class offers the following features:
■ Scanners can be constructed using files, streams, or strings as a source.
■ Tokenizing is performed within a loop so that you can exit the process at

any point.
■ Tokens can be converted to their appropriate primitive types automatically.

Let's look at a program that demonstrates several of Scanner's methods and
capabilities. Scanner's default delimiter is whitespace, which this program uses.
The program makes two Scanner objects: s1 is iterated over with the more generic
next() method, which returns every token as a String, while s2 is analyzed with
several of the specialized nextXxx() methods (where Xxx is a primitive type):
import java.util.Scanner;
class ScanNext {
public static void main(String [] args) {
boolean b2, b;
int i;
String s, hits = " ";
Scanner s1 = new Scanner(args[0]);
Scanner s2 = new Scanner(args[0]);
while(b = s1.hasNext()) {
s = s1.next(); hits += "s";
}
while(b = s2.hasNext()) {
if (s2.hasNextInt()) {
i = s2.nextInt(); hits += "i";
} else if (s2.hasNextBoolean()) {
b2 = s2.nextBoolean(); hits += "b";
} else {
s2.next(); hits += "s2";
}
}
System.out.println("hits " + hits);
}
}

If this program is invoked with
% java ScanNext "1 true 34 hi"

it produces
hits

ssssibis2

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Of course, we're not doing anything with the tokens once we've got them, but
you can see that s2's tokens are converted to their respective primitives. A key point
here is that the methods named hasNextXxx() test the value of the next token but
do not actually get the token, nor do they move to the next token in the source
data. The nextXxx() methods all perform two functions: They get the next token,
and then they move to the next token.
The Scanner class has nextXxx() (for instance, nextLong()) and hasNextXxx()
(for instance, hasNextDouble()) methods for every primitive type except char. In
addition, the Scanner class has a useDelimiter() method that allows you to set
the delimiter to be any valid regex expression.

Tokenizing with java.util.StringTokenizer
The java.util.StringTokenizer class is the rusty old Buick of tokenizing. These
days, when you want to do tokenizing, the Scanner class and String.split()are
the preferred approaches. In fact, in the API docs, the StringTokenizer class is
not recommended. The reason it's on the exam is because you'll often find it in older
code, and when you do, you'll want to understand how it works. The following list of
features summarizes the capabilities of StringTokenizer and relates them to the
Scanner class:
■ StringTokenizer objects are constructed using strings as a source.
■ StringTokenizer objects use whitespace characters by default as delimiters,

but they can be constructed with a custom set of delimiters (which are listed
as a string).
■ Tokenizing is performed within a loop so that you can exit the process at any

point.
■ The loop used for tokenizing uses the Enumerator interface, and typically

uses the hasMoreTokens() and nextToken() methods, which are very
similar to Scanner's next() and hasNext() methods. (Note: These days,
the Iterator interface is recommended instead of Enumerator.)
Let's look at a program that demonstrates several of StringTokenizer's methods
and capabilities:
import java.util.*;
public class STtest {
public static void main(String[] args) {
StringTokenizer st = new StringTokenizer("a bc d e");

Parsing, Tokenizing, and Formatting (OCP Objectives 5.1, 5.2, and 5.3)

451

System.out.println("\n " + st.countTokens());
while(st.hasMoreTokens()) {
System.out.print(">" + st.nextToken() + "< ");
}
System.out.println("\n " + st.countTokens());
// Second argument "a" is this StringTokenizer's delimiter
StringTokenizer st2 = new StringTokenizer("a b cab a ba d", "a");
System.out.println("\n " + st2.countTokens());
while(st2.hasMoreTokens()) {
System.out.print(">" + st2.nextToken() + "< ");
}
System.out.println("\n " + st2.countTokens());
}
}

which produces the output:
4
>a<
0

>bc<

4
> b c<
0

>d<

>b <

>e<

> b<

> d<

To recap, the first StringTokenizer, st, uses whitespace as its delimiter, and the
second StringTokenizer, st2, uses "a" as its delimiter. In both cases, we
surrounded the tokens with "> <" characters to show any whitespace included in
tokens. We also used the countTokens() method to display how many tokens were
Enumerator-able before and after each enumeration loop.
There are a few more details in the StringTokenizer class, but this is all you'll
need for the exam.

Formatting with printf() and format()
What fun would accounts receivable reports be if the decimal points didn't line up?
Where would you be if you couldn't put negative numbers inside of parentheses?
Burning questions like these caused the exam creation team to include formatting as
a part of the exam. The format() and printf() methods were added to java.io
.PrintStream in Java 5. These two methods behave exactly the same way, so
anything we say about one of these methods applies to both of them. (The rumor is
that printf() was added just to make old C programmers happy.)

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Behind the scenes, the format() method uses the java.util.Formatter class
to do the heavy formatting work. You can use the Formatter class directly if you
choose, but for the exam, all you have to know is the basic syntax of the arguments
you pass to the format() method. The documentation for these formatting
arguments can be found in the Formatter API. We're going to take the "nickel
tour" of the formatting String syntax, which will be more than enough to allow you
to do a lot of basic formatting work AND ace all the formatting questions on the
exam.
Let's start by paraphrasing the API documentation for format strings (for more
complete, way-past-what-you-need-for-the-exam coverage, check out the java.
util.Formatter API):
printf("format string", argument(s));

The format string can contain both normal string-literal information that isn't
associated with any arguments and argument-specific formatting data. The clue to
determining whether you're looking at formatting data is that formatting data will
always start with a percent sign (%). Let's look at an example, and don't panic, we'll
cover everything that comes after the % next:
System.out.printf("%2$d

+

%1$d", 123, 456);

This produces
456 + 123

Let's look at what just happened. Inside the double quotes there is a format string,
then a +, and then a second format string. Notice that we mixed literals in with the
format strings. Now let's dive in a little deeper and look at the construction of
format strings:
%[arg_index$][flags][width][.precision]conversion char

The values within [ ] are optional. In other words, the only required elements of
a format string are the % and a conversion character. In the previous example, the
only optional values we used were for argument indexing. The 2$ represents the
second argument, and the 1$ represents the first argument. (Notice that there's
no problem switching the order of arguments.) The d after the arguments is a
conversion character (more or less the type of the argument). Here's a rundown
of the format string elements you'll need to know for the exam:
■ arg_index

An integer followed directly by a $, this indicates which
argument should be printed in this position.

Parsing, Tokenizing, and Formatting (OCP Objectives 5.1, 5.2, and 5.3)

■ flags

453

While many flags are available, for the exam, you'll need to know:

■ -

Left-justify this argument

■ +

Include a sign (+ or -) with this argument

■ 0

Pad this argument with zeroes

■ ,

Use locale-specific grouping separators (i.e., the comma in 123,456)

■ (

Enclose negative numbers in parentheses

■ width

This value indicates the minimum number of characters to print.
(If you want nice, even columns, you'll use this value extensively.)

■ precision

For the exam, you'll only need this when formatting a floatingpoint number, and in the case of floating-point numbers, precision indicates
the number of digits to print after the decimal point.
■ conversion

The type of argument you'll be formatting. You'll need to know:

■ b

boolean

■ c

char

■ d

integer

■ f

floating point

■ s

string

Let's see some of these formatting strings in action:
int i1 = -123;
int i2 = 12345;
System.out.printf(">%1$(7d< \n", i1);
System.out.printf(">%0,7d< \n", i2);
System.out.format(">%+-7d< \n", i2);
System.out.printf(">%2$b + %1$5d< \n", i1, false);

This produces:
> (123)<
>012,345<
>+12345 <
>false + -123<

(We added the > and < literals to help show how minimum widths, zero padding,
and alignments work.) Finally, it's important to remember that—barring the use of
booleans—if you have a mismatch between the type specified in your conversion
character and your argument, you'll get a runtime exception:
System.out.format("%d", 12.3);

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This produces something like
Exception in thread "main" java.util.IllegalFormatConversionException: d !=
java.lang.Double

CERTIFICATION OBJECTIVE

Resource Bundles
(OCP Objectives 12.2, 12.3, and 12.5)
12.2

Build a resource bundle for each object.

12.3

Call a resource bundle from an application.

12.5

Describe the advantages of localizing an application.

Resource Bundles
Earlier, we used the Locale class to display numbers and dates for basic localization.
For full-fledged localization, we also need to provide language and country-specific
strings for display. There are only two parts to building an application with resource
bundles:
■ Locale

You can use the same Locale we used for DateFormat and
NumberFormat to identify which resource bundle to choose.

■ ResourceBundle

Think of a ResourceBundle as a map. You can use
property files or Java classes to specify the mappings.

Let's build up a simple application to be used in Canada. Since Canada has two
official languages, we want to let the user choose her favorite language. Designing
our application, we decided to have it just output "Hello Java" to show off how cool
it is. We can always add more text later.
We are going to externalize everything language specific to special files called
resource bundles. They're just property files that contain keys and string values to
display. Here are two simple resource bundle files:

Resource Bundles (OCP Objectives 12.2, 12.3, and 12.5)

455

A file named Labels_en.properties that contains a single line of data:
hello=Hello Java!

A second file named Labels_fr.properties that contains a single line of data:
hello=Bonjour Java!

Using a resource bundle has three steps: obtaining the Locale, getting the
ResourceBundle, and looking up a value from the resource bundle. First, we create
a Locale object. To review, this means one of the following:
new Locale("en")
new Locale("en", "CA")
Locale.CANADA
English

// language – English
// language and country – Canadian English
// constant for common locales – Canadian

Next, we need to create the resource bundle. We need to know the "title" of the
resource bundle and the locale. Then we pass those values to a factory, which creates
the resource bundle. The getBundle() method looks in the classpath for bundles
that match the bundle name ("Labels") and the provided locale.
ResourceBundle rb = ResourceBundle.getBundle("Labels", locale);

Finally, we use the resource bundle like a map and get a value based on the key:
rb.getString("hello");

Putting this together and adding java.util imports, we have everything we need
in order to read from a resource bundle:
import java.util.Locale;
import java.util.ResourceBundle;
public class WhichLanguage {
public static void main(String[] args) {
Locale locale = new Locale(args[0]);
ResourceBundle rb = ResourceBundle.getBundle("Labels", locale);
System.out.println(rb.getString("hello"));
}
}

Running the code twice, we get:
> java WhichLanguage en
Hello Java!
> java WhichLanguage fr
Bonjour Java!

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The Java API for java.util.ResourceBundle lists three good reasons to
use resource bundles. Using resource bundles "allows you to write programs than can
■
■
■

Be easily localized, or translated, into different languages
Handle multiple locales at once
Be easily modified later to support even more locales"

If you encounter any questions on the exam that ask about the advantages of using
resource bundles, this quote from the API will serve you well.

The most common use of localization in Java is web applications. You can
get the user's locale from information passed in the request rather than
hard-coding it.

Property Resource Bundles
Let's take a closer look at the property file. Aside from comments, a property file
contains key/value pairs:
# this file contains a single key/value
hello=Hello Java

As you can see, comments are lines beginning with #. A key is the first string on a
line. Keys and values are separated by an equal sign. If you want to break up a single
line into multiple lines, you use a backslash:
hello1 = Hello \
World!
System.out.println(rb.getString("hello1"));
Hello World!

If you actually want a line break, you use the standard Java \n escape sequence:
hello2 = Hello \nWorld !
System.out.println(rb.getString("hello2"));
Hello
World !

Resource Bundles (OCP Objectives 12.2, 12.3, and 12.5)

457

You can mix and match these to your heart's content. Java helpfully ignores any
whitespace before subsequent lines of a multiline property. This is so you can use
indentation for clarity:
hello3 =
123\
45
System.out.println(rb.getString("hello3"));
12345

Almost everyone uses # for comments and = to separate key/value pairs.There
are alternative syntax choices, though, which you should understand if you
come across them.
Property files can use two styles of commenting:
! comment

or
# comment

Property files can define key/value pairs in any of the following formats:
key=value
key:value
key value

These few rules are all you need to know about the PropertyResourceBundle.
While property files are the most common format for resource bundles, what
happens if we want to represent types of values other than String? Or if we want to
load the resource bundles from a database?

Java Resource Bundles
When we need to move beyond simple property file key to string value mappings, we
can use resource bundles that are Java classes. We write Java classes that extend
ListResourceBundle. The class name is similar to the one for property files. Only
the extension is different.
import java.util.ListResourceBundle;
public class Labels_en_CA extends ListResourceBundle {
protected Object[][] getContents() {
return new Object[][] {
{ "hello", new StringBuilder("from Java") }
};
}
}

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We implement ListResourceBundle's one required method that returns an array
of arrays. The inner array is key/value pairs. The outer array accumulates such pairs.
Notice that now we aren't limited to String values. We can call getObject() to
get a non-String value:
Locale locale = new Locale(args[0], "CA");
ResourceBundle rb = ResourceBundle.getBundle("Labels", locale);
System. out.println(rb.getObject("hello"));

Which prints "from Java".

Default Locale
What do you think happens if we call ResourceBundle.getBundle("Labels")
without any locale? It depends. Java will pick the resource bundle that matches the
locale the JVM is using. Typically, this matches the locale of the machine running
the program, but it doesn't have to. You can even change the default locale at
runtime. Which might be useful if you are working with people in different locales
so you can get the same behavior on all machines.
Exploring the API to get and set the default locale:
// store locale so can put it back at end
Locale initial = Locale.getDefault();
System.out.println(initial);
// set locale to Germany
Locale.setDefault(Locale.GERMANY);
System.out.println(Locale.getDefault());
// put original locale back
Locale.setDefault(initial);
System.out.println(Locale.getDefault());

which on our computer prints:
en_US
de_DE
en_US

For the first and last line, you may get different output depending on where you live.
The key is that the middle of the program executes as if it were in Germany,
regardless of where it is actually being run. It is good practice to restore the default
unless your program is ending right away. That way, the rest of your code works
normally—it probably doesn't expect to be in Germany.

Resource Bundles (OCP Objectives 12.2, 12.3, and 12.5)

459

Choosing the Right Resource Bundle
There are two main ways to get a resource bundle:
ResourceBundle.getBundle(baseName)
ResourceBundle.getBundle(baseName, locale)

Luckily, ResourceBundle.getBundle(baseName)is just shorthand for
ResourceBundle.getBundle(baseName, Locale.getDefault())and you only
have to remember one set of rules. There are a few other overloaded signatures for
getBundle(), such as taking a ClassLoader. But don't worry—these aren't on
the exam.
Now on to the rules. How does Java choose the right resource bundle to use? In a
nutshell, Java chooses the most specific resource bundle it can while giving
preference to Java ListResourceBundle.
Going back to our Canadian application, we decide to request the Canadian
French resource bundle:
Locale locale = new Locale("fr", "CA");
ResourceBundle rb = ResourceBundle.getBundle("RB", locale);

Java will look for the following files in the classpath in this order:
RB_fr_CA.java
RB_fr_CA.properties

// exactly what we asked for

RB_fr.java
RB_fr.properties
RB_en_US.java
RB_en_US.properties
RB_en.java
RB_en.properties
RB.java
RB.properties

//
//
//
//
//
//
//
//

couldn't find exactly what we asked for
now trying just requested language
couldn't find French
now trying default Locale
couldn't find full default Locale country
now trying default Locale language
couldn't find anything any matching Locale,
now trying default bundle

If none of these files exist, Java gives up and throws a MissingResourceException.
While this is a lot of things for Java to try, it is pretty easy to remember. Start with
the full Locale requested. Then fall back to just language. Then fall back to the
default Locale. Then fall back to the default bundle. Then cry.
Make sure you understand this because it is about to get more complicated.

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You don't have to specify all the keys in all the property files. They can inherit
from each other. This is a good thing, as it reduces duplication.
RB_en.properties
ride.in=Take a ride in the
RB_en_US.properties
elevator=elevator
RB_en_UK.properties
elevator=lift
Locale locale = new Locale("en", "UK");
ResourceBundle rb = ResourceBundle.getBundle("RB", locale);
System.out.println(rb.getString("ride.in") +
rb.getString("elevator"));

Outputs:
Take a ride in the lift

The common "ride.in" property comes from the parent noncountry-specific
bundle "RB_en.properties." The "elevator" property is different by country and
comes from the UK version that we specifically requested.
The parent hierarchy is more specific than the search order. A bundle's parent
always has a shorter name than the child bundle. If a parent is missing, Java just
skips along that hierarchy. ListResourceBundles and
PropertyResourcesBundles do not share a hierarchy. Similarly, the default locale's
resource bundles do not share a hierarchy with the requested locale's resource
bundles. Table 8-3 shows examples of bundles that do share a hierarchy.
TABLE 8-3

Name of Resource Bundle

Hierarchy

RB_fr_CA.java

RB.java
RB_fr.java
RB_fr_CA.java

RB_fr_CA.properties

RB.properties
RB_fr.properties
RB_fr_CA.properties

RB_en_US.java

RB.java
RB_en.java
RB_en_US.java

RB_en_US.properties

RB.properties
RB_en.properties
RB_en_US.properties

Resource Bundle
Lookups

Certification Summary

461

Remember that searching for a property file uses a linear list. However, once a
matching resource bundle is found, keys can only come from that resource bundle's
hierarchy.
One more example to make this clear. Think about which resource bundles will
be used from the previous code if I use the following code to request a resource
bundle:
Locale locale = new Locale("fr", "FR");
ResourceBundle rb = ResourceBundle.getBundle("RB", locale);

First, Java looks for RB_fr_FR.java and RB_fr_FR.properties. Since neither is
found, Java falls back to using RB_fr.java. Then as we request keys from rb, Java
starts looking in RB_fr.java and additionally looks in RB.java. Java started out
looking for a matching file and then switched to searching the hierarchy of that file.

CERTIFICATION SUMMARY
Dates, Numbers, and Currency Remember that the objective is a bit
misleading and that you'll have to understand the basics of five related classes:
java.util.Date, java.util.Calendar, java.util.Locale, java.text
.DateFormat, and java.text.NumberFormat. A Date is the number of
milliseconds since January 1, 1970, stored in a long. Most of Date's methods have
been deprecated, so use the Calendar class for your date-manipulation tasks.
Remember that in order to create instances of Calendar, DateFormat, and
NumberFormat, you have to use static factory methods like getInstance(). The
Locale class is used with DateFormat and NumberFormat to generate a variety of
output styles that are language and/or country specific.
Parsing,Tokenizing, and Formatting To find specific pieces of data in large
data sources, Java provides several mechanisms that use the concepts of regular
expressions (regex). Regex expressions can be used with the java.util.regex
package's Pattern and Matcher classes, as well as with java.util.Scanner and
with the String.split() method. When creating regex patterns, you can use
literal characters for matching or you can use metacharacters that allow you to
match on concepts like "find digits" or "find whitespace." Regex provides quantifiers
that allow you to say things like "find one or more of these things in a row." You won't
have to understand the Matcher methods that facilitate replacing strings in data.

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Tokenizing is splitting delimited data into pieces. Delimiters are usually as simple
as a comma, but they can be as complex as any other regex pattern. The java
.util.Scanner class provides full tokenizing capabilities using regex and allows you
to tokenize in a loop so that you can stop the tokenizing process at any point.
String.split() allows full regex patterns for tokenizing, but tokenizing is done in
one step; hence, large data sources can take a long time to process. The java.util
.StringTokenizer class is almost deprecated, but you might find it in old code. It's
similar to Scanner.
Formatting data for output can be handled by using the Formatter class, or more
commonly, by using the new PrintStream methods format() and printf().
Remember format() and printf() behave identically. To use these methods, you
create a format string that is associated with every piece of data you want to format.

Resource Bundles Finally, resource bundles allow you to move locale-specific
information (usually strings) out of your code and into external files where they can
easily be amended. This provides an easy way for you to localize your applications
across many locales.

Two-Minute Drill

✓

463

TWO-MINUTE DRILL
Here are some of the key points from the certification objectives in this chapter.

Dates, Numbers, and Currency
(OCP Objectives 12.1, 12.4, and 12.5)
❑ The classes you need to understand are java.util.Date, java.util.
Calendar, java.text.DateFormat, java.text.NumberFormat, and
java.util.Locale.
❑ Most of the Date class's methods have been deprecated.
❑ A Date is stored as a long, the number of milliseconds since January 1, 1970.
❑ Date objects are go-betweens for the Calendar and DateFormat classes.
❑ The Calendar provides a powerful set of methods to manipulate dates,
performing tasks such as getting days of the week or adding some number of
months or years (or other increments) to a date.
❑ Create Calendar instances using static factory methods (getInstance()).
❑ The Calendar methods you should understand are add(), which allows you
to add or subtract various pieces (minutes, days, years, and so on) of dates,
and roll(), which works like add() but doesn't increment a date's bigger
pieces. (For example, adding ten months to an October date changes the
month to August, but doesn't increment the Calendar's year value.)
❑ DateFormat instances are created using static factory methods
(getInstance() and getDateInstance()).
❑ There are several format "styles" available in the DateFormat class.
❑ DateFormat styles can be applied against various Locales to create a wide
array of outputs for any given date.
❑ The DateFormat.format() method is used to create strings containing
properly formatted dates.
❑ The Locale class is used in conjunction with DateFormat and
NumberFormat.
❑ Both DateFormat and NumberFormat objects can be constructed with a
specific, immutable Locale.
❑ For the exam, you should understand creating Locales using either language
or a combination of language and country.

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Parsing,Tokenizing, and Formatting
(OCP Objectives 5.1, 5.2, 5.3, 12.4)
❑ Regex is short for regular expressions, which are the patterns used to search
for data within large data sources.
❑ Regex is a sublanguage that exists in Java and other languages (such as Perl).
❑ Regex lets you create search patterns using literal characters or
metacharacters. Metacharacters allow you to search for slightly more abstract
data like "digits" or "whitespace."
❑ Study the \d, \s, \w, and . metacharacters.
❑ Regex provides for quantifiers, which allow you to specify concepts like "look
for one or more digits in a row."
❑ Study the ?, *, and + greedy quantifiers.
❑ Remember that metacharacters and strings don't mix well unless you
remember to "escape" them properly. For instance, String s = "\\d";.
❑ The Pattern and Matcher classes have Java's most powerful regex
capabilities.
❑ You should understand the Pattern compile() method and the Matcher
matches(), pattern(), find(), start(), and group() methods.
❑ You WON'T need to understand Matcher's replacement-oriented methods.
❑ You can use java.util.Scanner to do simple regex searches, but it is
primarily intended for tokenizing.
❑ Tokenizing is the process of splitting delimited data into small pieces.
❑ In tokenizing, the data you want is called tokens, and the strings that separate
the tokens are called delimiters.
❑ Tokenizing should be done with the Scanner class or with String.split().
❑ Delimiters are either single characters like commas or complex regex
expressions.
❑ The Scanner class allows you to tokenize data from within a loop, which
allows you to stop whenever you want to.

Two-Minute Drill

465

❑ The Scanner class allows you to tokenize strings or streams or files.
❑ The old StringTokenizer class allows you to tokenize strings.
❑ The String.split() method tokenizes the entire source data all at once, so
large amounts of data can be quite slow to process.
❑ As of Java 5 there are two methods used to format data for output. These
methods are format() and printf(). These methods are found in the
PrintStream class, an instance of which is the out in System.out.
❑ The format() and printf() methods have identical functionality.
❑ Formatting data with printf() (or format()) is accomplished using
formatting strings that are associated with primitive or string arguments.
❑ The format() method allows you to mix literals in with your format strings.
❑ The format string values you should know are
❑ Flags: -, +, 0, "," , and (
❑ Conversions: b, c, d, f, and s
❑ Barring booleans, if your conversion character doesn't match your argument
type, an exception will be thrown.

Resource Bundles
❑ A ListResourceBundle comes from Java classes, and a
PropertyResourceBundle comes from .property files.
❑ ResourceBundle.getBundle(name)uses the default Locale.
❑ Locale.getDefault()returns the JVM's default Locale
. Locale.setDefault(locale)can change the JVM's locale.
❑ Java searches for resource bundles in this order: requested language/country,
requested language, default locale language/country, default locale language,
default bundle. Within each item, Java ListResourceBundle is favored over
PropertyResourceBundle.
❑ Once a ResourceBundle is found, only parents of that bundle can be used to
look up keys.

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SELF TEST
Note: Both the OCA 7 and OCP 7 exams have objectives concerning Strings and StringBuilders.
In Chapter 5, we discussed these two classes. This chapter's self test includes questions about Strings
and StringBuilders for the sake of preparing you for the OCP exam. If you need a refresher on
Strings and Stringbuilders, head back to Chapter 5.
1. Given:
import java.util.regex.*;
class Regex2 {
public static void main(String[] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
boolean b = false;
while(b = m.find()) {
System.out.print(m.start() + m.group());
}
}
}

And the command line:
java Regex2 "\d*" ab34ef

What is the result?
A. 234
B. 334
C. 2334
D. 0123456
E. 01234456
F. 12334567
G. Compilation fails
2. Given:
public class Canada {
public static void main(String[] args) {
ResourceBundle rb = ResourceBundle.getBundle("Flag",
new Locale("en_CA"));
System.out.println(rb.getString("key"));
}
}

Self Test

467

Assume the default Locale is Italian. If each of the following is the only resource bundle on the
classpath and contains key=value, which will be used? (Choose all that apply.)
A. Flag.java
B. Flag_CA.properties
C. Flag_en.java
D. Flag_en.properties
E. Flag_en_CA.properties
F. Flag_fr_CA.properties
3. Given:
import java.util.regex.*;
class Quetico {
public static void main(String [] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
System.out.print("match positions: ");
while(m.find()) {
System.out.print(m.start() + " ");
}
System.out.println("");
}
}

Which invocation(s) will produce the output: 0 2 4 8 ? (Choose all that apply.)
A. java Quetico "\b" "^23 *$76 bc"
B. java Quetico "\B" "^23 *$76 bc"
C. java Quetico "\S" "^23 *$76 bc"
D. java Quetico "\W" "^23 *$76 bc"
E. None of the above
F. Compilation fails
G. An exception is thrown at runtime

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4. Given:
public class Banana {
public static void main(String[] args) {
String in = "1 a2 b 3 c4d 5e";
String[] chunks = in.split(args[0]);
System.out.println("count " + chunks.length);
for(String s : chunks)
System.out.print(">" + s + "< ");
}
}

And two invocations:
java Banana " "
java Banana "\d"

What is the result? (Choose all that apply.)
A. In both cases, the count will be 5
B. In both cases, the count will be 6
C. In one case, the count will be 5, and in the other case, 6
D. Banana cannot be invoked because it will not compile
E. At least one of the invocations will throw an exception
5. Given three resource bundles and a Java class:
Train_en_US.properties: train=subway
Train_en_UK.properties: train=underground
Train_en.properties: ride = ride
1: public class ChooChoo {
2:
public static void main(String[] args) {
3:
Locale.setDefault(new Locale("en", "US"));
4:
ResourceBundle rb = ResourceBundle.getBundle("Train",
5:
new Locale("en", "US"));
6:
System.out.print(rb.getString("ride")
+ " " + rb.getString("train"));
7:
}
8: }

Which of the following, when made independently, will change the output to "ride
underground"? (Choose all that apply.)

Self Test

A.
B.
C.
D.
E.

469

Add train=underground to Train_en.properties
Change line 1 to Locale.setDefault(new Locale("en", "UK"));
Change line 5 to Locale.ENGLISH);
Change line 5 to new Locale("en", "UK"));
Delete file Train_en_US.properties

6. Given that 1119280000000L is roughly the number of milliseconds from January 1, 1970, to
June 20, 2005, and that you want to print that date in German, using the LONG style such that
"June" will be displayed as "Juni," complete the code using the following fragments. Note: You
can use each fragment either zero or one times, and you might not need to fill all of the slots.
Code:
import java.___________
import java.___________
class DateTwo {
public static void main(String[] args) {
Date d = new Date(1119280000000L);
DateFormat df = ___________________________
________________ , _________________ );
System.out.println(________________
}
}

Fragments:
io.*;
nio.*;
util.*;
text.*;
date.*;

new DateFormat(
DateFormat.getInstance(
DateFormat.getDateInstance(
util.regex;
df.format(d));

Locale.LONG
Locale.GERMANY
DateFormat.LONG
DateFormat.GERMANY
d.format(df));

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7. Given:
public class Legos {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder(8);
System.out.print(sb.length() + " " + sb + " ");
sb.insert(0, "abcdef");
sb.append("789");
System.out.println(sb.length() + " " + sb);
}
}

What is the result?
A. 0 8 abcdef78
B. 0 8 789abcde
C. 0 9 abcdef789
D. 0 9 789abcdef
E. Compilations fails
F. 0, followed by an exception
8. Given two files:
package rb;
public class Bundle extends java.util.ListResourceBundle {
protected Object[][] getContents() {
return new Object[][] { { "123", 456 } };
}
}

package rb;
import java.util.*;
public class KeyValue {
public static void main(String[] args) {
ResourceBundle rb = ResourceBundle.getBundle("rb.Bundle",
Locale.getDefault());
// insert code here
}
}

Self Test

Which, inserted independently, will compile? (Choose all that apply.)
A. Object obj = rb.getInteger("123");
B. Object obj = rb.getInteger(123);
C. Object obj = rb.getObject("123");
D. Object obj = rb.getObject(123);
E. Object obj = rb.getString("123");
F. Object obj = rb.getString(123);
9. Given:
3. public class Theory {
4.
public static void main(String[] args) {
5.
String s1 = "abc";
6.
String s2 = s1;
7.
s1 += "d";
8.
System.out.println(s1 + " " + s2 + " " + (s1==s2));
9.
10.
StringBuffer sb1 = new StringBuffer("abc");
11.
StringBuffer sb2 = sb1;
12.
sb1.append("d");
13.
System.out.println(sb1 + " " + sb2 + " " + (sb1==sb2));
14.
}
15. }

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 is abcd abc false
E. The second line of output is abcd abc false
F. The second line of output is abcd abcd true
G. The second line of output is abcd abcd false

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10. Given:
public class Stone {
public static void main(String[] args) {
String s = "abc";
System.out.println(">" + doStuff(s) + "<");
}
static String doStuff(String s) {
s = s.concat(" ef h ");
return s.trim();
}
}

What is the result?
A. >abcefh<
B. >efhabc<
C. >abc ef h<
D. \>>ef h abc<
E. >abc ef h <
11. Given:
3. import java.text.*;
4. public class Slice {
5.
public static void main(String[] args) {
6.
String s = "987.123456";
7.
double d = 987.123456d;
8.
NumberFormat nf = NumberFormat.getInstance();
9.
nf.setMaximumFractionDigits(5);
10.
System.out.println(nf.format(d) + " ");
11.
try {
12.
System.out.println(nf.parse(s));
13.
} catch (Exception e) { System.out.println("got exc"); }
14.
}
15. }

Which are true? (Choose all that apply.)
A. The output is 987.12345 987.12345
B. The output is 987.12346 987.12345
C. The output is 987.12345 987.123456
D. The output is 987.12346 987.123456
E. The try/catch block is unnecessary
F. The code compiles and runs without exception
G. The invocation of parse() must be placed within a try/catch block

Self Test

12. Given:
3. import java.util.regex.*;
4. public class Archie {
5.
public static void main(String[] args) {
6.
Pattern p = Pattern.compile(args[0]);
7.
Matcher m = p.matcher(args[1]);
8.
int count = 0;
9.
while(m.find())
10.
count++;
11.
System.out.print(count);
12.
}
13. }

And given the command-line invocation:
java Archie "\d+" ab2c4d67

What is the result?
A. 0
B. 3
C. 4
D. 8
E. 9
F. Compilation fails
13. Given:
3. import java.util.*;
4. public class Looking {
5.
public static void main(String[] args) {
6.
String input = "1 2 a 3 45 6";
7.
Scanner sc = new Scanner(input);
8.
int x = 0;
9.
do {
10.
x = sc.nextInt();
11.
System.out.print(x + " ");
12.
} while (x!=0);
13.
}
14. }

What is the result?
A. 1 2
B. 1 2 3 45 6
C. 1 2 3 4 5 6
D. 1 2 a 3 45 6
E. Compilation fails
F. 1 2 followed by an exception

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SELF TEST ANSWERS
1. ☑ E is correct. The \d is looking for digits. The * is a quantifier that looks for 0 to many
occurrences of the pattern that precedes it. Because we specified *, the group()method returns
empty strings until consecutive digits are found, so the only time group() returns a value is
when it returns 34, when the matcher finds digits starting in position 2. The start() method
returns the starting position of the previous match because, again, we said find 0 to many
occurrences.
☐
✗ A, B, C, D, F, and G are incorrect based on the above. (OCP Objective 5.2)
2. ☑ A, C, D, and E are correct. The default Locale is irrelevant here since none of the choices
use Italian. A is the default resource bundle. C and D use the language but not the country from
the requested locale. E uses the exact match of the requested locale.
☐
✗ B is incorrect because the language code of CA does not match en. And CA isn't a valid
language code. F is incorrect because the language code "fr" does not match en. Even though
the country code of CA does match, the language code is more important. (OCP Objectives 12.2
and 12.3)
3. ☑ B is correct. Remember that the boundary metacharacters (\b and \B), act as though the
string being searched is bounded with invisible, non-word characters. Then remember that \B
reports on boundaries between word to word or non-word to non-word characters.
☐
✗ A, C, D, E, F, and G are incorrect based on the above. (OCP Objective 5.2)
4. ☑ B is correct. In the second case, the first token is empty. Remember that in the first case,
the delimiter is a space, and in the second case, the delimiter is any numeric digit.
☐
✗ A, C, D, and E are incorrect based on the above. (OCP Objectives 5.1 and 5.2)
5. ☑

D is correct. As is, the code finds resource bundle Train_en_US.properties, which uses
Train_en.properties as a parent. Choice D finds resource bundle Train_en_UK.properties,
which uses Train_en.properties as a parent.

