Java Program to Implement PriorityQueue API

Last Updated : 22 Jul, 2021

A PriorityQueue is a linear data structure in which the elements are ordered according to their natural ordering or by some custom comparator provided at the queue at construction time. In PriorityQueue, the front of the queue points to the least element, and the rear points to the greatest element according to the natural Ordering. For alphabetical PriorityQueue, their ASCII values will be considered for ordering.

Some important characteristics of PriorityQueue are as follows:

It does not allow the null elements to be inserted.
It is an unbounded Queue that means its size can be expanded.
It inherits the classes like Object, Abstract Collection, AbstractQueue.
It is not thread-safe.
It cannot be created for non-comparable objects.

Time Complexities of various operations:

Insertion and deletion are f order O(log(n))

remove() and contains() method is of order O(n)

Retrieval operations are the fastest which are of order O(1)

The PriorityQueue class inherits Queue Interface and all of its methods. PriorityQueue API implements serializable, Iterable, Collection, and Queue which can be perceived from the architecture shown below.

Serializable, Iterable<E>, Collection<E>, Queue<E>

Syntax:

public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable

Parameter: E — The type of elements held in this queue.

Methods:

Method	Type	Description
add(E e)	boolean	Inserts an element e to the PriorityQueue
clear()	void	Removes all the elements from the PriorityQueue
contains(Object O)	boolean	Return true if it contains the specified element
iterator()	Iterator<E>	Returns an iterator over all the elements
remove(Object o)	boolean	Removes the specified element from the Queue
comparator()	Comparator<E>	Returns the custom comparator used to order th elements
toArray()	Object[]	Returns an array that contains all the elements in the PriorityQueue.
peek()	<E>	Return the head of the PriorityQueue without deleting the element from the Queue
poll()	<E>	Removes and returns the head of the queue. Returns null if the queue is empty.
spliterator()	Spliterator<E>	Creates a late-binding and fail-fast Spliterator over the elements in the PriorityQueue.

Implementation:

Example

Java

// Java Program to implement Priority Queue API
 
// Importing all classes from java.util package
import java.util.*;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating(Declaring) an object of PriorityQueue of
        // Integer type i.e Integer elements will be
        // inserted in above object
        PriorityQueue<Integer> pq = new PriorityQueue<>();
 
        // Adding elements to the object created above
        // Custom inputs
        pq.add(89);
        pq.add(67);
        pq.add(78);
        pq.add(12);
        pq.add(19);
 
        // Printing the head of the PriorityQueue
        // using peek() method of Queues
        System.out.println("PriorityQueue Head:"
                           + pq.peek());
 
        // Display message
        System.out.println("\nPriorityQueue contents:");
 
        // Defining the iterator to traverse over elements of
        // object
        Iterator i = pq.iterator();
 
        // Condition check using hasNext() method which hold
        // true till single element is remaining in List
        while (i.hasNext()) {
 
            // Printing the elements of object
            System.out.print(i.next() + " ");
        }
 
        // Removing random element from above elements added
        // from the PriorityQueue
        // Custom removal be element equals 12
        pq.remove(12);
 
        // Display message
        System.out.print("\nPriorityQueue contents:");
 
        // Declaring iterator to traverse over object
        // elements
        Iterator it = pq.iterator();
 
        // Condition check using hasNext() method which hold
        // true till single element is remaining in List
        while (it.hasNext()) {
 
            // Printing the elements
            System.out.print(it.next() + " ");
        }
 
        // Removing all the elements from the PriorityQueue
        // using clear() method
        pq.clear();
 
        // Adding another different set of elements
        // to the Queue object
        // Custom different inputs
        pq.add(5);
        pq.add(7);
        pq.add(2);
        pq.add(9);
 
        // Checking a random element just inserted
        // using contains() which returns boolean value
        System.out.print("The queue has 7 = "
                         + pq.contains(7));
 
        // Display message for content in Priority queue
        System.out.print("\nPriorityQueue contents:");
 
        // Converting PriorityQueue to array
        // using toArray() method
        Object[] arr = pq.toArray();
 
        // Iterating over the array elements
        for (int j = 0; j < arr.length; j++) {
 
            // Printing all the elements in the array
            System.out.print(arr[j] + " ");
        }
    }
}

Output

PriorityQueue Head:12

PriorityQueue contents:
12 19 78 89 67 
PriorityQueue contents:19 67 78 89 The queue has 7 = true
PriorityQueue contents:2 7 5 9

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