3月29日，DoubleBufferDemo，每日20行。

import java.awt.*;
import java.awt.event.*;
class MyCanvas extends Canvas {
    private Image img;
    private Graphics og;
    public void init() {
        Dimension d = getSize();
        img = createImage(d.width, d.height);
        og = img.getGraphics();
        og.setColor(Color.blue);
        og.fillRect(30, 30, 60, 60);
        og.setColor(Color.yellow);
        og.fillOval(15, 15, 30, 30);
        og.setXORMode(Color.red);
        og.fillOval(75, 75, 30, 30);
        og.setPaintMode();
        og.setColor(Color.green);
        og.fillArc(150, 40, 60, 60, 30, 160);
    }
    public void paint(Graphics g) {
        if(img != null) {
            g.drawImage(img, 20, 20, this);
        }
    }
}
public class DoubleBufferDemo {

    public static void main(String[] args) {
        Frame f = new Frame("DoubleBufferDemo");
        f.setBounds(100, 100, 250, 200);
        MyCanvas mc = new MyCanvas();
        f.add(mc);
        f.setVisible(true);
        mc.init();
        mc.repaint();
        f.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }

}