Forcing failure from SimpleFileVisitor in a JUnit test

Question

I have implemented a class called InspectFiles which extends SimpleFileVisitor, for use with Files.walkFileTree .

I'm using InspectFiles to inspect the contents of files in a directory and its child directories. When one of any specific characters is detected can I fail the test from InspectFiles ? I am having a great deal of trouble doing so when InspectFiles.visitFile() is outside of the scope of the test class.

@Test
public void invalidPunctuationTest() throws IOException {
    Path startingDir = dir.toPath();
    InspectFiles inspector = new InspectFiles();

    Files.walkFileTree(startingDir, inspector);
}

and

public class InspectFiles extends SimpleFileVisitor<Path> {
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attr) throws IOException {

        try {
            File currentFile = new File(file.toString());
            InputStreamReader stream = new InputStreamReader(new FileInputStream(currentFile));
            for (int i = stream.read(); i != -1; i = stream.read()) {
                char c = (char) i;

                if (c == '’' || c == '–') {
                    throw new AssertionError();
                }
            }
        }
        catch (final IOException e) {
            e.printStackTrace();
        }
        catch (final AssertionError e) {
            System.err.println(e + " This file contains erroneous characters: " + file.toString());
            e.printStackTrace();
        }
        return FileVisitResult.CONTINUE;
    }
}

Answer 1

You can test whether the error message "This file contains erroneous characters" has been written to System.err by using the library System Rules

@Rule
public final StandardErrorStreamLog log = new StandardErrorStreamLog();

@Test
public void invalidPunctuationTest() throws IOException {
  Path startingDir = dir.toPath();
  InspectFiles inspector = new InspectFiles();
  Files.walkFileTree(startingDir, inspector);
  assertFalse(log.getLog().contains("This file contains erroneous characters"));
}