If the buffered image supports transparency : BufferedImage « 2D Graphics « Java Tutorial

1. Java Tutorial
2. 2D Graphics
3. BufferedImage

import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;

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

public class BasicShapes extends JPanel {

  public void paint(Graphics g) {
    BufferedImage bimage = new BufferedImage(200, 200,
        BufferedImage.TYPE_BYTE_INDEXED);
    Graphics2D g2d = bimage.createGraphics();

    Color transparent = new Color(0, 0, 0, 0);
    g2d.setColor(transparent);
    g2d.setComposite(AlphaComposite.Src);
    g2d.fill(new Rectangle2D.Float(20, 20, 100, 20));
    g2d.dispose();

  }

  public static void main(String[] args) {

    JFrame frame = new JFrame("Basic Shapes");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(new BasicShapes());
    frame.setSize(350, 250);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}