protected AbstractIterator() {}

  private enum State {
    READY,
    NOT_READY,
    DONE,
    FAILED,
  }

  @CheckForNull private T next;

  @CheckForNull
  protected abstract T computeNext();

  @CanIgnoreReturnValue
  @CheckForNull
  protected final T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {

   * @return the replacement string, or {@code null} if no escaping was needed
   */
  @CheckForNull
  public static String computeReplacement(UnicodeEscaper escaper, int cp) {
    return stringOrNull(escaper.escape(cp));
  }

  @CheckForNull
  private static String stringOrNull(@CheckForNull char[] in) {
    return (in == null) ? null : new String(in);
  }

    implements Serializable {
  final Ordering<? super T> ordering;

  NullsFirstOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override
  public int compare(@CheckForNull T left, @CheckForNull T right) {
    if (left == right) {
      return 0;
    }
    if (left == null) {
      return RIGHT_IS_GREATER;
    }
    if (right == null) {
      return LEFT_IS_GREATER;
    }

    implements Serializable {
  final Ordering<? super T> ordering;

  NullsLastOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override
  public int compare(@CheckForNull T left, @CheckForNull T right) {
    if (left == right) {
      return 0;
    }
    if (left == null) {
      return LEFT_IS_GREATER;
    }
    if (right == null) {
      return RIGHT_IS_GREATER;
    }

   * if there is no such edge.
   */
  @CheckForNull
  V value(N node);

  /** Remove {@code node} from the set of predecessors. */
  void removePredecessor(N node);

  /**
   * Remove {@code node} from the set of successors. Returns the value previously associated with
   * the edge connecting the two nodes.
   */
  @CanIgnoreReturnValue
  @CheckForNull
  V removeSuccessor(N node);

  /**

    implements Serializable {
  final Ordering<? super T> ordering;

  NullsLastOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override
  public int compare(@CheckForNull T left, @CheckForNull T right) {
    if (left == right) {
      return 0;
    }
    if (left == null) {
      return LEFT_IS_GREATER;
    }
    if (right == null) {
      return RIGHT_IS_GREATER;
    }

  }

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

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

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

  @Override
  @CheckForNull
  public E higher(E element) {

    }

    // TODO(lowasser): consider an optimized values() implementation

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return keyIndex.containsKey(key);
    }

    @CheckForNull
    @Override
    public V get(@CheckForNull Object key) {
      Integer index = keyIndex.get(key);
      if (index == null) {
        return null;
      } else {

        header);
  }

  private static final class Reference<T> {
    @CheckForNull private T value;

    @CheckForNull
    public T get() {
      return value;
    }

    public void checkAndSet(@CheckForNull T expected, @CheckForNull T newValue) {
      if (value != expected) {
        throw new ConcurrentModificationException();
      }
      value = newValue;

    int nextSegmentIndex;
    int nextTableIndex;
    @CheckForNull Segment<K, V> currentSegment;
    @CheckForNull AtomicReferenceArray<ReferenceEntry<K, V>> currentTable;
    @CheckForNull ReferenceEntry<K, V> nextEntry;
    @CheckForNull WriteThroughEntry nextExternal;
    @CheckForNull WriteThroughEntry lastReturned;

    HashIterator() {
      nextSegmentIndex = segments.length - 1;