5.1 An Overview of Inner Classes

Java 1.0 allowed classes and interfaces to be defined in exactly one context: at the "top level," as members of packages. Java 1.1 adds one new type of top-level classes and interfaces, and adds three new types of "inner classes," as outlined below. Later sections of this chapter describe each of these new types of classes and interfaces in more detail and present examples of their use.

Nested top-level classes and interfaces

A nested top-level class or interface is defined as a static member of an enclosing top-level class or interface. The definition of a nested top-level class uses the static modifier, just as the definition of a static method or static field does. Nested interfaces are implicitly static (though they may be declared static to make this explicit) and so are always top-level. A nested top-level class or interface behaves just like a "normal" class or interface that is a member of a package. The difference is that the name of a nested top-level class or interface includes the name of the class in which it is defined. Thus, a LinkedList class could define a nested top-level interface Linkable. This interface would be referred to as LinkedList.Linkable. Nested top-level classes and interfaces are typically used as a convenient way to group related classes.

Member classes

A member class is also defined as a member of an enclosing class, but unlike a nested top-level class, it is not defined with the static modifier. This means that it is an inner class, rather than a top-level class. Nested interfaces are always implicitly static, so they are always top-level; there is no such thing as a "member interface," or any kind of "inner interface." In many ways, a member class is analogous to the other members--the instance fields and methods--of a class. Member classes are of interest because the code within a member class can implicitly refer to any of the fields and methods, including private fields and methods, of its enclosing class. [1] Every instance of a member class is associated with an enclosing instance of the class that defines it. Because of the requirement for this enclosing instance, several new pieces of syntax have been introduced into the Java language.

[1] Unfortunately, in Java 1.1 and 1.1.1 there are compiler bugs that prevent access to the private fields and methods of enclosing classes from working correctly. It is not yet clear when these bugs will be fixed. So while access to private members of enclosing classes is part of the inner class specification, it is a feature that is currently best avoided. If a field or method must be visible to nested classes, you should give it package visibility rather than private visibility.

Local classes

A local class is an inner class defined within a block of Java code; it is visible only within that block. Interfaces can not be defined locally. Because a local class is defined within a block of code, it is analogous, in some ways, to a local variable. Local classes are not member classes, but can still use the fields and methods of enclosing classes. More important, however, the code within a local class definition can use any final local variables or parameters that are accessible in the scope of the block that defines the class. Local classes are useful primarily as "adapter classes" and are commonly used with the new event-handling model required by the Java 1.1 AWT and by JavaBeans. For example, a block of Java 1.1 code that creates a java.awt.Button object could use a local class to define a simple implementation of the java.awt.event.ActionListener interface. Then it could instantiate this simple implementation and pass the resulting object to the button's addActionListener() method, thereby connecting the button to the "callback" code that is executed when the button is pressed.

Anonymous classes

An anonymous class is an extension to the local class concept described above. Instead of declaring a local class with one Java statement, and then instantiating and using it in another statement, an anonymous class combines the two steps in a single Java expression. An anonymous class, as you might guess, does not have a name. And because it is instantiated in the same expression that defines it, it can only be instantiated once. Except for these differences, anonymous classes are quite similar to local classes in behavior and use. Interfaces cannot be defined anonymously, of course. When writing a simple adapter class, the choice between a named local class and an unnamed anonymous class typically comes down to a matter of style and code clarity, rather than any difference in functionality.

Table 5.1 summarizes the types of classes and interfaces that can be defined in Java 1.1; the remaining sections of the chapter document each type in more detail.