Design-Patterns

Advanced LLD Topics — A Deep Dive Who this is for: Anyone who has finished the SOLID principles and the three design-pattern modules (Creational, Structural, Behavioural) and is ready to learn how to use that knowledge well — in real systems, in real interviews, in real production code. What this module builds on: SOLID Principles (Module 2), Creational Patterns (Module 3), Structural Patterns (Module 4), Behavioural Patterns (Module 5). What this module feeds into: Case Studies (Module 9) and Testing (Module 8). The Dependency Injection section here is the foundation for every fake, mock, and stub you’ll write in Module 8. ...

Design Patterns, Part 1: Foundations and Creational Patterns Table of Contents What a Design Pattern Is Why Design Patterns Are Studied Classification of the GoF Catalogue The Template Used for Each Pattern The Problem Space of Creational Patterns Singleton Factory Method Abstract Factory Builder Prototype Comparative Summary of the Five Patterns Key Takeaways Practice Exercises with Solutions Self-Assessment Questions 1. What a Design Pattern Is 1.1 Definition Design pattern. A named, general, reusable solution to a problem that recurs within a particular context of software design. A pattern is a description of communicating objects and classes that are customised to solve a general design problem in that context. ...

Design Patterns — Part 2: Structural Patterns Who this is for: Anyone who has worked through the Creational patterns and is ready to learn how objects fit together — how to combine classes and objects into larger structures that stay flexible and maintainable. Table of Contents Structural Patterns — What and Why Adapter Bridge Composite Decorator Facade Flyweight Proxy Summary of Structural Patterns Practice Exercises with Solutions Self-Assessment Test 1. Structural Patterns Creational patterns answered: “how do we make objects flexibly?” Structural patterns answer: “how do we combine objects into larger structures flexibly?” ...

Design Patterns — Part 3: Behavioral Patterns Who this is for: Anyone who has worked through Creational and Structural patterns and is ready for the final, largest, and arguably most important family — the patterns that govern how objects communicate and divide responsibility. Table of Contents Behavioral Patterns — What and Why Chain of Responsibility Command Interpreter Iterator Mediator Memento Observer State Strategy Template Method Visitor Summary of Behavioral Patterns Practice Exercises with Solutions Self-Assessment Test Final Words — All 23 Patterns at a Glance 1. Behavioral Patterns Creational patterns answered “how do we make objects?” Structural patterns answered “how do we glue objects together?” Behavioral patterns answer: “how do objects talk to each other, and who is responsible for what?” ...

Design Patterns — Practice Workbook Who this is for: Anyone who has worked through the three teaching files — 03_design_patterns_creational.md, 04_design_patterns_structural.md, and 05_design_patterns_behavioral.md — and now wants to prove they actually understand by solving problems independently. Table of Contents Preface — How to Use This Workbook Section 1 — Creational Patterns Practice Problem 1.1 — Application Configuration (Level 1) Problem 1.2 — Document Export Flow (Level 1) Problem 1.3 — Food Delivery Order (Level 2) Problem 1.4 — Game Enemy Spawning (Level 2) Problem 1.5 — Cloud Environment Configurations (Level 3) Section 2 — Structural Patterns Practice Problem 2.1 — Third-Party Payment SDK (Level 1) Problem 2.2 — Coffee Shop Pricing (Level 2) Problem 2.3 — Smart Home Scenes (Level 2) Problem 2.4 — Library Book Database (Level 2) Problem 2.5 — File System with Virtual Files (Level 3) Problem 2.6 — UI Rendering Across Platforms (Level 3) Section 3 — Behavioural Patterns Practice Problem 3.1 — Stock Price Monitor (Level 1) Problem 3.2 — E-commerce Shipping (Level 1) Problem 3.3 — Text Editor Undo/Redo (Level 2) Problem 3.4 — Vending Machine (Level 2) Problem 3.5 — Order Validation Pipeline (Level 2) Problem 3.6 — News Aggregator (Level 3) Problem 3.7 — Chess Engine (Level 3) Section 4 — Combination Problems: Multiple Patterns Together Problem 4.1 — Ride-Sharing App (Level 3) Problem 4.2 — Hotel Booking Platform (Level 3) Problem 4.3 — Notification Service (Level 3) Problem 4.4 — Document Management System (Level 3 — Hardest) Section 5 — Anti-Pattern Traps: When NOT to Use a Pattern Problem 5.1 — A Simple CSV Script Problem 5.2 — TemperatureConverter Problem 5.3 — Internal Bulk-Rename Tool Summary — Pattern Selection Reference Card Preface You’ve read the three teaching files. Each one gave you the pattern: “Here is Observer, here is what it solves, here is the example.” Reading those files makes you familiar with patterns. It does not make you good at using them. ...

UML Deep Dive: Class Diagrams and Sequence Diagrams Who this is for: Anyone who has been designing classes and systems (perhaps through OOP, SOLID, and design patterns) and now wants a standardized visual language to communicate those designs to teammates, interviewers, and their own future self. Table of Contents Why UML Exists The 14 UML Diagram Types: A Complete Map Class Diagrams: Anatomy of a Class Box Abstract Classes vs. Interfaces: Are They the Same Thing? Class Diagrams: Relationships Between Classes Class Diagrams: Advanced Concepts Sequence Diagrams: Why We Need a Second Diagram Sequence Diagrams: Anatomy and Core Concepts Sequence Diagrams: Control Flow (alt, opt, loop, par) 9.5 More Sequence Diagram Examples Putting It All Together: A Simple E-Commerce System 10.1 The Complete Class Diagram 10.2 The Complete Sequence Diagram Summary and Key Takeaways Practice Questions with Solutions 1. Why UML Exists When a design exists only in someone’s head, or scattered across a codebase, it is expensive to communicate. Explaining a system in prose forces the listener to reconstruct a graph of classes and relationships from a linear sequence of sentences, and details get lost or misinterpreted along the way. Two people reading the same paragraph of design prose regularly walk away with two different mental models of the same system, particularly around exactly how strong a relationship between two entities is. ...