Design Patterns

Introduction to Design Patterns

Proven solutions to common software design problems. Learn the 23 Gang of Four patterns that every developer should know.

8 min readOverview

1What are Design Patterns?

🎯 Real-World Analogy
🧱 Building Architecture

An arch, a dome, or a flying buttress - architects don't reinvent these. They're proven patterns for solving structural problems.

♟️ Chess Openings

The Sicilian Defense, Queen's Gambit - chess masters don't invent new openings each game. They use battle-tested strategies.

Design Patterns are reusable solutions to commonly occurring problems in software design. They are templates that describe how to solve a problem in many different situations.

Why Learn Design Patterns?

🚀
Don't Reinvent the Wheel
Solutions to common problems already exist
🗣️
Common Vocabulary
'Let's use a Factory here' - everyone understands
📖
Readable Code
Patterns make code intention clear
🎯
Interview Essential
Frequently tested in LLD interviews

2The Gang of Four (GoF)

📚

Design Patterns: Elements of Reusable Object-Oriented Software

Published in 1994 by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (known as the "Gang of Four" or GoF).

This book cataloged 23 design patterns that are still used today, over 30 years later.

The 23 GoF patterns are divided into 3 categories:
Creational (5): Object creation
Structural (7): Object composition
Behavioral (11): Object interaction

3Creational Patterns

Deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.

Singleton
Essential
Ensure only one instance exists
Examples: Database connection pool, Logger, Configuration manager
Factory Method
Essential
Create objects without specifying exact class
Examples: NotificationFactory.create('email'), LoggerFactory.getLogger()
Abstract Factory
Create families of related objects
Examples: UI component factory (Windows vs Mac buttons, checkboxes)
Builder
Essential
Construct complex objects step by step
Examples: StringBuilder, QueryBuilder, HttpRequest.Builder
Prototype
Create objects by cloning existing ones
Examples: Object.clone(), copying game characters

4Structural Patterns

Deal with object composition - how classes and objects are combined to form larger structures.

Adapter
Essential
Convert interface to another expected interface
Examples: Legacy system integration, third-party library wrappers
Facade
Essential
Provide simplified interface to complex subsystem
Examples: Compiler facade, home theater system remote
Decorator
Essential
Add behavior to objects dynamically
Examples: I/O streams (BufferedReader), pizza toppings
Composite
Treat individual and group objects uniformly
Examples: File system (files and folders), UI components
Proxy
Control access to another object
Examples: Lazy loading, access control, logging proxy
Bridge
Separate abstraction from implementation
Examples: Device remote controls (TV, Radio), shapes and colors
Flyweight
Share common state between objects
Examples: Character objects in text editor, game particle systems

5Behavioral Patterns

Deal with communication between objects - how objects interact and distribute responsibilities.

Strategy
Essential
Define family of interchangeable algorithms
Examples: Payment methods, sorting algorithms, compression
Observer
Essential
Notify dependents of state changes
Examples: Event listeners, pub/sub systems, MVC
Command
Essential
Encapsulate requests as objects
Examples: Undo/redo, macro recording, task queues
State
Essential
Alter behavior when internal state changes
Examples: Vending machine, order status, TCP connection
Iterator
Access elements sequentially without exposing structure
Examples: For-each loops, database cursors
Template Method
Define algorithm skeleton, let subclasses fill in steps
Examples: Game loop, document parsing, test frameworks
Chain of Responsibility
Pass request along chain of handlers
Examples: Middleware, logging levels, event bubbling
Mediator
Centralize complex communications between objects
Examples: Chat room, air traffic control, dialog components
Memento
Capture and restore object state
Examples: Undo functionality, game saves
Visitor
Add operations to objects without changing them
Examples: Document export (PDF, HTML, XML)

6Interview Priority Guide

Focus on these patterns first - they appear most frequently in LLD interviews:

🔥 Must Know (90% of interviews)
  • • Singleton
  • • Factory
  • • Strategy
  • • Observer
  • • Builder
  • • Decorator
⭐ Good to Know
  • • State
  • • Command
  • • Adapter
  • • Facade
  • • Template Method
  • • Iterator
📚 Nice to Have
  • • Abstract Factory
  • • Composite
  • • Proxy
  • • Chain of Responsibility
  • • Mediator, Memento, Visitor

7How to Learn Design Patterns

1
Understand the Problem
Before learning the solution, understand what problem it solves. Why does this pattern exist?
2
Learn the Structure
Understand the participants: interface, concrete classes, client. Draw class diagrams.
3
Code It Yourself
Implement the pattern from scratch in your preferred language. Don't just read.
4
Find Real Examples
Look for the pattern in libraries you use: Java Collections, React hooks, Spring Framework.
5
Apply in LLD Problems
When solving Parking Lot or Elevator, actively think about which patterns apply.
⚠️ Don't Force Patterns!
Design patterns are tools, not goals. Use them when they solve a problem, not because you learned them. Over-engineering with patterns is as bad as not using them at all.

8Key Takeaways

1Design Patterns are reusable solutions to common software design problems.
2The Gang of Four book defined 23 patterns still used 30 years later.
3Patterns are grouped into Creational, Structural, and Behavioral.
4For interviews, focus on: Singleton, Factory, Strategy, Observer, Builder, Decorator.
5Don't force patterns - use them when they naturally fit the problem.
6Learn by coding, not just reading. Implement each pattern yourself.