Praise for the First Edition
“I sure wish I had this book ten years ago. Some might think that I don’t need
any Java books, but I need this one.”
—James Gosling, fellow and vice president, Sun Microsystems, Inc., and
inventor of the Java programming language
“An excellent book, crammed with good advice on using the Java program-
ming language and object-oriented programming in general.”
—Gilad Bracha, distinguished engineer, Cadence Design Systems,
and coauthor of The Java™ Language Specification,
Third Edition (Addison-Wesley, 2005)
“10/10—anyone aspiring to write good Java code that others will appreciate
reading and maintaining should be required to own a copy of this book. This
is one of those rare books where the information won’t become obsolete with
subsequent releases of the JDK library.”
—Peter Tran, bartender, JavaRanch.com
“The best Java book yet written Really great; very readable and eminently
useful. I can’t say enough good things about this book. At JavaOne 2001,
James Gosling said, ‘Go buy this book!’ I’m glad I did, and I couldn’t agree
more.”
—Keith Edwards, senior member of research staff,
Computer Science Lab at the Palo Alto Research Center (PARC),
and author of Core JINI (Prentice Hall, 2000)
“This is a truly excellent book done by the guy who designed several of the
better recent Java platform APIs (including the Collections API).”
—James Clark, technical lead of the XML Working Group
during the creation of the XML 1.0 Recommendation;
editor of the XPath and XSLT Recommendations
“Great content. Analogous to Scott Meyers’s classic Effective C++. If you know
the basics of Java, this has to be your next book.”
—Gary K. Evans, OO mentor and consultant, Evanetics, Inc.

Joshua Bloch
Effective Java,™ Second Edition
Stephanie Bodoff, Dale Green, Kim Haase, Eric Jendrock
The J2EE™ Tutorial, Second Edition
Mary Campione, Kathy Walrath, Alison Huml
The Java™ Tutorial, Third Edition: A Short Course on
the Basics
Mary Campione, Kathy Walrath, Alison Huml, The
Tutorial Team
The Java™ Tutorial Continued: The Rest of the JDK™
Patrick Chan
The Java™ Developers Almanac 1.4, Volume 1
Patrick Chan
The Java™ Developers Almanac 1.4, Volume 2
Patrick Chan, Rosanna Lee
The Java™ Class Libraries, Second Edition, Volume 2:
java.applet, java.awt, java.beans
Patrick Chan, Rosanna Lee, Doug Kramer
The Java
™
Class Libraries, Second Edition, Volume 1:
Supplement for the Java™ 2 Platform, Standard Edition,
v1.2
Kirk Chen, Li Gong
Programming Open Service Gateways with Java™
Embedded Server
Zhiqun Chen
Java Card™ Technology for Smart Cards: Architecture
and Programmer’s Guide
Maydene Fisher, Jon Ellis, Jonathan Bruce

Rosanna Lee, Scott Seligman
JNDI API Tutorial and Reference: Building Directory-
Enabled Java™ Applications
Sheng Liang
The Java™ Native Interface: Programmer’s Guide and
Specification
Tim Lindholm, Frank Yellin
The Java™ Virtual Machine Specification, Second Edition
Roger Riggs, Antero Taivalsaari, Jim Van Peursem, Jyri
Huopaniemi, Mark Patel, Aleksi Uotila
Programming Wireless Devices with the Java™ 2
Platform, Micro Edition, Second Edition
Rahul Sharma, Beth Stearns, Tony Ng
J2EE™ Connector Architecture and Enterprise
Application Integration
Inderjeet Singh, Beth Stearns, Mark Johnson, Enterprise
Team
Designing Enterprise Applications with the J2EE™
Platform, Second Edition
Inderjeet Singh, Sean Brydon, Greg Murray, Vijay
Ramachandran, Thierry Violleau, Beth Stearns
Designing Web Services with the J2EE™ 1.4 Platform:
JAX-RPC, SOAP, and XML Technologies
Kathy Walrath, Mary Campione, Alison Huml, Sharon
Zakhour
The JFC Swing Tutorial, Second Edition: A Guide to
Constructing GUIs
Steve Wilson, Jeff Kesselman
Java™ Platform Performance: Strategies and Tactics
Sharon Zakhour, Scott Hommel, Jacob Royal,

in connection with or arising out of the use of the information or programs contained herein.
The publisher offers excellent discounts on this book when ordered in quantity for bulk purchases or special sales, which
may include electronic versions and/or custom covers and content particular to your business, training goals, marketing
focus, and branding interests. For more information, please contact:
U.S. Corporate and Government Sales
(800) 382-3419

