1. In a computing environment, a method for specifying a hierarchy of user-defined classes of graphical objects comprising the steps of: graphically specifying a first class of graphical objects; and graphically specifying at least one descendant class of graphical objects that is a subclass of the first class of graphical objects for the hierarchy of classes of graphical objects, wherein at least one of the first or the at least one descendant class of graphical objects can be instantiated in an executable graphical model.