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 prior

Answer 1

Very simple implementation based on multiple lists and TreeMap I've done today to solve some task:

import javax.annotation.Nonnull;
import java.util.*;
import java.util.Map.Entry;
import java.util.function.Function;

public class PriorityFifo {

     protected TreeMap> storage = new TreeMap<>();

     public void push(E element, int priority) {
        storage.computeIfAbsent(priority, it -> new LinkedList<>()).addLast(element);
     }

     public Optional poll() {
        return doWithNextElement(LinkedList::pollFirst);
     }

     public Optional peek() {
        return doWithNextElement(LinkedList::getFirst);
     }

     protected Optional doWithNextElement(@Nonnull Function, E> f) {
         Entry> entry = storage.firstEntry();
         if (entry==null)
             return Optional.empty();
         LinkedList list = entry.getValue();
         E element = f.apply(list);
         if (list.isEmpty())
             storage.remove(entry.getKey());
         return Optional.of(Objects.requireNonNull(element));

     }

}

No comparator used for elements, but used internally by TreeMap for queues. My case is that I have only a few of priorities, but a lot of elements, so it should go faster than something using element comparison.