Question from Niko's blog on Collections.

[Alex Serna]

Hi everyone,

I don't understand why the answer to this question is c. What I understand from the code is that queue.toArray() returns an Object[] which isn't bounded to the collection. Therefore, it shouldn't affect the output and since PriorityQueue is sorted by natural order, in my opinion the otuput should be A A J V. Obviously I'm missing something...

public static void main(String[] args) {
    Queue<String> queue = new PriorityQueue<String>();
    queue.add("J");
    queue.add("A");
    queue.add("V");
    queue.add("A");
    Arrays.sort(queue.toArray());
    for (String element : queue) {
        System.out.format("%s ", element);
    }
}

   a. It prints “J A V A”.
   b. It prints “A A J V”.
   c The output order is not guaranteed.

[Ankit Garg]

Alex, as you might know, when you use the enhanced for loop on a Collection, then actually the for loop iterates over the collection using the iterator. So if you read the documentation of the iterate method of PriorityQueue class, then it says

Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.

[Alex Serna]

I didn't know that. I thought that when you used the iterator on a Collection that was sorted the iterator would follow the same order.
Thank you Ankit.