Comparable Interface in Java

Comparable Interface in Java

In Java, the Comparable interface is used to compare objects and define their natural ordering.

Examples:

Comparing students based on marks

Comparing products based on price

Sorting strings alphabetically

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Why Do We Need Comparable?

Primitive values like int or double can be compared easily.
But for custom objects like Student or Employee, Java does not know how to compare them by default.

So, we define the comparison logic using the Comparable interface.

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Definition

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

The key method is compareTo().

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*How compareTo() Works
*
Returns a negative value → current object is smaller

Returns 0 → both objects are equal

Returns a positive value → current object is greater

This is the basis for sorting.

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Example: Sorting Students by Marks

import java.util.*;

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

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

    public int compareTo(Student s) {
        return this.marks - s.marks; // ascending order
    }
}

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

        list.add(new Student("A", 80));
        list.add(new Student("B", 60));
        list.add(new Student("C", 90));

        Collections.sort(list);

        for (Student s : list) {
            System.out.println(s.name + " " + s.marks);
        }
    }
}

Output:

B 60
A 80
C 90

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Key Points

Comparable defines natural (default) ordering

It is implemented inside the class

Collections.sort() automatically uses it

You must override compareTo()

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Comparable vs Comparator

Comparable	Comparator
Implemented inside the class	Implemented in a separate class
Only one sorting logic	Multiple sorting logics possible
Uses compareTo()	Uses compare()

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Simple Analogy

Comparable: the object decides how it should be compared
Comparator: an external logic decides how objects are compared

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Final Line

The Comparable interface allows you to define a default sorting behavior for your objects.