code: protected final boolean checkForCompleteFrame ()
      {
          if (_length == -1 || _have < _length) {
              return false;
          }
  
          // prepare the buffer such that this frame can be read
          _buffer.position(HEADER_SIZE);
          _buffer.limit(_length);
          _haveCompleteFrame = true;
          return true;
      }
docs: Returns true if a complete frame is in the buffer, false otherwise.
  If a complete frame is in the buffer, the buffer will be prepared
  to deliver that frame via our {@link InputStream} interface.