Exploring semantics in aggregation hierarchies for object-oriented databases

An extended object-oriented model for exploring semantics in aggregation hierarchies is presented. The extension is mainly based on a general distinction between aggregation references and association references and a support for type inheritance in both specialization and aggregation abstractions. The author formally describes notions of aggregation reference and aggregation hierarchy and introduces the concept of aggregation inheritance as a type composition mechanism for sharing specifications among types. The similarities and differences between aggregation inheritance and subtype inheritance are analyzed. It is shown that a combination of these two types of inheritance provides a powerful mechanism for abstract implementation of behavior and for enhancing the extensibility of the object model