Welcome to the Ranch
I have added
code tags to your post; always use the,. Doesn't it look better. They do however highoight the incorrectly formatted comments. For comments longer than about ½ line use
/* comments */ and make sure to restrict the
line length. I shall have a go at correcting that later.
Don't throw plain Exception. If you don't want negative numbers throw an
IllegalArgumentException and don't use a throws clause. Describe the Exception in the
/** documentationcomments */ however. No, the possibility of throwing an Exception should not make any difference to your problem.
You never call a class. You call methods of its instances. There are no such things as start and launch keywords. Do you mean method names? Do you know how to create an instance of that JavaFXGUI class?