{"title":"Exercise: Law of Demeter","description":null,"content":"Try implementing the below code exercises to reinforce your understanding. Your goal is to implement the classes below so that they produce the expected output.\n\n---\n\n# Exercise 1: OrderSummary Printer\n\n\n**Problem:** You have an `OrderSummaryPrinter` that uses train wrecks to access nested data. Refactor it so the printer only calls methods on the `Order` object directly.\n\nThe current code accesses:\n\n- `order.getCustomer().getAddress().getCity()` for the customer's city\n- `order.getPayment().getCreditCard().getLast4Digits()` for the last 4 digits of the card\n\n**Requirements:**\n\n- Add delegation methods to `Order` so the printer never reaches past it\n- The delegation methods should have clear, descriptive names\n- Handle null/empty cases gracefully\n\n```java\nclass Address {\n private String city;\n\n public Address(String city) { this.city = city; }\n\n public String getCity() { return city; }\n}\n\nclass Customer {\n private Address address;\n\n public Customer(Address address) { this.address = address; }\n\n public Address getAddress() { return address; }\n}\n\nclass CreditCard {\n private String last4Digits;\n\n public CreditCard(String last4Digits) { this.last4Digits = last4Digits; }\n\n public String getLast4Digits() { return last4Digits; }\n}\n\nclass Payment {\n private CreditCard creditCard;\n\n public Payment(CreditCard creditCard) { this.creditCard = creditCard; }\n\n public CreditCard getCreditCard() { return creditCard; }\n}\n\nclass Order {\n private Customer customer;\n private Payment payment;\n\n public Order(Customer customer, Payment payment) {\n this.customer = customer;\n this.payment = payment;\n }\n\n // TODO: Add delegation methods\n\n public Customer getCustomer() { return customer; }\n public Payment getPayment() { return payment; }\n}\n\nclass OrderSummaryPrinter {\n public void print(Order order) {\n // CURRENT (violates LoD):\n // String city = order.getCustomer().getAddress().getCity();\n // String last4 = order.getPayment().getCreditCard().getLast4Digits();\n\n // TODO: Refactor to use delegation methods on Order\n String city = \"\"; // replace with order.getCustomerCity()\n String last4 = \"\"; // replace with order.getPaymentLast4Digits()\n\n System.out.println(\"Ship to: \" + city);\n System.out.println(\"Paid with card ending in: \" + last4);\n }\n}\n\nclass Main {\n public static void main(String[] args) {\n Address address = new Address(\"San Francisco\");\n Customer customer = new Customer(address);\n CreditCard card = new CreditCard(\"4242\");\n Payment payment = new Payment(card);\n Order order = new Order(customer, payment);\n\n OrderSummaryPrinter printer = new OrderSummaryPrinter();\n printer.print(order);\n }\n}\n```\n\n```python\nclass Address:\n def __init__(self, city):\n self._city = city\n\n def get_city(self):\n return self._city\n\nclass Customer:\n def __init__(self, address):\n self._address = address\n\n def get_address(self):\n return self._address\n\nclass CreditCard:\n def __init__(self, last_4_digits):\n self._last_4_digits = last_4_digits\n\n def get_last_4_digits(self):\n return self._last_4_digits\n\nclass Payment:\n def __init__(self, credit_card):\n self._credit_card = credit_card\n\n def get_credit_card(self):\n return self._credit_card\n\nclass Order:\n def __init__(self, customer, payment):\n self._customer = customer\n self._payment = payment\n\n # TODO: Add delegation methods\n\n def get_customer(self):\n return self._customer\n\n def get_payment(self):\n return self._payment\n\nclass OrderSummaryPrinter:\n def print_summary(self, order):\n # CURRENT (violates LoD):\n # city = order.get_customer().get_address().get_city()\n # last4 = order.get_payment().get_credit_card().get_last_4_digits()\n\n # TODO: Refactor to use delegation methods on Order\n city = \"\" # replace with order.get_customer_city()\n last4 = \"\" # replace with order.get_payment_last_4_digits()\n\n print(f\"Ship to: {city}\")\n print(f\"Paid with card ending in: {last4}\")\n\nif __name__ == \"__main__\":\n address = Address(\"San Francisco\")\n customer = Customer(address)\n card = CreditCard(\"4242\")\n payment = Payment(card)\n order = Order(customer, payment)\n\n printer = OrderSummaryPrinter()\n printer.print_summary(order)\n```\n\n```cpp\n#include \n#include \nusing namespace std;\n\nclass Address {\n string city;\npublic:\n Address(string city) : city(city) {}\n\n string getCity() const { return city; }\n};\n\nclass Customer {\n Address address;\npublic:\n Customer(Address address) : address(address) {}\n\n Address getAddress() const { return address; }\n};\n\nclass CreditCard {\n string last4Digits;\npublic:\n CreditCard(string last4Digits) : last4Digits(last4Digits) {}\n\n string getLast4Digits() const { return last4Digits; }\n};\n\nclass Payment {\n CreditCard creditCard;\npublic:\n Payment(CreditCard creditCard) : creditCard(creditCard) {}\n\n CreditCard getCreditCard() const { return creditCard; }\n};\n\nclass Order {\n Customer customer;\n Payment payment;\npublic:\n Order(Customer customer, Payment payment)\n : customer(customer), payment(payment) {}\n\n // TODO: Add delegation methods\n\n Customer getCustomer() const { return customer; }\n Payment getPayment() const { return payment; }\n};\n\nclass OrderSummaryPrinter {\npublic:\n void print(const Order& order) {\n // CURRENT (violates LoD):\n // string city = order.getCustomer().getAddress().getCity();\n // string last4 = order.getPayment().getCreditCard().getLast4Digits();\n\n // TODO: Refactor to use delegation methods on Order\n string city = \"\"; // replace with order.getCustomerCity()\n string last4 = \"\"; // replace with order.getPaymentLast4Digits()\n\n cout << \"Ship to: \" << city << endl;\n cout << \"Paid with card ending in: \" << last4 << endl;\n }\n};\n\nint main() {\n Address address(\"San Francisco\");\n Customer customer(address);\n CreditCard card(\"4242\");\n Payment payment(card);\n Order order(customer, payment);\n\n OrderSummaryPrinter printer;\n printer.print(order);\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Address struct {\n\tcity string\n}\n\nfunc NewAddress(city string) *Address {\n\treturn &Address{city: city}\n}\n\nfunc (a *Address) GetCity() string {\n\treturn a.city\n}\n\ntype Customer struct {\n\taddress *Address\n}\n\nfunc NewCustomer(address *Address) *Customer {\n\treturn &Customer{address: address}\n}\n\nfunc (c *Customer) GetAddress() *Address {\n\treturn c.address\n}\n\ntype CreditCard struct {\n\tlast4Digits string\n}\n\nfunc NewCreditCard(last4Digits string) *CreditCard {\n\treturn &CreditCard{last4Digits: last4Digits}\n}\n\nfunc (c *CreditCard) GetLast4Digits() string {\n\treturn c.last4Digits\n}\n\ntype Payment struct {\n\tcreditCard *CreditCard\n}\n\nfunc NewPayment(creditCard *CreditCard) *Payment {\n\treturn &Payment{creditCard: creditCard}\n}\n\nfunc (p *Payment) GetCreditCard() *CreditCard {\n\treturn p.creditCard\n}\n\ntype Order