Can a separate thread change static variable?

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.

Your code, in the current state, has a concurrency issue as two Threads try to access the static int variable at the same time (without synchronization):

• Main Thread tries to read the value of the variable
• t1 Thread tries to write the value of the variable

The Syso outputs 0, because the run method of the t1 thread hasn't started yet (no increments were done). But this is not 100% guaranteed to happen all the time.

If you want to be sure that the t1 thread finishes its execution before doing the Syso in main, you should use the join method:

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(); try { t1.join(); // Wait for thread to finish } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(parallelCounter.j); } } 

When you call System.out.println(parallelCounter.j);, your parallelCounter.inc(); function in your thread still hasn't ended. So j's value is still unincremented, at 0. Replace your for loop by a simple assignment, and add a Thread.sleep(1000); after starting your Thread t1, and you'll get the desired behaviour.

I did not run your code, but I assume that the method call System.out.println(parallelCounter.j); occured to soon within the program's execution flow.

You started the thread and then your code prints the value of j immediately. I think that the thread did not have enough time to start the run() method.

I suggest that you insert some waiting period before you print j, e.g.

Thread.currentThread().sleep(3000); // 3 seconds wait