Java Program to Find Sum of Digits in a Number using Recursion - Interview Question

Recently this question to asked was one of my reader, which inspired me
to write this tutorial. There was usual check to solve this problem using both recursion and iteration. To be frank, calculating sum of digit of an integral
number, is not difficult, but I have still seen quite a few programmers
fumbles, even after providing hint in terms of division and modules operator. Key
point here is to know how to use division and modules operator in Java. For
this kind of exercise including reversing a number, where you
need to find digits from a number, use division operator to remove right,
and use modules operator or % to get right most digits. For example if
you have number 1234 than 1234/10 will give you 123 i.e. right most digit 4 is
removed, while 1234%10 will give you 4, which is the right most digit in that
number. If you know this property, you can easily solve lots or problems which
are related to reversing numbers e.g. checking if a number is palindrome
or finding Armstrong numbers in Java.

Java program to calculate sum of digits in a
number

Here is complete Java code example to find sum of digits using recursion
in Java. This Java example, also includes an iterative solution of this problem
to prepare follow-up questions from Interviewer. By the way, you can also use
this example to learn Recursion in Java. It’s
a tricky concept, and examples like this, certainly helps to understand and apply recursion better.

/**
 * Java program to calculate sum of digits for a number using recursion and iteration.
 * Iterative solution uses while loop here.
 *
 * @author Javin Paul
 */
public class SumOfDigit {
 
    public static void main(String args[]) {
      
      System.out.println( "Sum of digit using recursion for number 123 is " + sumOfDigits(123));
      System.out.println( "Sum of digit using recursion for number 1234 is " + sumOfDigits(1234));
      System.out.println( "Sum of digit from recursive function for number 321 is " + sumOfDigits(321));
      System.out.println( "Sum of digit from recursive method for number 1 is " + sumOfDigits(1));
     
      System.out.println( "Sum of digit using Iteration for number 123 is " + sumOfDigitsIterative(123));
      System.out.println( "Sum of digit using while loop for number 1234 is " + sumOfDigitsIterative(1234));
     
    } 
   
   
  
    public static int sumOfDigits(int number){
        if(number/10 == 0) return number;
       
        return number%10 + sumOfDigits(number/10);
    }
   
    public static int sumOfDigitsIterative(int number){
        int result  = 0;
        while(number != 0){
            result = result + number%10;
            number = number/10;
        }
       
        return result;
    }
  
}

Output:
Sum of digit using recursion for number 123 is 6
Sum of digit using recursion for number 1234 is 10
Sum of digit from recursive function for number 321 is 6
Sum of digit from recursive method for number 1 is 1
Sum of digit using Iteration for number 123 is 6
Sum of digit using while loop for number 1234 is 10

That's all on How to find sum of digits of a number using recursion in
Java. You should be able to write this method using both Iteration i.e.
using loops, and using Recursion i.e. without using loops in Java.

Source:http://javarevisited.blogspot.com/2013/05/java-program-to-find-sum-of-digits-in-number-recursion.html