design-patterns/README.md

GoF Design Patterns in Java

Complete runnable Java 25 implementations of all 23 Gang of Four design patterns. Each pattern has a dedicated article at ankurm.com with a UML diagram, step-by-step explanation, and console output.

Structure

design-patterns/
├── 01-creational/
│   ├── factory-method/
│   ├── abstract-factory/
│   ├── builder/
│   ├── prototype/
│   └── singleton/
├── 02-structural/
│   ├── adapter/
│   ├── bridge/
│   ├── composite/
│   ├── decorator/
│   ├── facade/
│   ├── flyweight/
│   └── proxy/
└── 03-behavioral/
    ├── chain-of-responsibility/
    ├── command/
    ├── interpreter/
    ├── iterator/
    ├── mediator/
    ├── memento/
    ├── observer/
    ├── state/
    ├── strategy/
    ├── template-method/
    └── visitor/

Prerequisites

• Java 25+ (Eclipse Temurin recommended)
• No build tool required — plain javac / java

Run any pattern

# Linux / macOS / Git Bash (Windows)
javac 03-behavioral/strategy/*.java -d out/strategy
java -cp out/strategy strategy.Main

REM Windows Command Prompt
javac 03-behavioral\strategy\*.java -d out\strategy
java -cp out\strategy strategy.Main

Articles

Pattern	Category	Article
Factory Method	Creational	https://ankurm.com/factory-method-design-pattern-java/
Abstract Factory	Creational	https://ankurm.com/abstract-factory-design-pattern-java/
Builder	Creational	https://ankurm.com/builder-design-pattern-java/
Prototype	Creational	https://ankurm.com/prototype-design-pattern-java/
Singleton	Creational	https://ankurm.com/singleton-design-pattern-java/
Adapter	Structural	https://ankurm.com/adapter-design-pattern-java/
Bridge	Structural	https://ankurm.com/bridge-design-pattern-java/
Composite	Structural	https://ankurm.com/composite-design-pattern-java/
Decorator	Structural	https://ankurm.com/decorator-design-pattern-java/
Facade	Structural	https://ankurm.com/facade-design-pattern-java/
Flyweight	Structural	https://ankurm.com/flyweight-design-pattern-java/
Proxy	Structural	https://ankurm.com/proxy-design-pattern-java/
Chain of Responsibility	Behavioral	https://ankurm.com/chain-of-responsibility-design-pattern-java/
Command	Behavioral	https://ankurm.com/command-design-pattern-java/
Interpreter	Behavioral	https://ankurm.com/interpreter-design-pattern-java/
Iterator	Behavioral	https://ankurm.com/iterator-design-pattern-java/
Mediator	Behavioral	https://ankurm.com/mediator-design-pattern-java/
Memento	Behavioral	https://ankurm.com/memento-design-pattern-java/
Observer	Behavioral	https://ankurm.com/observer-design-pattern-java/
State	Behavioral	https://ankurm.com/state-design-pattern-java/
Strategy	Behavioral	https://ankurm.com/strategy-design-pattern-java/
Template Method	Behavioral	https://ankurm.com/template-method-design-pattern-java/
Visitor	Behavioral	https://ankurm.com/visitor-design-pattern-java/

Post ↔ Code Mapping

Each article is split into multiple runnable parts. This section maps every post section to the exact file(s) in this repository, so you can jump straight from "what I just read" to "the file that implements it." Updated pattern-by-pattern as each article is verified against Java 25.

Factory Method (01-creational/factory-method/)

Post Section	File(s)
The Problem: Object Creation Leaks Into Business Logic	NotificationService.java (anti-pattern shown for contrast — not called by Main)
Part 1 — The Notification Interface	Notification.java
Part 2 — The Concrete Notification Classes	EmailNotification.java, SmsNotification.java, PushNotification.java
Part 3 — The NotificationSender Class	NotificationSender.java
Part 4 — The Concrete Sender Subclasses	EmailSender.java, SmsSender.java, PushSender.java
Part 5 — Running It: Client Code and Output	Main.java
Adding a New Channel (Slack)	SlackNotification.java, SlackSender.java

Run it:

javac 01-creational/factory-method/*.java -d out/factory-method
java -cp out/factory-method factorymethod.Main

Abstract Factory (01-creational/abstract-factory/)

Post Section	File(s)
Part 1 — The Abstract Products: Button and TextField	Button.java, TextField.java
Part 2 — The Concrete Products (Light family)	LightButton.java, LightTextField.java
Part 2 — The Concrete Products (Dark family)	DarkButton.java, DarkTextField.java
Part 3 — The Abstract Factory: UIFactory Interface	UIFactory.java
Part 4 — The Concrete Factories	LightThemeFactory.java, DarkThemeFactory.java
Part 5 — The Client: Application	Application.java, Main.java

Run it:

javac 01-creational/abstract-factory/*.java -d out/abstract-factory
java -cp out/abstract-factory abstractfactory.Main
# or, for the dark theme family:
java -Dapp.theme=dark -cp out/abstract-factory abstractfactory.Main

Builder (01-creational/builder/)

Post Section	File(s)
Part 1 — The Product: HttpRequest (Immutable)	HttpRequest.java
Part 2 — The Director: Encapsulating Common Configurations	HttpClientConfig.java
Part 3 — Using the Builder: Client Code	Main.java

Note: the Lombok @Builder example and the JDK standard-library snippets (HttpRequest.newBuilder(), StringBuilder, ProcessBuilder) are illustrative only — they are not part of this repository's runnable example.

Run it:

javac 01-creational/builder/*.java -d out/builder
java -cp out/builder builder.Main

Prototype (01-creational/prototype/)

Post Section	File(s)
Part 1 — The Prototype Interface: Copyable	Copyable.java
Part 2 — The Nested Object: DocumentMetadata	DocumentMetadata.java
Part 3 — The Concrete Prototype: Document	Document.java
Part 4 — The Prototype Registry: DocumentRegistry	DocumentRegistry.java
Part 5 — Using the Prototype: Client Code	Main.java

Note: the ShallowVsDeepDemo snippet (shallow vs. deep copy) and the Cloneable-based example are illustrative only — they are not part of this repository's runnable example.

Run it:

javac 01-creational/prototype/*.java -d out/prototype
java -cp out/prototype prototype.Main

Reference

• Design Patterns: Elements of Reusable Object-Oriented Software — Gamma, Helm, Johnson, Vlissides
• https://refactoring.guru/design-patterns