Java Comparator Interface

Java comparator interface is used in a situation when we have to sort an object which does not implement Comparable or do sorting in a different way than the Comparable.

Comparator is a functional interface since it has only one abstract method compare(). It takes two augments and resides in java.util package.

Example of comparator:

import java.util.*;
public class Country implements Comparable<Country> {
private String name;
private  int polulation;
public Country(String name, int pop) {
this.name = name;
this.polulation = pop;
}
public int getPopulation() {
return polulation;
}
public String toString() {
return name;
}
public int compareTo(Country d) {
return name.compareTo(d.name);
}
static Comparator<Country> byPopulation = new Comparator<Country>() {
public int compare(Country c1, Country c2) {
return c1.getPopulation()- c2.getPopulation();
}};
public static void main(String[] args) {
List<Country> c = new ArrayList<>();
c.add(new Country("INDIA",1000));
c.add(new Country("CHINA",20000));
Collections.sort(c);// sort by name
System.out.println(c);// [CHINA, INDIA]
Collections.sort(c,byPopulation);
System.out.println(c);
}
}

Output:

[CHINA, INDIA]  // Comparable interface
[INDIA, CHINA]  // Comparator interface