PriorityQueue 只迭代一半的元素

Question

I have program which uses PriorityQueue. poll() is not giving all the values in the queue.

class Coffee {
    public static void main(String[] args) {
        PriorityQueue<Double> pq = new PriorityQueue<Double>();
        Random rand = new Random();
        for (int i = 0; i < 10; i++) {
            pq.offer(rand.nextDouble());
        }

        System.out.println(pq);
        System.out.print("size value " + pq.size());

        for (int i = 0; i < pq.size(); i++) {
            System.out.println(pq.poll());
        }
    }
}

Output:

[0.005756373546009885, 0.057563473207216886, 0.3415582636412481, 0.2026760924302
6186, 0.10792479235868724, 0.768845643547834, 0.5107848139799113, 0.758559713387
8311, 0.6437353209123445, 0.5156937257761389]
size value 10
0.005756373546009885
0.057563473207216886
0.10792479235868724
0.20267609243026186
0.3415582636412481

The size is 10, so why am I not getting all the 10 values with poll()?

Answer 1

for (int i = 0; i < pq.size(); i++) {
    System.out.println(pq.poll());
}

On each loop iteration you are removing an element by pq.poll() , which decrements pq.size() , and you are also incrementing i . So in the expression i < pq.size() the two values approach each other and meet in the middle, so it only loops half the times.

Instead do:

while (!pq.isEmpty()) {
    System.out.println(pq.poll());
}