struct {\n\tcustomer *Customer\n\tpayment *Payment\n}\n\nfunc NewOrder(customer *Customer, payment *Payment) *Order {\n\treturn &Order{\n\t\tcustomer: customer,\n\t\tpayment: payment,\n\t}\n}\n\n// TODO: Add delegation methods\n\nfunc (o *Order) GetCustomer() *Customer {\n\treturn o.customer\n}\n\nfunc (o *Order) GetPayment() *Payment {\n\treturn o.payment\n}\n\ntype OrderSummaryPrinter struct{}\n\nfunc NewOrderSummaryPrinter() *OrderSummaryPrinter {\n\treturn &OrderSummaryPrinter{}\n}\n\nfunc (p *OrderSummaryPrinter) Print(order *Order) {\n\t// CURRENT (violates LoD):\n\t// city := order.GetCustomer().GetAddress().GetCity()\n\t// last4 := order.GetPayment().GetCreditCard().GetLast4Digits()\n\n\t// TODO: Refactor to use delegation methods on Order\n\tcity := \"\" // replace with order.GetCustomerCity()\n\tlast4 := \"\" // replace with order.GetPaymentLast4Digits()\n\n\tfmt.Println(\"Ship to: \" + city)\n\tfmt.Println(\"Paid with card ending in: \" + last4)\n}\n\nfunc main() {\n\taddress := NewAddress(\"San Francisco\")\n\tcustomer := NewCustomer(address)\n\tcard := NewCreditCard(\"4242\")\n\tpayment := NewPayment(card)\n\torder := NewOrder(customer, payment)\n\n\tprinter := NewOrderSummaryPrinter()\n\tprinter.Print(order)\n}\n```\n\n```csharp\nusing System;\n\nclass Address\n{\n private string city;\n\n public Address(string city) { this.city = city; }\n\n public string GetCity() { return city; }\n}\n\nclass Customer\n{\n private Address address;\n\n public Customer(Address address) { this.address = address; }\n\n public Address GetAddress() { return address; }\n}\n\nclass CreditCard\n{\n private string last4Digits;\n\n public CreditCard(string last4Digits) { this.last4Digits = last4Digits; }\n\n public string GetLast4Digits() { return last4Digits; }\n}\n\nclass Payment\n{\n private CreditCard creditCard;\n\n public Payment(CreditCard creditCard) { this.creditCard = creditCard; }\n\n public CreditCard GetCreditCard() { return creditCard; }\n}\n\nclass Order\n{\n private Customer customer;\n private Payment payment;\n\n public Order(Customer customer, Payment payment)\n {\n this.customer = customer;\n this.payment = payment;\n }\n\n // TODO: Add delegation methods\n\n public Customer GetCustomer() { return customer; }\n public Payment GetPayment() { return payment; }\n}\n\nclass OrderSummaryPrinter\n{\n public void Print(Order order)\n {\n // CURRENT (violates LoD):\n // string city = order.GetCustomer().GetAddress().GetCity();\n // string last4 = order.GetPayment().GetCreditCard().GetLast4Digits();\n\n // TODO: Refactor to use delegation methods on Order\n string city = \"\"; // replace with order.GetCustomerCity()\n string last4 = \"\"; // replace with order.GetPaymentLast4Digits()\n\n Console.WriteLine(\"Ship to: \" + city);\n Console.WriteLine(\"Paid with card ending in: \" + last4);\n }\n}\n\nclass Program\n{\n static void Main(string[] args)\n {\n Address address = new Address(\"San Francisco\");\n Customer customer = new Customer(address);\n CreditCard card = new CreditCard(\"4242\");\n Payment payment = new Payment(card);\n Order order = new Order(customer, payment);\n\n OrderSummaryPrinter printer = new OrderSummaryPrinter();\n printer.Print(order);\n }\n}\n```\n\n```typescript\nclass Address {\n private city: string;\n\n constructor(city: string) { this.city = city; }\n\n getCity(): string { return this.city; }\n}\n\nclass Customer {\n private address: Address;\n\n constructor(address: Address) { this.address = address; }\n\n getAddress(): Address { return this.address; }\n}\n\nclass CreditCard {\n private last4Digits: string;\n\n constructor(last4Digits: string) { this.last4Digits = last4Digits; }\n\n getLast4Digits(): string { return this.last4Digits; }\n}\n\nclass Payment {\n private creditCard: CreditCard;\n\n constructor(creditCard: CreditCard) { this.creditCard = creditCard; }\n\n getCreditCard(): CreditCard { return this.creditCard; }\n}\n\nclass Order {\n private customer: Customer;\n private payment: Payment;\n\n constructor(customer: Customer, payment: Payment) {\n this.customer = customer;\n this.payment = payment;\n }\n\n // TODO: Add delegation methods\n\n getCustomer(): Customer { return this.customer; }\n getPayment(): Payment { return this.payment; }\n}\n\nclass OrderSummaryPrinter {\n print(order: Order): void {\n // CURRENT (violates LoD):\n // const city = order.getCustomer().getAddress().getCity();\n // const last4 = order.getPayment().getCreditCard().getLast4Digits();\n\n // TODO: Refactor to use delegation methods on Order\n const city = \"\"; // replace with order.getCustomerCity()\n const last4 = \"\"; // replace with order.getPaymentLast4Digits()\n\n console.log(`Ship to: ${city}`);\n console.log(`Paid with card ending in: ${last4}`);\n }\n}\n\n// --- Main ---\nconst address = new Address(\"San Francisco\");\nconst customer = new Customer(address);\nconst card = new CreditCard(\"4242\");\nconst payment = new Payment(card);\nconst order = new Order(customer, payment);\n\nconst printer = new OrderSummaryPrinter();\nprinter.print(order);\n```\n\n#### Solutions\n\n```java\nclass Address {\n private String city;\n\n public Address(String city) { this.city = city; }\n\n public String getCity() { return city; }\n}\n\nclass Customer {\n private Address address;\n\n public Customer(Address address) { this.address = address; }\n\n public Address getAddress() { return address; }\n\n public String getCity() {\n if (address == null) return \"Unknown\";\n return address.getCity();\n }\n}\n\nclass CreditCard {\n private String last4Digits;\n\n public CreditCard(String last4Digits) { this.last4Digits = last4Digits; }\n\n public String getLast4Digits() { return last4Digits; }\n}\n\nclass Payment {\n private CreditCard creditCard;\n\n public Payment(CreditCard creditCard) { this.creditCard = creditCard; }\n\n public CreditCard getCreditCard() { return creditCard; }\n\n public String getLast4Digits() {\n if (creditCard == null) return \"Unknown\";\n return creditCard.getLast4Digits();\n }\n}\n\nclass Order {\n private Customer customer;\n private Payment payment;\n\n public Order(Customer customer, Payment payment) {\n this.customer = customer;\n this.payment = payment;\n }\n\n public String getCustomerCity() {\n if (customer == null) return \"Unknown\";\n return customer.getCity();\n }\n\n public String getPaymentLast4Digits() {\n if (payment == null) return \"Unknown\";\n return payment.getLast4Digits();\n }\n\n public Customer getCustomer() { return customer; }\n public Payment getPayment() { return payment; }\n}\n\nclass OrderSummaryPrinter {\n public void print(Order order) {\n String city = order.getCustomerCity();\n String last4 = order.getPaymentLast4Digits();\n\n System.out.println(\"Ship to: \" + city);\n System.out.println(\"Paid with card ending in: \" + last4);\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n Address address = new Address(\"San Francisco\");\n Customer customer = new Customer(address);\n CreditCard card = new CreditCard(\"4242\");\n