Comparable interface in Java

The Comparable interface in Java is used to define the natural ordering of objects of a class. It enables objects to be compared and sorted automatically without using an external Comparator.

• It contains the compareTo() method, which compares the current object with another object.
• It is commonly used with Collections.sort() and Arrays.sort() for sorting custom objects.

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

Any class that implements Comparable must provide an implementation for the compareTo(T o) method.

Why Use Comparable?

Problem Without Comparable:

Let's say we have a Student class and a list of students. If we try to sort them using Collections.sort(), Java won’t know how to compare them.

import java.util.*;

class Student {
    int id;
    String name;

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

   public String toString() {
    return "Student[id=" + id + ", name='" + name + "]";
}
}

public class Main {
    public static void main(String[] args) {

        List<Student> container = new ArrayList<>();

        Student student1 = new Student(3, "Alice");
        Student student2 = new Student(2, "Charlie");
        Student student3 = new Student(4, "Jennie");

        container.add(student1);
        container.add(student2);
        container.add(student3);

        Collections.sort(container);

        System.out.println(container); 
    }
}

Output :
Main.java:22: error: no suitable method found for sort(List<Student>)
Collections.sort(container);    

Implementing Comparable:

To fix this, we implement Comparable and override compareTo()

Sorting Students by ID:

import java.util.*;

class Student implements Comparable<Student> {
    int id;
    String name;

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

    @Override
public int compareTo(Student other) {
    if (this.id > other.id) return 1;
    if (this.id < other.id) return -1;
    return 0;
}

    @Override
     public String toString() {
    return "Student[id=" + id + ", name='" + name + "]";
}
}

public class Main {
    public static void main(String[] args) {
        List<Student> container = new ArrayList<>();

      Student student1 = new Student(3, "Alice");
        Student student2 = new Student(2, "Charlie");
        Student student3 = new Student(4, "Jennie");

        container.add(student1);
        container.add(student2);
        container.add(student3);

        Collections.sort(container);

        System.out.println(container);
    }
}

Output:
[1 - Bob, 2 - Charlie, 3 - Alice]

What is happening?

implements Comparable → tells Java this class can be compared
compareTo() → defines comparison logic

Positive → current object is greater
Negative → current object is smaller
Zero → both are equal