🚀 go-pugleaf

I have once again one of these questions dealing with the professional usage
of the advices of school-books about java.
So here it is:
I try to implement an application, that in the whole manipulates a certain
class A. That means every action in the gui is to manipulate the central
object.
I have implemented the manipulation of the object in an apllication that is
to be used on the command-line.

Now I would like to use swing-components. But in which class do I now create
the central object?
Is it the "main" part, where I normally would only create the Jframe an call
frame.show; is it in the Frame while construction it or is it in the content
Pane in the JPanel, where I also implement the Buttons an so on?
And how do I make the central object known to all components of the GUI
(iow: Where do I implement the call of the methods of  the object?)

A long question, but as I said: My books only tell me where it is posible,
but not what is best.

Thanks
Thomas