迭代优先级队列引发ConcurrentModificationException

Question

PriorityQueue<Integer> queue = new PriorityQueue<Integer>(4);
queue.add(8);
queue.add(5);
queue.add(23);
queue.add(6);
for(int i:queue)
   System.out.println(queue.remove());

The preceding code throws ConcurrentModificationException. I don't see any threads of modifying the priority queue. I do understand it is unsynchronized collection as well. I am running Ubuntu 12.10 with Oracle Java SDK 6.0.
This problem looks lot interesting. Can somebody give an explanation? Is it a defect?

Answer 1

YOU SHOULD ONLY USE ITERATOR.remove() method .It is the only method that guarantee that you do not have a ConcurrentModificationException

There is only one thread.But when you iterate with for each you actually modify the queue by removing elements from it so you actually will eventually modify the queue at a different step then you want.

Answer 2

When you Iterate over any collection, you can only use Iterator.remove() to remove entries safely for that iterator (and no other) This is a know gotcha which has been around since Java 1.2 when Iterator was introduced.

Note: The Concurrent collections maintain "weak consistency" which allows you to iterate over the collection while it is being modified.