Java PriorityQueue

In this tutorial, we will learn about the PriorityQueue, its contructors and methods and how to implement it in Java.

Java Collection frameworks provide PriorityQueue class and it implements Queue Interface.It provides the implementation of heap data structure.

Unlink Queues, the elements are retrieved in a sorted order in PriorityQueue.

Constructors of the PriorityQueue

Constructors	Description
PriorityQueue():	Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
PriorityQueue(Collection c):	Creates a PriorityQueue containing the elements in the specified collection.
PriorityQueue(int initialCapacity):	Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.
PriorityQueue(int initialCapacity, Comparator comparator):	Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
PriorityQueue(PriorityQueue c):	Creates a PriorityQueue containing the elements in the specified priority queue.
PriorityQueue(SortedSet c):	Creates a PriorityQueue containing the elements in the specified sorted set.

Methods of PriorityQueue

Apart from the methods inherited from its parent class/interface ,it has following methods :

Methods	Description
boolean add(E element):	This method inserts the specified element into this priority queue.
public remove():	This method removes a single instance of the specified element from this queue, if it is present.
public poll():	This method retrieves and removes the head of this queue, or returns null if this queue is empty.
public peek():	This method retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Iterator iterator():	Returns an iterator over the elements in this queue.
boolean contains(Object o):	This method returns true if this queue contains the specified element.
void clear():	This method is used to remove all of the contents of the priority queue.
boolean offer(E e):	This method is used to insert a specific element into the priority queue.
int size():	The method is used to return the number of elements present in the set.
toArray():	This method is used to return an array containing all of the elements in this queue.
Comparator comparator():	The method is used to return the comparator that can be used to order the elements of the queue.

Example


import java.util.PriorityQueue;
import java.util.Iterator;

class Main 
{
    	public static void main(String[] args) 
	{

        	// Creating a priority queue
        	PriorityQueue integers = new PriorityQueue<>();

        	// Using the add() method
        	integers.add(4);
        	integers.add(2);
        	integers.add(16);
        	integers.add(11);
        	integers.add(28);
        	System.out.println("PriorityQueue: " + integers);

        	// Using the offer() method
        	integers.offer(1);
        	System.out.println("Updated PriorityQueue: " + integers);

		// Using the peek() method
        	int number1 = integers.peek();
        	System.out.println("Accessed Element: " + number1);

		// Using the remove() method
        	boolean result = integers.remove(2);
        	System.out.println("Is the element 2 removed? " + result);

        	// Using the poll() method
        	int number2 = integers.poll();
        	System.out.println("Removed Element Using poll(): " + number2);

		//Using the iterator() method
        	Iterator iterate = integers.iterator();
        	while(iterate.hasNext()) 
		{
            		System.out.print(iterate.next()+" ");
    		}
	}
}

Output:

PriorityQueue: [2, 4, 16, 11, 28]                                                                                              
Updated PriorityQueue: [1, 4, 2, 11, 28, 16]                                                                                   
Accessed Element: 1                                                                                                            
Is the element 2 removed? true                                                                                                 
Removed Element Using poll(): 1                                                                                                
4 11 16 28