problem whith custom classes


I am trying to make a program for mac OS X. I am using Java. I am trying to separate windows in different nib files, containing the window, the delegate and the controller of the window. In all the nib files, I must have only one NSApplication instance (logical). But when I try to apply a custom class to the fileOwner of a nib file, the program compiles, but can't run any more.
Does someone knows how to fix this problem ?

Thank you