import javax.swing.*;
import java.awt.*;

public class MovingBall {

    public static void main(String[] arg) {

        JFrame frame = new JFrame();
        frame.setSize(300, 300);

        final JPanel jpanel = new JPanel() {

            int x = 20;
            int y = 20;
            int radius = 20;


            public void paint(Graphics g) {
                x++;
                y++;
                g.setColor(Color.red);
                g.fillOval(x - radius, y - radius, 2 * radius, 2 * radius);
            }
        };

        jpanel.setBackground(Color.black);

        frame.add(jpanel);
        frame.setVisible(true);

        Thread a = new Thread(new Runnable() {

            public void run() {
                while (true) {

                    jpanel.repaint();

                    try {
                        Thread.sleep(20);
                    }
                    catch (InterruptedException ex) {
                        ex.printStackTrace();
                    }
                }
            }
        });

        a.start();
    }
}