Payment payment = new Payment(card);\n Order order = new Order(customer, payment);\n\n OrderSummaryPrinter printer = new OrderSummaryPrinter();\n printer.print(order);\n }\n}\n```\n\n```python\nclass Address:\n def __init__(self, city):\n self._city = city\n\n def get_city(self):\n return self._city\n\nclass Customer:\n def __init__(self, address):\n self._address = address\n\n def get_address(self):\n return self._address\n\n def get_city(self):\n if self._address is None:\n return \"Unknown\"\n return self._address.get_city()\n\nclass CreditCard:\n def __init__(self, last_4_digits):\n self._last_4_digits = last_4_digits\n\n def get_last_4_digits(self):\n return self._last_4_digits\n\nclass Payment:\n def __init__(self, credit_card):\n self._credit_card = credit_card\n\n def get_credit_card(self):\n return self._credit_card\n\n def get_last_4_digits(self):\n if self._credit_card is None:\n return \"Unknown\"\n return self._credit_card.get_last_4_digits()\n\nclass Order:\n def __init__(self, customer, payment):\n self._customer = customer\n self._payment = payment\n\n def get_customer_city(self):\n if self._customer is None:\n return \"Unknown\"\n return self._customer.get_city()\n\n def get_payment_last_4_digits(self):\n if self._payment is None:\n return \"Unknown\"\n return self._payment.get_last_4_digits()\n\n def get_customer(self):\n return self._customer\n\n def get_payment(self):\n return self._payment\n\nclass OrderSummaryPrinter:\n def print_summary(self, order):\n city = order.get_customer_city()\n last4 = order.get_payment_last_4_digits()\n\n print(f\"Ship to: {city}\")\n print(f\"Paid with card ending in: {last4}\")\n\nif __name__ == \"__main__\":\n address = Address(\"San Francisco\")\n customer = Customer(address)\n card = CreditCard(\"4242\")\n payment = Payment(card)\n order = Order(customer, payment)\n\n printer = OrderSummaryPrinter()\n printer.print_summary(order)\n```\n\n```cpp\n#include \n#include \nusing namespace std;\n\nclass Address {\n string city;\npublic:\n Address(string city) : city(city) {}\n\n string getCity() const { return city; }\n};\n\nclass Customer {\n Address address;\npublic:\n Customer(Address address) : address(address) {}\n\n Address getAddress() const { return address; }\n\n string getCity() const {\n return address.getCity();\n }\n};\n\nclass CreditCard {\n string last4Digits;\npublic:\n CreditCard(string last4Digits) : last4Digits(last4Digits) {}\n\n string getLast4Digits() const { return last4Digits; }\n};\n\nclass Payment {\n CreditCard creditCard;\npublic:\n Payment(CreditCard creditCard) : creditCard(creditCard) {}\n\n CreditCard getCreditCard() const { return creditCard; }\n\n string getLast4Digits() const {\n return creditCard.getLast4Digits();\n }\n};\n\nclass Order {\n Customer customer;\n Payment payment;\npublic:\n Order(Customer customer, Payment payment)\n : customer(customer), payment(payment) {}\n\n string getCustomerCity() const {\n return customer.getCity();\n }\n\n string getPaymentLast4Digits() const {\n return payment.getLast4Digits();\n }\n\n Customer getCustomer() const { return customer; }\n Payment getPayment() const { return payment; }\n};\n\nclass OrderSummaryPrinter {\npublic:\n void print(const Order& order) {\n string city = order.getCustomerCity();\n string last4 = order.getPaymentLast4Digits();\n\n cout << \"Ship to: \" << city << endl;\n cout << \"Paid with card ending in: \" << last4 << endl;\n }\n};\n\nint main() {\n Address address(\"San Francisco\");\n Customer customer(address);\n CreditCard card(\"4242\");\n Payment payment(card);\n Order order(customer, payment);\n\n OrderSummaryPrinter printer;\n printer.print(order);\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Address struct {\n\tcity string\n}\n\nfunc NewAddress(city string) *Address {\n\treturn &Address{city: city}\n}\n\nfunc (a *Address) GetCity() string {\n\treturn a.city\n}\n\ntype Customer struct {\n\taddress *Address\n}\n\nfunc NewCustomer(address *Address) *Customer {\n\treturn &Customer{address: address}\n}\n\nfunc (c *Customer) GetAddress() *Address {\n\treturn c.address\n}\n\nfunc (c *Customer) GetCity() string {\n\tif c.address == nil {\n\t\treturn \"Unknown\"\n\t}\n\treturn c.address.GetCity()\n}\n\ntype CreditCard struct {\n\tlast4Digits string\n}\n\nfunc NewCreditCard(last4Digits string) *CreditCard {\n\treturn &CreditCard{last4Digits: last4Digits}\n}\n\nfunc (c *CreditCard) GetLast4Digits() string {\n\treturn c.last4Digits\n}\n\ntype Payment struct {\n\tcreditCard *CreditCard\n}\n\nfunc NewPayment(creditCard *CreditCard) *Payment {\n\treturn &Payment{creditCard: creditCard}\n}\n\nfunc (p *Payment) GetCreditCard() *CreditCard {\n\treturn p.creditCard\n}\n\nfunc (p *Payment) GetLast4Digits() string {\n\tif p.creditCard == nil {\n\t\treturn \"Unknown\"\n\t}\n\treturn p.creditCard.GetLast4Digits()\n}\n\ntype Order struct {\n\tcustomer *Customer\n\tpayment *Payment\n}\n\nfunc NewOrder(customer *Customer, payment *Payment) *Order {\n\treturn &Order{customer: customer, payment: payment}\n}\n\nfunc (o *Order) GetCustomerCity() string {\n\tif o.customer == nil {\n\t\treturn \"Unknown\"\n\t}\n\treturn o.customer.GetCity()\n}\n\nfunc (o *Order) GetPaymentLast4Digits() string {\n\tif o.payment == nil {\n\t\treturn \"Unknown\"\n\t}\n\treturn o.payment.GetLast4Digits()\n}\n\nfunc (o *Order) GetCustomer() *Customer {\n\treturn o.customer\n}\n\nfunc (o *Order) GetPayment() *Payment {\n\treturn o.payment\n}\n\ntype OrderSummaryPrinter struct{}\n\nfunc NewOrderSummaryPrinter() *OrderSummaryPrinter {\n\treturn &OrderSummaryPrinter{}\n}\n\nfunc (p *OrderSummaryPrinter) Print(order *Order) {\n\tcity := order.GetCustomerCity()\n\tlast4 := order.GetPaymentLast4Digits()\n\n\tfmt.Println(\"Ship to: \" + city)\n\tfmt.Println(\"Paid with card ending in: \" + last4)\n}\n\nfunc main() {\n\taddress := NewAddress(\"San Francisco\")\n\tcustomer := NewCustomer(address)\n\tcard := NewCreditCard(\"4242\")\n\tpayment := NewPayment(card)\n\torder := NewOrder(customer, payment)\n\n\tprinter := NewOrderSummaryPrinter()\n\tprinter.Print(order)\n}\n```\n\n```csharp\nusing System;\n\nclass Address\n{\n private string city;\n\n public Address(string city) { this.city = city; }\n\n public string GetCity() { return city; }\n}\n\nclass Customer\n{\n private Address address;\n\n public Customer(Address address) { this.address = address; }\n\n public Address GetAddress() { return address; }\n\n public string GetCity()\n {\n if (address == null) return \"Unknown\";\n return address.GetCity();\n }\n}\n\nclass CreditCard\n{\n private string last4Digits;\n\n public CreditCard(string last4Digits) { this.last4Digits = last4Digits; }\n\n public string GetLast4Digits() { return last4Digits; }\n}\n\nclass Payment\n{\n private CreditCard creditCard;\n\n public Payment(CreditCard creditCard) { this.creditCard = creditCard; }\n\n public CreditCard GetCreditCard() { return creditCard; }\n\n public string GetLast4Digits()\n {\n if (creditCard == null) return \"Unknown\";\n return creditCard.GetLast4Digits();\n }\n}\n\nclass Order\n{\n private Customer customer;\n private Payment payment;\n\n public Order(Customer customer, Payment payment)\n {\n this.customer = customer;\n this.payment = payment;\n }\n\n public string GetCustomerCity()\n {\n if (customer == null) return \"Unknown\";\n return customer.GetCity();\n }\n\n public string GetPaymentLast4Digits()\n {\n if (payment == null) return \"Unknown\";\n return payment.GetLast4Digits();\n }\n\n public Customer GetCustomer() { return customer; }\n public Payment GetPayment() { return payment; }\n}\n\nclass OrderSummaryPrinter\n{\n public void Print(Order order)\n {\n string city = order.GetCustomerCity();\n string last4 = order.GetPaymentLast4Digits();\n\n Console.WriteLine(\"Ship to: \" + city);\n Console.WriteLine(\"Paid with card ending in: \" + last4);\n }\n}\n\nclass Program\n{\n static void Main(string[] args)\n {\n Address address = new Address(\"San Francisco\");\n Customer customer = new Customer(address);\n CreditCard card = new CreditCard(\"4242\");\n Payment payment = new Payment(card);\n Order order = new Order(customer, payment);\n\n OrderSummaryPrinter printer = new OrderSummaryPrinter();\n printer.Print(order);\n }\n}\n```\n\n```typescript\nclass Address {\n private city: string;\n\n constructor(city: string) { this.city = city; }\n\n getCity(): string { return this.city; }\n}\n\nclass Customer {\n private address: Address | null;\n\n constructor(address: Address) { this.address = address; }\n\n getAddress(): Address | null { return this.address; }\n\n getCity(): string {\n if (!this.address) return \"Unknown\";\n return this.address.getCity();\n }\n}\n\nclass CreditCard {\n private last4Digits: string;\n\n constructor(last4Digits: string) { this.last4Digits = last4Digits; }\n\n getLast4Digits(): string { return this.last4Digits; }\n}\n\nclass Payment {\n private creditCard: CreditCard | null;\n\n constructor(creditCard: CreditCard) { this.creditCard = creditCard; }\n\n getCreditCard(): CreditCard | null { return this.creditCard; }\n\n getLast4Digits(): string {\n if (!this.creditCard) return \"Unknown\";\n return this.creditCard.getLast4Digits();\n }\n}\n\nclass Order {\n private customer: Customer | null;\n private payment: Payment | null;\n\n constructor(customer: Customer, payment: Payment) {\n this.customer = customer;\n this.payment = payment;\n }\n\n getCustomerCity(): string {\n if (!this.customer) return \"Unknown\";\n return this.customer.getCity();\n }\n\n getPaymentLast4Digits(): string {\n if (!this.payment) return \"Unknown\";\n return this.payment.getLast4Digits();\n }\n\n getCustomer(): Customer | null { return this.customer; }\n getPayment(): Payment | null { return this.payment; }\n}\n\nclass OrderSummaryPrinter {\n print(order: Order): void {\n const city = order.getCustomerCity();\n const last4 = order.getPaymentLast4Digits();\n\n console.log(`Ship to: ${city}`);\n console.log(`Paid with card ending in: ${last4}`);\n }\n}\n\n// --- Main ---\nconst address = new Address(\"San Francisco\");\nconst customer = new Customer(address);\nconst card = new CreditCard(\"4242\");\nconst payment = new Payment(card);\nconst order = new Order(customer, payment);\n\nconst printer = new OrderSummaryPrinter();\nprinter.print(order);\n```\n\n\n---\n\n# Exercise 2: Library Checkout System\n\n\n**Problem:** Build a checkout system where a `CheckoutService` only talks to a `Library` object, never reaching into shelves, sections, or books directly.\n\n**Requirements:**\n\n- The `Library` contains `Sections` (Fiction, Non-Fiction, etc.)\n- Each `Section` contains a list of `Books`\n- Each `Book` has a title, author, and availability status\n- `CheckoutService` should be able to check out a book by title, but it should only call methods on `Library`\n- `Library` handles all the internal navigation\n\n```java\nimport java.util.List;\nimport java.util.ArrayList;\n\nclass Book {\n private String title;\n private String author;\n private boolean available;\n\n public Book(String title, String author) {\n this.title = title;\n this.author = author;\n this.available = true;\n }\n\n public String getTitle() { return title; }\n public String getAuthor() { return author; }\n public boolean isAvailable() { return available; }\n public void markCheckedOut() { this.available = false; }\n}\n\nclass Section {\n private String name;\n private List books;\n\n public Section(String name) {\n this.name = name;\n this.books = new ArrayList<>();\n }\n\n public void addBook(Book book) { books.add(book); }\n\n // TODO: Add a checkoutBook(String title) method that returns boolean\n // - Search through this section's books for a matching title\n // - If found AND available, mark it as checked out and return true\n // - If not found or already checked out, return false\n}\n\nclass Library {\n private List

sections;\n\n public Library() {\n this.sections = new ArrayList<>();\n }\n\n public void addSection(Section section) { sections.add(section); }\n\n // TODO: Add a checkoutBook(String title) method that returns boolean\n // - Iterate through all sections and delegate to each section's checkoutBook()\n // - Return true as soon as any section successfully checks out the book\n // - Return false if no section has the book or it's unavailable\n // - CheckoutService should only need to call this single method\n}\n\nclass CheckoutService {\n private Library library;\n\n public CheckoutService(Library library) {\n this.library = library;\n }\n\n public boolean checkout(String bookTitle) {\n // TODO: Replace with a single call to library's checkoutBook() method\n // This method should be one line - no direct access to sections or books\n return false;\n }\n}\n\nclass Main {\n public static void main(String[] args) {\n Library library = new Library();\n\n Section fiction = new Section(\"Fiction\");\n fiction.addBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.addBook(new Book(\"1984\", \"George Orwell\"));\n library.addSection(fiction);\n\n Section nonFiction = new Section(\"Non-Fiction\");\n nonFiction.addBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\n library.addSection(nonFiction);\n\n CheckoutService service = new CheckoutService(library);\n\n boolean result1 = service.checkout(\"The Great Gatsby\");\n System.out.println(\"Checking out \\\"The Great Gatsby\\\"... \" + (result1 ? \"Success!\" : \"Failed (not found)\"));\n\n boolean result2 = service.checkout(\"The Great Gatsby\");\n System.out.println(\"Checking out \\\"The Great Gatsby\\\"... \" + (result2 ? \"Success!\" : \"Failed (already checked out)\"));\n\n boolean result3 = service.checkout(\"Unknown Book\");\n System.out.println(\"Checking out \\\"Unknown Book\\\"... \" + (result3 ? \"Success!