What are Events?

• Java applets are mainly event driven programs.
• User activities related to keyboard and mouse  are called events.

Interfaces

• abstract class is a class, defined using the keyword abstract
• abstract class signatures of the methods but no body
• abstract class may contain some abstractmethods and some non-abstract methods.
•  to use these classes  the body of the abstractmethods must be completed.

• Interface is similar to an abstract class with two major differences:

(a) in defining an interface the key word interface is used instead of the keyword abstract
(b) none of the methods in an interface are defined, only signatures are included.
• Extend a superclass using the keyword extends
• Incorporate an interface into a class using the keyword implements.
• Syntax:

public class sample extends example implements interface1{

}
 

• An interface implementation must include definitions for all methods in the interface.

• java.awt.Event package : Listener interfaces to handle events.

All the interfaces and the methods in them.
 

Event	Generated by	Listener Interface	Methods in them
Action event	Pressing a button	ActionListener	actionPerformed()
Adjustment event	Scroll bar is manipulated	AdjustmentListener	adjustmentValueChanged()
Component event	A component is hidden, moved, resized, or shown	ComponentListener	componentHidden()  componentMoved()  componentResized()  componentShown()
Container event	A component is added or removed from a container	ContainerListener	componentAdded()  componentRemoved()
Focus event	A component gains or loses focus	FocusListener	focusGained()  focusLost()
Item event	An item is selected or deselected	ItemListener	itemStateChanged()
Key event	A key is pressed, released or typed	KeyListener	keyPressed()  keyReleased()  keyTyped()
Mouse event	Mouse button is clicked, pressed or released.  Mouse pointer enters leaves a component	MouseListener	mouseClicked()  mouseEntered()  mouseExited()  mousePressed()  mouseReleased()
Mouse event	Mouse pointer is dragged or moved	MouseMotionListener	mouseDragged()  mouseMoved()
Text event	Text value is changed	TextListener	textValueChanged()
Window event	A window is activated, closed, deactivated, deiconfied, opened, or quit	WindowListener	windowActivated()  windowClosed()  windowClosing()  windowDeactivated()  windowDeiconified()  windowIconified()  windowOpened()

Step 1: Import classes from the package:
 
import java.awt.event.*;
Step 2: Implement the appropriate listener interfaces.
 
public class sample extends Applet implements listenerinterface1{

} //Several listener interfaces can be listed, separated by commas.
 

Step 3: Register the listener object with the appropriate event source.
 
event_source_object.addevent_listenername(event_listener_object);
//Normally, you use the keyword this for event_listener_object.


Step 4: Define all the methods of the implemented listener interfaces. I

Mouse Events

 

Event Class	MouseEvent
Listener Interface	MouseListener
Listener Methods	public void mouseEntered(MouseEvent event)  public void mouseExited(MouseEvent event)  public void mouseClicked(MouseEvent event)  public void mousePressed(MouseEvent event)  public void mouseReleased(MouseEvent event)  By looking at the name of the methods, it is clear which activity invokes which method.  event.getX() returns (int) the x coordinate of mouse position.  event.getY() returns (int) the y coordinate of mouse position.  event.getClickCount() returns (int) the number of mouse clicks.

Four steps:

Step 1: Include the following import statement:
 

import java.awt.event.*;


Step 2: Include Implements MouseListener :
 

public class w2 extends Applet implements MouseListener {

}
 

Step 3: Register addMouseListener method in the applet.
 
this.addMouseListener (this);


Step 4: Define the five MouseListener methods:
 

public void mouseClicked(MouseEvent event){
}

public void mouseEntered(MouseEvent event){
}

public void mouseexited(MouseEvent event){
}

public void mousePressed(MouseEvent event){
}

public void mouseReleased(MouseEvent event){
}

The following program changes the applet background color, whenever the mouse event occurs.
 
import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class MouseEvent1 extends Applet implements MouseListener{

public void init() {

this.addMouseListener(this);
}

public void mouseClicked(MouseEvent e){

setBackground(Color.blue);
}

public void mouseEntered(MouseEvent e){

setBackground(Color.cyan);
}

public void mouseExited(MouseEvent e){

setBackground(Color.green);
}

public void mousePressed(MouseEvent e){

setBackground(Color.magenta);
}

public void mouseReleased(MouseEvent e){

setBackground(Color.yellow);
}

}

Mouse Motion Activities

• Moving and dragging the mouse generates  an object of the MouseEvent class.

 

Event Class	MouseEvent
Listener Interface	MouseMotionListener
Listener Methods	public void mouseMoved(MouseEvent event)  Invoked when the mouse button has been moved on a component (with no buttons down). public void mouseDragged(MouseEvent event) Invoked when a mouse button is pressed on a component and then dragged. event.getX() returns (int) the x coordinate of mouse position.  event.getY() returns (int) the y coordinate of mouse position.  event.getClickCount() returns (int) the number of mouse clicks.

