code: public ServiceFuture<VirtualMachineScaleSetExtensionInner> getAsync(String resourceGroupName, String vmScaleSetName, String vmssExtensionName, String expand, final ServiceCallback<VirtualMachineScaleSetExtensionInner> serviceCallback) {
          return ServiceFuture.fromResponse(getWithServiceResponseAsync(resourceGroupName, vmScaleSetName, vmssExtensionName, expand), serviceCallback);
      }
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.
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceFuture} object