Name: Comparable and Comparator in Java
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Author: rajubora

In java-8 comparable and comparator interface are used for sorting object using custom logic .
However there are some diffrences between them listed as below :

Comparable Interface :

For implementing comparable interface we have to override the compareTo() method.
CompareTo method takes only one arguement, which is the object being compared.
This CompareTo method has to be implemented in the class to define the natural ordering of the objects.
It allow us to define single sorting criteria.
Interface Signature :

public interface Comparable<T> {
    public int compareTo(T o);
}

Comparator Interface :

For implementing Comparator interface we have to override the compare() method of the interface.
This method takes two parameters and compares them (o1 and o2)
and returns:

a negative integer if o1 is less than o2.
zero if o1 is equal to o2.
a positive integer if o1 is greater than o2.

It allow is to define multiple sorting criteria and can be used to sort object in diffrent ways.
Interface Signature :

public interface Comparator<T> {
    public int compare(T o1, T o2);
}

Example :

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

// Example class representing a Student
class Student implements Comparable<Student> {
    private String name;
    private int id;
    private int age;

    // Constructor
    public Student(String name, int id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }

    // Getters
    public String getName() {
        return name;
    }

    public int getId() {
        return id;
    }

    public int getAge() {
        return age;
    }

    // Implementing Comparable interface based on student id
    @Override
    public int compareTo(Student other) {
        return Integer.compare(this.id, other.id);
    }

    // toString method for displaying student details
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

// Comparator to sort students by age
class AgeComparator implements Comparator<Student> {
    @Override
    public int compare(Student s1, Student s2) {
        return Integer.compare(s1.getAge(), s2.getAge());
    }
}

public class ComparableComparatorExample {
    public static void main(String[] args) {
        // Creating a list of students
        List<Student> students = new ArrayList<>();
        students.add(new Student("Ram", 101, 20));
        students.add(new Student("Shyam", 102, 22));
        students.add(new Student("Raj", 103, 21));
        // Sorting students using Comparable (by student id)
        Collections.sort(students);
        System.out.println("Students sorted by ID (Comparable):");
        for (Student student : students) {
            System.out.println(student);
        }

        // Sorting students using Comparator (by age)
        Collections.sort(students, new AgeComparator());
        System.out.println("\nStudents sorted by Age (Comparator):");
        for (Student student : students) {
            System.out.println(student);
        }
    }
}

In above Example :

The Student class implements the Comparable interface based on the student ID. This allows sorting a list of students by their IDs.
The AgeComparator class implements the Comparator interface to provide custom sorting logic based on student ages.
In the main method, we create a list of Student objects and sort them first using the natural ordering defined by Comparable and then using the custom ordering defined by Comparator.