Inheritance

Allows a class to derive shared state and behavior from a parent class, forming an "is-a" relationship.

The Problem

Duplicating fields and methods across similar classes leads to maintenance headaches — a fix or feature added in one place must be manually copied everywhere else.

How It Works

›A subclass inherits fields and methods from its superclass and can override behavior
›Models "is-a" relationships, e.g., a Dog is an Animal
›Deep hierarchies can become rigid and fragile — prefer composition when the relationship isn't truly "is-a"
›Abstract base classes define shared contracts that subclasses must fulfill

When to Use

✓Multiple types share common fields/behavior with specialization (Shape → Circle, Square)

✓Framework base classes that provide default behavior (UIViewController, Activity)

✓Modeling natural taxonomies with clear "is-a" relationships

Real-World Examples

🍎

iOS

UIButton inherits from UIControl, which inherits from UIView

🤖

Android

AppCompatActivity extends FragmentActivity extends Activity

🌐

Web

HTMLInputElement and HTMLButtonElement both extend HTMLElement

Code Examples

Swift

class Animal {
    let name: String
    init(name: String) { self.name = name }

    func makeSound() -> String {
        return "..."
    }
}

class Dog: Animal {
    override func makeSound() -> String {
        return "Woof!"
    }
}

class Cat: Animal {
    override func makeSound() -> String {
        return "Meow!"
    }
}

let animals: [Animal] = [Dog(name: "Rex"), Cat(name: "Whiskers")]
for animal in animals {
    print("\(animal.name): \(animal.makeSound())")
}