Java-Comparable and Comparator with example

Comparable Interface and Comparator Interface in java are very useful for sorting the collection of objects

The Comparable Interface in Java with Example
The Comparable interface has the compareTo(T obj) method which is used by sorting methods, you can check any Wrapper, String, or Date class to confirm this. We should override this method in such a way that it returns a negative integer, zero, or a positive integer if “this” object is less than, equal to, or greater than the object passed as an argument. Comparable is present in java.lang package. We can sort the list elements of Comparable type by Collections.sort(List) method.

Example:
Consider an Employee class that has members like, username,userid,   year of joining. Suppose we wish to sort a list of Employees based on the year of joining. We can implement the Comparable interface with the Employee class, and we override the method compareTo() of Comparable interface. 

Employee.java

//A Java program to demonstrate the use of Comparable

//A class 'Employee' that implements Comparable

class Employee implements Comparable<Employee> {

private String username;

private String userid;

private int yearofjoin;

/* Used to sort employees by year of join */

public int compareTo(Employee m) {

return this.yearofjoin - m.yearofjoin;

}

public String getUsername() {

return username;

}

public void setUsername(String username) {

this.username = username;

}

public String getUserid() {

return userid;

}

public void setUserid(String userid) {

this.userid = userid;

}

public int getYearofjoin() {

return yearofjoin;

}

public void setYearofjoin(int yearofjoin) {

this.yearofjoin = yearofjoin;

}

public Employee(String username, String userid, int yearofjoin) {

super();

this.username = username;

this.userid = userid;

this.yearofjoin = yearofjoin;

}

}

Main Class

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

//Test Class

public class TestClass {

public static void main(String[] args) {

List<Employee> list = new ArrayList<Employee>();

list.add(new Employee("sibin", "sibin@gmail.com", 2015));

list.add(new Employee("Rahul", "rahul@gmail.com", 1977));

list.add(new Employee("John", "John@gmail.com", 1980));

list.add(new Employee("Jannet", "Jannet@gmail.com", 1983));

Collections.sort(list);

System.out.println("Employees after sorting : ");

for (Employee employee : list) {

System.out.println(employee.getUserid() + " " + employee.getUsername() + " " + employee.getYearofjoin());

}

}

}

Output:

Employees after sorting : 

rahul@gmail.com Rahul 1977

John@gmail.com John 1980

Jannet@gmail.com Jannet 1983

sibin@gmail.com sibin 2015

The comparator in Java with example

In Comparator, we can sort the collection on the basis of multiple elements such as id, name, and age, etc.. It’s a separate class. We create multiple separate classes (that implement Comparator) to compare by different members.
Comparator provides compare() method to sort elements.A Comparator is present in the java. util package. We can sort the list elements of Comparator type by Collections.sort(List, Comparator) method.
Example:
Employee.java

// A class 'Employee' that implements Comparable

class Employee implements Comparable<Employee> {

private int age;

private String name;

private int yearofjoin;

// Used to sort employees by year of join

public int compareTo(Employee m) {

return this.yearofjoin - m.yearofjoin;

}

// Constructor

public Employee(int age, String name, int yearofjoin) {

super();

this.age = age;

this.name = name;

this.yearofjoin = yearofjoin;

}

// Getter methods for accessing private data

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getYearofjoin() {

return yearofjoin;

}

public void setYearofjoin(int yearofjoin) {

this.yearofjoin = yearofjoin;

}

}

import java.util.Comparator;

//Class to compare Employees by age

class AgeCompare implements Comparator<Employee> {

@Override

public int compare(Employee o1, Employee o2) {

if (o1.getAge() < o2.getAge())

return -1;

if (o1.getAge() > o2.getAge())

return 1;

else

return 0;

}

}

NameCompare.java

import java.util.Comparator;

//Class to compare Employees by name

class NameCompare implements Comparator<Employee> {

@Override

public int compare(Employee o1, Employee o2) {

return o1.getName().compareTo(o2.getName());

}

}

Main class

import java.util.ArrayList;

import java.util.Collections;

// Driver class

class Main {

public static void main(String[] args) {

ArrayList<Employee> list = new ArrayList<Employee>();

list.add(new Employee(17, "sibin", 1927));

list.add(new Employee(19, "Rahul", 1977));

list.add(new Employee(10, "Joy", 1980));

list.add(new Employee(9, "Scott", 1983));

// Sort by age : (1) Create an object of ageCompare

// (2) Call Collections.sort

// (3) Print Sorted list

System.out.println("Sorted by age");

AgeCompare ageCompare = new AgeCompare();

Collections.sort(list, ageCompare);

for (Employee employee : list)

System.out.println(employee.getAge() + " " + employee.getName() + " " + employee.getYearofjoin());

System.out.println("\nSorted by name");

NameCompare nameCompare = new NameCompare();

Collections.sort(list, nameCompare);

for (Employee employee : list)

System.out.println(employee.getName() + " " + employee.getAge() + " " + employee.getYearofjoin());

// Uses Comparable to sort by year of join

System.out.println("\nSorted by year");

Collections.sort(list);

for (Employee employee : list)

System.out.println(employee.getYearofjoin() + " " + employee.getAge() + " " + employee.getName() + " ");

}

}

Output:

Sorted by age

9 Scott 1983

10 Joy 1980

17 sibin 1927

19 Rahul 1977

Sorted by name

Joy 10 1980

Rahul 19 1977

Scott 9 1983

sibin 17 1927

Sorted by year

1927 17 sibin 

1977 19 Rahul 

1980 10 Joy 

1983 9 Scott 

by sibin