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Java Generics and Collections by Philip Wadler, Maurice Naftalin

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Java now supports generics, the most significant change to the language since the addition of inner classes in Java 1.2—some would say the most significant change to the language ever.

Say you wish to process lists. Some may be lists of integers, others lists of strings, and yet others lists of lists of strings. In Java before generics this is simple. You can represent all three by the same class, called List, which has elements of class Object:

list of integers


list of strings


list of lists of strings


In order to keep the language simple, you are forced to do some of the work yourself: you must keep track of the fact that you have a list of integers (or strings or lists of strings), and when you extract an element from the list you must cast it from Object back to Integer (or String or List). For instance, the Collections Framework before generics made extensive use of this idiom.

Einstein is reputed to have said, “Everything should be as simple as possible but no simpler”. And some might say the approach above is too simple. In Java with generics you may distinguish different types of lists:

list of integers


list of strings


list of lists of strings


Now the compiler keeps track of whether you have a list of integers (or strings or lists of strings), and no explicit cast back to Integer (or String or List<String>) is required. In some ways, this is similar to generics in Ada or templates in C++, but the actual inspiration is parametric polymorphism as found in functional languages such as ML and Haskell.

Part I of this book provides a thorough introduction to generics. We discuss the interactions between generics and subtyping, and how to use wildcards and bounds; we describe techniques for evolving your code; we explain subtleties connected with casts and arrays; we treat advanced topics such as the interaction between generics and security, and how to maintain binary compatibility; and we update common design patterns to exploit generics.

Much has been written on generics, and their introduction into Java has sparked some controversy. Certainly, the design of generics involves swings and roundabouts: making it easy to evolve code requires that objects not reify run-time information describing generic type parameters, but the absence of this information introduces corner cases into operations such as casting and array creation.We present a balanced treatment of generics, explaining how to exploit their strengths and work around their weaknesses.

Part II provides a comprehensive introduction to the Collections Framework. Newton is reputed to have said, “If I have seen farther than others, it is because I stand on the shoulders of giants”. The best programmers live by this motto, building on existing frameworks and reusable code wherever appropriate. The Java Collections Framework provides reusable interfaces and implementations for a number of common collection types, including lists, sets, queues, and maps. There is also a framework for comparing values, which is useful in sorting or building ordered trees. (Of course, not all programmers exploit reuse. As Hamming said of computer scientists, “Instead of standing on each other’s shoulders, we stand on each other’s toes.”)

Thanks to generics, code using collections is easier to read and the compiler will catch more type errors. Further, collections provide excellent illustrations of the use of generics. One might say that generics and collections were made for each other, and, indeed, ease of use of collections was one of the main reasons for introducing generics in the first place.

Java 5 and 6 not only update the Collections Framework to exploit generics, but also enhance the framework in other ways, introducing interfaces and classes to support concurrency and the new enum types. We believe that these developments mark the beginning of a shift in programming style, with heavier use of the Collections Framework and, in particular, increased use of collections in favor of arrays. In Part II, we describe the entire framework from first principles in order to help you use collections more effectively, flagging the new features of Java 5 and 6 as we present them.

Following common terminology, we refer to the successive versions of Java as 1.0 up to 1.4 and then 5 and 6. We say ‘Java before generics’ to refer to Java 1.0 through 1.4, and ‘Java with generics’ to refer to Java 5 and 6.

The design of generics for Java is influenced by a number of previous proposals—notably, GJ, by Bracha, Odersky, Stoutamire, and Wadler; the addition of wildcards to GJ, proposed by Igarashi and Viroli; and further development of wildcards, by Torgersen, Hansen, Ernst, von der Ahé, Bracha, and Gafter. Design of generics was carried out under the Java Community Process by a team led by Bracha, and including Odersky, Thorup, and Wadler (as parts of JSR 14 and JSR 201). Odersky’s GJ compiler is the basis of Sun’s current javac compiler.

Obtaining the Example Programs

Some of the example programs in this book are available online at:


If you can’t get the examples directly over the Internet but can send and receive email, you can use ftpmail to get them. For help using ftpmail, send an email to

with no subject and the single word “help” in the body of the message.

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Conventions Used in This Book

We use the following font and format conventions:

  • Code is shown in a fixed-width font, with boldface used for emphasis:

    class Client {
      public static void main(String[] args) {
        Stack<Integer> stack = new ArrayStack<Integer>();
        for (int i = 0; i<4; i++) stack.push(i);
        assert stack.toString().equals("stack[0, 1, 2, 3]");
  • We often include code that corresponds to the body of an appropriate main method:

    Stack<Integer> stack = new ArrayStack<Integer>();
    for (int i = 0; i<4; i++) stack.push(i);
    assert stack.toString().equals("stack[0, 1, 2, 3]");
  • Code fragments are printed in fixed-width font when they appear within a paragraph (as when we referred to a main method in the preceding item).

  • We often omit standard imports. Code that uses the Java Collection Framework or other utility classes should be preceded by the line:

    import java.util.*;
  • Sample interactive sessions, showing command-line input and corresponding output, are shown in constant-width font, with user-supplied input preceded by a percent sign:

    % javac g/Stack.java g/ArrayStack.java g/Stacks.java l/Client.java
    Note: Client.java uses unchecked or unsafe operations.
    Note: Recompile with -Xlint:unchecked for details.
  • When user-supplied input is two lines long, the first line is ended with a backslash:

    % javac -Xlint:unchecked g/Stack.java g/ArrayStack.java \
    %    g/Stacks.java l/Client.java
    l/Client.java:4: warning: [unchecked] unchecked call
    to push(E) as a member of the raw type Stack
          for (int i = 0; i<4; i++) stack.push(new Integer(i));

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: "Java Generics and Collections, by Maurice Naftalin and Philip Wadler. Copyright 2006 O’Reilly Media, Inc., 0-596-52775-6.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at .

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The folks at Sun (past and present) were fantastically good about answering our questions. They were always happy to explain a tricky point or mull over a design tradeoff. Thanks to Joshua Bloch, Gilad Bracha, Martin Buchholz, Joseph D. Darcy, Neal M. Gafter, Mark Reinhold, David Stoutamire, Scott Violet, and Peter von der Ahé.

It has been a pleasure to work with the following researchers, who contributed to the design of generics for Java: Erik Ernst, Christian Plesner Hansen, Atsushi Igarashi, Martin Odersky, Mads Torgersen, and Mirko Viroli.

We received comments and help from a number of people. Thanks to Brian Goetz, David Holmes, Heinz M. Kabutz, Deepti Kalra, Angelika Langer, Stefan Liebeg, Doug Lea, Tim Munro, Steve Murphy, and C K Shibin.

We enjoyed reading Heinz M. Kabutz’s The Java Specialists’ Newsletter and Angelika Langer’s Java Generics FAQ, both available online.

Our editor, Michael Loukides,was always ready with good advice. Paul C. Anagnostopoulos ofWindfall Software turned our LATEX into camera-ready copy, and JeremyYallop produced the index.

Our families kept us sane (and insane). Love to Adam, Ben, Catherine, Daniel, Isaac, Joe, Leora, Lionel, and Ruth.

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