How Much Can a Behavior-Preserving Changeset Be Decomposed into Refactoring Operations?

Abstract

Developers sometimes mix behavior-preserving modifications, such as refactorings, with behavior-altering modifications, such as feature additions. Several approaches have been proposed to support understanding such modifications by separating them into those two parts. Such refactoring-aware approaches are expected to be particularly effective when the behavior-preserving parts can be decomposed into a sequence of more primitive behavior-preserving operations, such as refactorings, but this has not been explored. In this paper, as an initial validation, we quantify how much of the behavior-preserving modifications can be decomposed into refactoring operations using a dataset of functionally-equivalent method pairs. As a result, when using an existing refactoring detector, only 33.9% of the changes could be identified as refactoring operations. In contrast, when including 67 newly defined functionally-equivalent operations, the coverage increased by over 128%. Further investigation into the remaining unexplained differences was conducted, suggesting improvement opportunities.