How to Customize the Ordering of Elements in a PriorityQueue Using a Comparator in Java?

A PriorityQueue is a data structure that allows elements to be processed based on their priority. By default, it orders elements according to their natural ordering (e.g., numeric values in ascending order). However, sometimes we need a different sorting order based on specific criteria.

In this article, we will learn to Customize the Ordering of Elements in a PriorityQueue Using a Comparator in Java.

Prerequisites:

• Java Collections Framework
• Comparator vs Comparable.

Customize the Ordering of Elements in a PriorityQueue

Using a Custom Comparator to customize the ordering of elements in a PriorityQueue, follow these steps:

• When constructing a PriorityQueue, provide a custom comparator a define the desired order.
• The constructor PriorityQueue (int capacity, Comparator<E> comparator) allows you to create a PriorityQueue with a specified initial capacity and a custom comparator.
• The comparator determines how elements are compared and ordered.

Program to Customize the Ordering of Elements in a PriorityQueue

Let’s create a PriorityQueue of students based on their CGPA (grade point average). We’ll define a custom comparator (StudentComparator) that compares students’ CGPA values

Below is the implementation of Customize the Ordering of Elements in a PriorityQueue:

Java

// Java Program to Customize the Ordering
// of Elements in a PriorityQueue
import java.util.*;

// Class representing a priority queue of students
public class GFG {
    // Main method
    public static void main(String[] args) {
        // Create a scanner object for input
        Scanner in = new Scanner(System.in);

        // Create a priority queue of students with custom comparator
        PriorityQueue<Student> pq = new PriorityQueue<>(5, new StudentComparator());

        // Create student objects and add them to the priority queue
        Student student1 = new Student("Nandini", 3.2);
        pq.add(student1);

        Student student2 = new Student("Anmol", 3.6);
        pq.add(student2);

        Student student3 = new Student("Palak", 4.0);
        pq.add(student3);

        // Print the students served in their priority order
        System.out.println("Students served in their priority order:");
        while (!pq.isEmpty()) {
            System.out.println(pq.poll().getName());
        }
    }
}

// Comparator class for comparing students based on their CGPA
class StudentComparator implements Comparator<Student> {
    // Compare method to compare two students
    public int compare(Student s1, Student s2) {
        if (s1.cgpa < s2.cgpa) return 1;
        else if (s1.cgpa > s2.cgpa) return -1;
        return 0;
    }
}

// Class representing a student
class Student {
    // Member variables
    public String name;
    public double cgpa;

    // Constructor
    public Student(String name, double cgpa) {
        this.name = name;
        this.cgpa = cgpa;
    }

    // Method to get the name of the student
    public String getName() {
        return name;
    }
}

Output

Students served in their priority order:
Palak
Anmol
Nandini

Explanation of the above Program:

• In the example above, we create a priorityQueue of Students.
• The custom comparator (StudentComparator) orders students based on their CGPA.
• Students with higher CGPA are served first, demonstrating customized ordering.

Let’s explore another approach for customizing the ordering of elements in a PriorityQueue using a Comparator in java.

Ordering Strings by Length

Suppose we want to create a PriorityQueue that orders strings based on their length (number of characters). we’ll define a custom comparator to achieve this.Create a new Java file (e.g., StringLengthPriorityQueue.java)

Below is the implementation of Ordering Strings by Length:

Java

// Java Program to Customize the Ordering
// of Elements in a PriorityQueue
import java.util.*;

// Class representing a priority queue based on string length
public class StringLengthPriorityQueue {
    // Main method
    public static void main(String[] args) {
        // Create a priority queue of strings with custom comparator
        PriorityQueue<String> pq = new PriorityQueue<>(5, new StringLengthComparator());

        // Add strings to the priority queue
        pq.add("apple");
        pq.add("banana");
        pq.add("grape");
        pq.add("kiwi");

        // Print the strings served in order of length
        System.out.println("Strings served in order of length:");
        while (!pq.isEmpty()) {
            System.out.println(pq.poll());
        }
    }
}

// Comparator class for comparing strings based on their length
class StringLengthComparator implements Comparator<String> {
    // Compare method to compare two strings based on their length
    public int compare(String s1, String s2) {
        return Integer.compare(s1.length(), s2.length());
    }
}

Output

Strings served in order of length:
kiwi
apple
grape
banana

Explanation of the above Program:

• In this example, we create a PriorityQueue of Strings.
• The custom comparator ( StringLengthComparator ) compares strings based on their length.
• Strings with shorter lengths are served first.