DrawPanel类---摘自《Java软件开发》Russel Winder & Graham Roberts

import java.awt.*;
import javax.swing.*;
/**
 * A <CODE>JPanel</CODE> with a default size. Subclasses must
 * override the <CODE>paint</CODE> method which has signature.
 *
 * <PRE> public void paint(final Graphics g) </PRE>
 *
 * @version 1.0 1999.09.04
 * @author Graham Roberts
 * @author Russel Winder
 */
public class DrawPanel extends JPanel{
    /**
     * The width of the panel.
     */
    private int width = 300;
    /**
     * The height of the panel
     */
    private int height = 300;
    /**
     * Default constructor, uses the default size.
     */
    protected DrawPanel() {
        setPreferredSize(new Dimension(width, height));
    }
    /**
     * Constructor for a size determined by the user.
     */
    protected DrawPanel(final int w, final int h) {
        width = w;
        height = h;
        setPreferredSize(new Dimension(width, height));
    }
    /**
     * Accessor for the width of the panel.
     */
    public int getHeight() {
        return height;
    }
}