☐
✗ A is incorrect because both the parent and child have the same property. In this scenario,
the more specific one (child) gets used. B is incorrect because the default locale only gets used if
the requested resource bundle can't be found. C is incorrect because it finds the resource bundle
Train_en.properties, which does not have any "train" key. E is incorrect because there is no
"ride" key once we delete the parent. F is incorrect based on the above. (OCP Objectives 12.2
and 12.3)
6. ☑

Answer:
import java.util.*;
import java.text.*;
class DateTwo {

Self Test Answers

475

public static void main(String[] args) {
Date d = new Date(1119280000000L);
DateFormat df = DateFormat.getDateInstance(
DateFormat.LONG, Locale.GERMANY);
System.out.println(df.format(d));
}
}

Notes: Remember that you must build DateFormat objects using static methods. Also,
remember that you must specify a Locale for a DateFormat object at the time of instantiation.
The getInstance() method does not take a Locale. (OCP Objective 12.4)
7. ☑ C is correct. The append()method appends to the end of the StringBuilder's current
value, and if you append past the current capacity, the capacity is automatically increased. Note:
Invoking insert() past the current capacity will cause an exception to be thrown.
☐
✗ A, B, D, E, and F are incorrect based on the above. (OCP Objective 5.1)
8. ☑ C and E are correct. When getting a key from a resource bundle, the key must be a string.
The returned result must be a string or an object. While that object may happen to be an
integer, the API is still getObject(). E will throw a ClassCastException since 456 is not a
string, but it will compile.
☐
✗ A, B, D, and F are incorrect because of the above. (OCP Objectives 12.2 and 12.3)
9. ☑ D and F are correct. While String objects are immutable, references to Strings
are mutable. The code s1 += "d"; creates a new String object. StringBuffer objects
are mutable, so the append() is changing the single StringBuffer object to which both
StringBuffer references refer.
☐
✗ A, B, C, E, and G are incorrect based on the above. (OCP Objective 5.1)
10. ☑ C is correct. The concat() method adds to the end of the String, not the front. The
trickier part is the return statement. Remember that Strings are immutable. The String
referred to by "s" in doStuff() is not changed by the trim() method. Instead, a new String
object is created via the trim() method, and a reference to that new String is returned to
main().
☐
✗ A, B, D, and E are incorrect based on the above. (OCP Objective 5.1)
11. ☑ D, F, and G are correct. The setMaximumFractionDigits() applies to the formatting,
but not the parsing. The try/catch block is placed appropriately. This one might scare you into
thinking that you'll need to memorize more than you really do. If you can remember that you're
formatting the number and parsing the string, you should be fine for the exam.
☐
✗ A, B, C, and E are incorrect based on the above. (OCP Objective 12.4)

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12. ☑ B is correct. The "\d" metacharacter looks for digits, and the + quantifier says look for
"one or more" occurrences. The find() method will find three sets of one or more consecutive
digits: 2, 4, and 67.
☐
✗ A, C, D, E, and F are incorrect based on the above. (OCP Objective 5.2)
13. ☑ F is correct. The nextXxx() methods are typically invoked after a call to a hasNextXxx(),
which determines whether the next token is of the correct type.
☐
✗ A, B, C, D, and E are incorrect based on the above. (OCP Objective 5.1)

9
I/O and NIO

CERTIFICATION OBJECTIVES

•
•

Read and Write Data from the Console

•

Use Streams to Read From and Write To
Files by Using Classes in the java
.io Package, Including BufferedReader,
BufferedWriter, File, FileReader, FileWriter,
DataInputStream, DataOutputStream,
ObjectOutputStream, ObjectInputStream,
and PrintWriter

Read and Change File and Directory
Attributes, Focusing on the
BasicFileAttributes, DosFileAttributes,
and PosixFileAttributes Interfaces

•

Recursively Access a Directory Tree Using
the DirectoryStream and FileVisitor
Interfaces

•

•
•

Find a File with the PathMatcher Interface

Operate on File and Directory Paths with
the Path Class (sic)

•

Check, Delete, Copy, or Move a File or
Directory with the Files Class

✓

Watch a Directory for Changes with the
WatchService Interface
Two-Minute Drill

Q&A Self Test

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I

/O (input/output) has been around since the beginning of Java.You could read and write
files along with some other common operations. Then with Java 1.4, Java added more I/O
functionality and cleverly named it NIO. That stands for "new I/O." Don't worry—you won't
be asked about those Java 1.4 additions on the exam.
The APIs prior to Java 7 still had a few limitations when you had to write
applications that focused heavily on files and file manipulation. Trying to write a
little routine listing all the files created in the past day within a directory tree would
give you some headaches. There was no support for navigating directory trees, and
just reading attributes of a file was also quite hard. In Java 7, this whole routine is
less than 15 lines of code!
Now what to name yet another I/O API? The name "new I/O" was taken, and
"new new I/O" would just sound silly. Since the Java 7 functionality was added to
package names that begin with java.nio, the new name was NIO.2. For the
purposes of this chapter and the exam, NIO is shorthand for NIO.2.
Since NIO (or NIO.2 if you like) builds upon the original I/O, some of those
concepts are still tested on the exam in addition to the new parts. Fortunately, you
won't have to become a total I/O or NIO guru to do well on the exam. The intention
of the exam team was to include just the basic aspects of these technologies, and
in this chapter, we cover more than you'll need to get through these objectives on
the exam.

CERTIFICATION OBJECTIVE

File Navigation and I/O
(OCP Objectives 7.1 and 7.2)
7.1

Read and write data from the console.

7.2 Use streams to read from and write to files by using classes in the java.io package,
including BufferedReader, BufferedWriter, File, FileReader, FileWriter, DataInputStream,
DataOutputStream, ObjectOutputStream, ObjectInputStream, and PrintWriter.

File Navigation and I/O (OCP Objectives 7.1 and 7.2)

479

I/O has had a strange history with the OCP certification. It was included in all
the versions of the exam, up to and including 1.2, then removed from the 1.4 exam,
reintroduced for Java 5, extended for Java 6, and extended still more for Java 7.
I/O is a huge topic in general, and the Java APIs that deal with I/O in one fashion
or another are correspondingly huge. A general discussion of I/O could include
topics such as file I/O, console I/O, thread I/O, high-performance I/O, byte-oriented
I/O, character-oriented I/O, I/O filtering and wrapping, serialization, and more.
Luckily for us, the I/O topics included in the Java 7 exam are fairly well restricted to
file I/O for characters and Serialization. Due to a late change in the Oracle objectives,
you WILL NOT find Serialization discussed in this chapter. Instead, we created
a complete "Serialization mini-chapter" (along with a Self Test) as Appendix A.
Here's a summary of the I/O classes you'll need to understand for the exam:
■ File

The API says that the File class is "an abstract representation of file
and directory pathnames." The File class isn't used to actually read or write
data; it's used to work at a higher level, making new empty files, searching for
files, deleting files, making directories, and working with paths.

■ FileReader

This class is used to read character files. Its read() methods
are fairly low-level, allowing you to read single characters, the whole stream
of characters, or a fixed number of characters. FileReaders are usually
wrapped by higher-level objects such as BufferedReaders, which improve
performance and provide more convenient ways to work with the data.

■ BufferedReader

This class is used to make lower-level Reader classes like
FileReader more efficient and easier to use. Compared to FileReaders,
BufferedReaders read relatively large chunks of data from a file at once
and keep this data in a buffer. When you ask for the next character or line
of data, it is retrieved from the buffer, which minimizes the number of
times that time-intensive, file-read operations are performed. In addition,
BufferedReader provides more convenient methods, such as readLine(),
that allow you to get the next line of characters from a file.

■ FileWriter

This class is used to write to character files. Its write()
methods allow you to write character(s) or strings to a file. FileWriters are
usually wrapped by higher-level Writer objects, such as BufferedWriters
or PrintWriters, which provide better performance and higher-level, more
flexible methods to write data.

■ BufferedWriter

This class is used to make lower-level classes like

FileWriters more efficient and easier to use. Compared to FileWriters,
BufferedWriters write relatively large chunks of data to a file at once,

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minimizing the number of times that slow, file-writing operations are
performed. The BufferedWriter class also provides a newLine() method to
create platform-specific line separators automatically.
■ PrintWriter

This class has been enhanced significantly in Java 5. Because
of newly created methods and constructors (like building a PrintWriter
with a File or a String), you might find that you can use PrintWriter
in places where you previously needed a Writer to be wrapped with a
FileWriter and/or a BufferedWriter. New methods like format(),
printf(), and append() make PrintWriters very flexible and powerful.

■ Console

This new Java 6 convenience class provides methods to read input
from the console and write formatted output to the console.

Stream classes are used to read and write bytes, and Readers and Writers
are used to read and write characters. Since all of the file I/O on the exam is related
to characters, if you see API class names containing the word "Stream"—for instance,
DataOutputStream—then the question is probably about serialization or something
unrelated to the actual I/O objective.

Creating Files Using the File Class
Objects of type File are used to represent the actual files (but not the data in the
files) or directories that exist on a computer's physical disk. Just to make sure we're
clear, when we talk about an object of type File, we'll say File, with a capital F.
When we're talking about what exists on a hard drive, we'll call it a file with a
lowercase f (unless it's a variable name in some code). Let's start with a few basic
examples of creating files, writing to them, and reading from them. First, let's create
a new file and write a few lines of data to it:
import java.io.*;

class Writer1 {
public static void main(String [] args) {
File file = new File("fileWrite1.txt");
}
}

// The section 7 objectives
// focus on classes from
// java.io

// There's no
// file yet!

File Navigation and I/O (OCP Objectives 7.1 and 7.2)

481

If you compile and run this program, when you look at the contents of your current
directory, you'll discover absolutely no indication of a file called fileWrite1.txt.
When you make a new instance of the class File, you're not yet making an actual file;
you're just creating a filename. Once you have a File object, there are several ways to
make an actual file. Let's see what we can do with the File object we just made:
import java.io.*;
class Writer1 {
public static void main(String [] args)
try {
boolean newFile = false;
File file = new File
("fileWrite1.txt");
System.out.println(file.exists());
newFile = file.createNewFile();
System.out.println(newFile);
System.out.println(file.exists());
} catch(IOException e) { }
}
}

{
// warning: exceptions possible
// it's only an object
//
//
//
//

look for a real file
maybe create a file!
already there?
look again

This produces the output
false
true
true

And also produces an empty file in your current directory. If you run the code a
second time, you get the output
true
false
true

Let's examine these sets of output:
■ First execution

The first call to exists() returned false, which we
expected… remember, new File() doesn't create a file on the disk! The
createNewFile() method created an actual file and returned true,
indicating that a new file was created and that one didn't already exist.
Finally, we called exists() again, and this time it returned true, indicating
that the file existed on the disk.

■ Second execution

The first call to exists() returns true because we
built the file during the first run. Then the call to createNewFile() returns

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false since the method didn't create a file this time through. Of course, the
last call to exists() returns true.

A couple of other new things happened in this code. First, notice that we had to
put our file creation code in a try/catch. This is true for almost all of the file I/O
code you'll ever write. I/O is one of those inherently risky things. We're keeping it
simple for now and ignoring the exceptions, but we still need to follow the handleor-declare rule, since most I/O methods declare checked exceptions. We'll talk more
about I/O exceptions later. We used a couple of File's methods in this code:
■ boolean exists()

This method returns true if it can find the actual file.

■ boolean createNewFile()

This method creates a new file if it doesn't

already exist.

Remember, the exam creators are trying to jam as much code as they can
into a small space, so in the previous example, instead of these three lines of code:
boolean newFile = false;
...
newFile = file.createNewFile();
System.out.println(newFile);

you might see something like the following single line of code, which is a bit harder to
read, but accomplishes the same thing:
System.out.println(file.createNewFile());

Using FileWriter and FileReader
In practice, you probably won't use the FileWriter and FileReader classes without
wrapping them (more about "wrapping" very soon). That said, let's go ahead and do
a little "naked" file I/O:
import java.io.*;
class Writer2 {
public static void main(String [] args) {
char[] in = new char[50];
// to store input
int size = 0;

File Navigation and I/O (OCP Objectives 7.1 and 7.2)

try {
File file = new File(
"fileWrite2.txt");
FileWriter fw =
new FileWriter(file);
fw.write("howdy\nfolks\n");
fw.flush();
fw.close();
FileReader fr =
new FileReader(file);
size = fr.read(in);
System.out.print(size + " ");
for(char c : in)
System.out.print(c);
fr.close();
} catch(IOException e) { }

483

// just an object

//
//
//
//
//
//

create an actual file
& a FileWriter obj
write characters to
the file
flush before closing
close file when done

//
//
//
//
//

create a FileReader
object
read the whole file!
how many bytes read
print the array

// again, always close

}
}

which produces the output:
12 howdy
folks

Here's what just happened:
1. FileWriter fw = new FileWriter(file) did three things:
a. It created a FileWriter reference variable, fw.
b. It created a FileWriter object and assigned it to fw.
c. It created an actual empty file out on the disk (and you can prove it).
2. We wrote 12 characters to the file with the write() method, and we did a
flush() and a close().
3. We made a new FileReader object, which also opened the file on disk for
reading.
4. The read() method read the whole file, a character at a time, and put it into
the char[] in.
5. We printed out the number of characters we read in size, and we looped
through the in array, printing out each character we read, and then we closed
the file.

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Before we go any further, let's talk about flush() and close(). When you write
data out to a stream, some amount of buffering will occur, and you never know for
sure exactly when the last of the data will actually be sent. You might perform many
write operations on a stream before closing it, and invoking the flush() method
guarantees that the last of the data you thought you had already written actually gets
out to the file. Whenever you're done using a file, either reading it or writing to it,
you should invoke the close() method. When you are doing file I/O, you're using
expensive and limited operating system resources, and so when you're done,
invoking close() will free up those resources.
Now, back to our last example. This program certainly works, but it's painful in a
couple of different ways:
1. When we were writing data to the file, we manually inserted line separators
(in this case \n) into our data.
2. When we were reading data back in, we put it into a character array. It being
an array and all, we had to declare its size beforehand, so we'd have been in
trouble if we hadn't made it big enough! We could have read the data in one
character at a time, looking for the end of the file after each read(), but
that's pretty painful too.
Because of these limitations, we'll typically want to use higher-level I/O classes
like BufferedWriter or BufferedReader in combination with FileWriter or
FileReader.

Combining I/O Classes
Java's entire I/O system was designed around the idea of using several classes in
combination. Combining I/O classes is sometimes called wrapping and sometimes
called chaining. The java.io package contains about 50 classes, 10 interfaces, and
15 exceptions. Each class in the package has a specific purpose (creating high
cohesion), and the classes are designed to be combined with each other in countless
ways to handle a wide variety of situations.
When it's time to do some I/O in real life, you'll undoubtedly find yourself poring
over the java.io API, trying to figure out which classes you'll need and how to
hook them together. For the exam, you'll need to do the same thing, but Oracle
artificially reduced the API (phew!). In terms of studying for Exam Objective 7.2,
we can imagine that the entire java.io package—consisting of the classes listed in
Exam Objective 7.2 and summarized in Table 9-1—is our mini I/O API.

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TABLE 9-1

java.io
Mini API

485

java.io Class

Extends Key Constructor(s)
From
Arguments

Key Methods

File

Object

File, String
String
String, String

createNewFile()
delete()
exists()
isDirectory()
isFile()
list()
mkdir()
renameTo()

FileWriter

Writer

File
String

close()
flush()
write()

BufferedWriter

Writer

Writer

close()
flush()
newLine()
write()

PrintWriter

Writer

File (as of Java 5)
String (as of Java 5)
OutputStream
Writer

close()
flush()
format(), printf()
print(), println()
write()

FileReader

Reader

File
String

read()

BufferedReader

Reader

Reader

read()
readLine()

Now let's say that we want to find a less painful way to write data to a file and
read the file's contents back into memory. Starting with the task of writing data to
a file, here's a process for determining what classes we'll need and how we'll hook
them together:
1. We know that ultimately we want to hook to a File object. So whatever
other class or classes we use, one of them must have a constructor that takes
an object of type File.
2. Find a method that sounds like the most powerful, easiest way to accomplish
the task. When we look at Table 9-1 we can see that BufferedWriter has
a newLine() method. That sounds a little better than having to manually
embed a separator after each line, but if we look further, we see that PrintWriter has a method called println(). That sounds like the easiest approach of all, so we'll go with it.

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3. When we look at PrintWriter's constructors, we see that we can build a
PrintWriter object if we have an object of type File, so all we need to do
to create a PrintWriter object is the following:
File file = new File("fileWrite2.txt");
PrintWriter pw = new PrintWriter(file);

//
//
//
//

create a File
pass file to
the PrintWriter
constructor

Okay, time for a pop quiz. Prior to Java 5, PrintWriter did not have constructors
that took either a String or a File. If you were writing some I/O code in Java 1.4,
how would you get a PrintWriter to write data to a file? Hint: You can figure this
out by studying the mini I/O API in Table 9-1.
Here's one way to go about solving this puzzle: First, we know that we'll create a
File object on one end of the chain, and that we want a PrintWriter object on
the other end. We can see in Table 9-1 that a PrintWriter can also be built using a
Writer object. Although Writer isn't a class we see in the table, we can see that
several other classes extend Writer, which for our purposes is just as good; any class
that extends Writer is a candidate. Looking further, we can see that FileWriter
has the two attributes we're looking for:
■ It can be constructed using a File.
■ It extends Writer.

Given all of this information, we can put together the following code (remember,
this is a Java 1.4 example):
File file = new File("fileWrite2.txt");
FileWriter fw = new FileWriter(file);

//
//
//
//

create a File object
create a FileWriter
that will send its
output to a File

PrintWriter pw = new PrintWriter(fw);

// create a PrintWriter
// that will send its
// output to a Writer

pw.println("howdy");
pw.println("folks");

// write the data

At this point, it should be fairly easy to put together the code to more easily read
data from the file back into memory. Again, looking through the table, we see a
method called readLine() that sounds like a much better way to read data. Going
through a similar process, we get the following code:

File Navigation and I/O (OCP Objectives 7.1 and 7.2)

File file =
new File("fileWrite2.txt");
FileReader fr =
new FileReader(file);
BufferedReader br =
new BufferedReader(fr);
String data = br.readLine();

487

// create a File object AND
// open "fileWrite2.txt"
// create a FileReader to get
// data from 'file'
// create a BufferReader to
// get its data from a Reader
// read some data

You're almost certain to encounter exam questions that test your
knowledge of how I/O classes can be chained. If you're not totally clear on this last
section, we recommend that you use Table 9-1 as a reference and write code to
experiment with which chaining combinations are legal and which are illegal.

Working with Files and Directories
Earlier, we touched on the fact that the File class is used to create files and directories.
In addition, File's methods can be used to delete files, rename files, determine whether
files exist, create temporary files, change a file's attributes, and differentiate between
files and directories. A point that is often confusing is that an object of type File is
used to represent either a file or a directory. We'll talk about both cases next.
We saw earlier that the statement
File file = new File("foo");

always creates a File object and then does one of two things:
1. If "foo" does NOT exist, no actual file is created.
2. If "foo" does exist, the new File object refers to the existing file.
Notice that File file = new File("foo"); NEVER creates an actual file.
There are two ways to create a file:
1. Invoke the createNewFile() method on a File object. For example:
File file = new File("foo");
file.createNewFile();

// no file yet
// make a file, "foo" which
// is assigned to 'file'

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2. Create a Writer or a Stream. Specifically, create a FileWriter, a
PrintWriter, or a FileOutputStream. Whenever you create an instance
of one of these classes, you automatically create a file, unless one already
exists; for instance
File file = new File("foo");
PrintWriter pw =
new PrintWriter(file);

// no file yet
//
//
//
//

make a PrintWriter object AND
make a file, "foo" to which
'file' is assigned, AND assign
'pw' to the PrintWriter

Creating a directory is similar to creating a file. Again, we'll use the convention
of referring to an object of type File that represents an actual directory as a
Directory object, with a capital D (even though it's of type File). We'll call an
actual directory on a computer a directory, with a small d. Phew! As with creating a
file, creating a directory is a two-step process; first we create a Directory (File)
object; then we create an actual directory using the following mkdir() method:
File myDir = new File("mydir");
myDir.mkdir();

// create an object
// create an actual directory

Once you've got a directory, you put files into it and work with those files:
File myFile = new File(myDir, "myFile.txt");
myFile.createNewFile();

This code is making a new file in a subdirectory. Since you provide the
subdirectory to the constructor, from then on, you just refer to the file by its
reference variable. In this case, here's a way that you could write some data to the
file myFile:
PrintWriter pw = new PrintWriter(myFile);
pw.println("new stuff");
pw.flush();
pw.close();

Be careful when you're creating new directories! As we've seen, constructing a
Writer or a Stream will often create a file for you automatically if one doesn't exist,

but that's not true for a directory.
File myDir = new File("mydir");
// myDir.mkdir();
// call to mkdir() omitted!
File myFile = new File(
myDir, "myFile.txt");
myFile.createNewFile();
// exception if no mkdir!

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489

This will generate an exception that looks something like
java.io.IOException: No such file or directory

You can refer a File object to an existing file or directory. For example, assume
that we already have a subdirectory called existingDir in which resides an existing
file existingDirFile.txt, which contains several lines of text. When you run the
following code:
File existingDir = new File("existingDir");
System.out.println(existingDir.isDirectory());
File existingDirFile = new File(
existingDir, "existingDirFile.txt");
System.out.println (existingDirFile.isFile());

// assign a dir

// assign a file

FileReader fr = new FileReader(existingDirFile);
BufferedReader br = new BufferedReader(fr);
// make a Reader
String s;
while( (s = br.readLine()) != null)
System.out.println(s);

// read data

br.close();

the following output will be generated:
true
true
existing sub-dir data
line 2 of text
line 3 of text

Take special note of what the readLine() method returns. When there is no
more data to read, readLine() returns a null—this is our signal to stop reading
the file. Also, notice that we didn't invoke a flush() method. When reading a file,
no flushing is required, so you won't even find a flush() method in a Reader kind
of class.
In addition to creating files, the File class lets you do things like renaming and
deleting files. The following code demonstrates a few of the most common ins and
outs of deleting files and directories (via delete()) and renaming files and
directories (via renameTo()):
File delDir = new File("deldir");
delDir.mkdir();
File delFile1 = new File(
delDir, "delFile1.txt");

// make a directory

// add file to directory

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delFile1.createNewFile();
File delFile2 = new File(
delDir, "delFile2.txt");
delFile2.createNewFile();
delFile1.delete();
System.out.println("delDir is "
+ delDir.delete());
File newName = new File(
delDir, "newName.txt");
delFile2.renameTo(newName);
File newDir = new File("newDir");
delDir.renameTo(newDir);

// add file to directory
// delete a file
// attempt to delete
// the directory
// a new object
// rename file
// rename directory

This outputs
delDir is false

and leaves us with a directory called newDir that contains a file called newName.txt.
Here are some rules that we can deduce from this result:
■ delete()

You can't delete a directory if it's not empty, which is why the
invocation delDir.delete() failed.

■ renameTo() You must give the existing File object a valid new File

object with the new name that you want. (If newName had been null, we
would have gotten a NullPointerException.)
■ renameTo()

It's okay to rename a directory, even if it isn't empty.

There's a lot more to learn about using the java.io package, but as far as the
exam goes, we only have one more thing to discuss, and that is how to search for a
file. Assuming that we have a directory named searchThis that we want to search
through, the following code uses the File.list() method to create a String array
of files and directories. We then use the enhanced for loop to iterate through and
print.
String[] files = new String[100];
File search = new File("searchThis");
files = search.list();
for(String fn : files)
System.out.println("found " + fn);

// create the list
// iterate through it

File Navigation and I/O (OCP Objectives 7.1 and 7.2)

491

On our system, we got the following output:
found
found
found
found
found

dir1
dir2
dir3
file1.txt
file2.txt

Your results will almost certainly vary : )
In this section, we've scratched the surface of what's available in the java.io
package. Entire books have been written about this package, so we're obviously
covering only a very small (but frequently used) portion of the API. On the other
hand, if you understand everything we've covered in this section, you will be in great
shape to handle any java.io questions you encounter on the exam, except for the
Console class, which we'll cover next. (Note: Serialization is covered in
Appendix A.)

The java.io.Console Class
New to Java 6 is the java.io.Console class. In this context, the console is the
physical device with a keyboard and a display (like your Mac or PC). If you're
running Java SE 6 from the command line, you'll typically have access to a console
object, to which you can get a reference by invoking System.console(). Keep in
mind that it's possible for your Java program to be running in an environment that
doesn't have access to a console object, so be sure that your invocation of System
.console() actually returns a valid console reference and not null.
The Console class makes it easy to accept input from the command line, both
echoed and nonechoed (such as a password), and makes it easy to write formatted
output to the command line. It's a handy way to write test engines for unit testing or
if you want to support a simple but secure user interaction and you don't need a GUI.
On the input side, the methods you'll have to understand are readLine and
readPassword. The readLine method returns a string containing whatever the
user keyed in—that's pretty intuitive. However, the readPassword method doesn't
return a string; it returns a character array. Here's the reason for this: Once you've
got the password, you can verify it and then absolutely remove it from memory. If a
string was returned, it could exist in a pool somewhere in memory, and perhaps some
nefarious hacker could find it.

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Let's take a look at a small program that uses a console to support testing
another class:
import java.io.Console;
public class NewConsole {
public static void main(String[] args) {
String name = "";
Console c = System.console();
char[] pw;
pw = c.readPassword("%s", "pw: ");
for(char ch: pw)
c.format("%c ", ch);
c.format("\n");
MyUtility mu = new MyUtility();
while(true) {
name = c.readLine("%s", "input?: ");

// #1: get a Console
// #2: return a char[]
// #3: format output

// #4: return a String

c.format("output: %s \n", mu.doStuff(name));
}
}
}
class MyUtility {
String doStuff(String arg1) {
// stub code
return "result is " + arg1;
}
}

// #5: class to test

Let's review this code:
■ At line 1, we get a new Console object. Remember that we can't say this:
Console c = new Console();

■ At line 2, we invoke readPassword, which returns a char[], not a string.

You'll notice when you test this code that the password you enter isn't echoed
on the screen.
■ At line 3, we're just manually displaying the password you keyed in,

separating each character with a space. Later on in this chapter, you'll read
about the format() method, so stay tuned.
■ At line 4, we invoke readLine, which returns a string.

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493

■ At line 5 is the class that we want to test. Later in this chapter, when you're

studying regex and formatting, we recommend that you use something like
NewConsole to test the concepts that you're learning.
The Console class has more capabilities than are covered here, but if you
understand everything discussed so far, you'll be in good shape for the exam.

CERTIFICATION OBJECTIVE

Files, Path, and Paths
(OCP Objectives 8.1 and 8.2)
8.1

Operate on file and directory paths with the Path class.

8.2

Check, delete, copy, or move a file or directory with the Files class.

Note: For coverage of Serialization, see Appendix A.
The OCP 7 exam has two sections devoted to I/O. The previous section Oracle
refers to as "Java I/O Fundamentals" (which we've referred to as the 7.x objectives),
and it was focused on the java.io package. Now we're going to look at the set of
objectives Oracle calls "Java File I/O (NIO.2)," whose specific objectives we'll refer
to as 8.x. The term NIO.2 is a bit loosely defined, but most people (and the exam
creators) define NIO.2 as being the key new features introduced in Java 7 that reside
in two packages:
■ java.nio.file
■ java.nio.file.attribute

We'll start by looking at the important classes and interfaces in the java.nio.file
package, and then we'll move to the java.nio.file.attribute package later in
the chapter.
As you read earlier in the chapter, the File class represents a file or directory at a
high level. NIO.2 adds three new central classes that you'll need to understand well
for the exam:
■ Path

This interface replaces File as the representation of a file or a directory
when working in NIO.2. It is a lot more powerful than a File though.

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■ Paths

This class contains static methods that create Path objects. (In the
next chapter, you'll learn this is called a factory.)

■ Files

This class contains static methods that work with Path objects.
You'll find basic operations in here like copying or deleting files.

The interface java.nio.file.Path is one of the key classes of file-based I/O
under NIO.2. Just like the good old java.io.File, a Path represents only a
location in the file system, like C:\java\workspace\ocpjp7 (a Windows
directory) or /home/nblack/docs (the docs directory of user nblack on UNIX).
When you create a Path to a new file, that file does not exist until you actually
create the file using Files.createFile(Path target). The Files utility class
will be covered in depth in the next section.
Let's take a look at these relationships another way. The Paths class is used to
create a class implementing the Path interface. The Files class uses Path objects as
parameters. All three of these are new to Java 7. Then there is the File class. It's
been around for a long time. File and Path objects know how to convert to the
other. This lets any older code interact with the new APIs in Files. But notice
what is missing. In the figure, there is no line between File and Files. Despite the
similarity in name, these two classes do not know about each other.

Path

converts

File

creates

Paths

uses

Files

The difference between File, Files, Path, and Paths is really important.
Read carefully on the exam. A one-letter difference can mean a big difference in what
the class does.
To make sure you know the difference between these key classes backward and
forward, make sure you can fill in the four rightmost columns in Table 9-2.

Files, Path, and Paths (OCP Objectives 8.1 and 8.2)

File

Files

Path

Paths

Existed in Java 6?

Yes

No

No

No

Concrete class or interface?

Concrete class

Concrete class Interface Concrete class

Create using "new"

Yes

No

No

No

Contains only static methods No

Yes

No

Yes

TABLE 9-2

Comparing the
Core Classes

495

Creating a Path
A Path object can be easily created by using the get methods from the Paths
helper class. Remember you are calling Paths.get() and not Path.get(). If you
don't remember why, study the last section some more. It's important to have this
down cold.
Taking a look at two simple examples, we have:
Path p1 = Paths.get("/tmp/file1.txt");
Path p2 = Paths.get("c:\\temp\\test");

// on UNIX
// On Windows

The actual method we just called is Paths.get(String first, String...
more). This means we can write it out by separating the parts of the path.
Path p3 = Paths.get("/tmp", "file1.txt");
Path p4 = Paths.get("c:", "temp", "test");
Path p5 = Paths.get("c:\\temp", "test") ;

// same as p1
// same as p2
// also same as p2

As you can see, you can separate out folder and filenames as much or as little as you
want when calling Paths.get(). For Windows, that is particularly cool because you
can make the code easier to read by getting rid of the backslash and escape
character.
Be careful when creating paths. The previous examples are absolute paths since
they begin with the root (/ on UNIX or c: on Windows). When you don't begin with
the root, the Path is considered a relative path, which means Java looks from the
current directory. Which file1.txt do you think p6 has in mind?
Path p6 = Paths.get("tmp", "file1.txt");
/ (root)
|-- tmp
| – file1.txt
| – tmp
| – file1.txt

// relative path - NOT same as p1

It depends. If the program is run from the root, it is the one in /tmp/file1.txt. If
the program is run from /tmp, it is the one in /tmp/tmp/file1.txt. If the program
is run from anywhere else, p6 refers to a file that does not exist.

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One more thing to watch for. If you are on Windows, you might deal with a URL
that looks like file:///c:/temp. The file:// is a protocol just like http:// is.
This syntax allows you to browse to a folder in Internet Explorer. Your program
might have to deal with such a String that a user copied/pasted from the browser.
No problem, right? We learned to code:
Path

p = Paths.get("file:///c:/temp/test");

Unfortunately, this doesn't work and you get an Exception about the colon being
invalid that looks something like this:
Exception in thread "main" java.nio.file.InvalidPathException:
Illegal char <:>
at index 4: file:///c:/temp

Paths provides another method that solves this problem. Paths.get(URI uri)
lets you (indirectly),convert the String to a URI (Uniform Resource Identifier)
before trying to create a Path.
Path

p = Paths.get(URI.create("file:///C:/temp"));

The last thing you should know is that the Paths.get() method we've been
discussing is really a shortcut. You won't need to code the longer version, but it is
good to understand what is going on under the hood. First, Java finds out what the
default file system is. For example, it might be WindowsFileSystemProvider.
Then Java gets the path using custom logic for that file system. Luckily, this all goes
on without us having to write any special code or even think about it.
Path short = Paths.get("c:", "temp");
Path longer = FileSystems.getDefault()
.getPath("c:", "temp");

// get default file system
// then get the Path

Now that you know how to create a Path instance, you can manipulate it in various
ways. We'll get back to that in a bit.
As far as the exam is concerned, Paths.get() is how to create a Path initially.
There is another way that is useful when working with code that was written
before Java 7:
Path
File

convertedPath = file.toPath();
convertedFile = path.toFile();

If you are updating older code that uses File, you can convert it to a Path
and start calling the new classes. And if your newer code needs to call older
code, it can convert back to a File.

