Application Design Patterns

This section covers fundamental design patterns used in application development, organized by their primary purpose and providing practical implementations with real-world examples.

Topics Covered

🏗️ Creational Patterns

• Mastering Creational Design Patterns - Complete guide to object creation patterns

  • Singleton Pattern - Ensures only one instance exists

  • Factory Method Pattern - Creates objects without specifying exact classes

  • Abstract Factory Pattern - Creates families of related objects

  • Builder Pattern - Constructs complex objects step by step

  • Prototype Pattern - Creates objects by cloning existing instances

  • Object Pool Pattern - Manages reusable object instances

  • Dependency Injection Pattern - Provides dependencies from external sources

🔗 Structural Patterns

• Mastering Structural Design Patterns - Complete guide to object composition

  • Adapter Pattern - Allows incompatible interfaces to work together

  • Bridge Pattern - Separates abstraction from implementation

  • Composite Pattern - Treats individual objects and compositions uniformly

  • Decorator Pattern - Adds behavior to objects dynamically

  • Facade Pattern - Provides simplified interface to complex subsystem

  • Flyweight Pattern - Shares common state between multiple objects

  • Proxy Pattern - Provides placeholder or surrogate for another object

  • Module Pattern - Encapsulates related methods and variables

  • Mixin Pattern - Adds features to objects without inheritance

🎯 Behavioral Patterns

• Mastering Behavioral Design Patterns - Complete guide to object communication and responsibility patterns

  • Observer Pattern - Notifies dependents of state changes

  • Strategy Pattern - Encapsulates algorithms and makes them interchangeable

  • Command Pattern - Encapsulates requests as objects

  • State Pattern - Changes object behavior based on internal state

  • Template Method Pattern - Defines skeleton of algorithm, subclasses fill details

  • Chain of Responsibility Pattern - Passes requests along chain of handlers

  • Mediator Pattern - Defines how objects interact with each other

  • Memento Pattern - Captures and restores object state

  • Visitor Pattern - Separates algorithms from object structure

  • Iterator Pattern - Provides way to access elements sequentially

  • Interpreter Pattern - Defines grammar and interpreter for language

��️ Architectural Patterns

• Model-View-Controller (MVC) - Separates concerns in user interfaces

• Model-View-Presenter (MVP) - Variant of MVC with passive view

• Model-View-ViewModel (MVVM) - Data binding and commanding patterns

• Repository Pattern - Encapsulates data access logic

• Unit of Work Pattern - Maintains list of objects affected by business transaction

Key Principles

• Single Responsibility - Each class should have one reason to change

• Open/Closed Principle - Open for extension, closed for modification

• Liskov Substitution - Subtypes must be substitutable for base types

• Interface Segregation - Clients shouldn't depend on unused interfaces

• Dependency Inversion - Depend on abstractions, not concretions

When to Use Design Patterns

✅ Good Reasons to Use Patterns

• Proven Solutions - Time-tested approaches to common problems

• Communication - Common vocabulary among developers

• Maintainability - Well-structured, predictable code

• Testability - Patterns often improve code testability

• Flexibility - Enable easier modifications and extensions

❌ When to Avoid Patterns

• Over-Engineering - Don't apply patterns for simple problems

• Premature Optimization - Add patterns when complexity justifies them

• Cargo Cult Programming - Understand why you're using a pattern

• Performance Critical - Some patterns add overhead

• Team Unfamiliarity - Consider team knowledge and training time

Best Practices

• Start Simple - Begin with straightforward solutions

• Refactor to Patterns - Introduce patterns as code evolves

• Understand Intent - Know the problem each pattern solves

• Consider Context - Patterns work differently in different environments

• Document Decisions - Explain why you chose specific patterns

• Test Thoroughly - Ensure patterns don't break functionality

• Review Regularly - Patterns may become unnecessary as code changes

Technology Stack

Languages & Frameworks

• TypeScript/JavaScript - Modern web applications and Node.js services

• Python - Data engineering, machine learning, and backend services

• Java - Enterprise applications and microservices

• C# - .NET applications and cloud services

Development Tools

• IDEs - VS Code, IntelliJ IDEA, Visual Studio

• Testing - Jest, PyTest, JUnit, NUnit

• Code Quality - ESLint, SonarQube, Code Climate

• Documentation - JSDoc, Sphinx, Swagger/OpenAPI

Design Tools

• UML Modeling - PlantUML, Lucidchart, Draw.io

• Architecture Diagrams - Miro, Figma, Visio

• Code Visualization - Dependency graphs, class diagrams

Learning Path

Beginner

1. Understand SOLID Principles - Foundation for good design

2. Start with Creational Patterns - Learn object creation best practices

3. Practice with Simple Examples - Implement patterns in isolation

4. Focus on Problem-Solution Fit - Understand when to use each pattern

Intermediate

1. Master Structural Patterns - Learn object composition techniques

2. Apply Behavioral Patterns - Handle complex interactions and algorithms

3. Combine Patterns - Use multiple patterns together effectively

4. Real-World Projects - Apply patterns in actual applications

Advanced

1. Architectural Patterns - Learn large-scale design patterns

2. Anti-Patterns - Recognize and avoid common mistakes

3. Pattern Languages - Understand pattern relationships and systems

4. Custom Patterns - Develop domain-specific patterns

Common Anti-Patterns to Avoid

• God Object - Objects that know too much or do too much

• Spaghetti Code - Code with complex, tangled control flow

• Copy-Paste Programming - Duplicating code instead of abstracting

• Golden Hammer - Using same solution for every problem

• Premature Optimization - Optimizing before identifying bottlenecks

• Feature Envy - Classes that use other classes' data excessively

Resources for Further Learning

• Books - "Design Patterns" by Gang of Four, "Head First Design Patterns"

• Online - Refactoring.Guru, SourceMaking, Design Patterns Explained

• Practice - Code katas, design pattern challenges

• Community - Stack Overflow, Reddit r/programming, local meetups