Java Design Patterns – Complete Guide

This repository contains a comprehensive explanation and implementation of all 23 classic Design Patterns using the Java programming language.

The goal of this repository is to help you deeply understand why design patterns exist, when to use them, and how to implement them cleanly in real-world Java applications.

Each design pattern is covered in extreme detail, with:

• Clear conceptual explanations
• Well-structured Java code examples
• Dedicated folders per pattern
• Step-by-step breakdowns for beginners

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What Are Design Patterns?

Design Patterns are proven, reusable solutions to commonly occurring problems in software design. They are not libraries or frameworks, but guidelines and templates for writing maintainable, flexible, and scalable code.

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Categories of Design Patterns

Design patterns are broadly classified into three major categories based on the type of problem they solve.

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1. Creational Design Patterns

Creational patterns focus on object creation mechanisms, helping create objects in a flexible, controlled, and reusable manner.

Covered Creational Patterns

• Singleton – Ensures only one instance of a class exists
• Factory – Delegates object creation logic to a factory class
• Abstract Factory – Creates families of related objects without specifying concrete classes
• Prototype – Creates new objects by cloning existing ones
• Builder – Constructs complex objects step by step

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2. Structural Design Patterns

Structural patterns focus on how classes and objects are composed to form larger structures while keeping them flexible and efficient.

Covered Structural Patterns

• Adapter – Converts one interface into another expected by the client
• Bridge – Separates abstraction from implementation
• Composite – Treats individual objects and object groups uniformly
• Decorator – Adds behavior dynamically to objects
• Facade – Provides a simplified interface to a complex subsystem
• Flyweight – Minimizes memory usage by sharing common object state
• Proxy – Controls access to another object

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3. Behavioral Design Patterns

Behavioral patterns focus on communication and responsibility distribution between objects.

Covered Behavioral Patterns

• Command – Encapsulates requests as objects
• Interpreter – Defines grammar and interprets sentences
• Mediator – Centralizes complex communication between objects
• Memento – Captures and restores object state
• Observer – Establishes a one-to-many dependency between objects
• Iterator – Provides a way to access elements sequentially
• Chain of Responsibility – Passes requests along a chain of handlers

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Repository Structure

Each design pattern is placed in its own dedicated folder, containing:

• Conceptual explanation
• UML-style reasoning (where applicable)
• Java implementation
• Practical examples

This structure allows you to learn patterns independently while also understanding how they fit together in larger systems.

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Who This Repository Is For

• Beginners learning Object-Oriented Design
• Developers preparing for interviews
• Engineers aiming to write clean, maintainable Java code
• Anyone wanting a deep, pattern-by-pattern understanding of software design

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How to Use This Repository

1. Start with Creational Patterns (especially Builder method)
2. Move to Structural Patterns to understand object composition
3. Finish with Behavioral Patterns for communication and flow control

Each pattern builds intuition that helps you understand the next.

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Additional Resources

• Gang of Four (GoF) Design Patterns – GeeksforGeeks Blog Blog article on GeeksforGeeks explaining the Gang of Four (GoF) design patterns.

• Gang of Four Design Patterns – YouTube Playlist Video playlist covering GoF design patterns.

• Design Patterns: Elements of Reusable Object-Oriented Software (Book PDF) Link to the classic GoF design patterns book.

Happy learning and clean coding!