// Purpose.  Producer & consumer threads, 1 Monitor [Source: Roberts, p212]

class Mailbox {
   private boolean request;
   private String  message;

   public Mailbox() {
      request = false;
   }
   public synchronized void storeMessage( String msg ) {
      while (request == true) {  // no room for another message
         try { wait(); } catch (InterruptedException e ) { }
      }
      request = true;
      message = msg;
      notifyAll();
   }
   public synchronized String retrieveMessage() {
      while (request == false) {  // no message to retrieve
         try { wait(); } catch (InterruptedException e ) { }
      }
      request = false;
      notifyAll();
      return message;
}  }

class ProducerThread extends Thread {
   private Mailbox mb;
   private int     id;
   private static int num = 1;

   public ProducerThread( Mailbox m ) {
      mb = m;  id = num++;
   }
   public void run() {
      for (int i=0; i < 9; i++) {
         System.out.println( "sending " + i + " from thread " + id );
         mb.storeMessage( "message " + i + " from thread " + id );
      }
      for (int i=0; i < 3; i++) {
         System.out.println( "sending DONE" );
         mb.storeMessage( "DONE" );
      }
      System.out.println( "%%% thread " + id + " exiting" );
}  }

class ConsumerThread extends Thread {
   private Mailbox mb;
   private int     id;
   private static int num = 2;

   public ConsumerThread( Mailbox m ) {
      mb = m;  id = num++;
   }
   public void run() {
      while (true) {
         String str = mb.retrieveMessage();
         if (str.equals( "DONE" )) break;
         System.out.println( "   thread " + id + " received --- " + str );
      }
      System.out.println( "%%% thread " + id + " exiting" );
}  }

class ThreadMailbox {
   public static void main( String[] args ) {
      Mailbox mb = new Mailbox();
      new ProducerThread( mb ).start();
      new ConsumerThread( mb ).start();
      new ConsumerThread( mb ).start();
      new ConsumerThread( mb ).start();
}  }

// sending 0 from thread 1
// sending 1 from thread 1
//    thread 4 received --- message 0 from thread 1
// sending 2 from thread 1
//    thread 2 received --- message 1 from thread 1
// sending 3 from thread 1
//    thread 3 received --- message 2 from thread 1
// sending 4 from thread 1
//    thread 4 received --- message 3 from thread 1
// sending 5 from thread 1
//    thread 2 received --- message 4 from thread 1
// sending 6 from thread 1
//    thread 3 received --- message 5 from thread 1
// sending 7 from thread 1
//    thread 4 received --- message 6 from thread 1
// sending 8 from thread 1
//    thread 2 received --- message 7 from thread 1
// sending DONE
//    thread 3 received --- message 8 from thread 1
// sending DONE
// %%% thread 4 exiting
// sending DONE
// %%% thread 2 exiting
// %%% thread 1 exiting
// %%% thread 3 exiting