Low Level Design Interview Mastery

Goal: Design and implement a working, extensible, object-oriented system in 45 minutes — while talking through your decisions.

What is a Low Level Design (LLD) Interview?

A Low Level Design interview (also called Object-Oriented Design or OOD interview) asks you to model a real-world system using classes, interfaces, and design patterns — and then write working code for it.

Unlike system design interviews (which focus on distributed systems, scalability, and infrastructure), LLD interviews zoom into the class-level structure of a solution.

Aspect	LLD Interview	System Design Interview
Focus	Classes, objects, patterns	Services, databases, networks
Output	Working code	Architecture diagram
Duration	45–60 min	45–60 min
Languages	Java, Python, C++	Language-agnostic
Evaluated on	OOP, code quality, patterns	Scale, reliability, trade-offs

The Interview Structure (45 minutes)

[0:00 – 0:05]  Requirements Clarification
[0:05 – 0:10]  Entity Identification & Relationships
[0:10 – 0:20]  Class Design (interfaces, hierarchy, patterns)
[0:20 – 0:40]  Implementation (core logic, clean Java code)
[0:40 – 0:45]  Edge Cases, Concurrency, Extensions

Interview Gold Rule

Narrate everything. Interviewers evaluate your thinking process, not just the final code. A great engineer who stays silent scores lower than a good engineer who explains each decision.

What Interviewers Look For

Junior Level (0–3 years)

Can you identify nouns → classes, verbs → methods?
Do you use private/public correctly (encapsulation)?
Can you avoid one giant class (SRP)?
Basic working implementation with no glaring bugs?

Mid Level (3–6 years)

Do you program to interfaces, not implementations?
Do you apply SOLID principles naturally?
Do you choose appropriate design patterns (and can justify why)?
Can you identify where concurrency could be an issue?

Senior Level (6+ years)

Do you immediately consider thread safety and make explicit choices?
Do you know when patterns add complexity without value?
Can you discuss trade-offs (e.g., eager vs. lazy loading, locking granularity)?
Do you proactively ask about extensibility requirements?
Can you sketch a path from class-level design to a distributed system?

Guide Structure

This guide is organized in the order you should learn things:

1. Foundation

OOP Concepts — The 4 pillars, with Java examples
Design Principles — SOLID + DRY, KISS, YAGNI

2. Design Patterns

Overview — When to use patterns, common mistakes
Creational Patterns — Singleton, Factory, Builder, Prototype
Structural Patterns — Adapter, Decorator, Composite, Facade
Behavioral Patterns — Strategy, Observer, Command, State

3. Concurrency

Correctness — Atomicity, visibility, data races
Coordination — Locks, semaphores, barriers, condition variables
Scarcity — Thread pools, rate limiting, backpressure

4. Problems

Full end-to-end walkthroughs of 8 classic LLD problems.

How to Use This Guide

If you have 2 weeks: Read everything in order. Do each problem from scratch before reading the solution.

If you have 3 days: Read OOP Concepts + Principles, skim Design Patterns overview, then focus on 3-4 problems relevant to your target company.

If you have 1 day: Read SOLID Principles, then do Parking Lot and Rate Limiter end-to-end.

Java Conventions Used

All code examples use Java 17+ with standard library (no frameworks, unless noted).

// Conventions used throughout:
// - Interfaces for all major abstractions
// - Enums for finite states
// - Records for immutable value objects (Java 16+)
// - Optional<T> instead of null returns
// - var for local type inference where it improves readability

Let's go! Start with OOP Concepts →