Output not visible !

Question

Can you see why the BouncingBalls are not visible ?
Something is wrong with the bounce() method in the BallDemo class
I tried to pick it apart one line at a time. No compilation errors.
Here is the bounce() method and the BallDemo class in which it resides and the BoungingBall class that defines the balls.
Norman
//bounce method
public void bounce()
           int ground = 400;   // position of the ground line
           int numberOfBalls = 0;
           myCanvas.setVisible(true);
           ArrayList balls = new ArrayList();
           // draw the ground
           myCanvas.drawLine(50, ground, 550, ground);
           //ex 5.50 (a)
           System.out.println("Type in the number of balls you would like to see bouncing and hit enter");
             numberOfBalls = input.readInt();
             BouncingBall myBall;
          myBall =   new BouncingBall( 100,  0,  24,  Color.blue,
                            ground,  myCanvas);
            //(b)
            for(int i = 0; i < numberOfBalls; i++)
              //(c) (d)
           Random rg = new Random(255);
            BouncingBall newBall =  new BouncingBall(i + 100, 0, 16, new Color(rg.nextInt(255),rg.nextInt(255),rg.nextInt(255)), ground, myCanvas);
            //add the ball to the ArrayList object
           balls.add(newBall);
          for(int i = 0; i < numberOfBalls; i++)
                 BouncingBall ball = (BouncingBall)balls.get(i);
                 ball.draw();
             // make the balls bounce
             boolean finished =  false;
             while(!finished) {
              myCanvas.wait(50);           // small delay
              ball.move();
               // stop once ball has travelled a certain distance on x axis
               if(ball.getXPosition() >= 550 )
                   finished = true;
           }//move loop
                 ball.erase();
         }//for loop
       }//bounce method
import java.awt.*;
//(d)
import java.awt.Color;
import java.util.Random;
import java.awt.geom.*;
//ex5.50 (a)
import java.util.*;
import java.io.*;
* Class BallDemo - provides two short demonstrations showing how to use the
* Canvas class.
//(a) Change the bounce() method in the BallDemo class to let the user
//choose how many balls should be bouncing
//(b) Use a collection to store the balls
//(c) Place the balls in a row along the top of the canvas.
public class BallDemo
    private Canvas myCanvas;
    FormattedInput input;
     * Create a BallDemo object. Creates a fresh canvas and makes it visible.
    public BallDemo()
        myCanvas = new Canvas("Ball Demo", 600, 500);
        input = new FormattedInput();
        myCanvas.setVisible(true);
     * This method demonstrates some of the drawing operations that are
     * available on a Canvas object.
    public void drawDemo()
        myCanvas.setFont(new Font("helvetica", Font.BOLD, 14));
        myCanvas.setForegroundColor(Color.red);
        myCanvas.setBackgroundColor(Color.yellow);
        myCanvas.drawString("We can draw text, ...", 20, 50);
        myCanvas.wait(1000);
        myCanvas.setForegroundColor(Color.black);
        myCanvas.drawString("...draw lines...", 60, 70);
        myCanvas.wait(500);
        myCanvas.setForegroundColor(Color.blue);
        myCanvas.drawLine(200, 20, 300, 450);
        myCanvas.wait(500);
        myCanvas.setForegroundColor(Color.blue);
        myCanvas.drawLine(220, 100, 570, 260);
        myCanvas.wait(500);
        myCanvas.setForegroundColor(Color.green);
        myCanvas.drawLine(290, 10, 620, 220);
        myCanvas.wait(1000);
        myCanvas.setForegroundColor(Color.white);
        myCanvas.drawString("...and shapes!", 110, 90);
    //    myCanvas.setForegroundColor(Color.red);
                // the shape to draw and move
        int xPos = 10;
        Rectangle rect = new Rectangle(xPos, 150, 30, 20);
        // move the rectangle across the screen
        for(int i = 0; i < 600; i ++) {
            myCanvas.fill(rect);
            myCanvas.wait(10);
            myCanvas.erase(rect);
            xPos++;
            rect.setLocation(xPos, 150);
        // at the end of the move, draw once more so that it remains visible
        myCanvas.fill(rect);
       //ex 5.48
       public void drawFrame()
         Dimension myDimension =new Dimension( myCanvas.getSize());
         //Rectangle rectangle = new Rectangle(10,10,myDimension.width,  myDimension.height);
         //page 136
        Rectangle rectangle = new Rectangle(10,10,580,480);
