assertThat(Stream.empty().collect(toOptional())).isEmpty();
  }

  public void testToOptionalSingleton() {
    assertThat(Stream.of(1).collect(toOptional())).hasValue(1);
  }

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

  public void testToOptionalMultiple() {

   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

   * iterable is empty.
   *
   * <p><b>Java 8+ users:</b> the {@code Stream} equivalent to this method is {@code
   * stream.collect(MoreCollectors.toOptional()).orElse(defaultValue)}.
   *
   * @throws IllegalArgumentException if the iterator contains multiple elements
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(

 */
@DoNotMock("Use Optional.of(value) or Optional.absent()")
@GwtCompatible(serializable = true)
@ElementTypesAreNonnullByDefault
public abstract class Optional<T> implements Serializable {
  /**
   * Returns an {@code Optional} instance with no contained reference.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
   * {@code Optional.empty}.
   */
  public static <T> Optional<T> absent() {

  // +optional
  optional k8s.io.apimachinery.pkg.apis.meta.v1.Time lastTransitionTime = 3;

  // The reason for the condition's last transition.
  // +optional
  optional string reason = 4;

  // A human readable message indicating details about the transition.
  // +optional
  optional string message = 5;
}

// DaemonSetList is a collection of daemon sets.
message DaemonSetList {

  // +optional
  optional string version = 4;

  // Resource is one of the existing resource types.  "*" means all.
  // +optional
  optional string resource = 5;

  // Subresource is one of the existing resource types.  "" means none.
  // +optional
  optional string subresource = 6;

  // Name is the name of the resource being requested for a "get" or deleted for a "delete". "" (empty) means all.
  // +optional

  // +optional
  optional k8s.io.apimachinery.pkg.apis.meta.v1.Time lastTransitionTime = 3;

  // The reason for the condition's last transition.
  // +optional
  optional string reason = 4;

  // A human readable message indicating details about the transition.
  // +optional
  optional string message = 5;
}

// DaemonSetList is a collection of daemon sets.
message DaemonSetList {
  // Standard list metadata.

  }

  public void testToJavaUtil_instance() {
    assertEquals(java.util.Optional.empty(), Optional.absent().toJavaUtil());
    assertEquals(java.util.Optional.of("abc"), Optional.of("abc").toJavaUtil());
  }

  @SuppressWarnings("NullOptional")
  public void testFromJavaUtil() {
    assertNull(Optional.fromJavaUtil(null));

  // +optional
  optional string reason = 4;

  // message is a human-readable explanation containing details about
  // the transition
  // +optional
  optional string message = 5;
}

// HorizontalPodAutoscalerList is a list of horizontal pod autoscaler objects.
message HorizontalPodAutoscalerList {
  // metadata is the standard list metadata.
  // +optional

  // one status to another
  // +optional
  optional k8s.io.apimachinery.pkg.apis.meta.v1.Time lastTransitionTime = 3;

  // reason is the reason for the condition's last transition.
  // +optional
  optional string reason = 4;

  // message is a human-readable explanation containing details about
  // the transition
  // +optional
  optional string message = 5;
}