The Luhn Check Data Algorithm in Java

The Luhn Check Digit Algorithm in Java

//---------------------------------------------------------
// Checks for valid credit card number using Luhn algorithm
//---------------------------------------------------------

public abstract class LuhnCheck {

  //--------------------------------
  // Filter out non-digit characters
  //--------------------------------

  private static String getDigitsOnly (String s) {
    StringBuffer digitsOnly = new StringBuffer ();
    char c;
    for (int i = 0; i < s.length (); i++) {
      c = s.charAt (i);
      if (Character.isDigit (c)) {
        digitsOnly.append (c);
      }
    }
    return digitsOnly.toString ();
  }

  //-------------------
  // Perform Luhn check
  //-------------------

  public static boolean isValid (String cardNumber) {
    String digitsOnly = getDigitsOnly (cardNumber);
    int sum = 0;
    int digit = 0;
    int addend = 0;
    boolean timesTwo = false;

    for (int i = digitsOnly.length () - 1; i >= 0; i--) {
      digit = Integer.parseInt (digitsOnly.substring (i, i + 1));
      if (timesTwo) {
        addend = digit * 2;
        if (addend > 9) {
          addend -= 9;
        }
      }
      else {
        addend = digit;
      }
      sum += addend;
      timesTwo = !timesTwo;
    }

    int modulus = sum % 10;
    return modulus == 0;

  }

  //-----
  // Test
  //-----

  public static void main (String[] args) {
    String cardNumber = "4408 0412 3456 7890";
    boolean valid = LuhnCheck.isValid (cardNumber);
    System.out.println (cardNumber + ": " + valid);
    cardNumber = "4408 0412 3456 7893";
    valid = LuhnCheck.isValid (cardNumber);
    System.out.println (cardNumber + ": " + valid);
    cardNumber = "4417 1234 5678 9112";
    valid = LuhnCheck.isValid (cardNumber);
    System.out.println (cardNumber + ": " + valid);
    cardNumber = "4417 1234 5678 9113";
    valid = LuhnCheck.isValid (cardNumber);
    System.out.println (cardNumber + ": " + valid);
  }

}