PriorityQueue in Java not deterministic?

问题

When I run the following priority queue test:

class Run {

public static void main(String[] args) {
    PriorityQueue<Entry> q = new PriorityQueue<>(8, Collections.reverseOrder(new Comparator<Entry>() {

        @Override
        public int compare(Entry o1, Entry o2) {
            return Integer.compare(o1.getValue(), o2.getValue());
        }
    }));

    q.offer(new Entry(100));
    q.offer(new Entry(0));
    q.offer(new Entry(1));
    q.offer(new Entry(-1));
    q.offer(new Entry(0));
    q.offer(new Entry(1));
    q.offer(new Entry(-100));
    q.offer(new Entry(100));

    while (q.peek() != null) {
        System.out.println(q.poll());
    }
}

private static class Entry {

    private static int GLOBAL_ID = 0;

    private final int value, id;

    public Entry(int value) {
        this.value = value;
        id = GLOBAL_ID++;
    }

    public int getValue() {
        return value;
    }

    @Override
    public String toString() {
        return "Entry[" + id + ", value = " + value + ']';
    }
}

}

I get the following result:

Entry[0, value = 100]
Entry[7, value = 100]
Entry[2, value = 1]
Entry[5, value = 1]
Entry[4, value = 0]
Entry[1, value = 0]
Entry[3, value = -1]
Entry[6, value = -100]

I expect equal elements to be output in the same order as they are being input, so when entry 0 is offered before entry 7, it is also polled before 7 is polled. But why is entry 4 suddenly polled before the 1? Is it the erroneous Comparator or does PriorityQueue not guarantee a deterministic behaviour?

回答1:

It is non-deterministic.

From the documentation for PriorityQueue

If multiple elements are tied for least value, the head is one of those elements -- ties are broken arbitrarily.

来源：https://stackoverflow.com/questions/27865331/priorityqueue-in-java-not-deterministic

标签

java

priority-queue