Chapter 18 Exercise 11, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

18.11 (Sum the digits in an integer using recursion)
Write a recursive method that computes the sum of
the digits in an integer. Use the following method header:

 public static int sumDigits(long n)

For example, sumDigits(234) returns 2 + 3 + 4 = 9. Write a test program
that prompts the user to enter an integer and displays its sum.

import java.util.Scanner;
 

public class Exercise_11 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        System.out.print("Enter an integer: ");
        long n = input.nextLong();
        System.out.println(sumDigits(n));

    }

    public static long sumDigits(long n) {

        if (n == 0)
            return n;

        return n % 10 + sumDigits(n / 10);
    }
}