{"title":"Exercise: Polymorphism","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: Discount Calculator\n\n\n**Problem:** Build a pricing system where an `OrderProcessor` applies discounts polymorphically. Different discount types share a common base class with shared formatting logic, but each one calculates the discounted price differently. The processor works with any discount through the abstract `Discount` type.\n\n**Requirements:**\n\n- Abstract `Discount` class with a `label` field (protected), an abstract `apply(price)` method that returns the discounted price, and a concrete `describe(originalPrice)` method that prints \"label: $originalPrice -> $discountedPrice\".\n- `PercentageDiscount`: takes a percentage (e.g., 20 means 20% off). Label is \"20.0% off\". Returns `price * (1 - percentage/100)`.\n- `FlatDiscount`: takes a fixed amount off. Label is \"$15.0 off\". Returns `price - amount` (minimum 0).\n- `BuyOneGetOneFree`: halves the price. Label is \"Buy 1 Get 1 Free\". Returns `price / 2`.\n- `OrderProcessor` class with a `processOrder(itemName, price, discount)` method that prints the item name and calls `describe()` on the discount.\n\n```java\nabstract class Discount {\n protected String label;\n\n public Discount(String label) {\n this.label = label;\n }\n\n abstract double apply(double price);\n\n public void describe(double originalPrice) {\n // TODO: call apply(originalPrice) and print:\n // \"label: $originalPrice -> $discountedPrice\"\n // Hint: use String.format(\"%.2f\", value) for formatting\n }\n}\n\nclass PercentageDiscount extends Discount {\n private double percentage;\n\n public PercentageDiscount(double percentage) {\n super(percentage + \"% off\");\n // TODO: initialize this.percentage\n }\n\n double apply(double price) {\n // TODO: return price * (1 - percentage / 100)\n return 0;\n }\n}\n\nclass FlatDiscount extends Discount {\n private double amount;\n\n public FlatDiscount(double amount) {\n super(\"$\" + amount + \" off\");\n // TODO: initialize this.amount\n }\n\n double apply(double price) {\n // TODO: return max(price - amount, 0)\n return 0;\n }\n}\n\nclass BuyOneGetOneFree extends Discount {\n public BuyOneGetOneFree() {\n super(\"Buy 1 Get 1 Free\");\n }\n\n double apply(double price) {\n // TODO: return price / 2\n return 0;\n }\n}\n\nclass OrderProcessor {\n public void processOrder(String itemName, double price, Discount discount) {\n // TODO: print \"Item: itemName\" then call discount.describe(price)\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n OrderProcessor processor = new OrderProcessor();\n\n processor.processOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\n processor.processOrder(\"Headphones\", 49.99, new FlatDiscount(15));\n processor.processOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n }\n}\n```\n\n```python\nfrom abc import ABC, abstractmethod\n\nclass Discount(ABC):\n def __init__(self, label: str):\n self._label = label\n\n @abstractmethod\n def apply(self, price: float) -> float:\n pass\n\n def describe(self, original_price: float):\n # TODO: call self.apply(original_price) and print:\n # \"label: $original_price -> $discounted_price\"\n # Hint: use f\"{value:.2f}\" for formatting\n pass\n\nclass PercentageDiscount(Discount):\n def __init__(self, percentage: float):\n super().__init__(f\"{percentage:.1f}% off\")\n # TODO: initialize self._percentage\n\n def apply(self, price: float) -> float:\n # TODO: return price * (1 - percentage / 100)\n return 0\n\nclass FlatDiscount(Discount):\n def __init__(self, amount: float):\n super().__init__(f\"${amount:.1f} off\")\n # TODO: initialize self._amount\n\n def apply(self, price: float) -> float:\n # TODO: return max(price - amount, 0)\n return 0\n\nclass BuyOneGetOneFree(Discount):\n def __init__(self):\n super().__init__(\"Buy 1 Get 1 Free\")\n\n def apply(self, price: float) -> float:\n # TODO: return price / 2\n return 0\n\nclass OrderProcessor:\n def process_order(self, item_name: str, price: float, discount: Discount):\n # TODO: print \"Item: item_name\" then call discount.describe(price)\n pass\n\nif __name__ == \"__main__\":\n processor = OrderProcessor()\n\n processor.process_order(\"Laptop\", 999.99, PercentageDiscount(20))\n processor.process_order(\"Headphones\", 49.99, FlatDiscount(15))\n processor.process_order(\"Keyboard\", 79.98, BuyOneGetOneFree())\n```\n\n```cpp\n#include \n#include \n#include \n#include \nusing namespace std;\n\nclass Discount {\nprotected:\n string label;\n\npublic:\n Discount(const string& label) : label(label) {}\n virtual ~Discount() {}\n\n virtual double apply(double price) = 0;\n\n void describe(double originalPrice) {\n // TODO: call apply(originalPrice) and print:\n // \"label: $originalPrice -> $discountedPrice\"\n // Hint: use printf(\"%.2f\", value) for formatting\n }\n};\n\nclass PercentageDiscount : public Discount {\n double percentage;\n\npublic:\n PercentageDiscount(double percentage)\n : Discount(string()) {\n char buf[32];\n snprintf(buf, sizeof(buf), \"%.1f\", percentage);\n this->label = string(buf) + \"% off\";\n // TODO: initialize this->percentage\n }\n\n double apply(double price) override {\n // TODO: return price * (1 - percentage / 100)\n return 0;\n }\n};\n\nclass FlatDiscount : public Discount {\n double amount;\n\npublic:\n FlatDiscount(double amount)\n : Discount(string()) {\n char buf[32];\n snprintf(buf, sizeof(buf), \"%.1f\", amount);\n this->label = \"$\" + string(buf) + \" off\";\n // TODO: