Worst Smells and Their Worst Reasons

Code bad smells are symptoms of poor design and implementation. There are several well-known smell types, such as large classes (aka God classes), code clones, etc. and they have been shown to lead to technical debt and hence to decrease code maintainability. Quality gates are a recent technology that prevents the automatic acceptance of push requests of code commits that have been identified as containing certain smells. However, it is a challenging activity to decide which smells should be included in the quality gate, as developers may choose to optimize short term benefits like time to market over long term benefits like maintainability. But some smells appear to provide no benefit to developers whatsoever and hence such smells should always be avoided. The aims of this paper are: 1) to identify "worst smells", i.e., bad smells that never have a good reason to exist, 2) to determine the frequency, change-proneness, and severity associated with worst smells, and 3) to identify the "worst reasons", i.e., the reasons for introducing these worst smells in the first place. To achieve these aims we ran a survey with 71 developers. We learned that 80 out of 314 catalogued code smells are "worst"; that is, developers agreed that these 80 smells should never exist in any code base. We then checked the frequency and change-proneness of these worst smells on 27 large Apache open-source projects. Our results show insignificant differences, in both frequency and change proneness, between worst and non-worst smells. That is to say, these smells are just as damaging as other smells, but there is never any justifiable reason to introduce them. Finally, in follow-up phone interviews with five developers we confirmed that these smells are indeed worst, and the interviewees proposed seven reasons for why they may be introduced in the first place. By explicitly identifying these seven reasons, project stakeholders can, through quality gates or reviews, ensure that such smells are never accepted in a code base, thus improving quality without compromising other goals such as agility or time to market.