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Thinking in Java
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Runnable revisited

In Chapter 13, I suggested that you think carefully before making a class as an implementation of Runnable. Of course, if you must inherit from a class and you want to add threading behavior to the class, Runnable is the correct solution. The following example exploits this by making a Runnable JPanel class that paints different colors on itself. This application is set up to take values from the command line to determine how big the grid of colors is and how long to sleep( ) between color changes. By playing with these values, you’ll discover some interesting and possibly inexplicable features of threads:

// Using the Runnable interface.
// <applet code=ColorBoxes width=500 height=400>
// <param name=grid value="12">
// <param name=pause value="50"></applet>
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import com.bruceeckel.swing.*;

class CBox extends JPanel implements Runnable {
  private Thread t;
  private int pause;
  private static final Color[] colors = {
    Color.BLACK, Color.BLUE, Color.CYAN,
    Color.DARK_GRAY, Color.GRAY, Color.GREEN,
    Color.LIGHT_GRAY, Color.MAGENTA,
    Color.ORANGE, Color.PINK, Color.RED,
    Color.WHITE, Color.YELLOW
  private static Random rand = new Random();
  private static final Color newColor() {
    return colors[rand.nextInt(colors.length)];
  private Color cColor = newColor();
  public void paintComponent(Graphics  g) {
    Dimension s = getSize();
    g.fillRect(0, 0, s.width, s.height);
  public CBox(int pause) {
    this.pause = pause;
    t = new Thread(this);
  public void run() {
    while(true) {
      cColor = newColor();
      try {
      } catch(InterruptedException e) {
        throw new RuntimeException(e);

public class ColorBoxes extends JApplet {
  private boolean isApplet = true;
  private int grid = 12;
  private int pause = 50;
  public void init() {
    // Get parameters from Web page:
    if(isApplet) {
      String gsize = getParameter("grid");
      if(gsize != null)
        grid = Integer.parseInt(gsize);
      String pse = getParameter("pause");
      if(pse != null)
        pause = Integer.parseInt(pse);
    Container cp = getContentPane();
    cp.setLayout(new GridLayout(grid, grid));
    for(int i = 0; i < grid * grid; i++)
      cp.add(new CBox(pause));
  public static void main(String[] args) {
    ColorBoxes applet = new ColorBoxes();
    applet.isApplet = false;
    if(args.length > 0)
      applet.grid = Integer.parseInt(args[0]);
    if(args.length > 1)
      applet.pause = Integer.parseInt(args[1]);, 500, 400);
} ///:~

ColorBoxes is the usual applet/application with an init( ) that sets up the GUI. This configures a GridLayout so that it has grid cells in each dimension. Then it adds the appropriate number of CBox objects to fill the grid, passing the pause value to each one. In main( ) you can see how pause and grid have default values that can be changed if you pass in command-line arguments, or by using applet parameters.

CBox is where all the work takes place. This is inherited from JPanel and it implements the Runnable interface so that each JPanel can also be a Thread. Remember that when you implement Runnable, you don’t make a Thread object, just a class that has a run( ) method. Thus, you must explicitly create a Thread object and hand the Runnable object to the constructor, then call start( ) (this happens in the constructor). In CBox this thread is called t.

Notice the array colors, which is an enumeration of all the colors in class Color. This is used in newColor( ) to produce a randomly selected color. The current cell color is cColor.

paintComponent( ) is quite simple; it just sets the color to cColor and fills the entire JPanel with that color.

In run( ), you see the infinite loop that sets the cColor to a new random color and then calls repaint( ) to show it. Then the thread goes to sleep( ) for the amount of time specified on the command line.

Precisely because this design is flexible and threading is tied to each JPanel element, you can experiment by making as many threads as you want. (In reality, there is a restriction imposed by the number of threads your JVM can comfortably handle.)

This program also makes an interesting benchmark, since it can and has shown dramatic performance and behavioral differences between one JVM threading implementation and another.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire