Java Design Patterns Practice

A collection of design pattern implementations in Java for learning and reference.

Patterns Implemented

Creational Patterns

• Singleton - Ensures a class has only one instance

  • DatabaseConnection: Double-checked locking implementation
  • AppConfig: Enum-based singleton (recommended approach)

• Factory Method - Creates objects without specifying exact class

  • NotificationFactory: Creates Email, SMS, or Push notifications

• Abstract Factory - Creates families of related objects

  • UIFactory: Creates themed UI components (Dark/Light themes)

• Builder - Constructs complex objects step by step

  • HttpRequest.Builder: Fluent API for building HTTP requests

Behavioral Patterns

• Observer - Notifies multiple objects about state changes

  • EventManager: Pub/sub event system with logging and alerting

• Strategy - Defines a family of interchangeable algorithms

  • Sorter: Swappable sorting strategies (Bubble Sort, Quick Sort)

• Template Method - Defines algorithm skeleton in base class

  • DataParser: CSV and JSON parsing with shared validation logic

• Chain of Responsibility - Passes requests along a handler chain

  • RequestHandler: Authentication, authorization, and logging chain

• Command - Encapsulates requests as objects

  • CommandHistory: Text editor with undo support

Structural Patterns

• Decorator - Adds behavior to objects dynamically

  • DataSourceDecorator: Encryption and compression wrappers

• Proxy - Controls access to another object

  • ImageProxy: Lazy-loading image proxy

• Adapter - Makes incompatible interfaces work together

• State - Allows object to change behavior when internal state changes

  • Order: Order lifecycle transitions (New -> Paid -> Shipped -> Delivered)
  • VlcPlayerAdapter: Adapts VLC player to MediaPlayer interface

Project Structure

src/
├── main/java/com/huweiming/patterns/
│   ├── singleton/
│   ├── factory/
│   ├── abstractfactory/
│   ├── builder/
│   ├── observer/
│   ├── strategy/
│   ├── templatemethod/
│   ├── chain/
│   ├── command/
│   ├── decorator/
│   ├── proxy/
│   └── adapter/
└── test/java/com/huweiming/patterns/
    └── (mirrors main structure)

Getting Started

# Compile
mvn compile

# Run tests
mvn test

# Package
mvn package

Requirements

• Java 17+
• Maven 3.8+