strategy-pattern

🧠 Concept

The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. This lets you change behavior at runtime without modifying the context class.

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🧱 Key Roles

Role	Description
Strategy (Interface)	Defines a common behavior (algorithm contract).
ConcreteStrategy	Implements a specific algorithm.
Context	Uses a Strategy to perform a task; can swap strategies dynamically.

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💻 Simple Java Example

// Strategy interface
interface PaymentStrategy {
    void pay(int amount);
}

// Concrete strategies
class CreditCardPayment implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Credit Card.");
    }
}

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

// Context
class ShoppingCart {
    private PaymentStrategy strategy;

    void setPaymentStrategy(PaymentStrategy strategy) {
        this.strategy = strategy;
    }

    void checkout(int amount) {
        strategy.pay(amount);
    }
}

✅ Usage Example:

ShoppingCart cart = new ShoppingCart();

cart.setPaymentStrategy(new CreditCardPayment());
cart.checkout(100);  // Paid 100 using Credit Card.

cart.setPaymentStrategy(new PayPalPayment());
cart.checkout(200);  // Paid 200 using PayPal.

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⚙️ Flow Summary (Step-by-Step)

Step	Action	Context (ShoppingCart)	Strategy Used	Output
1️⃣	setPaymentStrategy(CreditCard)	CreditCardPayment	CreditCardPayment	—
2️⃣	checkout(100)	→ calls pay(100)	CreditCardPayment	"Paid 100 using Credit Card."
3️⃣	setPaymentStrategy(PayPal)	PayPalPayment	PayPalPayment	—
4️⃣	checkout(200)	→ calls pay(200)	PayPalPayment	"Paid 200 using PayPal."

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🪜 Summary

• Encapsulates different algorithms behind a common interface.

• Context delegates behavior to the chosen Strategy.

• Easy to add new behaviors without touching existing code.

• Promotes Open/Closed Principle (OCP).

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🎯 Real-world Analogy

💳 Payment Methods – You can choose to pay by Credit Card, PayPal, or Cash, but the checkout process stays the same.