LLD Foundations in Low-Level Design

Lessons in this section

1. OOP Basics

   Learn classes and objects, then the four big ideas of object-oriented code: encapsulation, abstraction, inheritance and polymorphism, with QuickBite examples you can run.

2. SOLID Principles

   Learn the five SOLID rules with pictures and runnable examples, then fix QuickBite's god-class OrderService until hidden tests pass and the design smells are gone.

3. UML Diagrams

   Learn to read and draw the two UML diagrams teams really use, class diagrams and sequence diagrams, then turn a QuickBite class diagram into working code.

4. Creational Patterns

   Learn simple factory, factory method, builder and singleton with QuickBite examples, then replace an if-chain with a notifier factory and add WhatsApp.

5. Structural Patterns

   Learn adapter, decorator, facade and proxy with QuickBite examples, then adapt two map services to one interface and add a caching decorator.

6. Behavioural Patterns

   Learn strategy, observer, state, command and chain of responsibility, then turn QuickBite's order status if-chains into state classes with listeners.

7. API and Interface Design

   Design APIs other code can rely on: clear contracts, one answer shape, input checks, stable error codes and safe versioning, checked by contract tests.

8. Concurrency Basics

   See race conditions happen, find them with a scheduler that tries every order, and fix them with locks, one-step updates and queues.