\" : \"Failed (not found)\"));\n }\n}\n```\n\n```python\nclass Book:\n def __init__(self, title, author):\n self.title = title\n self.author = author\n self.available = True\n\n def mark_checked_out(self):\n self.available = False\n\nclass Section:\n def __init__(self, name):\n self.name = name\n self._books = []\n\n def add_book(self, book):\n self._books.append(book)\n\n # TODO: Add a checkout_book(title) method that returns bool\n # - Search through this section's books for a matching title\n # - If found AND available, mark it as checked out and return True\n # - If not found or already checked out, return False\n\nclass Library:\n def __init__(self):\n self._sections = []\n\n def add_section(self, section):\n self._sections.append(section)\n\n # TODO: Add a checkout_book(title) method that returns bool\n # - Iterate through all sections and delegate to each section's checkout_book()\n # - Return True as soon as any section successfully checks out the book\n # - Return False if no section has the book or it's unavailable\n # - CheckoutService should only need to call this single method\n\nclass CheckoutService:\n def __init__(self, library):\n self._library = library\n\n def checkout(self, book_title):\n # TODO: Replace with a single call to library's checkout_book() method\n # This method should be one line - no direct access to sections or books\n return False\n\nif __name__ == \"__main__\":\n library = Library()\n\n fiction = Section(\"Fiction\")\n fiction.add_book(Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"))\n fiction.add_book(Book(\"1984\", \"George Orwell\"))\n library.add_section(fiction)\n\n non_fiction = Section(\"Non-Fiction\")\n non_fiction.add_book(Book(\"Clean Code\", \"Robert C. Martin\"))\n library.add_section(non_fiction)\n\n service = CheckoutService(library)\n\n result1 = service.checkout(\"The Great Gatsby\")\n print(f'Checking out \"The Great Gatsby\"... {\"Success!\" if result1 else \"Failed (not found)\"}')\n\n result2 = service.checkout(\"The Great Gatsby\")\n print(f'Checking out \"The Great Gatsby\"... {\"Success!\" if result2 else \"Failed (already checked out)\"}')\n\n result3 = service.checkout(\"Unknown Book\")\n print(f'Checking out \"Unknown Book\"... {\"Success!\" if result3 else \"Failed (not found)\"}')\n```\n\n```cpp\n#include \n#include \n#include \nusing namespace std;\n\nclass Book {\n string title;\n string author;\n bool available;\npublic:\n Book(string title, string author)\n : title(title), author(author), available(true) {}\n\n string getTitle() const { return title; }\n string getAuthor() const { return author; }\n bool isAvailable() const { return available; }\n void markCheckedOut() { available = false; }\n};\n\nclass Section {\n string name;\n vector books;\npublic:\n Section(string name) : name(name) {}\n\n void addBook(const Book& book) { books.push_back(book); }\n\n // TODO: Add a checkoutBook(const string& title) method that returns bool\n // - Search through this section's books for a matching title\n // - If found AND available, mark it as checked out and return true\n // - If not found or already checked out, return false\n};\n\nclass Library {\n vector

sections;\npublic:\n void addSection(const Section& section) { sections.push_back(section); }\n\n // TODO: Add a checkoutBook(const string& title) method that returns bool\n // - Iterate through all sections and delegate to each section's checkoutBook()\n // - Return true as soon as any section successfully checks out the book\n // - Return false if no section has the book or it's unavailable\n // - CheckoutService should only need to call this single method\n};\n\nclass CheckoutService {\n Library* library;\npublic:\n CheckoutService(Library* library) : library(library) {}\n\n bool checkout(const string& bookTitle) {\n // TODO: Replace with a single call to library's checkoutBook() method\n // This method should be one line - no direct access to sections or books\n return false;\n }\n};\n\nint main() {\n Library library;\n\n Section fiction(\"Fiction\");\n fiction.addBook(Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.addBook(Book(\"1984\", \"George Orwell\"));\n library.addSection(fiction);\n\n Section nonFiction(\"Non-Fiction\");\n nonFiction.addBook(Book(\"Clean Code\", \"Robert C. Martin\"));\n library.addSection(nonFiction);\n\n CheckoutService service(&library);\n\n bool result1 = service.checkout(\"The Great Gatsby\");\n cout << \"Checking out \\\"The Great Gatsby\\\"... \" << (result1 ? \"Success!\" : \"Failed (not found)\") << endl;\n\n bool result2 = service.checkout(\"The Great Gatsby\");\n cout << \"Checking out \\\"The Great Gatsby\\\"... \" << (result2 ? \"Success!\" : \"Failed (already checked out)\") << endl;\n\n bool result3 = service.checkout(\"Unknown Book\");\n cout << \"Checking out \\\"Unknown Book\\\"... \" << (result3 ? \"Success!\" : \"Failed (not found)\") << endl;\n\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Book struct {\n\ttitle string\n\tauthor string\n\tavailable bool\n}\n\nfunc NewBook(title string, author string) *Book {\n\treturn &Book{\n\t\ttitle: title,\n\t\tauthor: author,\n\t\tavailable: true,\n\t}\n}\n\nfunc (b *Book) GetTitle() string {\n\treturn b.title\n}\n\nfunc (b *Book) GetAuthor() string {\n\treturn b.author\n}\n\nfunc (b *Book) IsAvailable() bool {\n\treturn b.available\n}\n\nfunc (b *Book) MarkCheckedOut() {\n\tb.available = false\n}\n\ntype Section struct {\n\tname string\n\tbooks []*Book\n}\n\nfunc NewSection(name string) *Section {\n\treturn &Section{\n\t\tname: name,\n\t\tbooks: []*Book{},\n\t}\n}\n\nfunc (s *Section) AddBook(book *Book) {\n\ts.books = append(s.books, book)\n}\n\n// TODO: Add a checkoutBook(title string) method that returns bool\n// - Search through this section's books for a matching title\n// - If found AND available, mark it as checked out and return true\n// - If not found or already checked out, return false\nfunc (s *Section) CheckoutBook(title string) bool {\n\t// TODO\n\treturn false\n}\n\ntype Library struct {\n\tsections []*Section\n}\n\nfunc NewLibrary() *Library {\n\treturn &Library{\n\t\tsections: []*Section{},\n\t}\n}\n\nfunc (l *Library) AddSection(section *Section) {\n\tl.sections = append(l.sections, section)\n}\n\n// TODO: Add a checkoutBook(title string) method that returns bool\n// - Iterate through all sections and delegate to each section's checkoutBook()\n// - Return true as soon as any section successfully checks out the book\n// - Return false if no section has the book or it's unavailable\n// - CheckoutService should only need to call this single method\nfunc (l *Library) CheckoutBook(title string) bool {\n\t// TODO\n\treturn false\n}\n\ntype CheckoutService struct {\n\tlibrary *Library\n}\n\nfunc NewCheckoutService(library *Library) *CheckoutService {\n\treturn &CheckoutService{\n\t\tlibrary: library,\n\t}\n}\n\nfunc (c *CheckoutService) Checkout(bookTitle string) bool {\n\t// TODO: Replace with a single call to library's checkoutBook() method\n\t// This method should be one line - no direct access to sections or books\n\treturn false\n}\n\nfunc main() {\n\tlibrary := NewLibrary()\n\n\tfiction := NewSection(\"Fiction\")\n\tfiction.AddBook(NewBook(\"The Great Gatsby\", \"F. Scott Fitzgerald\"))\n\tfiction.AddBook(NewBook(\"1984\", \"George Orwell\"))\n\tlibrary.AddSection(fiction)\n\n\tnonFiction := NewSection(\"Non-Fiction\")\n\tnonFiction.AddBook(NewBook(\"Clean Code\", \"Robert C. Martin\"))\n\tlibrary.AddSection(nonFiction)\n\n\tservice := NewCheckoutService(library)\n\n\tresult1 := service.Checkout(\"The Great Gatsby\")\n\tfmt.Printf(\"Checking out %q... %s\\n\", \"The Great Gatsby\", ternary(result1, \"Success!