Threads Q

[Kedar Dravid]

public class Test extends Thread
{
 static int x, y;
 public synchronized void run()
 {
  for(; :wink:    //AN INFINITE LOOP OR CRAB
  { 
   x++; y++; System.out.println(x+" "+y);
  }
 }
 public static void main(String[] args)
 {
   new Test().start();
   new Test().start();
 }
}

What will the above code print?

Select 1 correct option.
a It will keep on printin same values for x and y incrementing by 1 on each line.

b It will keep on printin same values for x and y but they may be incrementing by more than 1 on each line.

c It may print different values for x and y but both the values will be incrementing by 1 on each line.

d You cannot say anything about the values.

Given answer: d
I was expecting a as the o/p, since synchronized keyword is used.
Why is d) the answer?

[ March 13, 2005: Message edited by: Kedar Dravid ]
[ March 13, 2005: Message edited by: Kedar Dravid ]

[John Campbell]

I think your 'for' loop is ill-constructed, can you update your code please.

[Mike Gershman]

There are two Test objects created. Each thread calls run() synchronized on its own Test object. Not only can x be incremented, then x,y printed by the other thread, then y incremented, but x++ and y++ are not atomic or thread safe so you can get almost any result.

[Ipfa Ovart]

Answer a would have been ok if the code was looking like this:

 
public void run()
 {
  for(; ; )   //AN INFINITE LOOP OR CRAB
  { 
   process();
  }
 }
 private static synchronized process(){
   x++; y++; System.out.println(x+" "+y);
 }

Here the incrementation of x and y is synchronized by the call to the static method. The lock is the same for each Test instance, unlike in the original piece of code.

Another solution :

 
private static Object monitor = new Object();
public void run()
 {
  for(; ; )   //AN INFINITE LOOP OR CRAB
  { 
   synchronized(monitor){
    x++; y++; System.out.println(x+" "+y);
   }
  }
 }

[ March 13, 2005: Message edited by: Ipfa Ov ]