code: public void expectNumeric(String name, String message) {
          String value = Optional.ofNullable(get(name)).orElse("");
  
          if (!StringUtils.isNumeric(value)) {
              addError(name, Optional.ofNullable(message).orElse(messages.get(Validation.NUMERIC_KEY.name(), name)));
          }
      }
docs: Validates that a given field has a numeric value
  
  @param name The field to check
  @param message A custom error message instead of the default one