initialize this->amount\n }\n\n double apply(double price) override {\n // TODO: return max(price - amount, 0.0)\n return 0;\n }\n};\n\nclass BuyOneGetOneFree : public Discount {\npublic:\n BuyOneGetOneFree() : Discount(\"Buy 1 Get 1 Free\") {}\n\n double apply(double price) override {\n // TODO: return price / 2\n return 0;\n }\n};\n\nclass OrderProcessor {\npublic:\n void processOrder(const string& itemName, double price, Discount& discount) {\n // TODO: print \"Item: itemName\" then call discount.describe(price)\n }\n};\n\nint main() {\n OrderProcessor processor;\n\n PercentageDiscount pct(20);\n FlatDiscount flat(15);\n BuyOneGetOneFree bogo;\n\n processor.processOrder(\"Laptop\", 999.99, pct);\n processor.processOrder(\"Headphones\", 49.99, flat);\n processor.processOrder(\"Keyboard\", 79.98, bogo);\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Discount interface {\n\tapply(price float64) float64\n\tdescribe(originalPrice float64)\n}\n\ntype discountBase struct {\n\tlabel string\n}\n\nfunc (d *discountBase) describe(originalPrice float64) {\n\t// TODO: call apply(originalPrice) and print:\n\t// \"label: $originalPrice -> $discountedPrice\"\n\t// Hint: use fmt.Sprintf(\"%.2f\", value) for formatting\n}\n\ntype PercentageDiscount struct {\n\tdiscountBase\n\tpercentage float64\n}\n\nfunc NewPercentageDiscount(percentage float64) *PercentageDiscount {\n\td := &PercentageDiscount{}\n\td.label = fmt.Sprintf(\"%.1f%% off\", percentage)\n\t// TODO: initialize d.percentage using the parameter\n\treturn d\n}\n\nfunc (d *PercentageDiscount) apply(price float64) float64 {\n\t// TODO: return price * (1 - percentage / 100)\n\treturn 0\n}\n\ntype FlatDiscount struct {\n\tdiscountBase\n\tamount float64\n}\n\nfunc NewFlatDiscount(amount float64) *FlatDiscount {\n\td := &FlatDiscount{}\n\td.label = fmt.Sprintf(\"$%.1f off\", amount)\n\t// TODO: initialize d.amount using the parameter\n\treturn d\n}\n\nfunc (d *FlatDiscount) apply(price float64) float64 {\n\t// TODO: return max(price - amount, 0)\n\treturn 0\n}\n\ntype BuyOneGetOneFree struct {\n\tdiscountBase\n}\n\nfunc NewBuyOneGetOneFree() *BuyOneGetOneFree {\n\treturn &BuyOneGetOneFree{\n\t\tdiscountBase: discountBase{label: \"Buy 1 Get 1 Free\"},\n\t}\n}\n\nfunc (d *BuyOneGetOneFree) apply(price float64) float64 {\n\t// TODO: return price / 2\n\treturn 0\n}\n\ntype OrderProcessor struct{}\n\nfunc (o *OrderProcessor) processOrder(itemName string, price float64, discount Discount) {\n\t// TODO: print \"Item: itemName\" then call discount.describe(price)\n}\n\nfunc main() {\n\tprocessor := &OrderProcessor{}\n\n\tprocessor.processOrder(\"Laptop\", 999.99, NewPercentageDiscount(20))\n\tprocessor.processOrder(\"Headphones\", 49.99, NewFlatDiscount(15))\n\tprocessor.processOrder(\"Keyboard\", 79.98, NewBuyOneGetOneFree())\n}\n```\n\n```csharp\nusing System;\n\npublic abstract class Discount\n{\n protected string Label;\n\n public Discount(string label) { Label = label; }\n\n public abstract double Apply(double price);\n\n public void Describe(double originalPrice)\n {\n // TODO: call Apply(originalPrice) and print:\n // \"Label: $originalPrice -> $discountedPrice\"\n // Hint: use $\"{value:F2}\" for formatting\n }\n}\n\npublic class PercentageDiscount : Discount\n{\n private double _percentage;\n\n public PercentageDiscount(double percentage) : base($\"{percentage:F1}% off\")\n {\n // TODO: initialize _percentage\n }\n\n public override double Apply(double price)\n {\n // TODO: return price * (1 - _percentage / 100)\n return 0;\n }\n}\n\npublic class FlatDiscount : Discount\n{\n private double _amount;\n\n public FlatDiscount(double amount) : base($\"${amount:F1} off\")\n {\n // TODO: initialize _amount\n }\n\n public override double Apply(double price)\n {\n // TODO: return Math.Max(price - _amount, 0)\n return 0;\n }\n}\n\npublic class BuyOneGetOneFree : Discount\n{\n public BuyOneGetOneFree() : base(\"Buy 1 Get 1 Free\") {}\n\n public override double Apply(double price)\n {\n // TODO: return price / 2\n return 0;\n }\n}\n\npublic class OrderProcessor\n{\n public void ProcessOrder(string itemName, double price, Discount discount)\n {\n // TODO: print \"Item: itemName\" then call discount.Describe(price)\n }\n}\n\npublic class Program\n{\n public static void Main()\n {\n var processor = new OrderProcessor();\n\n processor.ProcessOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\n processor.ProcessOrder(\"Headphones\", 49.99, new FlatDiscount(15));\n processor.ProcessOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n }\n}\n```\n\n```typescript\nabstract class Discount {\n protected label: string;\n\n constructor(label: string) { this.label = label; }\n\n abstract apply(price: number): number;\n\n describe(originalPrice: number): void {\n // TODO: call this.apply(originalPrice) and print:\n // \"label: $originalPrice -> $discountedPrice\"\n // Hint: use value.toFixed(2) for formatting\n }\n}\n\nclass PercentageDiscount extends Discount {\n private percentage: number;\n\n constructor(percentage: number) {\n super(percentage.toFixed(1) + \"% off\");\n // TODO: initialize this.percentage using the parameter\n this.percentage = 0;\n }\n\n apply(price: number): number {\n // TODO: return price * (1 - percentage / 100)\n return 0;\n }\n}\n\nclass FlatDiscount extends Discount {\n private amount: number;\n\n constructor(amount: number) {\n super(\"$\" + amount.toFixed(1) + \" off\");\n // TODO: initialize this.amount using the parameter\n this.amount = 