Passing an entity from one method to another method located in different classes

I have an object, there are two values ​​in the object, username and email, and I have a controller class, the controller shows user.html, which has two inputs and one button, when the button is clicked, the data in the inputs is written to the data, the program works, but how can i move data writing function to UserService?

I tried to do this, but the method that I implement in UserService to implement database operation requires User data type, but the method in usersShow in UserController has String data type.

Example:

public User usersShow(User user) {
//Implementation of the database
}

The following is an example of existing code that is currently in my program.

MainController.class:

@Controller
@RequestMapping
public class MainController {

private final UserRepository userRepository;

@Autowired
public MainController(UserRepository userRepository) {
this.userRepository = userRepository;
}

@GetMapping(path = "/users")
public String usersPage(Model model) {
model.addAttribute("user", new User());
return "user";
	
}

@PostMapping(path = "/users")
public String usersShow(@ModelAttribute("user") User user) {
System.out.println(userRepository.findAll());
userRepository.save(user);
return "redirect:/user";
}

@Controller
@RequestMapping
public class MainController {

private final UserRepository userRepository;

@Autowired
public MainController(UserRepository userRepository) {
this.userRepository = userRepository;
}

@GetMapping(path = "/users")
public String usersPage(Model model) {
model.addAttribute("user", new User());
return "user";
	
}

@PostMapping(path = "/users")
public String usersShow(@ModelAttribute("user") User user) {
System.out.println(userRepository.findAll());
userRepository.save(user);
return "redirect:/user";
}

}

UserService.class (Here I want to transfer work with the database, but I don’t know how):

@Service
public class UserService {

private final UserRepository userRepository;

@Autowired
public UserService(UserRepository userRepository) {
this.userRepository = userRepository;
}

I couldn't do what I wanted to do, but I did better and I'm quite happy with it.

I remade the UserService into an interface and created a UserServiceImpl created the following methods, here is the code:

UserService.interface:

package authentication.service;

import java.util.List;
import java.util.Optional;

import authentication.entity.User;

public interface UserService {

User addUser(User user);

}

UserServiceImpl.class:

package authentication.service;

import java.util.List;
import java.util.Optional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import authentication.entity.User;
import authentication.repository.UserRepository;

@Service
public class UserServiceImpl implements UserService {

private final UserRepository userRepository;

@Autowired
public UserServiceImpl(UserRepository userRepository) {
this.userRepository = userRepository;
}

@Override
public User addUser(User user) {
User saveUser = userRepository.saveAndFlush(user);
	
return saveUser;
}

}

MainController.class:

package authentication.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;

import authentication.entity.User;
import authentication.service.UserService;

@Controller
@RequestMapping
public class MainController {

private final UserService userService;

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

@GetMapping(path = "/users")
public String usersPage(Model model) {
model.addAttribute("user", new User());
return "user";
	
}

@PostMapping(path = "/users")
public String usersShow(@ModelAttribute("user") User user) {
userService.addUser(user);
System.out.println(userService.findAll());
return "redirect:/user";
}

}