*
   * @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;

      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 {

      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 {

     * directly.
     */
    ByteSource source1 = asByteSource(path1);
    ByteSource source2 = asByteSource(path2);
    long len1 = source1.sizeIfKnown().or(0L);
    long len2 = source2.sizeIfKnown().or(0L);
    if (len1 != 0 && len2 != 0 && len1 != len2) {
      return false;
    }
    return source1.contentEquals(source2);
  }

  /**

     * directly.
     */
    ByteSource source1 = asByteSource(path1);
    ByteSource source2 = asByteSource(path2);
    long len1 = source1.sizeIfKnown().or(0L);
    long len2 = source2.sizeIfKnown().or(0L);
    if (len1 != 0 && len2 != 0 && len1 != len2) {
      return false;
    }
    return source1.contentEquals(source2);
  }

  /**