Java AWT Graphics2D clear rectangle

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JPanel {
  Random rnd = new Random();
  @Override//ww  w .  ja  v a 2s.  c om
  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    
    Graphics2D g2d = (Graphics2D) g;
    g2d.setBackground(Color.BLACK);
    g2d.clearRect(0, 0, getParent().getWidth(), getParent().getHeight());
    
    for (int i=0; i<3000; i++) {
        int red = rnd.nextInt(256);
        int green = rnd.nextInt(256);
        int blue = rnd.nextInt(256);
        g2d.setColor(new Color(red, green, blue));
    
        int rndX = rnd.nextInt(getSize().width);
        int rndY = rnd.nextInt(getSize().height);

        g2d.drawLine(rndX, rndY, rndX, rndY);
    }
  }

  public static void main(String[] args) {
    // create frame for Main
    JFrame frame = new JFrame("java2s.com");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    Main Main = new Main();
    frame.add(Main);
    frame.setSize(300, 210);
    frame.setVisible(true);
  }
}