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