Comparing within-and cross-project machine learning algorithms for code smell detection

Code smells represent a well-known problem in software engineering, since they are a notorious cause of loss of comprehensibility and maintainability. The most recent efforts in devising automatic machine learning-based code smell detection techniques have achieved unsatisfying results so far. This could be explained by the fact that all these approaches follow a within-project classification, i.e. training and test data are taken from the same source project, which combined with the imbalanced nature of the problem, produces datasets with a very low number of instances belonging to the minority class (i.e. smelly instances). In this paper, we propose a cross-project machine learning approach and compare its performance with a within-project alternative. The core idea is to use transfer learning to increase the overall number of smelly instances in the training datasets. Our results have shown that cross-project classification provides very similar performance with respect to within-project. Despite this finding does not yet provide a step forward in increasing the performance of ML techniques for code smell detection, it sets the basis for further investigations.