Explain the difference between the Decorator and Facade patterns, their purpose, and use cases.

Decorator and Facade are structural design patterns, but they solve different problems:

• Decorator allows dynamically adding new responsibilities to objects by wrapping them in other decorator objects. It is used when you need to extend functionality without modifying the original class.

• Facade provides a simplified interface to a complex system of classes, hiding internal complexity. It is used for ease of working with subsystems.

Example:

• Decorator: adding logging to an existing object without changing its code.
• Facade: a single interface for working with multiple services (e.g., network, database, cache).

// Decorator
interface Coffee { fun cost(): Double }
class SimpleCoffee : Coffee { override fun cost() = 2.0 }
class MilkDecorator(val coffee: Coffee) : Coffee {
    override fun cost() = coffee.cost() + 0.5
}

// Facade
class HomeTheaterFacade(val amp: Amplifier, val dvd: DvdPlayer) {
    fun watchMovie() {
        amp.on()
        dvd.play()
    }
}