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497

Creating Files and Directories
With I/O, we saw that a File doesn't exist just because you have a File object. You
have to call createNewFile()to bring the file into existence and exists()to
check if it exists. Rewriting the example from earlier in the chapter to use NIO.2
methods, we now have:
Path path = Paths.get("fileWrite1.txt");
System.out.println(Files.exists(path));
Files.createFile(path);
System.out.println(Files.exists(path));

//
//
//
//

it's only an object
look for a real file
create a file!
look again

NIO.2 has equivalent methods with two differences:
■ You call static methods on Files rather than instance methods on File.
■ Method names are slightly different.

See Table 9-3 for the mapping between old class/method names and new ones. You
can still continue to use the older I/O approach if you happen to be dealing with
File objects.
There is a new method Files.notExists() to supplement Files.exists().
In some incredibly rare situations, Java won't have enough permissions to
know whether the file exists. When this happens, both methods return false.
You can also create directories in Java. Suppose we have a directory named /java
and we want to create the file /java/source/directory/Program.java. We
could do this one at a time:
Path path1 = Paths.get("/java/source");
Path path2 = Paths.get("/java/source/directory");
Path
file = Paths.get("/java/source/directory/Program.java");
Files.createDirectory(path1);
// create first level of directory
Files.createDirectory(path2);
// create second level of directory
Files.createFile(file);
// create file

Or we could create all the directories in one go:
Files.createDirectories(path2);
Files.createFile(file);

// create all levels of directories
// create file

While both work, the second is clearly better if you have a lot of directories to
create. And remember that the directory needs to exist by the time the file is created.

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TABLE 9-3

I/O and NIO

I/O vs. NIO.2

Description

I/O Approach

NIO.2 Approach

Create an empty
file

File file = new File("test");
file.createNewFile():

Path path = Paths.get("test");
Files.createFile(path);

Create an empty
directory

File file

Path path = Paths.get("dir");
Files.createDirectory(path);

= new File("dir");

file.mkdir()

Create a directory, File file = new File("/a/b/c");
file.mkdirs():
including any
missing parent
directories

Path path = Paths.get("/a/b/c");
Files.createDirectories(path);

Check if a file or
directory exists

Path path = Paths.get("test");
Files.exists(path);

File file = new File("test");
file.exists();

Copying, Moving, and Deleting Files
We often copy, move, or delete files when working with the file system. Up until
Java 7, this was hard to do. In Java 7, however, each is one line. Let's look at some
examples:
Path source = Paths.get("/temp/test1");
Path target = Paths.get("/temp/test2.txt");
Files.copy(source, target);
Files.delete(target);
Files.move(source, target);

//
//
//
//
//

exists
doesn't yet exist
now two copies of the file
back to one copy
still one copy

This is all pretty self-explanatory. We copy a file, delete the copy, and then move the
file. Now, let's try another example:
Path one = Paths.get("/temp/test1");
Path two = Paths.get("/temp/test2.txt");
Path targ = Paths.get("/temp/test23.txt");
Files.copy(one, targ);
Files.copy(two, targ);

//
//
//
//
//
//

exists
exists
doesn't yet exist
now two copies of the file
oops,
FileAlreadyExistsException

Java sees it is about to overwrite a file that already exists. Java doesn't want us to lose
the file, so it "asks" if we are sure by throwing an Exception. copy()and move()
actually take an optional third parameter—zero or more CopyOptions. The most
useful option you can pass is StandardCopyOption.REPLACE_EXISTING.
Files.copy(two, target,
StandardCopyOption.REPLACE_EXISTING);

// ok. You know what
// you are doing

We have to think about whether a file exists when deleting the file too. Let's say
we wrote this test code:

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499

Path path = Paths.get("/java/out.txt");
try {
methodUnderTest();
// might throw an exception
Files.createFile(path);
// file only gets created
// if methodUnderTest() succeeds
} finally {
Files.delete(path);
// NoSuchFileException if no file
}

We don't know whether methodUnderTest works properly yet. If it does, the
code works fine. If it throws an Exception, we never create the file and Files.
delete() throws a NoSuchFileException. This is a problem, as we only want to
delete the file if it was created so we aren't leaving stray files around. There is an
alternative. Files.deleteIfExists(path) returns true and deletes the file only if
it exists. If not, it just quietly returns false. Most of the time, you can ignore this
return value. You just want the file to not be there. If it never existed, mission
accomplished.
If you have to work on pre-Java 7 code, you can use the FileUtils class in
Apache Commons IO (http://commons.apache.org/io.) It has methods similar
to many of the copy, move, and delete methods that are now built into Java 7.
To review, Table 9-4 lists the methods on Files that you are likely to come across
on the exam. Luckily, the exam doesn't expect you to know all 30 methods in the
API. The important thing to remember is to check the Files JavaDoc when you find
yourself dealing with files.
TABLE 9-4

Files Methods

Method

Description

Path copy(Path source, Path target,
CopyOption... options)

Copy the file from source to target and
return target

Path move(Path source, Path target,
CopyOption... options)

Move the file from source to target and
return target

void delete(Path path)

Delete the file and throw an Exception
if it does not exist

boolean deleteIfExists(Path path)

Delete the file if it exists and return
whether file was deleted

boolean exists(Path path,
LinkOption... options)

Return true if file exists

boolean notExists(Path path,
LinkOption... options)

Return true if file does not exist

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Retrieving Information about a Path
The Path interface defines a bunch of methods that return useful information about
the path that you're dealing with. In the following code listing, a Path is created
referring to a directory and then we output information about the Path instance:
Path path = Paths.get("C:/home/java/workspace");
System.out.println("getFileName: " + path.getFileName());
System.out.println("getName(1): " + path.getName(1));
System.out.println("getNameCount: " + path.getNameCount());
System.out.println("getParent: " + path.getParent());
System.out.println("getRoot: " + path.getRoot());
System.out.println("subpath(0, 2): " + path.subpath(0, 2));
System.out.println("toString: " + path.toString());

When you execute this code snippet on Windows, the following output is printed:
getFileName: workspace
getName(1): java
getNameCount: 3
getParent: C:\home\java
getRoot: C:\
subpath(0, 2): home\java
toString: C:\home\java\workspace

Based on this output, it is fairly simple to describe what each method does.
Table 9-5 does just that.
TABLE 9-5

Method

Description

String getFileName()

Returns the filename or the last element of the
sequence of name elements.

Path getName(int index)

Returns the path element corresponding to the
specified index. The 0th element is the one closest
to the root. (On Windows, the root is usually C:\
and on UNIX, the root is /.)

int getNameCount()

Returns the number of elements in this path,
excluding the root.

Path getParent()

Returns the parent path, or null if this path does
not have a parent.

Path getRoot()

Returns the root of this path, or null if this path
does not have a root.

Path subpath(int
beginIndex, int endIndex)

Returns a subsequence of this path (not including
a root element) as specified by the beginning
(included) and ending (not included) indexes.

String toString()

Returns the string representation of this path.

Path Methods

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501

Here is yet another interesting fact about the Path interface: It extends from
Iterable. At first sight, this seems anything but interesting. But every class
that (correctly) implements the Iterable interface can be used as an expression
in the enhanced for loop. So you know you can iterate through an array or a List,
but you can iterate through a Path as well. That's pretty cool!
Using this functionality, it's easy to print the hierarchical tree structure of a file
(or directory), as the following example shows:
int spaces = 1;
Path myPath = Paths.get("tmp", "dir1", "dir2", "dir3", "file.txt");
for (Path subPath : myPath) {
System.out.format("%" + spaces + "s%s%n", "", subPath);
spaces += 2; }

When you run this example, a (simplistic) tree is printed. Thanks to the variable
spaces (which is increased with each iteration by 2), the different subpaths are

printed like a directory tree.
tmp
dir1
dir2
dir3
file.txt

Normalizing a Path
Normally (no pun intended), when you create a Path, you create it in a direct way.
However, all three of these return the same logical Path:
Path p1 = Paths.get("myDirectory");
Path p2 = Paths.get("./myDirectory");
Path p3 = Paths.get("anotherDirectory", "..",
"myDirectory");

//
//
//
//

one dot means
current directory
two dots means go up
one directory

p1 is probably what you would type if you were coding. p2 is just plain redundant. p3
is more interesting. The two directories—anotherDirectory and myDirectory—

are on the same level, but we have to go up one level to get there:
/ (root)
|-- anotherDirectory
|-- myDirectory

You might be wondering why on earth we wouldn't just type myDirectory in the
first place. And you would if you could. Sometimes, that doesn't work out. Let's look
at a real example of why this might be.

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/ (root)
|-- Build_Project
|-- scripts
|-- buildScript.sh
|-- My_Project
|-- source
|-- MyClass.java

If you wanted to compile MyClass, you would cd to /My_Project/source and
run javac MyClass.java. Once your program gets bigger, it could be thousands of
classes and have hundreds of jar files. You don't want to type in all of those just to
compile, so someone writes a script to build your program. buildScript.sh now
finds everything that is needed to compile and runs the javac command for you.
The problem is that the current directory is now /Build_Project/scripts and
not /My_Project/source. The build script helpfully builds a path for you by doing
something like this:
String buildProject
= "/Build_Project/scripts";

// build scripts like to express
// paths in relation to themselves

String upTwoDirectories = "../..";

// remember what .. means?

String myProject = "/My_Project/source";
Path path = Paths.get(buildProject,
upTwoDirectories, myProject); // build path from variables
System.out.println("Original: " + path);
System.out.println("Normalized: " + path.normalize());

which outputs:
Original:/Build_Project/scripts/../../My_Project/source
Normalized:/My_Project/source

Whew. The second one is much easier to read. The normalize() method knows
that a single dot can be ignored. It also knows that any directory followed by two
dots can be removed from a path.
Be careful when using this normalize()! It just looks at the String equivalent
of the path and doesn't check the file system to see whether the directories or files
actually exist.
Let's practice and see what normalize returns for these paths. This time, we
aren't providing a directory structure to show that the directories and files don't
need to be present on the computer. What do you think the following prints out?
System.out.println(Paths.get("/a/./b/./c").normalize());
System.out.println(Paths.get(".classpath").normalize());
System.out.println(Paths.get("/a/b/c/..").normalize());
System.out.println(Paths.get("../a/b/c").normalize());

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503

The output is:
/a/b/c
.classpath
/a/b
../a/b/c

The first one removes all the single dots since they just point to the current directory.
The second doesn't change anything since the dot is part of a filename and not a
directory. The third sees one set of double dots, so it only goes up one directory. The
last one is a little tricky. The two dots do say to go up one directory. But since there
isn't a directory before it, Path can't simplify it.
To review, normalize() removes unneeded parts of the Path, making it more
like you'd normally type it. (That's not where the word "normalize" comes from, but
it is a nice way to remember it.)

Resolving a Path
So far, you have an overview of all methods that can be invoked on a single Path
object, but what if you need to combine two paths? You might want to do this if you
have one Path representing your home directory and another containing the Path
within that directory.
Path dir = Paths.get("/home/java");
Path file = Paths.get("models/Model.pdf");
Path result = dir.resolve(file);
System.out.println("result = " + result);

This produces the absolute path by merging the two paths:
result = /home/java/models/Model.pdf

path1.resolve(path2) should be read as "resolve path2 within path1's
directory." In this example, we resolved the path of the file within the directory
provided by dir.
Keeping this definition in mind, let's look at some more complex examples:
Path absolute = Paths.get("/home/java");
Path relative = Paths.get("dir");
Path file = Paths.get("Model.pdf");
System.out.println("1: " + absolute.resolve(relative));
System.out.println("2: " + absolute.resolve(file));
System.out.println("3: " + relative.resolve(file));
System.out.println("4: " + relative.resolve(absolute));
System.out.println("5: " + file.resolve(absolute));
System.out.println("6: " + file.resolve(relative));

// BAD
// BAD
// BAD

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The output is:
1:
2:
3:
4:
5:
6:

/home/java/dir
/home/java/Model.pdf
dir/Model.pdf
/home/java
/home/java
Model.pdf/dir

The first three do what you'd expect. They add the parameter to resolve to the
provided path object. The fourth and fifth ones try to resolve an absolute path
within the context of something else. The problem is that an absolute path doesn't
depend on other directories. It is absolute. Therefore, resolve just returns that
absolute path. The sixth one tries to resolve a directory within the context of a file.
Since that doesn't make any sense, Java just tries its best and gives you nonsense.
Just like normalize(), keep in mind that resolve() will not check that the
directory or file actually exists. To review, resolve() tells you how to resolve one
path within another.

Be careful with methods that come in two flavors: one with a Path
parameter and the other with a String parameter such as resolve().The tricky part
here is that null is a valid value for both a Path and a String. What will happen if you
pass just null as a parameter? Which method will be invoked?
Path path = Paths.get("/usr/bin/zip");
path.resolve(null);

The compiler can't decide which method to invoke: the one with the Path parameter or
the other one with the String parameter.That's why this code won't compile, and if you
see such code in an exam question, you'll know what to do.
The following examples will compile without any problem, because the compiler
knows which method to invoke thanks to the type of the variable other and the explicit
cast to String.
Path path = Paths.get("/usr/bin/zip");
Path other = null;
path.resolve(other);
path.resolve ((String) null);

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505

Relativizing a Path
Now suppose we want to do the opposite of resolve. We have the absolute path of
our home directory and the absolute path of the music file in our home directory.
We want to know just the music file directory and name.
Path dir = Paths.get("/home/java");
Path music = Paths.get("/home/java/country/Swift.mp3");
Path mp3 = dir.relativize(music);
System.out.println(mp3);

The output is: country/Swift.mp3. Java recognized that the /home/java part is
the same and returned a path of just the remainder.
path1.relativize(path2) should be read as "give me a path that shows how to
get from path1 to path2." In this example, we determined that music is a file in a
directory named country within dir.
Keeping this definition in mind, let's look at some more complex examples:
Path absolute1 = Paths.get("/home/java");
Path absolute2 = Paths.get("/usr/local");
Path absolute3 = Paths.get("/home/java/temp/music.mp3");
Path relative1 = Paths.get("temp");
Path relative2 = Paths.get("temp/music.pdf");
System.out.println("1: " + absolute1.relativize(absolute3));
System.out.println("2: " + absolute3.relativize(absolute1));
System.out.println("3: " + absolute1.relativize(absolute2));
System.out.println("4: " + relative1.relativize(relative2));
System.out.println("5: " + absolute1.relativize(relative1));//BAD

The output is
1: temp/music.mp3
2: ../..
3: ../../usr/local
4: music.pdf
Exception in thread "main" java.lang.IllegalArgumentException: 'other'
is different type of Path

Before you scratch your head, let's look at the logical directory structure here.
Keep in mind the directory doesn't actually need to exist; this is just to visualize it.
/root
| – usr
| – local
| – home
| -- java
| – temp
| – music.mp3

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Now we can trace it through. The first example is straightforward. It tells us how
to get to absolute3 from absolute1 by going down two directories. The second is
similar. We get to absolute1 from absolute3 by doing the opposite—going up two
directories. Remember from normalize() that a double dot means to go up a
directory.
The third output statement says that we have to go up two directories and then
down two directories to get from absolute1 to absolute2. Java knows this since
we provided absolute paths. The worst possible case is to have to go all the way up to
the root like we did here.
The fourth output statement is okay. Even though they are both relative paths,
there is enough in common for Java to tell what the difference in path is.
The fifth example throws an exception. Java can't figure out how to make a
relative path out of one absolute path and one relative path.
Remember, relativize() and resolve() are opposites. And just like
resolve(), relativize() does not check that the path actually exists. To review,
relativize() tells you how to get a relative path between two paths.

CERTIFICATION OBJECTIVE

File and Directory Attributes (OCP Objective 8.3)
8.3 Read and change file and directory attributes, focusing on the BasicFileAttributes,
DosFileAttributes, and PosixFileAttributes interfaces.

Reading and Writing Attributes the Easy Way
In this section, we'll add classes and interfaces from the java.nio.file.attribute
package to the discussion. Prior to NIO.2, you could read and write just a handful of
attributes. Just like we saw when creating files, there is a new way to do this using
Files instead of File. Oracle also took the opportunity to clean up the method
signatures a bit. The following example creates a file, changes the last modified date,
prints it out, and deletes the file using both the old and new method names. We
might do this if we want to make a file look as if it were created in the past. (As you
can see, there is a lesson about not relying on file timestamps here!)

File and Directory Attributes (OCP Objective 8.3)

Date januaryFirst = new GregorianCalendar(
2013, Calendar.JANUARY, 1).getTime();
// old way
File file = new File("c:/temp/file");
file.createNewFile();
file.setLastModified(januaryFirst.getTime());
System.out.println(file.lastModified());
file.delete();

507

// create a date

//
//
//
//

create the file
set time
get time
delete the file

// new way
Path path = Paths.get("c:/temp/file2");
Files.createFile(path);
FileTime fileTime = FileTime.fromMillis(
januaryFirst.getTime());
Files.setLastModifiedTime(path, fileTime);
System.out.println(Files.getLastModifiedTime(path));
Files.delete(path);

//
//
//
//
//
//

create another file
convert to the new
FileTime object
set time
get time
delete the file

As you can see from the output, the only change in functionality is that the new
Files.getLastModifiedTime() uses a human-friendly date format.
1357016400000
2013-01-01T05:00:00Z

The other common type of attribute you can set are file permissions. Both
Windows and UNIX have the concept of three types of permissions. Here's what
they mean:
■ Read

You can open the file or list what is in that directory.

■ Write

You can make a change to the file or add a file to that directory.

■ Execute

You can run the file if it is a runnable program or go into that

directory.
Printing out the file permissions is easy. Note that these permissions are just for
the user who is running the program—you! There are other types of permissions as
well, but these can't be set in one line.
System.out.println(Files.isExecutable(path));
System.out.println(Files.isReadable(path));
System.out.println(Files.isWritable(path));

Table 9-6 shows how to get and set these attributes that can be set in one line,
both using the older I/O way and the new Files class. You may have noticed that
setting file permissions isn't in the table. That's more code, so we will talk about
it later.

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I/O and NIO

I/O vs. NIO.2 Permissions

Description

I/O Approach

Approach

Get the last
modified date/
time

File file = new File("test");
file.lastModified();

Path path = Paths.get("test");
Files.getLastModifiedTime(path);

Is read
permission set

File file = new File("test");
file.canRead();

Path path = Paths.get("test");
Files.isReadable(path);

Is write
permission set

File file = new File("test");
file.canWrite();

Path path = Paths.get("test");
Files.isWritable(path);

Is executable
permission set

File file = new File("test");
file.canExecute();

Path path = Paths.get("test");
Files.isExecutable(path);

Set the last
modified
date/time
(Note:

File file = new File("test");
file.setLastModified(timeInMillis);

Path path = Paths.get("test");
FileTime fileTime = FileTime.fromMillis(timeInMillis);
Files.setLastModifiedTime(path, fileTime);

timeInMillis

is an appropriate
long.)

Types of Attribute Interfaces
The attributes you set by calling methods on Files are the most straightforward
ones. Beyond that, Java NIO.2 added attribute interfaces so that you could read
attributes that might not be on every operating system.
■ BasicFileAttributes

In the JavaDoc, Oracle says these are "attributes
common to many file systems." What they mean is that you can rely on these
attributes being available to you unless you are writing Java code for some
funky new operating system. Basic attributes include things like creation date.

■ PosixFileAttributes

POSIX stands for Portable Operating System
Interface. This interface is implemented by both UNIX- and Linux-based
operating systems. You can remember this because POSIX ends in "x," as do
UNIX and Linux.

■ DosFileAttributes

DOS stands for Disk Operating System. It is part of
all Windows operating systems. Even Windows 8 has a DOS prompt available.

There are also separate interfaces for setting or updating attributes. While the details
aren't in scope for the exam, you should be familiar with the purpose of each one.
■ BasicFileAttributeView

creation dates.

Used to set the last updated, last accessed, and

File and Directory Attributes (OCP Objective 8.3)

509

■ PosixFileAttributeView

Used to set the groups or permissions on UNIX/
Linux systems. There is an easier way to set these permissions though, so you
won't be using the attribute view.

■ DosFileAttributeView

Used to set file permissions on DOS/Windows
systems. Again, there is an easier way to set these, so you won't be using the
attribute view.

■ FileOwnerAttributeView
■ AclFileAttributeView

Used to set the primary owner of a file or directory.

Sets more advanced permissions on a file or directory.

Working with BasicFileAttributes
The BasicFileAttributes interface provides methods to get information about a
file or directory.
BasicFileAttributes basic = Files.readAttributes(path, // assume a valid path
BasicFileAttributes.class);
System.out.println("create: " + basic.creationTime());
System.out.println("access: " + basic.lastAccessTime());
System.out.println("modify: " + basic.lastModifiedTime());
System.out.println("directory: " + basic.isDirectory());

The sample output shows that all three date/time values can be different. A file is
created once. It can be modified many times. And it can be last accessed for reading
after that. The isDirectory method is the same as Files.isDirectory(path). It
is just an alternative way of getting the same information.
create: 2013-01-01T18:06:01Z
access: 2013-01-29T14:44:218
modify: 2014-01-13T16:13:21Z
directory: false

There are some more attributes on BasicFileAttributes, but they aren't on
the exam and you aren't likely to need them when coding. Just remember to check
the JavaDoc if you need more information about a file.
So far, you've noticed that all the attributes are read only. That is because Java
provides a different interface for updating attributes. Let's write code to update the
last accessed time:
BasicFileAttributes basic = Files.readAttributes(
path, BasicFileAttributes.class);
// attributes
FileTime lastUpdated = basic.lastModifiedTime();
// get current
FileTime created = basic.creationTime();
// values
FileTime now = FileTime.fromMillis(System.currentTimeMillis());
BasicFileAttributeView basicView = Files.getFileAttributeView(
path, BasicFileAttributeView.class);
// "view" this time
basicView.setTimes(lastUpdated, now, created);
// set all three

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In this example, we demonstrated getting all three times. In practice, when
calling setTimes(), you should pass null values for any of the times you don't want
to change, and only pass Filetimes for the times you want to change.
The key takeaways here are that the "XxxFileAttributes" classes are read only
and the "XxxFileAttributeView" classes allow updates.

The BasicFileAttributes and BasicFileAttributeView interfaces are
a bit confusing.They have similar names but different functionality, and you get them in
different ways.Try to remember these three things:
■ BasicFileAttributeView is singular, but BasicFileAttributes is not.
■ You get BasicFileAttributeView using Files.getFileAttributeView, and you get
BasicFileAttributes using Files.readAttributes.
■ You can ONLY update attributes in BasicFileAttributeView, not in
BasicFileAttributes. Remember that the view is for updating.

PosixFileAttributes and DosFileAttributes inherit from
BasicFileAttributes. This means that you can call Basic methods on a POSIX or

DOS subinterface.

BasicFileAttributes

inherits

PosixFileAttributes

DosFileAttributes

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511

Try to use the more general type if you can. For example, if you are only going to
use basic attributes, just get BasicFileAttributes. This lets your code remain
operating system independent. If you are using a mix of basic and POSIX attributes,
you can use PosixFileAttributes directly rather than calling readAttributes()
twice to get two different ones.

Working with DosFileAttributes
DosFileAttributes adds four more attributes to the basics. We'll look at the most
common ones here—hidden files and read-only files. Hidden files typically begin
with a dot and don't show up when you type dir to list the contents of a directory.
Read-only files are what they sound like—files that can't be updated. (The other
two attributes are "archive" and "system," which you are quite unlikely to ever use.)
Path path= Paths.get("C:/test");
Files.createFile(path);
// create file
Files.setAttribute(path, "dos:hidden", true);
// set attribute
Files.setAttribute(path, "dos:readonly", true);
// another one
DosFileAttributes dos = Files.readAttributes(path,
DosFileAttributes.class);
// dos attributes
System.out.println(dos.isHidden());
System.out.println(dos.isReadOnly());
Files.setAttribute(path, "dos:hidden", false);
Files.setAttribute(path, "dos:readonly", false);
dos = Files.readAttributes(path,
DosFileAttributes.class);
// get attributes again
System.out.println(dos.isHidden());
System.out.println(dos.isReadOnly());
Files.delete(path);

The output is:
true
true
false
false

The first tricky thing in this code is that the String "readonly" is lowercase
even though the method name is mixed case. If you forget and use the String
"readOnly," Java will silently ignore the statement and the file will still allow
anyone to write to it.

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The other tricky thing is that you cannot delete a read-only file. That's why the
code calls setAttribute a second time with false as a parameter, to make it no
longer "read only" so the code can clean up after itself. And you can see that we had
to call readAttributes again to see those updated values.
There is an alternative way to set these attributes where you don't have to
worry about the String values. However, the exam wants you to know how to
use Files. It is good to know both ways, though.
DosFileAttributeView view = Files.getFileAttributeView(path,
DosFileAttributeView.class);
view.setHidden(true);
view.setReadOnly(true);

Working with PosixFileAttributes
PosixFileAttributes adds two more attributes to the basics—groups and

permissions. On UNIX, every file or directory has both an owner and group name.
UNIX permissions are also more elaborate than the basic ones. Each file or
directory has nine permissions set in a String. A sample is "rwxrw-r--." Breaking
this into groups of three, we have "rwx", "rw-," and "r--." These sets of permissions
correspond to who gets them. In this example, the "user" (owner) of the file has
read, write, and execute permissions. The "group" only has read and write
permissions. UNIX calls everyone who is not the owner or in the group "other."
"Other" only has read access in this example.
Now let's look at some code to set the permissions and output them in humanreadable form:
Path path = Paths.get("/tmp/file2");
Files.createFile(path);
PosixFileAttributes posix = Files.readAttributes(path,
PosixFileAttributes.class);
//
Set perms =
PosixFilePermissions.fromString("rw-r--r--"); //
Files.setPosixFilePermissions(path, perms);
//
System.out.println(posix.permissions());
//

The output looks like this:
[OWNER_WRITE, GROUP_READ, OTHERS_READ, OWNER_READ]

get the Posix type
UNIX style
set permissions
get permissions

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513

It's not symmetric. We gave Java the permissions in cryptic UNIX format and got
them back in plain English. You can also output the group name:
System.out.println(posix.group());

// get group

which outputs something like this:
horse

Reviewing Attributes
Let's review the most common attributes information in Table 9-7.
TABLE 9-7

Common Attributes

Type

Read and Write an Attribute

Basic

// read
BasicFileAttributes basic = Files.readAttributes(path,
BasicFileAttributes.class);
FileTime lastUpdated = basic.lastModifiedTime();
FileTime created = basic.creationTime();
FileTime now = FileTime.fromMillis(System.currentTimeMillis());
// write
BasicFileAttributeView basicView =
Files.getFileAttributeView(path,
BasicFileAttributeView.class);
basicView.setTimes(lastUpdated, now, created);

Posix
(UNIX/Linux)

PosixFileAttributes posix = Files.readAttributes(path, PosixFileAttributes.class);
Set perms = PosixFilePermissions.fromString("rw-r--r--");
Files.setPosixFilePermissions(path, perms);
System.out.println(posix.group());
System.out.println(posix.permissions());

Dos
(Windows)

DosFileAttributes dos = Files.readAttributes(path,
DosFileAttributes.class);
System.out.println(dos.isHidden());
System.out.println(dos.isReadOnly());
Files.setAttribute(path, "dos:hidden", false);
Files.setAttribute(path, "dos:readonly", false);

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CERTIFICATION OBJECTIVE

DirectoryStream (OCP Objective 8.4)
8.4 Recursively access a directory tree using the DirectoryStream and FileVisitor
interfaces.
Now let's return to more NIO.2 capabilities that you'll find in the java.nio.file
package… You might need to loop through a directory. Let's say you were asked to
list out all the users with a home directory on this computer.
/home
| – users
| – vafi
| – eyra
Path dir = Paths.get("/home/users");
try (DirectoryStream stream =
// use try with resources
Files.newDirectoryStream(dir)) {
// so we don't have close()
for (Path path : stream)
// loop through the stream
System.out.println(path.getFileName());
}

As expected, this outputs
vafi
eyra

The DirectoryStream interface lets you iterate through a directory. But this is
just the tip of the iceberg. Let's say we have hundreds of users and each day we want
to only report on a few of them. The first day, we only want the home directories of
users whose names begin with either the letter v or the letter w.
Path dir = Paths.get("/home/users");
try (DirectoryStream stream = Files.newDirectoryStream(
dir, "[vw]*")) {
// "v" or "w" followed by anything
for (Path path : stream)
System.out.println(path.getFileName());
}

This time, the output is
vafi

Let's examine the expression [vw]*. [vw] means either of the characters v or w.
The * is a wildcard that means zero or more of any character. Notice this is not a
regular expression. (If it were, the syntax would be [vw].*—see the dot in there.)

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DirectoryStream uses something new called a glob. We will see more on globs

later in the chapter.
There is one limitation with DirectoryStream. It can only look at one directory.
One way to remember this is that it works like the dir command in DOS or the ls
command in UNIX. Or you can remember that DirectoryStream streams one
directory.

CERTIFICATION OBJECTIVE

FileVisitor (OCP Objective 8.4)
8.4 Recursively access a directory tree using the DirectoryStream and FileVisitor
interfaces.
Luckily, there is another class that does, in fact, look at subdirectories. Let's say
you want to get rid of all the .class files before zipping up and submitting your
assignment. You could go through each directory manually, but that would get
tedious really fast. You could write a complicated command in Windows and another
in UNIX, but then you'd have two programs that do the same thing. Luckily, you
can use Java and only write the code once.
Java provides a SimpleFileVisitor. You extend it and override one or more
methods. Then you can call Files.walkFileTree, which knows how to recursively
look through a directory structure and call methods on a visitor subclass. Let's try
our example:
/home
| – src
| – Test.java
| – Test.class
| – dir
| – AnotherTest.java
| – AnotherTest.class
public class RemoveClassFiles
extends SimpleFileVisitor {
// need to extend visitor
public FileVisitResult visitFile(
// called "automatically"
Path file, BasicFileAttributes attrs)
throws IOException {
if ( file.getFileName().endsWith(".class"))
Files.delete(file);
// delete the file
return FileVisitResult.CONTINUE;
// go on to next file
}
public static void main(String[] args) throws Exception {

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RemoveClassFiles dirs = new RemoveClassFiles();
Files.walkFileTree(
// kick off recursive check
Paths.get("/home/src"),
// starting point
dirs);
// the visitor
}
}

This is a simple file visitor. It only implements one method: visitFile. This
method is called for every file in the directory structure. It checks the extension of
the file and deletes it if appropriate. In our case, two .class files are deleted.
There are two parameters to visitFile(). The first one is the Path object
representing the current file. The other is a BasicFileAttributes interface.
Do you remember what this does? That's right—it lets you find out if the current
file is a directory, when it was created, and many other similar pieces of data.
Finally, visitFile()returns FileVisitResult.CONTINUE. This tells
walkFileTree() that it should keep looking through the directory structure for
more files.
Now that we have a feel for the power of this class, let's take a look at all the
methods available to us with another example:
/home
| – a.txt
| – emptyChild
| – child
| – b.txt
| – grandchild
| – c.txt
public class PrintDirs extends SimpleFileVisitor {
public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
System.out.println("pre: " + dir);
return FileVisitResult.CONTINUE; }
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
System.out.println("file: " + file);
return FileVisitResult.CONTINUE; }
public FileVisitResult visitFileFailed(Path file, IOException exc) {
return FileVisitResult.CONTINUE; }
public FileVisitResult postVisitDirectory(Path dir, IOException exc) {
System.out.println("post: " + dir);
return FileVisitResult.CONTINUE; }
public static void main(String[] args) throws Exception {
PrintDirs dirs = new PrintDirs();
Files.walkFileTree(Paths.get("/home"), dirs); } }

You might get the following output:
pre: /home
file: /home/a.txt
pre: /home/child
file: /home/child/b.txt
pre: /home/child/grandchild
file: /home/child/grandchild/c.txt

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517

post: /home/child/grandchild
post: /home/child
pre: /home/emptyChild
post: /home/emptyChild
post: /home

Note that Java goes down as deep as it can before returning back up the tree. This
is called a depth-first search. We said "might" because files and directories at the same
level can get visited in either order.
You can override as few or many of the four methods as you'd like. Note that the
second half of the methods have IOException as a parameter. This allows those methods
to handle problems that came earlier when walking through the tree. Table 9-8
summarizes the methods.
You actually do have some control, though, through those FileVisitResult
constants. Suppose we changed the preVisitDirectory method to the following:
public FileVisitResult preVisitDirectory(
Path dir, BasicFileAttributes attrs) {
System.out.println("pre: " + dir);
String name = dir.getFileName().toString();
if (name.equals("child"))
return FileVisitResult.SKIP_SUBTREE;
return FileVisitResult.CONTINUE;
}

Now the output is:
pre: /home
file: /home/a.txt
pre: /home/child
pre: /home/emptyChild
post: /home/emptyChild
post: /home

TABLE 9-8
FileVisitor

Method

Description

IOException
Parameter?

preVisitDirectory

Called before drilling down into the
directory

No

visitFile

Called once for each file (but not for
directories)

No

visitFileFailed

Called only if there was an error accessing
a file, usually a permissions issue

Yes

postVisitDirectory

Called when finished with the directory
on the way back up

Yes

Methods

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Since we instructed the program to skip the entire child subtree—i.e., we don't see
the file: b.txt or the sub-directory: grandchild—we also don't see the post visit call.
Now what do you think would happen if we changed FileVisitResult.SKIP_
SIBLINGS to FileVisitResult.TERMINATE? The output might be:
pre: /home
file: /home/a.txt
pre: /home/child

We see that as soon as the "child" directory came up, the program stopped walking
the tree. And again, we are using "might" in terms of the output. It's also possible
for emptyChild to come up first, in which case, the last line of the output would be
/home/emptyChild.
There's one more result type. What do you think would happen if we changed
FileVisitResult.TERMINATE to FileVisitResult.SKIP_SIBLINGS? The output
happens to be the same as the previous example:
pre: /home
file: /home/a.txt
pre: /home/child

SKIP_SIBLINGS is a combination of SKIP_SUBTREE and "don't look in any
folders at the same level." This means we skip everything under child and also skip
emptyChild.
One more example to make sure you really understand what is going on. What do
you think gets output if we use this method?
public FileVisitResult preVisitDirectory(Path dir,
BasicFileAttributes attrs) {
System.out.println("pre: " + dir);
String name = dir.getFileName().toString();
if (name.equals("grandchild"))
return FileVisitResult.SKIP_SUBTREE;
if ( name.equals("emptyChild"))
return FileVisitResult.SKIP_SIBLINGS;
return FileVisitResult.CONTINUE;
}

Assuming child is encountered before emptyChild, the output is:
pre: /home
file: /home/a.txt
pre: /home/child
file: /home/child/b.txt
pre: /home/child/grandchild
post: /home/child
pre: /home/emptyChild
post: /home

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519

We don't see file: c.txt or post: /home/child/grandchild because we skip
grandchild the subtree. We don't see "post: /home/emptyChild" because we
skip siblings of emptyChild. But wait. Isn't /home/child a sibling? It is. But the
visitor goes in order. Since child was seen before emptyChild, it is too late to skip
it. Just like when you print a document, it is too late to prevent pages from printing
that have already printed. File visitor can only skip subtrees that it has not
encountered yet.

