The order of the PriorityQueue is wrong

Question

I meet a problem about order of PriorityQueue in Java 8, Intellij Idea, when I add the third number in the queue, the order is wrong, but only the third one have this problem, here is my code.

import java.util.*;

public class vector {
    static Queue<Integer> q=new PriorityQueue<Integer>();
    public static void addNum(int num) {
        q.add(num);
    }
    public static void main(String args[]) {
        addNum(-1);
        addNum(-2);
        addNum(-3);
        addNum(-4);
        addNum(-5);
    }
}

I try to debug the code, after addNum(-3), the queue is -3,-1,-2, but after addNum(-4), the queue is -4, -3, -2, -1.

Answer 1

The contract for PriorityQueue does not guarantee iteration order, but rather it guarantees that each element removed from the priority queue will follow either the natural ordering of the queue's type, or the ordering of a custom comparator, should one be provided.

The Javadoc comments on what you are seeing:

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out).

The contract which Java appears to enforce for priority queues is that the first element removed from the queue will follow the natural order of the object in the queue, or using a custom comparator, should one be provided.

If we add to your current script to remove the five elements added, we will see that the items returned will be ordered from least to greatest:

public static void main(String args[]) {
    addNum(-1);
    addNum(-2);
    addNum(-3);
    addNum(-4);
    addNum(-5);

    // now remove all elements
    while (!q.isEmpty()) {
        System.out.println(q.remove());
    }
}

This prints:

-5
-4
-3
-2
-1

If you need a collection which maintains sorting order, then consider using something like TreeSet. Or, you could use a regular ArrayList, and then call Collections.sort() if you want to impose a certain order.