Strange ordering in PriorityQueue in Java [duplicate]

Question

This question already has answers here:

The built-in iterator for java's PriorityQueue does not traverse the data structure in any particular order. Why? (5 answers)

Closed 3 years ago.

I am trying to use priority queue to keep an ordered list of integers. in a simple exaple like this:

PriorityQueue<Integer> queue = new PriorityQueue<>();
queue.offer(3000);
queue.offer(1999);
queue.offer(999);
for(Integer i : queue)
    System.out.println(i);

This prints

999
3000
1999

This is not what I am expecting considering natural odering.

I simply want to iterate without removing or adding through the queue (which serves as a sorted list) WITH ordering. Can I still do that in a simple manner?

Answer 1

PriorityQueue is a collection optimized for finding the tail or head value quickly, using a partially ordered tree structure called heap (look it up on wikipedia). If you pop the elements, they will be ordered. If you want to iterate, use for example a SortedSet instead, which also stores the elements sorted.

Answer 2

This is a very sneaky problem with PriorityQueue: to quote the Api

The Iterator provided in method iterator() is not guaranteed to traverse the elements of the priority queue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

Use Poll instead to get the head which will be in order