Bouncing ball question

Question

Hi, I'm new to the forums and really new to java and I was wondering if anyone could help me with a simple java program that I'm writing.
It deals simply with a bouncing ball (yes I know there's a topics on this one heh) that bounces aimlessly inside a drawing window.
Here's my problem:
I can make the ball bounce off two boundaries (top and right, left and bottom, right and bottom etc.) but when the ball reaches the 3rd boundary it just passes through it. It wont bounce off opposite sides (left and right, top and bottom) it only bounces off the first.
I believe it has something to do with my if statements under the move method.
I need to make some kind of loop so it keeps checking if the ball comes to a new boundary.
Right now it's only checking it once and after it bounces the ball one time thats it.
(I've tested all the boundaries to see if they bounce the ball and they all do, its just I can't bounce it more than 2 times)
I'm stuck on how to write this loop statement. Any help would be greatly appreciated! Thanks.
My code (If this isnt enough let me know please):
(BallTester.java)
import element.*;
public class BallTester{
public static void main(String[] args){
int width = 500;
int height = 400;
DrawingWindow d = new DrawingWindow(width,height);      
Ball b = new Ball(70,30,width,height);
b.send(d);                                   
(Ball.java)
import element.*;
public class Ball{
int base; // center line of where the circle starts
int size; // radius of circle
int xPos,yPos; // circle ctr
Circle c;
     int width, height;
     int dx,dy;
public Ball(int base, int size, int width, int height) {
          this.base = base;
this.size = size;
c = new Circle(50,base,size);
          this.width = width; this.height = height;
          xPos = 50;
          yPos = base;
public void drawOn(DrawingWindow d){
          //draws the circle
          c.drawOn(d);
public void clearOn(DrawingWindow d){
          d.invertMode(); // black-> white
          drawOn(d); //redraw in white: erase!
          d.invertMode(); // white -> black
public void move(int dx, int dy){
          //Sets boundaries for x values
          if((xPos<=size)||(xPos>=width-size)){
          dx = -dx;
          else
          xPos += dx;
          //Sets boundaries for y values
          if((yPos<=size)||(yPos>=height-size)){
          dy = -dy;
          else     
          yPos += dy;
          c.move(dx,dy);
public void send(DrawingWindow d){
          Waiting w = new Waiting();
//This below keeps the ball drawing up until j = width)
for(int j = 0; j < width; j++){
          move(6,-3); // moves the circle based on dx, dy values
          this.drawOn(d);
w.snooze(60);
this.clearOn(d);}

Answer 1

You don't need a loop to check - you just need to check every loop (before you repaint).
I created an example for you. If this assignment is homework, don't hand it in - use it as a reference. Try to understand what's going on in the code and why it works.
import java.awt.*;
import javax.swing.*;
public class Test extends JFrame {
    public Test () {
        getContentPane ().setLayout (new BorderLayout ());
        getContentPane ().add (new TestPanel ());
        setDefaultCloseOperation (DISPOSE_ON_CLOSE);
        setTitle ("Test");
        pack ();
        setLocationRelativeTo (null);
        setVisible (true);
    public static void main (String[] parameters) {
        new Test ();
    private class TestPanel extends JPanel {
        private Ball ball;
        public TestPanel () {
            ball = new Ball ();
            Thread painter = new Thread (new Runnable () {
                public void run () {
                    while (true) {
                        try {
                            Thread.sleep (20);
                        } catch (InterruptedException exception) {}
                        repaint ();
            painter.setDaemon (true);
            painter.start ();
        public Dimension getPreferredSize () {
            return new Dimension (300, 200);
        public void paintComponent (Graphics g) {
            super.paintComponent (g);
            ball.move (getWidth (), getHeight ());
            ball.paint (g);
        private class Ball {
            private static final int W = 10;
            private static final int H = 10;
            private int x;
            private int y;
            private int dx;
            private int dy;
            public Ball () {
                x = 0;
                y = 0;
                dx = 2;
                dy = 2;
            public void move (int width, int height) {
                int x = this.x + dx;
                int y = this.y + dy;
                if ((x < 0) || (x + W > width)) {
                    dx = -dx;
                    x = this.x + dx;
                if ((y < 0) || (y + H > height)) {
                    dy = -dy;
                    y = this.y + dy;
                this.x = x;
                this.y = y;
            public void paint (Graphics g) {
                g.fillOval (x, y, W, H);
}