The following program changes an applet’s background color when any mouse activity take place.
 
import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class MouseEvent2 extends Applet implements
MouseMotionListener,MouseListener{

public void init() {

this.addMouseListener(this);
this.addMouseMotionListener(this);
}

public void mouseMoved(MouseEvent event){

setBackground(Color.white);
}

public void mouseDragged(MouseEvent event){

setBackground(Color.black);
}

public void mouseClicked(MouseEvent e){

setBackground(Color.blue);
}

public void mouseEntered(MouseEvent e){

setBackground(Color.cyan);
}

public void mouseExited(MouseEvent e){

setBackground(Color.green);
}

public void mousePressed(MouseEvent e){

setBackground(Color.magenta);
}

public void mouseReleased(MouseEvent e){

setBackground(Color.yellow);
}

}
 

Example

The following program displays the coordinates of the mouse.
 
import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class MouseEvent3 extends Applet implements
MouseMotionListener,MouseListener{

public void init() {

this.addMouseListener(this);
this.addMouseMotionListener(this);
}

public void mouseMoved(MouseEvent event){

setBackground(Color.white);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mouseDragged(MouseEvent event){

setBackground(Color.black);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mouseClicked(MouseEvent event){

setBackground(Color.blue);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mouseEntered(MouseEvent event){

setBackground(Color.cyan);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mouseExited(MouseEvent event){

setBackground(Color.green);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mousePressed(MouseEvent event){

setBackground(Color.magenta);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void mouseReleased(MouseEvent event){

setBackground(Color.yellow);
int x = event.getX();
int y = event.getY();
print(x,y);
}

public void print(int x, int y){

showStatus("x coordinate is = "+x+" y coordinate is = "+y);
}
}

Example

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

public class CircleDrawer2 extends Applet
                           implements MouseListener {
  private int radius = 25;

  public void init() {
    setForeground(Color.blue);
    addMouseListener(this);
  }

  // Remaining methods are from the MouseListener interface.

  public void mouseEntered(MouseEvent event) {}
  public void mouseExited(MouseEvent event) {}
  public void mouseReleased(MouseEvent event) {}
  public void mouseClicked(MouseEvent event) {}

  public void mousePressed(MouseEvent event) {
    Graphics g = getGraphics();
    g.fillOval(event.getX()-radius,
               event.getY()-radius,
               2*radius,
               2*radius);
  }
}
 
 
 

Example

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

// An applet that lets you perform freehand drawing.
 

public class SimpleWhiteboard extends Applet {
  protected int lastX=0, lastY=0;

  public void init() {
    setBackground(Color.white);
    setForeground(Color.blue);
    addMouseListener(new PositionRecorder());
    addMouseMotionListener(new LineDrawer());
  }

  protected void record(int x, int y) {
    lastX = x;
    lastY = y;
  }

  // Record position that mouse entered window or
  // where user pressed mouse button.

  private class PositionRecorder extends MouseAdapter {
    public void mouseEntered(MouseEvent event) {
      requestFocus(); // Plan ahead for typing
      record(event.getX(), event.getY());
    }

    public void mousePressed(MouseEvent event) {
      record(event.getX(), event.getY());
    }
  }

  // As user drags mouse, connect subsequent positions
  // with short line segments.

  private class LineDrawer extends MouseMotionAdapter {
    public void mouseDragged(MouseEvent event) {
      int x = event.getX();
      int y = event.getY();
      Graphics g = getGraphics();
      g.drawLine(lastX, lastY, x, y);
      record(x, y);
    }
  }
}
 
 

Keyboard Events

Whenever a key is pressed, an object of the KeyEvent class is generated.

Virtual Key Codes

• Refer to the key on the keyboard returned by event.getKeyCode()
• Usedas class variables.
• Produce a numeric value - but - use the symbol in the program. Nno guarantee that the numeric value produced is the same on all platforms.

Note

• To check for an upper case letter use:

if ( event.getKeyCode() == VK_A && KeyEvent.isShiftDown())
 
• Similarly use event.isControlDown()and event.isAltDown().

Example

The following program displays several text boxes.  Type any key in top box. The other boxes in the applet show information about the key pressed.

Example

The following program displays two boxes. Type each keyboard key in the first box and see result in the second box.

Example

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

public class Whiteboard extends SimpleWhiteboard {
  protected FontMetrics fm;

  public void init() {
    super.init();
    Font font = new Font("Serif", Font.BOLD, 20);
    setFont(font);
    fm = getFontMetrics(font);
    addKeyListener(new CharDrawer());
  }

  private class CharDrawer extends KeyAdapter {
    // When user types a printable character,
    // draw it and shift position rightwards.

    public void keyTyped(KeyEvent event) {
      String s = String.valueOf(event.getKeyChar());
      getGraphics().drawString(s, lastX, lastY);
      record(lastX + fm.stringWidth(s), lastY);
    }
  }
}
 

Output