Controller -> Service -> Repository, wired

package com.example.taskflow.model;

public class Task {

    private Long id;
    private String title;
    private boolean done;

    public Task() { }                       // no-arg constructor (Jackson needs it)

    public Task(Long id, String title, boolean done) {
        this.id = id;
        this.title = title;
        this.done = done;
    }

    // getters & setters — plain accessors
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }

    public String getTitle() { return title; }
    public void setTitle(String title) { this.title = title; }

    public boolean isDone() { return done; }
    public void setDone(boolean done) { this.done = done; }
}

package com.example.taskflow.repository;

import com.example.taskflow.model.Task;
import org.springframework.stereotype.Repository;

import java.util.*;
import java.util.concurrent.atomic.AtomicLong;

@Repository
public class TaskRepository {

    private final Map<Long, Task> store = new HashMap<>();
    private final AtomicLong nextId = new AtomicLong(1);   // hands out 1, 2, 3, ...

    public List<Task> findAll() {
        return new ArrayList<>(store.values());
    }

    public Optional<Task> findById(Long id) {
        return Optional.ofNullable(store.get(id));
    }

    public Task save(Task task) {
        if (task.getId() == null) {                 // new task → assign an id
            task.setId(nextId.getAndIncrement());
        }
        store.put(task.getId(), task);              // insert or overwrite
        return task;
    }

    public void deleteById(Long id) {
        store.remove(id);
    }
}

package com.example.taskflow.service;

import com.example.taskflow.model.Task;
import com.example.taskflow.repository.TaskRepository;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class TaskService {

    private final TaskRepository repository;

    // Spring injects the TaskRepository bean here — this is DI.
    public TaskService(TaskRepository repository) {
        this.repository = repository;
    }

    public List<Task> getAll() {
        return repository.findAll();
    }

    public Task getById(Long id) {
        return repository.findById(id).orElseThrow(() -> new TaskNotFoundException(id));   // a rule: missing = error
    }

    public Task create(String title) {
        Task task = new Task(null, title, false);   // new tasks start not-done — a rule
        return repository.save(task);
    }

    public Task update(Long id, String title, boolean done) {
        Task task = getById(id);                     // reuse the "must exist" rule
        task.setTitle(title);
        task.setDone(done);
        return repository.save(task);
    }

    public void delete(Long id) {
        getById(id);                                 // 404 if it doesn't exist, then delete
        repository.deleteById(id);
    }
}

package com.example.taskflow.service;

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

@ResponseStatus(HttpStatus.NOT_FOUND)   // Spring turns this exception into an HTTP 404
public class TaskNotFoundException extends RuntimeException {
    public TaskNotFoundException(Long id) {
        super("Task not found: " + id);
    }
}

package com.example.taskflow.dto;

// The shape a client sends when creating/updating. No id — the server owns ids.
public record TaskRequest(String title, boolean done) { }

package com.example.taskflow.controller;

import com.example.taskflow.dto.TaskRequest;
import com.example.taskflow.model.Task;
import com.example.taskflow.service.TaskService;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/tasks")            // every route here starts with /tasks
public class TaskController {

    private final TaskService service;

    public TaskController(TaskService service) {   // inject the service (DI again)
        this.service = service;
    }

    @GetMapping                                    // GET /tasks
    public List<Task> all() {
        return service.getAll();
    }

    @GetMapping("/{id}")                           // GET /tasks/5
    public Task one(@PathVariable Long id) {
        return service.getById(id);
    }

    @PostMapping                                   // POST /tasks  (body = TaskRequest)
    public Task create(@RequestBody TaskRequest req) {
        return service.create(req.title());
    }

    @PutMapping("/{id}")                           // PUT /tasks/5
    public Task update(@PathVariable Long id, @RequestBody TaskRequest req) {
        return service.update(id, req.title(), req.done());
    }

    @DeleteMapping("/{id}")                        // DELETE /tasks/5
    public void delete(@PathVariable Long id) {
        service.delete(id);
    }
}