LLD Lesson: The Strategy Pattern

1. The Problem: The "If-Else" Nightmare

Imagine designing a Pricing Engine for a ride-hailing app.

public double calculateFare(double distance, String type) {
    if (type.equals("Standard")) return distance * 10;
    if (type.equals("Premium")) return distance * 20;
    if (type.equals("Shared")) return distance * 5;
    // Every new ride type requires changing this code! (Violation of OCP)
    return 0;
}

2. The Solution: Strategy Pattern

We define a common interface for all algorithms and let the "Context" (the app) choose which one to use at runtime.

Step 1: The Strategy Interface

public interface PricingStrategy {
    double calculate(double distance);
}

Step 2: Concrete Strategies

public class StandardPricing implements PricingStrategy {
    public double calculate(double distance) { return distance * 10; }
}

public class PremiumPricing implements PricingStrategy {
    public double calculate(double distance) { return distance * 20; }
}

Step 3: The Context (PricingEngine)

public class PricingEngine {
    private PricingStrategy strategy;

    public void setStrategy(PricingStrategy strategy) {
        this.strategy = strategy;
    }

    public double getFare(double distance) {
        return strategy.calculate(distance);
    }
}

3. Real-world Intuition: The Multi-Charger

Think of a universal power adapter. The "Subject" is the wall outlet, and the "Strategies" are the different plugs (Type-C, Lightning, Micro-USB). You can switch the plug without changing the outlet.

4. Interview Discussion

Trade-offs: Strategy adds more classes to the project. Don't use it if you only have 2 constant behaviors.
Spring Boot Tip: Use a Map<String, PricingStrategy> injected via Constructor to automatically pick the right strategy based on a key.

Final Takeaway

The Strategy pattern is your #1 weapon against Conditional Bloat.