Uso de paneles en un applet de java

package masterdavidjavabasico;

import java.applet.Applet;
import java.awt.*;

/*
<APPLET
    CODE=paneles.class
    WIDTH=200
    HEIGHT=200 >
</APPLET>
*/


public class paneles extends Applet {

    buttonpanel panel1, panel2, panel3, panel4, panel5, panel6;

    public void init(){
        setLayout(new GridLayout(2, 3));
        panel1 = new buttonpanel();
        panel2 = new buttonpanel();
        panel3 = new buttonpanel();
        panel4 = new buttonpanel();
        panel5 = new buttonpanel();
        panel6 = new buttonpanel();
        add(panel1);
        add(panel2);
        add(panel3);
        add(panel4);
        add(panel5);
        add(panel6);
    }
}
class buttonpanel extends Panel
{
    Button button1, button2, button3, button4;

    buttonpanel()
    {
        button1 = new Button("1");
        add(button1);
        button2 = new Button("2");
        add(button2);
        button3 = new Button("3");
        add(button3);
        button4 = new Button("4");
        add(button4);
    }
}