Design Patterns & SOLID

By the end, you can

• Define what a design pattern is and distinguish it from a library or copy-paste code.
• Name the three GoF pattern categories and assign common patterns (Factory, Singleton, Builder, Adapter, Decorator, Facade, Observer, Strategy) to the right bucket.
• Explain the two foundational maxims and why they matter.
• Describe the Strategy pattern and explain why swapping a strategy is an example of the Open/Closed principle.
• Describe the Decorator pattern and explain how layers stack without modifying each other.
• Contrast Strategy (changes the algorithm inside) with Decorator (adds layers outside, same interface).
• State and explain each of the five SOLID principles in plain language.
• Identify the brittle if/elif payment design and explain why the interface-based design is more flexible.
• Recognize when applying a pattern would add unnecessary complexity.