Exploring the Effect of NULL Usage in Source Code

In this paper we propose to use causal inference (CI) to reason about code smells, or anti-patterns. CI provides methods to estimate the magnitude of effect of certain interventions on the studied system of variables based on observational data only. We would like to estimate the average effect of using a certain pattern on some code quality characteristic. If code quality characteristic systematically deteriorates when a certain pattern is used, it can serve as confirmation that it is in fact an anti-pattern. In the present study, we narrow down the scope and focus on one notorious case of code smells, the usage of NULL. We investigate the effect of using a selection of NULL-based patterns on code complexity metrics. The experiments on open source Java code show preliminary confirmation that NULL is in fact an anti-pattern under certain conditions. We come to a conclusion that in order to fully answer the research question, a better underlying model of the code development process is needed. Moreover, in order to confirm the detrimental effect of patterns, one should investigate their effect on a more diverse set of code characteristics, in addition to code complexity metrics.