code: public boolean next(K key, U value) throws IOException {
      if (hasNext()) {
        WritableUtils.cloneInto(key, khead);
        WritableUtils.cloneInto(value, vhead);
        next();
        return true;
      }
      return false;
    }
docs: Write key-value pair at the head of this stream to the objects provided;
  get next key-value pair from proxied RR.