}

  public void testToOptionalNull() {
    Stream<@Nullable Object> stream = Stream.of((Object) null);
    assertThrows(NullPointerException.class, () -> stream.collect(toOptional()));
  }

  public void testToOptionalMultiple() {
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> Stream.of(1, 2).collect(toOptional()));
    assertThat(expected).hasMessageThat().contains("1, 2");
  }