How to customize PriorityQueue.stream().foreach to iterate in priority order

Question

I have a class with PriorityQueue field inside:

public class MyClass<T>{
    Queue<T> queue = new PriorityQueue<>();

I want somehow get a stream from MyClass and use foreach and want the sequence behave in priority order of my PriorityQueue. The easiest way is to override stream() method:

@Override
public Stream stream() {
    return queue.stream();
}

but this will not expose the queue element in priority order. So the question is: how to make foreach stream method behave like:

    while(!queue.isEmpty()) 
        queue.poll();

Answer 1

You could use Stream::generate and Queue::poll method to get create a Stream with elements from PriorityQueue with keeping their order:

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll);
}

However this might be dangerous because Stream::generate will be invoking poll constantly so it is potentially an inifinite Stream. Therfore using Stream::limit with the queue size should be considered :

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll)
        .limit(queue.size());
}

Or you could simply return sorted stream :

@Override
public Stream<T> stream() {
    return queue.stream()
            .sorted(comparator);
}

where comparator is your comparator.

In Java 9 you could use Stream::takeWhile with predicate that rejects nulls. As Queue::poll will return null when queue is empty - the resulting Stream will contain elements from the queue in their order (this is alternative to using limit as described in the first solution) :

@Override
public Stream<T> stream() {
    return Stream.generate(queue::poll)
            .takeWhile(Objects::nonNull);
}