[hardwired]

Code:

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

public class ColourPanel extends JPanel implements Runnable {

    public void run() {
        try {
            for (int i=0; i<10; i++) {
                randomNumber = Math.random();
                if ((randomNumber >= 0) && (randomNumber < 0.25))
                    setBackground(Color.RED);
                if ((randomNumber >= 0.25) && (randomNumber < 0.5))
                    setBackground(Color.BLUE);
                if ((randomNumber >= 0.5) && (randomNumber < 0.75))
                    setBackground(Color.MAGENTA);
                if ((randomNumber >= 0.75) && (randomNumber <= 1))
                    setBackground(Color.ORANGE);

                repaint();
                System.out.printf("i = %s  isDisplayable = %b%n",
                                   i, isDisplayable());
                Thread.sleep(200);
            }
        } catch (InterruptedException e) {
            return;
        }
    }

    public ColourPanel() {
        setLayout(new BorderLayout());
        colourButton = new JButton("Change Background");
        add(colourButton, BorderLayout.SOUTH);

        colourButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                // This first line creates a new instance of ColourPanel
                // which is not the current instance that has been
                // instantiated and loaded into a gui. So this new
                // instance runs just fine but cannot be seen.
                //Runnable q = new ColourPanel();

                // If you get a reference to the current instance
                // that has been instantiated and added to a gui
                // you will see the colors change. Since we are inside
                // another class ("this" == this ActionListener anonymous
                // class) we use the static form of obtaining a reference.
                Runnable q = ColourPanel.this;
                Thread s = new Thread(q);
                s.start();
            }
        });
    }

    public static void main(String[] args) {
        ColourPanel panel = new ColourPanel();
        panel.setPreferredSize(new Dimension(200,200));
        JOptionPane.showMessageDialog(null, panel, "", -1);
    }

    private JButton colourButton;
    private double randomNumber;
}