How should I iterate a priority queue properly?

Question

I have a java assignment involving iterating a priority queue. The queue consists of objects with a string and an int in them and i need to have a way to check a seperate object's string against all the objects in the queue.

Would it be best way to do this be an iterator object? That seems too messy. I could dequeue and enqueue but that seems inefficient. Maybe a foreach loop?

Solution 1

Yes, if you need to check every single element in the collection, an iterator or for each is probably best.

Iterator<E> iter = myPriorityQueue.iterator();
while (iter.hasNext()) {
    current = iter.next();
    // do something with current
}

Or

for (Element e : myQueue) {
        // do something with e
}

Solution 2

One small detail: if there's any chance your queue could be modified during the loop, then both iterator and for each will cause a ConcurrentModificationException; if there's a chance the queue will get modified during the processing, you could use poll():

    Resource resource;
    while ((resource = resourceQueue.poll()) != null) {
        this.processIncludes(resourceQueue, resource);
    }

Solution 3

If you don't care about ordering (in that case - why are you dealing with PriorityQueue?), use Iterator. If you want to iterate by priority, then see the advice from Javadoc:

If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

Comments

[Thomas Jungblut]

Exactly, it is based on a heap which does prohibit complete ordering in itself. Thus you should call poll, because this will cause the heap to sift the min/max up to the root. But for his requirement, ordering doesn't matter, so he could iterate by a for each loop.

[Anon]

Im told I have to use a priority queue. It doesnt matter though what order I traverse it. I just need to check each objects string to make sure it doesnt match my seperate object's string