Introduction to Low-Level Design

Introduction

Welcome to Low-Level Design (LLD) on TechLearningPRO — a hands-on course for engineers preparing for object-oriented design interviews and building maintainable Java systems.

LLD sits between high-level architecture and production code. You translate requirements into classes, interfaces, relationships, and behavioral contracts that a team can implement and extend without rewriting the system every sprint.

Across 42 lessons you will master OOP fundamentals, SOLID principles, UML notation, creational and behavioral patterns, design guidelines, classic interview problems (parking lot, ATM, elevator, and more), and a structured approach to whiteboard design rounds.

Understanding the topic

Key concepts

• LLD defines classes, methods, and object interactions — not deployment topology or database sharding.
• Good LLD balances clarity, extensibility, and interview time constraints (typically 45–60 minutes).
• Java 21 is our reference language: records, sealed types, and modern APIs where they simplify design.
• Interviews test whether you can model entities, assign responsibilities, and justify trade-offs.
• UML sketches communicate intent faster than prose during live design sessions.
• Patterns are tools, not goals — apply them when a recurring problem appears.

flowchart LR
  Requirements --> UseCases
  UseCases --> ClassModel
  ClassModel --> Patterns
  Patterns --> Code

Step-by-step explanation

1. Clarify functional requirements and actors before drawing any class.
2. Identify nouns (entities) and verbs (operations) from the problem statement.
3. Assign each responsibility to exactly one primary class (Single Responsibility).
4. Define interfaces for variation points (payment types, vehicle sizes, pricing rules).
5. Walk through one happy path and one edge case on a sequence diagram.
6. Refactor names and relationships until the model reads like the problem domain.

Informative example

A minimal LLD entry point — a service interface and one implementation keeps the design testable and interview-friendly:

public interface BookingService {
    Confirmation book(BookingRequest request);
}

public record BookingRequest(String userId, String showId, int seats) {}

public record Confirmation(String bookingId, String status) {}

public final class DefaultBookingService implements BookingService {
    private final SeatAllocator allocator;
    private final PaymentGateway payments;

    public DefaultBookingService(SeatAllocator allocator, PaymentGateway payments) {
        this.allocator = allocator;
        this.payments = payments;
    }

    @Override
    public Confirmation book(BookingRequest request) {
        allocator.reserve(request.showId(), request.seats());
        payments.charge(request.userId(), request.seats());
        return new Confirmation("BK-" + System.nanoTime(), "CONFIRMED");
    }
}

Start interviews with interfaces and records for DTOs. Inject dependencies through constructors — interviewers notice testability immediately.

Real-world use

Real-world applications

• Backend engineer rounds at product companies (Amazon, Microsoft, Uber).
• Refactoring legacy Java modules into clearer bounded contexts.
• Writing RFCs that developers can implement without ambiguity.
• Code review discussions about where a new feature belongs.

Best practices

• Always restate requirements and assumptions before designing.
• Prefer composition over inheritance unless an is-a relationship is obvious.
• Name classes after domain concepts, not pattern names (use PaymentProcessor, not StrategyImpl).
• Keep methods short and intention-revealing.
• Design for extension at payment, notification, and pricing boundaries.
• Use enums for fixed state machines with few transitions.
• Practice explaining trade-offs aloud — communication counts as much as boxes and arrows.

Common mistakes

• Jumping to Singleton or Factory before identifying entities.
• Creating a God class that handles UI, persistence, and business rules.
• Ignoring concurrency when the problem mentions multiple users or threads.
• Over-engineering with micro-patterns for a one-screen CRUD problem.
• Skipping edge cases: null inputs, duplicate requests, insufficient resources.

Advanced interview questions

Summary

LLD bridges requirements and implementation through classes and interactions. TechLearningPRO's course covers OOP, SOLID, UML, patterns, and classic problems. Start with requirements, entities, and one core use case. Java 21 records and interfaces keep models concise. Patterns support the design — they are not the headline. Practice explaining decisions; communication wins tie-breakers.