java threads effect and race condition

Question

Possible Duplicate:
java threads effect on static classes

consider the following code:

    static class ThreadTest extends Thread {
        int x;
        int[] y;

        public ThreadTest(int x, int[] y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public void run() {
            while (x< 10) {
                ++x;
                System.out.print("0");
            }
            while (y[0] < 10) {
                ++y[0];
                System.out.print('1');
            }
        }
    }

    public static void main(String args[]) {
        int x = 0;
        int[] y = new int[1];
        y[0] = 0;

        Thread A = new ThreadTest(x, y);
        Thread B = new ThreadTest(x, y);

        B.start();
        A.start();
    }

there is clearly a race condition. how can i evaluate the maximum and minimum number of 1's that will be printed? what may cause change in the number of 1 printed?

Firstly the `y` array does not make much sense to me, can't it be just an `int`? Secondly, the code is not thread safe, so its behavior is undefined, I mean the counts of 0's and 1's are random. All those instance variables used in the `run` must be synchronized in some way. — neevek, May 19 '12 at 17:54
thats because we want to create race condition. that way both threads work on the same array. I'm interested in the limits of 1's — baby boom, May 19 '12 at 17:56
Oh, I just read the code in `main` method, and your code has no race condition, you just created 2 separated threads, which don't share the same `Runnable` object. And let's hypothesize your code has race condition, there will be at least **10** 1's and at most **20** 1's. — neevek, May 19 '12 at 18:14
You posted this question 8 hours ago: http://stackoverflow.com/questions/10664346/java-threads-effect-on-static-classes — Tudor, May 19 '12 at 18:35
Exact duplicate. Why ask the same question twice? Voted to close. — Jivings, May 19 '12 at 18:49
NOT THE SAME 2 lines missing! it's a completely different situation — baby boom, May 19 '12 at 19:12

score 0 · Answer 1 · answered May 19 '12 at 17:56

0

The max number is 10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 +2 + 1; lets say that one thread performed 10 times write 1. than the other thread with value y[0] == 0 perform 1 time. the first thread can again run the loop up to 10 from the place where the second thread stop.

answered May 19 '12 at 17:56

Horonchik

477
2
11

notice that they work on the same array, so when the 2nd thread starts y[0] in not necessarily ==0. – baby boom May 19 '12 at 18:00
but it could be. You are asking about the max – Esben Skov Pedersen May 19 '12 at 18:01
if the first thread writes 1 10 times, the value of y[0] should be 10 , no? – baby boom May 19 '12 at 18:07
Yes but then the second thread kicks in and set it to 1. It first read the value of 0, then the other thread write "1" 10 times. Only than it set set the value to y[0] = 0 + 1 – Horonchik May 20 '12 at 05:38

score 0 · Answer 2 · answered May 19 '12 at 18:23

First, see my comments on the question.

    public void run() {
        x = 0;

        y[0] = 0;             // line a
        while (x< 10) {      
            ++x;
            System.out.print("0");
        }

        while (y[0] < 10) {   // line b
            ++y[0];
            System.out.print('1');
        }
    }

If thread A and thread B reach line b and line a respectively at the same moment, there will be 10 1's printed. If thread A is running its loop and y[0] is just incremented to 10 but is not yet tested for truth/falsehood, and at this moment, thread B reaches line a, and it changes y[0] to 0, then thread A gains 10 more chances to print "1", so the count is 20.

score 0 · Answer 3 · answered May 19 '12 at 18:44

The min amount of 1s is obviously 10, the max amount will be 20.

20 because worst case will be that both threads reach

while (y[0] < 10)

at the same time every time, and then again reach

++y[0];

also at the same time every time, which will render one of the increments lost.

java threads effect and race condition

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