The Ideal Cart: 5 Design Patterns That Earn Their Keep (and 2 Refused)

Short summary

The author builds an ideal e-commerce checkout flow using five Gang of Four design patterns (Strategy, Decorator, State, Observer, Facade), each introduced only when a real requirement demands it. Two patterns (Singleton, Builder) were considered and rejected as unnecessary. The central insight: the Cart class itself uses no pattern — patterns live at the edges where change happens, and all concrete choices are made at assembly time.

• •Five GoF patterns applied to checkout only when real requirements demanded them
• •Singleton and Builder rejected as unnecessary complexity
• •The central Cart class is deliberately pattern-free; patterns live at change points

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