code: private void setupParse ()
      {
      if (parent == null) {
          throw new NullPointerException("No parent for filter");
      }
      parent.setEntityResolver(this);
      parent.setDTDHandler(this);
      parent.setContentHandler(this);
      parent.setErrorHandler(this);
      }
docs: Set up before a parse.
  
  <p>Before every parse, check whether the parent is
  non-null, and re-register the filter for all of the
  events.</p>