protected AbstractSequentialIterator(@CheckForNull T firstOrNull) {
    this.nextOrNull = firstOrNull;
  }

  /**
   * Returns the element that follows {@code previous}, or returns {@code null} if no elements
   * remain. This method is invoked during each call to {@link #next()} in order to compute the
   * result of a <i>future</i> call to {@code next()}.
   */
  @CheckForNull
  protected abstract T computeNext(T previous);

  @Override

  protected AbstractSequentialIterator(@CheckForNull T firstOrNull) {
    this.nextOrNull = firstOrNull;
  }

  /**
   * Returns the element that follows {@code previous}, or returns {@code null} if no elements
   * remain. This method is invoked during each call to {@link #next()} in order to compute the
   * result of a <i>future</i> call to {@code next()}.
   */
  @CheckForNull
  protected abstract T computeNext(T previous);

  @Override