i wrote a JPanel capable of drawing lines on it,
now i want to pop up a JPopupMenu when clicking MouseButton3,

here is my code:

import javax.swing.*;
import java.awt.event.MouseMotionListener;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.awt.*;


public class ScribblePane1 extends JPanel
        implements MouseListener, MouseMotionListener {

    private int last_x,last_y;
    private JPopupMenu pop = new JPopupMenu();

    public ScribblePane1() {

        this.addMouseListener(this);
        this.addMouseMotionListener(this);
        setPreferredSize(new Dimension(300, 500));
        pop.add(new JMenuItem("test"));
    }

    //mouse events
    public void mouseClicked(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

    public void mousePressed(MouseEvent e) {
        maybePopup(e);
            last_x = e.getX();
            last_y = e.getY();
    }

    public void mouseReleased(MouseEvent e) {
        maybePopup(e);
    }

    public void maybePopup(MouseEvent e) {
       if (e.isPopupTrigger()) {
            pop.show((Component)e.getSource(), e.getX(),e.getY());
        }
    }
    //mouse motion events
    public void mouseDragged(MouseEvent e) {
            this.getGraphics().drawLine(last_x, last_y, e.getX(),e.getY());
            last_x = e.getX();
            last_y = e.getY();
      }

    public void mouseMoved(MouseEvent e) {
    }
}

but when pop.show() is invoked,
the lined drew already will be covered and disappear,
what's the problem? and what should i do to fix it?

sorry my english is not good,
hope you can understand what i want to say,
thanks all :)