0;\n }\n\n apply(price: number): number {\n // TODO: return Math.max(price - amount, 0)\n return 0;\n }\n}\n\nclass BuyOneGetOneFree extends Discount {\n constructor() {\n super(\"Buy 1 Get 1 Free\");\n }\n\n apply(price: number): number {\n // TODO: return price / 2\n return 0;\n }\n}\n\nclass OrderProcessor {\n processOrder(itemName: string, price: number, discount: Discount): void {\n // TODO: print \"Item: itemName\" then call discount.describe(price)\n }\n}\n\nconst processor = new OrderProcessor();\n\nprocessor.processOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\nprocessor.processOrder(\"Headphones\", 49.99, new FlatDiscount(15));\nprocessor.processOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n```\n\n#### Solutions\n\n```java\nabstract class Discount {\n protected String label;\n\n public Discount(String label) {\n this.label = label;\n }\n\n abstract double apply(double price);\n\n public void describe(double originalPrice) {\n double discountedPrice = apply(originalPrice);\n System.out.println(label + \": $\" + String.format(\"%.2f\", originalPrice)\n + \" -> $\" + String.format(\"%.2f\", discountedPrice));\n }\n}\n\nclass PercentageDiscount extends Discount {\n private double percentage;\n\n public PercentageDiscount(double percentage) {\n super(percentage + \"% off\");\n this.percentage = percentage;\n }\n\n double apply(double price) {\n return price * (1 - percentage / 100);\n }\n}\n\nclass FlatDiscount extends Discount {\n private double amount;\n\n public FlatDiscount(double amount) {\n super(\"$\" + amount + \" off\");\n this.amount = amount;\n }\n\n double apply(double price) {\n return Math.max(price - amount, 0);\n }\n}\n\nclass BuyOneGetOneFree extends Discount {\n public BuyOneGetOneFree() {\n super(\"Buy 1 Get 1 Free\");\n }\n\n double apply(double price) {\n return price / 2;\n }\n}\n\nclass OrderProcessor {\n public void processOrder(String itemName, double price, Discount discount) {\n System.out.println(\"Item: \" + itemName);\n discount.describe(price);\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n OrderProcessor processor = new OrderProcessor();\n\n processor.processOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\n processor.processOrder(\"Headphones\", 49.99, new FlatDiscount(15));\n processor.processOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n }\n}\n```\n\n```python\nfrom abc import ABC, abstractmethod\n\nclass Discount(ABC):\n def __init__(self, label: str):\n self._label = label\n\n @abstractmethod\n def apply(self, price: float) -> float:\n pass\n\n def describe(self, original_price: float):\n discounted_price = self.apply(original_price)\n print(f\"{self._label}: ${original_price:.2f} -> ${discounted_price:.2f}\")\n\nclass PercentageDiscount(Discount):\n def __init__(self, percentage: float):\n super().__init__(f\"{percentage:.1f}% off\")\n self._percentage = percentage\n\n def apply(self, price: float) -> float:\n return price * (1 - self._percentage / 100)\n\nclass FlatDiscount(Discount):\n def __init__(self, amount: float):\n super().__init__(f\"${amount:.1f} off\")\n self._amount = amount\n\n def apply(self, price: float) -> float:\n return max(price - self._amount, 0)\n\nclass BuyOneGetOneFree(Discount):\n def __init__(self):\n super().__init__(\"Buy 1 Get 1 Free\")\n\n def apply(self, price: float) -> float:\n return price / 2\n\nclass OrderProcessor:\n def process_order(self, item_name: str, price: float, discount: Discount):\n print(f\"Item: {item_name}\")\n discount.describe(price)\n\nif __name__ == \"__main__\":\n processor = OrderProcessor()\n\n processor.process_order(\"Laptop\", 999.99, PercentageDiscount(20))\n processor.process_order(\"Headphones\", 49.99, FlatDiscount(15))\n processor.process_order(\"Keyboard\", 79.98, BuyOneGetOneFree())\n```\n\n```cpp\n#include \n#include \n#include \n#include \nusing namespace std;\n\nclass Discount {\nprotected:\n string label;\n\npublic:\n Discount(const string& label) : label(label) {}\n virtual ~Discount() {}\n\n virtual double apply(double price) = 0;\n\n void describe(double originalPrice) {\n double discountedPrice = apply(originalPrice);\n printf(\"%s: $%.2f -> $%.2f\\n\", label.c_str(), originalPrice, discountedPrice);\n }\n};\n\nclass PercentageDiscount : public Discount {\n double percentage;\n\npublic:\n PercentageDiscount(double percentage)\n : Discount(string()) {\n char buf[32];\n snprintf(buf, sizeof(buf), \"%.1f\", percentage);\n this->label = string(buf) + \"% off\";\n this->percentage = percentage;\n }\n\n double apply(double price) override {\n return price * (1 - percentage / 100);\n }\n};\n\nclass FlatDiscount : public Discount {\n double amount;\n\npublic:\n FlatDiscount(double amount)\n : Discount(string()) {\n char buf[32];\n snprintf(buf, sizeof(buf), \"%.1f\", amount);\n this->label = \"$\" + string(buf) + \" off\";\n this->amount = amount;\n }\n\n double apply(double price) override {\n return max(price - amount, 0.0);\n }\n};\n\nclass BuyOneGetOneFree : public Discount {\npublic:\n BuyOneGetOneFree() : Discount(\"Buy 1 Get 1 Free\") {}\n\n double apply(double price) override {\n return price / 2;\n }\n};\n\nclass OrderProcessor {\npublic:\n void processOrder(const string& itemName, double price, Discount& discount) {\n cout << \"Item: \" << itemName << endl;\n discount.describe(price);\n }\n};\n\nint main() {\n OrderProcessor processor;\n\n PercentageDiscount pct(20);\n FlatDiscount flat(15);\n BuyOneGetOneFree bogo;\n\n