Facade vs Strategy design pattern differences

While Facade and Strategy patterns are both design patterns, they are not the same. Let’s break it down clearly.

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1️⃣ Facade Pattern

Purpose

• Provides a simplified interface to a complex subsystem.
• Makes it easier for clients to interact with multiple classes without knowing the details.

Key Idea

“Hide complexity behind a simple interface.”

Example

Imagine a home theater system:

class Amplifier { void on() {} void setVolume(int v) {} }
class DVDPlayer { void on() {} void play(String movie) {} }
class Projector { void on() {} void wideScreenMode() {} }

class HomeTheaterFacade {
    private Amplifier amp;
    private DVDPlayer dvd;
    private Projector projector;

    public HomeTheaterFacade(Amplifier a, DVDPlayer d, Projector p) {
        amp = a; dvd = d; projector = p;
    }

    public void watchMovie(String movie) {
        amp.on(); amp.setVolume(5);
        projector.on(); projector.wideScreenMode();
        dvd.on(); dvd.play(movie);
    }
}

✅ Client code:

HomeTheaterFacade theater = new HomeTheaterFacade(amp, dvd, projector);
theater.watchMovie("Inception"); // one simple call

Key point: Client doesn’t need to know all the steps — Facade simplifies.

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2️⃣ Strategy Pattern

Purpose

• Defines a family of interchangeable algorithms and makes them pluggable at runtime.
• Lets the client choose the behavior dynamically.

Key Idea

“Encapsulate interchangeable behavior and make it swappable.”

Example

Payment processing:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    public void pay(int amount) { System.out.println("Paid " + amount + " with credit card"); }
}

class PayPalPayment implements PaymentStrategy {
    public void pay(int amount) { System.out.println("Paid " + amount + " via PayPal"); }
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;
    public ShoppingCart(PaymentStrategy ps) { paymentStrategy = ps; }
    public void checkout(int amount) { paymentStrategy.pay(amount); }
}

✅ Client code:

ShoppingCart cart = new ShoppingCart(new PayPalPayment());
cart.checkout(100); // Pays via PayPal

Key point: You can switch the strategy (CreditCardPayment, PayPalPayment) at runtime.

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3️⃣ Facade vs Strategy – Side by Side

Feature	Facade	Strategy
Intent	Simplify interface to a subsystem	Encapsulate interchangeable behaviors/algorithms
Client awareness	Hides complexity	Chooses behavior dynamically
Example	HomeTheaterFacade	PaymentStrategy (PayPal/CreditCard)
Number of variations	Usually single facade	Multiple strategies
Change at runtime	Not needed	Yes, dynamic swapping