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Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition by Ivor Horton

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6.14. Anonymous Classes

There are occasions where you need to define a class for which you will only ever want to define one object in your program, and the only use for the object is to pass it directly as an argument to a method. In this case, as long as your class extends an existing class, or implements an interface, you have the option of defining the class as an anonymous class. The definition for an anonymous class appears in the new expression, in the statement where you create and use the object of the class, so that there is no necessity to provide a name for the class.

I will illustrate how this is done using an example. Suppose you want to define an object of a class that implements the interface ActionListener for one-time use. You could do this as follows:

pickButton.addActionListener(new ActionListener() {
                                 // Code to define the class
                                 // that implements the ActionListener interface
                              }
                             );

The class definition appears in the new expression that creates the argument to the addActionListener() method. This method requires a reference of type ActionListener—in other words, a reference to a class that implements the ActionListener interface. The parentheses following the name of the interface indicate you are creating an object reference of this type, and the details of the class definition appear between the parentheses. The anonymous class can include data members as well as methods, but obviously not constructors because the class has no name. Here, all the methods declared ...

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