\", \"Failed (not found)\"))\n\n\tresult2 := service.Checkout(\"The Great Gatsby\")\n\tfmt.Printf(\"Checking out %q... %s\\n\", \"The Great Gatsby\", ternary(result2, \"Success!\", \"Failed (already checked out)\"))\n\n\tresult3 := service.Checkout(\"Unknown Book\")\n\tfmt.Printf(\"Checking out %q... %s\\n\", \"Unknown Book\", ternary(result3, \"Success!\", \"Failed (not found)\"))\n}\n\nfunc ternary(condition bool, trueValue string, falseValue string) string {\n\tif condition {\n\t\treturn trueValue\n\t}\n\treturn falseValue\n}\n```\n\n```csharp\nusing System;\nusing System.Collections.Generic;\n\nclass Book\n{\n public string Title { get; }\n public string Author { get; }\n public bool Available { get; private set; }\n\n public Book(string title, string author)\n {\n Title = title;\n Author = author;\n Available = true;\n }\n\n public void MarkCheckedOut() { Available = false; }\n}\n\nclass Section\n{\n public string Name { get; }\n private List books;\n\n public Section(string name)\n {\n Name = name;\n books = new List();\n }\n\n public void AddBook(Book book) { books.Add(book); }\n\n // TODO: Add a CheckoutBook(string title) method that returns bool\n // - Search through this section's books for a matching title\n // - If found AND available, mark it as checked out and return true\n // - If not found or already checked out, return false\n}\n\nclass Library\n{\n private List

sections;\n\n public Library()\n {\n sections = new List

();\n }\n\n public void AddSection(Section section) { sections.Add(section); }\n\n // TODO: Add a CheckoutBook(string title) method that returns bool\n // - Iterate through all sections and delegate to each section's CheckoutBook()\n // - Return true as soon as any section successfully checks out the book\n // - Return false if no section has the book or it's unavailable\n // - CheckoutService should only need to call this single method\n}\n\nclass CheckoutService\n{\n private Library library;\n\n public CheckoutService(Library library)\n {\n this.library = library;\n }\n\n public bool Checkout(string bookTitle)\n {\n // TODO: Replace with a single call to library's CheckoutBook() method\n // This method should be one line - no direct access to sections or books\n return false;\n }\n}\n\nclass Program\n{\n static void Main(string[] args)\n {\n Library library = new Library();\n\n Section fiction = new Section(\"Fiction\");\n fiction.AddBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.AddBook(new Book(\"1984\", \"George Orwell\"));\n library.AddSection(fiction);\n\n Section nonFiction = new Section(\"Non-Fiction\");\n nonFiction.AddBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\n library.AddSection(nonFiction);\n\n CheckoutService service = new CheckoutService(library);\n\n bool result1 = service.Checkout(\"The Great Gatsby\");\n Console.WriteLine($\"Checking out \\\"The Great Gatsby\\\"... {(result1 ? \"Success!\" : \"Failed (not found)\")}\");\n\n bool result2 = service.Checkout(\"The Great Gatsby\");\n Console.WriteLine($\"Checking out \\\"The Great Gatsby\\\"... {(result2 ? \"Success!\" : \"Failed (already checked out)\")}\");\n\n bool result3 = service.Checkout(\"Unknown Book\");\n Console.WriteLine($\"Checking out \\\"Unknown Book\\\"... {(result3 ? \"Success!\" : \"Failed (not found)\")}\");\n }\n}\n```\n\n```typescript\nclass Book {\n private title: string;\n private author: string;\n private available: boolean;\n\n constructor(title: string, author: string) {\n this.title = title;\n this.author = author;\n this.available = true;\n }\n\n getTitle(): string { return this.title; }\n getAuthor(): string { return this.author; }\n isAvailable(): boolean { return this.available; }\n markCheckedOut(): void { this.available = false; }\n}\n\nclass Section {\n private name: string;\n private books: Book[];\n\n constructor(name: string) {\n this.name = name;\n this.books = [];\n }\n\n addBook(book: Book): void { this.books.push(book); }\n\n // TODO: Add a checkoutBook(title: string) method that returns boolean\n // - Search through this section's books for a matching title\n // - If found AND available, mark it as checked out and return true\n // - If not found or already checked out, return false\n}\n\nclass Library {\n private sections: Section[];\n\n constructor() {\n this.sections = [];\n }\n\n addSection(section: Section): void { this.sections.push(section); }\n\n // TODO: Add a checkoutBook(title: string) method that returns boolean\n // - Iterate through all sections and delegate to each section's checkoutBook()\n // - Return true as soon as any section successfully checks out the book\n // - Return false if no section has the book or it's unavailable\n // - CheckoutService should only need to call this single method\n}\n\nclass CheckoutService {\n private library: Library;\n\n constructor(library: Library) {\n this.library = library;\n }\n\n checkout(bookTitle: string): boolean {\n // TODO: Replace with a single call to library's checkoutBook() method\n // This method should be one line - no direct access to sections or books\n return false;\n }\n}\n\n// --- Main ---\nconst library = new Library();\n\nconst fiction = new Section(\"Fiction\");\nfiction.addBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\nfiction.addBook(new Book(\"1984\", \"George Orwell\"));\nlibrary.addSection(fiction);\n\nconst nonFiction = new Section(\"Non-Fiction\");\nnonFiction.addBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\nlibrary.addSection(nonFiction);\n\nconst service = new CheckoutService(library);\n\nconst result1 = service.checkout(\"The Great Gatsby\");\nconsole.log(`Checking out \"The Great Gatsby\"... ${result1 ? \"Success!\" : \"Failed (not found)\"}`);\n\nconst result2 = service.checkout(\"The Great Gatsby\");\nconsole.log(`Checking out \"The Great Gatsby\"... ${result2 ? \"Success!\" : \"Failed (already checked out)\"}`);\n\nconst result3 = service.checkout(\"Unknown Book\");\nconsole.log(`Checking out \"Unknown Book\"... ${result3 ? \"Success!\" : \"Failed (not found)\"}`);\n```\n\n#### Solutions\n\n```java\nimport java.util.List;\nimport java.util.ArrayList;\n\nclass Book {\n private String title;\n private String author;\n private boolean available;\n\n public Book(String title, String author) {\n this.title = title;\n this.author = author;\n this.available = true;\n }\n\n public String getTitle() { return title; }\n public String getAuthor() { return author; }\n public boolean isAvailable() { return available; }\n public void markCheckedOut() { this.available = false; }\n}\n\nclass Section {\n private String name;\n private List books;\n\n public Section(String name) {\n this.name = name;\n this.books = new ArrayList<>();\n }\n\n public void addBook(Book book) { books.add(book); }\n\n public boolean checkoutBook(String title) {\n for (Book book : books) {\n if (book.getTitle().equals(title)) {\n if (book.isAvailable()) {\n book.markCheckedOut();\n return true;\n }\n return false;\n }\n }\n return false;\n }\n}\n\nclass Library {\n private List

