why is the above code printing different answers at different times? what should i do to get a same answer always?
Dismantle your system and change your processor. Just kidding. This is what expected regarding the behaviour of thread. Thereads dont run the way you like, it is totally under the control of the JVM. You don't have to worry about this.
I believe the problem is that you created a new Runnable object for each thread and you're using these different objects for synchronization. Since each thread is looking to obtain a different object lock (each thread is looking to obtain it's own Runnable object's lock) none of them will encounter a situation where they are blocked and cannot obtain the lock they're looking for, so really there's no sychronization.
I think one possible solution would be to create a static reference variable in the Runnable class (so just 1 lock is available) and sychronize the code in your run method on that (see below).