CSCI 146: Lab 8 Materials

The main goal of Lab 8 is to enhance the students' understanding of applets.

• You need to extend the following Java class. The code given to you can be compiled and run directly using appletviewer or your browser. The code generates an applet which has a button “start” and a ball. When the “start” button is pressed, the ball starts moving in a particular direction. The code has been written so that the ball never leaves the boundaries of the applet.
• You will have to extend this class by: 1. Creating a “stop” button, which, when pressed should halt the movement of the ball. The ball should just hold its position when the “stop” button is pressed, and should continue its motion when the “start” button is pressed again. 2. Writing some code that will create another ball, which behaves similarly as the first ball. Make it start from a different position. It should also be under the control of the “start” and the “stop” buttons. (You can make use of the existing code and logic).

  Here is the Ball class:
  

  
  import java.applet.*;
  
  import java.awt.*;
  
  import java.awt.event.*;
  
   
  
   // Ball.html will contain: applet code = Ball.class width = 500 height=500 /applet, 
   // with the appropriate angled brackets
  
    
  
    public class Ball extends Applet implements Runnable, ActionListener 
  
    {
  
      int x = 250, y = 250, r=30;           // Position and radius of the circle
  
      int dx = 8, dy = 5;                        // Trajectory of circle
  
      Dimension size;                           // The size of the applet
  
      Graphics g;                                  // A Graphics object for the buffer
  
      Thread animator = null;              // Thread that performs the animation
  
  	       
  
      Button start;
  
  		   
  
      public void init() 
  
       {
         size = this.size();
         g = getGraphics();
         start = new Button("start");
         add(start);
         start.addActionListener(this);
        }
      public void paint(Graphics g) 
       {
          g.setColor(Color.white);
  	g.fillRect(0, 0, size.width,
  	size.height); // clear the buffer
  
  	g = getGraphics();
  
          g.setColor(Color.blue);
          g.fillOval(x-r, y-r, r*2, r*2);            // draw the circle
        }
  
    public void update(Graphics g) 
      { 
  	paint(g);
      }
  
    /** The body of the animation thread */
  
    public void run() 
      {
        while(true) 
          {
  	    // Bounce the circle if we've hit an edge.
              if ((x - r + dx < 0) || (x + r + dx > size.width)) dx = -dx;
              if ((y - r + dy < 0) || (y + r + dy > size.height)) dy = -dy;
  
  
              // Move the circle.
              x += dx;  y += dy;
  
  
  
              // Ask the browser to call our paint() method to redraw the circle
    	    // at its new position.  Tell repaint what portion of the applet needs
              // be redrawn: the rectangle containing the old circle and the
              // the rectangle containing the new circle.  These two redraw requests
              // will be merged into a single call to paint()
              repaint(x-r-dx, y-r-dy, 2*r, 2*r);   // repaint old position of circle
              repaint(x-r, y-r, 2*r, 2*r);         // repaint new position of circle
  
              // Now pause 50 milliseconds before drawing the circle again.
              try 
                 {
  		    Thread.sleep(50); 
                 }
  	    catch (InterruptedException e) 
                 {
  		    e.printStackTrace(); 
                 }
       }
  }
  
  public void actionPerformed(ActionEvent e)
     {
       if(animator==null)
         {     
  	    animator = new Thread(this);
  	    animator.start();
         }
     }
  }