processor.processOrder(\"Laptop\", 999.99, pct);\n processor.processOrder(\"Headphones\", 49.99, flat);\n processor.processOrder(\"Keyboard\", 79.98, bogo);\n return 0;\n}\n```\n\n```go\npackage main\n\nimport (\n\t\"fmt\"\n\t\"math\"\n)\n\ntype Discount interface {\n\tapply(price float64) float64\n\tdescribe(originalPrice float64)\n}\n\ntype baseDiscount struct {\n\tlabel string\n}\n\nfunc (b *baseDiscount) describe(originalPrice float64, discountedPrice float64) {\n\tfmt.Printf(\"%s: $%.2f -> $%.2f\\n\", b.label, originalPrice, discountedPrice)\n}\n\ntype PercentageDiscount struct {\n\tbaseDiscount\n\tpercentage float64\n}\n\nfunc NewPercentageDiscount(percentage float64) *PercentageDiscount {\n\treturn &PercentageDiscount{\n\t\tbaseDiscount: baseDiscount{label: fmt.Sprintf(\"%.1f%% off\", percentage)},\n\t\tpercentage: percentage,\n\t}\n}\n\nfunc (p *PercentageDiscount) apply(price float64) float64 {\n\treturn price * (1 - p.percentage/100)\n}\n\nfunc (p *PercentageDiscount) describe(originalPrice float64) {\n\tp.baseDiscount.describe(originalPrice, p.apply(originalPrice))\n}\n\ntype FlatDiscount struct {\n\tbaseDiscount\n\tamount float64\n}\n\nfunc NewFlatDiscount(amount float64) *FlatDiscount {\n\treturn &FlatDiscount{\n\t\tbaseDiscount: baseDiscount{label: fmt.Sprintf(\"$%.1f off\", amount)},\n\t\tamount: amount,\n\t}\n}\n\nfunc (f *FlatDiscount) apply(price float64) float64 {\n\treturn math.Max(price-f.amount, 0)\n}\n\nfunc (f *FlatDiscount) describe(originalPrice float64) {\n\tf.baseDiscount.describe(originalPrice, f.apply(originalPrice))\n}\n\ntype BuyOneGetOneFree struct {\n\tbaseDiscount\n}\n\nfunc NewBuyOneGetOneFree() *BuyOneGetOneFree {\n\treturn &BuyOneGetOneFree{\n\t\tbaseDiscount: baseDiscount{label: \"Buy 1 Get 1 Free\"},\n\t}\n}\n\nfunc (b *BuyOneGetOneFree) apply(price float64) float64 {\n\treturn price / 2\n}\n\nfunc (b *BuyOneGetOneFree) describe(originalPrice float64) {\n\tb.baseDiscount.describe(originalPrice, b.apply(originalPrice))\n}\n\ntype OrderProcessor struct{}\n\nfunc (o *OrderProcessor) processOrder(itemName string, price float64, discount Discount) {\n\tfmt.Printf(\"Item: %s\\n\", itemName)\n\tdiscount.describe(price)\n}\n\nfunc main() {\n\tprocessor := &OrderProcessor{}\n\n\tprocessor.processOrder(\"Laptop\", 999.99, NewPercentageDiscount(20))\n\tprocessor.processOrder(\"Headphones\", 49.99, NewFlatDiscount(15))\n\tprocessor.processOrder(\"Keyboard\", 79.98, NewBuyOneGetOneFree())\n}\n```\n\n```csharp\nusing System;\n\npublic abstract class Discount\n{\n protected string Label;\n\n public Discount(string label) { Label = label; }\n\n public abstract double Apply(double price);\n\n public void Describe(double originalPrice)\n {\n double discountedPrice = Apply(originalPrice);\n Console.WriteLine($\"{Label}: ${originalPrice:F2} -> ${discountedPrice:F2}\");\n }\n}\n\npublic class PercentageDiscount : Discount\n{\n private double _percentage;\n\n public PercentageDiscount(double percentage) : base($\"{percentage:F1}% off\")\n {\n _percentage = percentage;\n }\n\n public override double Apply(double price)\n {\n return price * (1 - _percentage / 100);\n }\n}\n\npublic class FlatDiscount : Discount\n{\n private double _amount;\n\n public FlatDiscount(double amount) : base($\"${amount:F1} off\")\n {\n _amount = amount;\n }\n\n public override double Apply(double price)\n {\n return Math.Max(price - _amount, 0);\n }\n}\n\npublic class BuyOneGetOneFree : Discount\n{\n public BuyOneGetOneFree() : base(\"Buy 1 Get 1 Free\") {}\n\n public override double Apply(double price)\n {\n return price / 2;\n }\n}\n\npublic class OrderProcessor\n{\n public void ProcessOrder(string itemName, double price, Discount discount)\n {\n Console.WriteLine($\"Item: {itemName}\");\n discount.Describe(price);\n }\n}\n\npublic class Program\n{\n public static void Main()\n {\n var processor = new OrderProcessor();\n\n processor.ProcessOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\n processor.ProcessOrder(\"Headphones\", 49.99, new FlatDiscount(15));\n processor.ProcessOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n }\n}\n```\n\n```typescript\nabstract class Discount {\n protected label: string;\n\n constructor(label: string) { this.label = label; }\n\n abstract apply(price: number): number;\n\n describe(originalPrice: number): void {\n const discountedPrice = this.apply(originalPrice);\n console.log(`${this.label}: $${originalPrice.toFixed(2)} -> $${discountedPrice.toFixed(2)}`);\n }\n}\n\nclass PercentageDiscount extends Discount {\n private percentage: number;\n\n constructor(percentage: number) {\n super(percentage.toFixed(1) + \"% off\");\n this.percentage = percentage;\n }\n\n apply(price: number): number {\n return price * (1 - this.percentage / 100);\n }\n}\n\nclass FlatDiscount extends Discount {\n private amount: number;\n\n constructor(amount: number) {\n super(\"$\" + amount.toFixed(1) + \" off\");\n this.amount = amount;\n }\n\n apply(price: number): number {\n return Math.max(price - this.amount, 0);\n }\n}\n\nclass BuyOneGetOneFree extends Discount {\n constructor() {\n super(\"Buy 1 Get 1 Free\");\n }\n\n apply(price: number): number {\n return price / 2;\n }\n}\n\nclass OrderProcessor {\n processOrder(itemName: string, price: number, discount: Discount): void {\n console.log(`Item: ${itemName}`);\n discount.describe(price);\n }\n}\n\nconst processor = new OrderProcessor();\n\nprocessor.processOrder(\"Laptop\", 999.99, new PercentageDiscount(20));\nprocessor.processOrder(\"Headphones\", 49.99, new FlatDiscount(15));\nprocessor.processOrder(\"Keyboard\", 79.98, new BuyOneGetOneFree());\n```\n\n\n---\n\n# Exercise 2: Logging System\n\n\n**Problem:** Build a logging system where the application uses a `Logger` interface polymorphically. Different logger implementations send messages to different destinations, and the application doesn't know or care which one it's using.