Background: Code cloning - copying and reusing pieces of source code - is a common phenomenon in software development in practice. There have been several empirical studies on the effects of cloning, but there are contradictory results regarding the connection of cloning and faults. Objective: Our aim is to clarify the relationship between code clones and faults. In particular, we focus on inconsistent (or type-3) clones in this work. Method: We conducted a case study with TWT GmbH where we detected the code clones in three Java systems, set them into relation to information from issue tracking and version control and interviewed three key developers. Results: Of the type-3 clones, 17 contain faults. Developers modified most of the type-3 clones simultaneously and thereby fixed half of the faults in type-3 clones consistently. Type-2 clones with faults all evolved to fixed type-3 clones. Clone length is only weakly correlated with faultiness. Conclusion: There are indications that the developers in two cases have been aware of clones. It might be a reason for the weak relationship between type-3 clones and faults. Hence, it seems important to keep developers aware of clones, potentially with new tool support. Future studies need to investigate if the rate of faults in type-3 clones justifies using them as cues in defect detection.
@cite_12 is the only study known to us that analysed closed-source systems so far. They investigated four industrial systems from two companies and an academic open-source system for type-3 clones. The researchers showed all type-3 clones, which they considered to be true positives, to the developers of the systems and let them annotate them whether the inconsistency was intentional and it constitutes a fault. They found that 28 G "ode and Koschke @cite_18 investigated the clone genealogies in three open-source systems for changes to clones and unintentional inconsistent changes. They found that 57 inspected the remaining changes manually. They found that only 15 .
@cite_18 @cite_12
2114056383 2119887272
Code Clones - duplicated source fragments - are said to increase maintenance effort and to facilitate problems caused by inconsistent changes to identical parts. While this is certainly true for some clones and certainly not true for others, it is unclear how many clones are real threats to the system's quality and need to be taken care of. Our analysis of clone evolution in mature software projects shows that most clones are rarely changed and the number of unintentional inconsistent changes to clones is small. We thus have to carefully select the clones to be managed to avoid unnecessary effort managing clones with no risk potential. Code cloning is not only assumed to inflate maintenance costs but also considered defect-prone as inconsistent changes to code duplicates can lead to unexpected behavior. Consequently, the identification of duplicated code, clone detection, has been a very active area of research in recent years. Up to now, however, no substantial investigation of the consequences of code cloning on program correctness has been carried out. To remedy this shortcoming, this paper presents the results of a large-scale case study that was undertaken to find out if inconsistent changes to cloned code can indicate faults. For the analyzed commercial and open source systems we not only found that inconsistent changes to clones are very frequent but also identified a significant number of faults induced by such changes. The clone detection tool used in the case study implements a novel algorithm for the detection of inconsistent clones. It is available as open source to enable other researchers to use it as basis for further investigations.