code: public ServiceFuture<ExtensionInner> getAsync(String resourceGroupName, String clusterName, String extensionName, final ServiceCallback<ExtensionInner> serviceCallback) {
          return ServiceFuture.fromResponse(getWithServiceResponseAsync(resourceGroupName, clusterName, extensionName), serviceCallback);
      }
docs: Gets the extension properties for the specified HDInsight cluster extension.
  
  @param resourceGroupName The name of the resource group.
  @param clusterName The name of the cluster.
  @param extensionName The name of the cluster extension.
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceFuture} object