7.4 Java1640 java oop anonymous classes  (Page 3/13)

Anonymous classes versus local classes

Once again, according to David Flanagan,

"...an anonymous class behaves just like a local class, and is distinguished from a local class merely in the syntax used to define andinstantiate it."

Unlike a local class, however, an anonymous class cannot define a constructor. However, an anonymous class can define an instance initializer, which canprovide some of the benefits of a constructor.

(I discussed instance initializers in detail in an earlier tutorial titled The Essence of OOP using Java, Instance Initializers . As you may recall, a primary shortcoming of an instance initializer as compared to aconstructor is that an instance initializer cannot accept incoming parameters.)

Restrictions on the use of anonymous classes

Because an anonymous class has no name, and the definition and instantiation of the class appear in a single expression, only one instance of each anonymousclass can be created. If you need more than one instance of the class, you should probably use a local class, a member class, or a top-level class instead.

As mentioned above, it is not possible to define constructors for anonymous classes. If you need to use a constructor when you instantiate the class,you should probably use a local class, a member class, or a top-level class instead.

As with member classes and local classes, anonymous classes cannot contain static members.

As with local variables and local classes, anonymous classes cannot be declared public , protected , private , or static . In fact, no modifiers can be specified in the definition of an anonymous class.

Smoke and mirrors

As I told you in my earlier modules on local classes, the methods in an anonymous class don't really have access to local variables and methodparameters. Rather, when an object of the anonymous class is instantiated, copies of the final local variables and method parameters referred to by the object's methods are stored as instance variables in the object. Themethods in the object of the anonymous class really access those hidden instance variables.

Thus, the local variables and method parameters accessed by the methods of the local class must be declared final to prevent their values from changing after the object is instantiated.

There are some additional activities involving smoke and mirrors taking place behind the scenes when you define and instantiate an anonymousclass. Generally speaking, this involves the automatic generation of code to cause things to behave as they do. The good news is that you don't haveto write that extra code, and you don't have to maintain it. The extra code is written for you, and if you modify your class structure, the extra codeis automatically modified accordingly.

You can read about the code that is automatically generated in my earlier modules on local classes and member classes.

Syntax for anonymous classes

Before getting into actual code in the sample program, I want to explain the syntax used to define and instantiate an anonymous class.