Java : Peeka, Boo!

Our last program responded to mouse movements by polling: every time the Timer event "went off", we checked the current mouse position using MouseInfo.getPointerInfo().getLocation(). Today we're going to modify our program to respond directly to mouse events.

It's best if you make a new working directory, let's say "PeekaBoo", with its own "Images" sub-directory.
Now we need two new images: save them as "peeka.jpg" and "boo.jpg" in the new "Images" directory.

Instead of implementing "ActionListener" (listening for timer events), our class now implements "MouseListener" (listening for mouse events).

public class PeekaBoo extends JPanel implements MouseListener

We can drop the definition of the PERIOD constant, as we're not using a timer, and change LEFT & RIGHT to PEEKA & BOO.

private static final int PWIDTH =  237;
private static final int PHEIGHT=  315;
private static final int PEEKA =  0; // images[0] is Peeka
private static final int BOO =  1; // image [1] is Boo

Our constructor (initialization) method becomes:

public PeekaBoo()
{
 setPreferredSize(new Dimension(PWIDTH,PHEIGHT));
 images [PEEKA] = loadImage("Images/peeka.jpg");
 images [BOO] = loadImage("Images/boo.jpg");
  
 imageIndex = PEEKA;
  
 addMouseListener(this);  
}

Instead of starting a Timer, we add a mouse listener.
Now all that's required is to tell Java how to respond to mouse events.

// On mouse-click, display Boo image
public void mouseClicked(MouseEvent e) {imageIndex=BOO; repaint();}

// When mouse leaves window, display Peeka image
public void mouseExited(MouseEvent e) {imageIndex=PEEKA; repaint();} 

So, when a mouse-click is detected somewhere in the program window, display the "Boo" image. When the mouse leaves the program window, revert to the "Peeka" image.

And that's it. Here's the full program:

// PeekaBoo.java

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.swing.*;
import javax.imageio.*;
import java.io.*;

public class PeekaBoo extends JPanel implements MouseListener
{
 private static final int PWIDTH =  237;
 private static final int PHEIGHT=  315;
 private static final int PEEKA =  0; // images[0] is Peeka
 private static final int BOO =  1; // image [1] is Boo
 
 private BufferedImage [] images = new BufferedImage [2];
 
 private int imageIndex;
 
 public PeekaBoo()
 {
  setPreferredSize(new Dimension(PWIDTH,PHEIGHT));

  images [PEEKA] = loadImage("Images/peeka.jpg");
  images [BOO] = loadImage("Images/boo.jpg");
  
  imageIndex = PEEKA;
  
   addMouseListener(this);  
 }

 public BufferedImage loadImage(String fileName) 
 {
  try {
   BufferedImage im =  ImageIO.read(getClass().getResource(fileName));
   return im; 
  } catch(IOException e) {
   System.out.println("Load Image error for "+fileName+":\n"+e);
   return null;
  }
 }
  
 public void paintComponent(Graphics g){g.drawImage(images [imageIndex],0,0,this);}

 // stubs for you to play around
 public void mousePressed(MouseEvent e) {}
 public void mouseReleased(MouseEvent e) {}
 public void mouseEntered(MouseEvent e) {}
 
 // On mouse-click, display Boo image
 public void mouseClicked(MouseEvent e) {imageIndex=BOO; repaint();}
 
 // When mouse leaves window, display Peeka image
 public void mouseExited(MouseEvent e) {imageIndex=PEEKA; repaint();}
 
 public static void main(String args[])
 {
  final PeekaBoo showPanel = new PeekaBoo();

  JFrame showFrame = new JFrame("Peeka-Boo");
  showFrame.getContentPane().add(showPanel,BorderLayout.CENTER);
  showFrame.setDefaultCloseOperation (WindowConstants.EXIT_ON_CLOSE);
  showFrame.pack();
  showFrame.setResizable(false);  
  showFrame.setVisible(true); 
 }
}

And that's all there is too it. Save as "PeekaBoo.java" and compile and run in the usual way. Let's try it out. Click on the image below, then move the mouse away.

Something's different...

Apart from the obvious change of image, the program running above is an applet embedded in a web page, rather than a stand-alone app like our program listing.

Next time we'll go through the steps required to convert our app into an applet and post it on blogger.