Bouncing Ball

SAMPLE APPLET:

ASSIGNMENT: You will need to study the following code carefully!!! Here is what you should do:

Get the sample running
Notice the implements Runnable and the run methods. Figure out what the variables are for (especially xDir, incr, and sleepFor).
Understand why and when the ball changes directions. (That's the if statement in the run method ... the || means OR.)
Alter this applet so the ball moves up and down (in a tall and skinny applet area).

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

public class ball extends Applet implements Runnable{
  static int RADIUS = 15;
  int x = 0; //starting x position
  int y = 5; //starting y position
  int xDir = +1; //positive one
  int incr = 3;  //amount of each position change
  int sleepFor = 40; //delay between redraws (40/1000 sec)
  Graphics offG;
  Image offI;
  Dimension d;

public void init() {
    setBackground(Color.yellow); 
    d=getSize();
    offI=createImage(d.width,d.height);
    new Thread(this).start();
  }
 
   public void run() {
     while(true) {
       x+=(incr * xDir);  // xDir is either +1 or -1
       repaint();
       if(x < 1 || x > d.width-2*RADIUS)  //change directions
          xDir *= -1;  //direction left = neg, right = pos
       try {
         Thread.sleep(sleepFor);
       }catch(Exception e) {
       }
    }
  }

public void update(Graphics g){
    paint(g);
  }

public void paint(Graphics g) {
    offG=offI.getGraphics();
    offG.setColor(getBackground());
    offG.fillRect(0,0,d.width,d.height);
    offG.setColor(Color.blue);
    offG.drawOval(x, y, 2* RADIUS, 2* RADIUS);
    g.drawImage(offI,0,0,this);
    offG.dispose();
  }
}