How to create an REST API in Spring boot using Java

SpringApiApplication.java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringApiApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringApiApplication.class, args);
    }

}

User.java

public class User {

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

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

    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 int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getEmail() {
        return email;
    }

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

UserController.java

import com.example.springapi.api.model.User;
import com.example.springapi.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import java.util.Optional;

@RestController
public class UserController {

    private final UserService userService;

    @Autowired
    public UserController(UserService userService){
        this.userService = userService;
    }

    @GetMapping("/user")
    public User getUser(@RequestParam Integer id){
        Optional<User> user = userService.getUser(id);
        return (User) user.orElse(null);
    }
}

UserService.java

package com.example.springapi.service;

import com.example.springapi.api.model.User;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

@Service
public class UserService {

    private List<User> userList;

    public UserService() {
        userList = new ArrayList<>();

        User user1 = new User(1,"Ida", 32, "ida@mail.com");
        User user2 = new User(2,"Hans", 26, "hans@mail.com");
        User user3 = new User(3,"Lars", 45, "lars@mail.com");
        User user4 = new User(4,"Ben", 32, "ben@mail.com");
        User user5 = new User(5,"Eva", 59, "eva@mail.com");

        userList.addAll(Arrays.asList(user1,user2,user3,user4,user5));
    }

    public Optional<User> getUser(Integer id) {
        Optional<User> optional = Optional.empty();
        for (User user: userList) {
            if(id == user.getId()){
                optional = Optional.of(user);
                return optional;
            }
        }
        return optional;
    }
}