Muliple classes in applets

Question

am just starting to learn to use applets, and am a little confused with the structure. after imports, the applet generaly contines with pubic class xyz extend frame for example, which encomases most of the rest of the applet, so does that mean i can only use methords of xzy or can i put classes in side xzy.
If you can understand what i mean, or can point me to an easy resourse on how i should structure, an applet and how i should layout different class's inside the applet i would be great full. Exaclty 5 duke doller greatfull (cos thats what i got left).

Answer 1

Here's an example to illustrate.
You can just as well put the two outer classes inside the applet. If you do then you won't need to pass a reference into the ControlPanel class to call methods in the DisplayPanel class. Instead you would put the DisplayPanel instance variable in class scope. In java:
public class ExampleApplet extends JApplet
    DisplayPanel displayPanel;
    public void init()
        displayPanel = new DisplayPanel();
    class DisplayPanel...
    class ControlPanel extends JPanel
        JComboBox outerColor, innerColor;
        public ControlPanel(DisplayPanel dp)
            // create components for panel
    public static void main...
}To call methods or get access to variables in another class you need a way to get a reference to that class.
One benefit in keeping the classes separated is that you can isolate your event code, your drawing code and your gui display code. This can make it easier to conceptualize and maintain.
/*  <applet code="ExampleApplet" width="400" height="400"></applet>
*  use: >appletviewer ExampleApplet.java
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.Random;
import javax.swing.*;
public class ExampleApplet extends JApplet
    public void init()
        DisplayPanel displayPanel = new DisplayPanel();
        ControlPanel control = new ControlPanel(displayPanel);
        Container cp = getContentPane();
        cp.setLayout(new BorderLayout());
        cp.add(control, "North");
        cp.add(displayPanel);
     * Convenience method so you can run applet from command line
    public static void main(String[] args)
        JApplet applet = new ExampleApplet();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(applet);
        f.setSize(400,400);
        f.setLocation(200,200);
        applet.init();
        applet.start();
        f.setVisible(true);
class DisplayPanel extends JPanel
    Random seed;
    Color bgColor, outerColor, innerColor;
    public DisplayPanel()
        seed = new Random();
        bgColor = Color.white;
        outerColor = Color.blue;
        innerColor = Color.orange;
        setBackground(bgColor);
    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();
        int dia = Math.min(w, h)/4;
        g2.setPaint(outerColor);
        g2.fill(new Ellipse2D.Double(w/2 - dia/2, h/2 - dia/2, dia, dia));
        g2.setPaint(innerColor);
        g2.fill(new Ellipse2D.Double(w/2 - dia/4, h/2 - dia/4, dia/2, dia/2));
    public void setBackground()
        Color color = new Color(seed.nextInt(256), seed.nextInt(256), seed.nextInt(256));
        setBackground(color);
        repaint();
    public void setOuterColor(Color color)
        outerColor = color;
        repaint();
    public void setInnerColor(Color color)
        innerColor = color;
        repaint();
class ControlPanel extends JPanel
    DisplayPanel displayPanel;
    JComboBox outerColor, innerColor;
    public ControlPanel(DisplayPanel dp)
        displayPanel = dp;
        // create components for panel
        JButton bgColor = new JButton("background");
        bgColor.addActionListener(new ActionListener()
            public void actionPerformed(ActionEvent e)
                displayPanel.setBackground();
        String[] outerColorNames = {
            "blue", "yellow", "cyan", "magenta"
        outerColor = new JComboBox(outerColorNames);
        String[] innerColorNames = {
            "orange", "red", "green", "pink"
        innerColor = new JComboBox(innerColorNames);
        ColorListener l = new ColorListener();
        outerColor.addActionListener(l);
        innerColor.addActionListener(l);
        // layout components
        setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(2,2,2,2);
        gbc.weightx = 1.0;
        add(bgColor, gbc);
        gbc.anchor = gbc.EAST;
        add(new JLabel("outer"), gbc);
        gbc.anchor = gbc.WEST;
        add(outerColor, gbc);
        gbc.anchor = gbc.EAST;
        add(new JLabel("inner"), gbc);
        gbc.anchor = gbc.WEST;
        add(innerColor, gbc);
    class ColorListener implements ActionListener
        Color[] outerColors = {
            Color.blue, Color.yellow, Color.cyan, Color.magenta
        Color[] innerColors = {
            Color.orange, Color.red, Color.green, Color.pink
        public void actionPerformed(ActionEvent e)
            JComboBox combo = (JComboBox)e.getSource();
            int index = combo.getSelectedIndex();
            if(combo == outerColor)
                displayPanel.setOuterColor(outerColors[index]);
            if(combo == innerColor)
                displayPanel.setInnerColor(innerColors[index]);
}