For sales outside the United States please contact:
International Sales

Visit us on the Web: informit.com/aw
Library of Congress Control Number: 2008926278
Copyright © 2008 Sun Microsystems, Inc.
4150 Network Circle,
Santa Clara, California 95054 U.S.A.
All rights reserved. Printed in the United States of America. This publication is protected by copyright, and permis-
sion must be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or trans-
mission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information
regarding permissions, write to:
Pearson Education, Inc.
Rights and Contracts Department
501 Boylston Street, Suite 900
Boston, MA 02116
Fax: (617) 671-3447
ISBN-13: 978-0-321-35668-0
ISBN-10: 0-321-35668-3
Text printed in the United States on recycled paper at Courier in Stoughton, Massachusetts.
First printing, May 2008
To my family: Cindy, Tim, and Matt
This page intentionally left blank

Item 14: In public classes, use accessor methods,
not public fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Item 15: Minimize mutability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Item 16: Favor composition over inheritance . . . . . . . . . . . . . . . . . . 81
Item 17: Design and document for inheritance or else prohibit it . . 87
Item 18: Prefer interfaces to abstract classes . . . . . . . . . . . . . . . . . . 93
Item 19: Use interfaces only to define types. . . . . . . . . . . . . . . . . . . 98
Item 20: Prefer class hierarchies to tagged classes . . . . . . . . . . . . . 100
Item 21: Use function objects to represent strategies . . . . . . . . . . . 103
Item 22: Favor static member classes over nonstatic . . . . . . . . . . . 106
5 Generics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Item 23: Don’t use raw types in new code . . . . . . . . . . . . . . . . . . . 109
Item 24: Eliminate unchecked warnings. . . . . . . . . . . . . . . . . . . . . 116
Item 25: Prefer lists to arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Item 26: Favor generic types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Item 27: Favor generic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Item 28: Use bounded wildcards to increase API flexibility . . . . . 134
Item 29: Consider typesafe heterogeneous containers . . . . . . . . . . 142
6 Enums and Annotations . . . . . . . . . . . . . . . . . . . . . . . . 147
Item 30: Use enums instead of int constants. . . . . . . . . . . . . . . . . 147
Item 31: Use instance fields instead of ordinals . . . . . . . . . . . . . . . 158
Item 32: Use
EnumSet instead of bit fields . . . . . . . . . . . . . . . . . . . 159
Item 33: Use
EnumMap instead of ordinal indexing. . . . . . . . . . . . . 161
Item 34: Emulate extensible enums with interfaces . . . . . . . . . . . . 165
Item 35: Prefer annotations to naming patterns . . . . . . . . . . . . . . . 169
Item 36: Consistently use the
Override annotation. . . . . . . . . . . . 176
Item 37: Use marker interfaces to define types . . . . . . . . . . . . . . . 179

Item 59: Avoid unnecessary use of checked exceptions . . . . . . . . 246
Item 60: Favor the use of standard exceptions. . . . . . . . . . . . . . . . 248
Item 61: Throw exceptions appropriate to the abstraction. . . . . . . 250
Item 62: Document all exceptions thrown by each method. . . . . . 252
Item 63: Include failure-capture information in
detail messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
Item 64: Strive for failure atomicity . . . . . . . . . . . . . . . . . . . . . . . 256
Item 65: Don’t ignore exceptions . . . . . . . . . . . . . . . . . . . . . . . . . 258
10 Concurrency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .259
Item 66: Synchronize access to shared mutable data. . . . . . . . . . . 259
Item 67: Avoid excessive synchronization . . . . . . . . . . . . . . . . . . 265
Item 68: Prefer executors and tasks to threads. . . . . . . . . . . . . . . . 271
Item 69: Prefer concurrency utilities to wait and notify. . . . . . . 273
CONTENTS
xii
Item 70: Document thread safety . . . . . . . . . . . . . . . . . . . . . . . . . . 278
Item 71: Use lazy initialization judiciously . . . . . . . . . . . . . . . . . . 282
Item 72: Don’t depend on the thread scheduler . . . . . . . . . . . . . . . 286
Item 73: Avoid thread groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
11 Serialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
Item 74: Implement Serializable judiciously. . . . . . . . . . . . . . . 289
Item 75: Consider using a custom serialized form . . . . . . . . . . . . . 295
Item 76: Write
readObject methods defensively . . . . . . . . . . . . . 302
Item 77: For instance control, prefer enum types
to readResolve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
Item 78: Consider serialization proxies instead of serialized
instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
Appendix: Items Corresponding to First Edition . . . . . . 317
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

