我进行轮询时PriorityQueue如何工作？

Question

I have a predefined class named Pair with a key and a value. I store them in PriorityQueue based on the natural order of value of each Pair. When I change value of one of the Pair and then dequeue, what I expect did not happen. The test code is below. Please help. I feel confused!

import java.util.*;
public class Test {
  static class Pair {
      int key;
      int value;
      Pair (int key, int value) {
        this.key = key;
        this.value = value;
      }
  }
  public static void main(String[] args) {
      PriorityQueue<Pair> pq = new PriorityQueue<Pair>(3, new Comparator<Pair>() {
        @Override
        public int compare(Pair p1, Pair p2) {
            return p1.value - p2.value;
        }
    });
    Pair p1 = new Pair(1, 31);
    Pair p2 = new Pair(2, 32);
    Pair p3 = new Pair(3, 33);
    pq.offer(p1);
    pq.offer(p2);
    pq.offer(p3);
    p2.value = 31;
    p1.value = 32;
    Pair p0 = pq.poll(); // It shows the reference p0 is p1 not expected p2. 
                        // And what remain in pq are p2 with 31 and p3 with 33
  }
}

I know that PriorityQueue will sort the item when polling. It seems like the PriorityQueue in my example didn't work.

Answer 1

When you have any data structure such as a Hash Map/Set, TreeMap/Set or a sorted queue like PriorityQueue, if you mutate one of the elements (in particular a field used for hashCode/equals/compareTo depending on what methods(s) are used), the data structure doesn't know to re-arrange itself, instead what you are doing is corrupting the data structure so it won't work as expected.

In short, you can't mutate the key fields of an object in a data structure and expect it to behave in a reliable manner.