Explain the Strategy pattern with a concrete Java example. When would you use it?

Strategy defines a family of interchangeable algorithms behind a common interface and lets the client pick one at runtime, instead of hard-coding it with if/else or switch.

Example: interface ShippingCostStrategy { BigDecimal cost(Order o); } with implementations FlatRate, ByWeight and FreeOverThreshold. Checkout takes a strategy in its constructor and calls strategy.cost(order). Since the interface has one method, a lambda works too: new Checkout(o -> BigDecimal.ZERO). Comparator passed to List.sort is a built-in Strategy.

Use it when behavior varies by configuration, customer or context, when you want to add algorithms without modifying the client (open/closed principle), and to make each algorithm testable on its own.

Key points:
- Interchangeable algorithms behind a common interface
- The client receives and uses the strategy (composition), chosen at runtime
- Gives a concrete example (e.g. pricing, sorting with Comparator)
- Replaces conditionals; supports the open/closed principle
- Lambdas or functional interfaces can implement strategies