Low Level Design Interview Mastery
Goal: Design and implement a working, extensible, object-oriented system in 45 minutes โ while talking through your decisions.
What is a Low Level Design (LLD) Interview?โ
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 โ 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 | 45โ60 min | 45โ60 min |
| Languages | Java, Python, C++ | Language-agnostic |
| Evaluated on | OOP, code quality, patterns | Scale, reliability, trade-offs |
The Interview Structure (45 minutes)โ
[0:00 โ 0:05] Requirements Clarification
[0:05 โ 0:10] Entity Identification & Relationships
[0:10 โ 0:20] Class Design (interfaces, hierarchy, patterns)
[0:20 โ 0:40] Implementation (core logic, clean Java code)
[0:40 โ 0:45] Edge Cases, Concurrency, Extensions
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 Forโ
๐ข Junior Level (0โ3 years)โ
- Can you identify nouns โ classes, verbs โ methods?
- Do you use
private/publiccorrectly (encapsulation)? - Can you avoid one giant class (SRP)?
- Basic working implementation with no glaring bugs?
๐ก Mid Level (3โ6 years)โ
- 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?
๐ด Senior Level (6+ years)โ
- 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 Structureโ
This guide is organized in the order you should learn things:
1. Foundationโ
- OOP Concepts โ The 4 pillars, with Java examples
- Design Principles โ SOLID + DRY, KISS, YAGNI
2. Design Patternsโ
- Overview โ When to use patterns, common mistakes
- Creational Patterns โ Singleton, Factory, Builder, Prototype
- Structural Patterns โ Adapter, Decorator, Composite, Facade
- Behavioral Patterns โ Strategy, Observer, Command, State
3. Concurrencyโ
- Correctness โ Atomicity, visibility, data races
- Coordination โ Locks, semaphores, barriers, condition variables
- Scarcity โ Thread pools, rate limiting, backpressure
4. Problemsโ
Full end-to-end walkthroughs of 8 classic LLD problems.
How to Use This Guideโ
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 Usedโ
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 โ