ten ways to write code for some task T. Of those ten ways, seven will be awkward,
inefficient, or puzzling. Of the other three, which is most likely to be similar to the
code needed for the task T' in next year’s software release?
FOREWORD
xiv
There are numerous books from which you can learn the grammar of the Java
Programming Language, including The Java
™
Programming Language by Arnold,
Gosling, and Holmes [Arnold05] or The Java
™
Language Specification by Gos-
ling, Joy, yours truly, and Bracha [JLS]. Likewise, there are dozens of books on
the libraries and APIs associated with the Java programming language.
This book addresses your third need: customary and effective usage. Joshua
Bloch has spent years extending, implementing, and using the Java programming
language at Sun Microsystems; he has also read a lot of other people’s code,
including mine. Here he offers good advice, systematically organized, on how to
structure your code so that it works well, so that other people can understand it, so
that future modifications and improvements are less likely to cause headaches—
perhaps, even, so that your programs will be pleasant, elegant, and graceful.
Guy L. Steele Jr.
Burlington, Massachusetts
April 2001
xv
Preface
Preface to the Second Edition
A lot has happened to the Java platform since I wrote the first edition of this book
in 2001, and it’s high time for a second edition. The most significant set of changes
was the addition of generics, enum types, annotations, autoboxing, and the for-each

to work with. The changes in releases 5 and 6 have taken a good thing and made it
better. The platform is much bigger now than it was in 2001 and more complex,
but once you learn the patterns and idioms for using the new features, they make
your programs better and your life easier. I hope this edition captures my contin-
ued enthusiasm for the platform and helps make your use of the platform and its
new features more effective and enjoyable.
San Jose, California
April 2008
Preface to the First Edition
In 1996 I pulled up stakes and headed west to work for JavaSoft, as it was then
known, because it was clear that that was where the action was. In the intervening
five years I’ve served as Java platform libraries architect. I’ve designed, imple-
mented, and maintained many of the libraries and served as a consultant for many
others. Presiding over these libraries as the Java platform matured was a once-in-a-
lifetime opportunity. It is no exaggeration to say that I had the privilege to work with
some of the great software engineers of our generation. In the process, I learned a lot
about the Java programming language—what works, what doesn’t, and how to use
the language and its libraries to best effect.
This book is my attempt to share my experience with you so that you can imi-
tate my successes while avoiding my failures. I borrowed the format from Scott
Meyers’s Effective C++ [Meyers98], which consists of fifty items, each convey-
ing one specific rule for improving your programs and designs. I found the format
to be singularly effective, and I hope you do too.
In many cases, I took the liberty of illustrating the items with real-world
examples from the Java platform libraries. When describing something that could
have been done better, I tried to pick on code that I wrote myself, but occasionally
I pick on something written by a colleague. I sincerely apologize if, despite my
best efforts, I’ve offended anyone. Negative examples are cited not to cast blame
PREFACE
xvii

