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()}.
   */
  protected abstract @Nullable T computeNext(T previous);

  @Override
  public final boolean hasNext() {

     *
     * @param parentUrl the URL of the parent document
     * @param url the URL of the content that was not found
     */
    public ContentNotFoundException(final String parentUrl, final String url) {
        super("Not Found: " + url + " Parent: " + parentUrl, false, false);
    }

      } else {
        collectValueFromNonCancelledFuture(index, future);
      }
    } finally {
      /*
       * "null" means: There is no need to access `futures` again during
       * `processCompleted` because we're reading each value during a call to
       * handleOneInputDone.
       */
      decrementCountAndMaybeComplete(null);
    }
  }

  /**

     */
    @Size(max = 10)
    public String popularWord;

    /**
     * Character encoding to use for CSV file exports.
     * This setting affects the encoding of downloaded CSV files.
     */
    @Required
    @Size(max = 20)
    public String csvFileEncoding;

    /**
     * Number of days to keep search logs before purging.
     * Set to -1 to disable automatic purging of search logs.
     */
    @Min(-1)

        @Override
        public String toString() {
          return "healthy()";
        }
      };
  private static final ListenerCallQueue.Event<Listener> STOPPED_EVENT =
      new ListenerCallQueue.Event<Listener>() {
        @Override
        public void call(Listener listener) {
          listener.stopped();
        }

        @Override
        public String toString() {
          return "stopped()";

     * Map of query commands indexed by query class simple names.
     * Used to lookup appropriate command handlers for different query types.
     */
    protected Map<String, QueryCommand> queryCommandMap = new HashMap<>();

    /**
     * List of filters that will be applied during query processing.
     * Filters are applied in the order they are added to this list.
     */
    protected List<Filter> filterList = new ArrayList<>();

    /**

        }
        final String url = DocumentUtil.getValue(doc, fessConfig.getIndexFieldUrl(), String.class);
        if (url == null) {
            saveError(messages -> messages.addErrorsDocumentNotFound(GLOBAL, form.docId));
            return redirect(ErrorAction.class);
        }

        if (fessConfig.isSearchLog()) {
            final String userSessionId = userInfoHelper.getUserCode();

          sum += a.value;
          a.value = 0L;
        }
      }
    }
    return sum;
  }

  /**
   * Returns the String representation of the {@link #sum}.
   *
   * @return the String representation of the {@link #sum}
   */
  @Override
  public String toString() {
    return Long.toString(sum());
  }

  /**
   * Equivalent to {@link #sum}.
   *
   * @return the sum
   */

    }
  }

  /*
   * Notes on EnumSet and <E extends Enum<E>>:
   *
   * This class isn't an arbitrary ForwardingImmutableSet because we need to
   * know that calling {@code clone()} during deserialization will return an
   * object that no one else has a reference to, allowing us to guarantee
   * immutability. Hence, we support only {@link EnumSet}.
   */
  private final transient EnumSet<E> delegate;

   * entries will not be returned by the iterator. If existing entries are removed during iteration,
   * they will be absent (unless they were already returned).
   *
   * If there are I/O problems during iteration, this iterator fails silently. For example, if the
   * hosting filesystem becomes unreachable, the iterator will omit elements rather than throwing
   * exceptions.