*
   * @throws IOException if an I/O error occurs
   * @since 15.0
   */
  public boolean isEmpty() throws IOException {
    Optional<Long> sizeIfKnown = sizeIfKnown();
    if (sizeIfKnown.isPresent()) {
      return sizeIfKnown.get() == 0L;
    }
    Closer closer = Closer.create();
    try {
      InputStream in = closer.register(openStream());
      return in.read() == -1;

    assertEquals(expected.length, source.size());
  }

  public void testSizeIfKnown() throws IOException {
    Optional<Long> sizeIfKnown = source.sizeIfKnown();
    if (sizeIfKnown.isPresent()) {
      assertEquals(expected.length, (long) sizeIfKnown.get());
    }
  }

  public void testContentEquals() throws IOException {
    assertTrue(
        source.contentEquals(

    assertEquals(expected.length, source.size());
  }

  public void testSizeIfKnown() throws IOException {
    Optional<Long> sizeIfKnown = source.sizeIfKnown();
    if (sizeIfKnown.isPresent()) {
      assertEquals(expected.length, (long) sizeIfKnown.get());
    }
  }

  public void testContentEquals() throws IOException {
    assertTrue(
        source.contentEquals(

    // come close.
    USING_DECODER_WITH_SIZE_HINT {
      @Override
      String read(ByteSource byteSource, Charset cs) throws IOException {
        Optional<Long> size = byteSource.sizeIfKnown();
        // if we know the size and it fits in an int
        if (size.isPresent() && size.get().longValue() == size.get().intValue()) {
          // otherwise try to presize a StringBuilder

    // come close.
    USING_DECODER_WITH_SIZE_HINT {
      @Override
      String read(ByteSource byteSource, Charset cs) throws IOException {
        Optional<Long> size = byteSource.sizeIfKnown();
        // if we know the size and it fits in an int
        if (size.isPresent() && size.get().longValue() == size.get().intValue()) {
          // otherwise try to presize a StringBuilder

      Path dir = fs.getPath("dir");
      Files.createDirectory(dir);

      ByteSource source = MoreFiles.asByteSource(dir);

      assertThat(source.sizeIfKnown()).isAbsent();

      assertThrows(IOException.class, () -> source.size());
    }
  }

  public void testByteSource_size_ofSymlinkToDirectory() throws IOException {

    }

    @Override
    public FileInputStream openStream() throws IOException {
      return new FileInputStream(file);
    }

    @Override
    public Optional<Long> sizeIfKnown() {
      if (file.isFile()) {
        return Optional.of(file.length());
      } else {
        return Optional.absent();
      }
    }

    @Override
    public long size() throws IOException {