Towards a Client Driven Characterization of Class Hierarchies

Object-oriented legacy systems are hard to maintain because they are hard to understand. One of the main understanding problems is revealed by the so-called "yo-yo effect" that appears when a developer or maintainer wants to track a polymorphic method call. At least part of this understanding problem is due to the dual nature of the inheritance relation i.e., the fact that it can he used both as a code and/or as an interface reuse mechanism. Unfortunately, in order to find out the original intention for a particular hierarchy it is not enough to look at the hierarchy itself; rather than that, an in-depth analysis of the hierarchy's clients is required. In this paper we introduce a new metrics-based approach that helps us characterize the extent to which a base class was intended for interface reuse, by analyzing how clients use the interface of that base class. The idea of the approach is to quantify the extent to which clients treat uniformly the instances of the descendants of the base class, when invoking methods belonging to this common interface, We have evaluated our approach on two medium-sized case studies and we have found that the approach does indeed help to characterize the nature of a base class with respect to interface reuse. Additionally, the approach can be used to detect some interesting patterns in the way clients actually use the descendants through the interface of the base class