The Yin and Yang of Software Quality: On the Relationship between Design Patterns and Code Smells

Software reuse is considered the silver bullet of software engineering. It has been largely demonstrated that the proper implementation of design and reuse principles can substantially reduce the effort, time, and costs required to develop software systems. Design patterns are one of the most affirmed techniques for source code reuse. While previous work pointed out their benefits in terms of maintainability and understandability, some seem to raise the opposite concern, suggesting that they can negatively impact code quality from the developers’ perspectives. We recognize such discrepancy in the literature, and we aim to fill this gap by investigating whether and how design patterns are related to the emergence of issues compromising code understandability, namely the Complex Class, God Class, and Spaghetti Code smells, which have been also shown to increase the change- and fault-proneness of code. We perform an empirical evaluation on 15 Java projects evolving over 542 releases, and we find that, although design patterns are supposed to improve code quality without prejudice, they can be related to dangerous issues, as we observe the emergence of code smells in the classes participating in their implementation. From our findings, we distill a number of implications for developers and project managers to support them in dealing with design patterns.