import java.util.concurrent.Semaphore;

class Sync
{
   public static Semaphore ReadSema = new Semaphore(0);
   public static Semaphore WriteSema = new Semaphore(1);
}

class Writer implements Runnable 
{
   private String msg;

   public Writer(String s)
   {
      msg = s;
   }

   // Interface method
   public void run()
   {
      while ( true )
      {
         try
         {
            Sync.WriteSema.acquire();
         }
         catch ( InterruptedException e )
         {
         }
   
         System.out.println(msg+".. writing");
         Sync.ReadSema.release();
      }
   }
}


class Reader implements Runnable 
{
   private String msg;

   public Reader(String s)
   {
      msg = s;
   }

   // Interface method
   public void run() 
   {
      while ( true )
      {
         try
         {
            Sync.ReadSema.acquire();
         }
         catch ( InterruptedException e )
         {
         }
   
         System.out.println(msg+".. reading");
         Sync.WriteSema.release();
      }
   }
}



public class ThreadDemo2
{
   public static void main(String args[])  throws InterruptedException
   {
      Runnable r, w;
      Thread t1, t2;

      r = new Reader("Read");
      w = new Writer("Write");

      t1 = new Thread(r);
      t2 = new Thread(w);

      t1.start();
      t2.start();

      System.out.println("Waiting for threads to exit");
      t1.join();
      t2.join();
      System.out.println("Done");

   }
}