Java PriorityQueue Class Example

In Java, the PriorityQueue class is implemented as a priority heap. Heap is an important data structure in computer science. For a quick overview of heap, here is a very good tutorial.

1. Simple Example

The following examples shows the basic operations of PriorityQueue such as offer(), peek(), poll(), and size().

import java.util.Comparator;
import java.util.PriorityQueue;
 
public class PriorityQueueTest {
 
	static class PQsort implements Comparator<Integer> {
 
		public int compare(Integer one, Integer two) {
			return two - one;
		}
	}
 
	public static void main(String[] args) {
		int[] ia = { 1, 10, 5, 3, 4, 7, 6, 9, 8 };
		PriorityQueue<Integer> pq1 = new PriorityQueue<Integer>();
 
		// use offer() method to add elements to the PriorityQueue pq1
		for (int x : ia) {
			pq1.offer(x);
		}
 
		System.out.println("pq1: " + pq1);
 
		PQsort pqs = new PQsort();
		PriorityQueue<Integer> pq2 = new PriorityQueue<Integer>(10, pqs);
		// In this particular case, we can simply use Collections.reverseOrder()
		// instead of self-defined comparator
		for (int x : ia) {
			pq2.offer(x);
		}
 
		System.out.println("pq2: " + pq2);
 
		// print size
		System.out.println("size: " + pq2.size());
		// return highest priority element in the queue without removing it
		System.out.println("peek: " + pq2.peek());
		// print size
		System.out.println("size: " + pq2.size());
		// return highest priority element and removes it from the queue
		System.out.println("poll: " + pq2.poll());
		// print size
		System.out.println("size: " + pq2.size());
 
		System.out.print("pq2: " + pq2);
 
	}
}

Output:

pq1: [1, 3, 5, 8, 4, 7, 6, 10, 9]
pq2: [10, 9, 7, 8, 3, 5, 6, 1, 4]
size: 9
peek: 10
size: 9
poll: 10
size: 8
pq2: [9, 8, 7, 4, 3, 5, 6, 1]

2. Example of Solving Problems Using PriorityQueue

Merging k sorted list.

For more details about PriorityQueue, please go to doc.

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  • vikrant
  • Смотрящий

    It is a binary heap inside queue.

  • monotonique

    should the result be either ascending or descending?
    [1, 3, 5, 8, 4, 7, 6, 10, 9] -> [1, 3, 4, 5, 6, 7, 8, 9, 10]

  • darpan shah

    In compare method what if variable one or two has negative value? In such cases this compare method wont result correctly. Instead it should be:
    public int compare(Integer one, Integer two) {
    return two.compareTo(one);
    }

    It always recommends to use compareTo method on every object while comparing.

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  • ryanlr

    “Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.”

    (second-first) is equivalent to -(first-second). Consider the following node(ascending order), see if it helps.

    if (first == second)
    return 0;
    else if (first > second)
    return 1;
    else
    return -1;

  • omi

    public int compare(Integer one, Integer two) {
    return two – one;
    }
    how this is reversing??what is the logic behind?

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  • vikrant

    another link on tech forum: http://www.writeulearn.com/priority-queue/

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  • ryanlr

    From Java Doc –
    Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps), or to provide an ordering for collections of objects that don’t have a natural ordering.

  • deepu venkat

    Why do we use comparator?