Single Responsibility Principle (SRP)

// ─── EXAMPLE 1 ──────────────────────────────────────────────────────────────
// WHAT WE ARE IMPLEMENTING:
// An order processing workflow composed of highly cohesive, single-focus
// subservices.
//
// WHERE THE PRINCIPLE FITS IN:
// The principle separates OrderValidator (validation), PricingService
// (pricing), OrderRepository (DB persistence), and NotificationService
// (notifications) instead of keeping them in a single massive OrderService.
// ────────────────────────────────────────────────────────────────────────────
// --- Model ---
class Order {
    private final String id;
    private final String userId;
    private final double amount;
    private final String couponCode;

    public Order(String id, String userId, double amount, String couponCode) {
        this.id = id;
        this.userId = userId;
        this.amount = amount;
        this.couponCode = couponCode;
    }

    public String getId() { return id; }
    public String getUserId() { return userId; }
    public double getAmount() { return amount; }
    public String getCouponCode() { return couponCode; }
}

// --- Single Responsibility 1: Validation ---
class OrderValidator {
    public void validate(Order order) {
        if (order.getAmount() <= 0) throw new IllegalArgumentException("Amount must be positive");
        if (order.getUserId() == null || order.getUserId().isBlank())
            throw new IllegalArgumentException("User ID required");
        System.out.println("  [Validator] Order " + order.getId() + " validated");
    }
}

// --- Single Responsibility 2: Pricing ---
class PricingService {
    public double applyDiscount(Order order) {
        double discount = 0;
        if (order.getCouponCode() != null && order.getCouponCode().equals("SAVE10")) {
            discount = order.getAmount() * 0.10;
        }
        double finalAmount = order.getAmount() - discount;
        System.out.println("  [Pricing] Discount: " + discount + ", Final: " + finalAmount);
        return finalAmount;
    }
}

// --- Single Responsibility 3: Persistence ---
class OrderRepository {
    public void save(Order order) {
        System.out.println("  [Repository] Order " + order.getId() + " saved to DB");
    }
}

// --- Single Responsibility 4: Notification ---
class NotificationService {
    public void sendConfirmation(String userId, String orderId) {
        System.out.println("  [Notification] Confirmation sent to user " + userId + " for order " + orderId);
    }
}

// --- Orchestrator (thin coordinator) ---
class OrderApplicationService {
    private final OrderValidator validator;
    private final PricingService pricingService;
    private final OrderRepository repository;
    private final NotificationService notificationService;

    public OrderApplicationService(OrderValidator validator, PricingService pricingService,
                                   OrderRepository repository, NotificationService notificationService) {
        this.validator = validator;
        this.pricingService = pricingService;
        this.repository = repository;
        this.notificationService = notificationService;
    }

    public void placeOrder(Order order) {
        validator.validate(order);
        pricingService.applyDiscount(order);
        repository.save(order);
        notificationService.sendConfirmation(order.getUserId(), order.getId());
    }
}

// --- Client ---
public class Main {
    public static void main(String[] args) {
        OrderValidator validator = new OrderValidator();
        PricingService pricing = new PricingService();
        OrderRepository repo = new OrderRepository();
        NotificationService notif = new NotificationService();

        OrderApplicationService app = new OrderApplicationService(validator, pricing, repo, notif);
        Order order = new Order("ORD-001", "user-42", 250.0, "SAVE10");
        app.placeOrder(order);
    }
}