Example: Sum of Digits using Recursion in Java

// Sum of Digits using Recursion
import java.util.Scanner;

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

    // Recursive method to calculate the sum of digits
    public static int sumDigitsRecursive(int number) {
        // Base case: If the number is 0, the sum of digits is 0
        if (number == 0) {
            return 0;
        }
        // Recursive step:
        // 1. Get the last digit (number % 10)
        // 2. Add it to the sum of digits of the remaining number (number / 10)
        return (number % 10) + sumDigitsRecursive(number / 10);
    }

    public static void main(String[] args) {
        // Step 1: Create a Scanner object to read user input
        Scanner scanner = new Scanner(System.in);

        // Step 2: Prompt the user to enter a number
        System.out.print("Enter a non-negative integer: ");
        int num = scanner.nextInt();

        // Step 3: Validate input to ensure it's non-negative
        if (num < 0) {
            System.out.println("Please enter a non-negative integer.");
        } else {
            // Step 4: Call the recursive method to calculate the sum of digits
            int sum = sumDigitsRecursive(num);

            // Step 5: Print the result
            System.out.println("The sum of digits of " + num + " is: " + sum);
        }

        // Step 6: Close the scanner
        scanner.close();
    }
}