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Foundations - Low Level Design

Course Contents

What is Low Level Design? (LLD vs HLD)

Why LLD Matters — The 70% Maintenance Problem

How This Course is Structured

How to Get the Most Out of This Course

Imperative Programming

Procedural Programming

Object-Oriented Programming — The Big Picture

Functional Programming vs OOP — When to Use What

Classes and Objects — Blueprint vs Instance

Constructors — Default, Parameterized, Copy

Encapsulation — Data Hiding Done Right

Abstraction — Hiding Complexity

Inheritance — Code Reuse & Hierarchies

Polymorphism — Compile-Time vs Runtime

Method Overloading vs Overriding

Enums — Type-Safe Constants

Interfaces vs Abstract Classes

Association — Objects That Know Each Other

Aggregation — Has-A with Independent Lifecycle

Composition — Has-A with Dependent Lifecycle

Dependency — Objects That Use Each Other

Composition vs Inheritance — Favor Composition

Generalization vs Specialization

Introduction to UML — Why Visual Modeling Matters

Class Diagrams — Structure of Your System

Use Case Diagrams — Who Does What

Sequence Diagrams — How Objects Interact Over Time

Activity Diagrams — Workflow and Decision Points

State Machine Diagrams — Object Lifecycle States

Reading and Drawing UML — Practical Exercise

Why Design Principles Exist — The Cost of Bad Design

S — Single Responsibility Principle

O — Open/Closed Principle

L — Liskov Substitution Principle

I — Interface Segregation Principle

D — Dependency Inversion Principle

SOLID in Action — Refactoring a Bad Design

DRY — Don't Repeat Yourself

KISS — Keep It Simple, Stupid

YAGNI — You Aren't Gonna Need It

Law of Demeter (Principle of Least Knowledge)

GRASP Principles — General Responsibility Assignment

Composition Over Inheritance

What is Domain Modeling? — From Requirements to Objects

Entities vs Value Objects — Identity and Equality

Aggregates & Aggregate Roots — Consistency Boundaries

Domain Invariants — Rules Your Objects Must Enforce

The Object Discovery Process — Nouns, Verbs, Relationships

Domain Modeling Exercise — E-Commerce Domain

Code Smells — Recognizing Bad Design

Refactoring: God Object → Single Responsibility

Refactoring: Switch-Case → Strategy Pattern

Refactoring: Deep Inheritance → Composition

Refactoring: Tight Coupling → Dependency Injection

Before/After Case Study — Complete Refactoring Walk-through

What is Software Architecture? — Monolith, Layered, Clean

Layered Architecture — Presentation, Service, Repository

Dependency Injection — Constructor, Setter, Interface

DI Containers & Inversion of Control

Module Separation — Package Structure & Access Control

How to Approach Any LLD Problem — The 4-Step Framework

Step 1: Clarify Requirements & Define Scope

Step 2: Identify Core Objects and Relationships

Step 3: Draw UML Diagrams (Class, Use Case, Sequence)

Step 4: Implement with Patterns & Principles

Domain Modeling Exercise — E-Commerce Domain

Apply every technique from Section 8 — noun-verb analysis, entity vs value object classification, aggregate boundaries, and invariant enforcement — to model a complete e-commerce domain from scratch in a guided, hands-on exercise.

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