IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> Stream.of(1, 2).collect(toOptional()));
    assertThat(expected.getMessage()).contains("1, 2");
  }

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

  public void testToOptionalMany() {
    IllegalArgumentException expected =
        assertThrows(