this better be for a game.
offcourse you seperate your gui over and amount of classes, one big class is unmaintable/error prone. null Layout makes your layout static, limiting the amount of screens that your app is workable under greatly. See websites and absolute sizes.
you have to decide if you go the WYSIWYG editor or simply code it.
I take gui parts generaly in two ways; I have building blocks eg components, or parts that (pritty much) follow the navigation hierarchy. I used to have plain action listeners to capture functiononality (which tries to follows usecases/senario's) since that lead to cumsyness with respects to states.... anyways I then noticed the swing Action's and all was well.
that takes part of the coding end and then you have to scratch your head how to increase the accessability of your application.