What are the four pillars of Object-Oriented Programming?

Answer: Encapsulation, abstraction, inheritance, and polymorphism. Swift supports all four, though it leans on protocols and value types more than classic class-based OOP languages.

Pillar	What it means	Swift example
Encapsulation	Hide internal state, expose controlled access	private(set) var balance
Abstraction	Expose what something does, not how	protocol PaymentProcessor
Inheritance	Reuse/extend behavior from a base type	class SavingsAccount: Account
Polymorphism	Same interface, different underlying behavior	[PaymentProcessor] holding multiple conforming types

protocol PaymentProcessor {
    func charge(_ amount: Decimal) -> Bool
}

class CreditCardProcessor: PaymentProcessor {
    func charge(_ amount: Decimal) -> Bool { /* ... */ true }
}

class PayPalProcessor: PaymentProcessor {
    func charge(_ amount: Decimal) -> Bool { /* ... */ true }
}

let processors: [PaymentProcessor] = [CreditCardProcessor(), PayPalProcessor()]
processors.forEach { _ = $0.charge(9.99) } // polymorphic dispatch