Domain-Driven Design
Les principes essentiels du modèle de domaine.

public class Order {

    private final OrderId id;
    private final CustomerId customerId;
    private OrderStatus status;
    private final List<OrderLine> lines;
    private Money totalAmount;

    public Order(OrderId id, CustomerId customerId) {
        this.id = id;
        this.customerId = customerId;
        this.status = OrderStatus.DRAFT;
        this.lines = new ArrayList<>();
        this.totalAmount = Money.ZERO;
    }

    /** Ajoute une ligne en vérifiant les invariants métier. */
    public void addLine(Product product, int quantity) {
        if (status != OrderStatus.DRAFT) {
            throw new IllegalStateException("Cannot modify a confirmed order");
        }
        var line = new OrderLine(product, quantity);
        lines.add(line);
        totalAmount = recalculateTotal();
    }

    /** Confirme la commande : vérifie qu'elle contient au moins une ligne. */
    public OrderPlaced confirm() {
        if (lines.isEmpty()) {
            throw new IllegalStateException("Cannot confirm an empty order");
        }
        this.status = OrderStatus.CONFIRMED;
        return new OrderPlaced(id, customerId, totalAmount);
    }
}

/** Objet valeur représentant un montant monétaire. */
public record Money(BigDecimal amount, Currency currency) {

    public static final Money ZERO = new Money(BigDecimal.ZERO, Currency.EUR);

    public Money {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Amount must be non-negative");
        }
        Objects.requireNonNull(currency, "Currency is required");
    }

    public Money add(Money other) {
        if (!currency.equals(other.currency)) {
            throw new IllegalArgumentException("Cannot add different currencies");
        }
        return new Money(amount.add(other.amount), currency);
    }

    public Money multiply(int quantity) {
        return new Money(amount.multiply(BigDecimal.valueOf(quantity)), currency);
    }
}

/** Identifiant typé pour les commandes. */
public record OrderId(UUID value) {

    public OrderId {
        Objects.requireNonNull(value, "OrderId value is required");
    }

    public static OrderId generate() {
        return new OrderId(UUID.randomUUID());
    }

    public static OrderId of(String raw) {
        return new OrderId(UUID.fromString(raw));
    }
}

/** Événement émis lorsqu'une commande est confirmée. */
public record OrderPlaced(
    OrderId orderId,
    CustomerId customerId,
    Money totalAmount,
    Instant occurredAt
) {
    public OrderPlaced(OrderId orderId, CustomerId customerId, Money totalAmount) {
        this(orderId, customerId, totalAmount, Instant.now());
    }
}