code: public FormMappingOption yourCollection(FormYourCollectionResource resource) {
          if (resource == null) {
              throw new IllegalArgumentException("The argument 'resource' should not be null.");
          }
          if (yourCollectionResourceList == null) {
              yourCollectionResourceList = new ArrayList<>();
          }
          yourCollectionResourceList.add(resource);
          return this;
      }
docs: ---------------