code: public <ATTRIBUTE> OptionalThing<ATTRIBUTE> getAttribute(String key, Class<ATTRIBUTE> attributeType) {
          assertArgumentNotNull("key", key);
          assertArgumentNotNull("attributeType", attributeType);
          final Object original = findSuccessAttribute(key);
          final ATTRIBUTE attribute;
          if (original != null) {
              try {
                  attribute = attributeType.cast(original);
              } catch (ClassCastException e) {
                  final ExceptionMessageBuilder br = new ExceptionMessageBuilder();
                  br.addNotice("Cannot cast the validation success attribute");
                  br.addItem("Attribute Key");
                  br.addElement(key);
                  br.addItem("Specified Type");
                  br.addElement(attributeType);
                  br.addItem("Existing Attribute");
                  br.addElement(original.getClass());
                  br.addElement(original);
                  br.addItem("Attribute Map");
                  br.addElement(getSuccessAttributeMap());
                  final String msg = br.buildExceptionMessage();
                  throw new ValidationSuccessAttributeCannotCastException(msg);
              }
          } else {
              attribute = null;
          }
          return OptionalThing.ofNullable(attribute, () -> {
              final Set<String> keySet = getSuccessAttributeMap().keySet();
              final String msg = "Not found the validation success attribute by the string key: " + key + " existing=" + keySet;
              throw new ValidationSuccessAttributeNotFoundException(msg);
          });
      }
docs: Get the attribute value of the validation success by the key. <br>
  Basically saved by ActionMessages#saveSuccessAttribute().
  @param <ATTRIBUTE> The type of attribute object.
  @param key The string key of attribute saved in the scope. (NotNull)
  @param attributeType The generic type of the result for the attribute. (NotNull)
  @return The optional attribute object for the key. (NotNull, EmptyAllowed: when not found)