### Question:
OpScan implements CheckstyleCommand<Map<PsiFile, List<Problem>>> { @NotNull @Override public Map<PsiFile, List<Problem>> execute(@NotNull final Project project) throws CheckstyleException { if (scannableFiles.isEmpty()) { return Collections.emptyMap(); } return processAndAudit(filesOf(scannableFiles), createListener(mapFilesToElements(scannableFiles), project)) .getProblems(); } OpScan(@NotNull final CheckstyleInternalObject checkerWithConfig,
                  @NotNull final List<ScannableFile> scannableFiles,
                  final boolean suppressErrors,
                  final int tabWidth,
                  final Optional<String> baseDir); @NotNull @Override Map<PsiFile, List<Problem>> execute(@NotNull final Project project);  }### Answer:
@Test public void testEmptyListOfFiles() throws CheckstyleException { OpScan cmd = new OpScan(new CheckerWithConfig(null, null), Collections.emptyList(), false, 2, Optional.empty()); Assert.assertEquals(Collections.emptyMap(), cmd.execute(PROJECT)); }