7.4 Java1640 java oop anonymous classes  (Page 2/13)

• Member classes
• Local classes
• Anonymous classes
• Nested top-level classes and interfaces

The previous two modules explained member classes and local classes. This module will explain anonymous classes.

What is an anonymous class ?

I'm going to begin my discussion with a quotation from one of my favorite authors, David Flanagan, author of Java in a Nutshell .

"An anonymous class is essentially a local class without a name."

If you have read the previous module, you should know quite a lot about local classes at this point in time. Continuing with Flanagan's words,

"Instead of defining a local class and then instantiating it, you can often use an anonymous class to combine these two steps... an anonymousclass is defined by a Java expression , not a Java statement . This means that an anonymous class definition can be included within a larger Java expression..."

As you will see from the sample program in this module, anonymous class definitions are often included as arguments to method calls.

As is the case for an object of a member class or a local class (discussed in previous modules), an object of an anonymous class must be internally linked to an object of the enclosing class.

Thus, an anonymous class is truly an inner class, because an object of the anonymous class cannot exist in the absence of an object of the enclosing class.

What about an anonymous interface ?

Interfaces defined within classes are implicitly static. This means that they are always top-level. There is no such thing as a member interface, a local interface, or an anonymous interface.

Why use anonymous classes ?

As with local classes, objects instantiated from anonymous classes share many of the characteristics of objects instantiated from member classes.However, in some cases, an anonymous class can be defined closer to its point of use than would be possible with a member class or a local class. Once youbecome accustomed to the somewhat cryptic syntax used with anonymous classes, this can often lead to improved code readability.

Probably the most important benefit of anonymous classes has to do with accessing the members of enclosing classes. Just like with member classesand local classes, methods of an anonymous class have direct access to all the members of the enclosing classes, including private members. Thus the useof anonymous classes can often eliminate the requirement to connect objects together via constructor parameters.

In addition, although not demonstrated in this module, as with local classes, objects of anonymous classes have access to final local variables that are declared within the scope of the anonymous class.

Can be particularly useful when ...

An anonymous class can be particularly useful in those cases where

• There is no reason for an object of the anonymous class to exist in the absence of an object of the enclosing class.
• There is no reason for an object of the anonymous class to exist outside a method of the enclosing class.
• Methods of the object of the anonymous class need access to members of the object of the enclosing class.
• Methods of the object of the anonymous class need access to final local variables and method parameters belonging to the method in which the anonymous class is defined.
• Only one instance of the anonymous class is needed.
• There is no need for the class to have a name that is accessible elsewhere in the program.