# Java 21+ Style Guide for DDD

This guide covers Java conventions and modern language features for Domain-Driven Design implementations.

## Records vs Classes for Value Objects

### Use Records for Value Objects

Records are perfect for immutable value objects with validation:

```java
/**
 * Use record for simple value object.
 * Automatically generates equals, hashCode, toString.
 */
public record Money(
    java.math.BigDecimal amount,
    String currency
) {
    /**
     * Compact constructor performs validation.
     */
    public Money {
        if (amount == null) {
            throw new IllegalArgumentException("Amount cannot be null");
        }
        if (currency == null || currency.isBlank()) {
            throw new IllegalArgumentException("Currency cannot be empty");
        }
        if (amount.signum() < 0) {
            throw new IllegalArgumentException("Amount cannot be negative");
        }
        // Canonicalize to 2 decimal places
        amount = amount.setScale(2, java.math.RoundingMode.HALF_UP);
    }

    /**
     * Static factory for common currencies.
     */
    public static Money usd(long cents) {
        return new Money(
            java.math.BigDecimal.valueOf(cents).scaleByPowerOfTen(-2),
            "USD"
        );
    }

    public static Money eur(long cents) {
        return new Money(
            java.math.BigDecimal.valueOf(cents).scaleByPowerOfTen(-2),
            "EUR"
        );
    }

    /**
     * MustXxx variant for tests (panics on error).
     */
    public static Money mustUsd(String amount) {
        try {
            return usd(Long.parseLong(amount));
        } catch (NumberFormatException e) {
            throw new AssertionError("Invalid money for test: " + amount, e);
        }
    }

    public boolean isNegativeOrZero() {
        return amount.signum() <= 0;
    }

    public boolean isPositive() {
        return amount.signum() > 0;
    }

    /**
     * Domain operation: add money (must be same currency).
     */
    public Result<MoneyError, Money> add(Money other) {
        if (!currency.equals(other.currency)) {
            return Result.failure(
                new CurrencyMismatchError(currency, other.currency)
            );
        }
        return Result.success(
            new Money(amount.add(other.amount), currency)
        );
    }

    /**
     * Domain operation: multiply by factor.
     */
    public Money multiply(int factor) {
        if (factor < 0) {
            throw new IllegalArgumentException("Factor cannot be negative");
        }
        return new Money(
            amount.multiply(java.math.BigDecimal.valueOf(factor)),
            currency
        );
    }

    /**
     * Formatted display.
     */
    public String formatted() {
        return String.format("%s %s", currency, amount);
    }
}

public sealed interface MoneyError permits CurrencyMismatchError {
    String message();
}

public record CurrencyMismatchError(
    String from,
    String to
) implements MoneyError {
    @Override
    public String message() {
        return String.format("Currency mismatch: %s vs %s", from, to);
    }
}
```

### Use Classes for Aggregates/Entities

Keep mutable aggregates and entities as classes for encapsulation:

```java
/**
 * Aggregate root - use class for mutability.
 * Package-private constructor forces use of factory.
 */
public class Account {
    private final AccountId id;
    private Money balance;
    private AccountStatus status;
    private final java.time.Instant createdAt;
    private java.time.Instant updatedAt;

    /**
     * Private constructor - use factory method.
     */
    private Account(
        AccountId id,
        Money initialBalance,
        AccountStatus status,
        java.time.Instant createdAt
    ) {
        this.id = id;
        this.balance = initialBalance;
        this.status = status;
        this.createdAt = createdAt;
        this.updatedAt = createdAt;
    }

    /**
     * Factory method in aggregate.
     */
    public static Result<AccountError, Account> create(
        AccountId id,
        Money initialBalance
    ) {
        if (initialBalance.isNegative()) {
            return Result.failure(
                new InvalidBalanceError(initialBalance)
            );
        }

        Account account = new Account(
            id,
            initialBalance,
            AccountStatus.ACTIVE,
            java.time.Instant.now()
        );

        return Result.success(account);
    }

    // ✅ Getters with accessor method names (not get prefix)
    public AccountId id() { return id; }
    public Money balance() { return balance; }
    public AccountStatus status() { return status; }

    /**
     * Invariant: Cannot withdraw from closed account
     * Invariant: Cannot withdraw more than balance
     */
    public Result<AccountError, Void> withdraw(Money amount) {
        // Guard: Check status
        if (status == AccountStatus.CLOSED) {
            return Result.failure(new AccountClosedError(id));
        }

        // Guard: Check amount
        if (amount.isNegativeOrZero()) {
            return Result.failure(new InvalidAmountError(amount));
        }

        // Guard: Check balance
        if (balance.amount().compareTo(amount.amount()) < 0) {
            return Result.failure(
                new InsufficientFundsError(amount, balance)
            );
        }

        // Execute state change
        this.balance = new Money(
            balance.amount().subtract(amount.amount()),
            balance.currency()
        );
        this.updatedAt = java.time.Instant.now();

        return Result.success(null);
    }

    // Equality based on ID (entity identity)
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Account account)) return false;
        return Objects.equals(id, account.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }

    @Override
    public String toString() {
        return "Account{" +
            "id=" + id +
            ", balance=" + balance +
            ", status=" + status +
            '}';
    }
}
```

