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Chapter 6. Change Dynamics
growth in existing classes, which we did not consistently find, or (ii) new
clients exercise existing classes in new ways, thus uncovering previous
unknown defects. Further work is needed to discover which, if any of
these hypotheses is valid.
6.6 Summary
Change in software systems is unavoidable as they are adapted to meet
the changing needs of the users. Our study shows that when we look
at the latest version of a given system, around a third (or more) of the
classes are unchanged in their lifetime. Of the modified classes, very
few are changed multiple times, and the magnitude of change is small
suggesting an inherent resistance to change. These findings show that
maintenance effort, which is considered to be a substantial proportion
of the development effort (post initial versions) is spent on adding new
classes. In the absence of a substantial architectural shift or a rewrite
of the system, much of the code base resists change. Furthermore,
efforts to base new code on stable classes will inevitably make those
classes less stable as they need to be modified to meet the needs of the
new clients.
In the next chapter, we discuss the implications arising from our observations.
Specifically, we consider how our findings relate to the Laws
of Software Evolution, and how our work can help improve software
development practices.
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