code: public Observable<ServiceResponse<BuildStepInner>> beginCreateWithServiceResponseAsync(String resourceGroupName, String registryName, String buildTaskName, String stepName) {
          if (this.client.subscriptionId() == null) {
              throw new IllegalArgumentException("Parameter this.client.subscriptionId() is required and cannot be null.");
          }
          if (resourceGroupName == null) {
              throw new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null.");
          }
          if (registryName == null) {
              throw new IllegalArgumentException("Parameter registryName is required and cannot be null.");
          }
          if (buildTaskName == null) {
              throw new IllegalArgumentException("Parameter buildTaskName is required and cannot be null.");
          }
          if (stepName == null) {
              throw new IllegalArgumentException("Parameter stepName is required and cannot be null.");
          }
          final String apiVersion = "2018-02-01-preview";
          final BuildStepProperties properties = null;
          BuildStepInner buildStepCreateParameters = new BuildStepInner();
          buildStepCreateParameters.withProperties(null);
          return service.beginCreate(this.client.subscriptionId(), resourceGroupName, registryName, buildTaskName, stepName, apiVersion, this.client.acceptLanguage(), buildStepCreateParameters, this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<BuildStepInner>>>() {
                  @Override
                  public Observable<ServiceResponse<BuildStepInner>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<BuildStepInner> clientResponse = beginCreateDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
docs: Creates a build step for a build task.
  
  @param resourceGroupName The name of the resource group to which the container registry belongs.
  @param registryName The name of the container registry.
  @param buildTaskName The name of the container registry build task.
  @param stepName The name of a build step for a container registry build task.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the BuildStepInner object