Low-Level Design for MAANG Interviews

A book-shaped table of contents for LLD interview prep: OOP fundamentals through SOLID, design principles, UML, design patterns, dependency management, reliability, concurrency, domain modeling, API design, testing, refactoring, classic interview problems, advanced architecture, and performance — cross-linking Object-Oriented Programming and Patterns instead of duplicating them.

1 — Unit Testing

Covers writing isolated, fast, deterministic tests that verify a single unit of behavior without touching external dependencies.

2 — Testable Design

Explains how explicit dependency boundaries and small units of behavior make code inherently easier to exercise in isolation.

3 — Mocking

Covers using test doubles to isolate the unit under test from collaborators that are slow, external, or nondeterministic.

4 — Dependency Injection for Testing

Explains how injecting dependencies rather than constructing them internally lets tests substitute fakes without touching production code.