Proper Placement of Derived Classes in the Class Hierarchy

Users may derive new classes by defining views based on the current database contents. Some virtual classes are classified as brothers of existing classes, and others are either superclasses or subclasses of existing base and virtual classes. A base class is defined directly by the user using class definition constructs. A virtual class is classified as a brother of another class if it is derived from the latter class via a selection. To have a homogeneous system, virtual classes must be treated as first-class citizens in an object-oriented model. In this article, we handle reusability maximization by investigating the proper location of a derived class in the hierarchy by adjusting the list of superclasses and the set of subclasses for a given class to increase inheritance instead of duplication. Our aim is to maximize inherited, and minimize locally defined characteristics by adjusting the list of superclasses to include classes that maximize inherited and minimize locally defined characteristics. Depending on the specification of their superclasses and subclasses during query evaluation, virtual classes may be classified into four groups: (1) classes for which both superclasses and subclasses are specified; (2) classes for which only superclasses are specified; (3) classes for which only subclasses are specified; and (4) classes for which neither superclasses nor subclasses are specified. Reusability decreases from the first group to the fourth group. Two algorithms are presented to achieve the target of maximizing reusability when possible.