Comparable

Example

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

public class Main {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>(List.of(
                new Student("Ravi", 3), new Student("Asha", 1), new Student("Meena", 2)));
        Collections.sort(list);                     // uses compareTo
        System.out.println(list);
        System.out.println(new TreeSet<>(list).first());
    }
}

class Student implements Comparable<Student> {
    final String name;
    final int rollNo;

    Student(String name, int rollNo) {
        this.name = name;
        this.rollNo = rollNo;
    }

    @Override
    public int compareTo(Student other) {
        return Integer.compare(this.rollNo, other.rollNo);   // natural order: by roll number
    }

    @Override
    public String toString() {
        return rollNo + ":" + name;
    }
}

Output

[1:Asha, 2:Meena, 3:Ravi]
1:Asha