Java Simple Bouncing Ball, SingleThread, Mouse Listener, JFrame

Question

I've created a Bouncing Ball application in in Java. The goal is to have a ball appear on mousePressed() and have it bounce off the walls without leaving the frame. Only one Ball One Thread, should be easy.. My problem is that every time I click to make the ball appear it gets faster and I have no idea why. Can somebody help me please. PS: I'm new to threads.

import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class MyFrame extends JPanel {
public int xPos, yPos, xDir = 3, yDir = 4;
public int diameter = 50;

public MyFrame(){
    final JFrame thisFrame = new JFrame();
    thisFrame.add(this);
    thisFrame.setTitle("Bouncing Ball");
    thisFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    thisFrame.setLocationRelativeTo(null);
    thisFrame.setSize(500, 500);

    this.addMouseListener(new MouseListener() {
        @Override
        public void mouseReleased(MouseEvent e) {}
        @Override
        public void mousePressed(MouseEvent e) {
            xPos = e.getX();
            yPos = e.getY();
            Thread t = new Thread() {
                @Override
                public void run() {
                    while(true){
                        try{
                            Thread.sleep(10);
                        }catch(Exception e){};
                        xPos += xDir;
                        yPos += yDir;

                        if(xPos + diameter >= thisFrame.getWidth() - 25 || xPos <= 0) xDir = -xDir;
                        if(yPos + diameter >= thisFrame.getHeight() - diameter || yPos <= 0) yDir = -yDir;
                        repaint();
                    }
                } 
            };
            t.start();
        }
        @Override
        public void mouseExited(MouseEvent e) {}
        @Override
        public void mouseEntered(MouseEvent e) {}
        @Override
        public void mouseClicked(MouseEvent e) {}
        });

    thisFrame.setVisible(true);

    }
    public void paintComponent(Graphics g) { 
        super.paintComponent(g); 
        g.fillOval(xPos, yPos, diameter, diameter); 
    }
}

public class MyMain{

    public static void main(String[] args) {
        new MyFrame();
    }

}

Use MouseAdapter in the future when you don't need to override every mouse event method. — Tom Cammann

MadProgrammer MadProgrammer · Accepted Answer · 2014-01-17T20:21:38

Every time you click, you are starting a new Thread, which means you have another thread updating the x/y positions.

For example, 1 thread means your update the x/p once per sync, 2 means you're updating the x/y position at least twice per cycle and this just gets compounded each time you add a new thread.

A better solution would be to start the Thread at some time earlier and then use a List to maintain the position and direction of the ball.

This will require a little but synchronisation to keep things safe

FYI a delay of 40 milliseconds is roughly 25 fps, 16 milliseconds is roughly 60 fps. IMHO, for what you are doing, 10 milliseconds seems excessive...

Java Simple Bouncing Ball, SingleThread, Mouse Listener, JFrame

1 Answers