thread on different instances?

[Anthony Karta]

from K&B book, chapter 9, Self test Question 1.

    
public class messanger implements Runnable {
 
       public static void main(String[] args)  {
            new Thread(new messanger("Wallace")).start();
            new Thread(new messanger("Gromit")).start();
       }
 
       private String name;
 
      public messanger(String name) { this.name = name; }
 
      public void run() {
 
           message(1);
           message(2);
       }
 
      private synchronized void message(int n) {
           System.out.print(name + "-" + n + " ");
      }
}

Output:
Wallace-1 Gromit-1 Wallace-2 Gromit-2
OR
Gromit-1 Wallace-1 Gromit-2 Wallace-2



I'm expecting the output:

"Wallace-1 Wallace-2 Gromit-1 Gromit-2"
OR
"Gromit-1 Gromit-2 Wallace-1 Wallace-2 "

because there are 2 threads, and each has its "own" messanger instance, and won't block each other. please correct/explain me. thanks
[ November 27, 2006: Message edited by: Anthony Karta ]

[Aniket Patil]

You are right about there being 2 messenger instances and them not blocking each other.

It is possible, that the Wallace thread gets interrupted (not blocked by the other thread) in its execution by the scheduler. Hence a Wallace or a Gromit thread is not guaranteed to complete execution before the other thread begins.

[Wil Guo]

These two threads have the same default priority. So before thread 1 completes (output "Wallace-2"), thread 2 could start and then thread 1 finish, thread 2 finish. It is also possible that thread 2 completes before thread 1.

[Anthony Karta]

thanks guys... I get it now.

    
private [B]synchronized[/B] void message(int n) {
           System.out.print(name + "-" + n + " ");
      }

my next question is

do we need synchronized on message() method?? since they are running on different instances. I know there is no harmless but it's not necessary, am I correct??

I notice there is no different in the output.

[Aniket Patil]

With 2 different instances, each would have a separate copy of the instance method. Hence, these 2 instances would not block each other.

"synchronized" would only matter if the same object were being accessed by multiple threads.