Java Comparable Interface

The Comparable interface allows you to define the natural ordering of objects so they can be sorted easily using built-in methods like Collections.sort() or Arrays.sort().

It provides the compareTo() method, which is used to define the natural ordering of objects. This method is used by sorting algorithms (like Collections.sort()) to sort elements.

 

• Part of the java.lang package.
• Used to define the natural ordering of objects.
• Contains a single method: compareTo().
• Implemented by a class whose objects need to be sorted.
• Used by sorting utilities such as Collections.sort().

Uses of the Comparable Interface

The Comparable interface is used to impose a natural ordering on the objects of each class that implements it. For example, if you have a list of custom Student objects and you want to sort them by roll number, you can implement Comparable in the Student class and define the logic inside the compareTo() method.

Comparable Interface Method

Method	Description
int compareTo(T o)	Compares the current object with the specified object o for order. Returns a negative integer, zero, or a positive integer if the current object is less than, equal to, or greater than the specified object.

Code Example

import java.util.*;

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

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

 // Defining natural ordering based on roll number
 public int compareTo(Student s) {
 return this.rollNo - s.rollNo;
 }

 public String toString() {
 return rollNo + " - " + name;
 }
}

public class ComparableExample {
 public static void main(String[] args) {
 List<Student> list = new ArrayList<>();
 list.add(new Student(102, "Alice"));
 list.add(new Student(101, "Bob"));
 list.add(new Student(103, "Charlie"));

 Collections.sort(list); // Sorts using compareTo()

 for (Student s : list) {
 System.out.println(s);
 }
 }
}