An Empirical Study of Clone Removals

It is often claimed that duplicated source code is a threat to the maintainability of a software system and that developers should manage code duplication. A previous study analyzed the evolution of four software systems and found a remarkable discrepancy between code clones detected by a state-of-the-art clone detector and those deliberately removed by developers as the scope of the clones hardly ever matched. However, the results are based on a relatively small amount of data and need to be validated by a more extensive analysis. In this paper, we present an extension of this study by analyzing deliberate as well as accidental removals of code duplication in the evolution of eleven systems. Based on our findings, we could confirm the results of the previous study. Beyond that we found that accidental removals of cloned code occur slightly more often than deliberate removals and that many clone removals were in fact incomplete.