Tailoring Object-Oriented Data Models through Metaclasses

Object-oriented data models organize objects into classes that describe their structural and behavioral properties. Classes can be treated as objects themselves by organizing them into metaclasses. Usually, metaclasses have been employed to define the structure and behavior of classes. In this paper we extend the metaclass concept. Metaclasses specify not only the properties of their instances, which are classes, but also the common properties of the instances of their instances. Therefore, objects can be given a different predefined common behavior and structure at the metaclass level. In our paper we show the usefulness of this extended metaclass concept. It can be employed by the system designer to provide an open data model and to tailor a general object-oriented data model for special needs of an application designer. We illustrate the approach with examples. We define a new metaclass that introduces the semantics of ôrolesö, an additional modelling concept. We show how the semantics can be specified differently by different metaclasses.