11.6. Class Design Guidelines

[Page 363 ( continued )]

10.7. (Optional GUI) Handling GUI Events

When you run the program in Listing 9.12, it displays two buttons . If you click a button, there is no response. To respond, you need to write the code to process the clicking button action. The button is a source object where the action originates. You need to create an object capable of handling the action event on a button. This object is called a listener . Not all objects can be listeners for an action event. To be a listener, two requirements must be met:

1. The object must be an instance of the ActionListener interface. The ActionListener interface contains the actionPerformed method for processing the event.

2. The ActionListener object listener must be registered with the source using the method source.addActionListener(listener) .

Listing 10.6 gives the code that processes the ActionEvent on the two buttons. When you click the OK button, the message "OK button clicked" is displayed. When you click the Cancel button, the message "Cancel button clicked" is displayed, as shown in Figure 10.10.

Figure 10.10. The program responds to button-clicking action events.

Listing 10.6. HandleEvent.java

(This item is displayed on pages 363 - 364 in the print version)

 1   import   javax.swing.*; 2   import   java.awt.*; 3   import   java.awt.event.*; 4 5   public class    HandleEvent   extends   JFrame  { 6   public   HandleEvent() { 7  // Set FlowLayout for the frame  8 FlowLayout layout =   new   FlowLayout(); 9 setLayout(layout); 10 11  // Add two buttons to frame  12 JButton jbtOK =   new   JButton(   "OK"   ); 13 JButton jbtCancel =   new   JButton(   "Cancel"   ); 14 add(jbtOK); 15 add(jbtCancel); 16 17  // Register listeners  18  OKListenerClass listener1 =   new   OKListenerClass();  19 CancelListenerClass listener2 =   new   CancelListenerClass(); 20  jbtOK.addActionListener(listener1);  21 jbtCancel.addActionListener(listener2); 22 } 23 

---

[Page 364]

 24   public static void   main(String[] args) { 25  JFrame frame =   new   HandleEvent();  26 frame.setTitle(   "Handle Event"   ); 27 frame.setSize(   200   ,   150   ); 28 frame.setLocation(   200   ,   100   ); 29 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 30 frame.setVisible(   true   ); 31 } 32 } 33 34   class    OKListenerClass   implements   ActionListener  { 35   public void   actionPerformed(ActionEvent e) { 36 System.out.println(   "OK button clicked"   ); 37 } 38 } 39 40   class    CancelListenerClass   implements   ActionListener  { 41   public void   actionPerformed(ActionEvent e) { 42 System.out.println(   "Cancel button clicked"   ); 43 } 44 } 

Two listener classes are declared in lines 34 “44. Each listener class implements ActionListener to process ActionEvent . The object listener1 is an instance of OKListenerClass (line 18), which is registered with the button jbtOK in line 20. When the OK button is clicked, the actionPerformed(ActionEvent) method (line 36) in OKListenerClass is invoked to process the event. The object listener2 is an instance of CancelListenerClass (line 19), which is registered with the button jbtCancel in line 21. When the OK button is clicked, the actionPerformed(ActionEvent) method (line 42) in CancelListenerClass is invoked to process the event.