synchronized methods + Threads

[Parameswaran Thangavel]

class Counter {
int v = 0;
synchronized void inc() { v++; }
synchronized void dec() { v--; }
}
public class Q7ed5 {
Counter i;
Counter j;
Counter k;
public synchronized void a() {
i.inc();
System.out.println("a");
i.dec();
}
public synchronized void b() {
i.inc(); j.inc(); k.inc();
System.out.println("b");
i.dec(); j.dec(); k.dec();
}
public void c() {
k.inc();
System.out.println("c");
k.dec();
}
}

1. i.v guaranteed always to be 0 or 1
2. j.v guaranteed always to be 0 or 1
3. k.v guaranteed always to be 0 or 1
4. j.v will always be greater than or equal to k.v at any given time.
5. k.v will always be greater than or equal to j.v at any given time.

[Animesh Shrivastava]

I guess the answer should be both 1 and 2.
Whats the correct answer?

[Parameswaran Thangavel]

hi
can any please explain me

[Henry Wong]

Originally posted by Parameswaran Thangavel:
hi
can any please explain me

Let's look at the possible answers...

1. i.v guaranteed always to be 0 or 1
2. j.v guaranteed always to be 0 or 1
3. k.v guaranteed always to be 0 or 1
4. j.v will always be greater than or equal to k.v at any given time.
5. k.v will always be greater than or equal to j.v at any given time.

As for the methods, a() and b() are both synchronized. They can't be ran in parallel via different threads. The c() method is not synchronized, it can be ran in parallel, even while a() or b() is running.

(3) is not true as you can have two c() method running in parallel, and having increment the count before decrementing the count -- producing a value of 2. (albeit very short lived)

(4) is not true as a ramification of (3). K can have large values... In theory, hard to produce in practice.

(5) is not true based on how method b() works. It increments j before incrementing k... hence, j is larger than k for the really small period between the two methods.

Now that leaves (1) and (2) which is only modified from a() and b(), and which can only run via a single thread, and always runs in pairs, inc() followed by a dec().

Henry