Threads

[shashank dwivedi]

public class Chess implements Runnable{
public void run(){
    move(Thread.currentThread().getId());
   }
   synchronized void move(long id){
   System.out.print(id+ " ");
   System.out.print(id+ " ");
   }
public static void main(String args[]){
Chess ch =new Chess();
new Thread(ch).start();
new Thread(new Chess()).start();
   }
}

In K&B its written that output could be 4 2 4 2
My problem is if one thread runs and calls move() method which is synchronized ,it should print same id twice like 4 4 2 2
and why 2 2? it should better be 4 4 4 4. why priority of one thread is 2 and other is 4?what is going here? please tell me. 

[Aniruddh Joshi]

shashank dwivedi wrote:
why priority of one thread is 2 and other is 4

Thread ID is getting printed here, not the priority.
I did below to get the priority printed

   synchronized void move(long id){   
   System.out.print(" | " + id+ " > " + Thread.currentThread().getPriority());   
   System.out.print(" | " +id+ " > " + Thread.currentThread().getPriority());   
   }  

And got below-

| 7 > 5 | 7 > 5 | 8 > 5 | 8 > 5

Priority of both the threads is default since the code is not changing it.
And I think

4422 or 4242

depends on the machine(jvm and OS).

You'll definitely not get

4444

since these are 2 diffrent threads and will always always unique IDs.

[gurpeet singh]

shashank dwivedi wrote:

public class Chess implements Runnable{
public void run(){
    move(Thread.currentThread().getId());
   }
   synchronized void move(long id){
   System.out.print(id+ " ");
   System.out.print(id+ " ");
   }
public static void main(String args[]){
Chess ch =new Chess();
new Thread(ch).start();
new Thread(new Chess()).start();
   }
}

In K&B its written that output could be 4 2 4 2
My problem is if one thread runs and calls move() method which is synchronized ,it should print same id twice like 4 4 2 2
and why 2 2? it should better be 4 4 4 4. why priority of one thread is 2 and other is 4?what is going here? please tell me. 

first of all as Anirudh said we are not talking about priority here. what your code is printing is Thread Id. now coming to your question. here we have 2 threads running concurrently. let me put this way, you have started 2 threads , call them Worker A and Worker B . now these two workers are executing the same code of 2 different objects. In your code you have created 2 chess objects and have passed one to each thread worker. so Worker A will run its own copy of run method of say first chess object and Worker B will have its own copy of run method of second(different) chess object. now when worker A enters move method it acquires lock on first chess object. this won't affect other thread because other thread acquires lock on completely different chess object. so you have 2 threads running concurrently. which means output of 4 2 42 is possible. you can also get 4422 or 2244 or 2424.

[shashank dwivedi]

gurpeet singh wrote:

shashank dwivedi wrote:

public class Chess implements Runnable{
public void run(){
    move(Thread.currentThread().getId());
   }
   synchronized void move(long id){
   System.out.print(id+ " ");
   System.out.print(id+ " ");
   }
public static void main(String args[]){
Chess ch =new Chess();
new Thread(ch).start();
new Thread(new Chess()).start();
   }
}


In K&B its written that output could be 4 2 4 2
My problem is if one thread runs and calls move() method which is synchronized ,it should print same id twice like 4 4 2 2
and why 2 2? it should better be 4 4 4 4. why priority of one thread is 2 and other is 4?what is going here? please tell me. 

first of all as Anirudh said we are not talking about priority here. what your code is printing is Thread Id. now coming to your question. here we have 2 threads running concurrently. let me put this way, you have started 2 threads , call them Worker A and Worker B . now these two workers are executing the same code of 2 different objects. In your code you have created 2 chess objects and have passed one to each thread worker. so Worker A will run its own copy of run method of say first chess object and Worker B will have its own copy of run method of second(different) chess object. now when worker A enters move method it acquires lock on first chess object. this won't affect other thread because other thread acquires lock on completely different chess object. so you have 2 threads running concurrently. which means output of 4 2 42 is possible. you can also get 4422 or 2244 or 2424.

Thanks all. I GOT IT.