☕ Java Low Level Design Problems¶

A collection of 3 Low-Level Design (LLD) problems implemented in Java, following SOLID principles and design patterns — curated for backend developer interviews.

🎯 Preparing for the round itself? See the LLD Interview Playbook (language-neutral, with Java and Go concurrency equivalents).

📋 Project Index¶

#	Project	Domain	Key Patterns
1	Elevator System	State Machine	State, Strategy, Observer
2	Hotel Booking System	Reservations	Strategy, Factory, Observer
3	Meeting Scheduler	Calendar	Strategy, Observer, Command

🚀 How to Run¶

Each project is self-contained. Navigate to the project directory and compile/run:

cd java-low-level-design/<project-name>
javac <MainClass>.java
java <MainClass>

🧩 Design Patterns Used¶

Pattern	Usage
Strategy	Dispatching, pricing, booking policies
Observer	Notifications, monitoring, calendar sync
State	Elevator lifecycle, booking status
Facade	Unified service interfaces
Command	Request objects, undoable operations
Decorator	Compose pricing modifiers

📖 Interview Preparation¶

Each project includes: - THOUGHT_PROCESS.md — Design reasoning and trade-offs - CODE.md — Implementation walkthrough with code snippets - HIGH_LEVEL_DESIGN.md — Production architecture and scale - INTERVIEW_QUESTIONS.md — Common interview questions with answers