Doubt on PriorityQueue?

[Ashok Pradhan]

Can anyone explain me "PriorityQueue orders its elements using user-defined priority .The priority can be simple as natural ordering".

--Here what does it mean user-defined priority

--And when I run my program it produces:-
[1, 3, 6, 8, 4, 7]
AND
[1, 3, 6, 8, 4, 7] //with the use of comparator

So that I think this is not any kind of ordering.please explain me

import java.util.*;
 
class QueueTest
{
public static void main(String[] args)
{
Comparator<Integer> cp = new Comparator<Integer>()
{
public int compare(Integer int1,Integer int2)
{
return int1.compareTo(int2);
}
};
Queue<Integer> q = new PriorityQueue<Integer>();
//Queue<Integer> q = new PriorityQueue<Integer>(5,cp);

q.add(1);
q.add(3);
q.add(7);
q.add(8);
q.add(4);
q.add(6);
 
System.out.println(q);
}
}

[Seema Gaurav]

Interesting question Ashok, I thought this was strange too. I looked up the PriorityQueue API and this is what I found there:

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

Maybe that's the reason why printing 'q' or iterating through the queue doesn't display the order.

Having said that, I tried to poll the queue and it displays the correct order. Maybe polling is the way to go

Those were my 2 cents, any thoughts people?
Seema
[ October 07, 2008: Message edited by: Seema Gaurav ]

[Ankit Garg]

You are right Seema. When you use an PriorityQue object in System.out.println(), then the toString method of AbstractCollection class is executed.

The toString implementation in AbstractCollection class iterates through a collection and displays the output as [element1, element2,...]

But when you use poll() or peek(), then the elements are traversed in sorted order...