This handbook is a comprehensive guide to Low Level Design (LLD) / Object Oriented Design (OOD) for coding interviews at top tech companies — now with complete, runnable Java solutions to classic machine coding problems like Parking Lot, Splitwise, BookMyShow, Elevator System, Snake and Ladder, and Vending Machine. Each topic covers core principles, SOLID, design patterns, class diagrams, real-world examples, common interview questions, edge cases, and best practices to help you master system design fundamentals.
- 🎯 About This Handbook
- ⚡ Quick Start
- 🧩 LLD Solutions — Machine Coding Problems
- 📚 Topics Covered
- 🎯 What You'll Learn
- 📖 Must-Know References
- 📺 Video Resources
- ⭐ Star History
- 🤝 Contributing
- ⭐ Support the Project
This handbook is designed to help you master Low Level Design concepts for coding interviews. Each topic is presented in a structured format covering:
- 🔑 Core Principles - Fundamental OOP and design concepts
- 🏗️ Design Patterns - Common solutions to recurring problems
- 💡 Best Practices - Industry-standard coding guidelines
⚠️ Common Pitfalls - What to avoid in your designs- 🎯 Interview Questions - Practice with real scenarios
- 📝 Implementation Tips - Practical coding advice
- 🔗 Additional Resources - Further reading and examples
- Start with OOP Basics if you're new to object-oriented design
- Master SOLID Principles for robust software design
- Learn Design Patterns through practical examples
- Practice with the LLD Solutions — full Java implementations of classic machine coding problems
- Revise with the LLD Cheatsheet the night before your interview
Week 1-2: OOD Basics
┌─────────────────────────────────────────────────────────┐
│ Classes & Objects → Encapsulation → Inheritance │
│ → Polymorphism → Abstraction → Interfaces │
│ → Abstract Classes → Relationships │
└─────────────────────────────────────────────────────────┘
↓
Week 3-4: SOLID Principles
┌─────────────────────────────────────────────────────────┐
│ SRP → OCP → LSP → ISP → DIP │
└─────────────────────────────────────────────────────────┘
↓
Week 5-7: Design Patterns
┌─────────────────────────────────────────────────────────┐
│ Creational: Factory, Builder, Singleton │
│ Structural: Adapter, Decorator, Facade │
│ Behavioral: Strategy, Observer, Command │
└─────────────────────────────────────────────────────────┘
↓
Week 8+: Interview Practice
┌─────────────────────────────────────────────────────────┐
│ Easy Questions → Medium Questions → Hard Questions │
└─────────────────────────────────────────────────────────┘
| Your Level | Start Here | Focus Areas | Time Estimate |
|---|---|---|---|
| Beginner | OOD Basics | All pillars of OOP | 8-10 weeks |
| Know OOP | SOLID Principles | Design guidelines | 6-8 weeks |
| Know SOLID | Design Patterns | Pattern application | 4-6 weeks |
| Interview Ready | Interview Questions | Practice problems | 2-4 weeks |
- Clean Code - Apply throughout your learning
- Error Handling - Essential for robust code
- Testing - Validate your implementations
- Code Review - Improve code quality
⭐ Last-minute revision? Jump straight to the one-page LLD Cheatsheet.
Complete, runnable solutions to the classic LLD interview problems — requirements → class diagrams → full Java code → edge cases → follow-up questions:
| # | Problem | Difficulty | Key Concepts |
|---|---|---|---|
| 1 | 🚗 Parking Lot | 🟢 Easy | Singleton, Strategy, concurrency-safe spot allocation |
| 2 | 🎲 Snake and Ladder | 🟢 Easy | Rules engine, pluggable win strategies, seeded dice |
| 3 | 💰 Splitwise | 🟡 Medium | Strategy splits, balance ledger, debt simplification |
| 4 | 🛗 Elevator System | 🟡 Medium | State machines, LOOK scheduling, dispatch mediation |
| 5 | 🎟️ BookMyShow | 🔴 Hard | Seat locking with TTL, no double booking, compensation |
| 6 | 🥤 Vending Machine | 🟢 Easy | State pattern, change making, guard rails |
| 7 | ⭕ Tic-Tac-Toe | 🟢 Easy | O(1) win detection, pluggable move strategies |
| 8 | 🗃️ LRU Cache | 🟢 Easy | HashMap + doubly linked list, O(1) get/put |
| 9 | ☕ Coffee Vending Machine | 🟡 Medium | Recipe-as-data, atomic inventory, Observer alerts |
👉 View all 18 planned problems & the practice roadmap →
- 📌 Classes & Objects
- 🔄 Inheritance
- 🎭 Polymorphism
- 🔒 Encapsulation
- 🎯 Abstraction
- 🔌 Interfaces
- 🏗️ Abstract Classes
- 🤝 Association, Aggregation & Composition
- 🎯 Single Responsibility (SRP)
- 🔓 Open/Closed (OCP)
- 🔄 Liskov Substitution (LSP)
- 🔌 Interface Segregation (ISP)
↔️ Dependency Inversion (DIP)
- Object-oriented programming principles
- SOLID design principles
- Common design patterns
- System decomposition
- Requirements analysis
- System modeling
- Design trade-offs
- Performance considerations
- How to approach design problems
- Communication techniques
- Handling ambiguity
- Design documentation
- Refactoring Guru - Design patterns and principles
- Clean Code by Robert C. Martin
- Head First Design Patterns
- Martin Fowler's Blog - Architecture and design insights
We welcome contributions! Please check our Contributing Guidelines for details on:
- 📝 How to contribute
- 🎨 Style guidelines
- 💬 Commit message conventions
- 🔄 Pull request process
- 📚 Topic guidelines
- 🐛 Reporting issues
If this handbook helps you in your interview preparation, please consider:
📚 More interview handbooks by the author: DSA Handbook for Coding Interviews · HLD Handbook for Coding Interviews · Go Handbook for Software Engineers
Created with insights from industry best practices, design pattern resources, and real interview experiences. Happy System Designing! 🎨