code: public void addReaderListener(ReaderListener readerListener) {
          if (readerListener == null) {
              return;
          }
          synchronized (listeners) {
              if (!listeners.contains(readerListener)) {
                  listeners.add(readerListener);
              }
          }
      }
docs: Adds a reader listener to this reader that will be notified when
  new strings are read.
  
  @param readerListener a reader listener.