(JavaFX) How to call method of class between constructor and initialize()?

I have a class that implements an interface. From my base class, I'm creating an instance of the implemented class like this:

Class newTab;
TransactionTabInterface tabInterface = null;
try {
  newTab = Class.forName("[package] + title);
  tabInterface = (TransactionTabInterface) newTab.newInstance();
  tabInterface.setRootController(this);
} catch (Exception e) {
   e.printStackTrace();
}

This is necessary since I don't know the name of the class I need to instantiate (title is determined elsewhere).

The problem I'm having is that, within the new class, I need access to rootController immediately. I have a method within the class that allows me to pass the rootController reference, as seen in my above code.

Normally, I would pass rootController to the constructor when instantiating the class, but I don't know how to pass a parameter to the constructor in the newInstance() call above.