CERTIFICATION OBJECTIVE

PathMatcher (OCP Objective 8.5)
8.5

Find a file with PathMatcher interface.

DirectoryStream and FileVisitor allowed us to go through the files that

exist. Things can get complicated fast, though. Imagine you had a requirement to
print out the names of all text files in any subdirectory of "password." You might be
wondering why anyone would want to do this. Maybe a teammate foolishly stored
passwords for everyone to see and you want to make sure nobody else did that. You
could write logic to keep track of the directory structure, but that makes the code
harder to read and understand. By the end of this section, you'll know a better way.
Let's start out with a simpler example to see what a PathMatcher can do:
Path path1 = Paths.get("/home/One.txt");
Path path2 = Paths.get("One.txt");
PathMatcher matcher = FileSystems.getDefault()
.getPathMatcher(
"glob:*.txt");
System.out.println(matcher.matches(path1));
System.out.println(matcher.matches(path2));

// get the PathMatcher
// for the right file system
// wait. What's a glob?

which outputs:
false
true

We can see that the code checks if a Path consists of any characters followed by
".txt." To get a PathMatcher, you have to call FileSystems.getDefault()
.getPathMatcher because matching works differently on different operating
systems. PathMatchers use a new type that you probably haven't seen before called

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a glob. Globs are not regular expressions, although they might look similar at first.
Let's look at some more examples of globs using a common method so we don't have
to keep reading the same "boilerplate" code. (Boilerplate code is the part of the code
that is always the same.)
public void matches(Path path, String glob) {
PathMatcher matcher = FileSystems.getDefault().getPathMatcher(glob);
System.out.println(matcher.matches(path));
}

In the world of globs, one asterisk means "match any character except for a directory
boundary." Two asterisks means "match any character, including a directory boundary."
Path path = Paths.get("/com/java/One.java");
matches(path, "glob:*.java");
// false
matches(path, "glob:**/*.java");
// true
matches(path, "glob:*");
// false
matches(path, "glob:**");
// true

Remember that we are using a file system–specific PathMatcher.This means
slashes and backslashes can be treated differently, depending on what
operating system you happen to be running.The previous example does print
the same output on both Windows and UNIX because it uses forward slashes.
However, if you change just one line of code, the output changes:
Path path = Paths.get("com\\java\\One.java");

Now Windows still prints:
false
true
false
true

However, UNIX prints:
true
false
true
true

Why? Because UNIX doesn't see the backslash as a directory boundary.The
lesson here is to use / instead of \\ so your code behaves more predictably
across operating systems.
Now let's match files with a four-character extension. A question mark matches
any character. A character could be a letter or a number or anything else.

PathMatcher (OCP Objective 8.5)

Path path1 = Paths.get("One.java");
Path path2 = Paths.get("One.ja^a");
matches(path1, "glob:*.????");
matches(path1, "glob:*.???");
matches(path2, "glob:*.????");
matches(path2, "glob:*.???");

//
//
//
//

521

true
false
true
false

Globs also provide a nice way to match multiple patterns. Suppose we want to
match anything that begins with the names Kathy or Bert:
Path path1 = Paths.get("Bert-book");
Path path2 = Paths.get("Kathy-horse");
matches(path1, "glob:{Bert*,Kathy*}");
matches(path2, "glob:{Bert,Kathy}*");
matches(path1, "glob:{Bert,Kathy}");

// true
// true
// false

The first glob shows we can put wildcards inside braces to have multiple glob
expressions. The second glob shows that we can put common wildcards outside the
braces to share them. The third glob shows that without the wildcard, we will only
match the literal strings "Bert" and "Kathy."
You can also use sets of characters like [a-z] or [#$%] in globs just like in regular
expressions. You can also escape special characters with a backslash. Let's put this all
together with a tricky example:
Path path1 = Paths.get("0*b/test/1");
Path path2 = Paths.get("9\\*b/test/1");
Path path3 = Paths.get("01b/test/1");
Path path4 = Paths.get("0*b/1");
String glob = "glob:[0-9]\\*{A*,b}/**/1";
matches(path1, glob);
matches(path2, glob);
matches(path3, glob);
matches(path4, glob);

//
//
//
//

true
false
false
false

Spelling out what the glob does, we have the following:
■ [0-9] One single digit. Can also be read as any one character from 0 to 9.
■ \\*

The literal character asterisk rather than the asterisk that means to
match anything. A single backslash before * escapes it. However, Java won't
let you type a single backslash, so you have to escape the backslash itself with
another backslash.

■ {A*,b}
■ /**/
■ 1

Either a capital A followed by anything or the single character b.

One or more directories with any name.

The single character 1.

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The second path doesn't match because it has the literal backslash followed by the
literal asterisk. The glob was looking for the literal asterisk by itself. The third path
also doesn't match because there is no literal asterisk. The fourth path doesn't match
because there is no directory between "b" and "1" for the ** to match. Luckily, nobody
would write such a crazy, meaningless glob. But if you can understand this one, you are
all set. Globs tend to be simple expressions like {*.txt,*.html} when used for real.
Since globs are just similar enough to regular expressions to be tricky, Table 9-10
reviews the similarities and differences in common expressions. Regular expressions are
more powerful, but globs focus on what you are likely to need when matching filenames.
By now, you've probably noticed that we are dealing with Path objects, which
means they don't actually need to exist on the file system. But we wanted to print
out all the text files that actually exist in a subdirectory of password. Luckily, we can
combine the power of PathMatchers with what we already know about walking the
file tree to accomplish this.
public class MyPathMatcher extends SimpleFileVisitor {
private PathMatcher matcher =
FileSystems.getDefault().getPathMatcher(
"glob:**/password/**.txt");
// ** means any subdirectory
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
throws IOException {
if (matcher.matches(file)) {
System.out.println(file);
}
return FileVisitResult.CONTINUE;
}
public static void main(String[] args) throws Exception {
MyPathMatcher dirs = new MyPathMatcher();
Files.walkFileTree(Paths.get("/"),
dirs); // start with root
}
}

The code looks similar, regardless of what you want to do. You just change the
glob pattern to what you actually want to match.
TABLE 9-10

Glob vs. Regular
Expression

What to Match

In a Glob

In a Regular Expression

Zero or more of any character, including a
directory boundary

**

.*

Zero or more of any character, not
including a directory boundary

*

N/A – no special syntax

Exactly one character

?

.

Any digit

[0-9]

[0-9]

Begins with cat or dog

{cat, dog}*

(cat|dog).*

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523

CERTIFICATION OBJECTIVE

WatchService (OCP Objective 8.6)
8.6

Watch a directory for changes with the WatchService interface.

The last thing you need to know about in NIO.2 is WatchService. Suppose you
are writing an installer program. You check that the directory you are about to install
into is empty. If not, you want to wait until the user manually deletes that directory
before continuing. Luckily, you won't have to write this code from scratch, but you
should be familiar with the concepts. Here's the directory tree:
/dir
| – directoryToDelete
| – other

Here's the code snippet:
Path dir = Paths.get("/dir");

// get directory containing
// file/directory we care
// about
WatchService watcher = FileSystems.getDefault() // file system specific code
.newWatchService();
// create empty watch service
dir.register(watcher, ENTRY_DELETE);
// needs a static import!
// start watching for
// deletions
while (true) {
// loop until say to stop
WatchKey key;
try {
key = watcher.take();
// wait for a deletion
} catch (InterruptedException x) {
return;
// give up if something goes
// wrong
}
for (WatchEvent event : key.pollEvents()) {
WatchEvent.Kind kind = event.kind();
System.out.println(kind.name());
// create/delete/modify
System.out.println(kind.type());
// always a Path for us
System.out.println(event.context());
// name of the file
String name = event.context().toString();
if (name.equals("directoryToDelete")) {
// only delete right directory
System.out.format("Directory deleted, now we can proceed");
return;
// end program, we found what
// we were waiting for
}
}
key.reset();
// keep looking for events
}

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Supposing we delete directory "other" followed by directory directoryToDelete,
this outputs:
ENTRY_DELETE
interface java.nio.file.Path
other
ENTRY_DELETE
interface java.nio.file.Path
directoryToDelete
Directory deleted, now we can proceed

Notice that we had to watch the directory that contains the files or directories we are
interested in. This is why we watched /dir instead of /dir/directoryToDelete.
This is also why we had to check the context to make sure the directory we were
actually interested in is that one that was deleted.
The basic flow of WatchService stays the same, regardless of what you want to do:
1. Create a new WatchService
2. Register it on a Path listening to one or more event types
3. Loop until you are no longer interested in these events
4. Get a WatchKey from the WatchService
5. Call key.pollEvents and do something with the events
6. Call key.reset to look for more events
Let's look at some of these in more detail. You register the WatchService on a
Path using statements like the following:
dir1.register(watcher, ENTRY_DELETE);
dir2.register(watcher, ENTRY_DELETE, ENTRY_CREATE);
dir3.register(watcher, ENTRY_DELETE, ENTRY_CREATE, ENTRY_MODIFY);

(Note: These ENTRY_XXX constants can be found in the StandardWatchEventsKinds
class. Here and in later code, you'll probably want to create static imports for these
constants.) You can register one, two, or three of the event types. ENTRY_DELETE
means you want your program to be informed when a file or directory has been
deleted. Similarly, ENTRY_CREATE means a new file or directory has been created.
ENTRY_MODIFY means a file has been edited in the directory. These changes can be
made manually by a human or by another program on the computer.

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525

Renaming a file or directory is interesting, as it does not show up as ENTRY_MODIFY.
From Java's point of view, a rename is equivalent to creating a new file and deleting
the original. This means that two events will trigger for a rename—both ENTRY_
CREATE and ENTRY_DELETE. Actually editing a file will show up as ENTRY_MODIFY.
To loop through the events, we use while(true). It might seem a little odd to
write a loop that never ends. Normally, there is a break or return statement in the
loop so you stop looping once whatever event you were waiting for has occurred. It's
also possible you want the program to run until you kill or terminate it at the
command line.
Within the loop, you need to get a WatchKey. There are two ways to do this. The
most common is to call take(), which waits until an event is available. It throws an
InterruptedException if it gets interrupted without finding a key. This allows you
to end the program. The other way is to call poll(), which returns null if an event
is not available. You can provide optional timeout parameters to wait up to a specific
period of time for an event to show up.
watcher.take();
watcher.poll();
watcher.poll(10, TimeUnit.SECONDS);
watcher.poll(1, TimeUnit.MINUTES);

//
//
//
//

wait "forever" for an event
get event if present right NOW
wait up to 10 seconds for an event
wait up to 1 minute for an event

Next, you loop through any events on that key. In the case of rename, you'll get
one key with two events—the EVENT_CREATE and EVENT_DELETE. Remember that
you get all the events that happened since the last time you called poll()or
take(). This means you can get multiple seemingly unrelated events out of the
same key. They can be from different files but are for the same WatchService.
for (WatchEvent event : key.pollEvents()) {

Finally, you call key.reset(). This is very important. If you forget to call reset,
the program will work for the first event, but then you will not be notified of any
other events.
There are a few limitations you should be aware of with WatchService.
To begin with, it is slow. You could easily wait five seconds for the event to
register. It also isn't 100 percent reliable. You can add code to check if kind ==
OVERFLOW, but that just tells you something went wrong. You don't know what
events you lost. In practice, you are unlikely to use WatchService.

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WatchService only watches the files and directories immediately beneath it.
What if we want to watch to see if either p.txt or c.txt is modified?
/dir
| – parent
| - p.txt
| - child
| - c.txt

One way is to register both directories:
WatchService watcher =
FileSystems.getDefault().newWatchService();
Path dir = Paths.get("/dir/parent");
dir.register(watcher, ENTRY_MODIFY);
Path child = Paths.get("dir/parent/child");
child.register(watcher, ENTRY_MODIFY);

This works. You can type in all the directories you want to watch. If we had a lot
of child directories, this would quickly get to be too much work. Instead, we can
have Java do it for us:
Path myDir = Paths.get("/dir/parent");
final WatchService watcher =
// final so visitor can use it
FileSystems.getDefault().newWatchService();
Files.walkFileTree(myDir, new SimpleFileVisitor() {
public FileVisitResult preVisitDirectory(Path dir,
BasicFileAttributes attrs) throws IOException {
dir.register(watcher, ENTRY_MODIFY);
// watch each directory
return FileVisitResult.CONTINUE;
}
});

This code goes through the file tree recursively registering each directory with the
watcher. The NIO.2 classes are designed to work together. For example, we could
add PathMatcher to the previous example to only watch directories that have a
specific pattern in their path.

CERTIFICATION OBJECTIVE

Serialization (Objective 7.2)
7.2 Use streams to read from and write to files by using classes in the java.io package,
including BufferedReader, BufferedWriter, File, FileReader, FileWriter, DataInputStream,
DataOutputStream, ObjectOutputStream, ObjectInputStream, and PrintWriter.

Certification Summary

527

Over time, Oracle has fine-tuned the objectives of the OCP 7 exam. Serialization
was a topic on the old SCJP 5 and SCJP 6 exams, and recently (as of the summer of
2014), Oracle reintroduced serialization for the OCP 7 exam. Please see Appendix A
for in-depth, complete chapter coverage of serialization, right down to a self-test.

CERTIFICATION SUMMARY
File I/O Remember that objects of type File can represent either files or directories
but that until you call createNewFile() or mkdir() you haven't actually created
anything on your hard drive. Classes in the java.io package are designed to be
chained together. It will be rare that you'll use a FileReader or a FileWriter
without "wrapping" them with a BufferedReader or BufferedWriter object,
which gives you access to more powerful, higher-level methods. As of Java 5, the
PrintWriter class has been enhanced with advanced append(), format(), and
printf() methods, and when you couple that with new constructors that allow you
to create PrintWriters directly from a String name or a File object, you may use
BufferedWriters a lot less. The Console class allows you to read nonechoed input
(returned in a char[?]), and is instantiated using System.console().
NIO.2 Objects of type Path can be files or directories and are a replacement
of type File. Paths are created with Paths.get(). Utility methods in Files
allow you to create, delete, move, copy, or check information about a Path.
In addition, BasicFileAttributes, DosFileAttributes (Windows), and
PosixFileAttributes (UNIX/Linux/Mac) allow you to check more advanced
information about a Path. BasicFileAttributeView, DosFileAttributeView,
and PosixFileAttributeView allow you to update advanced Path attributes.
Using a DirectoryStream allows you to iterate through a directory. Extending
SimpleFileVisitor lets you walk a directory tree recursively looking at files and/or
directories. With a PathMatcher, you can search directories for files using regexesqu expressions called globs.
Finally, registering a WatchService provides notifications for new/changed/
removed files or directories.

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TWO-MINUTE DRILL
Here are some of the key points from the certification objectives in this chapter.

File I/O (OCP Objectives 7.1 and 7.2)
❑ The classes you need to understand in java.io are File, FileReader,
BufferedReader, FileWriter, BufferedWriter, PrintWriter, and
Console.
❑ A new File object doesn't mean there's a new file on your hard drive.
❑ File objects can represent either a file or a directory.
❑ The File class lets you manage (add, rename, and delete) files and
directories.
❑ The methods createNewFile() and mkdir() add entries to your file system.
❑ FileWriter and FileReader are low-level I/O classes. You can use them to
write and read files, but they should usually be wrapped.
❑ Classes in java.io are designed to be "chained" or "wrapped." (This is a
common use of the decorator design pattern.)
❑ It's very common to "wrap" a BufferedReader around a FileReader or a
BufferedWriter around a FileWriter to get access to higher-level (more
convenient) methods.
❑ PrintWriters can be used to wrap other Writers, but as of Java 5, they can
be built directly from Files or Strings.
❑ As of Java 5, PrintWriters have new append(), format(), and printf()
methods.
❑ Console objects can read nonechoed input and are instantiated using
System.console().

Path, Paths, and File (OCP Objectives 8.1 and 8.2)
❑ NIO.2 was introduced in Java 7.
❑ Path replaces File for a representation of a file or directory.
❑ Paths.get() lets you create a Path object.
❑ Static methods in Files let you work with Path objects.
❑ A Path object doesn't mean the file or directory exists on your hard drive.
❑ The methods Files.createFile() and Files.createDirectory() add
entries to your file system.

Two-Minute Drill

529

❑ The Files class provides methods to move, copy, and delete Path objects.
❑ Files.delete() throws an Exception if the file does not exist and
Files.deleteIfExists() returns false.
❑ On Path, normalize() simplifies the path representation.
❑ On Path, resolve() and relativize()work with the relationship between
two path objects.

File Attributes (OCP Objective 8.3)
❑ The Files class provides methods for common attributes such as whether the
file is executable and when it was last modified.
❑ For less common attributes the classes: BasicFileAttributes,
DosFileAttributes, and PosixFileAttributes read the attributes.
❑ DosFileAttributes works on Windows operating systems.
❑ PosixFileAttributes works on UNIX, Linux, and Mac operating systems.
❑ Attributes that can't be updated via the Files class are set using
the classes: BasicFileAttributeView, DosFileAttributeView,
PosixFileAttributeView, FileOwnerAttributeView, and
AclFileAttributeView.

Directory Trees, Matching, and Watching for Changes
(OCP Objectives 8.4, 8.5, and 8.6)
❑ DirectoryStream iterates through immediate children of a directory using
glob patterns.
❑ FileVisitor walks recursively through a directory tree.
❑ You can override one or all of the methods of SimpleFileVisitor—
preVisitDirectory, visitFile, visitFileFailed, and
postVisitDirectory.
❑ You can change the flow of a file visitor by returning one of the
FileVisitResult constants: CONTINUE, SKIP_SUBTREE, SKIP_
SIBLINGS, or TERMINATE.
❑ PathMatcher checks if a path matches a glob pattern.
❑ Know what the following expressions mean for globs: *, **, ?, and {a,b}.
❑ Directories register with WatchService to be notified about creation,
deletion, and modification of files or immediate subdirectories.
❑ PathMatcher and WatchService use FileSystem-specific implementations.

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SELF TEST
The following questions will help you measure your understanding of the material presented in this
chapter. Read all of the choices carefully, as there may be more than one correct answer. Choose all
correct answers for each question. Stay focused.
1. Note: The use of "drag-and-drop" questions has come and gone over the years. In case Oracle
brings them back into fashion, we threw a couple of them in the book.
Using the fewest fragments possible (and filling the fewest slots possible), complete the
following code so that the class builds a directory named "dir3" and creates a file named
"file3" inside "dir3." Note you can use each fragment either zero or one times.
Code:
import java.io.______________
class Maker {
public static void main(String[] args) {
___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

___________

} }

Fragments:
File;
try {
{ }
file
dir
} catch

FileDescriptor;
.createNewDir();
(Exception x)
.createNewFile();
(dir, "file3");
("dir3", "file3");

FileWriter;
File dir
("dir3");
= new File
(dir, file);
.mkdir();

Directory;
File
file
= new File
.createFile();
File file

Self Test

531

2. Given:
import java.io.*;
class Directories {
static String [] dirs = {"dir1", "dir2"};
public static void main(String [] args) {
for (String d : dirs) {
// insert code 1 here
File file = new File(path, args[0]);
// insert code 2 here
}
}
}

and that the invocation
java Directories file2.txt

is issued from a directory that has two subdirectories, "dir1" and "dir2," and that "dir1" has
a file "file1.txt" and "dir2" has a file "file2.txt," and the output is "false true," which
set(s) of code fragments must be inserted? (Choose all that apply.)
A. String path = d;
System.out.print(file.exists() + " ");

B. String path = d;
System.out.print(file.isFile() + " ");

C. String path = File.separator + d;
System.out.print(file.exists() + " ");

D. String path = File.separator + d;
System.out.print(file.isFile() + " ");

3. Given:
3. import java.io.*;
4. public class ReadingFor {
5. public static void main(String[] args) {
6.
String s;
7.
try {
8.
FileReader fr = new FileReader("myfile.txt");
9.
BufferedReader br = new BufferedReader(fr);
10.
while((s = br.readLine()) != null)

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11.
System.out.println(s);
12.
br.flush();
13.
} catch (IOException e) { System.out.println("io error"); }
16. }
17. }

And given that myfile.txt contains the following two lines of data:
ab
cd

What is the result?
A. ab
B. abcd
C. ab
cd

D. a
b
c
d

E. Compilation fails
4. Given:
3. import java.io.*;
4. public class Talker {
5. public static void main(String[] args) {
6.
Console c = System.console();
7.
String u = c.readLine("%s", "username: ");
8.
System.out.println("hello " + u);
9.
String pw;
10.
if(c != null && (pw = c.readPassword("%s", "password: ")) != null)
11.
// check for valid password
12. }
13. }

If line 6 creates a valid Console object and if the user enters fred as a username and 1234 as a
password, what is the result? (Choose all that apply.)
A. username:
password:

B. username: fred
password:

C. username: fred
password: 1234

D. Compilation fails
E. An Exception is thrown at runtime

Self Test

5. This question is about serialization, which Oracle reintroduced to the OCP 7 exam and
is covered in Appendix A.
Given:
3.
4.
5.
6.
7.
8.
9.
10.

import java.io.*;
class Vehicle { }
class Wheels { }
class Car extends Vehicle implements Serializable {
class Ford extends Car { }
class Dodge extends Car {
Wheels w = new Wheels();
}

}

Instances of which class(es) can be serialized? (Choose all that apply.)
A. Car
B. Ford
C. Dodge
D. Wheels
E. Vehicle
6. Which of the following creates a Path object pointing to c:/temp/exam?
(Choose all that apply.)
A. new Path("c:/temp/exam")
B. new Path("c:/temp", "exam")
C. Files.get("c:/temp/exam")
D. Files.get("c:/temp", "exam")
E. Paths.get("c:/temp/exam")
F. Paths.get("c:/temp", "exam")
7. Given a directory tree at the root of the C: drive and the fact that no other files exist:
dir x - |
..........| - dir y
..........| - file a

and these two paths:
Path one = Paths.get("c:/x");
Path two = Paths.get("c:/x/y/a");

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Which of the following statements prints out: y/a ?
A. System.out.println(one.relativize(two));
B. System.out.println(two.relativize(one));
C. System.out.println(one.resolve(two));
D. System.out.println(two.resolve(one));
E. System.out.println(two.resolve(two));
F. None of the above
8. Given the following statements:
I. A nonempty directory can usually be deleted using Files.delete
II. A nonempty directory can usually be moved using Files.move
III. A nonempty directory can usually be copied using Files.copy
Which of the following is true?
A. I only
B. II only
C. III only
D. I and II only
E. II and III only
F. I and III only
G. I, II, and III
9. Given:
new File("c:/temp/test.txt").delete();

How would you write this line of code using Java 7 APIs?
A. Files.delete(Paths.get("c:/temp/test.txt"));
B. Files.deleteIfExists(Paths.get("c:/temp/test.txt"));
C. Files.deleteOnExit(Paths.get("c:/temp/test.txt"));
D. Paths.get("c:/temp/test.txt").delete();
E. Paths.get("c:/temp/test.txt").deleteIfExists();
F. Paths.get("c:/temp/test.txt").deleteOnExit();
10. Given:
public void read(Path dir) throws IOException {
// CODE HERE
System.out.println(attr.creationTime());
}

Self Test

535

Which code inserted at // CODE HERE will compile and run without error on Windows?
(Choose all that apply.)
A. BasicFileAttributes attr = Files.readAttributes(dir, BasicFileAttributes.class);
B. BasicFileAttributes attr = Files.readAttributes(dir, DosFileAttributes.class);
C. DosFileAttributes attr = Files.readAttributes(dir, BasicFileAttributes.class);
D. DosFileAttributes attr = Files.readAttributes(dir, DosFileAttributes.class);
E. PosixFileAttributes attr = Files.readAttributes(dir, PosixFileAttributes.class);
F. BasicFileAttributes attr = new BasicFileAttributes(dir);
G. BasicFileAttributes attr =dir.getBasicFileAttributes();
11. Which of the following are true? (Choose all that apply.)
A. The class AbstractFileAttributes applies to all operating systems
B. The class BasicFileAttributes applies to all operating systems
C. The class DosFileAttributes applies to Windows-based operating systems
D. The class WindowsFileAttributes applies to Windows-based operating systems
E. The class PosixFileAttributes applies to all Linux/UNIX-based operating systems
F. The class UnixFileAttributes applies to all Linux/UNIX-based operating systems
12. Given a partial directory tree:
dir x - |
..........| - dir y
..........| - file a

In what order can the following methods be called if walking the directory tree from x?
(Choose all that apply.)
I: preVisitDirectory x
II: preVisitDirectory x/y
III: postVisitDirectory x/y
IV: postVisitDirectory x
V: visitFile x/a
A. I, II, III, IV, V
B. I, II, III, V, IV
C. I, V, II, III, IV
D. I, V, II, IV, III
E. V, I, II, III, IV
F. V, I, II, VI, III

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13. Given:
public class MyFileVisitor extends SimpleFileVisitor {
// more code here
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
throws IOException {
System.out.println("File " + file);
if ( file.getFileName().endsWith("Test.java")) {
// CODE HERE
}
return FileVisitResult.CONTINUE;
}
// more code here
}

Which code inserted at // CODE HERE would cause the FileVisitor to stop visiting files after
it sees the file Test.java?
A. return FileVisitResult.CONTINUE;
B. return FileVisitResult.END;
C. return FileVisitResult.SKIP_SIBLINGS;
D. return FileVisitResult.SKIP_SUBTREE;
E. return FileVisitResult.TERMINATE;
F. return null;
14. Assume all the files referenced by these paths exist:
Path a = Paths.get("c:/temp/dir/a.txt");
Path b = Paths.get("c:/temp/dir/subdir/b.txt");

What is the correct string to pass to PathMatcher to match both these files?
A. "glob:*/*.txt"
B. "glob:**.txt"
C. "glob:*.txt"
D. "glob:/*/*.txt"
E. "glob:/**.txt"
F. "glob:/*.txt"
G. None of the above

Self Test

537

15. Given a partial directory tree at the root of the drive:
dir x - |
..........| = file a.txt
..........| - dir y
....................| - file b.txt
....................| - dir y
..............................| - file c.txt

And the following snippet:
Path dir = Paths.get("c:/x");
try (DirectoryStream stream = Files.newDirectoryStream(dir, "**/*.txt")) {
for (Path path : stream) {
System.out.println(path);
} }

What is the result?
A. c:/x/a.txt
B. c:/x/a.txt
c:/x/y/b.txt
c:/x/y/z/c.txt

C. Code compiles but does not output anything
D. Does not compile because DirectoryStream comes from FileSystems, not Files
E. Does not compile for another reason
16. Given a partial directory tree:
dir x - |
..........| - dir y
..........| -file a

and given that a valid Path object, dir, points to x, and given this snippet:
WatchKey key = dir.register(watcher, ENTRY_CREATE);

If a WatchService is set using the given WatchKey, what would be the result if a file is added
to dir y?
A. No notice is given
B. A notice related to dir x is issued
C. A notice related to dir y is issued
D. Notices for both dir x and dir y are given
E. An Exception is thrown
F. The behavior depends on the underlying operating system

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SELF TEST ANSWERS
1. ☑

Answer:
import java.io.File;
class Maker {
public static void main(String[] args) {
try {
File dir = new File("dir3");
dir.mkdir();
File file = new File(dir, "file3");
file.createNewFile();
} catch (Exception x) { }
} }

Notes: The new File statements don't make actual files or directories, just objects. You need
the mkdir()and createNewFile()methods to actually create the directory and the file.
While drag-and-drop questions are no longer on the exam, it is still good to be able to complete
them. (OCP Objective 7.2)
2. ☑ A and B are correct. Because you are invoking the program from the directory whose direct
subdirectories are to be searched, you don't start your path with a File.separator character.
The exists()method tests for either files or directories; the isFile() method tests only for
files. Since we're looking for a file, both methods work.
☐
✗ C and D are incorrect based on the above. (OCP Objective 7.2)
3. ☑

E is correct. You need to call flush()only when you're writing data. Readers don't have
flush() methods. If not for the call to flush(), answer C would be correct.
☐
✗ A, B, C, and D are incorrect based on the above. (OCP Objective 7.2)

4. ☑ D is correct. The readPassword() method returns a char[]. If a char[] were used,
answer B would be correct.
☐
✗ A, B, C, and E are incorrect based on the above. (OCP Objective 7.1)
5. ☑ A and B are correct. Dodge instances cannot be serialized because they "have" an instance
of Wheels, which is not serializable. Vehicle instances cannot be serialized even though the
subclass Car can be.
☐
✗ C, D, and E are incorrect based on the above. (Pre-OCPJP 7 only)
6. ☑ E and F are correct since Paths must be created using the Paths.get() method. This
method takes a varargs String parameter, so you can pass as many path segments to it as you like.
☐
✗ A and B are incorrect because you cannot construct a Path directly. C and D are incorrect
because the Files class works with Path objects but does not create them from Strings.
(Objective 8.1)

Self Test Answers

539

7. ☑ A is correct because it prints the path to get to two from one.
☐
✗ B is incorrect because it prints out ../.. which is the path to navigate to one from two.
This is the reverse of what we want. C, D, and E are incorrect because it does not make sense to
call resolve with absolute paths. They might print out c:/x/c:/x/y/a, c:/x/y/a/c:/x, and
c:/x/y/a/c:/x/y/a, respectively. F is incorrect because of the above. Note that the directory
structure provided is redundant. Neither relativize() nor resolve() requires either path to
actually exist. (OCP Objective 8.1)
8. ☑ E is correct because a directory containing files or subdirectories is copied or moved in
its entirety. Directories can only be deleted if they are empty. Trying to delete a nonempty
directory will throw a DirectoryNotEmptyException. The question says "usually" because
copy and move success depends on file permissions. Think about the most common cases when
encountering words such as "usually" on the exam.
☐
✗ A, B, C, D, F, and G are incorrect because of the above. (OCP Objective 8.2)
9. ☑ B is correct because, like the Java 7 code, it returns false if the file does not exist.
☐
✗ A is incorrect because this code throws an Exception if the file does not exist.
C, D, E, and F are incorrect because they do not compile. There is no deleteOnExit()
method, and file operations such as delete occur using the Files class rather than the path
object directly. (OCP Objective 8.2)
10. ☑

A, B, and D are correct. Creation time is a basic attribute, which means you can read
BasicFileAttributes or any of its subclasses to read it. DosFileAttributes is one such
subclass.
☐
✗ C is incorrect because you cannot cast a more general type to a more specific type. E is
incorrect because this example specifies it is being run on Windows. While it would work on
UNIX, it throws an UnsupportedOperationException on Windows due to requesting the
WindowsFileSystemProvider to get a POSIX class. F and G are incorrect because those
methods do not exist. You must use the Files class to get the attributes. (OCP Objective 8.3)

11. ☑

B, C, and E are correct. BasicFileAttributes is the general superclass.
DosFileAttributes subclasses BasicFileAttributes for Windows operating systems.
PosixFileAttributes subclasses BasicFileAttributes for UNIX/Linux/Mac operating
systems.
☐
✗ A, D, and F are incorrect because no such classes exist. (Objective 8.3)

12. ☑ B and C are correct because file visitor does a depth-first search. When files and directories
are at the same level of the file tree, they can be visited in either order. Therefore, "y" and "a"
could be reversed. All of the subdirectories and files are visited before postVisit is called on
the directory.
☐
✗ A, D, and E are incorrect because of the above. (Objective 8.4)

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13. ☑ E is correct because it is the correct constant to end the FileVisitor.
☐
✗ B is incorrect because END is not defined as a result constant. A, C, and D are incorrect.
While they are valid constants, they do not end file visiting. CONTINUE proceeds as if nothing
special has happened. SKIP_SUBTREE skips the subdirectory, which doesn't even make sense
for a Java file. SKIP_SIBLINGS would skip any files in the same directory. Since we weren't
told what the file structure is, we can't assume there aren't other directories or subdirectories.
Therefore, we have to choose the most general answer of TERMINATE. F is incorrect because file
visitor throws a NullPointerException if null is returned as the result. (OCP Objective 8.4)
14. ☑ B is correct. ** matches zero or more characters, including multiple directories.
☐
✗ A is incorrect because */ only matches one directory. It will match "temp" but not "c:/temp,"
let alone "c:/temp/dir." C is incorrect because *.txt only matches filenames and not
directory paths. D, E, and F are incorrect because the paths we want to match do not begin
with a slash. G is incorrect because of the above. (Objective 8.5)
15. ☑ C is correct because DirectoryStream only looks at files in the immediate directory.
**/*.txt means zero or more directories followed by a slash, followed by zero or more
characters followed by .txt. Since the slash is in there, it is required to match, which makes
it mean one or more directories. However, this is impossible because DirectoryStream only
looks at one directory. If the expression were simply *.txt, answer A would be correct.
☐
✗ A, B, D, and E are incorrect because of the above. (OCP Objective 8.5)
16. ☑ A is correct because watch service only looks at a single directory. If you want to look at
subdirectories, you need to set recursive watch keys. This is usually done using a FileVisitor.
☐
✗ B, C, D, E, and F are incorrect because of the above. (OCP Objective 8.6)

10
Advanced OO and
Design Patterns

CERTIFICATION OBJECTIVES

•

Write Code that Declares, Implements,
and/or Extends Interfaces

•

Design a Class Using the Singleton Design
Pattern

•

Choose Between Interface Inheritance and
Class Inheritance

•

Write Code to Implement the DAO
Pattern

•

Apply Cohesion, Low-Coupling, IS-A, and
HAS-A Principles

•

Design and Create Objects Using a
Factory and Use Factories from the API

•

Apply Object Composition Principles
(Including HAS-A Relationships)

✓

Two-Minute Drill

Q&A Self Test

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Y

ou were introduced to object-oriented (OO) principles in Chapter 2. We will be looking
at some more advanced principles here, including coupling and cohesion. You'll also learn
what a design pattern is and dip your toe into the world of patterns by exploring three
of them. As a bit of a teaser, a design pattern is a reusable solution to problems. Which will come
in handy so you aren't reinventing new ways to solve common problems.

