Spring CRUD with no database

User.java

package kr.hs.dgsw.user;

public class User {

    private int id;
    private String name;
    private String email;

    public User(int id, String name, String email) {
        this.id = id;
        this.name = name;
        this.email = email;
    }

    public User() {
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

UserController.java

package kr.hs.dgsw.user;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
public class UserController {

    @Autowired
    private UserService userService;

    @GetMapping("/user")
    public @ResponseBody List<User> getAll() {
        return userService.getAll();
    }

    @GetMapping("/user/{id}")
    public @ResponseBody User getOne(@PathVariable String id) {
        return userService.getOne(Integer.parseInt(id));
    }

    @PostMapping(value = "/user", consumes = "application/json")
    public boolean add(@RequestBody User user) throws Exception {
        return userService.add(user);
    }

    @PutMapping(value = "/user", consumes = "application/json")
    public boolean update(@RequestBody User user) throws Exception {
        return userService.update(user);
    }

    @DeleteMapping("/user/{id}")
    public boolean delete(@PathVariable String id) {
        return userService.delete(Integer.parseInt(id));
    }
}

UserService.java

package kr.hs.dgsw.user;

import java.util.List;

public interface UserService {
    List<User> getAll();
    User getOne(int id);
    boolean add(User user);
    boolean update(User user);
    boolean delete(int id);
}

UserServiceImpl.java

package kr.hs.dgsw.user;

import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.List;

@Service
public class UserServiceImpl implements UserService {

    List<User> users;

    public UserServiceImpl() {
        users = new ArrayList<>();
        this.setupUsers();
    }

    private void setupUsers() {
        for (int i = 1; i <= 5; i++) {
            users.add(new User(i, "user" + i, "user" + i + "@aa.co.kr"));
        }
    }

    private User findOne(int id) {
        for (User u: users) {
            if (u.getId() == id) return u;
        }
        return null;
    }

    @Override
    public List<User> getAll() {
        return users;
    }

    @Override
    public User getOne(int id) {
        return this.findOne(id);
    }

    @Override
    public boolean add(User user) {
        if (this.findOne(user.getId()) != null) return false;

        users.add(user);
        return true;
    }

    @Override
    public boolean update(User user) {
        User updateUser = this.findOne(user.getId());
        if (updateUser == null) return false;

        updateUser.setName(user.getName());
        updateUser.setEmail(user.getEmail());
        return true;
    }

    @Override
    public boolean delete(int id) {
        User user = this.findOne(id);
        if (user == null) return false;

        users.remove(user);
        return true;
    }
}