  Well-designed and publicly available datasets of bugs are an invaluable asset
to advance research fields such as fault localization and program repair as
they allow directly and fairly comparison between competing techniques and also
the replication of experiments. These datasets need to be deeply understood by
researchers: the answer for questions like "which bugs can my technique
handle?" and "for which bugs is my technique effective?" depends on the
comprehension of properties related to bugs and their patches. However, such
properties are usually not included in the datasets, and there is still no
widely adopted methodology for characterizing bugs and patches. In this work,
we deeply study 395 patches of the Defects4J dataset. Quantitative properties
(patch size and spreading) were automatically extracted, whereas qualitative
ones (repair actions and patterns) were manually extracted using a thematic
analysis-based approach. We found that 1) the median size of Defects4J patches
is four lines, and almost 30% of the patches contain only addition of lines; 2)
92% of the patches change only one file, and 38% has no spreading at all; 3)
the top-3 most applied repair actions are addition of method calls,
conditionals, and assignments, occurring in 77% of the patches; and 4) nine
repair patterns were found for 95% of the patches, where the most prevalent,
appearing in 43% of the patches, is on conditional blocks. These results are
useful for researchers to perform advanced analysis on their techniques'
results based on Defects4J. Moreover, our set of properties can be used to
characterize and compare different bug datasets.
