return !queue.isEmpty();
    }

    @Override
    @ParametricNullness
    public T next() {
      PeekingIterator<T> nextIter = queue.remove();
      T next = nextIter.next();
      if (nextIter.hasNext()) {
        queue.add(nextIter);
      }
      return next;
    }
  }

  private static final class ConcatenatedIterator<T extends @Nullable Object>