code: public Observable<VirtualMachineScaleSetExtensionInner> getAsync(String resourceGroupName, String vmScaleSetName, String vmssExtensionName, String expand) {
          return getWithServiceResponseAsync(resourceGroupName, vmScaleSetName, vmssExtensionName, expand).map(new Func1<ServiceResponse<VirtualMachineScaleSetExtensionInner>, VirtualMachineScaleSetExtensionInner>() {
              @Override
              public VirtualMachineScaleSetExtensionInner call(ServiceResponse<VirtualMachineScaleSetExtensionInner> response) {
                  return response.body();
              }
          });
      }
docs: The operation to get the extension.
  
  @param resourceGroupName The name of the resource group.
  @param vmScaleSetName The name of the VM scale set containing the extension.
  @param vmssExtensionName The name of the VM scale set extension.
  @param expand The expand expression to apply on the operation.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the VirtualMachineScaleSetExtensionInner object