Re-visiting a Test Taxonomy with Refactoring and Defect-fix Data

In a previous empirical study by Bavota et al., multiple releases of three open-source systems reported the extent to which refactorings induced defect-fixes. In a much earlier study, van Deursen and Moonen (vD&M) provided a test taxonomy in which Fowler's seventy-two refactorings were categorized according to the post refactoring test burden of each (i.e., the changes required to unit tests after each refactoring had been undertaken). A refactoring was categorized as 'Type B' if it required no change to the original tests and 'Type E' if significant changes were necessary. In this paper, we investigate nine refactorings spread across vD&M's taxonomy and the corresponding defect-fix data provided by Bavota et al., to explore the relationship between defect-fixes due to refactoring and vD&M's taxonomy. Results showed that, in contrast to our intuition, the most defect-fix prone refactorings were of Types C and D and not, as we thought, of Type E. The 'Extract method' refactoring stood out as particularly 'defect-fix' inducing, suggesting that while it may solve one problem (i.e., in decomposing an excessively long method), it may well introduce other problems and required defect-fixes as a by-product.