🎯 Design Patterns Quick Reference

Real-World Guide to Factory, Strategy, Observer & Singleton Patterns

🏭

Factory

"Creates different things based on what you ask for"

Real-World Analogy

Restaurant kitchen - order by name, kitchen makes it

Problem It Solves

Creating many similar things without knowing all the complex details

Examples
Use When

✓ Many similar types to create
✓ Creation is complex
✓ Need to add new types easily

Don't Use When

✗ Only one type exists
✗ Creation is simple

🔄

Strategy

"Switching between different ways to do the same task"

Real-World Analogy

GPS routes - fastest, scenic, or avoid highways

Problem It Solves

Need different approaches for same goal that you can switch between

Examples
Use When

✓ Multiple ways to do same thing
✓ Need to switch at runtime
✓ Want to avoid messy if/else chains

Don't Use When

✗ Only one approach exists
✗ Approach never changes

📢

Observer

"When something happens, notify everyone who cares"

Real-World Analogy

YouTube subscriptions - upload once, all subscribers notified

Problem It Solves

One event needs to trigger multiple reactions automatically

Examples
Use When

✓ One change affects many objects
✓ Want loose coupling
✓ Listeners join/leave dynamically

Don't Use When

✗ Only one thing needs notification
✗ Direct communication is fine

⭐

Singleton

"Ensuring there's only ONE of something"

Real-World Analogy

Company CEO - one leader, everyone refers to same person

Problem It Solves

Need exactly one shared instance that everyone accesses

Examples
Use When

✓ Need exactly ONE instance
✓ Shared resource required
✓ Changes should be universal

Don't Use When

✗ Multiple instances might be needed
✗ "Just to be safe" (misused often!)

🎯 Quick Decision Guide: Which Pattern Do I Need?

Need to CREATE different types? → Use 🏭 Factory
Need to SWITCH between methods? → Use 🔄 Strategy
Need to NOTIFY multiple objects? → Use 📢 Observer
Need EXACTLY ONE shared instance? → Use ⭐ Singleton

💡 Key Principles