How Inner Classes Work (Java in a Nutshell)

3.13. How Inner Classes Work

The preceding sections have explained the features and behavior of the various types of inner classes. Strictly speaking, that should be all you need to know about inner classes. In practice, however, some programmers find it easier to understand the details of inner classes if they understand how they are implemented.

Inner classes were introduced in Java 1.1. Despite the dramatic changes to the Java language, the introduction of inner classes did not change the Java Virtual Machine or the Java class file format. As far as the Java interpreter is concerned, there is no such thing as an inner class: all classes are normal top-level classes. In order to make an inner class behave as if it is actually defined inside another class, the Java compiler ends up inserting hidden fields, methods, and constructor arguments into the classes it generates. You may want to use the javap disassembler to disassemble some of the class files for inner classes so you can see what tricks the compiler has used to make inner classes work. (See Chapter 8, "Java Development Tools", for information on javap.)

3.13.1. Static Member Class Implementation

Recall our first LinkedStack example (Example 3-8), which defined a static member interface named Linkable. When you compile this LinkedStack class, the compiler actually generates two class files. The first one is LinkedStack.class, as expected. The second class file, however, is called LinkedStack$Linkable.class. The $ in this name is automatically inserted by the Java compiler. This second class file contains the implementation of the static member interface.

As we discussed earlier, a static member class can access all the static members of its containing class. If a static member class does this, the compiler automatically qualifies the member access expression with the name of the containing class. A static member class is even allowed to access the privatestatic fields of its containing class. Since the static member class is compiled into an ordinary top-level class, however, there is no way it can directly access the private members of its container. Therefore, if a static member class uses a private member of its containing class (or vice versa), the compiler automatically generates non-private access methods and converts the expressions that access the private members into expressions that access these specially generated methods. These methods are given the default package access, which is sufficient, as the member class and its containing class are guaranteed to be in the same package.

3.13.2. Member Class Implementation

A member class is implemented much like a static member class. It is compiled into a separate top-level class file, and the compiler performs various code manipulations to make interclass member access work correctly.

The most significant difference between a member class and a static member class is that each instance of a member class is associated with an instance of the enclosing class. The compiler enforces this association by defining a synthetic field named this$0 in each member class. This field is used to hold a reference to the enclosing instance. Every member class constructor is given an extra parameter that initializes this field. Every time a member class constructor is invoked, the compiler automatically passes a reference to the enclosing class for this extra parameter.

As we've seen, a member class, like any member of a class, can be declared public, protected, or private, or given the default package visibility. However, as I mentioned earlier, there have been no changes to the Java Virtual Machine to support member classes. Member classes are compiled to class files just like top-level classes, but top-level classes can only have public or package access. Therefore, as far as the Java interpreter is concerned, member classes can only have public or package visibility. This means that a member class declared protected is actually treated as a public class, and a member class declared private actually has package visibility. This does not mean you should never declare a member class as protected or private. Although the interpreter cannot enforce these access control modifiers, the modifiers are noted in the class file. This allows any conforming Java compiler to enforce the access modifiers and prevent the member classes from being accessed in unintended ways.

3.13.3. Local and Anonymous Class Implementation

A local class is able to refer to fields and methods in its containing class for exactly the same reason that a member class can; it is passed a hidden reference to the containing class in its constructor and saves that reference away in a private field added by the compiler. Also, like member classes, local classes can use private fields and methods of their containing class because the compiler inserts any required accessor methods.

What makes local classes different from member classes is that they have the ability to refer to local variables in the scope that defines them. The crucial restriction on this ability, however, is that local classes can only reference local variables and parameters that are declared final. The reason for this restriction becomes apparent from the implementation. A local class can use local variables because the compiler automatically gives the class a private instance field to hold a copy of each local variable the class uses. The compiler also adds hidden parameters to each local class constructor to initialize these automatically created private fields. Thus, a local class does not actually access local variables, but merely its own private copies of them. The only way this can work correctly is if the local variables are declared final, so that they are guaranteed not to change. With this guarantee, the local class can be assured that its internal copies of the variables are always in sync with the real local variables.

Since anonymous classes have no names, you may wonder what the class files that represent them are named. This is an implementation detail, but the Java compiler from Sun uses numbers to provide anonymous class names. If you compile the example shown in Example 3-11, you'll find that it produces a file with a name like LinkedStack$1.class. This is the class file for the anonymous class.