code: public void addObserver(OutputObserver observer, Object data)
      {
          if (_observers==null)
              _observers=new ArrayList(4);
          _observers.add(observer);
          _observers.add(data);
      }
docs: Add an Output Observer.
  Output Observers get notified of significant events on the
  output stream. Observers are called in the reverse order they
  were added.
  They are removed when the stream is closed.
  @param observer The observer.
  @param data Data to be passed wit notify calls.