Does a PriorityQueue allow elements already within the queue to be reordered?

Question

I want to augment or lower the priority of items in a PriorityQueue: for example, I might be downloading a long list of images and suddenly want the thirtieth one t

Answer 1

None of the collections in Java automatically reorders elements if you change the property that determine their order. For collections that depend on the .hashCode() , .equals() or some comparator, you are not allowed to change the object while it resides in the collection so that the hashcode/equals or comparator would yield different values.

You have to remove, change, re-insert the object if you want to change its priority within a PriorityQueue.

Answer 2

If you iterate over a priority queue, you will find it is in no particular order (except the first element) If you want to change the order, I suggest you create another priority queue.

If you wan to change the position of one entry, I suggest you remove it, change its fields as required and add it again.

Answer 3

If you look at the source code, every time you poll() on a PriorityQueue it resifts, but it always returns the item that was at the top before the sift.

public class PQ {

  int priority;

  public PQ(int priority) {
    this.priority = priority;
  }

  public static void main(String[] args) {

    PQ one = new PQ(1);
    PQ two = new PQ(2);
    PQ three = new PQ(3);
    PQ four = new PQ(4);
    PQ five = new PQ(5);

    PriorityQueue<PQ> q = new PriorityQueue<PQ>(3, new Comparator<PQ>() {
      @Override
      public int compare(PQ o1, PQ o2) {
        return o1.priority-o2.priority;
      }
    });

    q.add(three);
    q.add(one);
    q.add(four);
    q.add(two);
    q.add(five);

    //Prints;
    //PQ-1
    //PQ-2
    //PQ-3
    //PQ-4
    //PQ-5
    while (!q.isEmpty()) {
      System.out.println(q.poll());
    }

    q.add(three);
    q.add(one);
    q.add(four);
    q.add(two);
    q.add(five);

    //Change the priority after it has been queued
    four.priority = 10;

    //Prints;
    //PQ-1
    //PQ-2
    //PQ-3
    //PQ-5
    //PQ-10
    while (!q.isEmpty()) {
      System.out.println(q.poll());
    }

    //Reset the priority
    four.priority = 4;

    q.add(three);
    q.add(one);
    q.add(four);
    q.add(two);
    q.add(five);

    //Change the priority after it has been queued
    four.priority = 0;

    //Prints;
    //PQ-1
    //PQ-0
    //PQ-2
    //PQ-3
    //PQ-5
    while (!q.isEmpty()) {
      System.out.println(q.poll());
    }
  }

  public String toString() {
    return "PQ-" + priority;
  }

}