assertEquals(0, stopwatch.elapsed(MILLISECONDS));
    ticker.advance(1);
    assertEquals(1, stopwatch.elapsed(MILLISECONDS));
  }

  @J2ktIncompatible // TODO(b/259213718): Switch J2kt to String.format("%.4g") once that's supported
  public void testToString() {
    stopwatch.start();
    assertEquals("0.000 ns", stopwatch.toString());
    ticker.advance(1);
    assertEquals("1.000 ns", stopwatch.toString());

     */
    return ref == null ? Optional.absent() : Optional.fromNullable(enumClass.cast(ref.get()));
  }

  static String formatCompact4Digits(double value) {
    return String.format(Locale.ROOT, "%.4g", value);
  }

  static boolean stringIsNullOrEmpty(@Nullable String string) {
    return string == null || string.isEmpty();
  }

  /**
   * Returns the string if it is not null, or an empty string otherwise.