CERTIFICATION OBJECTIVE

IS-A and HAS-A (OCP Objectives 3.3 and 3.4)
3.3

Apply cohesion, low-coupling, IS-A, and HAS-A principles.

3.4

Apply object composition principles (including HAS-A relationships).

You learned the difference between IS-A and HAS-A in Chapter 2. As a brief
review, how many IS-A/HAS-A statements can you write about BeachUmbrella?
class BeachUmbrella extends Umbrella implements SunProtector {
Stand stand;
}
class Umbrella{}
interface SunProtector {};
class Stand{}

We can make four statements about BeachUmbrella:
■ BeachUmbrella IS-A Umbrella
■ BeachUmbrella IS-A SunProtector
■ BeachUmbrella HAS-A Stand
■ And, of course, as always, BeachUmbrella IS-A Object

In a nutshell, IS-A happens when a class uses inheritance—e.g., when a class extends
another class or implements an interface. HAS-A happens when a class has instance
variables of a class.

IS-A and HAS-A (OCP Objectives 3.3 and 3.4)

543

Coupling and Cohesion
We're going to admit it up front: The Oracle exam's definitions for cohesion and
coupling are somewhat subjective, so what we discuss in this chapter is from the
perspective of the exam and is by no means The One True Word on these two OO
design principles. It may not be exactly the way that you've learned it, but it's what
you need to understand to answer the questions. You'll have very few questions
about coupling and cohesion on the real exam.
These two topics, coupling and cohesion, have to do with the quality of an OO
design. In general, good OO design calls for loose coupling and shuns tight coupling,
and good OO design calls for high cohesion and shuns low cohesion. As with most
OO design discussions, the goals for an application are
■ Ease of creation
■ Ease of maintenance
■ Ease of enhancement

Coupling
Let's start by attempting to define coupling. Coupling is the degree to which one
class knows about another class. If the only knowledge that class A has about class B
is what class B has exposed through its interface, then class A and class B are said to
be loosely coupled… that's a good thing. If, on the other hand, class A relies on
parts of class B that are not part of class B's interface, then the coupling between the
classes is tighter… not a good thing. In other words, if A knows more than it should
about the way in which B was implemented, then A and B are tightly coupled.
Using this second scenario, imagine what happens when class B is enhanced. It's
quite possible that the developer enhancing class B has no knowledge of class
A—why would she? Class B's developer ought to feel that any enhancements that
don't break the class's interface should be safe, so she might change some
noninterface part of the class, which then causes class A to break.
At the far end of the coupling spectrum is the horrible situation in which class A
knows non-API stuff about class B, and class B knows non-API stuff about class
A—this is REALLY BAD. If either class is ever changed, there's a chance that the
other class will break. Let's look at an obvious example of tight coupling that has
been enabled by poor encapsulation.

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class DoTaxes {
float rate;
float doColorado() {
SalesTaxRates str = new SalesTaxRates();
rate = str.salesRate;
// ouch this should be a method call like:
// rate = str.getSalesRate("CO");
// do stuff with rate
}
}
class SalesTaxRates {
public float salesRate;
public float adjustedSalesRate;
public float getSalesRate(String region) {
salesRate = new DoTaxes().doColorado();
// do region-based calculations
return adjustedSalesRate;
}

// should be private
// should be private

// ouch again!

}

All nontrivial OO applications are a mix of many classes and interfaces working
together. Ideally, all interactions between objects in an OO system should use the
APIs—in other words, the contracts of the objects' respective classes. Theoretically, if
all of the classes in an application have well-designed APIs, then it should be possible for
all interclass interactions to use those APIs exclusively. As we discussed in Chapter 2,
an aspect of good class and API design is that classes should be well encapsulated.
The bottom line is that coupling is a somewhat subjective concept. Because of
this, the exam will test you on really obvious examples of tight coupling; you won't
be asked to make subtle judgment calls.

Cohesion
While coupling has to do with how classes interact with each other, cohesion is all
about how a single class is designed. The term cohesion is used to indicate the degree
to which a class has a single, well-focused purpose. Keep in mind that cohesion is a
subjective concept. The more focused a class is, the higher its cohesiveness—a good
thing. The key benefit of high cohesion is that such classes are typically much easier
to maintain (and less frequently changed) than classes with low cohesion. Another
benefit of high cohesion is that classes with a well-focused purpose tend to be more
reusable than other classes. Let's look at a pseudo-code example:
class BudgetReport {
void connectToRDBMS(){ }
void generateBudgetReport() { }
void saveToFile() { }
void print() { }
}

Object Composition Principles (OCP Objective 3.4)

545

Now imagine your manager comes along and says, "Hey, you know that accounting
application we're working on? The clients just decided that they're also going to want
to generate a revenue projection report, oh and they want to do some inventory
reporting also. They do like our reporting features, however, so make sure that all of
these reports will let them choose a database, choose a printer, and save generated
reports to data files…." Ouch!
Rather than putting all the printing code into one report class, we probably would
have been better off with the following design right from the start:
class BudgetReport {
Options getReportingOptions() { }
void generateBudgetReport(Options o) { }
}
class RDBMSmanager {
DBconnection getRDBMS() { }
}
class PrintStuff {
PrintOptions getPrintOptions() { }
}
class FileSaver {
SaveOptions getFileSaveOptions() { }
}

This design is much more cohesive. Instead of one class that does everything,
we've broken the system into four main classes, each with a very specific, or cohesive,
role. Because we've built these specialized, reusable classes, it'll be much easier to
write a new report since we already have the database connection class, the printing
class, and the file saver class, and that means they can be reused by other classes that
might want to print a report.

CERTIFICATION OBJECTIVE

Object Composition Principles
(OCP Objective 3.4)
3.4

Apply object composition principles.

Object composition principles build on IS-A and HAS-A. If you aren't 100 percent
comfortable with the differences between IS-A and HAS-A, go back and reread
Chapter 2 before continuing on.

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Object composition refers to one object having another as an instance variable
(HAS-A). Sometimes, that instance variable might be the same type as the object
we are writing. Think about when you get that package from Amazon that is a box
containing some bubble wrap, a receipt, and yet another box. That is composition at
work. The outer (containing class) box contains an inner (instance) box.
Let's build out this box example. We want to reuse as much code as possible.
After all, the procedure for sealing a box with some tape doesn't change from box to
box. Let's start with the concept of a Box:
public interface Box {
void pack();
void seal();
}

Wait. Boxes are simple. Why do we need an interface? We realize there are many
types of boxes. There are gift boxes, jewelry boxes, small boxes, large boxes, etc.
Now we create a concrete type of Box:
public class GiftBox implements Box {
public void pack() {
System.out.println("pack box");
}
public void seal() {
System.out.println("seal box");
}
}

// from
// interface
// from
// interface

GiftBox implements Box by implementing the two methods Box requires. Providing
an interface lets us keep the Box logic where it belongs—in the relevant subclasses.
And to review, GiftBox IS-A Box.
Now that we've figured out Box, it's time to build a MailerBox:
public class MailerBox implements Box {
public void pack() {
System.out.println("pack box");
}
public void seal() {
System.out.println("seal box");
}
public void addPostage() {
System.out.println("affix stamps");
}
public void ship() {
System.out.println("put in mailbox");
}
}

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See any problems? That's right, we've duplicated the logic to pack and seal the
Box. All two lines of it. Our real Box logic would be a lot longer, though. And when
we start manufacturing different types of boxes, we'd have that Box logic all over the

place.
One thought is to solve this by having MailerBox extend GiftBox. It doesn't
take long to see the problem here. We would need MailerGiftBox, MailerSmallBox,
MailerMediumBox, etc. That's a lot of classes! And this technique would repeat
for other types of functionality we create. Which means we would also need
WrappedGiftBox, MailerWrappedGiftBox. Uh oh. We can only extend one class
in Java. We can't inherit both Mailer and GiftBox functionality. Clearly, IS-A isn't
going to work for us here.
Instead, we can use HAS-A. First, we create the interface for our desired
functionality:
public interface Mailer {
void addPostage();
void ship();
}

Then we can create the object that is both a Box and Mailer:
public class MailerBox implements Box, Mailer {
private Box box;
public MailerBox(Box box) {
this.box = box;
}
public void pack() {
box.pack();
}
public void seal() {
box.seal();
}
public void addPostage() {
System.out.println("affix stamps");
}
public void ship() {
System.out.println("put in mailbox");
}
}

// pass in a Box

// from Box
// delegate to box
// from Box
// delegate to box
// from Mailer

// from Mailer

The first thing to notice is that the logic to pack and seal a box is only in one
place—in the Box hierarchy where it belongs. In fact, the MailerBox doesn't even
know what kind of Box it has. This allows us to be very flexible.

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Next, notice the implementation of pack() and seal(). That's right—each is one
line. We delegate to Box to actually do the work. This is called method forwarding
or method delegation. These two terms mean the same thing.
Finally, notice that MailerBox is both a Box and a Mailer. This allows us to pass
it to any method that needs a Box or a Mailer.

Polymorphism
Looking at these classes graphically, we have the following:
Box

Mailer

pack()
seal()

addPostage()
ship()

implements
implements

implements
GiftBox

MailerBox

pack()
seal()

pack()
seal()
addPostage()
ship()

Think about which of the objects can be passed to this method:
public void load(Box b) {
b.pack();
}

GiftBox can because it implements Box. So can MailerBox for the same reason.
MailerBox knows how to pack—by delegating to the Box instance. This is why it is

important for the composing class to both contain and implement the same
interface. Repeating the relevant parts here, we have:
public class MailerBox implements Box, Mailer {
private Box box;

You can see the composition part. MailerBox both IS-A Box and HAS-A Box.
MailerBox is composed of a Box and delegates to Box for logic. That's the
terminology for object composition.

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Benefits of Composition
Benefits of composition include
■ Reuse

An object can delegate to another object rather than repeating the
same code.

■ Preventing a proliferation of subclasses

We can have one class
per functionality rather than needing one for every combination of
functionalities.

CERTIFICATION OBJECTIVE

Singleton Design Pattern (OCP Objective 3.5)
3.5

Design a class using the singleton design pattern.

In a nutshell, the singleton design pattern ensures we only have one instance of a
class of an object within the application. It's called a creational design pattern
because it deals with creating objects. But wait, what's this "design pattern"?

What Is a Design Pattern?
Wikipedia currently defines a design pattern as "a general reusable solution to a
commonly occurring problem within a given context." What does that mean? As
programmers, we frequently need to solve the same problem repeatedly. Such as how
to only have one of a class of an object in the application. Rather than have
everyone come up with their own solution, we use a "best practice" type solution
that has been documented and proven to work. The word "general" is important.
We can't just copy and paste a design pattern into our code. It's just an idea. We can
write an implementation for it and put that in our code.
Using a design pattern has a few advantages. We get to use a solution that is known
to work. The tradeoffs, if any, are well documented so we don't stumble over problems
that have already been solved. Design patterns also serve as a communication aid.
Your boss can say, "We will use a singleton," and that one word is enough to tell you
what is expected.

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When books or web pages document patterns, they do so using consistent
sections. In this book, we have sections for the "Problem," "Solution," and
"Benefits." The "Problem" section explains why we need the pattern—what problem
we are trying to solve. The "Solution" section explains how to implement the
pattern. The "Benefits" section reviews why we need the pattern and how it has
helped us solve the problem. Some of the benefits are hinted at in the "Problem"
section. Others are additional benefits that come from the pattern.
While the exam only covers three patterns, this is just to get your feet wet.
Whole books are written on the topic of design patterns. Head First Design
Patterns (O'Reilly Media, 2004) covers more patterns. And the most famous
book on patterns, Design Patterns: Elements of Reusable Object-Oriented Software
(Addison-Wesley Professional, 1994)—also known as "Gang of Four"—covers
23 design patterns. You may notice that these books are over 10 years old.
That's because the classic patterns haven't changed.
While each book does use a consistent set of sections, there isn't one
common set of names. You will see synonyms used such as "Problem" versus
"Motivation." You will also see additional sections such as "Consequences."
The exam picks simpler patterns so you can use the simpler sections.
When talking about patterns, they are usually presented in a problem/solution
format. Then, depending on the level of detail, other sections are added. In this
book, each pattern will cover the problem, solution, and benefits.

Problem
Let's suppose we are going to put on a show. We have one performance of this show
and we only have a few seats in the theater.
import java.util.*;
public class Show {
private Set availableSeats;
public Show() {
availableSeats = new HashSet();
availableSeats.add("1A");
availableSeats.add("1B");
}
public boolean bookSeat(String seat) {
return availableSeats.remove(seat);
}

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public static void main(String[] args) {
ticketAgentBooks("1A");
ticketAgentBooks("1A");
}
private static void ticketAgentBooks(String seat) {
Show show = new Show();
// a new Show gets created
// each time we call the method
System.out.println(show.bookSeat(seat));
}
}

This code prints out true twice. That's a problem. We just put two people in the
same seat. Why? We created a new Show object every time we needed it. Even
though we want to use the same theater and seats, Show deals with a new set of seats
each time. Which causes us to double-book seats.

Solution
There are a few ways to implement the singleton pattern. The simplest is
import java.util.*;
public class Show {
private static final Show INSTANCE // store one instance
= new Show();
// (this is the singleton)
private Set availableSeats;
public static Show getInstance() { // callers can get to
return INSTANCE;
// the instance
}
private Show() {
// callers can't create
// directly anymore.
// Must use getInstance()
availableSeats = new HashSet();
availableSeats.add("1A");
availableSeats.add("1B");
}
public boolean bookSeat(String seat) {
return availableSeats.remove(seat);
}
public static void main(String[] args) {
ticketAgentBooks("1A");
ticketAgentBooks("1A");
}
private static void ticketAgentBooks(String seat) {
Show show = Show.getInstance();
System.out.println(show.bookSeat(seat));
}
}

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Now the code prints true and false. Much better! We are no longer going to have
two people in the same seat. The bolded bits in the code call attention to the
implementation of the singleton pattern.
The key parts of the singleton pattern are
■ A private static variable to store the single instance called the singleton. This

variable is usually final to keep developers from accidentally changing it.
■ A public static method for callers to get a reference to the instance.
■ A private constructor so no callers can instantiate the object directly.

Remember, the code doesn't create a new Show each time, but merely returns the
singleton instance of Show each time getInstance() is called.
To understand this a little better, consider what happens if we change parts of the
code.
If the constructor weren't private, we wouldn't have a singleton. Callers would be
free to ignore getInstance() and instantiate their own instances. Which would
leave us with multiple instances in the program and defeat the purpose entirely.
If getInstance() weren't public, we would still have a singleton. However, it
wouldn't be as useful because only static methods of the class Show would be able to
use the singleton.
If getInstance() weren't static, we'd have a bigger problem. Callers couldn't
instantiate the class directly, which means they wouldn't be able to call
getInstance() at all.
If INSTANCE weren't static and final, we could have multiple instances at
different points in time. These keywords signal that we assign the field once and it
stays that way for the life of the program.
When talking about design patterns, it is common to also communicate the
pattern in diagram form. The singleton pattern diagram looks like this:
Show
private static Show INSTANCE
private Show()
public static Show getInstance()

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A format called UML (Unified Modeling Language) is used.The diagrams
in this book use some aspects of UML, such as a box with three sections
representing each class. Actual UML uses more notation, such as showing
public versus private visibility. You can think of this as faux-UML.
As long as the method in the diagram keeps the same signature, we can change
our logic to other implementations of the singleton pattern. One "feature" of the
above implementation is that it creates the Show object before we need it. This is
called eager initialization, which is good if the object isn't expensive to create or we
know it will be needed for every run of the program. Sometimes, however, we want
to create the object only on the first use. This is called lazy initialization.
private static Show INSTANCE;
private Set availableSeats;
public static Show getInstance() {
if (INSTANCE == null) {
INSTANCE = new Show();
}
return INSTANCE;
}

In this case, INSTANCE isn't set to be a Show until the first time getInstance() is
called. Walking through what happens, the first time getInstance() is called, Java
sees INSTANCE is still null and creates the singleton. The second time getInstance()
is called, Java sees INSTANCE has already been set and simply returns it. In this
example, INSTANCE isn't final because that would prevent the code from compiling.
The singleton code here assumes you are only running one thread at a time.
It is NOT thread-safe.Think about if this were a web site and two users
managed to be booking a seat at the exact same time. If getInstance() were
running at the exact same time, it would be possible for both of them to see
that INSTANCE was null and create a new Show at the same time.There are
a few ways to solve this. One is to add synchronized to the getInstance()
method.This works, but comes with a small performance hit. We're getting way
beyond the scope of the exam, but you can Google "double checked locked
pattern" for more information.

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You might have noticed that the code for getInstance()can get a bit complicated.
In Java 5, there became a much shorter way of creating a singleton:
public enum ShowEnum {
INSTANCE;

// this is an enum
// instead of a class

private Set availableSeats;
private ShowEnum() {
availableSeats = new HashSet();
availableSeats.add("1A");
availableSeats.add("1B");
}
public boolean bookSeat(String seat) {
return availableSeats.remove(seat);
}
public static void main(String[] args) {
ticketAgentBooks("1A");
ticketAgentBooks("1A");
}
private static void ticketAgentBooks(String seat) {
ShowEnum show = ShowEnum.INSTANCE;
// we don't even
// need a method to
// get the instance
System.out.println(show.bookSeat(seat));
}
}

Short and sweet. By definition, there is only one instance of an enum constant.
You are probably wondering why we've had this whole discussion of the singleton
pattern when it can be written so easily. The main reason is that enums were
introduced with Java 5 and there is a ton of older code out there that you need to be
able to understand. Another reason is that sometimes the older versions of the
pattern are still needed.

Benefits
Benefits of the singleton pattern include the following:
■ The primary benefit is that there is only one instance of the object in

the program. When an object's instance variables are keeping track of
information that is used across the program, this becomes useful. For example,
consider a web site visitor counter. You only want one count that is shared.
■ Another benefit is performance. Some objects are expensive to create. For

example, maybe we need to make a database call to look up the state for the
object.

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CERTIFICATION OBJECTIVE

DAO Design Pattern (OCP Objective 3.6)
3.6

Write code to implement the DAO pattern.

DAO stands for "Data Access Object." A DAO is only responsible for storing
data. Nothing else. Why can't we do this in the object with everything else, you ask?
Suppose we have three objects in our program as shown in Table 10-1.
Already there is a problem. These classes aren't cohesive. Remember cohesion? We
want each class to have a single purpose. Storing and searching objects in the database is
NOT that purpose. Having that database code all over makes it hard to focus on the
classes' core purpose for existing, which is clearly for our entertainment. Since dealing
with a database is very common, separating out that responsibility is a pattern—the DAO.

Problem
Let's drill down into just the Book class. This is the poorly written, noncohesive
version. Pay particular attention to the two responsibilities.
import java.util.*;
public class Book {
private static Map bookstore // storage: extra
= new HashMap();
// responsibility
private String isbn;
private String title;
private String author;

// core responsibility:
// book instance
// variables

public Collection findAllBooks() {
// more storage
return bookstore.values();
// extra responsibility
}
public Book findBookByIsbn(String isbn) { // more storage
return bookstore.get(isbn);
}
public void create() {
bookstore.put(isbn, this);
}
public void delete() {
// still more storage
bookstore.remove(isbn);
}
public void update() {
// yet still more storage
// no operation - for an in-memory database,
// we update automatically in real time
}
// omitted getters and setters
}

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TABLE 10-1

Object
Responsibilities

Advanced OO and Design Patterns

Object

Responsibilities

Still More Responsibilities

Book

Store book information, be read

Store and search in database

CD

Store CD information, be listened to

Store and search in database

DVD

Store DVD information, be watched

Store and search in database

Counting the getters and setters we didn't want to bore you with, the Book class
is over 50 lines. And it hardly does anything! A real Book class would have a lot
more fields. A bookstore needs to tell you when the book was written, the edition,
the price, and all sorts of other information. A bookstore also needs to be able to
keep track of books somewhere other than a map. After all, we don't want our
bookstore to forget everything when we reboot.
The problem is that our class is responsible for two things. The first is keeping
track of being a book. This seems like a good responsibility for a class to have that is
named Book. The other is keeping track of storage responsibilities.
A datastore is the name of—wait for it—where data is stored. In the real world,
we'd use a database or possibly a file containing the books. For testing, we might use
an in-memory database. The map in Book is actually a bare-bones in-memory
datastore. As you'll see in Chapter 15, using a real database would make the Book
class MUCH longer.
This is a problem. We want our code to be easy to read and focused on one
responsibility.

Solution
The DAO pattern has us split up these two responsibilities. We start by letting our
Book class focus on being a book:
public class Book {
private String isbn;
private String title;
private String author;

// core responsibility:
// book instance
// variables

// omitted getters and setters
}

There can be other methods in Book such as toString(), hashCode(), and
equals(). These methods have to do with the Book object. Methods that have to
do with a bookstore or database are now gone. Much better. Now we can go on to
the data access code:

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557

import java.util.*;
public class InMemoryBookDao {
private static Map bookstore
= new HashMap();

// storage:
// core responsibility

public Collection findAllBooks() {
return bookstore.values();
}
public Book findBookByIsbn(Book book) {
return bookstore.get(book.getIsbn());
}
public void create(Book book) {
bookstore.put(book.getIsbn(), book);
}
public void delete(Book book) {
bookstore.remove(book.getIsbn());
}
public void update(Book book) {
// no operation - for an in-memory
// database,
// we update automatically in real time
} }

The new InMemoryBookDao class only knows how to do one thing—deal with
the datastore. This is such a common technique that it has a name: the single
responsibility principle. The method names in the DAO are actually standard. You'll
see them again when you get to Chapter 15.
When everything was in the Book object, we just created a Book and started
calling methods. Now that Book and DAO are separate objects, the caller deals with
two objects:
public class Student {
public static void main(String[] args) {
BookDao dao = new BookDao();
// dao
Book book = new Book();
// call setters
dao.create(book);
// dao - storage
book.setTitle("Updated");
dao.update(book);
// dao - storage
dao.delete(book);
// dao - storage
} }

The new DAO object gets all the calls that have to do with the datastore. Table 10-2
shows why each method call is associated with each class.

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Book

TABLE 10-2

Deals with datastore

dao.create(book)

DAO Method
Call Associations

book.setTitle("updated")

DAO

Changes a Book instance variable

dao.update(book)

Deals with datastore

dao.delete(book)

Deals with datastore

Good so far? The DAO pattern only has one more part. Our datastore is pretty
wimpy right now. Every time we restart the program, it forgets what books we have.
At some point, we are going to want to change that. But when we do, we want to
make it easier for callers to change.
It's time to add an interface!
import java.util.*;
public interface BookDao {
Collection findAllBooks();
Book findBookByIsbn(Book book);
void create(Book book);
void delete(Book book);
void update(Book book);
}

Since all the method names in the interface match our existing DAO, all we have to
do is have it implement the new interface:
public class InMemoryBookDao implements BookDao {

And we can use the interface type when declaring the DAO:
BookDao dao = new InMemoryBookDao();

Wait a minute. We still have InMemoryBookDao in the line of code that instantiates
the DAO. It is a bit like writing Collection c = new ArrayList();. It just so
happens to be an ArrayList right now, but we could change it at any time. It is a
bit like signifying that the surrounding code shouldn't get too cozy with any particular
implementation. We can always change the specific DAO implementation later
without changing the interface. And we will learn in the next section how to get rid
of even the one reference to InMemoryBookDao.
To review the classes involved in the DAO pattern, we have the following
illustration:

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559

BookDao
findAllBooks()
findByIsbn()
create()
delete()
update()

uses

implements
InMemoryBookDao

Book

findAllBooks()
findByIsbn()
create()
delete()
update()

Assorted getters
and setters
uses

Now we have three objects, each responsible for one thing. We have the public
interface BookDao, which specifies the contract. Next, we have the implementation
of that interface, InMemoryBookDao. Finally, we have the Book class itself, which
focuses on the object state and any methods related to Book.
In addition to making the code easier to read, this pattern makes it easy
for us to organize code. We could put all the JavaBeans in one package,
the interfaces in another package, and the implementations in still another
package.This approach allows us to have one package for in-memory
implementations and another for JDBC implementations.

Benefits
To review, the benefits of the DAO pattern are as follows:
■ The main object (Book in this case) is cohesive and doesn't have database

code cluttering it up.
■ All the database code is in one part of the program, making it easy to find.
■ We can change the database implementation without changing the business

object.
■ Reuse is easier. As the database code grows, we can create helper classes and

even helper superclasses.

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CERTIFICATION OBJECTIVE

Factory Design Pattern (OCP Objective 3.7)
3.7

Design and create objects using a factory and use factories from the API.

Like the singleton design pattern, the factory design pattern is a creational design
pattern. Unlike the singleton, it doesn't limit you to only having one copy of an
object. The factory design pattern creates new objects of whatever implementation
it chooses.

Problem
So far, we only have one implementation of our BookDAO called InMemoryBookDao.
It isn't very robust since it only stores objects in memory. We will need to create a
version of it that uses JDBC or writes to a file or does something else where we can
remember state. We want to be able to change the DAO implementation without
having to change the caller code (Student). Remember coupling? This is loose
coupling. Interfaces are part of loose coupling, but we want to go a step further.

Solution
The simplest factory we can write while still implementing the pattern is an abstract
class and implementation with one method:
public abstract class Factory {
public abstract BookDao createDao();
}
public class FactoryImpl extends Factory
public BookDao createDao() {
return new InMemoryBookDao();
}
}
public class Student {
public static void main(String[] args)
Factory factory = new FactoryImpl();
BookDao dao = factory.createDao();
// work with dao
}
}

{
// right now, we only
// have one DAO

{
// create the DAO

This is very simple. The Factory is an abstract class with one method. Its
implementation simply returns an in-memory DAO. From Student's point of view,
this is all that exists—the Factory class and the BookDao interface. Note that
Student no longer has the code new InMemoryBookDao.

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561

In diagram form, here is how our classes fit together:
Student

uses

uses
Factory

BookDao

createDao()
uses

findAllBooks()
findByIsbn()
create()
delete()
update()
implements

implements
FactoryImpl

InMemoryBookDao

createDao()
uses

findAllBooks()
findByIsbn()
create()
delete()
update()

To review, Student only interacts with the two abstract classes Factory and
BookDao. All implementation is in the concrete subclasses.
This setup frees us up to change the implementation of FactoryImpl without
affecting the caller.
Let's try an example to show how we can change the factory. Suppose we write a
DAO implementation OracleBookDao that uses a real database. We might change
FactoryImpl to:
public class FactoryImpl extends Factory {
public BookDao createDao() {
if (Util.isTestMode()) {
return new InMemoryBookDao();
} else {
return new OracleBookDao();
}
}
}

// factory subclass

// for test
// for real

Just like that—nothing changes in Student. Yet it starts using the real database
implementation. This is good design. A change only needs to be made in one place.

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You might be wondering why Factory is an abstract class rather than an
interface. It is common with the factory method pattern to work "around" the
creation logic, or at least recognize that it might happen later.
As an example here, we could decide that we want to include the test logic check
in the superclass so any future subclasses use it:
public abstract class Factory {
public BookDao createDao() {
if (Util.isTestMode()) {
return new InMemoryBookDao();
} else {
return createDatabaseBookDao();
// for subclass
}
// to implement
}
protected abstract BookDao createDatabaseBookDao();
}
public class FactoryImpl extends Factory {
protected BookDao createDatabaseBookDao() {
return new OracleBookDao();
}
}

// fills in the
// missing part

In this case, the superclass Factory has all the common logic, and the subclass
FactoryImpl merely creates the relevant object. Notice how the API createDao()
hasn't changed its signature at all despite our extensive changes to the method
implementation. That is why we are using the factory pattern. So the caller Student
isn't affected by any changes to our factory and DAO.
There are three patterns with factory in their name:
■ Factory method This is the pattern we are talking about in this chapter
and is on the exam.
■ Abstract factory This takes the factory method pattern a bit further and
is used to create families of related classes.
■ Factory It's debatable whether this is even a pattern. It's not in the
"Gang of Four" book. However, on the job, when developers say "factory,"
they are often referring to a method like
public Foo createFoo() {return new Foo(); }

rather than a full-fledged factory method pattern.The method may
return Foo or SubclassOfFoo, but it doesn't have the superclass/subclass
relationship for the creator object that the factory method pattern has.

Certification Summary

563

You might have noticed we didn't say anything about making the DAO constructors
private. In the singleton pattern, we needed to force callers to use getInstance()to
prevent multiple copies. The factory pattern is merely a convenience. At times, it is
a pretty big convenience. However, callers can still instantiate the DAO directly
without breaking our logic, so we let them.
In fact, Oracle uses the factory pattern in the Java API in many places. When
we learned how to create a DateFormat, we used DateFormat.getInstance(),
DateFormat.getDateInstance(), and other similar factory methods. If you
wanted more control over the format string, you could still write new
SimpleDateFormat("yyyy MM"). Oracle leaves the constructor available for
when you need it.
Similarly, when we learned how to create a Calendar, we wrote Calendar.
getInstance() or Calendar.getInstance(Locale). You will see many more
examples of the factory pattern as you explore the Java API.

Benefits
Benefits of the factory design pattern include the following
■ The caller doesn't change when the factory returns different subclasses. This

is useful when the final implementation isn't ready yet. For example, maybe
the database isn't yet available. It's also useful when we want to use different
implementations for unit testing and production code. For example, you want
to write code that behaves the same way, regardless of what happens to be in
the database.
■ Centralizes creation logic outside the calling class. This prevents duplication

and makes the code more cohesive.
■ Allows for extra logic in the object creation process. For example, an object is

time-consuming to create, and you want to reuse the same one each time.

CERTIFICATION SUMMARY
We started the chapter by reviewing the difference between IS-A and HAS-A. To
review the review, IS-A is implemented using inheritance, and HAS-A is implemented
using instance variables that refer to other objects.
We discussed the OO concepts of coupling and cohesion. Loose coupling is the
desirable state of two or more classes that interact with each other only through
their respective APIs. Tight coupling is the undesirable state of two or more classes

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that know inside details about another class, details not revealed in the class's API.
High cohesion is the desirable state of a single class whose purpose and responsibilities
are limited and well focused.
Then we built on those concepts and learned about object composition principles. In
particular, we learned how to build objects out of other objects. We saw how method
delegation and method forwarding prevent the need to duplicate code. For example:
public class MailerBox implements Box {
private Box box;
...
public void pack() { box.pack(); }

Next, we moved on to design patterns. We learned that design patterns are
reusable solutions to common problems.
We saw the singleton pattern used to ensure we only have one instance of a given
class within the application. We created a private static variable to store the single
instance, which we called the singleton. We then created a public static method for
callers to get a reference to the instance. Finally, we made the constructor private so
no callers can instantiate the object directly.
We also looked at the DAO design pattern. DAO stands for Data Access Object
and provides a way to separate database functionality from the main business object.
We saw how using an interface allows us to easily change the data access
implementation. A DAO interface typically looks like this:
public interface BookDao {
void create(Book book);
void delete(Book book);
...
}

Finally, we looked at the factory design pattern as another way of creating objects.
We learned how to create an abstract and concrete factory object. We also saw that
we could have common logic in the abstract class. For example:
public abstract class Factory {
public BookDao createDao() {
BookDao dao = createDatabaseBookDao();
// more setup on DAO
return dao;
}
public abstract BookDao createDatabaseBookDao();
}
public class FactoryImpl extends Factory {
public BookDao createDatabaseBookDao() {
return new OracleBookDao();
}
}

Two-Minute Drill

✓

565

TWO-MINUTE DRILL
Here are some of the key points from each certification objective in this chapter.

