assertThrows(NoSuchElementException.class, () -> Stream.empty().collect(onlyElement()));
  }

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

  public void testOnlyElementNull() {
    assertThat(Stream.<@Nullable Object>of((Object) null).collect(onlyElement())).isNull();
  }

  public void testOnlyElementMultiple() {

   * more elements, and a {@code NoSuchElementException} if the stream is empty.
   */
  @SuppressWarnings("unchecked")
  public static <T extends @Nullable Object> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */