code: @Override
      public ReadStream<Buffer> handler(Handler<Buffer> handler) {
          if (handler == null) {
              throw new IllegalArgumentException("handler");
          }
          this.dataHandler = handler;
          doRead();
          return this;
      }
docs: Set a data handler. As data is read, the handler will be called with the data.
  
  @param handler the handler.
  @return a reference to this, so the API can be used fluently