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Question

After seaching a lot and not finding a concrete answer

If I have two threads started:

Thread t1 = new Thread();
Thread t2 = new Thread();

t1.start();
t2.start();

After starting the threads I need the main thread to wait for these two thread to finish before printing the final result

How can I make the main thread wait for both t1 and t2?

A plain wait() would be enough?

Answer 1

Add

t1.join();
t2.join();

in your thread which should wait till t1 and t2 will finish their tasks (in your case call it from your main thread).

Answer 2

The answer depends...

You could

Use a CountDownLatch which is probably the simplest solution.

This way you would simply wait on the latch until it has been signaled the prescribed number of times (by each Thread terminating). This scales quite nicely as you increase the number of threads...

You could

Use join , but it would become tedious as you add more threads

You could

Add each Thread to a List , loop through the list, removing those threads that are no longer alive and keep looping until the List is empty, but that's a rather heavy handed approach.

You could

Combine the above solution with some kind of monitor lock which the loop would wait on and each Thread would notify when they complete, but it's not much cleaner and you could still end up waiting for non-existent threads...

You could

Use an ExecutorService and either use it's invokeAll and/or shutdown methods. See Executors for more details. This also scales quite nicely and even has the added benefit of allowing you to use a Thread pool to better manage the system resourcs

Answer 3

Check out the Thread#join method.

Also, you might find using an ExecutorService (and friends) helpful. Its essentially thread pool/management and provides a lot of conveniences and IMO a cleaner API than threads. Barrier to entry is low...

Answer 4

You want Thread#join() . wait() is for signalling, join() is to wait for the thread to finish.