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();

     *
     * @return an {@link Optional} containing the map of user properties, or empty if not set
     */
    @Nonnull
    Optional<Map<String, String>> userProperties();

    /**
     * Indicates whether to show the version information and exit.
     *
     * @return an {@link Optional} containing the boolean flag, or empty if not set
     */
    @Nonnull
    Optional<Boolean> showVersionAndExit();

    /**

  // handle values up to 64kB.
  // +optional
  optional string note = 10;

  // type is the type of this event (Normal, Warning), new types could be added in the future.
  // It is machine-readable.
  // +optional
  optional string type = 11;

  // deprecatedSource is the deprecated field assuring backward compatibility with core.v1 Event type.
  // +optional
  optional k8s.io.api.core.v1.EventSource deprecatedSource = 12;

  // +k8s:conversion-gen=false
  // +optional
  optional bool hostNetwork = 11;

  // Use the host's pid namespace.
  // Optional: Default to false.
  // +k8s:conversion-gen=false
  // +optional
  optional bool hostPID = 12;

  // Use the host's ipc namespace.
  // Optional: Default to false.
  // +k8s:conversion-gen=false
  // +optional
  optional bool hostIPC = 13;

  // +optional
  optional string timeZone = 8;

  // Optional deadline in seconds for starting the job if it misses scheduled
  // time for any reason.  Missed jobs executions will be counted as failed ones.
  // +optional
  optional int64 startingDeadlineSeconds = 2;

  // Specifies how to treat concurrent executions of a Job.

message ClusterRole {
  // Standard object's metadata.
  // +optional
  optional k8s.io.apimachinery.pkg.apis.meta.v1.ObjectMeta metadata = 1;

  // Rules holds all the PolicyRules for this ClusterRole
  // +optional
  repeated PolicyRule rules = 2;

  // AggregationRule is an optional field that describes how to build the Rules for this ClusterRole.

  // +optional
  optional k8s.io.apimachinery.pkg.apis.meta.v1.ObjectMeta metadata = 1;

  // eventTime is the time when this Event was first observed. It is required.
  optional k8s.io.apimachinery.pkg.apis.meta.v1.MicroTime eventTime = 2;

  // series is data about the Event series this event represents or nil if it's a singleton Event.
  // +optional
  optional EventSeries series = 3;

message BoundObjectReference {
  // Kind of the referent. Valid kinds are 'Pod' and 'Secret'.
  // +optional
  optional string kind = 1;

  // API version of the referent.
  // +optional
  optional string apiVersion = 2;

  // Name of the referent.
  // +optional
  optional string name = 3;

  // UID of the referent.
  // +optional
  optional string uID = 4;
}

// ExtraValue masks the value so protobuf can generate

  // +optional
  optional string namespace = 3;

  // Name is the name of the object being referenced.
  // +required
  optional string name = 4;

  // UID is the uid of the object being referenced.
  // +optional
  optional string uid = 5;