4.2 Inner Classes

While the bulk of the changes in Java 1.1 are additions to the core Java API, there has also been a major addition to the language itself. The language has been extended to allow class definitions to be nested within other classes, and even to be defined locally, within blocks of code. Altogether, there are four new types of classes that can be defined in Java 1.1; these four new types are sometimes loosely referred to as "inner classes."

Chapter 5, Inner Classes and Other New Language Features explains in detail how to define and use each of the four new types of classes. As we'll see, inner classes are useful primarily for defining simple "helper" or "adaptor" classes that serve a very specific function at a particular place in a program, and are not intended to be general-purpose "top-level" classes. By using inner classes nested within other classes, you can place the definition of these special-purpose helper classes closer to where they are actually used in your programs. This makes your code clearer, and also prevents you from cluttering up the package namespace with small special purpose classes that are not of interest to programmers using your package. We'll also see that inner classes are particularly useful in conjunction with the new AWT event model in Java 1.1.

One important feature of inner classes is that no changes to the Java Virtual Machine are required to support them. When a Java 1.1 compiler encounters an inner class, it transforms the Java 1.1 source code in a way that converts the nested class to a regular top-level class. Once that transformation has been performed, the code can be compiled just as it would have been in Java 1.0.