Section 13.5. The Java Event Model

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13.5. The Java Event Model

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 is 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.

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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. Events that belong to the program must be handled by the program (refer to Fig. 4.18). 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 registered to handle the component's events. You have 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);  // Add clickme to the GUI and assign it a listener   clickme.addActionListener(this); } 

In this case, we have designated the GUI itself (this) as an ActionListener for clickme (Fig. 13.5). A listener is any object that implements a listener interface, which is one of the interfaces derived from java.util.EventListener. An ActionListener is an object that listens for and receives ActionEvents.

Figure 13.5. The GUI listens for action events on the JButton.

In order to complete the event-handling code, the GUI must implement the ActionListener interface. As Figure 13.6 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.

Figure 13.6. A simple GUI application that handles action events on a JButton. (This item is displayed on page 611 in the print version)

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  class 

Now that we have implemented the code in Figure 13.6, 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 13.7 depicts the sequence of actions and events that occur when the user clicks a button.

Figure 13.7. A UML depiction of the sequence of actions and events that take place when a button is clicked. The vertical lines represent time lines, with time running from top to bottom. The arrows between lines represent messages passing between objects. (This item is displayed on page 611 in the print version)

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The methods used to handle the ActionEvent are derived from the java.util.EventObject class, the root class for all events (Fig. 13.8). Our example (Fig. 13.6) uses the get- Source() method to get a reference to the object that generated the event. To see what information is contained in an event object, we can use the toString() method to print a string representation of the event that was generated. Here's what it displays:

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

Figure 13.8. An EventObject. The getSource() method is used to get the object that caused the event.

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.

13.5.1. Event Classes

Although the event model is the same for both AWT and Swing classes, the Swing package introduces many additional events. Table 13.1 lists the events generated by both AWT and Swing components. You worked with some of these when we wrote GUIs that handled ActionEvents for JButtons and JTextFields in preceding chapters.

Table 13.1. Java's AWTEvents for each Component type

Component	Event	Description
Button, JButton	ActionEvent	User clicked button
CheckBox, JCheckBox	ItemEvent	User toggled a checkbox
CheckboxMenuItem, JCheckboxMenuItem	ItemEvent	User toggled a checkbox
Choice, JPopupMenu	ItemEvent	User selected a choice
Component, JComponent	ComponentEvent	Component was moved or resized
	FocusEvent	Component acquired or lost focus
	KeyEvent	User typed a key
	MouseEvent	User manipulated the mouse
Container, JContainer	ContainerEvent	Component added/removed from container
List, JList	ActionEvent	User double-clicked a list item
	ItemEvent	User clicked a list item
Menu, JMenu	ActionEvent	User selected menu item
Scrollbar, JScrollbar	AdjustmentEvent	User moved scrollbar
TextComponent, JTextComponent	TextEvent	User edited text
TextField, JTextField	ActionEvent	User typed Enter key
Window, JWindow	WindowEvent	User manipulated window
Original source: David Flanagan, Java in a Nutshell, 2d ed., O'Reilly Associates, 1997. Modified for Swing components.

In viewing Table 13.1, remember that the classes it lists are arranged in a hierarchy. This will affect the events any given object can generate. For example, a JButton is a JComponent (Fig. 13.2), so in addition to generating ActionEvents when the user clicks on it, it can also generate MouseEvents when the user moves the mouse over it. Similarly, because a JTextField is also a JComponent, it can generate KeyEvents as well as ActionEvents.

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Note that the more generic events, such as those that involve moving, focusing, or resizing a component, are associated with the more generic components. For example, the JComponent class contains methods that are used to manage ComponentEvents. Because they are subclasses of JComponent, JButtons and JTextFields can also use these methods. Defining the more generic methods in the JComponent superclass is another example of the effective use of inheritance.

Effective Design: Inheritance

The higher a method is defined in the inheritance hierarchy, the broader is its use.

Table 13.2 lists events that are new with the Swing classes. Some of the events apply to new components. For example, JTable and Jtree do not have AWT counterparts. Other events provide Swing components with capabilities not available in their AWT counterparts. For example, a CaretEvent allows the programmer to have control over mouse clicks that occur within a text component.

Table 13.2. Some of the events defined in the Swing library

Component	Event	Description
JPopupMenu	PopupMenuEvent	User selected a choice
JComponent	AncestorEvent	Event occurred in an ancestor
JList	ListSelectionEvent	User double-clicked a list item
	ListDataEvent	List's contents were changed
JMenu	MenuEvent	User selected menu item
JTextComponent	CaretEvent	Mouse clicked in text
	UndoableEditEvent	Undoable edit occurred
JTable	TableModelEvent	Items added/removed from table
	TableColumnModelEvent	Table column was moved
Jtree	TReeModelEvent	Items added/removed from tree
	treeSelectionEvent	User selected a tree node
	TReeExpansionEvent	User expanded or collapsed a tree node
JWindow	WindowEvent	User manipulated window

Tables 13.1 and 13.2 provide only a brief summary of these classes and Swing components. For further details, consult the JDK online documentation at

http://java.sun.com/docs/

Self-Study Exercises

Exercise 13.2	Is it possible to register a component with more than one listener?
Exercise 13.3	Is it possible for a component to have two different kinds of listeners?