Date: April 16, 2001

Defines standard and consistent mechanism to generate and process events.

A source generates an event and sends it to one or more listeners.

Listener simply waits until it receives an event.

Listener processes events and then returns.

User interface element is able to "delegate" the processing of an event to a separate piece of code.

Notifications are sent only to those listeners that want to receive them.

Event was propagated up the containment hierarchy until it was handled by a component.

In the delegation model, an event is an object that describes a state change in a source.

A source is an object that generates an event. This occurs when object changes in some way.

Source may generate more than one type of event.

A source must register listeners in order for the listeners to receive notifications about a specific type of event.

Each type of event has its own registration method.

General form:

public void addTypeListener (TypeListener el)

When an event occurs, all registered listeners are notified and receive a copy of the event object.

It is called Multicasting the event.

Some sources may allow only one listener to register.

General form:

public void addTypeListener (TypeListener el) throws java.util.TooManyListenersException

To remove Listener

Public void removeTypeListener (TypeListener el)

A listener is an object that is notified when an event occurs.

• Must have registered with one or more sources to receive notifications about specific types of events
• Must implement methods to receive and process these notifications

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

EventObject (Object src)

Contains 2 methods:

Object getSource()

String toString()

AWTEvent class defined in java.awt package is a subclass of EventObject.

It is superclass (directly or indirectly) of all AWT-based events handled by delegation model.

Method getID () can be used to determine the type of event.

Event Source

Button

Checkbox

Choice

List

Menu Item

Scrollbar

Text Components

Window 

void actionPerformed (ActionEvent ae)

void adjustmentValueChanged (AdjustmentEvent ae)

void componentResized (ComponentEvent ce)

void componentMoved (ComponentEvent ce)

void componentShown (ComponentEvent ce)

void componentHidden (ComponentEvent ce)

void componentAdded (ContainerEvent ce)

void componentRemoved (ContainerEvent ce)

void focusGained (FocusEvent fe)

void focusLost (FocusEvent fe)

void itemStateChanged (ItemEvent ie)

void keyPressed (KeyEvent ke)

void keyReleased (KeyEvent ke)

void keyTyped (KeyEvent ke)

void mouseClicked (MouseEvent me)

void mouseEntered (MouseEvent me)

void mouseExited (MouseEvent me)

void mousePressed (MouseEvent me)

void mouseReleased (MouseEvent me)

void mouseDragged (MouseEvent me)

void mouseMoved (MouseEvent me)

void textChanged (TextEvent te)

void windowActivated (WindowEvent we)

void windowClosed (WindowEvent we)

void windowClosing (WindowEvent we)

void windowDeactivated (WindowEvent we)

void windowDeiconified (WindowEvent we)

void windowIconified (WindowEvent we)

void windowOpened (WindowEvent we)

Provides an empty implementation of all methods in an event listener interface.

Useful when u want to listen and process only some of the events that are handled by one particular event listener interface.

Define your own class as subclass of this class and provide desired implementation.

Listener Interfaces implemented by Adapter classes

import java.awt.event.*;

import java.applet.*;

/*

<applet code="AdapterDemo" width=300 height=100>

</applet>

*/

public class AdapterDemo extends Applet {

public void init () {

}

}

class MyMouseAdapter extends MouseAdapter {

AdapterDemo adapterDemo;

public MyMouseAdapter(AdapterDemo adapterDemo) {

}

// Handle mouse clicked

public void mouseClicked (MouseEvent me) {

adapterDemo.showStatus ("Mouse Clicked");

}

}

class MyMouseMotionAdapter extends MouseMotionAdapter {

AdapterDemo adapterDemo;

public MyMouseMotionAdapter (AdapterDemo adapterDemo) {

}

// Handle Mouse Drag

public void mouseDragged (MouseEvent me) {

adapterDemo.showStatus("Mouse Dragged");

}

}

import java.awt.event.*;

import java.applet.*;

/*

<applet code="MousePressedDemo" width=300 height=100>

</applet>

*/

public class MousePressedDemo extends Applet {

public void init () {

}

}

class MyMouseAdapter extends MouseAdapter {

MousePressedDemo mousePressedDemo;

public MyMouseAdapter (MousePressedDemo mousePressedDemo) {

}

// Handle mouse clicked

public void mouseClicked (MouseEvent me) {

mousePressedDemo.showStatus ("Mouse Clicked");

}

}

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

/*

<applet code="InnerClassDemo" width=300 height=100>

</applet>

*/

public class InnerClassDemo extends Applet {

public void init () {

addMouseListener(new MyMouseAdapter ());

}

class MyMouseAdapter extends MouseAdapter {

public void mouseClicked (MouseEvent me) {

showStatus ("Mouse Clicked");

}

}

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

/*

<applet code=" AnonymousInnerClassDemo" width=300 height=100>

</applet>

*/

public class AnonymousInnerClassDemo extends Applet {

public void init () {

addMouseListener(new MyMouseAdapter () {

showStatus ("Mouse Clicked");

}

}