Java: RadioButtonDemo

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

/*
* RadioButtonDemo.java requires these files:
*   public/images/article/Bird.gif
*   public/images/article/Cat.gif
*   public/images/article/Dog.gif
*   public/images/article/Rabbit.gif
*   public/images/article/Pig.gif
*/
public class RadioButtonDemo extends JPanel
                             implements ActionListener {
    static String birdString = "Bird";
    static String catString = "Cat";
    static String dogString = "Dog";
    static String rabbitString = "Rabbit";
    static String pigString = "Pig";

    JLabel picture;

    public RadioButtonDemo() {
        super(new BorderLayout());

        //Create the radio buttons.
        JRadioButton birdButton = new JRadioButton(birdString);
        birdButton.setMnemonic(KeyEvent.VK_B);
        birdButton.setActionCommand(birdString);
        birdButton.setSelected(true);

        JRadioButton catButton = new JRadioButton(catString);
        catButton.setMnemonic(KeyEvent.VK_C);
        catButton.setActionCommand(catString);

        JRadioButton dogButton = new JRadioButton(dogString);
        dogButton.setMnemonic(KeyEvent.VK_D);
        dogButton.setActionCommand(dogString);

        JRadioButton rabbitButton = new JRadioButton(rabbitString);
        rabbitButton.setMnemonic(KeyEvent.VK_R);
        rabbitButton.setActionCommand(rabbitString);

        JRadioButton pigButton = new JRadioButton(pigString);
        pigButton.setMnemonic(KeyEvent.VK_P);
        pigButton.setActionCommand(pigString);

        //Group the radio buttons.
        ButtonGroup group = new ButtonGroup();
        group.add(birdButton);
        group.add(catButton);
        group.add(dogButton);
        group.add(rabbitButton);
        group.add(pigButton);

        //Register a listener for the radio buttons.
        birdButton.addActionListener(this);
        catButton.addActionListener(this);
        dogButton.addActionListener(this);
        rabbitButton.addActionListener(this);
        pigButton.addActionListener(this);

        //Set up the picture label.
        picture = new JLabel(createImageIcon("public/images/article/"
                                             + birdString
                                             + ".gif"));

        //The preferred size is hard-coded to be the width of the
        //widest image and the height of the tallest image.
        //A real program would compute this.
        picture.setPreferredSize(new Dimension(177, 122));

        //Put the radio buttons in a column in a panel.
        JPanel radioPanel = new JPanel(new GridLayout(0, 1));
        radioPanel.add(birdButton);
        radioPanel.add(catButton);
        radioPanel.add(dogButton);
        radioPanel.add(rabbitButton);
        radioPanel.add(pigButton);

        add(radioPanel, BorderLayout.LINE_START);
        add(picture, BorderLayout.CENTER);
        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
    }

    /** Listens to the radio buttons. */
    public void actionPerformed(ActionEvent e) {
        picture.setIcon(createImageIcon("public/images/article/"
                                        + e.getActionCommand()
                                        + ".gif"));
    }

    /** Returns an ImageIcon, or null if the path was invalid. */
    protected static ImageIcon createImageIcon(String path) {
        java.net.URL imgURL = RadioButtonDemo.class.getResource(path);
        if (imgURL != null) {
            return new ImageIcon(imgURL);
        } else {
            System.err.println("Couldn't find file: " + path);
            return null;
        }
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("RadioButtonDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new RadioButtonDemo();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}