Object class definition by generalization

Abstract

The principle of generalization has been used in the database world for schema integration [BL84, DW84, NEL86, LR82, MK85]. Schema integration may refer to the integration of individual user views to a single conceptual schema, or to the integration of preexisting database schemas into a global view schema. In this chapter, we have been interested in the the second aspect and we have shown how the process of object class definition by generalization can be incorporated into object-oriented systems. The presented approach for the definition of new object classes on existing previously defined ones is applicable whenever predefined object classes have to be combined, be it in bottom-up design in centralized systems or in the integration of distributed systems.

Upward inheritance is used to achieve a maximum of reusablity of methods and attributes of previously defined object classes. The distinction of different types of generalization classes and different types of semantic relationships between attributes reduces the integration and development effort to a minimum. In a great number of cases only the types of semantic relationships that hold between object classes to be integrated and that hold between object class's attriutes have to be specified for a successful integration of predefined object classes. Based on the type of semantic relationship the correct property propagations from the local instance(s) to the generalized instance can be automatically infered.