}

    /**
     * Checks if a next page exists for pagination.
     *
     * @return true if a next page exists, false otherwise
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets the flag indicating whether a next page exists.
     *
     * @param existNextPage true if a next page exists, false otherwise
     */

    Iterator<Entry<E>> entryIterator = entryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public @Nullable Entry<E> lastEntry() {
    Iterator<Entry<E>> entryIterator = descendingEntryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public @Nullable Entry<E> pollFirstEntry() {

        this.existPrePage = existPrePage;
    }

    /**
     * Check if the next page exists.
     * @return true if the next page exists.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Set if the next page exists.
     * @param existNextPage true if the next page exists.
     */
    public void setExistNextPage(final boolean existNextPage) {

    }

    /**
     * Checks if there is a next page.
     * @return True if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets the existence of a next page.
     * @param existNextPage True if a next page exists.
     */
    public void setExistNextPage(final boolean existNextPage) {

        this.existPrePage = existPrePage;
    }

    /**
     * Checks if next page exists.
     *
     * @return true if next page exists
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether next page exists.
     *
     * @param existNextPage true if next page exists
     */
    public void setExistNextPage(final boolean existNextPage) {

        this.existPrePage = existPrePage;
    }

    /**
     * Checks if a next page exists.
     * @return true if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether a next page exists.
     * @param existNextPage true if a next page exists, false otherwise.
     */

    }

    /**
     * Checks if a next page exists.
     *
     * @return true if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets the flag for the existence of a next page.
     *
     * @param existNextPage true if a next page exists.
     */

    @Override
    public DfsReferralDataImpl next() {
        return this.next;
    }

    /**
     * Append another referral to this referral chain
     *
     * @param dr the referral to append
     */
    @Override
    public void append(final DfsReferralDataInternal dr) {
        final DfsReferralDataImpl dri = (DfsReferralDataImpl) dr;
        dri.next = this.next;
        this.next = dri;
    }

   */
  public abstract @Nullable C previous(C value);

  /**
   * Returns a signed value indicating how many nested invocations of {@link #next} (if positive) or
   * {@link #previous} (if negative) are needed to reach {@code end} starting from {@code start}.
   * For example, if {@code end = next(next(next(start)))}, then {@code distance(start, end) == 3}
   * and {@code distance(end, start) == -3}. As well, {@code distance(a, a)} is always zero.
   *

                return Integer.valueOf(from);
              }
            });

    Iterator<Integer> resultIterator = result.iterator();
    resultIterator.next();

    assertThrows(NumberFormatException.class, () -> resultIterator.next());
  }

  public void testNullFriendlyTransform() {
    List<@Nullable Integer> input = asList(1, 2, null, 3);
    Iterable<String> result =
        Iterables.transform(