How to implement asynchronous queue?

Question

Given following variation of queue:

interface AsyncQueue<T> {
    //add new element to the queue
    void add(T elem); 
    //request single element from the queue via callback
    //callback will be called once for single polled element when it is available
    //so, to request multiple elements, poll() must be called multiple times with (possibly) different callbacks
    void poll(Consumer<T> callback);
}

I found out i do not know how to implement it using java.util.concurrent primitives! So questions are:

• What is the right way to implement it using java.util.concurrent package?
• Is it possible to do this w/o using additional thread pool?

Answer 1

Your AsyncQueue is very similar to a BlockingQueue such as ArrayBlockingQueue. The Future returned would simply delegate to the ArrayBlockingQueue methods. Future.get would call blockingQueue.poll for instance.

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As for your update, I'm assuming the thread that calls add should invoke the callback if there's one waiting? If so it's a simple task of creating one queue for elements, and one queue for callbacks.

• Upon add, check if there's a callback waiting, then call it, otherwise put the element on the element queue
• Upon poll, check if there's an element waiting, then call the callback with that element, otherwise put the callback on the callback queue

Code outline:

class AsyncQueue<E> {

    Queue<Consumer<E>> callbackQueue = new LinkedList<>();
    Queue<E> elementQueue = new LinkedList<>();

    public synchronized void add(E e) {
        if (callbackQueue.size() > 0)
            callbackQueue.remove().accept(e);
        else
            elementQueue.offer(e);
    }

    public synchronized void poll(Consumer<E> c) {
        if (elementQueue.size() > 0)
            c.accept(elementQueue.remove());
        else
            callbackQueue.offer(c);
    }
}