Last Minute Notes

Object-oriented low-level design and machine coding: an approach, UML in Mermaid, design patterns, twelve worked problems with runnable Java, and concurrency in LLD.

LLD is about classes, interfaces, relationships and behavior inside one service. Interviews ask you to design and often code something like a parking lot, an LRU cache or a rate limiter, with clean, extensible object-oriented code. Every Java solution on these pages was compiled and run.

Start with LLD Fundamentals

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01
LLD Fundamentals
How to approach low-level design and machine coding: an 8-step method, requirements to classes, UML class, sequence and state diagrams in Mermaid, SOLID and design principles in practice, code quality checklist, and a 60-minute plan.
Beginner
Table of Contents 1. What LLD Interviews Test 2. The 8-Step Method 3. From Requirements to Classes 4. UML With Mermaid 5. Principles in Practice 6. Code Quality Checklist 7. Common Mistakes 8. A 60-Minute Plan 9. Questions and Answers
02
Design Patterns
A practical design pattern catalog for LLD interviews: when to use each pattern, a chooser flowchart, and runnable Java for Builder, State, Command, Chain of Responsibility, Template Method, Adapter, Facade, Proxy, Composite and Mediator, plus TypeScript versions.
Beginner to mid
Table of Contents 1. Catalog and Chooser 2. Creational Patterns 3. Structural Patterns 4. Behavioral Patterns 5. TypeScript Versions 6. Anti-Patterns and Misuse 7. Patterns in the Machine Coding Problems 8. Questions and Answers
03
Machine Coding 1
Six machine coding problems with requirements, class diagrams and complete runnable Java: Parking Lot, LRU Cache, Elevator, Vending Machine, Tic-Tac-Toe, and a Logger framework.
Mid
Table of Contents 1. Parking Lot 2. LRU Cache 3. Elevator System 4. Vending Machine 5. Tic-Tac-Toe 6. Logger Framework
04
Machine Coding 2
Six more machine coding problems with complete runnable Java: Movie Ticket Booking with seat holds, Splitwise with debt simplification, an in-memory Pub/Sub broker, pluggable Rate Limiter, ATM with State and Chain of Responsibility, and an in-memory File System.
Mid to senior
Table of Contents 1. Movie Ticket Booking 2. Splitwise 3. Pub/Sub Broker 4. Rate Limiter 5. ATM 6. In-Memory File System
05
Concurrency in LLD
Thread safety for low-level design: race conditions, the Java concurrency toolbox, lock granularity, deadlock and lock ordering, producer-consumer, bounded buffers, thread-safe singletons, optimistic CAS, testing concurrent code, and async races in JavaScript. All Java runs.
Senior
Table of Contents 1. The Three Problems 2. The Java Toolbox 3. Lost Updates and Atomic Counters 4. Check-Then-Act and Lock Granularity 5. Deadlock 6. Producer-Consumer 7. Thread-Safe Singletons and Safe Publication 8. Optimistic Concurrency 9. Design Strategies 10. Testing Concurrent Code 11. JavaScript: Async Races +2 more

Prerequisites: the OOPS notes (pillars, SOLID, UML, five core patterns) and Java basics. For the architecture side of interviews, see HLD.

Low-Level Design (LLD)