Draw only part of a shape transparent

Question

Hi,
I have two shapes neither of which is transparent. However, the shapes overlap and I want the intersecting region on the foreground shape to be transparent so that we can see the background shape.
So how do I draw part of a shape transparently and the rest of it not transparent? If it is any help, I have a curve in front and a rectangle behind it. I want the curve to be semi-transparent whenever it covers the rectangle.
Maybe there is a much easier way to do this?

Answer 1

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
public class SeeThru extends JPanel {
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int w = getWidth();
        int h = getHeight();
        g2.setPaint(Color.red);
        g2.fill(new Ellipse2D.Double(w/3, h/3, w/4, h/4));
        Rectangle2D r = new Rectangle2D.Double(w/4, h/4, w/4, h/4);
        Rectangle2D clip = new Rectangle2D.Double(w/4, h/4, w/4+1, h/4+1);
        g2.setClip(clip);
        g2.setPaint(getBackground());
        g2.fill(clip);
        g2.setPaint(Color.blue);
        g2.draw(r);
        int rule = AlphaComposite.SRC_OVER;
        float alpha = 0.25f;
        AlphaComposite ac = AlphaComposite.getInstance(rule, alpha);
        g2.setComposite(ac);
        g2.setPaint(Color.red);
        g2.fill(new Ellipse2D.Double(w/3, h/3, w/4, h/4));
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new SeeThru());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
}