sections;\n\n public Library() {\n this.sections = new ArrayList<>();\n }\n\n public void addSection(Section section) { sections.add(section); }\n\n public boolean checkoutBook(String title) {\n for (Section section : sections) {\n if (section.checkoutBook(title)) {\n return true;\n }\n }\n return false;\n }\n}\n\nclass CheckoutService {\n private Library library;\n\n public CheckoutService(Library library) {\n this.library = library;\n }\n\n public boolean checkout(String bookTitle) {\n return library.checkoutBook(bookTitle);\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n Library library = new Library();\n\n Section fiction = new Section(\"Fiction\");\n fiction.addBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.addBook(new Book(\"1984\", \"George Orwell\"));\n library.addSection(fiction);\n\n Section nonFiction = new Section(\"Non-Fiction\");\n nonFiction.addBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\n library.addSection(nonFiction);\n\n CheckoutService service = new CheckoutService(library);\n\n boolean result1 = service.checkout(\"The Great Gatsby\");\n System.out.println(\"Checking out \\\"The Great Gatsby\\\"... \" + (result1 ? \"Success!\" : \"Failed (not found)\"));\n\n boolean result2 = service.checkout(\"The Great Gatsby\");\n System.out.println(\"Checking out \\\"The Great Gatsby\\\"... \" + (result2 ? \"Success!\" : \"Failed (already checked out)\"));\n\n boolean result3 = service.checkout(\"Unknown Book\");\n System.out.println(\"Checking out \\\"Unknown Book\\\"... \" + (result3 ? \"Success!\" : \"Failed (not found)\"));\n }\n}\n```\n\n```python\nclass Book:\n def __init__(self, title, author):\n self.title = title\n self.author = author\n self.available = True\n\n def mark_checked_out(self):\n self.available = False\n\nclass Section:\n def __init__(self, name):\n self.name = name\n self._books = []\n\n def add_book(self, book):\n self._books.append(book)\n\n def checkout_book(self, title):\n for book in self._books:\n if book.title == title:\n if book.available:\n book.mark_checked_out()\n return True\n return False\n return False\n\nclass Library:\n def __init__(self):\n self._sections = []\n\n def add_section(self, section):\n self._sections.append(section)\n\n def checkout_book(self, title):\n for section in self._sections:\n if section.checkout_book(title):\n return True\n return False\n\nclass CheckoutService:\n def __init__(self, library):\n self._library = library\n\n def checkout(self, book_title):\n return self._library.checkout_book(book_title)\n\nif __name__ == \"__main__\":\n library = Library()\n\n fiction = Section(\"Fiction\")\n fiction.add_book(Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"))\n fiction.add_book(Book(\"1984\", \"George Orwell\"))\n library.add_section(fiction)\n\n non_fiction = Section(\"Non-Fiction\")\n non_fiction.add_book(Book(\"Clean Code\", \"Robert C. Martin\"))\n library.add_section(non_fiction)\n\n service = CheckoutService(library)\n\n result1 = service.checkout(\"The Great Gatsby\")\n print(f'Checking out \"The Great Gatsby\"... {\"Success!\" if result1 else \"Failed (not found)\"}')\n\n result2 = service.checkout(\"The Great Gatsby\")\n print(f'Checking out \"The Great Gatsby\"... {\"Success!\" if result2 else \"Failed (already checked out)\"}')\n\n result3 = service.checkout(\"Unknown Book\")\n print(f'Checking out \"Unknown Book\"... {\"Success!\" if result3 else \"Failed (not found)\"}')\n```\n\n```cpp\n#include \n#include \n#include \nusing namespace std;\n\nclass Book {\n string title;\n string author;\n bool available;\npublic:\n Book(string title, string author)\n : title(title), author(author), available(true) {}\n\n string getTitle() const { return title; }\n string getAuthor() const { return author; }\n bool isAvailable() const { return available; }\n void markCheckedOut() { available = false; }\n};\n\nclass Section {\n string name;\n vector books;\npublic:\n Section(string name) : name(name) {}\n\n void addBook(const Book& book) { books.push_back(book); }\n\n bool checkoutBook(const string& title) {\n for (auto& book : books) {\n if (book.getTitle() == title) {\n if (book.isAvailable()) {\n book.markCheckedOut();\n return true;\n }\n return false;\n }\n }\n return false;\n }\n};\n\nclass Library {\n vector

sections;\npublic:\n void addSection(const Section& section) { sections.push_back(section); }\n\n bool checkoutBook(const string& title) {\n for (auto& section : sections) {\n if (section.checkoutBook(title)) {\n return true;\n }\n }\n return false;\n }\n};\n\nclass CheckoutService {\n Library* library;\npublic:\n CheckoutService(Library* library) : library(library) {}\n\n bool checkout(const string& bookTitle) {\n return library->checkoutBook(bookTitle);\n }\n};\n\nint main() {\n Library library;\n\n Section fiction(\"Fiction\");\n fiction.addBook(Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.addBook(Book(\"1984\", \"George Orwell\"));\n library.addSection(fiction);\n\n Section nonFiction(\"Non-Fiction\");\n nonFiction.addBook(Book(\"Clean Code\", \"Robert C. Martin\"));\n library.addSection(nonFiction);\n\n CheckoutService service(&library);\n\n bool result1 = service.checkout(\"The Great Gatsby\");\n cout << \"Checking out \\\"The Great Gatsby\\\"... \" << (result1 ? \"Success!\" : \"Failed (not found)\") << endl;\n\n bool result2 = service.checkout(\"The Great Gatsby\");\n cout << \"Checking out \\\"The Great Gatsby\\\"... \" << (result2 ? \"Success!\" : \"Failed (already checked out)\") << endl;\n\n bool result3 = service.checkout(\"Unknown Book\");\n cout << \"Checking out \\\"Unknown Book\\\"... \" << (result3 ? \"Success!