Cloned and non-cloned Java methods: a comparative study

Reusing code via copy-and-paste, with or without modification is a common behavior observed in software engineering. Traditionally, cloning has been considered as a bad smell suggesting flaws in design decisions. Many studies exist targeting clone discovery, removal, and refactoring. However there are not many studies which empirically investigate and compare the quality of cloned code to that of the code which has not been cloned. To this end, we present a statistical study that shows whether qualitative differences exist between cloned methods and non-cloned methods in Java projects. The dataset consists of 3562 open source Java projects containing 412,705 cloned and 616,604 non-cloned methods. The study uses 27 software metrics as a proxy for quality, spanning across complexity, modularity, and documentation (code-comments) categories. When controlling for size, no statistically significant differences were found between cloned and non-cloned methods for most of the metrics, except for three of them. The main statistically significant difference found was that cloned methods are on an average 18% smaller than non-cloned methods. After doing a mixed method analysis, we provide some insight for why cloned methods are smaller.