//cd c:\HPBasic\Toppage-sample3\MySwing\Try
//javac ColorTest.java
//set CLASSPATH=.	
//appletviewer ColorTest.java

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/*
    <applet code="ColorTest.class" width=400 height=300>
    </applet>
*/

public class ColorTest extends JApplet implements ActionListener {
    JPanel panel;

     public void init(){
        Container contentPane = getContentPane();
        panel = new JPanel();
        contentPane.add(panel);
 
       contentPane.setLayout(new FlowLayout());
        JButton yellow = new JButton("Yellow");
        JButton blue = new JButton("Blue");
        JButton red = new JButton("Red");
 
        contentPane.add(yellow);
        contentPane.add(blue);
        contentPane.add(red);
        yellow.addActionListener(this);
        blue.addActionListener(this);
        red.addActionListener(this);
    }
 
   public void actionPerformed(ActionEvent event) {
        Color c;
        String source = event.getActionCommand();
        if (source == "Yellow") {
            c=Color.yellow;
        } else if  (source == "Blue") {
            c=Color.blue;
        } else  {
            c=Color.red;
        }
        panel.setBackground(c);
    }
} 

/*
1  panel.setBackground(c);は、panelの大きさが意図とは異なるが、一応OK。
2  contentPane.setLayout(new FlowLayout());はボタンの大きさはいいぐあい。
   単に、setBackground(c)だと、JPanelもJLabel同様、色が変わらない。
3 BorderLayout.WESTとすると、ボタンが異常に大きくなる。
       contentPane.add(yellow,BorderLayout.WEST);
        contentPane.add(blue,BorderLayout.CENTER);
        contentPane.add(red,BorderLayout.EAST);
4   contentPane.setLayout(new GridLayout());も同じ。
5 下記は、GridBagのAPI
Button1, Button2, Button3: weightx = 1.0 
Button4: weightx = 1.0, gridwidth = GridBagConstraints.REMAINDER 
Button5: gridwidth = GridBagConstraints.REMAINDER 
Button6: gridwidth = GridBagConstraints.RELATIVE 
Button7: gridwidth = GridBagConstraints.REMAINDER 
Button8: gridheight = 2, weighty = 1.0 
Button9, Button 10: gridwidth = GridBagConstraints.REMAINDER 
Here is the code that implements the example shown above: 
-------------------------------------------------------------------------------- import java.awt.*;
 import java.util.*;
 import java.applet.Applet;

 public class GridBagEx1 extends Applet {

     protected void makebutton(String name,
                               GridBagLayout gridbag,
                               GridBagConstraints c) {
         Button button = new Button(name);
         gridbag.setConstraints(button, c);
         add(button);
     }

     public void init() {
         GridBagLayout gridbag = new GridBagLayout();
         GridBagConstraints c = new GridBagConstraints();

         setFont(new Font("Helvetica", Font.PLAIN, 14));
         setLayout(gridbag);

         c.fill = GridBagConstraints.BOTH;
         c.weightx = 1.0;
         makebutton("Button1", gridbag, c);
         makebutton("Button2", gridbag, c);
         makebutton("Button3", gridbag, c);

     	   c.gridwidth = GridBagConstraints.REMAINDER; //end row
         makebutton("Button4", gridbag, c);

         c.weightx = 0.0;		   //reset to the default
         makebutton("Button5", gridbag, c); //another row

 	   c.gridwidth = GridBagConstraints.RELATIVE; //next-to-last in row
         makebutton("Button6", gridbag, c);

 	   c.gridwidth = GridBagConstraints.REMAINDER; //end row
         makebutton("Button7", gridbag, c);

 	   c.gridwidth = 1;	   	   //reset to the default
 	   c.gridheight = 2;
         c.weighty = 1.0;
         makebutton("Button8", gridbag, c);

         c.weighty = 0.0;		   //reset to the default
 	   c.gridwidth = GridBagConstraints.REMAINDER; //end row
 	   c.gridheight = 1;		   //reset to the default
         makebutton("Button9", gridbag, c);
         makebutton("Button10", gridbag, c);

         setSize(300, 100);
     }
*/