\n\n**Requirements:**\n\n- `Logger` interface with `log(level, message)` and `getDestination()` methods.\n- `ConsoleLogger`: prints formatted log messages to the console.\n- `FileLogger`: simulates writing to a file (print with file path prefix).\n- `DatabaseLogger`: simulates inserting into a database (print with table name prefix).\n- `Application` class that takes a `Logger` in its constructor and uses it throughout.\n\n```java\nimport java.util.List;\n\ninterface Logger {\n void log(String level, String message);\n String getDestination();\n}\n\nclass ConsoleLogger implements Logger {\n public void log(String level, String message) {\n // TODO: print \"[level] message\" to console\n }\n\n public String getDestination() {\n // TODO: return \"Console\"\n return \"\";\n }\n}\n\nclass FileLogger implements Logger {\n private String filePath;\n\n public FileLogger(String filePath) {\n // TODO: initialize this.filePath\n }\n\n public void log(String level, String message) {\n // TODO: print \"Writing to filePath: [level] message\"\n }\n\n public String getDestination() {\n // TODO: return \"File: filePath\"\n return \"\";\n }\n}\n\nclass DatabaseLogger implements Logger {\n private String tableName;\n\n public DatabaseLogger(String tableName) {\n // TODO: initialize this.tableName\n }\n\n public void log(String level, String message) {\n // TODO: print \"INSERT INTO tableName: [level] message\"\n }\n\n public String getDestination() {\n // TODO: return \"Database: tableName\"\n return \"\";\n }\n}\n\nclass Application {\n private Logger logger;\n\n public Application(Logger logger) {\n // TODO: initialize this.logger\n }\n\n public void run() {\n // TODO: log three messages with level \"INFO\":\n // \"Application starting...\"\n // \"Processing data...\"\n // \"Application shutting down.\"\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n List loggers = List.of(\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\")\n );\n\n for (Logger logger : loggers) {\n System.out.println(\"--- Using \" + logger.getDestination() + \" ---\");\n Application app = new Application(logger);\n app.run();\n System.out.println();\n }\n }\n}\n```\n\n```python\nfrom abc import ABC, abstractmethod\n\nclass Logger(ABC):\n @abstractmethod\n def log(self, level: str, message: str) -> None:\n pass\n\n @abstractmethod\n def get_destination(self) -> str:\n pass\n\nclass ConsoleLogger(Logger):\n def log(self, level: str, message: str) -> None:\n # TODO: print \"[level] message\" to console\n pass\n\n def get_destination(self) -> str:\n # TODO: return \"Console\"\n return \"\"\n\nclass FileLogger(Logger):\n def __init__(self, file_path: str):\n # TODO: initialize self._file_path\n pass\n\n def log(self, level: str, message: str) -> None:\n # TODO: print \"Writing to file_path: [level] message\"\n pass\n\n def get_destination(self) -> str:\n # TODO: return \"File: file_path\"\n return \"\"\n\nclass DatabaseLogger(Logger):\n def __init__(self, table_name: str):\n # TODO: initialize self._table_name\n pass\n\n def log(self, level: str, message: str) -> None:\n # TODO: print \"INSERT INTO table_name: [level] message\"\n pass\n\n def get_destination(self) -> str:\n # TODO: return \"Database: table_name\"\n return \"\"\n\nclass Application:\n def __init__(self, logger: Logger):\n # TODO: initialize self._logger\n pass\n\n def run(self):\n # TODO: log three messages with level \"INFO\":\n # \"Application starting...\"\n # \"Processing data...\"\n # \"Application shutting down.\"\n pass\n\nif __name__ == \"__main__\":\n loggers = [\n ConsoleLogger(),\n FileLogger(\"/var/log/app.log\"),\n DatabaseLogger(\"app_logs\"),\n ]\n\n for logger in loggers:\n print(f\"--- Using {logger.get_destination()} ---\")\n app = Application(logger)\n app.run()\n print()\n```\n\n```cpp\n#include \n#include \n#include \nusing namespace std;\n\nclass Logger {\npublic:\n virtual ~Logger() {}\n virtual void log(const string& level, const string& message) = 0;\n virtual string getDestination() = 0;\n};\n\nclass ConsoleLogger : public Logger {\npublic:\n void log(const string& level, const string& message) override {\n // TODO: print \"[level] message\" to console\n }\n\n string getDestination() override {\n // TODO: return \"Console\"\n return \"\";\n }\n};\n\nclass FileLogger : public Logger {\nprivate:\n string filePath;\n\npublic:\n FileLogger(const string& filePath) {\n // TODO: initialize this->filePath\n }\n\n void log(const string& level, const string& message) override {\n // TODO: print \"Writing to filePath: [level] message\"\n }\n\n string getDestination() override {\n // TODO: return \"File: filePath\"\n return \"\";\n }\n};\n\nclass DatabaseLogger : public Logger {\nprivate:\n string tableName;\n\npublic:\n DatabaseLogger(const string& tableName) {\n // TODO: initialize this->tableName\n }\n\n void log(const string& level, const string& message) override {\n // TODO: print \"INSERT INTO tableName: [level] message\"\n }\n\n string getDestination() override {\n // TODO: return \"Database: tableName\"\n return \"\";\n }\n};\n\nclass Application {\nprivate:\n Logger* logger;\n\npublic:\n Application(Logger* logger) {\n // TODO: initialize this->logger\n }\n\n void run() {\n // TODO: log three messages with level \"INFO\":\n // \"Application starting...