Moving ball inside rectangle program

Question

I am creating a moving ball program that features a ball moving and bouncing off the walls of a rectangle with go and stop buttons at the bottom.

What I am having problems with is I want the ball to start off moving when the program is run and bouncing off the lines inside the rectangle which is my main problem. Below is my code:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

//Does the drawing
class MyDrawing extends JPanel {

private int xpos;
private int ypos;

public void setXPos(final int x) {
    this.xpos = x;
}

public void setYPos(final int y) {
    this.ypos = y;
}

public int getXpos() {
    return xpos;
}

public int getYpos() {
    return ypos;
}

@Override
public void paintComponent(final Graphics g) {
    super.paintComponent(g);
    final Graphics2D g2 = (Graphics2D) g;
    g.setColor(Color.red);
    final Ellipse2D.Double circle = new Ellipse2D.Double(xpos, ypos, 50, 50);
    g2.draw(circle);
    g2.fill(circle);
    final Rectangle box1 = new Rectangle(10, 10, 380, 300);
    g.setColor(Color.BLACK);
    g2.draw(box1);
}
}


public class ControlledBall extends JFrame {
private final JButton       flash = new JButton("Go");
private final JButton       steady = new JButton("Stop");
private final JPanel        panel = new JPanel(new GridBagLayout());
private final MyDrawing     drawing = new MyDrawing();
private final Timer         timer;


//direction
private int dx = 3;
private int dy = 2;

public ControlledBall() {
    panel.add(flash);
    panel.add(steady);
    this.add(panel, BorderLayout.SOUTH);
    this.add(drawing, BorderLayout.CENTER);
    drawing.setXPos(300);
    drawing.setYPos(150);
    steady.addActionListener(new SteadyListener());
    final MoveListener ml = new MoveListener();
    flash.addActionListener(ml);
    timer = new Timer(15, ml);
}

class MoveListener implements ActionListener {
    @Override
    public void actionPerformed(final ActionEvent event) {
        if (!timer.isRunning()){
            timer.start();
        }
        move();
    }
}

class SteadyListener implements ActionListener {
    @Override
    public void actionPerformed(final ActionEvent event) {
        if (timer.isRunning()){
            timer.stop();
        }
    }
}

private void move() {

    int x = drawing.getXpos();
    int y = drawing.getYpos();
    final int dia = 30;
    if (x + dx < 0 || x + dia + dx > getWidth()) {
        dx *= -1;
    }
    if (y + dy < 0 || y + dia + dy > getHeight()) {
        dy *= -1;
    }
    x += dx;
    y += dy;
    drawing.setXPos(x);
    drawing.setYPos(y);
    repaint();
}

public static void main(final String[] args) {
    final JFrame window = new ControlledBall();
    window.setSize(400, 400);
    window.setTitle("Controlled Ball");
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setVisible(true);
}
}

If want them to bounce you ll also have to save the direction of the ball and use some basic trigo to get the new direction — Erwan C., Aug 19 '15 at 11:53

score 1 · Accepted Answer · answered Aug 19 '15 at 11:44

The problem is the boundaries you check against. If you want to check against the Rectangle you have to implement the size as variables and enter them in your "bounce"-check or you add it manually like (take the size of the rectangle from your code):

 if (x + dx < 10 || x + dia + dx > 380) {
        dx *= -1;
    }
    if (y + dy < 10 || y + dia + dy > 300) {
        dy *= -1;
    }

If you like to take the real distanz use an offset of the size of hallf of the ball and add it to this code. i think this is better than the speed-vector dxand dyonly.

Moving ball inside rectangle program

1 Answers1