优先队列轮询

Question

Looks like PriorityQueue is a LIFO structure (if all the elements in it have same priority), am I right?

 public E poll() {
    if (size == 0)
        return null;
    modCount++;

    E result = (E) queue[1];
    queue[1] = queue[size];
    queue[size--] = null;  // Drop extra ref to prevent memory leak
    if (size > 1)
        fixDown(1);

    return result;
}

Answer 1

The statement in the Javadoc that 'ties are broken arbitrarily' means the answer to your question is 'no'.

Answer 2

From the documentation:

An unbounded priority queue based on a priority heap. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used. A priority queue does not permit null elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result in ClassCastException).

The head of this queue is the least element with respect to the specified ordering. If multiple elements are tied for least value, the head is one of those elements -- ties are broken arbitrarily. The queue retrieval operations poll, remove, peek, and element access the element at the head of the queue.

http://docs.oracle.com/javase/6/docs/api/java/util/PriorityQueue.html

Basically, there's always a "priority", by default, it's the natural ordering of elements.