code: public static boolean isVisa(String cc) {
          if (((cc.length() == 16) || (cc.length() == 13)) && (cc.substring(0, 1).equals("4")))
              return isCreditCard(cc);
          return false;
      }
docs: Checks to see if the cc number is a valid Visa number
  
  @param cc a string representing a credit card number; Sample number: 4111 1111 1111 1111(16 digits)
  @return true, if the credit card number is a valid VISA number, false otherwise