  We propose NOPOL, an approach to automatic repair of buggy conditional
statements (i.e., if-then-else statements). This approach takes a buggy program
as well as a test suite as input and generates a patch with a conditional
expression as output. The test suite is required to contain passing test cases
to model the expected behavior of the program and at least one failing test
case that reveals the bug to be repaired. The process of NOPOL consists of
three major phases. First, NOPOL employs angelic fix localization to identify
expected values of a condition during the test execution. Second, runtime trace
collection is used to collect variables and their actual values, including
primitive data types and objected-oriented features (e.g., nullness checks), to
serve as building blocks for patch generation. Third, NOPOL encodes these
collected data into an instance of a Satisfiability Modulo Theory (SMT)
problem, then a feasible solution to the SMT instance is translated back into a
code patch. We evaluate NOPOL on 22 real-world bugs (16 bugs with buggy IF
conditions and 6 bugs with missing preconditions) on two large open-source
projects, namely Apache Commons Math and Apache Commons Lang. Empirical
analysis on these bugs shows that our approach can effectively fix bugs with
buggy IF conditions and missing preconditions. We illustrate the capabilities
and limitations of NOPOL using case studies of real bug fixes.
