Need help, I need to randomly change the size of programs balls

I am starting to learn java at home and one of the tasks in the book is to create a program which randomly changes the size of balls which are bouncing off the screen, but all i seem to be able to do is get them to randomly start in different places not change size, can anyone help please?

here is my code:

Code:

---

import java.awt.*;
import java.awt.geom.*;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Random;
import java.util.ArrayList;

/**
 * Class BallDemo - a short demonstration showing animation with the
 * Canvas class.
 *
 * @author Michael Kölling and David J. Barnes
 * @version 2011.07.31
 */

public class BallDemo 
{
    private Canvas myCanvas;

    /**
    * Create a BallDemo object. Creates a fresh canvas and makes it visible.
    */
    public BallDemo()
    {
        myCanvas = new Canvas("Ball Demo", 600, 500);
    }

        public void drawFrame() {
        int borderSize = 20;
        Dimension size = myCanvas.getSize();
        Rectangle r = new Rectangle(borderSize, borderSize, (int) size.getWidth() - 2*borderSize, (int) size.getHeight() - 2*borderSize);
        myCanvas.draw(r);
    }
 
     
    /**
    * Simulate two bouncing balls
    */
      public void bounce(int numberOfBalls)
    {
        int ground = 400;  // position of the ground line
        myCanvas.setVisible(true);
        // draw the ground
        myCanvas.drawLine(50, ground, 550, ground);
        // create and show the balls
        Random random = new Random();
        HashSet<BouncingBall> balls = new HashSet<BouncingBall>();
        for(int i=0; i<numberOfBalls; i++) {
            Dimension size = myCanvas.getSize();
            int x = random.nextInt((int) size.getWidth());           
            int y = random.nextInt((int) size.getHeight());
            BouncingBall ball = new BouncingBall(x, y, 16, Color.blue, ground, myCanvas);
            balls.add(ball);
            ball.draw();
        }

       
        // make them bounce
        boolean finished =  false;
        while(!finished) {
            myCanvas.wait(50);          // small delay
            for(BouncingBall ball : balls) {
                ball.move();
                // stop once ball has travelled a certain distance on x axis
                if(ball.getXPosition() >= 550 + 32*numberOfBalls) {
                    finished = true;
                }
            }
        }
        for(BouncingBall ball : balls) {
            ball.erase();
        }
    }
}

---

You are missing the BouncingBall class here. To make a guess when you call the line:

BouncingBall ball = new BouncingBall(x, y, 16, Color.blue, ground, myCanvas);

... the x and y are the coordinates of the ball, and 16 is the radius of the ball.
So you have to make an random integer within reasonable limits (not to make the ball too big or small) - assign it to a variable and then replace 16 with the variable. How to make an random integer within limits - thats for you to google.
Luck