IllegalArgumentException.class,
            () -> Stream.of(1, 2, 3, 4, 5, 6).collect(toOptional()));
    assertThat(expected.getMessage()).contains("1, 2, 3, 4, 5, ...");
  }

  public void testOnlyElement() {
    assertThrows(NoSuchElementException.class, () -> Stream.empty().collect(onlyElement()));
  }

  public void testOnlyElementSingleton() {
    assertThat(Stream.of(1).collect(onlyElement())).isEqualTo(1);
  }