Yes. You just need to get access to a reference to the controller that you want to manipulate, which you can do by:
- When one window creates the other, it can get the Controller from the FXMLLoader (just like in the Day14_DemoOpenNextWindow project), and then it could pass a reference to itself (
this) into a method that it calls on the recently created controller. That recently created controller could store a reference back to the "parent controller", and call methods whatever methods on it as necessary.
(There may be other solutions to your underlying problem too, however...)