A large-scale empirical study on the lifecycle of code smell co-occurrences

Context: Code smells are suboptimal design or implementation choices made by programmers during the development of a software system that possibly lead to low code maintainability and higher maintenance costs. Objective: Previous research mainly studied the characteristics of code smell instances affecting a source code file, while only few studies analyzed the magnitude and effects of smell co-occurrence, i.e., the co-occurrence of different types of smells on the same code component. This paper aims at studying in details this phenomenon. Method: We analyzed 13 code smell types detected in 395 releases of 30 software systems to firstly assess the extent to which code smells co-occur, and then we analyze (i) which code smells co-occur together, and (ii) how and why they are introduced and removed by developers. Results: 59% of smelly classes are affected by more than one smell, and in particular there are six pairs of smell types (e.g., Message Chains and Spaghetti Code) that frequently co-occur. Furthermore, we observed that method-level code smells may be the root cause for the introduction of class-level smells. Finally, code smell co-occurrences are generally removed together as a consequence of other maintenance activities causing the deletion of the affected code components (with a consequent removal of the code smell instances) as well as the result of a major restructuring or scheduled refactoring actions. Conclusions: Based on our findings, we argue that more research aimed at designing co-occurrence-aware code smell detectors and refactoring approaches is needed.