7.4 Java1640 java oop anonymous classes  (Page 6/13)

An anonymous class that extends an existing class

The program illustrates the use of an anonymous class that extends the WindowAdapter class, to instantiate an object to handle window events fired by the close button in theupper-right corner of the Frame object shown in Figure 2 . This class also uses the source-listener event model, and uses an instance initializer todisplay the name of the class file.

The screen output

The program produces the screen output shown in Figure 4 when

• The program is started
• Each button shown in Figure 2 is clicked once in succession, going from left to right
• The close button in the upper-right corner of the Frame object in Figure 2 is clicked

When the close button is clicked, the program produces the last line of text in Figure 4 and terminates. I will identify the code that produces each line of output text in the discussion of the program that follows.

The controlling class

The controlling class for the program is shown in Listing 1 .

As you can see, the controlling class is very simple, with the main method instantiating an object of the GUI class. This results in the GUI that is pictured in Figure 2 .

Local and anonymous classes inside GUI constructor

The local class and the three anonymous classes are defined inside the constructor for the GUI class.

(Recall that local classes and anonymous classes are defined inside code blocks, which often place them inside methods and constructors, but you canalso place them inside static initializer blocks and instance initializers.)

The first four lines of the output text in Figure 4 are produced by constructors and instance initializers in the local and anonymous classes.Therefore, those four lines of text are produced when the new object of the GUI class is instantiated.

The GUI class

As is often the case, the GUI class used to create the visual GUI shown in Figure 2 consists solely of a constructor. Basically, this constructor

• places three buttons in the frame and
• registers event handlers on the buttons and on the frame.

Once the GUI object is constructed and appears on the screen, all furtheractivity in the program occurs under control of the event handlers associated with the buttons and the frame.

(You can learn more about event handling here .)

The GUI constructor

The GUI class, and the constructor for that class begin in Listing 2 .

As you can see, the GUI class extends Frame , so that an object of the class is a frame.

The constructor code shown in Listing 2 simply sets values for the layout, size, and title properties of the frame.

The BaldButton class

The definition of the BaldButton class begins in Listing 3 . This is a local class that extends Button .

Extending the Button class makes it possible to override the processMouseEvent method in order to handle mouse events that are fired by the button.

This is a form of low-level event handling that will be contrasted with source-listenerevent handling later in the program.

Listing 3 shows the constructor for the BaldButton class.

Enable mouse events

The most important code in the constructor is the statement that enables mouse events on the button.