IS-A/HAS-A (OCP Objective 3.3)
❑ IS-A refers to inheritance.
❑ IS-A is expressed with either the keyword extends or implements.
❑ 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.

Coupling and Cohesion (OCP Objective 3.3)
❑ Coupling refers to the degree to which one class knows about or uses
members of another class.
❑ Loose coupling is the desirable state of having classes that are well encapsulated,
minimize references to each other, and limit the breadth of API usage.
❑ Tight coupling is the undesirable state of having classes that break the rules of
loose coupling.
❑ Cohesion refers to the degree to which a class has a single well-defined role or
responsibility.
❑ High cohesion is the desirable state of a class whose members support a single
well-focused role or responsibility.
❑ Low cohesion is the undesirable state of a class whose members support
multiple unfocused roles or responsibilities.

Object Composition Principles (OCP Objective 3.4)
❑ Object composition takes advantage of IS-A, HAS-A, and polymorphism.
❑ Object composition prevents proliferation of subclasses by having each class
responsible for one thing.

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❑ Object composition delegates to objects to which it "has" to implement
functionality.
❑ The terms method forwarding and method delegation are used interchangeably.

Singleton Design Pattern (OCP Objective 3.5)
❑ Design pattern is "a general reusable solution to a commonly occurring
problem within a given context."
❑ Having only one instance of the object allows a program to share its state.
❑ This pattern might improve performance by not repeating the same work.
❑ This pattern often stores a single instance as a static variable.
❑ We can instantiate right away (eager) or when needed (lazy).

DAO Design Pattern (OCP Objective 3.6)
❑ DAO stands for Data Access Object.
❑ DAO separates datastore responsibilities from the core responsibilities of the
object.
❑ DAO uses an interface so we can change the implementation.
❑ DAO is only responsible for database operations. The main object remains
cohesive.
❑ DAO facilitates reuse.

Factory Design Pattern (OCP Objective 3.7)
❑ Factory is a creational design pattern.
❑ Factory can create any subclass of an interface or abstract class.
❑ Factory is an abstract class.
❑ Factory subclassing allows for multiple factories.
❑ The factory method return type is an interface or abstract class.
❑ Factory method implementation returns subclasses of the target object.
❑ There may be common logic in the abstract class that all factory
subclasses share.

Self Test

567

SELF TEST
The following questions will help you measure your understanding of the material presented in this
chapter. Read all of the choices carefully, as there may be more than one correct answer. Choose all
correct answers for each question. Stay focused.
1. Given:
class A extends B {
C tail;
}

Which is true?
A. A HAS-A B and A HAS-A C
B. A HAS-A B and A IS-A C
C. A IS-A B and A HAS-A C
D. A IS-A B and A IS-A C
E. B IS-A A and A-HAS-A C
F. B IS-A A and A IS-A C
2. Which statements are true? (Choose all that apply.)
A. Method delegation relies on IS-A relationships
B. Method forwarding relies on HAS-A relationships
C. The DAO pattern limits you to one instance of the DAO object
D. The singleton pattern relies on IS-A relationships
E. To use object composition, classes must be final
3. Given:
public class F {
private static final F f = new F();
public static F c() {
return f;
}
public void update(F a) { }
public void delete(F a) { }
}

Which design pattern or principle is implemented?
A. Coupling
B. DAO

568
C.
D.
E.
F.

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Factory
IS-A
Object composition
Singleton

4. Given:
public class E {
private D d;
public void m() {
d.m();
}
public static E getInstance() {
return new E();
}
}
class D {
public void m() {}
}

Which design pattern or principle is implemented?
A. DAO
B. Factory
C. IS-A
D. Object composition
E. Singleton
5. Given:
class A {}
abstract class G {
A m() { return n(); }
abstract A n() ;
}

Which design pattern or principle is implemented?
A. DAO
B. Factory
C. IS-A
D. Object composition
E. Singleton

Self Test

569

6. Which design patterns are classified as creational design patterns? (Choose all that apply.)
A. Coupling
B. DAO
C. Factory
D. IS-A
E. Object composition
F. Singleton
7. Which statements indicate the need to use the factory pattern? (Choose all that apply.)
A. You don't want the caller to depend on a specific implementation
B. You have two classes that do the same thing
C. You only want one instance of the object to exist
D. You want one class to be responsible for database operations
E. You want to build a chain of objects
8. Given:
public class Dao {
Collection
void create(String
void delete(String
void update(String
}

findAll() { return null;}
a) {}
a) {}
a){}

And the following statements:
I – This is a good use of the DAO pattern
II – The DAO needs an interface
III – The DAO is missing a method
IV – The DAO must use a type other than String
Which of these statements are true?
A. Statement I only
B. Statement II only
C. Statement III only
D. Statement IV only
E. Statements II and III
F. Statements III and IV

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9. Which is a benefit of the DAO pattern? (Choose all that apply.)
A. Reuse is easier
B. The database code is automatically generated
C. We can change the database implementation independently
D. Your business object extends the DAO pattern to reduce coding
E. You are limited to one DAO object
10. Which are true of design patterns? (Choose all that apply.)
A. Design patterns are chunks of code you can copy into your application unchanged
B. Design patterns are conceptual reusable solutions
C. Design patterns are shortcuts to talking about code
D. There are three design patterns defined for Java
E. You can only use each design pattern once per application
F. Design patterns are libraries you can call from your code
11. Which statement is true? (Choose all that apply.)
A. Cohesion is the OO principle most closely associated with hiding implementation details
B. Cohesion is the OO principle most closely associated with making sure that classes know
about other classes only through their APIs
C. Cohesion is the OO principle most closely associated with making sure that a class is
designed with a single well-focused purpose
D. Cohesion is the OO principle most closely associated with allowing a single object to be
seen as having many types
12. Given:
1) ClassA has a ClassD
2) Methods in ClassA use public methods in ClassB
3) Methods in ClassC use public methods in ClassA
4) Methods in ClassA use public variables in ClassB
Which is most likely true? (Choose only one.)
A. ClassD has low cohesion.
B. ClassA has weak encapsulation.
C. ClassB has weak encapsulation.
D. ClassB has strong encapsulation.
E. ClassC is tightly coupled to ClassA.

Self Test Answers

571

SELF TEST ANSWERS
1. ☑ C is correct. Since A extends B, it IS-A B. Since C is an instance variable in A, A HAS-A C.
☐
✗ A, B, D, E, and F are incorrect because of the above. (OCP Objective 3.3)
2. ☑ B is correct. Method forwarding is an object composition principle and calls methods on an
instance variable of an object.
☐
✗ A is incorrect because method delegation and method forwarding are the same thing. C is
incorrect because it is the singleton pattern that limits you to one object. D is incorrect because
singleton classes typically don't have a superclass (other than Object). E is incorrect because
there is no such requirement. (OCP Objective 3.4)
3. ☑ F is correct. The singleton pattern is identifiable by the static variable for the single
instance and the accessor returning it.
☐
✗ B is incorrect because there is no interface. The class just happens to have methods
update() and delete(), which are similar to those found in a DAO. A, C, D, and E are
incorrect because of the above. (OCP Objective 3.5)
4. ☑ D is correct. The object composition principle of method forwarding is shown.
☐
✗ E is tricky, but incorrect. Although getInstance() is a common name for a method in
a singleton, the method doesn't return a static object. While it does create an object, it isn't
a factory either, since there is no superclass. A, B, and C are incorrect because of the above.
(OCP Objective 3.4)
5. ☑ B is correct. Class A is the object we are creating using the factory method. Class G is
the abstract superclass for the factory. Not shown is a class implementing class G that actually
creates the object.
☐
✗ A, C, D, and E are incorrect because of the above. (OCP Objective 3.7)
6. ☑ C and F are correct. The factory design pattern creates new objects for each call, and the
singleton design pattern creates one object, returning it each time.
☐
✗ A, B, D, and E are incorrect because of the above. (OCP Objectives 3.5 and 3.7)
7. ☑ A is correct. The factory design pattern decouples the caller from the implementation
class name.
☐
✗ B is incorrect because that would be poor design. C is incorrect because it describes the
singleton pattern. D is incorrect because it describes the DAO pattern. E is incorrect because of
the above. (OCP Objective 3.7)
8. ☑ B is correct. The Data Access Object pattern uses an interface so callers aren't dependent
on a specific implementation class.
☐
✗ A, C, D, E, and F are incorrect because of the above. (OCP Objective 3.6)

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9. ☑ A and C are correct. The DAO pattern centralizes logic for the data access code, making
reuse easier and allowing you to switch out implementations.
☐
✗ B is incorrect because you still have to code the DAO. D is incorrect because you call a
DAO from your business object; you do not inherit from it. E is incorrect because you can have
many DAO objects. (OCP Objective 3.6)
10. ☑ B and C are correct. Design patterns are conceptual and design level. You have to code the
implementation for each use.
☐
✗ D is incorrect because there are dozens of patterns defined for Java. Only three of them are
tested on the exam, but you should be aware that more exist. E is incorrect because it makes
sense to reuse the same pattern. For example, you might have multiple DAO objects. A and F
are incorrect because of the above. (OCP Objectives 3.5, 3.6, and 3.7)
11. ☑ C is correct.
☐
✗ A, B, and D are incorrect. A refers to encapsulation, B refers to coupling, and D refers to
polymorphism. (OCP Objective 3.3)
12. ☑ C is correct. Generally speaking, public variables are a sign of weak encapsulation.
☐
✗ A, B, D, and E are incorrect because based on the information given, none of these
statements can be supported. (OCP Objective 3.3)

11
Generics and
Collections

CERTIFICATION OBJECTIVES

•
•

Create a Generic Class

•

Analyze the Interoperability of Collections
that Use Raw and Generic Types

•
•

Use Wrapper Classes and Autoboxing

Use the Diamond Syntax to Create
a Collection

Create and Use a List, a Set, and a Deque

•
•

Create and Use a Map

•
✓

Sort and Search Arrays and Lists

Use java.util.Comparator and
java.lang.Comparable

Two-Minute Drill

Q&A Self Test

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G

enerics were the most talked about feature of Java 5. Some people love 'em, some
people hate 'em, but they're here to stay. At their simplest, they can help make code
easier to write and more robust. At their most complex, they can be very, very hard
to create and maintain. Luckily, the exam creators stuck to the simple end of generics, covering the
most common and useful features and leaving out most of the especially tricky bits.

CERTIFICATION OBJECTIVE

toString(), hashCode(), and equals()
(OCP Objectives 4.7 and 4.8)
4.X

toString() will show up in numerous places throughout the exam.

4.7

Use java.util.Comparator and java.lang.Comparable.

4.8

Sort and search arrays and lists.

It might not be immediately obvious, but understanding hashCode() and equals()
is essential to working with Java collections, especially when using Maps and when
searching and sorting in general.
You're an object. Get used to it. You have state, you have behavior, you have a
job. (Or at least your chances of getting one will go up after passing the exam.) If
you exclude primitives, everything in Java is an object. Not just an object, but an
Object with a capital O. Every exception, every event, every array extends from
java.lang.Object. For the exam, you don't need to know every method in class
Object, but you will need to know about the methods listed in Table 11-1.
Chapter 13 covers wait(), notify(), and notifyAll(). The finalize()
method was covered in Chapter 3. In this section, we'll look at the hashCode() and
equals() methods because they are so often critical when using collections. Oh,
that leaves toString(), doesn't it? Okay, we'll cover that right now because it takes
two seconds.

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

TABLE 11-1

Methods of Class
Object Covered
on the Exam

575

Method

Description

boolean equals (Object obj)

Decides whether two objects are meaningfully
equivalent

void finalize()

Called by the garbage collector when the garbage
collector sees that the object cannot be referenced

int hashCode()

Returns a hashcode int value for an object
so that the object can be used in Collection
classes that use hashing, including Hashtable,
HashMap, and HashSet

final void notify()

Wakes up a thread that is waiting for this object's
lock

final void notifyAll()

Wakes up all threads that are waiting for this
object's lock

final void wait()

Causes the current thread to wait until another
thread calls notify() or notifyAll() on
this object

String toString()

Returns a "text representation" of the object

The toString() Method
Override toString() when you want a mere mortal to be able to read something
meaningful about the objects of your class. Code can call toString() on your
object when it wants to read useful details about your object. When you pass an
object reference to the System.out.println() method, for example, the object's
toString() method is called, and the return of toString() is shown in the
following example:
public class HardToRead {
public static void main (String [] args) {
HardToRead h = new HardToRead();
System.out.println(h);
}
}

Running the HardToRead class gives us the lovely and meaningful
% java HardToRead
HardToRead@a47e0

The preceding output is what you get when you don’t override the toString()
method of class Object. It gives you the class name (at least that’s meaningful)
followed by the @ symbol, followed by the unsigned hexadecimal representation of
the object’s hashcode.

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Trying to read this output might motivate you to override the toString()
method in your classes, for example:
public class BobTest {
public static void main (String[] args) {
Bob f = new Bob("GoBobGo", 19);
System.out.println(f);
}
}
class Bob {
int shoeSize;
String nickName;
Bob(String nickName, int shoeSize) {
this.shoeSize = shoeSize;
this.nickName = nickName;
}
public String toString() {
return ("I am a Bob, but you can call me " + nickName +
". My shoe size is " + shoeSize);
}
}

This ought to be a bit more readable:
% java BobTest
I am a Bob, but you can call me GoBobGo. My shoe size is 19

Some people affectionately refer to toString() as the "spill-your-guts method"
because the most common implementations of toString() simply spit out the
object's state (in other words, the current values of the important instance variables).
That's it for toString(). Now we'll tackle equals() and hashCode().

Overriding equals()
As we mentioned earlier, you might be wondering why we decided to talk about
Object.equals()near the beginning of the chapter on collections. We'll be
spending a lot of time answering that question over the next pages, but for now, it's
enough to know that whenever you need to sort or search through a collection of
objects, the equals() and hashCode() methods are essential. But before we go
there, let's look at the more common uses of the equals() method.
You learned a bit about the equals() method in Chapter 4. We discussed how
comparing two object references using the == operator evaluates to true only when
both references refer to the same object because == simply looks at the bits in the
variable, and they're either identical or they're not. You saw that the String class
has overridden the equals() method (inherited from the class Object), so you

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

577

could compare two different String objects to see if their contents are meaningfully
equivalent. Later in this chapter, we'll be discussing the so-called wrapper classes
when it's time to put primitive values into collections. For now, remember that there
is a wrapper class for every kind of primitive. The folks who created the Integer
class (to support int primitives) decided that if two different Integer instances
both hold the int value 5, as far as you're concerned, they are equal. The fact that
the value 5 lives in two separate objects doesn't matter.
When you really need to know if two references are identical, use ==. But when
you need to know if the objects themselves (not the references) are equal, use the
equals() method. For each class you write, you must decide if it makes sense to
consider two different instances equal. For some classes, you might decide that two
objects can never be equal. For example, imagine a class Car that has instance
variables for things like make, model, year, configuration—you certainly don't want
your car suddenly to be treated as the very same car as someone with a car that has
identical attributes. Your car is your car and you don't want your neighbor Billy
driving off in it just because "hey, it's really the same car; the equals() method said
so." So no two cars should ever be considered exactly equal. If two references refer to
one car, then you know that both are talking about one car, not two cars that have
the same attributes. So in the case of class Car you might not ever need, or want,
to override the equals() method. Of course, you know that isn't the end of the story.

What It Means If You Don't Override equals()
There's a potential limitation lurking here: If you don't override a class's equals()
method, you won't be able to use those objects as a key in a hashtable and you
probably won't get accurate Sets such that there are no conceptual duplicates.
The equals() method in class Object uses only the == operator for comparisons,
so unless you override equals(), two objects are considered equal only if the two
references refer to the same object.
Let's look at what it means to not be able to use an object as a hashtable key.
Imagine you have a car, a very specific car (say, John's red Subaru Outback as
opposed to Mary's purple Mini) that you want to put in a HashMap (a type of
hashtable we'll look at later in this chapter) so that you can search on a particular
car and retrieve the corresponding Person object that represents the owner. So you
add the car instance as the key to the HashMap (along with a corresponding Person
object as the value). But now what happens when you want to do a search? You want
to say to the HashMap collection, "Here's the car; now give me the Person object
that goes with this car." But now you're in trouble unless you still have a reference to
the exact object you used as the key when you added it to the Collection. In other
words, you can't make an identical Car object and use it for the search.

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The bottom line is this: If you want objects of your class to be used as keys for a
hashtable (or as elements in any data structure that uses equivalency for searching
for—and/or retrieving—an object), then you must override equals() so that two
different instances can be considered the same. So how would we fix the car? You
might override the equals() method so that it compares the unique VIN (Vehicle
Identification Number) as the basis of comparison. That way, you can use one
instance when you add it to a Collection and essentially re-create an identical
instance when you want to do a search based on that object as the key. Of course,
overriding the equals() method for Car also allows the potential for more than one
object representing a single unique car to exist, which might not be safe in your
design. Fortunately, the String and wrapper classes work well as keys in hashtables—
they override the equals() method. So rather than using the actual car instance as
the key into the car/owner pair, you could simply use a String that represents the
unique identifier for the car. That way, you'll never have more than one instance
representing a specific car, but you can still use the car—or rather, one of the car's
attributes—as the search key.

Implementing an equals() Method
Let's say you decide to override equals() in your class. It might look like this:
public class EqualsTest {
public static void main (String [] args) {
Moof one = new Moof(8);
Moof two = new Moof(8);
if (one.equals(two)) {
System.out.println("one and two are equal");
}
}
}
class Moof {
private int moofValue;
Moof(int val) {
moofValue = val;
}
public int getMoofValue() {
return moofValue;
}
public boolean equals(Object o) {
if ((o instanceof Moof) && (((Moof)o).getMoofValue()
== this.moofValue)) {
return true;
} else {
return false;
}
}
}

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

579

Let's look at this code in detail. In the main() method of EqualsTest, we create
two Moof instances, passing the same value 8 to the Moof constructor. Now look at
the Moof class and let's see what it does with that constructor argument—it assigns
the value to the moofValue instance variable. Now imagine that you've decided two
Moof objects are the same if their moofValue is identical. So you override the
equals() method and compare the two moofValues. It is that simple. But let's
break down what's happening in the equals() method:
1. public boolean equals(Object o) {
2.
if ((o instanceof Moof) && (((Moof)o).getMoofValue()
== this.moofValue)) {
3.
return true;
4.
} else {
5.
return false;
6.
}
7. }

First of all, you must observe all the rules of overriding, and in line 1 we are
indeed declaring a valid override of the equals() method we inherited from Object.
Line 2 is where all the action is. Logically, we have to do two things in order to
make a valid equality comparison.
First, be sure that the object being tested is of the correct type! It comes in
polymorphically as type Object, so you need to do an instanceof test on it.
Having two objects of different class types be considered equal is usually not a good
idea, but that's a design issue we won't go into here. Besides, you'd still have to do
the instanceof test just to be sure that you could cast the object argument to the
correct type so that you can access its methods or variables in order to actually do
the comparison. Remember, if the object doesn't pass the instanceof test, then
you'll get a runtime ClassCastException. For example:
public boolean equals(Object o) {
if (((Moof)o).getMoofValue() == this.moofValue){
// the preceding line compiles, but it's BAD!
return true;
} else {
return false;
}
}

The (Moof)o cast will fail if o doesn't refer to something that IS-A Moof.
Second, compare the attributes we care about (in this case, just moofValue).
Only the developer can decide what makes two instances equal. (For best
performance, you're going to want to check the fewest number of attributes.)
In case you were a little surprised by the whole ((Moof)o).getMoofValue()
syntax, we're simply casting the object reference, o, just-in-time as we try to call a

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method that's in the Moof class but not in Object. Remember, without the cast, you
can't compile because the compiler would see the object referenced by o as simply,
well, an Object. And since the Object class doesn't have a getMoofValue()
method, the compiler would squawk (technical term). But then, as we said earlier,
even with the cast, the code fails at runtime if the object referenced by o isn't
something that's castable to a Moof. So don't ever forget to use the instanceof test
first. Here's another reason to appreciate the short-circuit && operator—if the
instanceof test fails, we'll never get to the code that does the cast, so we're always
safe at runtime with the following:
if ((o instanceof Moof) && (((Moof)o).getMoofValue()
== this.moofValue)) {
return true;
} else {
return false;
}

So that takes care of equals()…
Whoa… not so fast. If you look at the Object class in the Java API spec, you'll
find what we call a contract specified in the equals() method. A Java contract is a
set of rules that should be followed, or rather must be followed, if you want to
provide a "correct" implementation as others will expect it to be. Or to put it
another way: If you don't follow the contract, your code may still compile and run,
but your code (or someone else's) may break at runtime in some unexpected way.

Remember that the equals(), hashCode(), and toString() methods are
all public.The following would not be a valid override of the equals() method, although
it might appear to be if you don't look closely enough during the exam:
class Foo { boolean equals(Object o) { } }

And watch out for the argument types as well.The following method is an overload, but
not an override of the equals() method:
class Boo { public boolean equals(Boo b) { } }

Be sure you're very comfortable with the rules of overriding so that you can identify
whether a method from Object is being overridden, overloaded, or illegally redeclared in
a class.The equals() method in class Boo changes the argument from Object to Boo, so it
becomes an overloaded method and won't be called unless it's from your own code that
knows about this new, different method that happens to also be named equals().

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

581

The equals() Contract
Pulled straight from the Java docs, the equals() contract says
■ It is reflexive. For any reference value x, x.equals(x) should return true.
■ It is symmetric. For any reference values x and y, x.equals(y) should

return true if and only if y.equals(x) returns true.
■ It is transitive. For any reference values x, y, and z, if x.equals(y) returns
true and y.equals(z) returns true, then x.equals(z) must return true.

■ It is consistent. For any reference values x and y, multiple invocations of
x.equals(y) consistently return true or consistently return false, provided
no information used in equals() comparisons on the object is modified.

■ For any non-null reference value x, x.equals(null) should return false.

And you're so not off the hook yet. We haven't looked at the hashCode()
method, but equals() and hashCode() are bound together by a joint contract that
specifies if two objects are considered equal using the equals() method, then they
must have identical hashcode values. So to be truly safe, your rule of thumb should
be if you override equals(), override hashCode() as well. So let's switch over to
hashCode() and see how that method ties in to equals().

Overriding hashCode()
Hashcodes are typically used to increase the performance of large collections of data.
The hashcode value of an object is used by some collection classes (we'll look at the
collections later in this chapter). Although you can think of it as kind of an object
ID number, it isn't necessarily unique. Collections such as HashMap and HashSet
use the hashcode value of an object to determine how the object should be stored in
the collection, and the hashcode is used again to help locate the object in the collection.
For the exam, you do not need to understand the deep details of how the collection
classes that use hashing are implemented, but you do need to know which collections
use them (but, um, they all have "hash" in the name, so you should be good there).
You must also be able to recognize an appropriate or correct implementation of
hashCode(). This does not mean legal and does not even mean efficient. It's
perfectly legal to have a terribly inefficient hashcode method in your class, as long as
it doesn't violate the contract specified in the Object class documentation (we'll
look at that contract in a moment). So for the exam, if you're asked to pick out an
appropriate or correct use of hashcode, don't mistake appropriate for legal or efficient.

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Understanding Hashcodes
In order to understand what's appropriate and correct, we have to look at how some
of the collections use hashcodes.
Imagine a set of buckets lined up on the floor. Someone hands you a piece of
paper with a name on it. You take the name and calculate an integer code from it by
using A is 1, B is 2, and so on, adding the numeric values of all the letters in the
name together. A given name will always result in the same code; see Figure 11-1.
We don't introduce anything random; we simply have an algorithm that will
always run the same way given a specific input, so the output will always be identical
for any two identical inputs. So far, so good? Now the way you use that code (and
we'll call it a hashcode now) is to determine which bucket to place the piece of
paper into (imagine that each bucket represents a different code number you might
get). Now imagine that someone comes up and shows you a name and says, "Please
retrieve the piece of paper that matches this name." So you look at the name they
show you and run the same hashcode-generating algorithm. The hashcode tells you
in which bucket you should look to find the name.
You might have noticed a little flaw in our system, though. Two different names
might result in the same value. For example, the names Amy and May have the same
letters, so the hashcode will be identical for both names. That's acceptable, but it
does mean that when someone asks you (the bucket clerk) for the Amy piece of
paper, you'll still have to search through the target bucket, reading each name until
we find Amy rather than May. The hashcode tells you only which bucket to go into
and not how to locate the name once we're in that bucket.

FIGURE 11-1

A simplified
hashcode
example

Key

Hashcode Algorithm

Hashcode

Alex
Bob
Dirk
Fred

A(1) + L(12) + E(5) + X(24)
B(2) + O(15) + B(2)
D(4) + I(9) + R(18) + K(11)
F(6) + R(18) + E(5) + D(4)

= 42
= 19
= 42
= 33

HashMap Collection
Hashcode Buckets

19

33

42

“Bob”

“Fred”

“Alex”
“Dirk”

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

583

In real-life hashing, it's not uncommon to have more than one entry in a
bucket. Hashing retrieval is a two-step process.
1.
2.

Find the right bucket (using hashCode()).
Search the bucket for the right element (using equals()).

So, for efficiency, your goal is to have the papers distributed as evenly as possible
across all buckets. Ideally, you might have just one name per bucket so that when
someone asked for a paper, you could simply calculate the hashcode and just grab the
one paper from the correct bucket, without having to flip through different papers in
that bucket until you locate the exact one you're looking for. The least efficient (but
still functional) hashcode generator would return the same hashcode (say, 42),
regardless of the name, so that all the papers landed in the same bucket while the
others stood empty. The bucket clerk would have to keep going to that one bucket
and flipping painfully through each one of the names in the bucket until the right
one was found. And if that's how it works, they might as well not use the hashcodes
at all, but just go to the one big bucket and start from one end and look through
each paper until they find the one they want.
This distributed-across-the-buckets example is similar to the way hashcodes are
used in collections. When you put an object in a collection that uses hashcodes, the
collection uses the hashcode of the object to decide in which bucket/slot the object
should land. Then when you want to fetch that object (or, for a hashtable, retrieve
the associated value for that object), you have to give the collection a reference to
an object, which it then compares to the objects it holds in the collection. As long
as the object stored in the collection, like a paper in the bucket, you're trying to
search for has the same hashcode as the object you're using for the search (the name
you show to the person working the buckets), then the object will be found. But—
and this is a Big One—imagine what would happen if, going back to our name
example, you showed the bucket worker a name and they calculated the code based
on only half the letters in the name instead of all of them. They'd never find the
name in the bucket because they wouldn't be looking in the correct bucket!
Now can you see why if two objects are considered equal, their hashcodes must
also be equal? Otherwise, you'd never be able to find the object, since the default
hashcode method in class Object virtually always comes up with a unique number

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for each object, even if the equals() method is overridden in such a way that two
or more objects are considered equal. It doesn't matter how equal the objects are if
their hashcodes don't reflect that. So one more time: If two objects are equal, their
hashcodes must be equal as well.

Implementing hashCode()
What the heck does a real hashcode algorithm look like? People get their PhDs on
hashing algorithms, so from a computer science viewpoint, it's beyond the scope of
the exam. The part we care about here is the issue of whether you follow the
contract. And to follow the contract, think about what you do in the equals()
method. You compare attributes because that comparison almost always involves
instance variable values (remember when we looked at two Moof objects and
considered them equal if their int moofValues were the same?). Your hashCode()
implementation should use the same instance variables. Here's an example:
class HasHash {
public int x;
HasHash(int xVal) { x = xVal; }
public boolean equals(Object o) {
HasHash h = (HasHash) o;
// Don't try at home without
// instanceof test
if (h.x == this.x) {
return true;
} else {
return false;
}
}
public int hashCode() { return (x * 17); }
}

This equals() method says two objects are equal if they have the same x value,
so objects with the same x value will have to return identical hashcodes.

A hashCode() that returns the same value for all instances, whether
they're equal or not, is still a legal—even appropriate—hashCode() method! For example:
public int hashCode() { return 1492; }

This does not violate the contract.Two objects with an x value of 8 will have the same
hashcode. But then again, so will two unequal objects, one with an x value of 12 and the

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

585

other with a value of -920.This hashCode() method is horribly inefficient, remember,
because it makes all objects land in the same bucket. Even so, the object can still be
found as the collection cranks through the one and only bucket—using equals()—
trying desperately to finally, painstakingly, locate the correct object. In other words, the
hashcode was really no help at all in speeding up the search, even though improving
search speed is hashcode's intended purpose! Nonetheless, this one-hash-fits-all method
would be considered appropriate and even correct because it doesn't violate the
contract. Once more, correct does not necessarily mean good.

Typically, you'll see hashCode() methods that do some combination of ^-ing
(XOR-ing) a class's instance variables (in other words, twiddling their bits), along
with perhaps multiplying them by a prime number. In any case, while the goal is to
get a wide and random distribution of objects across buckets, the contract (and
whether or not an object can be found) requires only that two equal objects have
equal hashcodes. The exam does not expect you to rate the efficiency of a hashCode()
method, but you must be able to recognize which ones will and will not work
("work" meaning "will cause the object to be found in the collection").
Now that we know that two equal objects must have identical hashcodes, is the
reverse true? Do two objects with identical hashcodes have to be considered equal?
Think about it—you might have lots of objects land in the same bucket because
their hashcodes are identical, but unless they also pass the equals() test, they won't
come up as a match in a search through the collection. This is exactly what you'd
get with our very inefficient, everybody-gets-the-same-hashcode method. It's legal
and correct, just slooooow.
So in order for an object to be located, the search object and the object in the
collection must both have identical hashcode values and return true for the
equals() method. So there's just no way out of overriding both methods to be
absolutely certain that your objects can be used in Collections that use hashing.

The hashCode() Contract
Now coming to you straight from the fabulous Java API documentation for class
Object, may we present (drumroll) the hashCode() contract:
■ Whenever it is invoked on the same object more than once during an

execution of a Java application, the hashCode() method must consistently
return the same integer, provided that no information used in equals()

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comparisons on the object is modified. This integer need not remain consistent
from one execution of an application to another execution of the same
application.
■ If two objects are equal according to the equals(Object) method, then

calling the hashCode() method on each of the two objects must produce the
same integer result.
■ It is NOT required that if two objects are unequal according to the
equals(java.lang.Object) method, then calling the hashCode() method

on each of the two objects must produce distinct integer results. However,
the programmer should be aware that producing distinct integer results for
unequal objects may improve the performance of hashtables.
And what this means to you is…
Condition

Required

x.equals(y) == true

x.hashCode() ==
y.hashCode()

Not Required
(But Allowed)

x.hashCode() ==
y.hashCode()

x.equals(y) == true

x.equals(y) == false

No hashCode()
requirements

x.hashCode() !=
y.hashCode()

x.equals(y) == false

So let's look at what else might cause a hashCode() method to fail. What happens
if you include a transient variable in your hashCode() method? While that's legal
(the compiler won't complain), under some circumstances, an object you put in a
collection won't be found. As you might know, serialization saves an object so that it
can be reanimated later by deserializing it back to full objectness. But danger, Will
Robinson—transient variables are not saved when an object is serialized. A bad
scenario might look like this:
class SaveMe implements Serializable{
transient int x;
int y;
SaveMe(int xVal, int yVal) {
x = xVal;
y = yVal;
}

toString(), hashCode(), and equals() (OCP Objectives 4.7 and 4.8)

public int hashCode() {
return (x ^ y);
}
public boolean equals(Object o) {
SaveMe test = (SaveMe)o;
if (test.y == y && test.x == x) {
return true;
} else {
return false;
}
}

587

// Legal, but not correct to
// use a transient variable

// Legal, not correct

}

Here's what could happen using code like the preceding example:
1. Give an object some state (assign values to its instance variables).
2. Put the object in a HashMap, using the object as a key.
3. Save the object to a file using serialization without altering any of its state.
4. Retrieve the object from the file through deserialization.
5. Use the deserialized (brought back to life on the heap) object to get the
object out of the HashMap.
Oops. The object in the collection and the supposedly same object brought back
to life are no longer identical. The object's transient variable will come back with a
default value rather than the value the variable had at the time it was saved (or put
into the HashMap). So using the preceding SaveMe code, if the value of x is 9 when
the instance is put in the HashMap, then since x is used in the calculation of the
hashcode, when the value of x changes, the hashcode changes too. And when that
same instance of SaveMe is brought back from deserialization, x == 0, regardless of
the value of x at the time the object was serialized. So the new hashcode calculation
will give a different hashcode and the equals() method fails as well since x is used
to determine object equality.
Bottom line: transient variables can really mess with your equals() and
hashCode() implementations. Keep variables non-transient or, if they must be
marked transient, don't use them to determine hashcodes or equality.

