Thread: Question about Program Output

Hello,

a) I have a Ball Object which implements the Runnable interface and traces the various positions of a ball.

b) I then have a Ball_Bounce JPanel inside a JFrame which creates two instances of the Ball object and then paints them to the JPanel.

As per my understanding, when the main() program in Ball_Bounce.java is started, there a total of three threads running in this program, one for each ball and one for the main(). What I cannot understand is whenever the balls collide, I end up getting the "Collision" message twice even though the collision is checked only in the main() thread.

I would be grateful for an explanation for why I am getting the collision message outputted twice.

Thank you!

[#]public class Ball implements Runnable
{
    private boolean xUp, yUp, xUp1, yUp1;
    private int x, y, xDx, yDy;
    private final int MAX_X = 500, MAX_Y = 500;
    private boolean flag = true;
    private static Thread ball;
 
 
    public  Ball(int xCoordinate, int yCoordinate)
    {
        x = xCoordinate;
        y = yCoordinate;
        xUp = false;
        yUp = false;
        xDx = 1;
        yDy = 1;
 
        ball = new Thread(this);
        ball.start();
 
    }
 
    public int getX()
    {
        return x;
    }
 
    public int getY()
    {
        return y;
    }
 
    public void run()
    {
        while ( flag == true )
        {
            try
            {
                ball.sleep(12);
            }
            catch ( InterruptedException exception )
            {
                System.err.println( exception.toString() );
            }
 
            if ( y <= 0 ) {
                yUp = true;
                yDy = ( int ) ( Math.random() * 5 + 2 );
            }
 
            else if ( y >= MAX_Y - 50 ) {
                yDy = ( int ) ( Math.random() * 5 + 2 );
                yUp = false;
            }
 
            if ( x <= 0 ) {
                xUp = true;
                xDx = ( int ) ( Math.random() * 5 + 2 );
            }
 
            else if ( x >= MAX_X - 50 ) {
                xUp = false;
                xDx = ( int ) ( Math.random() * 5 + 2 );
            }
 
 
            if ( xUp == true )
                x += xDx;
            else
                x -= xDx;
 
            if ( yUp == true )
                y += yDy;
            else
                y -= yDy;
 
 
        }
    }
 
 
}[/#]----------------------------------------------------------------------------------------------------------------------------------------------
[#]import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.lang.Thread;
 
public class Ball_Bounce extends JPanel implements ActionListener
{
    private Ball ball[] = new Ball[2];
    private Timer timer;
    private int count = 0;
 
    public Ball_Bounce()
    {
        timer = new Timer (12, this);
        timer.start();
        ball[0] = new Ball( 300, 250);
        ball[1] = new Ball (450, 450);
 
    }
 
    public void actionPerformed (ActionEvent e)
    {
        repaint();
    }
 
    public void paintComponent( Graphics g )
    {
        super.paintComponent(g);
 
        for (int i = 0; i < 2; i++)
        {
            if (i == 0)
            {
                Color c = Color.RED;
                g.setColor (c);
                g.fillOval( ball[i].getX(), ball[i].getY(), 50, 50 );
            }
            if ( i == 1)
            {
                Color c = Color.BLUE;
                g.setColor (c);
                g.fillOval( ball[i].getX(), ball[i].getY(), 50, 50 );
            }
 
        }
        if (Math.abs(ball[0].getX()-ball[1].getX()) <= 50 && Math.abs(ball[0].getY()-ball[1].getY()) <= 50)
        {
             JOptionPane.showMessageDialog (null, "Collision");  
        }
    }
 
    public static void main (String args [])
    {
        JFrame f = new JFrame ("Bouncing Ball");
        Ball_Bounce b = new Ball_Bounce ();
        f.add (b);
        f.setSize (500,500);
        f.setVisible(true);
    }
 
}[/#]

Swing is not thread safe. Animation in Swing is done using javax.swing.Timer. You can find many examples on the Internet that use Timer to do animation in Swing. Start simple and grow from there.