The Delegation Event Model

1. The modern approach to handling events is based on the delegation event model, which defines standard and consistent mechanisms to generate and process events.
2. Its concept is quite simple: a source generates an event and sends it to one or more listeners.
3. In the scheme, the listener simply waits until it receives an event.  Once received, the listener processes event and then returns.  The advantage of this design is that the application logic that processes events is clearly separated from the user interface logic that generates those events.
4. In the delegation event model, listeners must register with a source in order to receive an event notification.  This provides an important benefit: notifications are sent only to listeners that want to receive them.

Event:

1. An event is an object that describes some change in source.  It can be generated when a person interacts with an element in a GUI.  For example pressing a button, clicking a mouse, double clicking on a list box entry or closing window.

1. Events may also occur that are not directly caused by interactions with a user interface.  For example, an event may be generated when a time expires, counter exceeds a value, a software or hardware failure occurs.

Event Source:

A source generates events.  It has three main responsibilities

1. It must provide methods that allow listeners to register and unregister for notifications about a specific type of events.
2. It must generate the event
3. It must send the event to all registered listeners.  The event may be a unicast to a single listeners or multicast to several listeners.  It is possible for a source to generate several types of events.
4. Each type of event has its own registration method. Here is the general form:

                                 public void addTypeListener(TypeListener el)

Event Listeners:

A listener receives event notifications.  It has three main responsibilities

1. It must register to receive notifications account specific events.  It does so by calling the appropriate registration method of source.
2. It must implement an interface to receive events of that type.
3. It must unregister if it is no longer to proceed those notifications.  It does so by calling appropriate unregistration method of the source.

Event Classes

The classes that represent events are at the core of Java’s event handling mechanism.

                                                         Object

                                                    Event Object

                                                      AWT Event 

ActionEvent       AdjustmentEvent     ComponentEvent       ItemEvent    TextEvent

ContainerEvent       FocusEvent           InputEvent                WindowEvent

                                          KeyEvent             MouseEvent

At the root of the Java event class hierarchy is EventObject, which is in java.util.

It is the superclass for all events. Its one constructor is shown here:

EventObject(Object src). Here, src is the object that generates this event.

To summarize:

■ EventObject is a superclass of all events.

■ AWTEvent is a superclass of all AWT events that are handled by the delegation

event model.

The package java.awt.event defines several types of events that are generated by

various user interface elements

Main Event Classes in java.awt.event

Sources of Events

The table lists some of the user interface components that can generate the events

Event Listener Interfaces

The delegation event model has two parts: sources and listeners. Listeners are created by implementing one or more of the interfaces defined by the java.awt.event package. When an event occurs, the event source invokes the appropriate method defined by the listener and provides an event object as its argument. Table lists commonly used listener interfaces and provides a brief description of the methods that they define.

// Demonstrate the mouse event handlers.

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;

public class MouseEventEx extends JFrame implements MouseListener

{

Container cp;

public MouseEventEx()

{

cp=getContentPane();

addMouseListener(this);

setSize(300,300);

setTitle(“Mouse Listener”);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

setVisible(true);

}

public void mouseClicked(MouseEvent e)

{

cp.setBackground(Color.pink);

System.out.println(e.getX()+”,”+e.getY());

System.out.println(“mouseClicked”);

}

public void mouseEntered(MouseEvent e)

{

cp.setBackground(Color.yellow);

System.out.println(“mouseEntered”);

}

public void mouseExited(MouseEvent e)

{

cp.setBackground(Color.cyan);

System.out.println(“mouseExited”);

}

public void mousePressed(MouseEvent e)

{

cp.setBackground(Color.red);

System.out.println(“mousePressed”);

}

public void mouseReleased(MouseEvent e)

{

cp.setBackground(Color.blue);

System.out.println(“mouseReleased”);

}

public static void main(String[] args)

{

new MouseEventEx();

}

}

// Demonstrate the key event handlers.

import java.awt.*;

import java.awt.event.*;

public class SimpleKey extends JFrame implements KeyListener {

String msg = “”;

int X = 10, Y = 20; // output coordinates

public SimpleKey () {

addKeyListener(this);

requestFocus(); // request input focus

setSize(300,300);

setTitle(“Mouse Listener”);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

setVisible(true);

}

public void keyPressed(KeyEvent ke) {

showStatus(“Key Down”);

}

public void keyReleased(KeyEvent ke) {

showStatus(“Key Up”);

}

public void keyTyped(KeyEvent ke) {

msg += ke.getKeyChar();

repaint();

}

// Display keystrokes.

public void paint(Graphics g) {

g.drawString(msg, X, Y);

}

public static void main(String[] args)

{

new SimpleKey ();

}

}

 JAVA LIBRARY