Tools of the trade, Part 3

// Lines.java
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;
public class Lines extends Applet implements Runnable
{
   static Lines l;
   static Stub s;
   private int width = 0, height = 0;
   private Thread runner = null;
   private final static int MAXCOLORS = 256;
   private final static int SLEEPMILLIS = 50;
   public void init ()
   {
      // Acquire the applet's width and height.
      width = getSize ().width;
      height = getSize ().height;
   }
   public void start ()
   {
      // If no thread exists, create a Thread object that associates with the
      // current Lines object, and start a new thread that invokes the current
      // Lines object's run() method.
      if (runner == null)
      {
          runner = new Thread (this);
          runner.start ();
      }
   }
   public void paint (Graphics g)
   {
      // Generate a random color and establish that color as the Graphics
      // context's drawing color.
      Color c = new Color (rnd (MAXCOLORS), rnd (MAXCOLORS), rnd (MAXCOLORS));
      g.setColor (c);
      // Generate a random set of coordinates in the upper-left quadrant of
      // the drawing surface, and use those coordinates as the starting
      // coordinates for a new line.
      int x1 = rnd (width / 2);
      int y1 = rnd (height / 2);
      // Compute the line's ending coordinates by mirroring the starting
      // coordinates in the lower-right quadrant.
      int x2 = width - x1;
      int y2 = height - y1;
      // Draw the line.
      g.drawLine (x1, y1, x2, y2);
      // Draw an inverse of the line (for symmetry).
      g.drawLine (x1, y2, x2, y1);
   }
   static int rnd (int limit)
   {
      // Return a random integer that ranges from 0 through limit-1.
      return (int) (Math.random () * limit);
   }
   public void run ()
   {
      // Obtain a reference to the thread that was started in the applet's
      // start() method.
      Thread current = Thread.currentThread ();
      // As long as runner contains the same reference, keep looping. The
      // reference in runner becomes null when the applet's stop() method
      // executes.
      while (current == runner)
      {
         // Invoke the paint(Graphics g) method to draw another
         // randomly colored and randomly positioned line.
         repaint ();
         // Pause for SLEEPMILLIS milliseconds to achieve an eye-pleasing
         // display.
         try
         {
             Thread.sleep (SLEEPMILLIS);
         }
         catch (InterruptedException e)
         {
             System.err.println ("Thread was interrupted.");
         }
      }
   }
   public void stop ()
   {
      // Tell the line-drawing thread to terminate.
      runner = null;
   }
   // The following method is overridden to prevent the drawing surface from
   // being automatically cleared after a line is drawn. Stay tuned to Java
   // 101 to learn more about this method.
   public void update (Graphics g)
   {
      paint (g);
   }
   public static void main (String [] args)
   {
      Frame frame = new Frame ("Lines as an Application");
      l = new Lines ();
      // NOTE:
      //
      // It is not necessary to specify frame.add (new Panel ().add (l));.
      frame.add (l);
      // Create the frame's peer. Peer is not visible. This is necessary
      // for when a background buffer must be created in the init() method
      // (e.g., Image bk = createImage (100, 100);).
      frame.addNotify ();
      l.setStub (s = new Stub (args));
      frame.setSize (300, 300);
      frame.setVisible (true);
      // NOTES:
      //
      // 1) l.init (); must follow frame.setVisible (true); or graphics
      //    do not display.
      //
      // 2) The l.doLayout (); call is included in case GUI components
      //    will be added to applet's panel -- because frame window has
      //    already been laid out.
      l.init ();
      l.doLayout ();
      s.setActive (true);
      l.start ();
      frame.addWindowListener (new WindowAdapter ()
                               {
                                   public void windowClosing
                                                       (WindowEvent w)
                                   {
                                      s.setActive (false);
                                      l.stop ();
                                      l.destroy ();
                                      System.exit (0);
                                   }
                               });
   }
}
/* The Stub class provides a mechanism for obtaining information from
   the runtime environment.  Typically, this environment is maintained
   by a Web browser.  For this program, a Web browser environment is
   being simulated. */
class Stub implements AppletStub
{
   private boolean active = false;
   private Hashtable ht = new Hashtable ();
   private Context c;
   // Create a new Stub object.  The application's array of command
   // arguments are passed to this constructor, where they are saved
   // in a Hashtable object for later retrieval by the getParameter()
   // method.
