Yield and sleep

Question

Hello!
I have some questions.
In the code below i use sleep instead of yield to make it look like both threads are executing at the same time because the yield function does not have that effect on a unix system.
It works even thougt i just sleep for 0 milliseconds. Is it correct to do it this way or are there any problems with it. I guess the program will be slower if i sleep for ie 50 ms.
Lets assume that i have two processors. Is it then unneccesary sleep?
Why does the yield function not work on sun os?
class Threads
                     public static void main(String[] args)
          Thread1 t1 = new Thread1();
          Thread2 t2 = new Thread2();
          Thread thread1 = new Thread(t1);
          thread1.start();
          Thread thread2 = new Thread(t2);
          thread2.start();
class Thread1 implements Runnable
         synchronized public void run()
          for (int i = 0; i < 10; i++)
                   System.out.println("Thread1");
                   try
                                                                Thread.sleep(0);
                   catch (InterruptedException e){}
                   //Thread.yield();
class Thread2 implements Runnable
         synchronized public void run()
          for (int i = 0; i < 10; i++)
                   System.out.println("Thread2");
                   try
                                                                  Thread.sleep(0);
                   catch (InterruptedException e){}
                   //Thread.yield();
}

Answer 1

The rules for threads don't state that threads of equal priority will be dispatched in a round-robin fashion, its strictly first come, first served, so if your initial thread is always ready to go again, yield may well not release control to the other thread. sleep - even sleep(0) apparently pauses the action long enough that the other thread is the only one despatchable and off it goes.
As hiwa mentioned, as soon as you have real logic in each thread this issue will probably go away.