Making a Java PriorityQueue into a stable priority queue

Question

I'm trying to implement a stable (first in first out) priority queue in Java. Supposing that the key is a name and the value is an age, I know I can make an unstable priority queue like this:

Queue<Map.Entry<String, Integer>> pq = new PriorityQueue<Map.Entry<String, Integer>>(100, ageComparator);

This does pretty much everything that I need it to, except that it doesn't maintain order of key-value pairs as I insert them (or remove them).

I've found a "work around" by making a LinkedList, which offers essentially all of the same functionality, except that it doesn't include a constructor with a comparator option, and I feel that it must be slower since I maintain value ordering by calling Collections.sort() after each queue operation.

So I guess that really there are two options that I'm interested in. First, how could I edit the PriorityQueue above to maintain insertion and removal order? Or second, how could I force my LinkedList option to use a comparator immediately rather than having to call a sort on each operation? Thanks!

EDIT:

Thanks for the good question in the first comment that was posted. By FIFO, I mean that for key-value pairs with equal values, the pair which is put in first should be extracted first.

Answer 1

You need something like this:

import java.util.AbstractMap;
import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.concurrent.atomic.AtomicInteger;

public class PriorityTest {
  @SuppressWarnings("serial")
  private static class Entry extends AbstractMap.SimpleEntry<String, Integer> {
    private final static AtomicInteger seq = new AtomicInteger(0);
    final int order;
    public Entry(final String _key, final Integer _value) {
      super(_key, _value);
      order = seq.incrementAndGet();
    }
  }

  private static class OrderedComparator implements Comparator<Entry> {
    @Override
    public int compare(final Entry _e1, final Entry _e2) {
      int r = _e1.getValue().compareTo(_e2.getValue());
      if (r == 0)
        return Integer.compare(_e1.order, _e2.order);
      return r;
    }
  }

  public static void main(String[] args) {
    final PriorityQueue<Entry> pq = new PriorityQueue<Entry>(10, new OrderedComparator());
    pq.add(new Entry("Jane", 22));
    pq.add(new Entry("John", 15));
    pq.add(new Entry("Bill", 45));
    pq.add(new Entry("Bob", 22));
    while(!pq.isEmpty()) {
      System.out.println(pq.remove());
    }
  }
}

Answer 2

Keap-based PriorityQueue is naturally stable. It's written in Kotlin, so it can replace java.util.PriorityQueue in Java code.