}

  public void testToOptionalMultipleWithNull() {
    assertThrows(NullPointerException.class, () -> Stream.of(1, null).collect(toOptional()));
  }

  public void testToOptionalMany() {
    IllegalArgumentException expected =
        assertThrows(
            IllegalArgumentException.class,
            () -> Stream.of(1, 2, 3, 4, 5, 6).collect(toOptional()));