code: public void bindTo(Object proxy) {
      if (handle != null) {
        throw new IllegalStateException(
            "Attempted to rebind a default method handler that was already bound");
      }
      handle = unboundHandle.bindTo(proxy);
    }
docs: Bind this handler to a proxy object. After bound, DefaultMethodHandler#invoke will act as if it
  was called on the proxy object. Must be called once and only once for a given instance of
  DefaultMethodHandler