PriorityQueue in Java

A PriorityQueue in Java is a queue where elements are ordered based on their priority, rather than the order of insertion. By default, it uses natural ordering (min-heap), but a custom comparator can be used to define different priorities.

Key feature

• The PriorityQueue is based on the Priority Heap.
• The elements of the priority queue are ordered according to the natural ordering and elements must implement Comparable or by a Comparator provided at queue construction time, depending on which constructor is used.  
• The size of the Priority Queue is dynamic, which means it will increase or decrease as per the requirement.

Example:

import java.util.PriorityQueue;

public class Geeks 
{
    public static void main(String[] args) 
    {
      	// Priority Queue Min Type
        PriorityQueue<Integer> p = new PriorityQueue<>();

        // Add elements to the queue
        p.add(3);
        p.add(10);
        p.add(7);
        p.add(2);

        // Print the head of the queue
        System.out.println("Head of Queue: " + p.peek());
    }
}

Head of Queue: 2

PriorityQueue Class Hierarchy in Java

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

Declaration of PriorityQueue

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

where, E is the type of elements held in this queue.

The class implements Serializable, Iterable<E>, Collection<E>, Queue<E> interfaces.

• PriorityQueue does not allow null values.
• We cannot create a 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.
• Since PriorityQueue is not thread-safe, Java provides PriorityBlockingQueue class that implements the BlockingQueue interface to use in a Java multithreading environment.
• The queue retrieval operations poll, remove, peek and element access the element at the head of the queue.
• It provides O(log(n)) time for add and poll methods.
• It inherits methods from AbstractQueue, AbstractCollection, Collection and Object class.

Constructors

1. PriorityQueue(): This method creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.

PriorityQueue<E> pq = new PriorityQueue<E>();

2. PriorityQueue(Collection<E> c): This creates a PriorityQueue containing the elements in the specified collection.

PriorityQueue<E> pq = new PriorityQueue<E>(Collection<E> c);

3. PriorityQueue(int initialCapacity): This creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.

PriorityQueue<E> pq = new PriorityQueue<E>(int initialCapacity);

4. PriorityQueue(int initialCapacity, Comparator<E> comparator): This creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.

PriorityQueue<E> pq = new PriorityQueue<E>(int initialCapacity, Comparator<E> comparator);

5. PriorityQueue(PriorityQueue<E> c): This creates a PriorityQueue containing the elements in the specified priority queue.

PriorityQueue<E> pq = new PriorityQueue<E>(PriorityQueue<E> c);

6. PriorityQueue(SortedSet<E> c): This creates a PriorityQueue containing the elements in the specified sorted set.

PriorityQueue<E> pq = new PriorityQueue<E>(SortedSet<E> c);

7. PriorityQueue(Comparator<E> comparator): This creates a PriorityQueue with the default initial capacity and whose elements are ordered according to the specified comparator.

PriorityQueue<E> pq = new PriorityQueue<E>(Comparator<E> c);

Different Operations on PriorityQueue

Let’s see how to perform a few frequently used operations on the Priority Queue class.

1. Adding Elements

To add an element in a priority queue, we can use the add() method. The insertion order is not retained in the PriorityQueue. The elements are stored based on the priority order which is ascending by default. 

Example:

import java.util.*;
  
public class Geeks
{  
    public static void main(String args[])
    {
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        
      	for(int i=0;i<3;i++){
            pq.add(i);
            pq.add(1);
        }
      
        System.out.println(pq);
    }
}

[0, 1, 1, 1, 2, 1]

Note: We will not get sorted elements by printing PriorityQueue. Below is an example for this

Example:

import java.util.*;
import java.io.*;
  
public class Geeks {
  
    public static void main(String args[])
    {
        PriorityQueue<String> pq = new PriorityQueue<>();
  
        pq.add("Geeks");
        pq.add("For");
        pq.add("Geeks");
  
        System.out.println(pq);
    }
}

[For, Geeks, Geeks]

2. Removing Elements

To remove an element from a priority queue, we can use the remove() method. If there are multiple such objects, then the first occurrence of the object is removed. Apart from that, the poll() method is also used to remove the head and return it.

import java.util.*;
import java.io.*;

public class Geeks {

    public static void main(String args[])
    {
        PriorityQueue<String> pq = new PriorityQueue<>();

        pq.add("Geeks");
        pq.add("For");
        pq.add("Geeks");

        System.out.println("Initial PriorityQueue " + pq);

        // using the method
        pq.remove("Geeks");

        System.out.println("After Remove: " + pq);

        System.out.println("Poll Method: " + pq.poll());

        System.out.println("Final PriorityQueue: " + pq);
    }
}

Initial PriorityQueue [For, Geeks, Geeks]
After Remove: [For, Geeks]
Poll Method: For
Final PriorityQueue: [Geeks]

3. Accessing the Elements

Queue follows the First In First Out principle, we can access only the head of the queue. To access elements from a priority queue, we can use the peek() method.

Example: 

import java.util.*;

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

        PriorityQueue<String> pq = new PriorityQueue<>();
        pq.add("Geeks");
        pq.add("For");
        pq.add("Geeks");
        System.out.println("PriorityQueue: " + pq);

        // Using the peek() method
        String element = pq.peek();
        System.out.println("Accessed Element: " + element);
    }
}

PriorityQueue: [For, Geeks, Geeks]
Accessed Element: For

4. Iterating the PriorityQueue

There are multiple ways to iterate through the PriorityQueue. The most famous way is converting the queue to the array and traversing using an iterator. Iterator does not traverse in priority order.

Example:

import java.util.*;

public class Geeks {
    public static void main(String args[])
    {
        PriorityQueue<String> pq = new PriorityQueue<>();

        pq.add("Geeks");
        pq.add("For");
        pq.add("Geeks");

        Iterator iterator = pq.iterator();

        while (iterator.hasNext()) {
            System.out.print(iterator.next() + " ");
        }
    }
}

For Geeks Geeks 

Demonstrating Common Operations on PriorityQueue

Example:

import java.util.PriorityQueue;

public class Geeks {

    public static void main(String[] args) {
        
        // Create a priority queue with initial capacity 10
        PriorityQueue<Integer> pq = new PriorityQueue<>(10);
        
        // Add elements to the queue
        pq.add(3);
        pq.add(1);
        pq.add(2);
        pq.add(5);
        pq.add(4);
        
        // Print the queue
        System.out.println("Priority queue: " + pq);
        
        // Peek at the top element of the queue
        System.out.println("Peek: " + pq.peek());
        
        // Remove the top element of the queue
        pq.poll();
        
        // Print the queue again
        System.out.println("Priority queue after removing top element: " + pq);
        
        // Check if the queue contains a specific element
        System.out.println("Does the queue contain 3? " + pq.contains(3));
        
        // Get the size of the queue
        System.out.println("Size of queue: " + pq.size());
        
        // Remove all elements from the queue
        pq.clear();
        
        // Check if the queue is empty
        System.out.println("Is the queue empty? " + pq.isEmpty());
    }
}

Priority queue: [1, 3, 2, 5, 4]
Peek: 1
Priority queue after removing top element: [2, 3, 4, 5]
Does the queue contain 3? true
Size of queue: 4
Is the queue empty? true

Methods in PriorityQueue Class

Methods Declared in Interface java.util.Queue

Methods Declared in Class java.util.AbstractQueue

Methods Declared in Class java.util.AbstractCollection

Methods Declared in Interface java.util.Collection

Applications of PriorityQueue

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

PriorityQueue in Java

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