Happens-before rules in Java Memory Model

Question

I am currently studying for a concurrent programming exam and don\'t understand why the output of this program is 43. Why is x = y + 1 executed before t.s

Answer 1

I would like to add that the code in the main method runs in the Main thread and Thread t doesn't block execution of Main in your example. That's why the line x = y + 1 might be executed faster than the body of the Thread t (as @davidxxx already pointed out).

You can observe different behavior if you add t.join() after t.start():

...
t.start();
t.join();

In this situation, the Main thread will wait for the Thread t to complete and the output will be 1.