@GetMapping("/users/{id}")
    public User retrieveUser(@PathVariable int id) {
        return service.retrieveUser(id);
    }
Copy

public User retrieveUser( int id) {

        // either use for loop to iterate through the list and find by id
        // or use functional programming :
        Predicate<? super User> predicate = user -> user.getId().equals(id);
        return users.stream().filter(predicate).findFirst().get();

    }
Copy

We'll create a usersCount variable to add the ID automatially.

UserDaoService.java

package org.example.rest.webservices.restfulwebservices.user;

import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.PathVariable;

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

@Component // since we want spring to manage it therefore we annotate the class as a component
public class UserDaoService {

    private static List<User> users = new ArrayList<>();

    private static int usersCount = 0;

    static {
        users.add(new User(++usersCount, "Adam", LocalDate.now()));
        users.add(new User(++usersCount, "Eve", LocalDate.now()));
        users.add(new User(++usersCount, "Jim", LocalDate.now()));
    }

    public List<User> findAll() {
        return users;
    }

    public User retrieveUser(int id) {

        // either use for loop to iterate through the list and find by id
        // or use functional programming :
        Predicate<? super User> predicate = user -> user.getId().equals(id);
        return users.stream().filter(predicate).findFirst().get();

    }

    public User save(User user) {
        user.setId(++usersCount);
        users.add(user);
        return user;
    }

}
Copy

@PostMapping("/users")
    public void createUser(@RequestBody User user) {
        service.save(user);
    }
Copy

public User save(User user) {
        user.setId(++usersCount);
        users.add(user);
        return user;
    }
Copy

@PostMapping("/users")
    public ResponseEntity<User> createUser(@RequestBody User user) {
         service.save(user);

        return ResponseEntity.created(null).build(); // returns a status of 201 created, .build() builds the response entity object
    }
Copy

• Now we can see that the response status is 201

Returning URI of the newly created user :

@PostMapping("/users")
    public ResponseEntity<User> createUser(@RequestBody User user) {
         service.save(user);

        // /users/4 => /users  /{id} => user.getId()
        URI location = ServletUriComponentsBuilder.fromCurrentRequest()
                .path("/{id}") // to append the id to the current request
                .buildAndExpand(user.getId()) // to replace the {id} with the actual id
                .toUri(); // to convert it to a proper URI
        return ResponseEntity.created(location).build(); // returns a status of 201 created, .build() builds the response entity object
    }
Copy

• We can see the URI here now in the location header.

public User retrieveUser(int id) {

        // either use for loop to iterate through the list and find by id
        // or use functional programming :
        Predicate<? super User> predicate = user -> user.getId().equals(id);
        return users.stream().filter(predicate).findFirst().get();
		
    }
Copy

in the code above, if there is no element of a given ID then get() throws an NoSuchElementException error. If we want to return null value when no ID exists we can replace that line with : return users.stream().filter(predicate).findFirst().orElse(null);
However this is not good as well because we are still returning something which is null and the status code will be 200. We'll have to fix that :

UserResource.java

package org.example.rest.webservices.restfulwebservices.user;

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.servlet.support.ServletUriComponentsBuilder;

import java.net.URI;
import java.util.List;

@RestController
public class UserResource {

    private UserDaoService service;

    public UserResource(UserDaoService service) {
        this.service = service;
    }

    @GetMapping("/users")
    public List<User> retrieveAllUsers() {
        return service.findAll();
    }

    @GetMapping("/users/{id}")
    public User retrieveUser(@PathVariable int id) {
        User retrievedUser = service.retrieveUser(id);
        if (retrievedUser == null) {
            throw new UserNotFoundException("id: " + id);
        }
        else {
            return retrievedUser;
        }
    }

    @PostMapping("/users")
    public ResponseEntity<User> createUser(@RequestBody User user) {
         service.save(user);

        // /users/4 => /users  /{id} => user.getId()
        URI location = ServletUriComponentsBuilder.fromCurrentRequest()
                .path("/{id}") // to append the id to the current request
                .buildAndExpand(user.getId()) // to replace the {id} with the actual id
                .toUri(); // to convert it to a proper URI
        return ResponseEntity.created(location).build(); // returns a status of 201 created, .build() builds the response entity object
    }

}
Copy

UserNotFoundException.java

package org.example.rest.webservices.restfulwebservices.user;

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ResponseStatus;

@ResponseStatus(code = HttpStatus.NOT_FOUND) // to return a status of 404 not found, by default it returns 500 internal server error
public class UserNotFoundException extends RuntimeException {
    public UserNotFoundException(String message) {
            super(message); // pass the message to the super class, which is the RuntimeException
    }
}
Copy

• We now get a proper response back.

Video hi dekh lo bhai

If for UserNotFoundException we wish to return a 400 bad request then we'll do this

@DeleteMapping("/users/{id}")
    public void deleteUser(@PathVariable int id) {
        User retrievedUser = service.retrieveUser(id);
        if (retrievedUser == null) {
            throw new UserNotFoundException("id: " + id);
        }
        else {
            service.deleteById(id);
        }
    }
Copy

public void deleteById(int id) {
        Predicate<? super  User> predicate = user -> user.getId().equals(id);
        users.removeIf(predicate);
    }
Copy

• We'll be using jakarta validation constraints for setting up constraints in the User class.
• We'll also use the following dependency to validate the constraints

<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-validation</artifactId>
		<scope>runtime</scope>
		<optional>true</optional>
</dependency>
Copy

    @Override
    protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers, HttpStatusCode status, WebRequest request) {

//        ErrorDetails errorDetails = new ErrorDetails(LocalDate.now(), ex.getMessage(), request.getDescription(false));

        ErrorDetails errorDetails = new ErrorDetails(LocalDate.now(), ex.getFieldError().getDefaultMessage(), request.getDescription(false)); // → to return the message of the validation error



        return new ResponseEntity(errorDetails, HttpStatus.BAD_REQUEST);
    }
}
Copy

• We use different URLs for different versions :