         myCanvas.fill(rectangle);
         public void bounce()
           int ground = 400;   // position of the ground line
           int numberOfBalls = 0;
           myCanvas.setVisible(true);
           ArrayList balls = new ArrayList();
           // draw the ground
           myCanvas.drawLine(50, ground, 550, ground);
           //ex 5.50 (a)
           System.out.println("Type in the number of balls you would like to see bouncing and hit enter");
             numberOfBalls = input.readInt();
             BouncingBall myBall;
          myBall =   new BouncingBall( 100,  0,  24,  Color.blue,
                            ground,  myCanvas);
            //(b)
            for(int i = 0; i < numberOfBalls; i++)
              //(c) (d)
           Random rg = new Random(255);
            BouncingBall newBall =  new BouncingBall(i + 100, 0, 16, new Color(rg.nextInt(255),rg.nextInt(255),rg.nextInt(255)), ground, myCanvas);
            //add the ball to the ArrayList object
           balls.add(newBall);
          for(int i = 0; i < numberOfBalls; i++)
                 BouncingBall ball = (BouncingBall)balls.get(i);
                 ball.draw();
             // make the balls bounce
             boolean finished =  false;
             while(!finished) {
              myCanvas.wait(50);           // small delay
              ball.move();
               // stop once ball has travelled a certain distance on x axis
               if(ball.getXPosition() >= 550 )
                   finished = true;
           }//move loop
                 ball.erase();
         }//for loop
       }//bounce method
     }//endof class
import java.awt.*;
import java.awt.geom.*;
* Class BouncingBall - a graphical ball that observes the effect of gravity. The ball
* has the ability to move. Details of movement are determined by the ball itself. It
* will fall downwards, accelerating with time due to the effect of gravity, and bounce
* upward again when hitting the ground.
* This movement can be initiated by repeated calls to the "move" method.
* @version 1.1  (23-Jan-2002)
public class BouncingBall
    private static final int gravity = 3;  // effect of gravity
    private int ballDegradation = 2;
    private Ellipse2D.Double circle;
    private Color color;
    private int diameter;
    private int xPosition;
    private int yPosition;
    private final int groundPosition;      // y position of ground
    private Canvas canvas;
    private int ySpeed = 1;                // initial downward speed
     * Constructor for objects of class BouncingBall
     * @param xPos  the horizontal coordinate of the ball
     * @param yPos  the vertical coordinate of the ball
     * @param ballDiameter  the diameter (in pixels) of the ball
     * @param ballColor  the color of the ball
     * @param groundPos  the position of the ground (where the wall will bounce)
     * @param drawingCanvas  the canvas to draw this ball on
    public BouncingBall(int xPos, int yPos, int ballDiameter, Color ballColor,
                        int groundPos, Canvas drawingCanvas)
        xPosition = xPos;
        yPosition = yPos;
        color = ballColor;
        diameter = ballDiameter;
        groundPosition = groundPos;
        canvas = drawingCanvas;
     * Draw this ball at its current position onto the canvas.
    public void draw()
        canvas.setForegroundColor(color);
        canvas.fillCircle(xPosition, yPosition, diameter);
     * Erase this ball at its current position.
    public void erase()
        canvas.eraseCircle(xPosition, yPosition, diameter);
     * Move this ball according to its position and speed and redraw.
    public void move()
        // remove from canvas at the current position
        erase();
        // compute new position
        ySpeed += gravity;
        yPosition += ySpeed;
        xPosition +=2;
        // check if it has hit the ground
        if(yPosition >= (groundPosition - diameter) && ySpeed > 0) {
            yPosition = (int)(groundPosition - diameter);
            ySpeed = -ySpeed + ballDegradation;
        // draw again at new position
        draw();
     * return the horizontal position of this ball
    public int getXPosition()
        return xPosition;
     * return the vertical position of this ball
    public int getYPosition()
        return yPosition;
}

Answer 1

I don't even have to study your code - all I have to do is search the HTML on this screen and see there is no reference to run() or threads. You won't make squat move unless you use threads.