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CERTIFICATION OBJECTIVE

Collections Overview
(OCP Objectives 4.5 and 4.6)
4.5 Create and use a List, a Set, and a Deque.
4.6 Create and use a Map.
In this section, we're going to present a relatively high-level discussion of the
major categories of collections covered on the exam. We'll be looking at their
characteristics and uses from an abstract level. In the section after this one, we'll
dive into each category of collection and show concrete examples of using each.
Can you imagine trying to write object-oriented applications without using data
structures like hashtables or linked lists? What would you do when you needed to
maintain a sorted list of, say, all the members in your Simpsons fan club? Obviously,
you can do it yourself; Amazon.com must have thousands of algorithm books you
can buy. But with the kind of schedules programmers are under today, it's almost too
painful to consider.
The Collections Framework in Java, which took shape with the release of JDK 1.2
and was expanded in 1.4 and again in Java 5, and yet again in Java 6 and Java 7,
gives you lists, sets, maps, and queues to satisfy most of your coding needs. They've
been tried, tested, and tweaked. Pick the best one for your job, and you'll get
reasonable performance. And when you need something a little more custom, the
Collections Framework in the java.util package is loaded with interfaces and
utilities.

So What Do You Do with a Collection?
There are a few basic operations you'll normally use with collections:
■ Add objects to the collection.
■ Remove objects from the collection.
■ Find out if an object (or group of objects) is in the collection.
■ Retrieve an object from the collection without removing it.
■ Iterate through the collection, looking at each element (object) one after

another.

Collections Overview (OCP Objectives 4.5 and 4.6)

589

Key Interfaces and Classes of the Collections Framework
The Collections API begins with a group of interfaces, but also gives you a truckload
of concrete classes. The core interfaces you need to know for the exam (and life in
general) are the following nine:
Collection

Set

SortedSet

List

Map

SortedMap

Queue

NavigableSet

NavigableMap

In Chapter 14, which deals with concurrency, we will discuss several classes
related to the Deque interface. Other than those, the concrete implementation
classes you need to know for the exam are the following 13 (there are others, but the
exam doesn't specifically cover them).
Maps

Sets

Lists

Queues

HashMap

HashSet

ArrayList

PriorityQueue

Hashtable

LinkedHashSet

Vector

TreeMap

TreeSet

LinkedList

Utilities
Collections
Arrays

LinkedHashMap

Not all collections in the Collections Framework actually implement the Collection
interface. In other words, not all collections pass the IS-A test for Collection.
Specifically, none of the Map-related classes and interfaces extend from Collection.
So while SortedMap, Hashtable, HashMap, TreeMap, and LinkedHashMap are all
thought of as collections, none are actually extended from Collection-with-acapital-C (see Figure 11-2). To make things a little more confusing, there are really
three overloaded uses of the word "collection":
■ collection (lowercase c), which represents any of the data structures in which

objects are stored and iterated over.
■ Collection (capital C), which is actually the java.util.Collection

interface from which Set, List, and Queue extend. (That's right, extend,
not implement. There are no direct implementations of Collection.)
■ Collections (capital C and ends with s) is the java.util.Collections

class that holds a pile of static utility methods for use with collections.

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FIGURE 11-2

Generics and Collections

The interface and class hierarchy for collections
<>
Collection

<>

HashSet

LinkedHashSet

<>

<>

Set

Queue

List

<>
SortedSet

<>
NavigableSet

ArrayList

Vector

LinkedList

PriorityQueue

TreeSet

<>

Object

Map

<>
SortedMap

Arrays

Collections

implements

Hashtable

HashMap

LinkedHashMap

<>
NavigableMap

TreeMap

extends

You can so easily mistake "Collections" for "Collection"—be careful.
Keep in mind that Collections is a class, with static utility methods, while Collection is an
interface with declarations of the methods common to most collections, including add(),
remove(), contains(), size(), and iterator().

Collections Overview (OCP Objectives 4.5 and 4.6)

591

Collections come in four basic flavors:
■ Lists

Lists of things (classes that implement List)

■ Sets

Unique things (classes that implement Set)

■ Maps

Things with a unique ID (classes that implement Map)

■ Queues

Things arranged by the order in which they are to be processed

Figure 11-3 illustrates the relative structures of a List, a Set, and a Map.
But there are subflavors within those four flavors of collections:
Sorted

Unsorted

Ordered

Unordered

An implementation class can be unsorted and unordered, ordered but unsorted, or
both ordered and sorted. But an implementation can never be sorted but unordered,
because sorting is a specific type of ordering, as you'll see in a moment. For example,
a HashSet is an unordered, unsorted set, while a LinkedHashSet is an ordered (but
not sorted) set that maintains the order in which objects were inserted.
FIGURE 11-3

Examples of a
List, a Set,
and a Map

Index:

0

1

2

3

4

5

Value:

“Boulder”

“Ft. Collins”

“Greeley”

“Boulder”

“Denver”

“Boulder”

List: The salesman’s itinerary (Duplicates allowed)

Boulder

Greeley

Ft. Collins
Denver

Vail

Idaho Springs
Dillon

Set: The salesman’s territory (No duplicates allowed)

Hashcode Buckets:
Values: “Sky Hook” “MonkeyWrench” “Phase Inverter” “Warp Core”
“Flux Capacitor”
HashMap: The salesman’s products (Keys generated from product IDs)

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Maybe we should be explicit about the difference between sorted and ordered, but
first we have to discuss the idea of iteration. When you think of iteration, you may
think of iterating over an array using, say, a for loop to access each element in the
array in order ([0], [1], [2], and so on). Iterating through a collection usually means
walking through the elements one after another, starting from the first element.
Sometimes, though, even the concept of first is a little strange—in a Hashtable,
there really isn't a notion of first, second, third, and so on. In a Hashtable, the
elements are placed in a (seemingly) chaotic order based on the hashcode of the key.
But something has to go first when you iterate; thus, when you iterate over a
Hashtable, there will indeed be an order. But as far as you can tell, it's completely
arbitrary and can change in apparently random ways as the collection changes.

Ordered When a collection is ordered, it means you can iterate through the
collection in a specific (not random) order. A Hashtable collection is not ordered.
Although the Hashtable itself has internal logic to determine the order (based on
hashcodes and the implementation of the collection itself), you won't find any order
when you iterate through the Hashtable. An ArrayList, however, keeps the order
established by the elements' index position (just like an array). LinkedHashSet
keeps the order established by insertion, so the last element inserted is the last
element in the LinkedHashSet (as opposed to an ArrayList, where you can insert
an element at a specific index position). Finally, there are some collections that keep
an order referred to as the natural order of the elements, and those collections are
then not just ordered, but also sorted. Let's look at how natural order works for
sorted collections.
Sorted A sorted collection means that the order in the collection is determined
according to some rule or rules, known as the sort order. A sort order has nothing to
do with when an object was added to the collection or when was the last time it was
accessed, or what "position" it was added at. Sorting is done based on properties of
the objects themselves. You put objects into the collection, and the collection will
figure out what order to put them in, based on the sort order. A collection that keeps
an order (such as any List, which uses insertion order) is not really considered sorted
unless it sorts using some kind of sort order. Most commonly, the sort order used is
something called the natural order. What does that mean?
You know how to sort alphabetically—A comes before B, F comes before G, and
so on. For a collection of String objects, then, the natural order is alphabetical. For
Integer objects, the natural order is by numeric value—1 before 2, and so on. And
for Foo objects, the natural order is… um… we don't know. There is no natural

Collections Overview (OCP Objectives 4.5 and 4.6)

593

order for Foo unless or until the Foo developer provides one through an interface
(Comparable) that defines how instances of a class can be compared to one another
(does instance a come before b, or does instance b come before a?). If the developer
decides that Foo objects should be compared using the value of some instance
variable (let's say there's one called bar), then a sorted collection will order the Foo
objects according to the rules in the Foo class for how to use the bar instance
variable to determine the order. Of course, the Foo class might also inherit a natural
order from a superclass rather than define its own order in some cases.
Aside from natural order as specified by the Comparable interface, it's also
possible to define other, different sort orders using another interface: Comparator.
We will discuss how to use both Comparable and Comparator to define sort orders
later in this chapter. But for now, just keep in mind that sort order (including natural
order) is not the same as ordering by insertion, access, or index.
Now that we know about ordering and sorting, we'll look at each of the four
interfaces, and we'll dive into the concrete implementations of those interfaces.

List Interface
A List cares about the index. The one thing that List has that non-lists don't is a
set of methods related to the index. Those key methods include things like get(int
index), indexOf(Object o), add(int index, Object obj), and so on. All
three List implementations are ordered by index position—a position that you
determine either by setting an object at a specific index or by adding it without
specifying position, in which case the object is added to the end. The three List
implementations are described in the following sections.

ArrayList Think of this as a growable array. It gives you fast iteration and fast
random access. To state the obvious: It is an ordered collection (by index), but not
sorted. You might want to know that as of version 1.4, ArrayList now implements
the new RandomAccess interface—a marker interface (meaning it has no methods)
that says, "This list supports fast (generally constant time) random access." Choose
this over a LinkedList when you need fast iteration but aren't as likely to be doing
a lot of insertion and deletion.
Vector Vector is a holdover from the earliest days of Java; Vector and Hashtable
were the two original collections—the rest were added with Java 2 versions 1.2 and
1.4. A Vector is basically the same as an ArrayList, but Vector methods are
synchronized for thread safety. You'll normally want to use ArrayList instead of

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Vector because the synchronized methods add a performance hit you might not need.
And if you do need thread safety, there are utility methods in class Collections
that can help. Vector is the only class other than ArrayList to implement
RandomAccess.

LinkedList A LinkedList is ordered by index position, like ArrayList, except
that the elements are doubly linked to one another. This linkage gives you new
methods (beyond what you get from the List interface) for adding and removing
from the beginning or end, which makes it an easy choice for implementing a stack
or queue. Keep in mind that a LinkedList may iterate more slowly than an
ArrayList, but it's a good choice when you need fast insertion and deletion. As of
Java 5, the LinkedList class has been enhanced to implement the java.util.
Queue interface. As such, it now supports the common queue methods peek(),
poll(), and offer().

Set Interface
A Set cares about uniqueness—it doesn't allow duplicates. Your good friend the
equals() method determines whether two objects are identical (in which case, only
one can be in the set). The three Set implementations are described in the
following sections.

HashSet A HashSet is an unsorted, unordered Set. It uses the hashcode of the
object being inserted, so the more efficient your hashCode() implementation, the
better access performance you'll get. Use this class when you want a collection with
no duplicates and you don't care about order when you iterate through it.
LinkedHashSet A LinkedHashSet is an ordered version of HashSet that
maintains a doubly linked List across all elements. Use this class instead of
HashSet when you care about the iteration order. When you iterate through a
HashSet, the order is unpredictable, while a LinkedHashSet lets you iterate
through the elements in the order in which they were inserted.

When using HashSet or LinkedHashSet, the objects you add to them must
override hashCode(). If they don't override hashCode(), the default Object.hashCode()
method will allow multiple objects that you might consider "meaningfully equal" to be
added to your "no duplicates allowed" set.

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595

TreeSet The TreeSet is one of two sorted collections (the other being
TreeMap). It uses a Red-Black tree structure (but you knew that), and guarantees
that the elements will be in ascending order, according to natural order. Optionally,
you can construct a TreeSet with a constructor that lets you give the collection
your own rules for what the order should be (rather than relying on the ordering
defined by the elements' class) by using a Comparator. As of Java 6, TreeSet
implements NavigableSet.

Map Interface
A Map cares about unique identifiers. You map a unique key (the ID) to a specific
value, where both the key and the value are, of course, objects. You're probably quite
familiar with Maps since many languages support data structures that use a key/value
or name/value pair. The Map implementations let you do things like search for a
value based on the key, ask for a collection of just the values, or ask for a collection
of just the keys. Like Sets, Maps rely on the equals() method to determine whether
two keys are the same or different.

HashMap The HashMap gives you an unsorted, unordered Map. When you need a
Map and you don't care about the order when you iterate through it, then HashMap is
the way to go; the other maps add a little more overhead. Where the keys land in
the Map is based on the key's hashcode, so, like HashSet, the more efficient your
hashCode() implementation, the better access performance you'll get. HashMap
allows one null key and multiple null values in a collection.
Hashtable Like Vector, Hashtable has existed from prehistoric Java times. For
fun, don't forget to note the naming inconsistency: HashMap vs. Hashtable. Where's
the capitalization of t? Oh well, you won't be expected to spell it. Anyway, just as
Vector is a synchronized counterpart to the sleeker, more modern ArrayList,
Hashtable is the synchronized counterpart to HashMap. Remember that you don't
synchronize a class, so when we say that Vector and Hashtable are synchronized,
we just mean that the key methods of the class are synchronized. Another difference,
though, is that while HashMap lets you have null values as well as one null key, a
Hashtable doesn't let you have anything that's null.
LinkedHashMap Like its Set counterpart, LinkedHashSet, the LinkedHashMap
collection maintains insertion order (or, optionally, access order). Although it will

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be somewhat slower than HashMap for adding and removing elements, you can
expect faster iteration with a LinkedHashMap.

TreeMap You can probably guess by now that a TreeMap is a sorted Map. And
you already know that, by default, this means "sorted by the natural order of the
elements." Like TreeSet, TreeMap lets you define a custom sort order (via a
Comparator) when you construct a TreeMap that specifies how the elements should
be compared to one another when they're being ordered. As of Java 6, TreeMap
implements NavigableMap.

Queue Interface
A Queue is designed to hold a list of "to-dos," or things to be processed in some way.
Although other orders are possible, queues are typically thought of as FIFO (first-in,
first-out). Queues support all of the standard Collection methods and they also have
methods to add and subtract elements and review queue elements.

PriorityQueue This class is new as of Java 5. Since the LinkedList class has
been enhanced to implement the Queue interface, basic queues can be handled with
a LinkedList. The purpose of a PriorityQueue is to create a "priority-in, priority
out" queue as opposed to a typical FIFO queue. A PriorityQueue's elements are
ordered either by natural ordering (in which case the elements that are sorted first
will be accessed first) or according to a Comparator. In either case, the elements'
ordering represents their relative priority.

You can easily eliminate some answers right away if you recognize
that, for example, a Map can't be the class to choose when you need a name/value pair
collection, since Map is an interface and not a concrete implementation class.The wording
on the exam is explicit when it matters, so if you're asked to choose an interface, choose
an interface rather than a class that implements that interface.The reverse is also
true—if you're asked to choose a class, don't choose an interface type.

Table 11-2 summarizes 11 of the 13 concrete collection-oriented classes you'll
need to understand for the exam. (Arrays and Collections are coming right up!)

Collections Overview (OCP Objectives 4.5 and 4.6)

TABLE 11-2

Collection
Interface
Concrete
Implementation
Classes

Class

Map

HashMap

Set

List

597

Ordered

Sorted

X

No

No

Hashtable

X

No

No

TreeMap

X

Sorted

By natural order or
custom comparison rules

LinkedHashMap

X

By insertion order
or last access order

No

HashSet

X

No

No

TreeSet

X

Sorted

By natural order or
custom comparison rules

LinkedHashSet

X

By insertion order

No

ArrayList

X

By index

No

Vector

X

By index

No

LinkedList

X

By index

No

Sorted

By to-do order

PriorityQueue

Be sure you know how to interpret Table 11-2 in a practical way. For the
exam, you might be expected to choose a collection based on a particular requirement,
where that need is expressed as a scenario. For example, which collection would you
use if you needed to maintain and search on a list of parts identified by their unique
alphanumeric serial number where the part would be of type Part? Would you change
your answer at all if we modified the requirement such that you also need to be able
to print out the parts in order by their serial number? For the first question, you can see
that since you have a Part class but need to search for the objects based on a serial
number, you need a Map.The key will be the serial number as a String, and the value will
be the Part instance.The default choice should be HashMap, the quickest Map for access.
But now when we amend the requirement to include getting the parts in order of their
serial number, then we need a TreeMap—which maintains the natural order of the keys.
Since the key is a String, the natural order for a String will be a standard alphabetical
sort. If the requirement had been to keep track of which part was last accessed, then
we'd probably need a LinkedHashMap. But since a LinkedHashMap loses the natural order
(replacing it with last-accessed order), if we need to list the parts by serial number, we'll
have to explicitly sort the collection using a utility method.

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CERTIFICATION OBJECTIVE

Using Collections
(OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)
4.3

Use the diamond syntax to create a collection.

4.4

Use wrapper classes and autoboxing.

4.5

Create and use a List, a Set, and a Deque.

4.6

Create and use a Map.

4.7

Use java.util.Comparator and java.lang.Comparable.

4.8

Sort and search arrays and lists.

We've taken a high-level theoretical look at the key interfaces and classes in the
Collections Framework; now let's see how they work in practice.

ArrayList Basics
Let's start with a quick review of what we learned about ArrayLists from Chapter 5.
The java.util.ArrayList class is one of the most commonly used classes in the
Collections Framework. It's like an array on vitamins. Some of the advantages
ArrayList has over arrays are
■ It can grow dynamically.
■ It provides more powerful insertion and search mechanisms than arrays.

Let's take a look at using an ArrayList that contains strings. A key design goal
of the Collections Framework was to provide rich functionality at the level of the
main interfaces: List, Set, and Map. In practice, you'll typically want to instantiate
an ArrayList polymorphically, like this:
List myList = new ArrayList();

As of Java 5, you'll want to say
List myList = new ArrayList();

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This kind of declaration follows the object-oriented programming principle of
"coding to an interface," and it makes use of generics. We'll say lots more about
generics later in this chapter, but for now, just know that, as of Java 5, the 
syntax is the way that you declare a collection's type. (Prior to Java 5, there was no
way to specify the type of a collection, and when we cover generics, we'll talk about
the implications of mixing Java 5 [typed] and pre-Java 5 [untyped] collections.)

Why we're still talking about Java 5:
■ Understanding how collections worked before Java 5 makes generics easier to
understand now.
■ On the exam, and in the real world, you'll have to understand how code written
before Java 5 works and how such code interacts with more recent code.

In many ways, ArrayList is similar to a String[] in that it declares a
container that can hold only strings, but it's more powerful than a String[]. Let's
look at some of the capabilities that an ArrayList has:
List test = new ArrayList();
String s = "hi";
test.add("string");
test.add(s);
test.add(s+s);
System.out.println(test.size());
System.out.println(test.contains(42));
System.out.println(test.contains("hihi"));
test.remove("hi");
System.out.println(test.size());

// declare the ArrayList
// add some strings

// use ArrayList methods

which produces
3
false
true
2

There's a lot going on in this small program. Notice that when we declared the
ArrayList we didn't give it a size. Then we were able to ask the ArrayList for its
size, we were able to ask whether it contained specific objects, we removed an object
right out from the middle of it, and then we rechecked its size.

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Autoboxing with Collections
In general, collections 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. With Java 5, primitives still have to be wrapped, but autoboxing takes
care of it for you.
List myInts = new ArrayList();
myInts.add(new Integer(42));

// pre Java 5 declaration
// Use Integer to "wrap" an int

In the previous example, we create an instance of class Integer with a value of 42.
We've created an entire object to "wrap around" a primitive value. As of Java 5, we
can say:
myInts.add(42);

// autoboxing handles it!

In this last example, we are still adding an Integer object to myInts (not an int
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:
Integer y = new Integer(567);
int x = y.intValue();
x++;
y = new Integer(x);
System.out.println("y = " + y);

//
//
//
//
//

make it
unwrap it
use it
rewrap it
print it

Now, with new and improved Java 5, you can say
Integer y = new Integer(567);
y++;
System.out.println("y = " + y);

//
//
//
//

make it
unwrap it, increment it,
rewrap it
print it

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 from 567 to 568. What actually

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601

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:
Integer y = 567;
Integer x = y;

// make a wrapper
// assign a second ref
// var to THE wrapper

System.out.println(y==x);

//
//
//
//

y++;
System.out.println(x + " " + y);
System.out.println(y==x);

verify that they refer
to the same object
unwrap, use, "rewrap"
print values

// verify that they refer
// to different objects

Which produces the output:
true
567 568
false

So, under the covers, when the compiler got to the line y++; it had to substitute
something like this:
int x2 = y.intValue();
x2++;
y = new Integer(x2);

// unwrap it
// use it
// rewrap it

Just as we suspected, there's gotta be a call to new in there somewhere.

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
Integer i1 = 1000;
Integer i2 = 1000;
if(i1 != i2) System.out.println("different objects");
if(i1.equals(i2)) System.out.println("meaningfully equal");

produces the output
different objects
meaningfully equal

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It's just two wrapper objects that happen to have the same value. Because they
have the same int value, the equals() method considers them to be "meaningfully
equivalent," and therefore returns true. How about this one:
Integer i3 = 10;
Integer i4 = 10;
if(i3 == i4) System.out.println("same object");
if(i3.equals(i4)) System.out.println("meaningfully equal");

This example produces the output:
same object
meaningfully equal

Yikes! The equals() method seems to be working, but what happened with ==
and !=? Why is != telling us that i1 and i2 are different objects, when == is saying
that i3 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 is 127 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:
class UseBoxing {
public static void main(String [] args) {
UseBoxing u = new UseBoxing();
u.go(5);
}
boolean go(Integer i) {
// boxes the int it was passed
Boolean ifSo = true;
// boxes the literal
Short s = 300;
// boxes the primitive

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

if(ifSo) {
System.out.println(++s);
}
return !ifSo;

603

// unboxing
// unboxes, increments, reboxes
// unboxes, returns the inverse

}
}

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:
class Boxing2 {
static Integer x;
public static void main(String [] args) {
doStuff(x);
}
static void doStuff(int z) {
int z2 = 5;
System.out.println(z2 + z);
} }

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
In the OCA part of 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 in this book is the "diamond syntax." We've already seen
several examples of declaring type-safe collections, and as we go deeper into
collections, we'll see lots more like this:
ArrayList stuff = new ArrayList();
List myDogs = new ArrayList();
Map dogMap = new HashMap();

Notice that the type parameters are duplicated in these declarations. As of Java 7,
these declarations could be simplified to:
ArrayList stuff = new ArrayList<>();
List myDogs = new ArrayList<>();
Map dogMap = new HashMap<>();

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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:
List<> stuff = new ArrayList();

// NOT a legal diamond syntax

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!

Sorting Collections and Arrays
Sorting and searching topics were added to the exam as of Java 5. Both collections
and arrays can be sorted and searched using methods in the API.

Sorting Collections
Let's start with something simple, like sorting an ArrayList of strings
alphabetically. What could be easier? Okay, we'll wait while you go find ArrayList's
sort() method… got it? Of course, ArrayList doesn't give you any way to sort its
contents, but the java.util.Collections class does
import java.util.*;
class TestSort1 {
public static void main(String[] args) {
ArrayList stuff = new ArrayList(); // #1
stuff.add("Denver");
stuff.add("Boulder");
stuff.add("Vail");
stuff.add("Aspen");
stuff.add("Telluride");
System.out.println("unsorted " + stuff);
Collections.sort(stuff);
// #2
System.out.println("sorted
" + stuff);
}
}

This produces something like this:
unsorted [Denver, Boulder, Vail, Aspen, Telluride]
sorted
[Aspen, Boulder, Denver, Telluride, Vail]

Line 1 is declaring an ArrayList of Strings, and line 2 is sorting the ArrayList
alphabetically. We'll talk more about the Collections class, along with the Arrays
class, in a later section; for now, let's keep sorting stuff.

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605

Let's imagine we're building the ultimate home-automation application. Today
we're focused on the home entertainment center, and more specifically, the DVD
control center. We've already got the file I/O software in place to read and write data
between the dvdInfo.txt file and instances of class DVDInfo. Here are the key
aspects of the class:
class DVDInfo {
String title;
String genre;
String leadActor;
DVDInfo(String t, String g, String a) {
title = t; genre = g; leadActor = a;
}
public String toString() {
return title + " " + genre + " " + leadActor + "\n";
}
// getters and setter go here
}

Here's the DVD data that's in the dvdinfo.txt file:
Donnie Darko/sci-fi/Gyllenhall, Jake
Raiders of the Lost Ark/action/Ford, Harrison
2001/sci-fi/??
Caddyshack/comedy/Murray, Bill
Star Wars/sci-fi/Ford, Harrison
Lost in Translation/comedy/Murray, Bill
Patriot Games/action/Ford, Harrison

In our home-automation application, we want to create an instance of DVDInfo
for each line of data we read in from the dvdInfo.txt file. For each instance, we
will parse the line of data (remember String.split()?) and populate DVDInfo's
three instance variables. Finally, we want to put all of the DVDInfo instances into an
ArrayList. Imagine that the populateList() method (shown next) does all of this.
Here is a small piece of code from our application:
ArrayList dvdList = new ArrayList();
populateList();
// adds the file data to the ArrayList
System.out.println(dvdList);

You might get output like this:
[Donnie Darko sci-fi Gyllenhall, Jake
, Raiders of the Lost Ark action Ford, Harrison
, 2001 sci-fi ??
, Caddyshack comedy Murray, Bill
, Star Wars sci-fi Ford, Harrison
, Lost in Translation comedy Murray, Bill
, Patriot Games action Ford, Harrison
]

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(Note: We overrode DVDInfo's toString() method, so when we invoked
println() on the ArrayList, it invoked toString() for each instance.)
Now that we've got a populated ArrayList, let's sort it:
Collections.sort(dvdlist);

Oops! You get something like this:
TestDVD.java:13: cannot find symbol
symbol : method sort(java.util.ArrayList)
location: class java.util.Collections
Collections.sort(dvdlist);

What's going on here? We know that the Collections class has a sort() method,
yet this error implies that Collections does NOT have a sort() method that can
take a dvdlist. That means there must be something wrong with the argument we're
passing (dvdlist).
If you've already figured out the problem, our guess is that you did it without the
help of the obscure error message shown earlier… How the heck do you sort
instances of DVDInfo? Why were we able to sort instances of String? When you
look up Collections.sort() in the API, your first reaction might be to panic.
Hang tight—once again, the generics section will help you read that weird-looking
method signature. If you read the description of the one-arg sort() method, you'll
see that the sort() method takes a List argument, and that the objects in the
List must implement an interface called Comparable. It turns out that String
implements Comparable, and that's why we were able to sort a list of Strings using
the Collections.sort() method.

The Comparable Interface
The Comparable interface is used by the Collections.sort() method and the
java.util.Arrays.sort() method to sort Lists and arrays of objects, respectively.
To implement Comparable, a class must implement a single method, compareTo().
Here's an invocation of compareTo():
int x = thisObject.compareTo(anotherObject);

The compareTo() method returns an int with the following characteristics:
■ Negative
■ Zero

If thisObject < anotherObject

If thisObject == anotherObject

■ Positive

If thisObject > anotherObject

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607

The sort() method uses compareTo() to determine how the List or object
array should be sorted. Since you get to implement compareTo() for your own
classes, you can use whatever weird criteria you prefer to sort instances of your
classes. Returning to our earlier example for class DVDInfo, we can take the easy way
out and use the String class's implementation of compareTo():
class DVDInfo implements Comparable {
// existing code
public int compareTo(DVDInfo d) {
return title.compareTo(d.getTitle());
} }

// #1

// #2

In line 1, we declare that class DVDInfo implements Comparable in such a way
that DVDInfo objects can be compared to other DVDInfo objects. In line 2, we
implement compareTo() by comparing the two DVDInfo object's titles. Since we
know that the titles are strings and that String implements Comparable, this is an
easy way to sort our DVDInfo objects by title. Before generics came along in Java 5,
you would have had to implement Comparable using something like this:
class DVDInfo implements Comparable {
// existing code
public int compareTo(Object o) {
// takes an Object rather
// than a specific type
DVDInfo d = (DVDInfo)o;
return title.compareTo(d.getTitle());
} }

This is still legal, but you can see that it's both painful and risky because you have
to do a cast, and you need to verify that the cast will not fail before you try it.

It's important to remember that when you override equals(), you MUST
take an argument of type Object, but that when you override compareTo(), you should
take an argument of the type you're sorting.

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Putting it all together, our DVDInfo class should now look like this:
class DVDInfo implements Comparable {
String title;
String genre;
String leadActor;
DVDInfo(String t, String g, String a) {
title = t; genre = g; leadActor = a;
}
public String toString() {
return title + " " + genre + " " + leadActor + "\n";
}
public int compareTo(DVDInfo d) {
return title.compareTo(d.getTitle());
}
public String getTitle() {
return title;
}
// other getters and setters
}

Now, when we invoke Collections.sort(dvdList), we get
[2001 sci-fi ??
, Caddyshack comedy Murray, Bill
, Donnie Darko sci-fi Gyllenhall, Jake
, Lost in Translation comedy Murray, Bill
, Patriot Games action Ford, Harrison
, Raiders of the Lost Ark action Ford, Harrison
, Star Wars sci-fi Ford, Harrison
]

Hooray! Our ArrayList has been sorted by title. Of course, if we want our
home-automation system to really rock, we'll probably want to sort DVD collections
in lots of different ways. Since we sorted our ArrayList by implementing the
compareTo() method, we seem to be stuck. We can only implement compareTo()
once in a class, so how do we go about sorting our classes in an order different from
what we specify in our compareTo() method? Good question. As luck would have
it, the answer is coming up next.

Sorting with Comparator
While you were looking up the Collections.sort() method, you might have
noticed that there is an overloaded version of sort() that takes both a List AND
something called a Comparator. The Comparator interface gives you the capability
to sort a given collection any number of different ways. The other handy thing about
the Comparator interface is that you can use it to sort instances of any class—even

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609

classes you can't modify—unlike the Comparable interface, which forces you to
change the class whose instances you want to sort. The Comparator interface is also
very easy to implement, having only one method, compare(). Here's a small class
that can be used to sort a List of DVDInfo instances by genre:
import java.util.*;
class GenreSort implements Comparator {
public int compare(DVDInfo one, DVDInfo two) {
return one.getGenre().compareTo(two.getGenre());
}
}

The Comparator.compare() method returns an int whose meaning is the same
as the Comparable.compareTo() method's return value. In this case, we're taking
advantage of that by asking compareTo() to do the actual comparison work for us.
Here's a test program that lets us test both our Comparable code and our new
Comparator code:
import java.util.*;
import java.io.*;
// populateList() needs this
public class TestDVD {
ArrayList dvdlist = new ArrayList();
public static void main(String[] args) {
new TestDVD().go();
}
public void go() {
populateList();
System.out.println(dvdlist);
// output as read from file
Collections.sort(dvdlist);
System.out.println(dvdlist);
// output sorted by title
GenreSort gs = new GenreSort();
Collections.sort(dvdlist, gs);
System.out.println(dvdlist);

// output sorted by genre
}
public void populateList() {
// read the file, create DVDInfo instances, and
// populate the ArrayList dvdlist with these instances
}
}

You've already seen the first two output lists; here's the third:
[Patriot Games action Ford, Harrison
, Raiders of the Lost Ark action Ford, Harrison
, Caddyshack comedy Murray, Bill
, Lost in Translation comedy Murray, Bill
, 2001 sci-fi ??
, Donnie Darko sci-fi Gyllenhall, Jake
, Star Wars sci-fi Ford, Harrison
]

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Because the Comparable and Comparator interfaces are so similar, expect the
exam to try to confuse you. For instance, you might be asked to implement the
compareTo() method in the Comparator interface. Study Table 11-3 to burn into
your mind the differences between these two interfaces.

Sorting with the Arrays Class
We've been using the java.util.Collections class to sort collections; now
let's look at using the java.util.Arrays class to sort arrays. The good news
is that sorting arrays of objects is just like sorting collections of objects. The
Arrays.sort() method is overloaded in the same way the Collections.sort()
method is:
■ Arrays.sort(arrayToSort)
■ Arrays.sort(arrayToSort, Comparator)

In addition, the Arrays.sort() method (the one argument version), is
overloaded about a million times to provide a couple of sort methods for every type
of primitive. The Arrays.sort(myArray) methods that sort primitives always sort
based on natural order. Don't be fooled by an exam question that tries to sort a
primitive array using a Comparator.
Finally, remember that the sort() methods for both the Collections class and
the Arrays class are static methods, and that they alter the objects they are
sorting instead of returning a different sorted object.

TABLE 11-3

Comparing
Comparable to
Comparator

java.lang.Comparable

java.util.Comparator

int objOne.compareTo(objTwo)

int compare(objOne, objTwo)

Returns
negative if objOne < objTwo
zero
if objOne == objTwo
positive if objOne > objTwo

Same as Comparable

You must modify the class whose instances you
want to sort.

You build a class separate from the class
whose instances you want to sort.

Only one sort sequence can be created.

Many sort sequences can be created.

Implemented frequently in the API by:
String, Wrapper classes, Date, Calendar…

Meant to be implemented to sort
instances of third-party classes.

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We've talked a lot about sorting by natural order and using Comparators
to sort.The last rule you'll need to burn in your mind is that whenever you want to sort
an array or a collection, the elements inside must all be mutually comparable. In other
words, if you have an Object[] and you put Cat and Dog objects into it, you won't be
able to sort it. In general, objects of different types should be considered NOT mutually
comparable unless specifically stated otherwise.

