It all starts with a review question. I was reworking my notes on the Design Patterns book, the Gang of Four one, with examples that all came from the same universe: an online shop. Shipping fees for Strategy, the order for State, stock for Observer. And the question landed: "so what would the ideal cart be? One class that uses all the patterns?"
Excellent question. Wrong direction. And the answer deserves more than a paragraph, because it contains roughly everything I know about design.
The trap: the ultimate Cart class
The starting intuition is healthy: if all five examples come from the same shop, why not assemble everything? The trap is the word "class". A Cart class stacking Strategy, Decorator, State, Observer and a Facade "to show off" is exactly what the 1994 book calls pattern fever, and it warns against it as early as page 31: a pattern should only be applied when the flexibility it brings is actually needed.
The ideal cart is not a class. It is a small architecture where each pattern holds the exact post where it earns its keep. And the hiring criterion fits in one question, the most useful one in the book: what is going to change?