I thank the readers of the first edition of this book for giving it such a kind and
enthusiastic reception, for taking its ideas to heart, and for letting me know what a
positive influence it had on them and their work. I thank the many professors who
used the book in their courses, and the many engineering teams that adopted it.
I thank the whole team at Addison-Wesley for the their kindness, profession-
alism, patience, and grace under pressure. Through it all, my editor Greg Doench
remained unflappable: a fine editor and a perfect gentleman. My production man-
ager, Julie Nahil, was everything that a production manager should be: diligent,
prompt, organized, and friendly. My copy editor, Barbara Wood, was meticulous
and tasteful.
I have once again been blessed with the best team of reviewers imaginable,
and I give my sincerest thanks to each of them. The core team, who reviewed
every chapter, consisted of Lexi Baugher, Cindy Bloch, Beth Bottos, Joe Bowbeer,
Brian Goetz, Tim Halloran, Brian Kernighan, Rob Konigsberg, Tim Peierls, Bill
Pugh, Yoshiki Shibata, Peter Stout, Peter Weinberger, and Frank Yellin. Other
reviewers included Pablo Bellver, Dan Bloch, Dan Bornstein, Kevin Bourrillion,
Martin Buchholz, Joe Darcy, Neal Gafter, Laurence Gonsalves, Aaron Green-
house, Barry Hayes, Peter Jones, Angelika Langer, Doug Lea, Bob Lee, Jeremy
Manson, Tom May, Mike McCloskey, Andriy Tereshchenko, and Paul Tyma.
Again, these reviewers made numerous suggestions that led to great improve-
ments in this book and saved me from many embarrassments. And again, any
remaining embarrassments are my responsibility.
I give special thanks to Doug Lea and Tim Peierls, who served as sounding
boards for many of the ideas in this book. Doug and Tim were unfailingly gener-
ous with their time and knowledge.
I thank my manager at Google, Prabha Krishna, for her continued support and
encouragement.
ACKNOWLEDGMENTS
xx
Finally, I thank my wife, Cindy Bloch, for encouraging me to write, for read-

Steffen Grarup, Peter Kessler, Richard Roda, John Rose, and David Stoutamire
ACKNOWLEDGMENTS
xxi
contributed useful insights. A special thanks is due Doug Lea, who served as a
sounding board for many of the ideas in this book. Doug has been unfailingly
generous with his time and his knowledge.
I thank Julie Dinicola, Jacqui Doucette, Mike Hendrickson, Heather Olszyk,
Tracy Russ, and the whole team at Addison-Wesley for their support and profes-
sionalism. Even under an impossibly tight schedule, they were always friendly
and accommodating.
I thank Guy Steele for writing the Foreword. I am honored that he chose to
participate in this project.
Finally, I thank my wife, Cindy Bloch, for encouraging and occasionally
threatening me to write this book, for reading each item in its raw form, for help-
ing me with Framemaker, for writing the index, and for putting up with me while I
wrote.
This page intentionally left blank
1
CHAPTER 1
Introduction
T
HIS book is designed to help you make the most effective use of the Java
™
programming language and its fundamental libraries, java.lang, java.util,
and, to a lesser extent,
java.util.concurrent and java.io. The book discusses
other libraries from time to time, but it does not cover graphical user interface
programming, enterprise APIs, or mobile devices.
This book consists of seventy-eight items, each of which conveys one rule.
The rules capture practices generally held to be beneficial by the best and most

sible to anyone with a working knowledge of the language, it should provide food
for thought even for advanced programmers.
Most of the rules in this book derive from a few fundamental principles. Clar-
ity and simplicity are of paramount importance. The user of a module should
never be surprised by its behavior. Modules should be as small as possible but no
smaller. (As used in this book, the term module refers to any reusable software
component, from an individual method to a complex system consisting of multiple
packages.) Code should be reused rather than copied. The dependencies between
modules should be kept to a minimum. Errors should be detected as soon as possi-
ble after they are made, ideally at compile time.
While the rules in this book do not apply 100 percent of the time, they do
characterize best programming practices in the great majority of cases. You
should not slavishly follow these rules, but violate them only occasionally and
with good reason. Learning the art of programming, like most other disciplines,
consists of first learning the rules and then learning when to break them.
For the most part, this book is not about performance. It is about writing pro-
grams that are clear, correct, usable, robust, flexible, and maintainable. If you can
do that, it’s usually a relatively simple matter to get the performance you need
(Item 55). Some items do discuss performance concerns, and a few of these items
provide performance numbers. These numbers, which are introduced with the
phrase “On my machine,” should be regarded as approximate at best.
For what it’s worth, my machine is an aging homebuilt 2.2 GHz dual-core
AMD Opteron
™
170 with 2 gigabytes of RAM, running Sun’s 1.6_05 release of
the Java SE Development Kit (JDK) atop Microsoft Windows
®
XP Professional
SP2. This JDK has two virtual machines, the Java HotSpot
™