Searching Arrays and Collections
The Collections class and the Arrays class both provide methods that allow you
to search for a specific element. When searching through collections or arrays, the
following rules apply:
■ Searches are performed using the binarySearch() method.
■ Successful searches return the int index of the element being searched.
■ Unsuccessful searches return an int index that represents the insertion

point. The insertion point is the place in the collection/array where the
element would be inserted to keep the collection/array properly sorted.
Because positive return values and 0 indicate successful searches, the
binarySearch() method uses negative numbers to indicate insertion
points. Since 0 is a valid result for a successful search, the first available
insertion point is -1. Therefore, the actual insertion point is represented as
(-(insertion point) -1). For instance, if the insertion point of a search is at
element 2, the actual insertion point returned will be -3.
■ The collection/array being searched must be sorted before you can search it.
■ If you attempt to search an array or collection that has not already been

sorted, the results of the search will not be predictable.
■ If the collection/array you want to search was sorted in natural order, it must

be searched in natural order. (Usually, this is accomplished by NOT sending a
Comparator as an argument to the binarySearch() method.)
■ If the collection/array you want to search was sorted using a Comparator, it

must be searched using the same Comparator, which is passed as the second
argument to the binarySearch() method. Remember that Comparators
cannot be used when searching arrays of primitives.

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Let's take a look at a code sample that exercises the binarySearch() method:
import java.util.*;
class SearchObjArray {
public static void main(String [] args) {
String [] sa = {"one", "two", "three", "four"};
Arrays.sort(sa);
for(String s : sa)
System.out.print(s + " ");
System.out.println("\none = "
+ Arrays.binarySearch(sa,"one"));
System.out.println("now reverse sort");
ReSortComparator rs = new ReSortComparator();
Arrays.sort(sa,rs);
for(String s : sa)
System.out.print(s + " ");
System.out.println("\none = "
+ Arrays.binarySearch(sa,"one"));
System.out.println("one = "
+ Arrays.binarySearch(sa,"one",rs));
}
static class ReSortComparator
implements Comparator {
public int compare(String a, String b) {
return b.compareTo(a);
}
}

// #1

// #2

// #3

// #4
// #5

// #6
// #7

}

which produces something like this:
four one three two
one = 1
now reverse sort
two three one four
one = -1
one = 2

Here's what happened:
■ #1

Sort the sa array, alphabetically (the natural order).

■ #2

Search for the location of element "one", which is 1.

■ #3

Make a Comparator instance. On the next line, we re-sort the array
using the Comparator.

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

613

■ #4

Attempt to search the array. We didn't pass the binarySearch()
method the Comparator we used to sort the array, so we got an incorrect
(undefined) answer.

■ #5

Search again, passing the Comparator to binarySearch(). This time,
we get the correct answer, 2.

■ #6

We define the Comparator; it's okay for this to be an inner class. (We'll
be discussing inner classes in Chapter 12.)

■ #7

By switching the use of the arguments in the invocation of compareTo(),
we get an inverted sort.

When solving, searching, and sorting questions, two big gotchas are
1. Searching an array or collection that hasn't been sorted.
2. Using a Comparator in either the sort or the search, but not both.

Converting Arrays to Lists to Arrays
A couple of methods allow you to convert arrays to Lists and Lists to arrays. The
List and Set classes have toArray() methods, and the Arrays class has a method
called asList().
The Arrays.asList() method copies an array into a List. The API says,
"Returns a fixed-size list backed by the specified array. (Changes to the returned list
'write through' to the array.)" When you use the asList() method, the array and
the List become joined at the hip. When you update one of them, the other is
updated automatically. Let's take a look:
String[] sa = {"one", "two", "three", "four"};
List sList = Arrays.asList(sa);
// make a List
System.out.println("size " + sList.size());
System.out.println("idx2 " + sList.get(2));
sList.set(3,"six");
// change List
sa[1] = "five";
// change array
for(String s : sa)
System.out.print(s + " ");
System.out.println("\nsl[1] " + sList.get(1));

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This produces
size 4
idx2 three
one five three six
sl[1] five

Notice that when we print the final state of the array and the List, they have
both been updated with each other's changes. Wouldn't something like this
behavior make a great exam question?
Now let's take a look at the toArray() method. There's nothing too fancy going
on with the toArray() method; it comes in two flavors: one that returns a new
Object array, and one that uses the array you send it as the destination array:
List iL = new ArrayList();
for(int x=0; x<3; x++)
iL.add(x);
Object[] oa = iL.toArray();
// create an Object array
Integer[] ia2 = new Integer[3];
ia2 = iL.toArray(ia2);
// create an Integer array

Using Lists
Remember that Lists are usually used to keep things in some kind of order. You can
use a LinkedList to create a first-in, first-out queue. You can use an ArrayList to
keep track of what locations were visited and in what order. Notice that in both of
these examples, it's perfectly reasonable to assume that duplicates might occur. In
addition, Lists allow you to manually override the ordering of elements by adding
or removing elements via the element's index. Before Java 5 and the enhanced for
loop, the most common way to examine a List "element by element" was through
the use of an Iterator. You'll still find Iterators in use in the Java code you
encounter, and you might just find an Iterator or two on the exam. An Iterator is
an object that's associated with a specific collection. It lets you loop through the
collection step by step. The two Iterator methods you need to understand for the
exam are
■ boolean hasNext()

Returns true if there is at least one more element in
the collection being traversed. Invoking hasNext() does NOT move you to
the next element of the collection.

■ Object next()

This method returns the next object in the collection
AND moves you forward to the element after the element just returned.

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

615

Let's look at a little code that uses a List and an Iterator:
import java.util.*;
class Dog {
public String name;
Dog(String n) { name = n; }
}
class ItTest {
public static void main(String[] args) {
List d = new ArrayList();
Dog dog = new Dog("aiko");
d.add(dog);
d.add(new Dog("clover"));
d.add(new Dog("magnolia"));
Iterator i3 = d.iterator(); // make an iterator
while (i3.hasNext()) {
Dog d2 = i3.next();
// cast not required
System.out.println(d2.name);
}
System.out.println("size " + d.size());
System.out.println("get1 " + d.get(1).name);
System.out.println("aiko " + d.indexOf(dog));
d.remove(2);
Object[] oa = d.toArray();
for(Object o : oa) {
Dog d2 = (Dog)o;
System.out.println("oa " + d2.name);
}
}
}

This produces
aiko
clover
magnolia
size 3
get1 clover
aiko 0
oa aiko
oa clover

First off, we used generics syntax to create the Iterator (an Iterator of type Dog).
Because of this, when we used the next() method, we didn't have to cast the Object
returned by next() to a Dog. We could have declared the Iterator like this:
Iterator i3 = d.iterator();

// make an iterator

But then we would have had to cast the returned value:
Dog d2 = (Dog)i3.next();

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The rest of the code demonstrates using the size(), get(), indexOf(), and
toArray() methods. There shouldn't be any surprises with these methods. Later in
the chapter, Table 11-7 will list all of the List, Set, and Map methods you should be
familiar with for the exam. As a last warning, remember that List is an interface!

Using Sets
Remember that Sets are used when you don't want any duplicates in your
collection. If you attempt to add an element to a set that already exists in the set,
the duplicate element will not be added, and the add() method will return false.
Remember, HashSets tend to be very fast because, as we discussed earlier, they use
hashcodes.
You can also create a TreeSet, which is a Set whose elements are sorted. You
must use caution when using a TreeSet (we're about to explain why):
import java.util.*;
class SetTest {
public static void main(String[] args) {
boolean[] ba = new boolean[5];
// insert code here
ba[0] = s.add("a");
ba[1] = s.add(new Integer(42));
ba[2] = s.add("b");
ba[3] = s.add("a");
ba[4] = s.add(new Object());
for(int x=0; x m = new HashMap();
m.put("k1", new Dog("aiko"));
m.put("k2", Pets.DOG);

// add some key/value pairs

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m.put(Pets.CAT, "CAT key");
Dog d1 = new Dog("clover");
m.put(d1, "Dog key");
m.put(new Cat(), "Cat key");

// let's keep this reference

System.out.println(m.get("k1"));
String k2 = "k2";
System.out.println(m.get(k2));
Pets p = Pets.CAT;
System.out.println(m.get(p));
System.out.println(m.get(d1));
System.out.println(m.get(new Cat()));
System.out.println(m.size());

// #1
// #2
//
//
//
//

#3
#4
#5
#6

}
}

which produces something like this:
Dog@1c
DOG
CAT key
Dog key
null
5

Let's review the output. The first value retrieved is a Dog object (your value will
vary). The second value retrieved is an enum value (DOG). The third value retrieved
is a String; note that the key was an enum value. Pop quiz: What's the implication
of the fact that we were able to successfully use an enum as a key?
The implication of this is that enums override equals() and hashCode(). And,
if you look at the java.lang.Enum class in the API, you will see that, in fact, these
methods have been overridden.
The fourth output is a String. The important point about this output is that the
key used to retrieve the String was made of a Dog object. The fifth output is null.
The important point here is that the get() method failed to find the Cat object
that was inserted earlier. (The last line of output confirms that, indeed, 5 key/value
pairs exist in the Map.) Why didn't we find the Cat key String? Why did it work to
use an instance of Dog as a key, when using an instance of Cat as a key failed?
It's easy to see that Dog overrode equals() and hashCode() while Cat didn't.
Let's take a quick look at hashcodes. We used an incredibly simplistic hashcode
formula in the Dog class—the hashcode of a Dog object is the length of the instance's
name. So in this example, the hashcode = 6. Let's compare the following two
hashCode() methods:
public int hashCode() {return name.length(); }
public int hashCode() {return 4; }

// #1
// #2

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

619

Time for another pop quiz: Are the preceding two hashcodes legal? Will they
successfully retrieve objects from a Map? Which will be faster?
The answer to the first two questions is Yes and Yes. Neither of these hashcodes
will be very efficient (in fact, they would both be incredibly inefficient), but they are
both legal, and they will both work. The answer to the last question is that the first
hashcode will be a little bit faster than the second hashcode. In general, the more
unique hashcodes a formula creates, the faster the retrieval will be. The first hashcode
formula will generate a different code for each name length (for instance, the name
Robert will generate one hashcode and the name Benchley will generate a different
hashcode). The second hashcode formula will always produce the same result, 4, so
it will be slower than the first.
Our last Map topic is what happens when an object used as a key has its values
changed? If we add two lines of code to the end of the earlier MapTest.main(),
d1.name = "magnolia";
System.out.println(m.get(d1));

we get something like this:
Dog@4
DOG
CAT key
Dog key
null
5
null

The Dog that was previously found now cannot be found. Because the Dog.name
variable is used to create the hashcode, changing the name changed the value of the
hashcode. As a final quiz for hashcodes, determine the output for the following lines
of code if they're added to the end of MapTest.main():
d1.name = "magnolia";
System.out.println(m.get(d1));
d1.name = "clover";
System.out.println(m.get(new Dog("clover")));
d1.name = "arthur";
System.out.println(m.get(new Dog("clover")));

// #1
// #2
// #3

Remember that the hashcode is equal to the length of the name variable. When
you study a problem like this, it can be useful to think of the two stages of retrieval:
1. Use the hashCode() method to find the correct bucket.
2. Use the equals() method to find the object in the bucket.

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In the first call to get(), the hashcode is 8 (magnolia) and it should be 6
(clover), so the retrieval fails at step 1 and we get null. In the second call to
get(), the hashcodes are both 6, so step 1 succeeds. Once in the correct bucket (the
"length of name = 6" bucket), the equals() method is invoked, and since Dog's
equals() method compares names, equals() succeeds, and the output is Dog key.
In the third invocation of get(), the hashcode test succeeds, but the equals() test
fails because arthur is NOT equal to clover.

Navigating (Searching)
TreeSets and TreeMaps
Note: This section and the next ("Backed Collections") are fairly complex, and there
is a good chance that OCP 7 candidates will NOT get any questions on these topics.
On the other hand, OCPJP 6 candidates are likely to be tested on these topics.
We've talked about searching lists and arrays. Let's turn our attention to
searching TreeSets and TreeMaps. Java 6 introduced (among other things) two new
interfaces: java.util.NavigableSet and java.util.NavigableMap. For the
purposes of the exam, you're interested in how TreeSet and TreeMap implement
these interfaces.
Imagine that the Santa Cruz–Monterey ferry has an irregular schedule. Let's say
that we have the daily Santa Cruz departure times stored in military time in a
TreeSet. Let's look at some code that determines two things:
1. The last ferry that leaves before 4 pm (1600 hours)
2. The first ferry that leaves after 8 pm (2000 hours)
import java.util.*;
public class Ferry {
public static void main(String[] args) {
TreeSet times = new TreeSet();
times.add(1205);
// add some departure times
times.add(1505);
times.add(1545);
times.add(1830);
times.add(2010);
times.add(2100);
// Java 5 version
TreeSet subset = new TreeSet();
subset = (TreeSet)times.headSet(1600);
System.out.println("J5 - last before 4pm is: " + subset.last());
TreeSet sub2 = new TreeSet();

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

621

sub2 = (TreeSet)times.tailSet(2000);
System.out.println("J5 - first after 8pm is: " + sub2.first());
// Java 6 version using the new lower() and higher() methods
System.out.println("J6 - last before 4pm is: " + times.lower(1600));
System.out.println("J6 - first after 8pm is: " + times.higher(2000));
}
}

This should produce the following:
J5
J5
J6
J6

-

last before
first after
last before
first after

4pm
8pm
4pm
8pm

is:
is:
is:
is:

1545
2010
1545
2010

As you can see in the preceding code, before the addition of the NavigableSet
interface, zeroing in on an arbitrary spot in a Set—using the methods available in
Java 5—was a compute-expensive and clunky proposition. On the other hand, using
the new Java 6 methods lower() and higher(), the code becomes a lot cleaner.
For the purpose of the exam, the NavigableSet methods related to this type of
navigation are lower(), floor(), higher(), and ceiling(), and the mostly
parallel NavigableMap methods are lowerKey(), floorKey(), ceilingKey(), and
higherKey(). The difference between lower() and floor() is that lower()
returns the element less than the given element, and floor() returns the element
less than or equal to the given element. Similarly, higher() returns the element
greater than the given element, and ceiling() returns the element greater than or
equal to the given element. Table 11-4 summarizes the methods you should know for
the exam.

Other Navigation Methods
In addition to the methods we just discussed, there are a few more new Java 6
methods that could be considered "navigation" methods. (Okay, it's a little bit of a
stretch to call these "navigation" methods, but just play along.)

Polling
Although the idea of polling isn't new to Java 6 (as you'll see in a minute,
PriorityQueue had a poll() method before Java 6), it is new to TreeSet and
TreeMap. The idea of polling is that we want both to retrieve and remove an element
from either the beginning or the end of a collection. In the case of TreeSet,
pollFirst() returns and removes the first entry in the set, and pollLast()

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returns and removes the last. Similarly, TreeMap now provides pollFirstEntry()
and pollLastEntry() to retrieve and remove key/value pairs.

Descending Order
Also new to Java 6 for TreeSet and TreeMap are methods that return a
collection in the reverse order of the collection on which the method was
invoked. The important methods for the exam are TreeSet.descendingSet()
and TreeMap.descendingMap().
Table 11-4 summarizes the "navigation" methods you'll need to know for the exam.

Backed Collections
Some of the classes in the java.util package support the concept of "backed
collections." We'll use a little code to help explain the idea:
TreeMap map = new TreeMap();
map.put("a", "ant"); map.put("d", "dog"); map.put("h", "horse");
SortedMap submap;
submap = map.subMap("b", "g");

// #1 create a backed collection

System.out.println(map + " " + submap);

// #2 show contents

map.put("b", "bat");
submap.put("f", "fish");

// #3 add to original
// #4 add to copy

map.put("r", "raccoon");
// submap.put("p", "pig");

// #5 add to original - out of range
// #6 add to copy - out of range

System.out.println(map + " " + submap);

// #7 show final contents

This should produce something like this:
{a=ant, d=dog, h=horse} {d=dog}
{a=ant, b=bat, d=dog, f=fish, h=horse, r=raccoon} {b=bat, d=dog, f=fish}

The important method in this code is the TreeMap.subMap() method. It's easy
to guess (and it's correct) that the subMap() method is making a copy of a portion
of the TreeMap named map. The first line of output verifies the conclusions we've
just drawn.
What happens next is powerful, and a little bit unexpected (now we're getting to
why they're called backed collections). When we add key/value pairs to either the
original TreeMap or the partial-copy SortedMap, the new entries were automatically
added to the other collection—sometimes. When submap was created, we provided

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

TABLE 11-4

Important
“Navigation”Related Methods

Method

Description

TreeSet.ceiling(e)

Returns the lowest element >= e

TreeMap.ceilingKey(key)

Returns the lowest key >= key

TreeSet.higher(e)

Returns the lowest element > e

TreeMap.higherKey(key)

Returns the lowest key > key

TreeSet.floor(e)

Returns the highest element <= e

TreeMap.floorKey(key)

Returns the highest key <= key

TreeSet.lower(e)

Returns the highest element < e

TreeMap.lowerKey(key)

Returns the highest key < key

TreeSet.pollFirst()

Returns and removes the first entry

TreeMap.pollFirstEntry()

Returns and removes the first key/value pair

TreeSet.pollLast()

Returns and removes the last entry

TreeMap.pollLastEntry()

Returns and removes the last key/value pair

TreeSet.descendingSet()

Returns a NavigableSet in reverse order

TreeMap.descendingMap()

Returns a NavigableMap in reverse order

623

a value range for the new collection. This range defines not only what should be
included when the partial copy is created, but also defines the range of values that
can be added to the copy. As we can verify by looking at the second line of output,
we can add new entries to either collection within the range of the copy, and the
new entries will show up in both collections. In addition, we can add a new entry to
the original collection, even if it's outside the range of the copy. In this case, the
new entry will show up only in the original—it won't be added to the copy because
it's outside the copy's range. Notice that we commented out line 6. If you attempt to
add an out-of-range entry to the copied collection, an exception will be thrown.
For the exam, you'll need to understand the basics just explained, plus a few more
details about three methods from TreeSet—headSet(), subSet(), and tailSet()—
and three methods from TreeMap—headMap(), subMap(), and tailMap(). As
with the navigation-oriented methods we just discussed, we can see a lot of parallels
between the TreeSet and the TreeMap methods. The headSet()/headMap()
methods create a subset that starts at the beginning of the original collection and
ends at the point specified by the method's argument. The tailSet()/tailMap()
methods create a subset that starts at the point specified by the method's argument
and goes to the end of the original collection. Finally, the subSet()/subMap()

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methods allow you to specify both the start and end points for the subset collection
you're creating.
As you might expect, the question of whether the subsetted collection's end
points are inclusive or exclusive is a little tricky. The good news is that for the exam
you have to remember only that when these methods are invoked with end point
and boolean arguments, the boolean always means "is inclusive". A little more good
news is that all you have to know for the exam is that, unless specifically indicated
by a boolean argument, a subset's starting point will always be inclusive. Finally,
you'll notice when you study the API that all of the methods we've been discussing
here have an overloaded version that's new to Java 6. The older methods return
either a SortedSet or a SortedMap; the new Java 6 methods return either a
NavigableSet or a NavigableMap. Table 11-5 summarizes these methods.

Let’s say that you’ve created a backed collection using either a
tailXxx() or subXxx() method. Typically in these cases, the original and copy collections
have different “first” elements. For the exam, it’s important that you remember that the
pollFirstXxx() methods will always remove the first entry from the collection on which
they’re invoked, but they will remove an element from the other collection only if it has
the same value. So it’s most likely that invoking pollFirstXxx() on the copy will remove
an entry from both collections, but invoking pollFirstXxx() on the original will remove
only the entry from the original.

TABLE 11-5

Important
“Backed
Collection”
Methods for
TreeSet and

Method

Description

headSet(e, b*)

Returns a subset ending at element e and exclusive of e

headMap(k, b*)

Returns a submap ending at key k and exclusive of key k

tailSet(e, b*)

Returns a subset starting at and inclusive of element e

tailMap(k, b*)

Returns a submap starting at and inclusive of key k

subSet(s, b*, e, b*)

Returns a subset starting at element s and ending just
before element e

subMap(s, b*, e, b*)

Returns a submap starting at key s and ending just
before key e

TreeMap

* Note: These boolean arguments are optional. If they exist, it’s a Java 6 method that lets you specify whether the
start point and/or end point are exclusive, and these methods return a NavigableXxx. If the boolean argument(s)
don’t exist, the method returns either a SortedSet or a SortedMap.

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

625

Using the PriorityQueue Class and the Deque Interface
Note: Having completed the Navigable Collections and Backed Collections
discussions, we’re now back to topics that all candidates (OCPJP 5 and 6 and
OCP 7), are likely to be tested on.
For the exam, you'll need to understand several of the classes that implement
the Deque interface. These classes will be discussed in Chapter 14, the
concurrency chapter.
Other than those concurrency-related classes, the last collection class you'll need
to understand for the exam is the PriorityQueue. Unlike basic queue structures
that are first-in, first-out by default, a PriorityQueue orders its elements using a
user-defined priority. The priority can be as simple as natural ordering (in which, for
instance, an entry of 1 would be a higher priority than an entry of 2). In addition, a
PriorityQueue can be ordered using a Comparator, which lets you define any
ordering you want. Queues have a few methods not found in other collection
interfaces: peek(), poll(), and offer().
import java.util.*;
class PQ {
static class PQsort
implements Comparator {
public int compare(Integer one, Integer
return two - one;
}
}
public static void main(String[] args) {
int[] ia = {1,5,3,7,6,9,8 };
PriorityQueue pq1 =
new PriorityQueue();

// inverse sort
two) {
// unboxing

// unordered data
// use natural order

for(int x : ia)
pq1.offer(x);
for(int x : ia)
System.out.print(pq1.poll() + " ");
System.out.println("");

// load queue

PQsort pqs = new PQsort();
PriorityQueue pq2 =
new PriorityQueue(10,pqs);

// get a Comparator

// review queue

// use Comparator

for(int x : ia)
// load queue
pq2.offer(x);
System.out.println("size " + pq2.size());
System.out.println("peek " + pq2.peek());
System.out.println("size " + pq2.size());
System.out.println("poll " + pq2.poll());
System.out.println("size " + pq2.size());
for(int x : ia)
// review queue
System.out.print(pq2.poll() + " ");
}
}

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This code produces something like this:
1 3 5 6 7 8 9
size 7
peek 9
size 7
poll 9
size 6
8 7 6 5 3 1 null

Let's look at this in detail. The first for loop iterates through the ia array and
uses the offer() method to add elements to the PriorityQueue named pq1. The
second for loop iterates through pq1 using the poll() method, which returns the
highest-priority entry in pq1 AND removes the entry from the queue. Notice that
the elements are returned in priority order (in this case, natural order). Next, we
create a Comparator—in this case, a Comparator that orders elements in the
opposite of natural order. We use this Comparator to build a second PriorityQueue,
pq2, and we load it with the same array we used earlier. Finally, we check the size of
pq2 before and after calls to peek() and poll(). This confirms that peek() returns
the highest-priority element in the queue without removing it, and poll() returns
the highest-priority element AND removes it from the queue. Finally, we review the
remaining elements in the queue.

Method Overview for Arrays and Collections
For these two classes, we've already covered the trickier methods you might encounter
on the exam. Table 11-6 lists a summary of the methods you should be aware of.
(Note: The T[] syntax will be explained later in this chapter; for now, think of it as
meaning "any array that's NOT an array of primitives.")

Method Overview for List, Set, Map, and Queue
For these four interfaces, we’ve already covered the trickier methods you might
encounter on the exam. Table 11-7 lists a summary of the List, Set, and Map
methods you should be aware of, and—if you’re an OCPJP 6 candidate—don’t
forget the new “Navigable” methods floor, lower, ceiling, and higher that we
discussed a few pages back.

Using Collections (OCP Objectives 4.2, 4.4, 4.5, 4.6, 4.7, and 4.8)

TABLE 11-6

627

Key Methods in Arrays and Collections

Key Methods in java.util.Arrays

Descriptions

static List asList(T[])

Convert an array to a List
(and bind them).

static int binarySearch(Object[], key)
static int binarySearch(primitive[], key)

Search a sorted array for a given value;
return an index or insertion point.

static int binarySearch(T[], key, Comparator)

Search a Comparator-sorted array for
a value.

static boolean equals(Object[], Object[])
static boolean equals(primitive[], primitive[])

Compare two arrays to determine if their
contents are equal.

static void sort(Object[ ] )
static void sort(primitive[ ] )

Sort the elements of an array by natural
order.

static void sort(T[], Comparator)

Sort the elements of an array using
a Comparator.

static String toString(Object[])
static String toString(primitive[])

Create a String containing the contents
of an array.

Key Methods in java.util.Collections

Descriptions

static int binarySearch(List, key)

Search a "sorted" List for a given value;
return an index or insertion point.

static int binarySearch(List, key, Comparator)
static void reverse(List)

Reverse the order of elements in a List.

static Comparator reverseOrder()
static Comparator reverseOrder(Comparator)

Return a Comparator that sorts the reverse
of the collection's current sort sequence.

static void sort(List)
static void sort(List, Comparator)

Sort a List either by natural order or by
a Comparator.

For the exam, the PriorityQueue methods that are important to understand are
offer() (which is similar to add()), peek() (which retrieves the element at the
head of the queue but doesn’t delete it), and poll() (which retrieves the head
element and removes it from the queue).

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TABLE 11-7

Generics and Collections

Key Methods in List, Set, and Map

Key Interface Methods

List

Set

boolean add(element)
boolean add(index, element)

X
X

X

boolean contains(object)
boolean containsKey(object key)
boolean containsValue(object value)

X

X

object get(index)
object get(key)

X

int indexOf(object)

X

Iterator iterator()

X

Map Descriptions
Add an element. For Lists, optionally
add the element at an index point.
X
X

Search a collection for an object (or,
optionally for Maps, a key); return the
result as a boolean.

X

Get an object from a collection via an
index or a key.
Get the location of an object in a List.

X

Get an Iterator for a List or a Set.

Set keySet()

X

Return a Set containing a Map's keys.

put(key, value)

X

Add a key/value pair to a Map.

element remove(index)
element remove(object)
element remove(key)

X
X

int size()

X

X

Object[] toArray()
T[] toArray(T[])

X

X

Remove an element via an index, or via
the element's value, or via a key.

X
X
X

Return the number of elements in a
collection.
Return an array containing the
elements of the collection.

It's important to know some of the details of natural ordering.The
following code will help you understand the relative positions of uppercase characters,
lowercase characters, and spaces in a natural ordering:
String[] sa = {">ff<", "> f<", ">f <", ">FF<" }; // ordered?
PriorityQueue pq3 = new PriorityQueue();
for(String s : sa)
pq3.offer(s);
for(String s : sa)
System.out.print(pq3.poll() + " ");

This produces
> f< >FF< >f < >ff<

If you remember that spaces sort before characters and that uppercase letters sort
before lowercase characters, you should be good to go for the exam.

Generic Types (OCP Objectives 4.1 and 4.3)

629

CERTIFICATION OBJECTIVE

Generic Types (OCP Objectives 4.1 and 4.3)
4.1

Create a generic class.

4.3

Analyze the interoperability of collections that use raw and generic types.

Now would be a great time to take a break. Those two innocent-sounding
objectives unpack into a world of complexity. When you're well rested, come on
back and strap yourself in—the next several pages might get bumpy.
Arrays in Java have always been type-safe—an array declared as type String
(String []) can't accept Integers (or ints), Dogs, or anything other than
Strings. But remember that before Java 5 there was no syntax for declaring a
type-safe collection. To make an ArrayList of Strings, you said,
ArrayList myList = new ArrayList();

or the polymorphic equivalent
List myList = new ArrayList();

There was no syntax that let you specify that myList will take Strings and only
Strings. And with no way to specify a type for the ArrayList, the compiler couldn't
enforce that you put only things of the specified type into the list. As of Java 5, we
can use generics, and while they aren't only for making type-safe collections, that's
just about all most developers use generics for. So, while generics aren't just for
collections, think of collections as the overwhelming reason and motivation for
adding generics to the language.
And it was not an easy decision, nor has it been an entirely welcome addition.
Because along with all the nice, happy type-safety, generics come with a lot of
baggage—most of which you'll never see or care about—but there are some gotchas
that come up surprisingly quickly. We'll cover the ones most likely to show up in
your own code, and those are also the issues that you'll need to know for the exam.
The biggest challenge for the Java engineers in adding generics to the language
(and the main reason it took them so long) was how to deal with legacy code built
without generics. The Java engineers obviously didn't want to break everyone's
existing Java code, so they had to find a way for Java classes with both type-safe
(generic) and nontype-safe (nongeneric/pre–Java 5) collections to still work

630

Chapter 11:

Generics and Collections

together. Their solution isn't the friendliest, but it does let you use older nongeneric
code, as well as use generic code that plays with nongeneric code. But notice we said
"plays" and not "plays WELL."
While you can integrate Java 5 and later generic code with legacy, nongeneric
code, the consequences can be disastrous, and unfortunately, most of the disasters
happen at runtime, not compile time. Fortunately, though, most compilers will
generate warnings to tell you when you're using unsafe (meaning nongeneric)
collections.
The Java 7 exam covers both pre–Java 5 (nongeneric) and generic-style collections,
and you'll see questions that expect you to understand the tricky problems that can
come from mixing nongeneric and generic code together. And like some of the other
topics in this book, you could fill an entire book if you really wanted to cover every
detail about generics, but the exam (and this book) covers more than most
developers will ever need to use.

The Legacy Way to Do Collections
Here's a review of a pre–Java 5 ArrayList intended to hold Strings. (We say
"intended" because that's about all you had—good intentions—to make sure that
the ArrayList would hold only Strings.)
List myList = new ArrayList();

// can't declare a type

myList.add("Fred");

// OK, it will hold Strings

myList.add(new Dog());

// and it will hold Dogs too

myList.add(new Integer(42));

// and Integers...

A nongeneric collection can hold any kind of object! A nongeneric collection is
quite happy to hold anything that is NOT a primitive.
This meant it was entirely up to the programmer to be… careful. Having no way
to guarantee collection type wasn't very programmer-friendly for such a strongly
typed language. We're so used to the compiler stopping us from, say, assigning an int
to a boolean or a String to a Dog reference, but with collections, it was, "Come on
in! The door is always open! All objects are welcome here any time!"
And since a collection could hold anything, the methods that get objects out of the
collection could have only one kind of return type—java.lang.Object. That meant
that getting a String back out of our only-Strings-intended list required a cast:
String s = (String) myList.get(0);

Generic Types (OCP Objectives 4.1 and 4.3)

631

And since you couldn't guarantee that what was coming out really was a String
(since you were allowed to put anything in the list), the cast could fail at runtime.
So generics takes care of both ends (the putting in and getting out) by enforcing
the type of your collections. Let's update the String list:
List myList = new ArrayList();
myList.add("Fred");
// OK, it will hold Strings
myList.add(new Dog());
// compiler error!!

Perfect. That's exactly what we want. By using generics syntax—which means
putting the type in angle brackets —we're telling the compiler that this
collection can hold only String objects. The type in angle brackets is referred to as
the "parameterized type," "type parameter," or, of course, just old-fashioned "type." In
this chapter, we'll refer to it both new ways.
So now that what you put IN is guaranteed, you can also guarantee what comes
OUT, and that means you can get rid of the cast when you get something from the
collection. Instead of
String s = (String)myList.get(0);

// pre-generics, when a
// String wasn't guaranteed

we can now just say
String s = myList.get(0);

The compiler already knows that myList contains only things that can be
assigned to a String reference, so now there's no need for a cast. So far, it seems
pretty simple. And with the new for loop, you can, of course, iterate over the
guaranteed-to-be-String list:
for (String s : myList) {
int x = s.length();
// no need for a cast before calling a String method! The
// compiler already knew "s" was a String coming from myList
}

And, of course, you can declare a type parameter for a method argument, which
then makes the argument a type-safe reference:
void takeListOfStrings(List strings) {
strings.add("foo"); // no problem adding a String
}

The previous method would NOT compile if we changed it to
void takeListOfStrings(List strings) {
strings.add(new Integer(42)); // NO!! strings is type safe
}

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Chapter 11:

Generics and Collections

Return types can obviously be declared type-safe as well:
public List getDogList() {
List dogs = new ArrayList();
// more code to insert dogs
return dogs;
}

The compiler will stop you from returning anything not compatible with a
List (although what is and is not compatible is going to get very interesting
in a minute). And since the compiler guarantees that only a type-safe Dog List is
returned, those calling the method won't need a cast to take Dogs from the List:
Dog d = getDogList().get(0);

// we KNOW a Dog is coming out

With pre–Java 5 nongeneric code, the getDogList() method would be
public List getDogList() {
List dogs = new ArrayList();
// code to add only Dogs... fingers crossed...
return dogs; // a List of ANYTHING will work here
}

and the caller would need a cast:
Dog d = (Dog) getDogList().get(0);

(The cast in this example applies to what comes from the List's get() method;
we aren't casting what is returned from the getDogList() method, which is a List.)
But what about the benefit of a completely heterogeneous collection? In other
words, what if you liked the fact that before generics you could make an ArrayList
that could hold any kind of object?
List myList = new ArrayList();

// old-style, non-generic

is almost identical to
List
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