In java, how to wait on multiple `Conditions` until any one of them is signaled

Question

Suppose an elevator simulation program, visitors about to take a ride are to wait until any one of the elevator doors opens. i.e. I want to wait on multiple Conditions until any one of them is signaled.

Actually, it doesn't have to be Conditions, other approaches that can fulfill my need is welcome.

How can this be done in Java?

Answer 1

You might find CountDownLatch does the job you need. You would instantiate the latch with a count of 1:

CountDownLatch latch = new CountDownLatch(1);

and then share it between your threads. All the threads that wait for the doors to open will do latch.await(). This method will not return until another thread calls latch.countDown().

Answer 2

You might want to check out Observer and Observable. You will still have to handle treading issues but with Observer you at least have an easy way for the simulator to know when a door opens (triggers an event)

Answer 3

Rather than a set of conditions, I'd use a BlockingQueue<Door>, (Door is an enum of the doors in the lift) where threads which want to use a door call take() on the queue, and threads which are opening a door call put(Door.ONE). and then uses drainTo to remove any other open doors (presumably there's another mechanism to tell the door opening threads that the lift has left and that they can't open any more doors).