help with this multithreading program using Swing

[VarunS Singh]

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;
import java.util.ArrayList;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class BounceThread {
    
    public static void main(String[] args) {
        
        JFrame frame = new BounceFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.show();
               }
}

    class BounceFrame extends JFrame {

        public BounceFrame() {
            
            setSize(WIDTH, HEIGHT);
            setTitle("Bounce Ball Using Thread Application");
            
            Container contentPane = getContentPane();
            canvas = new BallCanvas();
            
            contentPane.add(canvas);
            JPanel buttonPanel = new JPanel();
           
            addButton(buttonPanel,"Start",
                    new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    
                    addBall();
                    
                }
            }
                    );
            
              addButton(buttonPanel, "Close",
                new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    
                    System.exit(0);
                   
                }
            });
              
              contentPane.add(buttonPanel, BorderLayout.SOUTH);
              
        }

/*Adds a button to a container*/      

    private void addButton(Container c, String title, ActionListener listener) {
                    
        JButton button = new JButton(title);
        c.add(button);
        button.addActionListener(listener);
        
    }
    
    /*Adds a bouncing ball to the canvas and makes it bounce 1000 times*/
    
    public void addBall(){
        try{
            
          Ball b = new Ball(canvas);
          canvas.add(b);
          
          for(int i=1;i<=50;i++){
              b.move();
              Thread.sleep(5);
          }
            
        }catch(InterruptedException exception){
            System.out.println(exception);
        }
    }
    private BallCanvas canvas;
    private static final int WIDTH=450;
    private static final int HEIGHT=450;
    }

/*The canvas that draws the ball*/

class BallThread extends Thread{
    public BallThread(Ball aBall){
        b = aBall;
    }
    public void run(){
        try{
            b.move();
            sleep(5);
        }catch(InterruptedException ex){
            System.out.println(ex);
        }
    }
    private Ball b;
}

class BallCanvas extends JPanel{

   
    /*Ad a ball to thecanvas
        @param b the Ball to add
    */
    public void add(Ball b){
        balls.add(b);
    }
    
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        for(int i=0;i<balls.size();i++){
            Ball b = (Ball)balls.get(i);
            b.draw(g2);
        }
        
   }
    private  ArrayList balls = new ArrayList();
    
    
}

/*A ball that moves and bounces of the edges*/
   class Ball {

        public Ball(Component c) {canvas=c;  }
        
        public void draw(Graphics2D g2){g2.fill(new Ellipse2D.Double(x, y, XSIZE, YSIZE));  }
        
        public void move(){
            x += dx;
            y += dy;
            if(x<0){
                x = 0;
                dx = -dx;
            }
            if(x +XSIZE <= canvas.getWidth()){
                x = canvas.getWidth() - XSIZE;
                dx = -dx;
            }
             if(y<0){
                y = 0;
                dy = -dy;
            }
            if(y +YSIZE <= canvas.getWidth()){
                y = canvas.getWidth() - YSIZE;
                dy = -dy;
            }
            
            canvas.paint(canvas.getGraphics());
        }
        
        private final Component canvas;
        private static final int XSIZE = 15;
        private static final int YSIZE = 15;
        private int x=0;
        private int y=0;
        private int dx=0;
        private int dy=0;
        
    }
    

The ball does not appears when start is clicked.

[VarunS Singh]

I don't know what the problem is, but the ball is not appearing. Is there something wrong with the code?Please help

Thanks in advance ..

[Stephan van Hulst]

I didn't look at your code close enough to locate a definite reason for your problem, but there are many issues which you should address:

You're not initializing your components on the EDT. Call your constructor inside a lambda passed to SwingUtilities.invokeLater().

Don't extend JFrame. Your class should use a private field that holds a reference to a JFrame.

You have public members and constructors in a class that's not public.

Make your classes final.

Don't call Thread.sleep() on the event dispatch thread. You shouldn't call Thread.sleep() ever in the first place, but even if you do, NEVER do it on the EDT.

Don't extend Thread. Either use a ExecutorService, or in this case, a SwingWorker.

Don't use a for-loop with an index when an enhanced for-loop will do.

Don't use raw generic types.

Don't call paint() on Swing components. Call repaint() instead, and let the EDT sort the rest out.

Put your fields at the start of your class, not the end.

[VarunS Singh]

Thanks for your reply...I will correct all these problems you addressed and be right back

[Campbell Ritchie]

Stephan van Hulst wrote:. . . Don't call Thread.sleep() on the event dispatch thread. You shouldn't call Thread.sleep() ever . . .

No, use a Timer which calls an action listener to move the balls.
Don't use System.exit. In this instance nothing will happen, but it can cause serious harm in a threaded environment. Imagine you have a T‍hread which saves all your data in a file when you close your GUI. If you use System.exit, there is a risk of nothing being saved. Or even worse, part of it is saved and the records are corrupted. Use something like DISPOSE_ON_EXIT instead. The close button can include instructions to save any data when you close the GUI.

[VarunS Singh]

Thanks Campbell Ritchie, I will surely do that...

[Campbell Ritchie]

That's a pleasure . Try setting the Timer to fire more often than every 40ms and less often than every 20ms; I think you will get good movement like that. Remember that moving 10px every 20ms will take the ball right across an average‑sized display in about 1″. Consider doing that animation in JavaFX, where pixel numbers are denominated in doubles, so you can have fractional pixels. Or use doubles yourself, but you may have to cast all the doubles to ints when you paint the display.