\"\n // \"Processing data...\"\n // \"Application shutting down.\"\n }\n};\n\nint main() {\n ConsoleLogger console;\n FileLogger file(\"/var/log/app.log\");\n DatabaseLogger db(\"app_logs\");\n\n vector loggers = {&console, &file, &db};\n\n for (auto* logger : loggers) {\n cout << \"--- Using \" << logger->getDestination() << \" ---\" << endl;\n Application app(logger);\n app.run();\n cout << endl;\n }\n\n return 0;\n}\n```\n\n```go\npackage main\n\ntype Logger interface {\n\tLog(level string, message string)\n\tGetDestination() string\n}\n\ntype ConsoleLogger struct{}\n\nfunc (c *ConsoleLogger) Log(level string, message string) {\n\t// TODO: print \"[level] message\" to console\n}\n\nfunc (c *ConsoleLogger) GetDestination() string {\n\t// TODO: return \"Console\"\n\treturn \"\"\n}\n\ntype FileLogger struct {\n\tfilePath string\n}\n\nfunc NewFileLogger(filePath string) *FileLogger {\n\t// TODO: initialize this.filePath using the parameter\n\treturn &FileLogger{}\n}\n\nfunc (f *FileLogger) Log(level string, message string) {\n\t// TODO: print \"Writing to filePath: [level] message\"\n}\n\nfunc (f *FileLogger) GetDestination() string {\n\t// TODO: return \"File: filePath\"\n\treturn \"\"\n}\n\ntype DatabaseLogger struct {\n\ttableName string\n}\n\nfunc NewDatabaseLogger(tableName string) *DatabaseLogger {\n\t// TODO: initialize this.tableName using the parameter\n\treturn &DatabaseLogger{}\n}\n\nfunc (d *DatabaseLogger) Log(level string, message string) {\n\t// TODO: print \"INSERT INTO tableName: [level] message\"\n}\n\nfunc (d *DatabaseLogger) GetDestination() string {\n\t// TODO: return \"Database: tableName\"\n\treturn \"\"\n}\n\ntype Application struct {\n\tlogger Logger\n}\n\nfunc NewApplication(logger Logger) *Application {\n\t// TODO: initialize this.logger\n\treturn &Application{}\n}\n\nfunc (a *Application) Run() {\n\t// TODO: log three messages with level \"INFO\":\n\t// \"Application starting...\"\n\t// \"Processing data...\"\n\t// \"Application shutting down.\"\n}\n\nfunc main() {\n\tloggers := []Logger{\n\t\t&ConsoleLogger{},\n\t\tNewFileLogger(\"/var/log/app.log\"),\n\t\tNewDatabaseLogger(\"app_logs\"),\n\t}\n\n\tfor _, logger := range loggers {\n\t\tprintln(\"--- Using \" + logger.GetDestination() + \" ---\")\n\t\tapp := NewApplication(logger)\n\t\tapp.Run()\n\t\tprintln()\n\t}\n}\n```\n\n```csharp\nusing System;\nusing System.Collections.Generic;\n\npublic interface ILogger\n{\n void Log(string level, string message);\n string GetDestination();\n}\n\npublic class ConsoleLogger : ILogger\n{\n public void Log(string level, string message)\n {\n // TODO: print \"[level] message\" to console\n }\n\n public string GetDestination()\n {\n // TODO: return \"Console\"\n return \"\";\n }\n}\n\npublic class FileLogger : ILogger\n{\n private string _filePath;\n\n public FileLogger(string filePath)\n {\n // TODO: initialize _filePath\n }\n\n public void Log(string level, string message)\n {\n // TODO: print \"Writing to _filePath: [level] message\"\n }\n\n public string GetDestination()\n {\n // TODO: return \"File: _filePath\"\n return \"\";\n }\n}\n\npublic class DatabaseLogger : ILogger\n{\n private string _tableName;\n\n public DatabaseLogger(string tableName)\n {\n // TODO: initialize _tableName\n }\n\n public void Log(string level, string message)\n {\n // TODO: print \"INSERT INTO _tableName: [level] message\"\n }\n\n public string GetDestination()\n {\n // TODO: return \"Database: _tableName\"\n return \"\";\n }\n}\n\npublic class Application\n{\n private ILogger _logger;\n\n public Application(ILogger logger)\n {\n // TODO: initialize _logger\n }\n\n public void Run()\n {\n // TODO: log three messages with level \"INFO\":\n // \"Application starting...\"\n // \"Processing data...\"\n // \"Application shutting down.\"\n }\n}\n\npublic class Program\n{\n public static void Main()\n {\n var loggers = new List\n {\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\")\n };\n\n foreach (var logger in loggers)\n {\n Console.WriteLine($\"--- Using {logger.GetDestination()} ---\");\n var app = new Application(logger);\n app.Run();\n Console.WriteLine();\n }\n }\n}\n```\n\n```typescript\ninterface Logger {\n log(level: string, message: string): void;\n getDestination(): string;\n}\n\nclass ConsoleLogger implements Logger {\n log(level: string, message: string): void {\n // TODO: print \"[level] message\" to console\n }\n\n getDestination(): string {\n // TODO: return \"Console\"\n return \"\";\n }\n}\n\nclass FileLogger implements Logger {\n private filePath: string;\n\n constructor(filePath: string) {\n // TODO: initialize this.filePath using the parameter\n this.filePath = \"\";\n }\n\n log(level: string, message: string): void {\n // TODO: print \"Writing to filePath: [level] message\"\n }\n\n getDestination(): string {\n // TODO: return \"File: filePath\"\n return \"\";\n }\n}\n\nclass DatabaseLogger implements Logger {\n private tableName: string;\n\n constructor(tableName: string) {\n // TODO: initialize this.tableName using the parameter\n this.tableName = \"\";\n }\n\n log(level: string, message: string): void {\n // TODO: print \"INSERT INTO tableName: [level] message\"\n }\n\n getDestination(): string {\n // TODO: return \"Database: tableName\"\n return \"\";\n }\n}\n\nclass Application {\n private logger: Logger;\n\n constructor(logger: Logger) {\n this.logger = logger;\n }\n\n run(): void {\n // TODO: log three messages with level \"INFO\":\n // \"Application starting...