Event Handling In Java - Naughty Examples For Those Little Mice

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Mouse Events, MouseListener and MouseMotionListener:

These are generated when a mouse is moved, clicked, pressed and released etc. The MouseEvent class represents these events as six constant values and has methods to get the co-ordinates at which a mouse event occurred. The mouse move and mouse drag are treated differently from the rest four mouse pressed, mouse released, mouse entered and mouse exited. Hence there are two types of mouse event listeners- MouseListener and MouseMotionListener.

The following code illustrates the usage of mouse event listeners. Watch out for messages on the status line of applet as and when the user performs a mouse event. (Example 3 also shows the co-ordinates at which the event occurred.

// Example 3: MouseEvents & MouseListener
/*
<Applet code = "mouseEvent.class" height= 400 width = 400 >
</applet>
*/
Import java.awt.*;
Import java.awt.event.*;
Import javax.swing.*;
Public class mouseEvent extends JApplet implements MouseListener, MouseMotionListener
{
Public void init()
{
addMouseListener(this);
addMouseMotionListener(this);
}
Public void mouseClicked(MouseEvent e)
{
showStatus("Mouse has been clicked at " + e.getX()+ "," + e.getY());
}
Public void mouseEntered(MouseEvent e)
{
showStatus("Mouse has been Entered at " + e.getX()+ "," + e.getY());
// For loop:to make sure mouse entered is on status bar for a few sec
For (int i= 0; i<1000000; i++);
} 
Public void mouseExited (MouseEvent e)
{
showStatus ("Mouse has been Exited at " + e.getX()+ "," + e.getY());
}
Public void mousePressed(MouseEvent e)
{
showStatus ("Mouse pressed at " + e.getX()+ "," + e.getY());
}
Public void mouseReleased(MouseEvent e)
{
showStatus("Mouse released at " + e.getX()+ "," + e.getY());
}
Public void mouseDragged(MouseEvent e)
{
showStatus("Mouse dragged at " + e.getX()+ "," + e.getY());
}
Public void mouseMoved(MouseEvent e)
{
showStatus("Mouse moved at " + e.getX()+ "," + e.getY());
}
}

Since the applet implements MouseListener and MouseMotionListener it has to provide for the functionality of all their methods. Observe the mouseEntered() method has a small loop to ensure the mouse entered message remains on the status bar for a while. {mospagebreak title=Key Frienldy Codes} Keyboard Events and KeyListener: They are generated on pressing or releasing a key. The KeyEvent class contains constants to represent the keys pressed or typed. The event listener corresponding to these types of event is the KeyListener.

The following example puts forth key event handling. The applet contains a label and a text field. The recently typed in character in the text field is displayed in the status bar. Example 4.

// Key events and KeyListener.
/*
<applet code = "keyTest.class" width = 400 height = 400>
</applet>
*/
Import java.awt.*;
Import java.awt.event.*;
Import java.applet.Applet;
Public class keyTest extends Applet implements KeyListener
{
Public void init()
{
Label lab = new Label ("Enter Characters :");
add(lab);
TextField tf = new TextField(20);
add(tf);
tf.addKeyListener(this);
}
Public void keyPressed(KeyEvent e)
{}
Public void keyReleased(KeyEvent e)
{}
Public void keyTyped(KeyEvent e)
{
showStatus(" Recently typed characters are : " + e.getKeyChar());
}
}

Here the text field delegates its key events to the applet. The add() method adds the components Label and TextField to the applet.

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