code: public Observable<RouteFilterInner> getByResourceGroupAsync(String resourceGroupName, String routeFilterName, String expand) {
          return getByResourceGroupWithServiceResponseAsync(resourceGroupName, routeFilterName, expand).map(new Func1<ServiceResponse<RouteFilterInner>, RouteFilterInner>() {
              @Override
              public RouteFilterInner call(ServiceResponse<RouteFilterInner> response) {
                  return response.body();
              }
          });
      }
docs: Gets the specified route filter.
  
  @param resourceGroupName The name of the resource group.
  @param routeFilterName The name of the route filter.
  @param expand Expands referenced express route bgp peering resources.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the RouteFilterInner object