Thanks Boudewijn,

Sorry that I do not express myself very clearly. I mean yield method will
make current thread idle on CPU (current thread will not occupy CPU time)
and it will give chances to other threads to run on CPU. Am I correct?

regards,
George

Not expert by any means, just dimly aware that different
implementations do take different approaches to scheduling when
there's more than one thread with the same priority.  Below is a
quickly-hacked-together little program that should give some clues
about what a particular implementation does.  If the implementation
is doing timeslicing at all, then for a "large enough" value of
N you should observe that the messages produced by the first loop
("no yield") in both threads are interspersed.  If it's not doing
timeslicing, then the first message from thread "pong" won't show up
until after thread "ping" has finished that first loop.

As for getting into the details of how particular implementations do
scheduling -- I'm not sure I'd advise this unless you need very
precise control over the scheduling of your threads and/or don't
care about portability.  The standard advice is to write your code
to work with any implementation that meets the published specs,
which are probably available at Sun's Web site, somewhere.  Sorry
I don't have time to track down more details.

/* No claims are made about the beauty or generality of this code,
  only that it compiles and executes for me and I think produces
  some useful information.
*/

public class PingPong {

   public static void main(String[] args) {

       if (args.length < 1) {
           System.err.println("usage:  PingPong count");
           System.exit(1);
       }
       int count = Integer.parseInt(args[0]);

       new Thread(new CodeForThread(count, "ping")).start();
       new Thread(new CodeForThread(count, "pong")).start();
   }

   private static class CodeForThread implements Runnable {

       private int count;
       private String name;

       public CodeForThread(int count, String name) {
           this.count = count;
           this.name = name;
       }

       public void run() {
           System.out.println("thread " + name + " will now print "
                   + count + " messages without using Thread.yield");
           for (int i = 0; i < count; ++i) {
               System.out.println("in thread " + name + " (no yield)");
           }
           System.out.println("thread " + name + " will now print "
                   + count + " messages using Thread.yield");
           for (int i = 0; i < count; ++i) {
               System.out.println("in thread " + name + " (yield)");
               Thread.yield();
           }
       }
   }
}