   Stub (String [] args)
   {
      c = new Context (); // Create an applet context.
      // Make sure an even number of arguments has been passed.
      if ((args.length & 1) != 0)
          return;
      for (int i = 0; i < args.length; i += 2)
           ht.put (args [i], args [i + 1]);
   }
   // Return the current state of an applet.  During initialization,
   // the applet is not active (and this method returns false).  The
   // applet's active state is set to true just before the start
   // method is called.
   public boolean isActive ()
   {
      return active;
   }
   // Return the complete URL of the HTML document containing the
   // applet.  This URL includes the name of the document's file.
   public URL getDocumentBase ()
   {
      URL u = null;
      try
      {
          u = new URL ("file:///" + (new File ("").getAbsolutePath ()) +
                       "/x.html"); // Use a fake document.
      }
      catch (MalformedURLException e) {}
      return u;
   }
   // Return the complete URL of the applet's .class file(s).  This
   // method is often used with the getImage and getAudioClip
   // methods to load image/audio files relative to the .class files.
   public URL getCodeBase ()
   {
      URL u = null;
      try
      {
          u = new URL ("file:///" + new File ("").getAbsolutePath () + "/");
      }
      catch (MalformedURLException e) {}
      return u;
   }
   // Return the value of the applet parameter, identified by the
   // name argument.  If not present, null is returned.  The Applet
   // class contains a getParameter() method that calls this method.
   public String getParameter (String name)
   {
      return (String) ht.get (name);
   }
   // Return a reference to the applet's context.  The Applet class
   // contains a getAppletContext() method that calls this method.
   public AppletContext getAppletContext ()
   {
      return c; // Return current applet context.
   }
   // Resize the applet.  The Applet class contains a pair of resize
   // methods that call this method. Note: Web browsers like Netscape 
   // don't allow applets to be resized. 
   public void appletResize (int width, int height)
   {
   }
   // The following method is an extra method that is called to set
   // the value of the private active variable.
   public void setActive (boolean active)
   {
      this.active = active;
   }
}
/* The Context class provides a mechanism to control the environment
   in which the program is running.  Typically, this environment is
   maintained by a Web browser.  For this program, a Web browser
   environment is being simulated. */
class Context implements AppletContext
{
   // Load the file located by the url argument.  The Applet
   // class contains a pair of getAudioClip() methods that call
   // this method.
   public AudioClip getAudioClip (URL url)
   {
      return Applet.newAudioClip (url);
   }
   // Prepare to load the image located by the url argument.  The
   // image is loaded when needed (by one of Graphics' drawImage()
   // methods).  The Applet class contains a pair of getImage()
   // methods that call this method.
   public Image getImage (URL url)
   {
      Toolkit tk = Toolkit.getDefaultToolkit ();
      return tk.getImage (url);
   }
   // Fetch the Applet (identified by name) from the current HTML
   // document.
   public Applet getApplet (String name)
   {
      return null;
   }
   // Return an enumeration to all Applets located on the current HTML
   // page.
   public Enumeration getApplets ()
   {
      return null;
   }
   // Show the HTML document, located by the url argument, in the
   // current Web browser window.
   public void showDocument (URL url)
   {
      System.out.println ("Showing document " + url);
   }
   // Show the HTML document, located by the url argument, in the
   // Web browser window, identified by the frame argument.
   public void showDocument (URL url, String frame)
   {
      try
      {
         showDocument (new URL (url.toString () + frame));
      }
      catch (MalformedURLException e) {}
   }
   // Show a status message, identified by the message argument, in
   // the Web browser's status bar.  The Applet class contains a
   // showStatus() method that calls this method.
   public void showStatus (String message)
   {
      System.out.println (message);
   }
   // The following three methods are required by SDK 1.4.  To learn
   // about those methods, please refer to the SDK 1.4 documentation.
   public InputStream getStream (String key)
   {
      return null;
   }
   public Iterator getStreamKeys ()
   {
      return null;
   }
   public void setStream (String key, InputStream stream)
   {
   }
}