code: public String outputString(final WireFeed feed, final boolean prettyPrint) throws IllegalArgumentException, FeedException {
          final Document doc = outputJDom(feed);
          final String encoding = feed.getEncoding();
          Format format;
          if (prettyPrint) {
              format = Format.getPrettyFormat();
          } else {
              format = Format.getCompactFormat();
          }
          if (encoding != null) {
              format.setEncoding(encoding);
          }
          final XMLOutputter outputter = new XMLOutputter(format);
          return outputter.outputString(doc);
      }
docs: Creates a String with the XML representation for the given WireFeed.
  <p>
  If the feed encoding is not NULL, it will be used in the XML prolog encoding attribute. It is
  the responsibility of the developer to ensure that if the String is written to a character
  stream the stream charset is the same as the feed encoding property.
  <p>
  NOTE: This method delages to the 'Document WireFeedOutput#outputJDom(WireFeed)'.
  <p>
  
  @param feed Abstract feed to create XML representation from. The type of the WireFeed must
  match the type given to the FeedOuptut constructor.
  @param prettyPrint pretty-print XML (true) oder collapsed
  @return a String with the XML representation for the given WireFeed.
  @throws IllegalArgumentException thrown if the feed type of the WireFeedOutput and WireFeed
  don't match.
  @throws FeedException thrown if the XML representation for the feed could not be created.