这将编译

 public ResponseEntity<User> getUserById(@PathVariable(value = "id") Long userId) throws UserNotFoundException {
        ResponseEntity u = userRepository.findById(userId)
                .map(p->ResponseEntity.ok(new UserResource(p)))
                .orElseThrow(() -> new UserNotFoundException(userId));
        return u;
    }

但这不会

 public ResponseEntity<User> getUserById(@PathVariable(value = "id") Long userId) throws UserNotFoundException {
        return userRepository.findById(userId)
                .map(p->ResponseEntity.ok(new UserResource(p)))
                .orElseThrow(() -> new UserNotFoundException(userId));
    }

怎么来？