Internal Server Error on Post Request Spring Boot [Java]

I am trying to send a post request to add an User object to the list of users (no database).
getUser, getAllUsers Api endpoints are working fine and giving desired results.

When I send a Post request with User, it is always giving 500 Internal Server Error.

I have also printed out the User and data is fine.

Below is my code -

User Entity (User.java) -

public class User {

	
	private Integer userId;
	
	private String name;
	private String email;
	
	private Account account;

Getters and Setters along with constructors are also there.

My User Service and its implementation -

UserService.java -

@Service
public interface UserService {
	public User getUser(Integer userId);
	public List<User> getAllUsers();
	<T> ResponseEntity<T> deleteUser(Integer userId);
	<T> ResponseEntity<User> addUser(User user);
	
}

UserServiceImpl.java

@Service
public class UserServiceImpl implements UserService{
	List<User> userList = List.of(
			new User(1311, "Vaibhav Shrivastava", "innomightmail@gmail.com"),
			new User(1312, "Varun Shrivastava", "varun@gmail.com"),
			new User(1313, "Rajesh Shrivastava", "rajesh@gmail.com")
		);
	
	@Override
	public User getUser(Integer id) {
		// TODO Auto-generated method stub
		System.out.println(this.userList.stream().filter(user -> user.getUserId().equals(id)).findAny().orElse(null));
		return this.userList.stream().filter(user -> user.getUserId().equals(id)).findAny().orElse(null);
	}

	@Override
	public List<User> getAllUsers() {
		// TODO Auto-generated method stub
		return userList;
	}

	
	@Override
	public <T> ResponseEntity<User> addUser(User user) {
		// TODO Auto-generated method stub
		
		userList.add(user);
		return new ResponseEntity<User>((User)user, HttpStatus.OK);
		
	}

Controller (UserController.java) -

@RestController
@RequestMapping("/user")
public class UserController {
	
@Autowired
private UserService userService;

@Autowired
private RestTemplate restTemplate;

@GetMapping("/{userId}")
public User getUser(@PathVariable("userId") Integer userId) {
	User user = this.userService.getUser(userId);
	Account account = this.restTemplate.getForObject("http://account-service/account/user/" + user.getUserId(), Account.class);
	
	user.setAccount(account);
	
	return user;
	
}

@GetMapping("/all-users")
public List<User> getAllUsers(){
	return this.userService.getAllUsers();
}

@PostMapping("/add-user")
public ResponseEntity<Object> addUser(@RequestBody User user) {
	try {
		  User newUser = new User(user.getUserId(), user.getName(), user.getEmail());
	      this.userService.addUser(newUser);
	      return new ResponseEntity<>(user, HttpStatus.CREATED);
	    } catch (Exception e) {
	      User newUser = new User(user.getUserId(), user.getName(), user.getEmail());
	      return new ResponseEntity<>(newUser, HttpStatus.INTERNAL_SERVER_ERROR);
	    }
	
}

getUser and getAllUsers are working fine (dont get confused by account in getUser that is my another microservice)

When I am sending Post request using postman -

{
    "userId": 1314,
    "name": "James",
    "email": "james@gmail.com"
}

I am getting response -
500 Internal server error and User object which I wanted to check so I printed it out -

{
    "userId": 1314,
    "name": "Kavita Shrivastava",
    "email": "kavita@gmail.com",
    "account": null
}

I do not know why I am getting Internal server error.

The request should add the new User to the list in UserServiceImpl.

Stack trace has all the information you are looking for. UserServiceImple.addUser tries to add an object to the userList . but userList is an immutable list. so it throws exception. code flows to "catch" block and it returns 500 with newly created object.

In my opinion That's happens because
when U use List.of it equals to
Collections.unmodifiableList so it will throw an exception as Its Immutable.