Why am I getting a copy of a button in my JPanel?

[Luigi Plinge]

You will notice that I only add one button to my frame, but when I add the animation loop in, another button mysteriously appears on the left!

What's going on?

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

public class findbug {

    JButton midiButton;
    JFrame frame;
    int x, y;

    public static void main(String[] args) {
        new findbug().runGui();
    }

    public void runGui() {

        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        midiButton = new JButton("Make a note");
        frame.add(BorderLayout.EAST, midiButton);

        JPanel panel = new MyDrawPanel();
        frame.add(BorderLayout.CENTER, panel);

        frame.setSize(500, 500);
        frame.setVisible(true);

        x = 140;
        for (y = 10; y <= 200; y = y + 3){
            try{Thread.sleep(50);
            }catch (Exception e){}
            panel.repaint();
        }
    }

    class MyDrawPanel extends JPanel{
        @Override
        public void paintComponent(Graphics g){
            g.fillOval(x, y, 60, 60);
        }
    }
}

[Luigi Plinge]

More bizarreness if I put it on the WEST:



Similar things happen for NORTH and SOUTH as well.

[Christophe Verré]

If you override the paintComponent method, you have to be careful. Call super.paintComponent(g), otherwise :

API wrote:
If you do not invoker super's implementation you must honor the opaque property, that is if this component is opaque, you must completely fill in the background in a non-opaque color. If you do not honor the opaque property you will likely see visual artifacts.

[Luigi Plinge]

Cool, thanks. That works.