Example: Sum of Natural Numbers (Recursive) in Java

// Sum of Natural Numbers (Recursive)
import java.util.Scanner;

// Main class containing the entry point of the program
public class Main {

    // Recursive method to calculate the sum of natural numbers
    public static int sumNaturalNumbers(int n) {
        // Step 1: Define the base case
        // If n is 0, the sum is 0. This stops the recursion.
        if (n == 0) {
            return 0;
        }
        // Step 2: Define the recursive step
        // Sum of n natural numbers = n + sum of (n-1) natural numbers
        else {
            return n + sumNaturalNumbers(n - 1);
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Step 3: Prompt user for input
        System.out.print("Enter a positive integer (n): ");
        int n = scanner.nextInt();

        // Step 4: Validate input
        if (n < 0) {
            System.out.println("Please enter a non-negative integer.");
        } else {
            // Step 5: Call the recursive method
            int sum = sumNaturalNumbers(n);

            // Step 6: Display the result
            System.out.println("The sum of natural numbers up to " + n + " is: " + sum);
        }

        scanner.close();
    }
}