Java : Priority Queue

Question

I have a java program which goes like this

public class PriorityQueueExample {

public static void main(String[] args) {
    PriorityQueue<Integer> pq = new PriorityQueue<Integer>();
    pq.add(10);
    pq.add(1);
    pq.add(9);
    pq.add(2);
    pq.add(8);
    pq.add(3);
    pq.add(7);
    pq.add(4);
    pq.add(6);
    pq.add(5);
System.out.println(pq);

}

}

My Question is why does not the priority queue sort them. As per the java specs it implements comparable and maintains the sorting order(natural sorting)

My output of the program is as follows : [1, 2, 3, 4, 5, 9, 7, 10, 6, 8]

Konrad Garus · Accepted Answer · 2011-10-28 09:11:03Z

7

It is sorted, but internally the elements are stored in a heap. If you call peek(), poll(), or remove(), you will get the right order (and that's how you access queues).

answered Oct 28, 2011 at 9:11

Konrad Garus

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Comments

Fred Foo · Accepted Answer · 2011-10-28 09:15:51Z

7

Insertion into a priority queue is not enough to sort a list of elements, since it doesn't store them in sorted order; it stores them in the partially sorted heap order. You have to remove the elements in a loop to sort them:

while (pq.size() > 0)
    System.out.println(pq.remove());

edited Oct 28, 2011 at 9:15

answered Oct 28, 2011 at 9:09

Fred Foo

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Comments

ZF007 · Accepted Answer · 2019-11-28 09:21:38Z

0

poll() and remove() will give sorted order not peek() as per java8.

 PriorityQueue<Integer> pq = new PriorityQueue<Integer>();
    pq.add(10);
    pq.add(1);
    pq.add(9);
    pq.add(2);
    pq.add(8);
    pq.add(3);
    pq.add(7);
    pq.add(4);
    pq.add(6);
    pq.add(5);
        // Remove items from the Priority Queue (DEQUEUE)
        while (!pq.isEmpty()) {
          //  System.out.println(pq.remove());
          System.out.println(pq.poll());
        }
Output for poll() & remove():
1 
2
3 
4 
5 
6 
7 
8 
9 
10
output for peek():
1
1
1
1
1
1
1
1
1
1

edited Nov 28, 2019 at 9:21

ZF007

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answered Nov 27, 2019 at 16:47

Anil

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1 Comment

ZF007 Over a year ago

code example is more needed here. Please add it in context. (End of Low_Quality Review).

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