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.
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.
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.
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 |
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 >
