Software Component Design-for-reuse: a Language-independent Discipline Applied

Software engineers often espouse the importance of using abstraction and encapsulation in developing software components. They advocate the \layering" of new components on top of existing components, using only information about the functionality and interfaces provided by the existing components. This layering approach is in contrast to a \direct implementation" of new components, utilizing unencapsulated access to the representation data structures and code present in the existing components. However, there is no empirical evidence that indicates whether the layering approach improves the productivity or quality of the development activity. We discuss three controlled experiments designed to gather such empirical evidence. The results support the contention that layering signi cantly reduces the e ort required to build new components. Furthermore, the quality of the components, in terms of the number of defects introduced during their development, is at least as good using the layered approach. We also discuss some interesting issues related to the statistical analysis of experiments such as these.