Design Patterns

Design Patterns

All 23 Gang of Four patterns — Creational, Structural, and Behavioral — with multi-class UML diagrams, full Java implementations, anti-patterns, and interview preparation guides.

23 patterns 3 categories 4 reference guides Java + Python implementations
01
Category One · Object Creation

Creational Patterns

📦

Singleton

  • Ensure a class has only one instance
  • Tier 1 · Foundational
🏭

Factory Method

  • Defer instantiation to subclasses
  • Tier 1 · Foundational
🏗️

Abstract Factory

  • Create families of related objects
  • Tier 2 · Intermediate
🧱

Builder

  • Construct complex objects step by step
  • Tier 2 · Intermediate
🐑

Prototype

  • Clone existing objects without coupling
  • Tier 3 · Intermediate
02
Category Two · Object Composition

Structural Patterns

🔌

Adapter

  • Convert one interface to another
  • Tier 2 · Foundational
🌉

Bridge

  • Separate abstraction from implementation
  • Tier 3 · Intermediate
🌲

Composite

  • Compose objects into tree structures
  • Tier 2 · Intermediate
🎁

Decorator

  • Add responsibilities dynamically
  • Tier 1 · Intermediate
🏠

Facade

  • Provide a simplified interface
  • Tier 2 · Foundational
🪶

Flyweight

  • Share fine-grained objects efficiently
  • Tier 3 · Advanced
🛡️

Proxy

  • Control access to another object
  • Tier 2 · Intermediate
03
Category Three · Communication

Behavioral Patterns

🔔

Observer

  • Notify dependents of state changes
  • Tier 1 · Foundational
♟️

Strategy

  • Encapsulate interchangeable algorithms
  • Tier 1 · Foundational
📜

Command

  • Encapsulate a request as an object
  • Tier 2 · Intermediate
📋

Template Method

  • Define algorithm skeleton, defer steps
  • Tier 2 · Foundational
⛓️

Chain of Responsibility

  • Pass request along a chain of handlers
  • Tier 3 · Intermediate
🚦

State

  • Alter behavior when internal state changes
  • Tier 2 · Intermediate
📡

Mediator

  • Centralize complex communications
  • Tier 3 · Intermediate
🔁

Iterator

  • Access elements sequentially
  • Tier 2 · Foundational
🧑‍💼

Visitor

  • Add operations without changing classes
  • Tier 3 · Advanced
📸

Memento

  • Capture and restore object state
  • Tier 3 · Intermediate
🔤

Interpreter

  • Define a grammar and interpret sentences
  • Tier 3 · Advanced
04
Category Four · Reference Guides

Reference & Guides

🎯

Interview Patterns

  • Tier 1–3 pattern ranking
  • What interviewers test
⚠️

Anti-Patterns

  • The wrong approach for each pattern
  • How to recognise and fix them
🔗

Pattern Relationships

  • Confused-with pairs
  • Compose-together trios
☕

Java Stdlib Map

  • Every pattern in the JDK
  • Real class and method references