패턴 매칭 — instanceof, switch, sealed class와의 조합

// Before
if (obj instanceof String) {
    String s = (String) obj;  // 명시적 캐스팅 필요
    System.out.println(s.length());
}

// After — 캐스팅 자동
if (obj instanceof String s) {
    System.out.println(s.length()); // s가 이미 String 타입
}

// && 뒤에서 바인딩 변수 사용 가능
if (obj instanceof String s && s.length() > 5) {
    process(s);
}

// || 에서는 불가 — s가 바인딩되지 않을 수 있으므로
// if (obj instanceof String s || s.length() > 5) { } // 컴파일 에러

// Before
String describe(Object obj) {
    if (obj instanceof Integer i) return "정수: " + i;
    else if (obj instanceof String s) return "문자열: " + s;
    else if (obj instanceof List<?> l) return "리스트 크기: " + l.size();
    else return "알 수 없음";
}

// After
String describe(Object obj) {
    return switch (obj) {
        case Integer i -> "정수: " + i;
        case String s  -> "문자열: " + s;
        case List<?> l -> "리스트 크기: " + l.size();
        case null      -> "null";
        default        -> "알 수 없음";
    };
}

String classify(Object obj) {
    return switch (obj) {
        case Integer i when i < 0   -> "음수";
        case Integer i when i == 0  -> "영";
        case Integer i              -> "양수";
        case String s when s.isEmpty() -> "빈 문자열";
        case String s               -> "문자열: " + s;
        default                     -> "기타";
    };
}

record Point(int x, int y) { }

// Record 패턴 — 필드를 바로 추출
void print(Object obj) {
    if (obj instanceof Point(int x, int y)) {
        System.out.println("x=" + x + ", y=" + y);
    }
}

record Circle(double radius) { }
record Rectangle(double w, double h) { }

double area(Object shape) {
    return switch (shape) {
        case Circle(var r)      -> Math.PI * r * r;
        case Rectangle(var w, var h) -> w * h;
        default -> 0;
    };
}

record Address(String city, String street) { }
record Customer(String name, Address address) { }

String getCity(Object obj) {
    return switch (obj) {
        case Customer(var name, Address(var city, _)) -> city;
        default -> "unknown";
    };
}

sealed interface PaymentMethod
    permits CreditCard, BankTransfer, DigitalWallet { }

record CreditCard(String number, String cvv) implements PaymentMethod { }
record BankTransfer(String accountNo) implements PaymentMethod { }
record DigitalWallet(String provider) implements PaymentMethod { }

String processPayment(PaymentMethod method) {
    return switch (method) {
        case CreditCard(var number, _) -> "카드 결제: " + maskCard(number);
        case BankTransfer(var account)  -> "계좌 이체: " + account;
        case DigitalWallet(var provider) -> provider + " 결제";
        // default 불필요 — sealed class이므로 모든 경우가 커버됨
    };
}

String process(Object obj) {
    return switch (obj) {
        case null        -> "null 입력"; // null을 명시적으로 처리
        case String s    -> s.toUpperCase();
        default          -> obj.toString();
    };
}

sealed interface DomainEvent permits OrderCreated, OrderCancelled, PaymentReceived { }
record OrderCreated(Long orderId, BigDecimal amount) implements DomainEvent { }
record OrderCancelled(Long orderId, String reason) implements DomainEvent { }
record PaymentReceived(Long orderId, BigDecimal paid) implements DomainEvent { }

void handle(DomainEvent event) {
    switch (event) {
        case OrderCreated(var id, var amount) -> {
            log.info("주문 생성: {} ({}원)", id, amount);
            notifyWarehouse(id);
        }
        case OrderCancelled(var id, var reason) -> {
            log.info("주문 취소: {} ({})", id, reason);
            refundPayment(id);
        }
        case PaymentReceived(var id, var paid) -> {
            log.info("결제 수신: {} ({}원)", id, paid);
            updateOrderStatus(id);
        }
    }
}