\" : \"Failed (not found)\") << endl;\n\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Book struct {\n\ttitle string\n\tauthor string\n\tavailable bool\n}\n\nfunc NewBook(title, author string) *Book {\n\treturn &Book{\n\t\ttitle: title,\n\t\tauthor: author,\n\t\tavailable: true,\n\t}\n}\n\nfunc (b *Book) GetTitle() string {\n\treturn b.title\n}\n\nfunc (b *Book) GetAuthor() string {\n\treturn b.author\n}\n\nfunc (b *Book) IsAvailable() bool {\n\treturn b.available\n}\n\nfunc (b *Book) MarkCheckedOut() {\n\tb.available = false\n}\n\ntype Section struct {\n\tname string\n\tbooks []*Book\n}\n\nfunc NewSection(name string) *Section {\n\treturn &Section{\n\t\tname: name,\n\t\tbooks: make([]*Book, 0),\n\t}\n}\n\nfunc (s *Section) AddBook(book *Book) {\n\ts.books = append(s.books, book)\n}\n\nfunc (s *Section) CheckoutBook(title string) bool {\n\tfor _, book := range s.books {\n\t\tif book.GetTitle() == title {\n\t\t\tif book.IsAvailable() {\n\t\t\t\tbook.MarkCheckedOut()\n\t\t\t\treturn true\n\t\t\t}\n\t\t\treturn false\n\t\t}\n\t}\n\treturn false\n}\n\ntype Library struct {\n\tsections []*Section\n}\n\nfunc NewLibrary() *Library {\n\treturn &Library{\n\t\tsections: make([]*Section, 0),\n\t}\n}\n\nfunc (l *Library) AddSection(section *Section) {\n\tl.sections = append(l.sections, section)\n}\n\nfunc (l *Library) CheckoutBook(title string) bool {\n\tfor _, section := range l.sections {\n\t\tif section.CheckoutBook(title) {\n\t\t\treturn true\n\t\t}\n\t}\n\treturn false\n}\n\ntype CheckoutService struct {\n\tlibrary *Library\n}\n\nfunc NewCheckoutService(library *Library) *CheckoutService {\n\treturn &CheckoutService{library: library}\n}\n\nfunc (cs *CheckoutService) Checkout(bookTitle string) bool {\n\treturn cs.library.CheckoutBook(bookTitle)\n}\n\nfunc main() {\n\tlibrary := NewLibrary()\n\n\tfiction := NewSection(\"Fiction\")\n\tfiction.AddBook(NewBook(\"The Great Gatsby\", \"F. Scott Fitzgerald\"))\n\tfiction.AddBook(NewBook(\"1984\", \"George Orwell\"))\n\tlibrary.AddSection(fiction)\n\n\tnonFiction := NewSection(\"Non-Fiction\")\n\tnonFiction.AddBook(NewBook(\"Clean Code\", \"Robert C. Martin\"))\n\tlibrary.AddSection(nonFiction)\n\n\tservice := NewCheckoutService(library)\n\n\tresult1 := service.Checkout(\"The Great Gatsby\")\n\tfmt.Printf(\"Checking out \\\"The Great Gatsby\\\"... %s\\n\", statusMessage(result1, \"not found\"))\n\n\tresult2 := service.Checkout(\"The Great Gatsby\")\n\tfmt.Printf(\"Checking out \\\"The Great Gatsby\\\"... %s\\n\", statusMessage(result2, \"already checked out\"))\n\n\tresult3 := service.Checkout(\"Unknown Book\")\n\tfmt.Printf(\"Checking out \\\"Unknown Book\\\"... %s\\n\", statusMessage(result3, \"not found\"))\n}\n\nfunc statusMessage(success bool, failureReason string) string {\n\tif success {\n\t\treturn \"Success!\"\n\t}\n\treturn fmt.Sprintf(\"Failed (%s)\", failureReason)\n}\n```\n\n```csharp\nusing System;\nusing System.Collections.Generic;\n\nclass Book\n{\n public string Title { get; }\n public string Author { get; }\n public bool Available { get; private set; }\n\n public Book(string title, string author)\n {\n Title = title;\n Author = author;\n Available = true;\n }\n\n public void MarkCheckedOut() { Available = false; }\n}\n\nclass Section\n{\n public string Name { get; }\n private List books;\n\n public Section(string name)\n {\n Name = name;\n books = new List();\n }\n\n public void AddBook(Book book) { books.Add(book); }\n\n public bool CheckoutBook(string title)\n {\n foreach (var book in books)\n {\n if (book.Title == title)\n {\n if (book.Available)\n {\n book.MarkCheckedOut();\n return true;\n }\n return false;\n }\n }\n return false;\n }\n}\n\nclass Library\n{\n private List

sections;\n\n public Library()\n {\n sections = new List

();\n }\n\n public void AddSection(Section section) { sections.Add(section); }\n\n public bool CheckoutBook(string title)\n {\n foreach (var section in sections)\n {\n if (section.CheckoutBook(title))\n {\n return true;\n }\n }\n return false;\n }\n}\n\nclass CheckoutService\n{\n private Library library;\n\n public CheckoutService(Library library)\n {\n this.library = library;\n }\n\n public bool Checkout(string bookTitle)\n {\n return library.CheckoutBook(bookTitle);\n }\n}\n\nclass Program\n{\n static void Main(string[] args)\n {\n Library library = new Library();\n\n Section fiction = new Section(\"Fiction\");\n fiction.AddBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\n fiction.AddBook(new Book(\"1984\", \"George Orwell\"));\n library.AddSection(fiction);\n\n Section nonFiction = new Section(\"Non-Fiction\");\n nonFiction.AddBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\n library.AddSection(nonFiction);\n\n CheckoutService service = new CheckoutService(library);\n\n bool result1 = service.Checkout(\"The Great Gatsby\");\n Console.WriteLine($\"Checking out \\\"The Great Gatsby\\\"... {(result1 ? \"Success!\" : \"Failed (not found)\")}\");\n\n bool result2 = service.Checkout(\"The Great Gatsby\");\n Console.WriteLine($\"Checking out \\\"The Great Gatsby\\\"... {(result2 ? \"Success!\" : \"Failed (already checked out)\")}\");\n\n bool result3 = service.Checkout(\"Unknown Book\");\n Console.WriteLine($\"Checking out \\\"Unknown Book\\\"... {(result3 ? \"Success!\" : \"Failed (not found)\")}\");\n }\n}\n```\n\n```typescript\nclass Book {\n private title: string;\n private author: string;\n private available: boolean;\n\n constructor(title: string, author: string) {\n this.title = title;\n this.author = author;\n this.available = true;\n }\n\n getTitle(): string { return this.title; }\n getAuthor(): string { return this.author; }\n isAvailable(): boolean { return this.available; }\n markCheckedOut(): void { this.available = false; }\n}\n\nclass Section {\n private name: string;\n private books: Book[];\n\n constructor(name: string) {\n this.name = name;\n this.books = [];\n }\n\n addBook(book: Book): void { this.books.push(book); }\n\n checkoutBook(title: string): boolean {\n for (const book of this.books) {\n if (book.getTitle() === title) {\n if (book.isAvailable()) {\n book.markCheckedOut();\n return true;\n }\n return false;\n }\n }\n return false;\n }\n}\n\nclass Library {\n private sections: Section[];\n\n constructor() {\n this.sections = [];\n }\n\n addSection(section: Section): void { this.sections.push(section); }\n\n checkoutBook(title: string): boolean {\n for (const section of this.sections) {\n if (section.checkoutBook(title)) {\n return true;\n }\n }\n return false;\n }\n}\n\nclass CheckoutService {\n private library: Library;\n\n constructor(library: Library) {\n this.library = library;\n }\n\n checkout(bookTitle: string): boolean {\n return this.library.checkoutBook(bookTitle);\n }\n}\n\n// --- Main ---\nconst library = new Library();\n\nconst fiction = new Section(\"Fiction\");\nfiction.addBook(new Book(\"The Great Gatsby\", \"F. Scott Fitzgerald\"));\nfiction.addBook(new Book(\"1984\", \"George Orwell\"));\nlibrary.addSection(fiction);\n\nconst nonFiction = new Section(\"Non-Fiction\");\nnonFiction.addBook(new Book(\"Clean Code\", \"Robert C. Martin\"));\nlibrary.addSection(nonFiction);\n\nconst service = new CheckoutService(library);\n\nconst result1 = service.checkout(\"The Great Gatsby\");\nconsole.log(`Checking out \"The Great Gatsby\"... ${result1 ? \"Success!\" : \"Failed (not found)\"}`);\n\nconst result2 = service.checkout(\"The Great Gatsby\");\nconsole.log(`Checking out \"The Great Gatsby\"... ${result2 ? \"Success!\" : \"Failed (already checked out)\"}`);\n\nconst result3 = service.checkout(\"Unknown Book\");\nconsole.log(`Checking out \"Unknown Book\"... ${result3 ? \"Success!\" : \"Failed (not found)\"}`);\n```\n","pageType":"lld-interviews"}

Exercise: Law of Demeter

Exercise 1: OrderSummary Printer

Exercise 2: Library Checkout System

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