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Section 13.4 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 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 FigureĀ 4.4.9 in ChapterĀ 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 FigureĀ 13.4.1.

Figure 13.4.1. ActionListener
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 an ActionEvent.

In order to complete the event-handling code, the GUI must implement the ActionListener interface. As ListingĀ 13.4.2 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
Listing 13.4.2. A simple GUI application that handles action events on a JButton.

Now that we have implemented the code in ListingĀ 13.4.2, 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. FigureĀ 13.4.3 depicts the sequence of actions and events that occur when the the user clicks a button.

Figure 13.4.3. 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.

The methods used to handle the ActionEvent are derived from the java.util.EventObject class, the root class for all events (FigureĀ 13.4.4). The getSource() method is used to get the object that caused the event.

Figure 13.4.4. An EventObject

Our example (ListingĀ 13.4.2) uses the getSource() 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:

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.

Subsection 13.4.1 Event Classes

Although the event model is the same for both AWT and Swing classes, the Swing package introduces many additional events. TableĀ 13.1 lists the events that are generated by both AWT and Swing components. You already have worked with some of these. We have written GUIs that handled ActionEvents for JButtons and JTextFields in preceding chapters.

Table 13.4.5. Java's AWTEvent for each Component type (Original source: David Flanagan, Java in a Nutshell, 2d ed., O'Reilly Associates, 1997. Modified for Swing components.)
Components Events 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

In viewing TableĀ 13.4.5, it's important to remember that the classes listed there are arranged in a hierarchy. This will affect the events that a particular object can generate. For example, a JButton is a JComponent( FigureĀ 13.2.2), so in addition to generating an ActionEvent when the user clicks on it, it can also generate a MouseEvent when the user moves the mouse over it. Similarly, because a JTextField is also a JComponent, it can generate a KeyEvent as well as an ActionEvent.

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 ComponentEvent s. Because they are subclasses of JComponent, JButton and JTextField can also use these methods. Defining the more generic methods in the JComponent superclass is another example of the effective use of inheritance.

TableĀ 13.4.7 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 that are 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 13.4.7. Some of the events that are defined in the Swing library.
Component Events Description
JPopupMenu PopupMenuEvent User selected a choice
JComponent AncestorEvent An 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 An undoable edit has occurred
JTable TableModelEvent Items added/removed from table
TableColumnModelEvent A 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

These tables provide only a brief summary of these classes and Swing components. For further details you should consult the JDK online documentation at https://docs.oracle.com/javase/7/docs/api/javax/swing/package-summary.html.

Exercises 13.4.2 Self-Study Exercises

1. Listeners 1.

    Is it possible to register a component with more than one listener?

  • Yes

  • A component can indeed be registered with more than one listener.

  • No

  • A component can be registered with more than one listener.

2. Listeners 2.

    Is it possible for a component to have two different kinds of listeners?

  • Yes

  • For example, ActionEvent and MouseEvent.

  • No

  • What about listening both to a click and a mouse event?

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