Exercise 9: Clocks

The source code:

import java.applet.*;
import java.awt.*;
import java.util.*;
import java.text.*;

public class Clock1 extends Applet implements Runnable {

   int width, height;
   Thread t = null;
   boolean threadSuspended;
   int hours=0, minutes=0, seconds=0;
   String timeString = "";

   public void init() {
      width = getSize().width;
      height = getSize().height;
      setBackground( Color.black );
   }

   public void start() {
      if ( t == null ) {
         t = new Thread( this );
         t.setPriority( Thread.MIN_PRIORITY );
         threadSuspended = false;
         t.start();
      }
      else {
         if ( threadSuspended ) {
            threadSuspended = false;
            synchronized( this ) {
               notify();
            }
         }
      }
   }

   public void stop() {
      threadSuspended = true;
   }

   public void run() {
      try {
         while (true) {

            // Here's where the thread does some work
            Calendar cal = Calendar.getInstance();
            hours = cal.get( Calendar.HOUR_OF_DAY );
            if ( hours > 12 ) hours -= 12;
            minutes = cal.get( Calendar.MINUTE );
            seconds = cal.get( Calendar.SECOND );

            SimpleDateFormat formatter
               = new SimpleDateFormat( "hh:mm:ss", Locale.getDefault() );
            Date date = cal.getTime();
            timeString = formatter.format( date );

            // Now the thread checks to see if it should suspend itself
            if ( threadSuspended ) {
               synchronized( this ) {
                  while ( threadSuspended ) {
                     wait();
                  }
               }
            }
            repaint();
            t.sleep( 1000 );  // interval given in milliseconds
         }
      }
      catch (InterruptedException e) { }
   }

   void drawHand( double angle, int radius, Graphics g ) {
      angle -= 0.5 * Math.PI;
      int x = (int)( radius*Math.cos(angle) );
      int y = (int)( radius*Math.sin(angle) );
      g.drawLine( width/2, height/2, width/2 + x, height/2 + y );
   }

   void drawWedge( double angle, int radius, Graphics g ) {
      angle -= 0.5 * Math.PI;
      int x = (int)( radius*Math.cos(angle) );
      int y = (int)( radius*Math.sin(angle) );
      angle += 2*Math.PI/3;
      int x2 = (int)( 5*Math.cos(angle) );
      int y2 = (int)( 5*Math.sin(angle) );
      angle += 2*Math.PI/3;
      int x3 = (int)( 5*Math.cos(angle) );
      int y3 = (int)( 5*Math.sin(angle) );
      g.drawLine( width/2+x2, height/2+y2, width/2 + x, height/2 + y );
      g.drawLine( width/2+x3, height/2+y3, width/2 + x, height/2 + y );
      g.drawLine( width/2+x2, height/2+y2, width/2 + x3, height/2 + y3 );
   }

   public void paint( Graphics g ) {
      g.setColor( Color.gray );
      drawWedge( 2*Math.PI * hours / 12, width/5, g );
      drawWedge( 2*Math.PI * minutes / 60, width/3, g );
      drawHand( 2*Math.PI * seconds / 60, width/2, g );
      g.setColor( Color.white );
      g.drawString( timeString, 10, height-10 );
   }
}

The resulting applet:

Note that the time displayed in the lower left corner may not be correct -- certain browsers (perhaps only older versions?) seem to interpret the "default" time zone as something different from the local time zone. (The hands, however, always seem to display the correct time.) If this problem bothers you, try using

instead of