}

  private Absent() {}

  @Override
  public boolean isPresent() {
    return false;
  }

  @Override
  public T get() {
    throw new IllegalStateException("Optional.get() cannot be called on an absent value");
  }

  @Override
  public T or(T defaultValue) {
    return checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)");
  }

    return forward.iterator();
  }

  @Override
  @GwtIncompatible // NavigableSet
  ImmutableSortedSet<E> createDescendingSet() {
    throw new AssertionError("should never be called");
  }

  @Override
  public @Nullable E lower(E element) {
    return forward.higher(element);
  }

  @Override
  public @Nullable E floor(E element) {
    return forward.ceiling(element);
  }

  virtual absl::Status InitializeLocalOnlyContext(const ServerDef& server_def,
                                                  int keep_alive_secs) = 0;

  // Set up a multi-client distributed execution environment. Must be called
  // on all tasks in the cluster. This call internally coordinates with other
  // tasks to initialize the eager context and TF server for multi-client
  // execution.

    if (op_) {
      return errors::FailedPrecondition("Reset called on already built op.");
    }
    if (raw_device_name) {
      device_name_ = raw_device_name;
    }
    op_type_ = op;
    return absl::OkStatus();
  }
  absl::Status SetOpName(const char* const op_name) override {
    if (op_) {
      return errors::FailedPrecondition(
          "SetOpName called on already built op.");
    }
    if (op_type_.empty()) {

 * resulting in notification could have occurred to an entry being manually removed or replaced, or
 * due to eviction resulting from timed expiration, exceeding a maximum size, or garbage collection.
 *
 * <p>An instance may be called concurrently by multiple threads to process different entries.
 * Implementations of this interface should avoid performing blocking calls or synchronizing on
 * shared resources.
 *

   * tags are computed.
   */
  fun <T> tag(type: Class<out T>): T?

  /**
   * Returns the tag attached with [type] as a key. If it is absent, then [computeIfAbsent] is
   * called and that value is both inserted and returned.
   *
   * If multiple calls to this function are made concurrently with the same [type], multiple values

    private boolean expectCall = false;
    private final CountDownLatch calledCountDown = new CountDownLatch(1);

    @Override
    public void run() {
      assertTrue("Listener called before it was expected", expectCall);
      assertFalse("Listener called more than once", wasCalled());
      calledCountDown.countDown();
    }

    void expectCall() {
      assertFalse("expectCall is already true", expectCall);
      expectCall = true;

    @Nullable ImmutableMapEntry<K, V>[] keyTable = createEntryArray(tableSize);
    @Nullable ImmutableMapEntry<K, V>[] valueTable = createEntryArray(tableSize);
    /*
     * The cast is safe: n==entryArray.length means that we have filled the whole array with Entry
     * instances, in which case it is safe to cast it from an array of nullable entries to an array
     * of non-null entries.
     */
    @SuppressWarnings("nullness")
    Entry<K, V>[] entries =

 * family. <b>Avoid</b> creating a mock or stub {@code Future}. Mock and stub implementations are
 * fragile because they assume that only certain methods will be called and because they often
 * implement subtleties of the API improperly.
 *
 * <p><b>Custom implementation</b>: Avoid implementing {@code ListenableFuture} from scratch. If you

// current row and stores the result.
//
// On success, it returns (nil, nil).
func (e *FuncExpr) evalAggregationNode(r Record, tableAlias string) error {
	// It is assumed that this function is called only when
	// `e` is an aggregation function.

	var val *Value
	var err error
	funcName := e.getFunctionName()
	if aggFnCount == funcName {
		if e.Count.StarArg {
			// Handle COUNT(*)