Multithreading: Thread.join waits for a worker to finish

join() blocks the calling thread until the target thread has terminated, so any state the worker wrote is safely visible afterwards. Without it you could read the result before the worker ever set it.

Code

int[] result = new int[1];
Thread worker = new Thread(() -> result[0] = 6 * 7);
worker.start();
worker.join();
System.out.println("Answer: " + result[0]);

Output

Answer: 42