## Sealed Interfaces for Type Hierarchies

Use sealed interfaces for error types and domain concepts:

```java
/**
 * Sealed interface - only permitted implementations.
 */
public sealed interface AccountError permits
    AccountClosedError,
    InsufficientFundsError,
    InvalidAmountError,
    AccountNotFoundError {
    String message();
}

/**
 * Only this class can implement AccountError.
 */
public record AccountClosedError(AccountId id) implements AccountError {
    @Override
    public String message() {
        return "Account closed: " + id;
    }
}

// More implementations...

/**
 * Sealed with final subclasses.
 */
public sealed interface OrderStatus permits
    DraftStatus,
    PlacedStatus,
    ShippedStatus,
    DeliveredStatus,
    CancelledStatus {
    String description();
}

public final record DraftStatus() implements OrderStatus {
    @Override
    public String description() {
        return "Draft - being composed";
    }
}

public final record PlacedStatus(java.time.Instant placedAt) implements OrderStatus {
    @Override
    public String description() {
        return "Placed on " + placedAt;
    }
}
```

## Pattern Matching

### Instance Checking

```java
// ❌ Old style
public void process(OrderError error) {
    if (error instanceof InvalidAmountError) {
        InvalidAmountError e = (InvalidAmountError) error;
        System.out.println(e.amount());
    }
}

// ✅ New style - pattern matching
public void process(OrderError error) {
    if (error instanceof InvalidAmountError e) {
        System.out.println(e.amount());
    }
}
```

### Switch with Pattern Matching

```java
public String formatError(OrderError error) {
    return switch (error) {
        case InvalidAmountError e ->
            "Invalid amount: " + e.amount();
        case LineItemNotFoundError e ->
            "Line item not found: " + e.lineItemId();
        case OrderAlreadyPlacedError e ->
            "Order already placed: " + e.orderNumber();
        case OrderCancelledError e ->
            "Order cancelled: " + e.orderNumber();
        case EmptyOrderError ->
            "Cannot place order with no items";
    };
}

/**
 * Pattern matching with Result.
 */
public void handleResult(Result<OrderError, Order> result) {
    switch (result) {
        case Success(Order order) -> {
            System.out.println("Order created: " + order.orderNumber());
        }
        case Failure(InvalidAmountError e) -> {
            logger.warn("Invalid amount in order: " + e.amount());
        }
        case Failure(EmptyOrderError) -> {
            logger.warn("User attempted to place empty order");
        }
        case Failure(OrderError e) -> {
            logger.error("Unexpected order error: " + e.message());
        }
    }
}
```

### Record Pattern Matching

```java
// Record patterns (Java 21+)
public void processTransfer(Object obj) {
    if (obj instanceof Transfer(
        Money amount,
        AccountId from,
        AccountId to
    )) {
        System.out.println("Transfer " + amount + " from " + from + " to " + to);
    }
}

// In switch expressions
public String describeTransfer(Object obj) {
    return switch (obj) {
        case Transfer(Money amount, AccountId from, AccountId to) ->
            String.format("Transfer %s from %s to %s", amount, from, to);
        case Withdrawal(Money amount, AccountId account) ->
            String.format("Withdrawal %s from %s", amount, account);
        default -> "Unknown operation";
    };
}
```

## Static Imports

Use static imports for readability:

```java
// ❌ Verbose without static import
Result<AccountError, Account> account = Result.success(newAccount);

// ✅ With static imports
import static com.example.shared.result.Result.success;
import static com.example.shared.result.Result.failure;

Result<AccountError, Account> account = success(newAccount);
Result<AccountError, Void> error = failure(new AccountClosedError(id));
```

