PriorityQueue in Java - GeeksforGeeks

A PriorityQueue is used when the objects are supposed to be processed based on the priority. It is known that a queue follows First-In-First-Out algorithm, but sometimes the elements of the queue are needed to be processed according to the priority, that’s when the PriorityQueue comes into play. The PriorityQueue is based on the priority heap. The elements of the priority queue are ordered according to the natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used.

In the below priority queue, element with maximum ASCII value will have the highest priority.

Few important points on Priority Queue are as follows:

PriorityQueue doesn’t permit NULL pointers.
We can’t create PriorityQueue of Objects that are non-comparable
PriorityQueue are unbound queues.
The head of this queue is the least element with respect to the specified ordering. If multiple elements are tied for least value, the head is one of those elements — ties are broken arbitrarily.
The queue retrieval operations poll, remove, peek, and element access the element at the head of the queue.
It inherits methods from AbstractQueue, AbstractCollection, Collection and Object class.

Constructors of PriorityQueue class

PriorityQueue(): Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
PriorityQueue(Collection<E> 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<E> comparator): Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
PriorityQueue(PriorityQueue<E> c): Creates a PriorityQueue containing the elements in the specified priority queue.
PriorityQueue(SortedSet<E> c): Creates a PriorityQueue containing the elements in the specified sorted set.

Below program illustrates the basic operations of PriorityQueue:

// Java program to demonstrate working of priority queue in Java 
import java.util.*; 
  
class Example 
{ 
    public static void main(String args[]) 
    { 
        // Creating empty priority queue 
        PriorityQueue<String> pQueue = 
                          new PriorityQueue<String>(); 
  
        // Adding items to the pQueue using add() 
        pQueue.add("C"); 
        pQueue.add("C++"); 
        pQueue.add("Java"); 
        pQueue.add("Python"); 
  
        // Printing the most priority element 
        System.out.println("Head value using peek function:"
                                           + pQueue.peek()); 
  
        // Printing all elements 
        System.out.println("The queue elements:"); 
        Iterator itr = pQueue.iterator(); 
        while (itr.hasNext()) 
            System.out.println(itr.next()); 
  
        // Removing the top priority element (or head) and 
        // printing the modified pQueue using poll() 
        pQueue.poll(); 
        System.out.println("After removing an element" + 
                           "with poll function:"); 
        Iterator<String> itr2 = pQueue.iterator(); 
        while (itr2.hasNext()) 
            System.out.println(itr2.next()); 
  
        // Removing Java using remove() 
        pQueue.remove("Java"); 
        System.out.println("after removing Java with" + 
                           " remove function:"); 
        Iterator<String> itr3 = pQueue.iterator(); 
        while (itr3.hasNext()) 
            System.out.println(itr3.next()); 
  
        // Check if an element is present using contains() 
        boolean b = pQueue.contains("C"); 
        System.out.println ( "Priority queue contains C " + 
                             "or not?: " + b); 
  
        // Getting objects from the queue using toArray() 
        // in an array and print the array  
        Object[] arr = pQueue.toArray(); 
        System.out.println ( "Value in array: "); 
        for (int i = 0; i<arr.length; i++) 
          System.out.println ( "Value: " + arr[i].toString()) ; 
    } 
} 

Output:

Head value using peek function:C
The queue elements:
C
C++
Java
Python
After removing an elementwith poll function:
C++
Python
Java
after removing Java with remove function:
C++
Python
Priority queue contains C or not?: false
Value in array: 
Value: C++
Value: Python

Methods in PriorityQueue class:

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.

Applications:

Implementing Dijkstra’s and Prim’s algorithms.
Maximize array sum after K negations

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This article is contributed by Mehak Kumar. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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