code: public boolean validateCVC() {
          if (StripeTextUtils.isBlank(cvc)) {
              return false;
          }
          String cvcValue = cvc.trim();
          String updatedType = getBrand();
          boolean validLength =
                  (updatedType == null && cvcValue.length() >= 3 && cvcValue.length() <= 4)
                  || (AMERICAN_EXPRESS.equals(updatedType) && cvcValue.length() == 4)
                  || cvcValue.length() == 3;
  
          return ModelUtils.isWholePositiveNumber(cvcValue) && validLength;
      }
docs: Checks whether or not the {@link #cvc} field is valid.
  
  @return {@code true} if valid, {@code false} otherwise