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Escher is a collection of libraries for X Window System written purely in Java.
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package gnu.app.displayhack;

import gnu.x11.Color;
import gnu.x11.GC;
import gnu.x11.Input;
import gnu.x11.Rectangle;
import gnu.x11.Window;
import gnu.app.displayhack.eraser.Eraser;
import gnu.x11.event.ButtonPress;
import gnu.x11.event.ClientMessage;
import gnu.x11.event.ConfigureNotify;
import gnu.x11.event.Event;
import gnu.x11.event.Expose;
import gnu.x11.event.KeyPress;
import java.util.Random;


/**
 * Base class for display hacks. 
 * 
 * The implementation uses two threads so that one can be painting and
 * one can be waiting for user input. Only with two threads can {@link
 * #sleep(long)} be interrupted (with {@link #restart()} or {@link
 * #exit()}) to give responsive user interface.
 */
public abstract class DisplayHack extends gnu.x11.Application
  implements Runnable {

  public boolean stop_now;
  public static final Random random = new Random ();

  public int eraser_delta, eraser_mode;
  public long delay, eraser_delay;
  public Color [] colors;
  public GC gc, eraser_gc;
  public GC.Values gc_values = new GC.Values ();
  public Window window;
  public Thread thread = new Thread (this);


  /** 
   * Clear window before the first iteration. Hacks that play with screen
   * content or those that wish to improve initial perceived drawing speed
   * (such as {@link Deco}) should set this to false.
   */
  public boolean clear;


  /** Erase window between iterations. */
  public boolean erase;


  public DisplayHack (String [] args, boolean clear, boolean erase,
    boolean rainbow_color, int default_color_count, int default_delay) {

    super (args);
    this.clear = clear;
    this.erase = erase;

    int color_count = option.intt ("color-count", 
      "total number of random colors", default_color_count);
    delay = option.longg ("delay",
      "delay between screens in ms", default_delay);

    if (erase) {
      eraser_delay = option.longg ("eraser-delay", 
        "delay between iterations of eraser in ms", 10);
      eraser_delta = option.intt ("eraser-delta",
        "granularity of eraser", 5, 1, 10);
      eraser_mode = option.enumerate ("eraser-mode",
        "which eraser", Eraser.ALL_STRINGS, 
        Eraser.RANDOM_ERASER_INDEX);
    }

    Rectangle geometry = option.rectangle ("geometry", 
      "initial geometry of main window",
      new Rectangle (10, 10, 600, 480));

    if (help_option) return;

    gc = GC.build (display);
    if (erase) eraser_gc = GC.build (display);


    colors = new Color [color_count];

    if (rainbow_color)
      for (int i=0; iExpose. It is responsible for user interaction.
   */
  public void exec () { 
    if (help_option) return;

    window.map ();
    while (!exit_now) dispatch_event ();

    /* Do not close display here. The other thread (painting thread) is
     * reponsible for closing display, because it always outlasts the main
     * thread.
     */
  }


  public void exit () {
    super.exit ();
    restart ();
  }


  public Color random_color () {
    return colors [random.nextInt (colors.length)];
  }   


  /** {@link java.util.Random#nextInt(int)} accepting zero as argument. */
  public int random_int (int n) {
    if (n == 0) return 0;
    return random.nextInt (n);
  }


  public int random_sign () {
    return random.nextBoolean () ? 1 : -1;
  }


  public void restart () {
    stop_now = true;            // stop paint, if painting
    thread.interrupt ();        // stop sleep, if sleeping
  }


  public void run () {
    while (!exit_now) {
      stop_now = false;
      paint ();
      sleep (delay);

      // erase despite `stop_now'
      if (erase && !exit_now) erase ();
    }

    display.close ();
  }


  public boolean sleep (long millis) {
    if (millis != 0) display.flush ();
    if (!stop_now) gnu.util.Misc.sleep (millis);
    return stop_now;
  }
}