protected AbstractIterator() {}

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

  private @Nullable T next;

  protected abstract @Nullable T computeNext();

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

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);

          Set<N> seenNodes = new HashSet<>();
          return new AbstractIterator<N>() {
            @Override
            protected @Nullable N computeNext() {
              while (nodeConnections.hasNext()) {
                NodeConnection<N> nodeConnection = nodeConnections.next();
                boolean added = seenNodes.add(nodeConnection.node);

  }

  @Override
  public UnmodifiableIterator<C> iterator() {
    return new AbstractSequentialIterator<C>(first()) {
      final C last = last();

      @Override
      protected @Nullable C computeNext(C previous) {
        return equalsOrThrow(previous, last) ? null : domain.next(previous);
      }
    };
  }

  @GwtIncompatible // NavigableSet
  @Override

                  .values()
                  .iterator();
        }
      }
      return new AbstractIterator<Entry<Cut<C>, Range<C>>>() {
        @Override
        protected @Nullable Entry<Cut<C>, Range<C>> computeNext() {
          if (!backingItr.hasNext()) {
            return endOfData();
          }
          Range<C> range = backingItr.next();
          if (upperBoundWindow.upperBound.isLessThan(range.upperBound)) {

   */
  private static final class Directed<N> extends EndpointPairIterator<N> {
    private Directed(BaseGraph<N> graph) {
      super(graph);
    }

    @Override
    protected @Nullable EndpointPair<N> computeNext() {
      while (true) {
        if (successorIterator.hasNext()) {
          // requireNonNull is safe because successorIterator is empty until we set this.node.

  private final Queue<T> queue;

  ConsumingQueueIterator(Queue<T> queue) {
    this.queue = checkNotNull(queue);
  }

  @Override
  protected @Nullable T computeNext() {
    // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
    if (queue.isEmpty()) {
      return endOfData();
    }
    return queue.remove();
  }

  }

  @Override
  public UnmodifiableIterator<C> iterator() {
    return new AbstractSequentialIterator<C>(first()) {
      final C last = last();

      @Override
      protected @Nullable C computeNext(C previous) {
        return equalsOrThrow(previous, last) ? null : domain.next(previous);
      }
    };
  }

  @GwtIncompatible // NavigableSet
  @Override

  private final Queue<T> queue;

  ConsumingQueueIterator(Queue<T> queue) {
    this.queue = checkNotNull(queue);
  }

  @Override
  protected @Nullable T computeNext() {
    // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
    if (queue.isEmpty()) {
      return endOfData();
    }
    return queue.remove();
  }

  @Override
  public UnmodifiableIterator<E> iterator() {
    Iterator<? extends Entry<E, ?>> entries = outEdgeToNode.entrySet().iterator();
    return new AbstractIterator<E>() {
      @Override
      protected @Nullable E computeNext() {
        while (entries.hasNext()) {
          Entry<E, ?> entry = entries.next();
          if (targetNode.equals(entry.getValue())) {
            return entry.getKey();
          }
        }

        // TODO(lowasser): consider making the entries live views
        return new AbstractIterator<Entry<E>>() {
          @Override
          protected @Nullable Entry<E> computeNext() {
            if (iterator1.hasNext()) {
              Entry<? extends E> entry1 = iterator1.next();
              E element = entry1.getElement();