Promise.java

public class Promise<T> {

    private final AtomicReference<T> ref = new AtomicReference<T>();
    private final CountDownLatch latch = new CountDownLatch(1);

    public final void deliver(T object) {
        if (this.ref.compareAndSet(null, object)) {
            latch.countDown();
        }
    }

    public final T deref() throws InterruptedException {
            latch.await();
            return ref.get();
    }

    public final T deref(long timeout, TimeUnit unit) throws InterruptedException {
        latch.await(timeout, unit);
        return ref.get();
    }
}