为什么我可以通过Iterator看到PriorityQueue元素，而不能通过poll看到？

Question

I have the following Java code, which creates a PriorityQueue and adds some elements to it:

public class Main {
    public static void main(String[] args) {
        PrintWriter out = new PrintWriter(System.out);
        PriorityQueue<Integer> q = new PriorityQueue<>();

        Integer[] integers = new Integer[10];
        Integer[] integers1 = new Integer[10];
        for (int i = 0; i < 10; i++) {
            integers[i] = i;
            integers1[i] = i + 10;
        }


        for (int i = 0; i < integers.length; i++) {
            q.add(integers[i]);
            q.add(integers1[i]);
        }

        out.println(q.size());

        for(int i = 0; i < integers.length; i++)
            out.println(q.poll());

        out.close();
    }    
}

In this code segment q.size() return the size of the queue, which is 20 . However, when I retrieve elements using the poll() function in the last loop, it only retrieves the first 10 elements. However, when I use an Iterator to access the elements, it retrieves all the elements from the queue. It seems like the poll() method always retrieves half of the elements in the queue.

What's going on here?

Answer 1

Your for loop has the wrong bounds here:

for(int i = 0; i < integers.length; i++)
    out.println(q.poll());

You've added integers.length + integers1.length elements to the priority queue, but you're only looping integers.length times. Try changing this to use a while loop:

while (!q.isEmpty())
    out.println(q.poll());

Hope this helps!

Answer 2

You add 20 elements:

                          10
                     _____________
                    /             \
for (int i = 0; i < integers.length; i++) {
    q.add(integers[i]);        \________________ x 2
    q.add(integers1[i]);       /
}

But you only retrieve 10 elements:

                          10
                     _____________
                    /             \
for(int i = 0; i < integers.length; i++)
    out.println(q.poll());    ------------  x 1

To drain the priority queue completely, do:

while (!q.isEmpty())
    out.println(q.poll());

Answer 3

因为integers.length等于10。这就是最后一个for语句的中断条件。