Hi, I am sending a oval shaped ball across a screen and use the repaint() method controlled by a timer to move it.

public class Court extends JFrame implements Runnable
{

    private int x = 10;
    private int y = 200;
    private int r = 3;

    /** Creates new form MainContainer */
    public Court()
    {
        this.setBackground(Color.BLACK);
        this.setVisible(true);
        start();
       
    }
    public void run()
    {
        
        while(true)
        {
            x++;
            repaint();

            //this.paint(this.getGraphics());
            
                try
                {
                    Thread.sleep(10);
                }
               catch (InterruptedException ex)
                {
                    
                }
            
        }
        
    }
   
    public void start()
    {
        Thread thread = new Thread(this);
        thread.start();

    }
    void p(Graphics g)
    {
        super.paintComponents(g);
    }
    public void paint(Graphics g)
    {
        g.setColor(Color.GREEN);
        g.fillOval(x-r, y-r, 2*r, 2*r);

    }
}

The green ball does move across the screen but draws a line in its wake. Only when I resize the window does the wake vanish.

Is there something new? I have seen suggestions of a revalidate() method but that method does not seem to exist.

Any help would be welcome.

Many thanks

That's because you aren't repainting the background rectangle before you paint the oval. Even so, you really should do your custom painting on a lightweight component such as a JPanel or small JComponent class that overrides paintComponent(), rather then painting directly on the JFrame. Here is your code with a small canvas component for the painting:

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class Court extends JFrame implements Runnable {

    private int x = 10;
    private int y = 20;
    private int r = 3;

    public Court() {
        setBounds(0, 0, 200, 200);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setBackground(Color.BLACK);

        // adding the new component here
        getContentPane().add(new CanvasComponent());

        this.setVisible(true);
        start();

    }

    // Custom component for your paint work.
    // JPanel is also a good candidate for this.
    class CanvasComponent extends JComponent {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.BLACK);
            g.fillRect(0, 0, getWidth(), getHeight());
            g.setColor(Color.GREEN);
            g.fillOval(x - r, y - r, 2 * r, 2 * r);
        }
    }

    public void run() {
        while (true) {
            x++;
            repaint();

            try {
                Thread.sleep(10);
            } catch (InterruptedException ex) {}
        }
    }

    public void start() {
        Thread thread = new Thread(this);
        thread.start();
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Court();
            }
        });
    }
}//end

So I have used a JPanel and a JFrame and paint() the JFrame as well! This has worked.

Many thanks

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