JavaAlgorithms/Maths/SumOfDigits.java

package Maths;

public class SumOfDigits {
  public static void main(String[] args) {
    assert sumOfDigits(-123) == 6 && sumOfDigitsRecursion(-123) == 6 && sumOfDigitsFast(-123) == 6;

    assert sumOfDigits(0) == 0 && sumOfDigitsRecursion(0) == 0 && sumOfDigitsFast(0) == 0;

    assert sumOfDigits(12345) == 15
        && sumOfDigitsRecursion(12345) == 15
        && sumOfDigitsFast(12345) == 15;
  }

  /**
   * Calculate the sum of digits of a number
   *
   * @param number the number contains digits
   * @return sum of digits of given {@code number}
   */
  public static int sumOfDigits(int number) {
    number = number < 0 ? -number : number; /* calculate abs value */
    int sum = 0;
    while (number != 0) {
      sum += number % 10;
      number /= 10;
    }
    return sum;
  }

  /**
   * Calculate the sum of digits of a number using recursion
   *
   * @param number the number contains digits
   * @return sum of digits of given {@code number}
   */
  public static int sumOfDigitsRecursion(int number) {
    number = number < 0 ? -number : number; /* calculate abs value */
    return number < 10 ? number : number % 10 + sumOfDigitsRecursion(number / 10);
  }

  /**
   * Calculate the sum of digits of a number using char array
   *
   * @param number the number contains digits
   * @return sum of digits of given {@code number}
   */
  public static int sumOfDigitsFast(int number) {
    number = number < 0 ? -number : number; /* calculate abs value */
    char[] digits = (number + "").toCharArray();
    int sum = 0;
    for (int i = 0; i < digits.length; ++i) {
      sum += digits[i] - '0';
    }
    return sum;
  }
}