UserDetailsController.java

@RestController
@RequestMapping("/api/users")
class UserDetailsController {

    private final UserRepository userRepository;

    UserDetailsController(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @GetMapping("/{userId}")
    ResponseEntity<UserDetails> getByUserId(@PathVariable("userId") String userId) {
        return userRepository.findById(UserId.of(userId))
                             .map(toUserDetails())
                             .map(ResponseEntity::ok)
                             .orElseGet(() -> ResponseEntity.notFound().build());
    }

    private Function<User, UserDetails> toUserDetails() {
        return (user) -> new UserDetails(idOf(user), displayNameOf(user));
    }

    private String idOf(User user) {
        return user.getId().value();
    }

    private String displayNameOf(User user) {
        return user.getFirstName() + " " + user.getLastName();
    }

}