X Design Properties and Evolvability of Object-Oriented Systems

Over the years, cumulative data have shown that maintenance is a major cost concern, as a matter of fact, a growing cost concern (Pigosky, 1997). The maintainability of a system seems to have much influence on the ease or difficulty to implement changes. A consensus has emerged that the maintainability of a software system is dependent on its design (Rombach, 1990), in the procedural paradigm as well as in the object-oriented (OO) paradigm. In ISO 9126, maintainability has four components, namely, analyzability, testability, stability, and changeability (ISO, 1992). In application areas like telecommunications, software systems are evolving constantly. There are organizations , which do not develop the software they operate, but purchase it. These organizations are not directly interested in testability or diagnosis, but in the software's ability to sustain an ongoing flow of changes. In this research, the focus will be on that single aspect of maintainability, i.e., changeability. In the SPOOL project (cf. chapters Y and Z), we are investigating the dependency between the changeability of software systems and their design. One way of assessing changeability is to assess the impact of changes (change impact analysis). The approach we have taken is both analytical and experimental. It involves defining a systematic change impact model that is more complete and general than those presented in the literature, and applying it on industrial software systems to assess their changeability. In Section 2 of this chapter, we present work that is related to maintainability and changeability. The change impact model, and its application to C++, are described in Section 3. A first case study for testing the practicality of the model