### Import Aliases

```java
// For classes with same name in different packages
import com.example.domain.account.AccountError;
import com.example.application.account.AccountError as AppAccountError;
```

## Private Constructors + Public Static Factories

### Aggregate Pattern

```java
/**
 * Aggregates use private constructor + public factory.
 * Ensures validation always happens.
 */
public class Order {
    private final OrderNumber number;
    private final CustomerId customerId;
    private final List<OrderLine> lineItems;
    private OrderStatus status;

    /**
     * Private - access only via factory.
     */
    private Order(
        OrderNumber number,
        CustomerId customerId,
        OrderStatus status
    ) {
        this.number = number;
        this.customerId = customerId;
        this.lineItems = new ArrayList<>();
        this.status = status;
    }

    /**
     * Create new order (in DRAFT status).
     */
    public static Result<OrderError, Order> create(
        OrderNumber number,
        CustomerId customerId
    ) {
        // Validation before construction
        if (number == null) {
            return Result.failure(new InvalidOrderNumberError());
        }
        if (customerId == null) {
            return Result.failure(new InvalidCustomerIdError());
        }

        Order order = new Order(number, customerId, OrderStatus.DRAFT);
        return Result.success(order);
    }

    /**
     * Reconstruct from database (internal use).
     * Skips validation since data is from trusted source.
     */
    static Order reconstruct(
        OrderNumber number,
        CustomerId customerId,
        List<OrderLine> lineItems,
        OrderStatus status
    ) {
        Order order = new Order(number, customerId, status);
        order.lineItems.addAll(lineItems);
        return order;
    }

    /**
     * For tests - panics on error.
     */
    public static Order mustCreate(String number, String customerId) {
        return create(
            new OrderNumber(number),
            new CustomerId(customerId)
        ).orElseThrow(e ->
            new AssertionError("Failed to create test order: " + e.message())
        );
    }
}
```

### Value Object Pattern

```java
/**
 * Value object - sealed to prevent subclassing.
 */
public final record Money(
    java.math.BigDecimal amount,
    String currency
) {
    /**
     * Compact constructor validates.
     */
    public Money {
        if (amount == null || currency == null) {
            throw new IllegalArgumentException("Cannot be null");
        }
        if (amount.signum() < 0) {
            throw new IllegalArgumentException("Amount cannot be negative");
        }
    }

    /**
     * Factory for USD (common case).
     */
    public static Money usd(long cents) {
        return new Money(
            java.math.BigDecimal.valueOf(cents, 2),
            "USD"
        );
    }

    /**
     * Factory for arbitrary currency.
     */
    public static Result<MoneyError, Money> of(String amount, String currency) {
        try {
            return Result.success(
                new Money(new java.math.BigDecimal(amount), currency)
            );
        } catch (NumberFormatException e) {
            return Result.failure(new InvalidMoneyFormatError(amount));
        }
    }

    /**
     * Must-variant for tests.
     */
    public static Money mustUsd(long cents) {
        return usd(cents);
    }

    public static Money mustOf(String amount, String currency) {
        return of(amount, currency)
            .orElseThrow(e ->
                new AssertionError("Test money construction failed: " + e.message())
            );
    }
}
```

## Package-Private for Entities

Hide child entities from outside aggregates:

```java
package com.example.domain.order;

/**
 * Aggregate root - public.
 */
public class Order {
    private final List<OrderLine> lineItems;

    /**
     * Public factory - creates Order and its children.
     */
    public static Result<OrderError, Order> create(...) {
        return Result.success(new Order(...));
    }

    /**
     * Package-private - only Order and other aggregate classes access this.
     */
    public Result<OrderError, Void> addLineItem(...) {
        // Internal validation
        OrderLine line = new OrderLine(...);  // Package-private constructor
        lineItems.add(line);
        return Result.success(null);
    }
}

/**
 * Entity - package-private constructor.
 * Only Order aggregate can create it.
 */
public class OrderLine {
    private final OrderLineId id;
    private final ProductId productId;
    private int quantity;

    /**
     * Package-private - only Order can use.
     */
    OrderLine(
        OrderLineId id,
        ProductId productId,
        String name,
        Money unitPrice,
        int quantity
    ) {
        this.id = id;
        this.productId = productId;
        this.quantity = quantity;
    }

    /**
     * Package-private factory - used by Order.
     */
    static OrderLine create(
        OrderLineId id,
        ProductId productId,
        String name,
        Money unitPrice,
        int quantity
    ) {
        if (quantity <= 0) {
            throw new IllegalArgumentException("Quantity must be positive");
        }
        return new OrderLine(id, productId, name, unitPrice, quantity);
    }

    // ✅ Package-private accessor (no public getter for modification)
    int getQuantity() { return quantity; }

    /**
     * Package-private mutation - only Order calls this.
     */
    void updateQuantity(int newQuantity) {
        if (newQuantity <= 0) {
            throw new IllegalArgumentException("Quantity must be positive");
        }
        this.quantity = newQuantity;
    }
}

// Outside the package - cannot access OrderLine directly
Order order = Order.create(...).success();
// ❌ OrderLine line = new OrderLine(...);  // Compile error
// ❌ order.lineItems().get(0);  // No public getter

// ✅ Access through Order aggregate only
order.addLineItem(...);
order.removeLineItem(...);
```

