1. Why Tech Companies Test LLD for Freshers

While DSA proves algorithmic agility, Low-Level Design (LLD) evaluates how an engineer writes maintainable code in a team. Interviewers at top product companies assess whether you can design extensible classes that don't crumble when business requirements evolve.

A common failure pattern among candidates is jumping directly to writing code without clarifying the scope. When asked to design a "Parking Lot" or "Elevator System", successful candidates first establish clear boundaries: What vehicle types are supported? How is pricing calculated? Are there concurrent spot allocations?

2. Applying SOLID Principles in Real Scenarios

Candidates often memorize SOLID acronyms but fail to identify violations during coding exercises. The two most critical principles in technical interviews are:

Open/Closed Principle (OCP): Software entities should be open for extension, but closed for modification. If adding a new payment method (UPI, Crypto, Credit Card) requires modifying an existing processPayment() method with endless if/else chains, your design violates OCP.

// Applying Strategy Pattern to satisfy Open/Closed Principle
public interface PaymentStrategy {
    PaymentReceipt pay(double amount);
}

public class UpiPaymentStrategy implements PaymentStrategy {
    private final String vpaAddress;
    public UpiPaymentStrategy(String vpa) { this.vpaAddress = vpa; }
    
    @Override
    public PaymentReceipt pay(double amount) {
        // Authenticate with UPI gateway
        return new PaymentReceipt(UUID.randomUUID().toString(), true);
    }
}

public class CheckoutService {
    public PaymentReceipt completeOrder(double amount, PaymentStrategy strategy) {
        return strategy.pay(amount); // Decoupled execution
    }
}

3. The 3 Most Frequently Asked LLD Problems

Focus your preparation on these industry-standard interview problems:

• Design a Parking Lot: Tests vehicle inheritance, spot management, and Strategy Pattern for dynamic hourly vs. flat-rate fee calculations.
• Design an Elevator System: Tests state machines (Moving, Idle, Maintenance) and scheduling algorithms (LOOK/SCAN algorithms).
• Design a Notification Engine: Tests Observer Pattern and Factory Pattern to dispatch SMS, Email, and Push alerts dynamically.