You want your GUI to remember its location and/or size when the user restarts the application.
Set a ComponentListener on the main Window which will call you back to fetch the new size and/or location; save the value in Preferences (See XX); restore them from Preferences at application startup time.
This is a basic feature that users expect in a production GUI.
How ... (see UtilGUI.monitor method)
Swing Hacks for more preferences stuff.