عرض عملي: استبدال switch على النوع بنمط Strategy

خطوة بخطوة

1. Show a switch (shippingType) in ShippingCalculator with three cases; tests pass.
2. Extract the ShippingStrategy interface and one class per case.
3. Choose the strategy through a small factory.
4. Planned mistake: create a strategy for a case with one line of code and a new file for each trivial detail; the design is heavier than the switch. Fix: use lambdas for tiny strategies, and keep only the meaningful classes.
5. Run the tests; add a fourth shipping type with no edit to old classes.
6. Say in one line why Strategy was justified here.

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