JPanel doesn't repaint well using transparent JFrame

Question

I'm trying to repaint a JPanel over a transparent JFrame but it lets the mark of the previous repaint:

previous repaint keeps showing

I know this is a known problem and there's a similar question already posted, but the answers only explain why it doesn't work and they don't give a solution.

Can anyone please make a change in the code or add some, so it gives the desired effect? (a cross that follows the mouse over a transparent background)

I don't want to take a picture of the desktop and draw it in the JPanel .

The code is the following

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

final class Transparency extends JFrame{
    private NewPanel np;

    Transparency(){
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setUndecorated(true);
        setLocation(200,200);
        setBackground(new Color(0,0,0,0));

        np = new NewPanel();
        setContentPane(np);

        pack();
        setVisible(true);
    }

    public static void main(String []args){
        Transparency li = new Transparency();
    }
}

final class NewPanel extends JPanel{
    private Point p = new Point(0,0);
    private final int length = 25;

    NewPanel(){
        setPreferredSize(new Dimension(400,400));
        setOpaque(false);
        addMouseMotionListener(new MouseAdapter(){
            public void mouseMoved(MouseEvent e){
                p = e.getPoint();
                repaint();
            }
        });
    }

    public void paintComponent(Graphics g){
        super.paintComponent(g);

        g.setColor(Color.GREEN);
        g.drawLine(p.x-length,p.y,p.x+length,p.y);
        g.drawLine(p.x,p.y-length,p.x,p.y+length);

        g.setColor(Color.WHITE);
        g.drawRect(0,0,399,399);
    }
}

Answer 1

Since there is no heavyweight top level component, you will need to manage the background yourself.

Add this to your paintComponent method, right after super.paintComponent:

g.clearRect(0, 0, getWidth(), getHeight());