Comparator interface

In this article, we will discuss Comparator interface with its methods in detail


Comparator interface:

  • Comparator interface is used to order the objects
  • This is part of original collection framework introduced in Java 1.2 version
  • Present in java.util package
  • Fully qualified name is: java.util.Comparator
  • Contains two methods namely: compare() and equals()
  • Method signature: public int compare(Object object1, Object object2);
  • We can sort according to our need, either natural ordering or reverse the natural ordering by coding our own logic for compare() method
  • This is mainly used to customize the sorting order [Customized Sorting Order]
  • Based on the integer value returned by compare(object2, object2) method, elements are stored/sorted
    1. returns negative integer, if object1 less than object2 (object1 < object2)
    2. returns positive integer, if object1 greater than object2 (object1 > object2)
    3. returns 0, if object1 is equal to object2 (object1 == object2)
  • We can customize Collection classes like TreeSet & TreeMap in natural or reverse order as per business requirement


Example on Comparator interface:

  • Customer POJO with 2 member variables of Integer and String type
  • 2-arg constructor
  • Overriding toString() method

public class Customer {

	// member variables
	int customerId;
	String customerName;

	// 2-arg parameterized constructor
	public Customer(int customerId, String customerName) {
		this.customerId = customerId;
		this.customerName = customerName;

	// override toString() method
	public String toString() {
		return "Customer [customerId=" + customerId + ", customerName="
				+ customerName + "]";

  • This is a separate class which implements Comparator interface providing customized sorting logic
  • compare() method provides reverse order sorting logic, according to customer Id

import java.util.Comparator;

public class CustomerIdComparator implements Comparator<Customer> {

	public int compare(Customer o1, Customer o2) {
		return o2.customerId - o1.customerId;


Main class

  • This class uses above customer POJO and customized sorting logic class implementing comparator interface, to store objects inside TreeSet
  • TreeSet stores customer objects in descending order of customer Id
  • prints customer objects in descending order of customer Id


import java.util.TreeSet;

public class CustomerReverseOrder {

	// main() method
	public static void main(String[] args) {

		// creating TreeSet object of type String
		TreeSet<Customer> ts = 
				new TreeSet<Customer>(new CustomerIdComparator());

		// adding elements to TreeSet object
		ts.add(new Customer(101, "Sundar Pichai"));
		ts.add(new Customer(107, "Satya Nadella"));
		ts.add(new Customer(103, "Shiv Nadar"));
		ts.add(new Customer(102, "Shantanu Narayen"));
		ts.add(new Customer(104, "Francisco D’Souza"));
		ts.add(new Customer(106, "Vishal Sikka"));
		ts.add(new Customer(105, "Chanda Kochhar"));

		System.out.println("Customized sorting on basis of CustomerId\n");

		// natural ordering of customer name
		for(Customer cust : ts){
		   System.out.println(cust.customerId + "  " + cust.customerName);


Customized sorting on basis of Customer Id

107  Satya Nadella
106  Vishal Sikka
105  Chanda Kochhar
104  Francisco D’Souza
103  Shiv Nadar
102  Shantanu Narayen
101  Sundar Pichai




Happy Coding !!
Happy Learning !!

Comparable v/s Comparator
Comparable interface