Behaviour of PriorityQueue using Java 6

Question

Hi,
I have a below code to test PriorityQueue ordering of elements using it's natural order.
Output of SOP1 is clear to me.
I was expecting
SOP2 as : [a, b, d]
SOP3 as : [a, b, c, d]
import java.util.*;
class PQTest {
public static void main(String... args) {
PriorityQueue<String> pq = new PriorityQueue<String>();
     pq.add("b");
     pq.add("d");
     System.out.println(pq); //SOP1 output [b, d]
     pq.add("a");
     System.out.println(pq); //SOP2 output [a, d, b]
     pq.add("c");
     System.out.println(pq); //SOP3 output [a, c, b, d]
Could somebody shed the light?
Appreciate your help.

Answer 1

The toString() method of a PriorityQueue in no way reflect the order of the elements in the queue. Since the PriorityQueue is based on a 'heap' ( see http://download.oracle.com/javase/6/docs/api/java/util/PriorityQueue.html and http://www.cs.auckland.ac.nz/software/AlgAnim/heaps.html ) the order you see in the toString() is probably the physical order of the elements in the array used to back the 'heap' which is not the logical queue order.