Main Page

Previous Next

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.

We will illustrate how this is done using an example. Supposing we want to define an object of a class that implements the interface ActionListener for one time use. We 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 we 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 in the ActionListener interface would need to be defined.

If the anonymous class extends an existing class, the syntax is much the same. In this case you are calling a constructor for the base class and, if this is not a default constructor, you can pass arguments to it by specifying them between the parentheses following the base class name. The definition of the anonymous class must appear between braces, just as in the previous example.

An anonymous class can be convenient where the class definition is short and simple. This technique should not be used extensively as it tends to make the code very difficult to understand.

Previous Next
JavaScript Editor Java Tutorials Free JavaScript Editor