🧑💻 Author: RK ROY
🎨 Design Patterns
Design patterns are reusable solutions to common problems in software design. They represent best practices evolved over time by experienced object-oriented software developers. This section covers all 23 Gang of Four (GoF) design patterns with detailed explanations, examples, and real-world implementations.
🎯 What are Design Patterns?
Design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.
- Gang of Four
Key Benefits
- Reusability: Proven solutions that can be applied to similar problems
- Communication: Common vocabulary for developers
- Best Practices: Encapsulate expert knowledge and experience
- Flexibility: Make code more flexible and maintainable
📚 Pattern Categories
🏗️ Creational Patterns
Purpose: Deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.
| Pattern | Purpose | Use When |
|---|---|---|
| Singleton | Ensure only one instance exists | Need exactly one instance (DB connection, logger) |
| Factory Method | Create objects without specifying exact classes | Object creation logic is complex or varies |
| Abstract Factory | Create families of related objects | Need to create groups of related objects |
| Builder | Construct complex objects step by step | Objects have many optional parameters |
| Prototype | Create objects by cloning existing instances | Object creation is expensive |
Creational Patterns Overview
🏛️ Structural Patterns
Purpose: Deal with object composition, forming larger structures from individual objects.
| Pattern | Purpose | Use When |
|---|---|---|
| Adapter | Allow incompatible interfaces to work together | Integrate existing classes with incompatible interfaces |
| Decorator | Add behavior to objects dynamically | Need to add responsibilities without subclassing |
| Facade | Provide simplified interface to complex subsystem | Want to hide complexity of subsystem |
| Composite | Compose objects into tree structures | Need to treat individual and composite objects uniformly |
| Bridge | Separate abstraction from implementation | Want to avoid permanent binding between abstraction and implementation |
| Flyweight | Share objects efficiently to support large numbers | Need to support large numbers of fine-grained objects |
| Proxy | Provide placeholder/surrogate for another object | Need to control access to another object |
Structural Patterns Overview
🎭 Behavioral Patterns
Purpose: Focus on communication between objects and the assignment of responsibilities.
| Pattern | Purpose | Use When |
|---|---|---|
| Observer | Define one-to-many dependency between objects | Changes to one object require updating multiple objects |
| Strategy | Define family of algorithms and make them interchangeable | Have multiple ways to perform a task |
| Command | Encapsulate requests as objects | Need to parameterize objects with operations |
| State | Allow object to alter behavior when internal state changes | Object behavior depends on its state |
| Template Method | Define skeleton of algorithm, let subclasses override steps | Algorithm structure is fixed but steps vary |
| Chain of Responsibility | Pass requests along chain of handlers | Multiple objects can handle request |
| Mediator | Define how objects interact without direct references | Complex interactions between multiple objects |
| Visitor | Define new operations without changing object structure | Need to perform operations on objects of different classes |
| Iterator | Provide way to access elements sequentially | Need to traverse collection without exposing structure |
| Memento | Capture and restore object state | Need to save/restore object state |
Behavioral Patterns Overview
🚀 Pattern Selection Guide
By Problem Type
Object Creation Problems
Structural Problems
Behavioral Problems
🎯 Popular Pattern Combinations
Model-View-Controller (MVC)
- Observer: View observes Model
- Strategy: Controller strategies
- Composite: View hierarchies
Enterprise Applications
- Factory Method: Create business objects
- Decorator: Add cross-cutting concerns
- Command: Encapsulate business operations
- Observer: Event-driven architecture
Game Development
- State: Character states
- Observer: Game events
- Command: Player actions
- Flyweight: Game objects
🛠️ Implementation Guidelines
When to Use Patterns
✅ DO use patterns when:
- You have a recurring design problem
- The pattern fits naturally
- It improves code maintainability
- Team understands the pattern
❌ DON'T use patterns when:
- The problem is simple
- It adds unnecessary complexity
- You're forcing a pattern to fit
- Team is unfamiliar with the pattern
Anti-Patterns to Avoid
- Golden Hammer: Using same pattern everywhere
- Over-Engineering: Using complex patterns for simple problems
- Pattern Fever: Using patterns just to use them
- Copy-Paste Patterns: Not understanding the pattern before using
📊 Pattern Comparison Matrix
| Pattern | Complexity | Flexibility | Performance | Use Frequency |
|---|---|---|---|---|
| Singleton | Low | Low | High | High |
| Factory Method | Medium | High | Medium | High |
| Observer | Medium | High | Medium | High |
| Strategy | Low | High | High | High |
| Decorator | Medium | High | Medium | Medium |
| Command | Medium | High | Medium | Medium |
| Adapter | Low | Medium | High | Medium |
| Facade | Low | Low | High | Medium |
🎓 Learning Path
Beginner Path (Start Here)
- Singleton - Understand the concept of patterns
- Factory Method - Learn about object creation
- Observer - Understand behavioral patterns
- Strategy - Learn algorithm selection
- Decorator - Understand structural composition
Intermediate Path
- Builder - Complex object creation
- Adapter - Interface integration
- Command - Request encapsulation
- State - State-dependent behavior
- Facade - System simplification
Advanced Path
- Abstract Factory - Complex creation patterns
- Proxy - Access control patterns
- Visitor - Complex behavioral patterns
- Mediator - Communication patterns
- Flyweight - Performance optimization
🔗 Pattern Relationships
📚 Further Reading
Books:
- "Design Patterns: Elements of Reusable Object-Oriented Software" by Gang of Four
- "Head First Design Patterns" by Freeman & Robson
- "Patterns of Enterprise Application Architecture" by Martin Fowler
Online Resources:
- Refactoring.Guru Design Patterns
- SourceMaking Design Patterns
- DoFactory Design Pattern Framework
🎯 Quick Reference
Most Common Patterns (80/20 Rule)
- Singleton - Single instance
- Factory Method - Object creation
- Observer - Event notification
- Strategy - Algorithm selection
- Decorator - Add behavior
- Command - Encapsulate requests
- Adapter - Interface conversion
- Facade - Simplify interface