Where the Bugs are: A Quasi-replication Study of the Effect of Inheritance Depth and Width in Java Systems

Abstract

The role of inheritance in the OO paradigm and its inherent complexity has caused conflicting results in the software engineering community. In a seminal empirical study, Basili et al., suggest that, based on a critique of the Chidamber and Kemerer OO metrics suite, a class located deeper in an inheritance hierarchy will introduce more bugs because it inherits a large number of definitions from its ancestors. Equally, classes with a large number of children (i.e., descendants) are difficult to modify and usually require more testing because the class potentially affects all of its children. In this paper, we use a large data set containing bug and inheritance data from eleven Java systems (seven open-source and four commercial) to explore these two research questions. We explore whether it is the case that a class deeper in the hierarchy is more buggy because of its deep position. Equally, we explore whether there is a positive relationship between the number of children and bugs, if classes with large numbers of children are indeed more difficult to modify. Results showed no specific trend for classes deeper in the hierarchy to be more buggy vis-a-vis shallower classes; the four commercial systems actually showed a negative relationship. The majority of classes across the hierarchy were also found to have no children and those classes included the most buggy.