code: public Observable<DomainTopicInner> getAsync(String resourceGroupName, String domainName, String topicName) {
          return getWithServiceResponseAsync(resourceGroupName, domainName, topicName).map(new Func1<ServiceResponse<DomainTopicInner>, DomainTopicInner>() {
              @Override
              public DomainTopicInner call(ServiceResponse<DomainTopicInner> response) {
                  return response.body();
              }
          });
      }
docs: Get a domain topic.
  Get properties of a domain topic.
  
  @param resourceGroupName The name of the resource group within the user's subscription.
  @param domainName Name of the domain
  @param topicName Name of the topic
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the DomainTopicInner object