Delegation Event Model
Sun Microsystems Discussion
http://java.sun.com/j2se/1.3/docs/guide/awt/designspec/events.html
Introduction
Graphical user interfaces (GUIs) are event driven - they generate events when the user interacts with them by clicking etc. GUIs have event handlers associated with them - explained below.
When the interaction occurs, an event is automatically sent to the program for processing. The event information is stored in an object that inherits from AWTEvent. The hierarchy of classes involved is illustrated below.
Note that the Swing component uses these event types. Swing has added new ones that
are defined in javax.swing.event.
Listener Interfaces of awt.event
An event listener for a GUI is an object of a class that implements one or more of the event-listener interfaces from package java.awt.event and package javax.swing.event. Many of the event listeners are common to these two packages.
In order to process one of these user interface events, the programmer must perform two important tasks.
Register an event listener
Implement an event handler
An event listener object then "listens" for specific types of event generated in the object or by other objects (usually GUIs) in the program.
An event handler is a method that is automatically called in response to a particular type of event.
Each event listener interface specifies one or more event handling methods that must be defined in the class that implements the event-listener interface.
Delegation Event Model means that the processing of an event is delegated to a particular object in the program.
// Demonstrating the JTextField class and the Delegation Event Model
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class TextFieldTest extends JFrame
{
private JTextField text1, text2, text3;
private JPasswordField password;
public TextFieldTest()
{
super( "Testing JTextField and JPasswordField" );
Container c = getContentPane();
c.setLayout( new FlowLayout() );
// construct textfield with default sizing
text1 = new JTextField( 10 ); //2. Is 10 width or height or neither?
c.add( text1 );
// construct textfield with default text
text2 = new JTextField( "Enter text here" );
c.add( text2 );
// construct textfield with default text and 20 visible elements and no event handler
text3 = new JTextField( "Uneditable text field", 20 );
text3.setEditable( false );
c.add( text3 );
// construct textfield with default text
password = new JPasswordField( "Hidden text" );
c.add( password );
TextFieldHandler handler = new TextFieldHandler();
text1.addActionListener( handler );
text2.addActionListener( handler );
text3.addActionListener( handler );
password.addActionListener( handler ); //3. What is handler?
setSize( 325, 100 );
show();
}
public static void main( String args[] )
{
TextFieldTest app = new TextFieldTest();
app.addWindowListener(new WindowAdapter()
{
public void windowClosing( WindowEvent e )
{
System.exit( 0 ); //4. What is the purpose of this code?
}
}
);
}
// inner class for event handling
private class TextFieldHandler implements ActionListener
{
public void actionPerformed( ActionEvent e ) //5. What is ActionEvent?
{
String s = "";
if ( e.getSource() == text1 )
s = "text1: " + e.getActionCommand();
else if ( e.getSource() == text2 )
s = "text2: " + e.getActionCommand();
else if ( e.getSource() == text3 )
s = "text3: " + e.getActionCommand();
else if ( e.getSource() == password )
{
JPasswordField pwd =
(JPasswordField) e.getSource();
s = "password: " +
new String( pwd.getPassword() );
}
JOptionPane.showMessageDialog( null, s );
}
}
}
