Java program output - concurrent

Question

It might be a silly question but, can the output of this program ( the way it is) be zero?

public class Test2{

    int a = 0;
    AtomicInteger b = new AtomicInteger();
    public static Test2  c = new Test2();

    public static void main(String[] args){

        Thread t1 = new Thread(new MyTest1());
        Thread t2 = new Thread (new Mytest2());

        t1.start();
        t2.start();
    }

}

class MyTest1 implements Runnable{

    public void run(){
        Test2.c.a = 1;
        Test2.c.b.compareAndSet(0,1);

    }
}

class Mytest2 implements Runnable{
    public void run(){
        int x = 0;

        if(Test2.c.b.get() == 1)
            x = Test2.c.a;

        System.out.println("Value of x = "+x);
    }

}

The reason I'm asking this is, although I'm using AtomicInteger, the if() statement in MyTest2 might be executed first and then x will have the value zero..right?

or am I not thinking straight.

Any help would be highly appreciated.

Answer 1

can the output of this program ( the way it is) be zero?

Yes, that's possible. The code does not guarantee in any way that t1 will finish before t2 finishes. If that's your intent, you may find CountdownLatch or CyclicBarrier useful. Click the links, their javadocs contains code examples.

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That said, I'd rather pass a reference of the AtomicInteger as constructor argument of the both runnables instead of accessing it the static way.

Answer 2

With one small change you can guarantee that the output will be 1:

public void run(){
    int x = 1;

    if(Test2.c.b.get() == 1)
        x = Test2.c.a;

    System.out.println("Value of x = "+x);
}

Because of the ordering guarantees of volatile reads/writes:

• x is initally 1
• The only other value that can be assigned to x is the value of Test2.c.a
• Test2.c.a can be copied to x only if 1 was previously read from Test2.c.b
• 1 is written to Test2.c.b only after writing 1 to Test2.c.a
• Because Test2.c.b is volatile, any thread that reads it also observes all previous writes to all variables (volatile or not) by the same thread that wrote to Test2.c.b. That means t2 also sees the write of 1 to Test2.c.a
• If the test fails (i.e. the read of Test2.c.b returns something other than 1) then x retains its initial value of 1, so the output is still 1.

I suspect this is what the author of this program was trying to illustrate, and the default value of 0 for x was a typo.