\"\n // \"Processing data...\"\n // \"Application shutting down.\"\n }\n}\n\nconst loggers: Logger[] = [\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\"),\n];\n\nfor (const logger of loggers) {\n console.log(`--- Using ${logger.getDestination()} ---`);\n const app = new Application(logger);\n app.run();\n console.log();\n}\n```\n\n#### Solutions\n\n```java\nimport java.util.List;\n\ninterface Logger {\n void log(String level, String message);\n String getDestination();\n}\n\nclass ConsoleLogger implements Logger {\n public void log(String level, String message) {\n System.out.println(\"[\" + level + \"] \" + message);\n }\n\n public String getDestination() {\n return \"Console\";\n }\n}\n\nclass FileLogger implements Logger {\n private String filePath;\n\n public FileLogger(String filePath) {\n this.filePath = filePath;\n }\n\n public void log(String level, String message) {\n System.out.println(\"Writing to \" + filePath + \": [\" + level + \"] \" + message);\n }\n\n public String getDestination() {\n return \"File: \" + filePath;\n }\n}\n\nclass DatabaseLogger implements Logger {\n private String tableName;\n\n public DatabaseLogger(String tableName) {\n this.tableName = tableName;\n }\n\n public void log(String level, String message) {\n System.out.println(\"INSERT INTO \" + tableName + \": [\" + level + \"] \" + message);\n }\n\n public String getDestination() {\n return \"Database: \" + tableName;\n }\n}\n\nclass Application {\n private Logger logger;\n\n public Application(Logger logger) {\n this.logger = logger;\n }\n\n public void run() {\n logger.log(\"INFO\", \"Application starting...\");\n logger.log(\"INFO\", \"Processing data...\");\n logger.log(\"INFO\", \"Application shutting down.\");\n }\n}\n\npublic class Main {\n public static void main(String[] args) {\n List loggers = List.of(\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\")\n );\n\n for (Logger logger : loggers) {\n System.out.println(\"--- Using \" + logger.getDestination() + \" ---\");\n Application app = new Application(logger);\n app.run();\n System.out.println();\n }\n }\n}\n```\n\n```python\nfrom abc import ABC, abstractmethod\n\nclass Logger(ABC):\n @abstractmethod\n def log(self, level: str, message: str) -> None:\n pass\n\n @abstractmethod\n def get_destination(self) -> str:\n pass\n\nclass ConsoleLogger(Logger):\n def log(self, level: str, message: str) -> None:\n print(f\"[{level}] {message}\")\n\n def get_destination(self) -> str:\n return \"Console\"\n\nclass FileLogger(Logger):\n def __init__(self, file_path: str):\n self._file_path = file_path\n\n def log(self, level: str, message: str) -> None:\n print(f\"Writing to {self._file_path}: [{level}] {message}\")\n\n def get_destination(self) -> str:\n return f\"File: {self._file_path}\"\n\nclass DatabaseLogger(Logger):\n def __init__(self, table_name: str):\n self._table_name = table_name\n\n def log(self, level: str, message: str) -> None:\n print(f\"INSERT INTO {self._table_name}: [{level}] {message}\")\n\n def get_destination(self) -> str:\n return f\"Database: {self._table_name}\"\n\nclass Application:\n def __init__(self, logger: Logger):\n self._logger = logger\n\n def run(self):\n self._logger.log(\"INFO\", \"Application starting...\")\n self._logger.log(\"INFO\", \"Processing data...\")\n self._logger.log(\"INFO\", \"Application shutting down.\")\n\nif __name__ == \"__main__\":\n loggers = [\n ConsoleLogger(),\n FileLogger(\"/var/log/app.log\"),\n DatabaseLogger(\"app_logs\"),\n ]\n\n for logger in loggers:\n print(f\"--- Using {logger.get_destination()} ---\")\n app = Application(logger)\n app.run()\n print()\n```\n\n```cpp\n#include \n#include \n#include \nusing namespace std;\n\nclass Logger {\npublic:\n virtual ~Logger() {}\n virtual void log(const string& level, const string& message) = 0;\n virtual string getDestination() = 0;\n};\n\nclass ConsoleLogger : public Logger {\npublic:\n void log(const string& level, const string& message) override {\n cout << \"[\" << level << \"] \" << message << endl;\n }\n\n string getDestination() override {\n return \"Console\";\n }\n};\n\nclass FileLogger : public Logger {\nprivate:\n string filePath;\n\npublic:\n FileLogger(const string& filePath) {\n this->filePath = filePath;\n }\n\n void log(const string& level, const string& message) override {\n cout << \"Writing to \" << filePath << \": [\" << level << \"] \" << message << endl;\n }\n\n string getDestination() override {\n return \"File: \" + filePath;\n }\n};\n\nclass DatabaseLogger : public Logger {\nprivate:\n string tableName;\n\npublic:\n DatabaseLogger(const string& tableName) {\n this->tableName = tableName;\n }\n\n void log(const string& level, const string& message) override {\n cout << \"INSERT INTO \" << tableName << \": [\" << level << \"] \" << message << endl;\n }\n\n string getDestination() override {\n return \"Database: \" + tableName;\n }\n};\n\nclass Application {\nprivate:\n Logger* logger;\n\npublic:\n Application(Logger* logger) {\n this->logger = logger;\n }\n\n void run() {\n logger->log(\"INFO\", \"Application starting...\");\n logger->log(\"INFO\", \"Processing data...\");\n logger->log(\"INFO\", \"Application shutting down.