Comparable and Comparator Interface in Java

Hello Friends,

In java, when we want to sort a collection, we use Class “Collections” and static method “sort()”
For example : List list = new ArraysList();
Collections.sort(list);
Now all the elements of the list will be sorted as per alphabetic order or numeric order.
But what if a list has objects then list items will be compared and which one will go first in list??
So , to sort the objects, we need to specify a rule based on that object should be sorted.
The “Comparator” and “Comparable” interface allow us to specify what rule is used to sort the objects.

Feature	Comparable	Comparator
Interface	java.lang.Comparable	java.util.Comparator
Method	compareTo(T o)	compare(T o1, T o2)
Natural Order	Defines natural ordering within the class	Defines custom ordering outside the class
Single/Multiple	Can only have one natural order	Can define multiple comparison methods
Usage	Used when you have one "natural" way to compare	Used for custom sorting logic or multiple comparisons

1. Comparable Interface

Purpose: Used to define a natural ordering of objects.
Method: Requires implementation of the method compareTo(Object o).
Usage: A class implements Comparable when its objects are supposed to be ordered in a natural way (e.g., alphabetical for strings, numerical for numbers).
Modification: This affects the class itself, meaning the comparison logic is part of the class definition.



2. Comparator Interface

Purpose: Used to define a custom or multiple orderings for objects.
Method: Requires implementation of the method compare(Object o1, Object o2).
Usage: Comparator is typically used when you want to define multiple ways of comparing objects or when you want to compare objects of a class that you do not control (e.g., a library class).
Modification: The comparison logic is external to the class being compared.

Comparable with example :

public class Vehicle implements Comparable<Vehicle>{
private int vehiclenumber;
private String vehiclename;
private int vehicleyear;
public Vehicle(int vehiclenumber, String vehiclename, int vehicleyear){
this.vehiclenumber = vehiclenumber;
this.vehiclename = vehiclename;
this.vehicleyear = vehicleyear;
}
public String toString(){
return "vehicle{"+
"vehiclenumber='" + vehiclenumber+ '\''+
", vehiclename='" + vehiclename+ '\''+
", vehicleyear=" + vehicleyear+
'}';
}

@Override
public int compareTo(@NotNull Vehicle o) {
return this.vehiclenumber - o.vehiclenumber;
}
}

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

public class comparatordemoa {
public static void main(String[] args){
List list = new ArrayList();
list.add(10);
list.add(20);
list.add(9);
list.add(15);
System.out.println(list);
Collections.sort(list);
System.out.println(list);
List list2 = new ArrayList();
list2.add(new vehical(2345, "ford", 2024));
list2.add(new vehical(3456, "kia", 2023));
list2.add(new vehical(5678, "suzuki", 2025));
System.out.println(list2);
Collections.sort(list2);
System.out.println(list2);
}
}

Comparator with example :

public class Vehicle2 {
public int getVehiclenumber() {
return vehiclenumber;
}
public void setVehiclenumber(int vehiclenumber) {
this.vehiclenumber = vehiclenumber;
}
public String getVehiclename() {
return vehiclename;
}
public void setVehiclename(String vehiclename) {
this.vehiclename = vehiclename;
}
public int getVehicleyear() {
return vehicleyear;
}

public void setVehicleyear(int vehicleyear) {
this.vehicleyear = vehicleyear;
}

private int vehiclenumber;
private String vehiclename;
private int vehicleyear;

public Vehicle2(int vehiclenumber, String vehiclename, int vehicleyear){
this.vehiclenumber = vehiclenumber;
this.vehiclename = vehiclename;
this.vehicleyear = vehicleyear;
}

public String toString(){
return "vehicle{"+
"vehiclenumber='" + vehiclenumber+ '\''+
", vehiclename='" + vehiclename+ '\''+
", vehicleyear=" + vehicleyear+
'}';
}
}

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class ComparableDemo {
public static void main(String[] args){
List list = new ArrayList();
list.add("Amit");
list.add("Baji");
list.add("Nishant");
list.add("Diraaj");
list.add("Chandan");
System.out.println(list);
Collections.sort(list);
System.out.println(list);
List list1 = new ArrayList();
list1.add(new Vehicle(10,"ford", 2022));
list1.add(new Vehicle(15,"Hundai", 2023));
list1.add(new Vehicle(5,"Honda", 2026));
System.out.println(list1);
Collections.sort(list1);
System.out.println(list1);
List list2 = new ArrayList();
list2.add(new Vehicle2(10,"City", 2022));
list2.add(new Vehicle2(15,"Baleno", 2023));
list2.add(new Vehicle2(5,"Accent", 2026));
System.out.println(list2);
Collections.sort(list2, new CompratorDemo());
System.out.println(list2);
}
}

public class CompratorDemo implements Comparator<Vehicle2> {

@Override
public int compare(Vehicle2 o1, Vehicle2 o2) {
return o1.getVehiclename().compareTo(o2.getVehiclename());
}
}

Example :

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Example of Comparable interface in java

Example of Comparator interface in java

Comparable vs comparator in java

Adavntage of Comparator over Comparable interface in java