Comparable

 It is an interface, we need to implement this interface to the class which we are working with.

and we need to import the java.util.Comparator package in order to work with comparable interface.

and also we need to override the compareTo method. Comparable will be used when the source code file is available, In our Example, It is Student class.

import java.util.*;
import java.util.Collections;
import java.util.Comparator;

// class Student implements Comparable<Student>
// {
//     int age;
//     String name;
//     String tech;
//     Integer i;
//     public Student(int age, String name, String tech) {
//         this.age = age;
//         this.name = name;
//         this.tech = tech;
//     }
//     @Override
//     public String toString() {
//         return "Student [age=" + age + ", name=" + name + ", tech=" + tech + "]";
//     }

//     @Override
//     public int compareTo(Student that)
//     {
//             return this.name.compareTo(that.name);
//     }


// }

public class Demo
{
    public static void main(String a[])
    {
    

        List<Integer> nums = Arrays.asList(5,7,2,9);
        
        Collections.sort(nums);
        Collections.reverse(nums);

        System.out.println(nums);


        // ArrayList<Student> nums = new ArrayList<>();
        // nums.add(new Student(21, "Nagesh", "Java"));
        // nums.add(new Student(17, "Ashwini", "JS"));
        // nums.add(new Student(18, "Souvik", "Blockchain"));
        // Collections.sort(nums, (s1, s2) -> s1.tech.length() - s2.tech.length());
        // // 21 17 18
        // // 17 21 18
        // // 17 18 21

        // // 17 18 21
 
        // System.out.println(nums);

    }
}