StackOverflow sample for file traversal and a few tests

FileUtils.java

public final class FileUtils {

  //Private constructor

  static List<File> fetchSongs(File rootDir){
    final File[] files = rootDir.listFiles();
    if(isEmpty(files))
      return Collections.emptyList();
    return fetchSongs(files);
  }

  private static List<File> fetchSongs(File[] songs) {
    ArrayList<File> out = new ArrayList<>();
    for(File f: songs)
      addSongsIfNotHidden(f, out);
    return out;
  }

  private static void addSongsIfNotHidden(File file, List<File> out) {
    if(!isHidden(file))
      addIfFileIsDirectoryOrMp3(file, out);
  }

  private static void addIfFileIsDirectoryOrMp3(File f, List<File> out) {
    if(f.isDirectory())
      out.addAll(fetchSongs(f));
    else if(hasMp3FileType(f))
      out.add(f);
  }

  private static boolean hasMp3FileType(File f) {
    return f.getName().endsWith(".mp3");
  }

  private static boolean isHidden(File f) {
    return f.isHidden() || f.getName().startsWith(".");
  }

  private static boolean isEmpty(Object[] arr) {
    return arr == null || arr.length == 0;
  }
  
}

FileUtilsTest.java

public class FileUtilsTest {

  @Rule
  public final TemporaryFolder tmp = new TemporaryFolder();

  @Test
  public void whenDirectoryIsEmpty_thenReturnEmptySongsList() {
    Assertions.assertThat(getSongs()).isEmpty();
  }

  @Test
  public void whenDirectoryContainsSingleMp3_thenReturnSongsListForSingleMp3() throws IOException {
    final File song = tmp.newFile("song.mp3");
    Assertions.assertThat(getSongs()).hasSize(1).containsOnly(song);
  }

  @Test
  public void whenDirectoryContainsSingleMp3_andJpgFile_thenReturnSongsListForSingleMp3Only() throws IOException {
    final File song = tmp.newFile("song.mp3");
    final File img = tmp.newFile("image.jpg");
    Assertions.assertThat(getSongs()).hasSize(1).containsOnly(song).doesNotContain(img);
  }

  @Test
  public void whenDirectoryContainsNestedDirectoriesWithMp3s_thenReturnSongsListForAllNestedMp3s() throws IOException {
    tmp.newFolder("spotify");
    tmp.newFolder("spotify", "favourites");
    final File song1 = tmp.newFile("song1.mp3");
    final File song2 = tmp.newFile("spotify/song2.mp3");
    final File song3 = tmp.newFile("spotify/favourites/song3.mp3");
    Assertions.assertThat(getSongs()).hasSize(3).containsOnly(song1, song2, song3);
  }

  private List<File> getSongs() {
    return fetchSongs(tmp.getRoot());
  }
  
}