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

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 q = new PriorityQueue(3, new Comparator() {
      @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;
  }

}