String无法转换为T java lambda表达式

Question

如何解决此错误？ 我的代码是：

  public Future<Set<Client>> getAllClients(){
    Future<Set<Client>> result = executorService.submit(() -> {
        Message request = new Message(HelloService.GETALL, "");
        Message response = tcpClient.sendAndReceive(request);
        String body =response.body();
        return body;
    });
    return result;
}