code: private static <E extends Enum<E>> Class<E> asEnum(final Class<E> enumClass) {
          Validate.notNull(enumClass, ENUM_CLASS_MUST_BE_DEFINED);
          Validate.isTrue(enumClass.isEnum(), S_DOES_NOT_SEEM_TO_BE_AN_ENUM_TYPE, enumClass);
          return enumClass;
      }
docs: Validate {@code enumClass}.
  @param <E> the type of the enumeration
  @param enumClass to check
  @return {@code enumClass}
  @throws NullPointerException if {@code enumClass} is {@code null}
  @throws IllegalArgumentException if {@code enumClass} is not an enum class
  @since 3.2