Java JFrame create transparent window via transparent panel

import java.awt.Color;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Paint;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

class TranslucentJPanel extends JPanel {
  private int red = 240;
  private int green = 240;
  private int blue = 240;

  public TranslucentJPanel(Color bgColor) {
    this.red = bgColor.getRed();
    this.green = bgColor.getGreen();
    this.blue = bgColor.getBlue();
  }/*from w  ww .ja va  2  s  .  c o  m*/

  @Override
  protected void paintComponent(Graphics g) {
    if (g instanceof Graphics2D) {
      int width = this.getWidth();
      int height = this.getHeight();
      float startPointX = 0.0f;
      float startPointY = 0.0f;
      float endPointX = width;
      float endPointY = 0.0f;
      Color startColor = new Color(red, green, blue, 255);
      Color endColor = new Color(red, green, blue, 0);

      // Create a GradientPaint object
      Paint paint = new GradientPaint(startPointX, startPointY, startColor, endPointX, endPointY, endColor);

      Graphics2D g2D = (Graphics2D) g;
      g2D.setPaint(paint);
      g2D.fillRect(0, 0, width, height);
    }
  }
}

public class Main extends JFrame {
  private JButton closeButton = new JButton("Close");

  public Main(String title) {
    super(title);
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);

    // Make sure the frame is undecorated
    this.setUndecorated(true);

    // Set the background color with all components as zero,
    // so per-pixel translucency is used
    this.setBackground(new Color(0, 0, 0, 0));
    this.setSize(200, 200);
    this.getContentPane().setLayout(new GridLayout(0, 1));

     this.add(new TranslucentJPanel(Color.RED));
    this.add(new TranslucentJPanel(Color.GREEN));
    this.add(new TranslucentJPanel(Color.BLUE));

     this.add(closeButton);
    closeButton.addActionListener(e -> System.exit(0));
  }

  public static void main(String[] args) {
    SwingUtilities.invokeLater(() -> {
      Main frame = new Main("Per-Pixel Translucent Frame");
      frame.setVisible(true);
    });
  }
}