}

  public void testToOptionalSingleton() {
    assertThat(Stream.of(1).collect(MoreCollectors.toOptional())).hasValue(1);
  }

  public void testToOptionalNull() {
    Stream<@Nullable Object> stream = Stream.of((Object) null);
    try {
      stream.collect(MoreCollectors.toOptional());
      fail("Expected NullPointerException");
    } catch (NullPointerException expected) {