Refactoring with Unit Testing: A Match Made in Heaven?

Unit testing is a basic principle of agile development. Its benefits include early defect detection, defect cause localization and removal of fear to apply changes to the code. As such, unit tests seem to be ideal companions during refactoring, as they provide a safety net which enables to quickly verify that behaviour is indeed preserved. In this study we investigate whether having unit tests available during refactoring actually leads to quicker refactorings and more high-quality code after refactoring. For this, we set up a two-group controlled experiment involving 42 participants. Results indicate that having unit tests available during refactoring does not lead to quicker refactoring or to higher-quality code after refactoring.