JavaFX | Having access of other Controllers from a specific Controller

There are some good ways to do this:

1. Create a reference of a Controller

private ControllerClass controller;

/*...

Parent root;
FXMLLoader fxmlLoader = new FXMLLoader();
fxmlLoader.setLocation(getClass().getResource("/path/ofYourFxml.fxml"));

controller = fxmlloader.getController();
//Now you can use a method like getter and setter to set or get any value of variable
controller.yourMethodToCall()

Stage stage = new Stage();
stage.setScene(new Scene(root, 1200, 700));
stage.show()

2. For dirt and dirty solution: declare any variable you want to use in other class as static (Not recommended for a complex application(static problem))
3. Use a shared data Model among Controllers

Explaination follows...