## Naming Conventions

| Element | Convention | Example |
|---------|-----------|---------|
| Class | PascalCase | `Account`, `Order`, `Transfer` |
| Record | PascalCase | `Money`, `OrderNumber`, `CustomerId` |
| Interface | PascalCase | `Repository`, `AccountError`, `OrderStatus` |
| Variable | camelCase | `accountId`, `initialBalance`, `orderNumber` |
| Constant | UPPER_SNAKE_CASE | `MAX_AMOUNT`, `DEFAULT_CURRENCY` |
| Method | camelCase (no get/set prefix) | `balance()`, `withdraw()`, `transfer()` |
| Enum | PascalCase values | `ACTIVE`, `DRAFT`, `PLACED` |

## Accessor Methods (Property-Style)

```java
// ✅ Preferred in DDD - property-style accessors
public class Account {
    private Money balance;

    public Money balance() {  // Property name, not getBalance()
        return balance;
    }

    public AccountId id() {  // Not getId()
        return id;
    }
}

// ❌ Avoid getter/setter naming in domain
public class Account {
    public Money getBalance() { ... }  // Too verbose for domain
    public void setBalance(Money m) { ... }  // Never use setters
}
```

## Immutability

```java
// ✅ Immutable record
public record Money(
    java.math.BigDecimal amount,
    String currency
) {
    // No setters, fields are final automatically
}

// ✅ Defensive copy for mutable collection
public class Order {
    private List<OrderLine> lineItems;

    public List<OrderLine> lineItems() {
        return List.copyOf(lineItems);  // Returns unmodifiable copy
    }

    public void addLineItem(OrderLine line) {
        lineItems.add(line);  // Internal modification only
    }
}

// ❌ Avoid direct exposure of mutable collections
public class Order {
    public List<OrderLine> getLineItems() {  // Caller could modify!
        return lineItems;
    }
}
```

## Documentation

```java
/**
 * Order aggregate root.
 *
 * Represents a customer's order with multiple line items.
 * State transitions: DRAFT -> PLACED -> SHIPPED -> DELIVERED
 * Or: DRAFT/PLACED -> CANCELLED
 *
 * Invariant: Cannot modify order after PLACED
 * Invariant: Cannot place order with zero line items
 * Invariant: Order total = sum of line item totals
 *
 * @see OrderLine
 * @see OrderRepository
 */
public class Order {

    /**
     * Creates new order in DRAFT status.
     *
     * @param number unique order identifier
     * @param customerId customer who placed the order
     * @return new Order wrapped in Result
     * @throws nothing - errors are in Result type
     */
    public static Result<OrderError, Order> create(
        OrderNumber number,
        CustomerId customerId
    ) {
        ...
    }

    /**
     * Places the order (transitions DRAFT -> PLACED).
     *
     * Invariant: Can only place DRAFT orders
     * Invariant: Order must have at least one line item
     *
     * @return success if placed, failure with reason if not
     */
    public Result<OrderError, Void> place() {
        ...
    }
}
```

## Best Practices Summary

1. **Use records** for value objects, IDs, errors
2. **Use classes** for aggregates and entities
3. **Private constructors** on aggregates, **public factories** for validation
4. **Sealed interfaces** for closed type hierarchies (errors, statuses)
5. **Pattern matching** instead of instanceof + casting
6. **Static imports** for Result.success/failure and common factories
7. **Property-style accessors** (method name = property name)
8. **No setters** in domain objects (return Result instead)
9. **Defensive copies** for mutable collections
10. **Package-private** for child entities (enforce aggregate boundaries)
11. **Final records** by default
12. **Compact constructors** for validation
13. **Document invariants** in Javadoc
14. **Use Result<E, T>** instead of throwing exceptions