Java, Java, Java: Object-Oriented Problem Solving, 2022E

As we saw in Chapter 4, whatever happens while the computer is running is classified as an event. Every keystroke and mouse click, every time a disk is inserted into a disk drive, an event is generated. The handling of events are an important element of GUI programming. Therefore, before we begin discussing how to design GUIs, it will be useful to review the main concepts of Java's event model.

When a Java program is running, events generated by the hardware are passed up through the operating system (and through the browser, for applets) to the program. Those events that belong to the program must be handled by the program (refer to Fig. 4.18 in Chapter 4). For example, if you click your browser's menu bar, that event will be handled by the browser itself. If you click a button contained in the Java program, that event should be handled by the program.

In Java, whenever something happens within a GUI component, an event object is generated and passed to the event listener that has been registered to handle that component's events. You've seen numerous examples of this process in earlier chapters, but we've included a simple example to serve as a reminder.

Suppose you create a JButton in a GUI as follows:

private JButton clickme = new JButton("ClickMe");

Whenever the user clicks the JButton, an ActionEvent is generated. In order to handle these events, the GUI must register the JButton with a listener object that listens for action events. This can be done in an applet's init() method or in an application's constructor method, as in this example:

public MyGUI() {
  // Add clickme to the GUI and assign it a listener
  add(clickme);
  clickme.addActionListener(this); }

In this case, we have designated the GUI itself (this) as an ActionListener for clickme(Fig. 13.5).

In order to complete the event-handling code, the GUI must implement the ActionListener interface. As Figure 13.5.1 shows, implementing an interface is a matter of declaring the interface in the class heading and implementing the methods contained in the interface, in this case the actionPerformed() method.

import javax.swing.*;
import java.awt.event.*;
public class MyGUI extends JFrame
                              implements ActionListener {
    private JButton clickme = new JButton("ClickMe");
    public MyGUI() {
     // Add clickme to the GUI and assign it a listener
        getContentPane().add(clickme);
        clickme.addActionListener(this);
        setSize(200,200);
        setVisible(true);
    } // init()
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == clickme) {
      clickme.setText(clickme.getText()+"*");
        }
    } // actionPerformed()
    public static void main(String args[]) {
        MyGUI gui = new MyGUI();
    }} // MyGUI

Now that we have implemented the code in Figure 13.5.1, whenever the user clicks clickme, that action is encapsulated within an ActionEvent object and passed to the actionPerformed() method. This method contains Java code that will handle the user's action in an appropriate way. For this example, it modifies the button's label by appending an asterisk to it each time it is clicked. Figure 13.5.2 depicts the sequence of actions and events that occur when the the user clicks a button.

The methods used to handle the ActionEvent are derived from the

java.awt.event.ActionEvent[ACTION_PERFORMED,cmd=ClickMe]
  on javax.swing.JButton[,58,5,83x27,
  layout=javax.swing.OverlayLayout]

As you can see, the event generated was an ACTION_PERFORMED event, in response to the ClickMe command. The source of the event was the JButton.