code: public Observable<List<DomainTopicInner>> listByDomainAsync(String resourceGroupName, String domainName) {
          return listByDomainWithServiceResponseAsync(resourceGroupName, domainName).map(new Func1<ServiceResponse<List<DomainTopicInner>>, List<DomainTopicInner>>() {
              @Override
              public List<DomainTopicInner> call(ServiceResponse<List<DomainTopicInner>> response) {
                  return response.body();
              }
          });
      }
docs: List domain topics.
  List all the topics in a domain.
  
  @param resourceGroupName The name of the resource group within the user's subscription.
  @param domainName Domain name.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the List&lt;DomainTopicInner&gt; object