Can a separate thread change static variable?

Question

After surfing the web, I am still confused about the following thread behavior. I am aware that static variables are shared within the same classloader, however there's sth definitely missing in this extract:

public class parallelCounter {
    public static final int N = 100000000;
    public static int j = 0;
    public static void inc() {
        for (int i = 0; i < N; i++) {
            j++;
        }
        System.out.println(j); // 10000000
    }
}

class parallelCounterDemo {
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                parallelCounter.inc();
            }
        });
        t1.start();
        System.out.println(parallelCounter.j); // 0 Why?
    }
}

Answer 1

There are two things to note here:

1. Your code has a race-condition in that the state when printing depends on the execution speed of the two independent threads. Most times t1 will not even have started executing inc when your println executes. You might try adding a sleep(100) or something after t1.start.

2. You have to take care that not all changes to variables made by one thread will be instantly visible to other threads - this is a pretty complex topic where you have to check which constucts will result in data-synchronization between threads. For you example the easiest way could be to declare j as public static volatile int.