A Comparative Study of Bug Patterns in Java Cloned and Non-cloned Code

Code cloning via copy-and-paste is a common practice in software engineering. Traditionally, this practice has been considered harmful, and a symptom that some important design abstraction is being ignored. As such, many previous studies suggest approaches to facilitate the discovery, removal, and refactoring of clones. However, not many studies exist that empirically investigate the relationship of code clones with code quality. In this paper, we conduct an empirical study of 31 open source Java projects (1.7 MSLOC) to explore the relationship between code clones and a set of bug patterns reported by Find Bugs. We found that: (i) the defect density in cloned code is 3.7 times less than that of the rest of the code, (ii) 66% of the bug patterns associated with code clones are related to issues in coding style and practice, the two least problematic of the Find Bugs' categories, while that number is 49% for non-cloned code, and (iii) 75% of the bug patterns in cloned code are duplicated without any changes, while 25% are only present in one of the clones. These results show that, when using Find Bugs to detect bug patterns, there is a positive differentiation of cloned code with respect to the rest of the code: the cloned code has considerably less, and less problematic, bug patterns. While our study does not unveil any explanation for this, results from other, more qualitative studies indicate that developers use copy-and-paste intentionally and wisely, which may explain the quantitative observations of our study. Overall, these research results suggest that the practice of code cloning in Java, and possibly in all other object-oriented languages, needs to be given serious consideration on the part of tool designers.