java Event Handling ~ GNIITHELP

Any program that uses GUI (graphical user interface) such as Java application written for windows, is event driven. Event describes the change of state of any object. Example : Pressing a button, Entering a character in Textbox.

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Components of Event Handling

Event handling has three main components,

• Events : An event is a change of state of an object.
• Events Source : Event source is an object that generates an event.
• Listeners : A listener is an object that listens to the event. A listener gets notified when an event occurs.

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How Events are handled ?

A source generates an Event and send it to one or more listeners registered with the source. Once event is received by the listener, they processe the event and then return. Events are supported by a number of Java packages, like java.util, java.awt and java.awt.event.

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Important Event Classe and Interface

Event Classe	Description	Listener Interface
ActionEvent	generated when button is pressed, menu-item is selected, list-item is double clicked	ActionListener
MouseEvent	generated when mouse is dragged, moved,clicked,pressed or released also when the enters or exit a component	MouseListener
KeyEvent	generated when input is received from keyboard	KeyListener
ItemEvent	generated when check-box or list item is clicked	ItemListener
TextEvent	generated when value of textarea or textfield is changed	TextListener
MouseWheelEvent	generated when mouse wheel is moved	MouseWheelListener
WindowEvent	generated when window is activated, deactivated, deiconified, iconified, opened or closed	WindowListener
ComponentEvent	generated when component is hidden, moved, resized or set visible	ComponentEventListener
ContainerEvent	generated when component is added or removed from container	ContainerListener
AdjustmentEvent	generated when scroll bar is manipulated	AdjustmentListener
FocusEvent	generated when component gains or loses keyboard focus	FocusListener

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Example of Event Handling

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.applet.*;
import java.awt.event.*;
import java.awt.*;

public class Test extends Applet implements KeyListener
{
 String msg="";
 public void init()
 {
   addKeyListener(this);       
 }
 public void keyPressed(KeyEvent k)
 {
   showStatus("KeyPressed");
 }
 public void keyReleased(KeyEvent k)
 {
   showStatus("KeyRealesed");
 }
 public void keyTyped(KeyEvent k)
 {
   msg = msg+k.getKeyChar();
   repaint();
 }
 public void paint(Graphics g)
 {
   g.drawString(msg, 20, 40);
 }
}

HTML code :

< applet code="Test" width=300, height=100 >