RecursiveDigitAdder.java

import java.util.Scanner; // to read data

public class RecursiveDigitAdder {

   /**
   *    Program:    RecursiveDigitAdder.java
   *    Purpose:    Keep adding sum of digits until sum <= 9
   *    Creator:    Chris Clarke
   *    Created:    17.12.2012
   */

   public static void main (String[] args) {

      // set up scanner to read input from keyboard
      Scanner scan = new Scanner (System.in);
      // prompt user
      System.out.print("Enter a number: ");
      // read number from keyboard
      long num =  scan.nextLong();

      System.out.println("The recursive sum of the digits is "
       +getRecursiveSumOfDigits( num));

   }// end main

   public static long getRecursiveSumOfDigits(long n) {
      long sum = 0;
      String strNum = ""+n;

      // iterate through digits of strNum
      for (int i=0; i<strNum.length(); i++) {
         // add the value of digit i of strNum, to sum
         sum += Integer.valueOf( strNum.substring( i, i+1));
      }

      // recursive function call
      if (sum>9) return getRecursiveSumOfDigits(sum);

      return sum;

   }// end getRecursiveSumOfDigits
}// end class