ddd-clean-architecture

com.example.app/
├── entity/              # Entities — business rules, no dependencies
├── usecase/             # Use Cases — application logic, depends only on entity
│   ├── port/            # Input/output port interfaces
│   └── interactor/      # Use case implementations
├── adapter/             # Interface Adapters — presenters, gateways
│   ├── controller/      # Web controllers
│   ├── presenter/       # Response formatting
│   └── gateway/         # Gateway implementations
└── framework/           # Frameworks — DB, web server, external APIs
    ├── web/
    └── persistence/

// Use case port (input boundary)
public interface CreateOrderUseCase {
    OrderOutput execute(CreateOrderInput input);
}

// Use case interactor
public class CreateOrderInteractor implements CreateOrderUseCase {
    private final OrderGateway orderGateway;
    private final PaymentGateway paymentGateway;

    public CreateOrderInteractor(OrderGateway orderGateway, PaymentGateway paymentGateway) {
        this.orderGateway = orderGateway;
        this.paymentGateway = paymentGateway;
    }

    @Override
    public OrderOutput execute(CreateOrderInput input) {
        Order order = Order.create(input.getItems());
        paymentGateway.charge(order.totalAmount());
        orderGateway.save(order);
        return OrderOutput.from(order);
    }
}

public interface OrderGateway {
    void save(Order order);
    Optional<Order> findById(String id);
}