docs: public boolean validateAllFields() {
          boolean cardNumberIsValid =
                  CardUtils.isValidCardNumber(mCardNumberEditText.getCardNumber());
          boolean expiryIsValid = mExpiryDateEditText.getValidDateFields() != null &&
                  mExpiryDateEditText.isDateValid();
          boolean cvcIsValid = isCvcLengthValid();
          mCardNumberEditText.setShouldShowError(!cardNumberIsValid);
          mExpiryDateEditText.setShouldShowError(!expiryIsValid);
          mCvcEditText.setShouldShowError(!cvcIsValid);
          boolean postalCodeIsValidOrGone;
          if (mShouldShowPostalCode) {
              postalCodeIsValidOrGone = isPostalCodeMaximalLength(true,
                      mPostalCodeEditText.getText().toString());
              mPostalCodeEditText.setShouldShowError(!postalCodeIsValidOrGone);
          } else {
              postalCodeIsValidOrGone = true;
          }
  
          return cardNumberIsValid
                  && expiryIsValid
                  && cvcIsValid
                  && postalCodeIsValidOrGone;
      }