Design Patterns

Design patterns are proven, reusable solutions to common problems that arise in software design. First cataloged by the “Gang of Four” (Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides) in their landmark 1994 book Design Patterns: Elements of Reusable Object-Oriented Software, these patterns remain foundational knowledge for every software engineer.

Why Design Patterns Matter

Design patterns provide a shared vocabulary for developers. Instead of explaining a complex design decision from scratch, you can say “we used a Strategy pattern here” and your team immediately understands the architecture. They also:

• Accelerate development by providing tested solutions to recurring problems
• Improve code quality through well-structured, maintainable designs
• Facilitate communication with a common language among developers
• Reduce risk by leveraging solutions that have been refined over decades
• Support SOLID principles and clean architecture naturally

The Three Categories

Design patterns are organized into three categories based on their purpose:

Creational Patterns

Deal with object creation mechanisms, abstracting the instantiation process to make systems independent of how objects are created, composed, and represented.

• Singleton
• Factory Method
• Abstract Factory
• Builder
• Prototype

View Creational Patterns

Structural Patterns

Concerned with class and object composition, using inheritance and interfaces to compose objects into larger structures while keeping them flexible and efficient.

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Proxy

View Structural Patterns

Behavioral Patterns

Focus on communication between objects, defining how objects interact and distribute responsibility among themselves.

• Observer
• Strategy
• Command
• State
• Iterator
• Template Method

View Behavioral Patterns

When to Use Design Patterns

Use Patterns When

• You recognize a recurring problem that a pattern solves cleanly
• You need to decouple components for flexibility and testability
• Your code is becoming rigid or fragile under changing requirements
• You want to communicate intent clearly to other developers
• You need to follow the Open/Closed Principle (open for extension, closed for modification)

Avoid Patterns When

• The problem is simple enough that a pattern adds unnecessary complexity
• You are forcing a pattern where a direct solution is clearer
• The pattern introduces over-engineering for a prototype or throwaway code
• You do not fully understand the pattern and its trade-offs
• A language feature (closures, first-class functions, generics) already solves the problem natively

Pattern Overuse

The most common mistake is applying patterns everywhere. A pattern is a tool, not a goal. If your solution is clear without a pattern, do not add one just for the sake of it. As the saying goes: “When all you have is a hammer, everything looks like a nail.”

Quick Reference Table

Creational Patterns

Pattern	Intent	Common Use Cases
Singleton	Ensure a class has only one instance	Configuration managers, logging, connection pools
Factory Method	Define an interface for creating objects, letting subclasses decide which class to instantiate	UI component libraries, plugin systems, document parsers
Abstract Factory	Create families of related objects without specifying concrete classes	Cross-platform UI toolkits, database access layers
Builder	Construct complex objects step by step	Query builders, configuration objects, HTTP requests
Prototype	Create new objects by copying existing ones	Game entities, document templates, cached objects

Structural Patterns

Pattern	Intent	Common Use Cases
Adapter	Convert an interface into another that clients expect	Legacy system integration, third-party library wrappers
Bridge	Separate abstraction from implementation so both can vary	Cross-platform rendering, device drivers, persistence layers
Composite	Compose objects into tree structures for part-whole hierarchies	File systems, UI component trees, organizational charts
Decorator	Attach additional responsibilities to objects dynamically	I/O streams, middleware pipelines, logging wrappers
Facade	Provide a simplified interface to a complex subsystem	API gateways, library wrappers, service orchestration
Proxy	Provide a surrogate or placeholder to control access	Lazy loading, access control, caching, remote objects

Behavioral Patterns

Pattern	Intent	Common Use Cases
Observer	Define a one-to-many dependency so dependents are notified of changes	Event systems, reactive UI, pub/sub messaging
Strategy	Define a family of interchangeable algorithms	Sorting algorithms, payment processing, validation rules
Command	Encapsulate a request as an object	Undo/redo systems, task queues, macro recording
State	Allow an object to alter its behavior when its internal state changes	Workflow engines, TCP connections, game character AI
Iterator	Provide a way to access elements of a collection sequentially	Collection traversal, database cursors, stream processing
Template Method	Define the skeleton of an algorithm, deferring steps to subclasses	Frameworks, data pipelines, test fixtures

Relationships Between Patterns

Patterns often complement each other. Understanding these relationships helps you choose the right combination:

Creational Relationships:
  Abstract Factory  ──uses──►  Factory Method
  Builder           ──can build──►  Composite
  Prototype         ──alternative to──►  Factory Method

Structural Relationships:
  Adapter       ──similar to──►  Bridge (but different intent)
  Composite     ──often uses──►  Iterator
  Decorator     ──similar to──►  Proxy (but different intent)
  Facade        ──simplifies──►  Complex subsystems

Behavioral Relationships:
  Strategy      ──similar to──►  State (but different intent)
  Observer      ──often uses──►  Mediator
  Command       ──can use──►  Memento (for undo)
  Iterator      ──traverses──►  Composite
  Template Method ──alternative to──►  Strategy (inheritance vs composition)

How to Study Design Patterns

1. Understand the problem first — Know what problem each pattern solves before memorizing the structure
2. Study the UML/structure — Understand the participants and their relationships
3. Read real-world examples — See how patterns are used in frameworks you already know
4. Implement from scratch — Code each pattern yourself in your preferred language
5. Recognize patterns in existing code — Look for patterns in open-source libraries
6. Practice refactoring — Take messy code and refactor it using appropriate patterns

Recommended Learning Path

Phase 1: Essential Patterns

Duration: 1-2 weeks

Start with the most commonly used patterns that you will encounter in everyday development.

Start with: Singleton, Factory Method, Observer, Strategy, Decorator

Prerequisites: Basic OOP knowledge (classes, interfaces, inheritance)

Phase 2: Structural Mastery

Duration: 1-2 weeks

Learn how to compose objects into flexible, maintainable structures.

Focus on: Adapter, Facade, Composite, Proxy, Builder

Prerequisites: Phase 1 patterns

Phase 3: Advanced Patterns

Duration: 2-3 weeks

Master the remaining behavioral patterns and learn to combine patterns effectively.

Focus on: Command, State, Template Method, Iterator, Abstract Factory, Bridge, Prototype

Prerequisites: Phases 1 and 2

Begin Learning

Creational Patterns Singleton, Factory Method, Abstract Factory, Builder, Prototype

Structural Patterns Adapter, Bridge, Composite, Decorator, Facade, Proxy

Behavioral Patterns Observer, Strategy, Command, State, Iterator, Template Method