\");\n }\n};\n\nint main() {\n ConsoleLogger console;\n FileLogger file(\"/var/log/app.log\");\n DatabaseLogger db(\"app_logs\");\n\n vector loggers = {&console, &file, &db};\n\n for (auto* logger : loggers) {\n cout << \"--- Using \" << logger->getDestination() << \" ---\" << endl;\n Application app(logger);\n app.run();\n cout << endl;\n }\n\n return 0;\n}\n```\n\n```go\npackage main\n\nimport \"fmt\"\n\ntype Logger interface {\n\tLog(level, message string)\n\tGetDestination() string\n}\n\ntype ConsoleLogger struct{}\n\nfunc (c ConsoleLogger) Log(level, message string) {\n\tfmt.Printf(\"[%s] %s\\n\", level, message)\n}\n\nfunc (c ConsoleLogger) GetDestination() string {\n\treturn \"Console\"\n}\n\ntype FileLogger struct {\n\tfilePath string\n}\n\nfunc NewFileLogger(filePath string) *FileLogger {\n\treturn &FileLogger{filePath: filePath}\n}\n\nfunc (f *FileLogger) Log(level, message string) {\n\tfmt.Printf(\"Writing to %s: [%s] %s\\n\", f.filePath, level, message)\n}\n\nfunc (f *FileLogger) GetDestination() string {\n\treturn \"File: \" + f.filePath\n}\n\ntype DatabaseLogger struct {\n\ttableName string\n}\n\nfunc NewDatabaseLogger(tableName string) *DatabaseLogger {\n\treturn &DatabaseLogger{tableName: tableName}\n}\n\nfunc (d *DatabaseLogger) Log(level, message string) {\n\tfmt.Printf(\"INSERT INTO %s: [%s] %s\\n\", d.tableName, level, message)\n}\n\nfunc (d *DatabaseLogger) GetDestination() string {\n\treturn \"Database: \" + d.tableName\n}\n\ntype Application struct {\n\tlogger Logger\n}\n\nfunc NewApplication(logger Logger) *Application {\n\treturn &Application{logger: logger}\n}\n\nfunc (a *Application) Run() {\n\ta.logger.Log(\"INFO\", \"Application starting...\")\n\ta.logger.Log(\"INFO\", \"Processing data...\")\n\ta.logger.Log(\"INFO\", \"Application shutting down.\")\n}\n\nfunc main() {\n\tloggers := []Logger{\n\t\tConsoleLogger{},\n\t\tNewFileLogger(\"/var/log/app.log\"),\n\t\tNewDatabaseLogger(\"app_logs\"),\n\t}\n\n\tfor _, logger := range loggers {\n\t\tfmt.Printf(\"--- Using %s ---\\n\", logger.GetDestination())\n\t\tapp := NewApplication(logger)\n\t\tapp.Run()\n\t\tfmt.Println()\n\t}\n}\n```\n\n```csharp\nusing System;\nusing System.Collections.Generic;\n\npublic interface ILogger\n{\n void Log(string level, string message);\n string GetDestination();\n}\n\npublic class ConsoleLogger : ILogger\n{\n public void Log(string level, string message)\n {\n Console.WriteLine($\"[{level}] {message}\");\n }\n\n public string GetDestination()\n {\n return \"Console\";\n }\n}\n\npublic class FileLogger : ILogger\n{\n private string _filePath;\n\n public FileLogger(string filePath)\n {\n _filePath = filePath;\n }\n\n public void Log(string level, string message)\n {\n Console.WriteLine($\"Writing to {_filePath}: [{level}] {message}\");\n }\n\n public string GetDestination()\n {\n return \"File: \" + _filePath;\n }\n}\n\npublic class DatabaseLogger : ILogger\n{\n private string _tableName;\n\n public DatabaseLogger(string tableName)\n {\n _tableName = tableName;\n }\n\n public void Log(string level, string message)\n {\n Console.WriteLine($\"INSERT INTO {_tableName}: [{level}] {message}\");\n }\n\n public string GetDestination()\n {\n return \"Database: \" + _tableName;\n }\n}\n\npublic class Application\n{\n private ILogger _logger;\n\n public Application(ILogger logger)\n {\n _logger = logger;\n }\n\n public void Run()\n {\n _logger.Log(\"INFO\", \"Application starting...\");\n _logger.Log(\"INFO\", \"Processing data...\");\n _logger.Log(\"INFO\", \"Application shutting down.\");\n }\n}\n\npublic class Program\n{\n public static void Main()\n {\n var loggers = new List\n {\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\")\n };\n\n foreach (var logger in loggers)\n {\n Console.WriteLine($\"--- Using {logger.GetDestination()} ---\");\n var app = new Application(logger);\n app.Run();\n Console.WriteLine();\n }\n }\n}\n```\n\n```typescript\ninterface Logger {\n log(level: string, message: string): void;\n getDestination(): string;\n}\n\nclass ConsoleLogger implements Logger {\n log(level: string, message: string): void {\n console.log(`[${level}] ${message}`);\n }\n\n getDestination(): string {\n return \"Console\";\n }\n}\n\nclass FileLogger implements Logger {\n private filePath: string;\n\n constructor(filePath: string) {\n this.filePath = filePath;\n }\n\n log(level: string, message: string): void {\n console.log(`Writing to ${this.filePath}: [${level}] ${message}`);\n }\n\n getDestination(): string {\n return \"File: \" + this.filePath;\n }\n}\n\nclass DatabaseLogger implements Logger {\n private tableName: string;\n\n constructor(tableName: string) {\n this.tableName = tableName;\n }\n\n log(level: string, message: string): void {\n console.log(`INSERT INTO ${this.tableName}: [${level}] ${message}`);\n }\n\n getDestination(): string {\n return \"Database: \" + this.tableName;\n }\n}\n\nclass Application {\n private logger: Logger;\n\n constructor(logger: Logger) {\n this.logger = logger;\n }\n\n run(): void {\n this.logger.log(\"INFO\", \"Application starting...\");\n this.logger.log(\"INFO\", \"Processing data...\");\n this.logger.log(\"INFO\", \"Application shutting down.\");\n }\n}\n\nconst loggers: Logger[] = [\n new ConsoleLogger(),\n new FileLogger(\"/var/log/app.log\"),\n new DatabaseLogger(\"app_logs\"),\n];\n\nfor (const logger of loggers) {\n console.log(`--- Using ${logger.getDestination()} ---`);\n const app = new Application(logger);\n app.run();\n console.log();\n}\n```\n","pageType":"lld-interviews"}

Exercise 1: Discount Calculator

Exercise 2: Logging System

Get Premium