Strategy Design Pattern

The Strategy pattern is a behavioural design pattern that defines a family of interchangeable algorithms, puts each in its own class, and lets the client choose one at run time.

Instead of long if-else or switch statements, the context object holds a reference to a strategy interface and delegates the work to it. Examples include choosing a payment method, a sorting algorithm or a discount rule without changing the code that uses it.

interface DiscountStrategy { double apply(double amount); }

class FestiveDiscount implements DiscountStrategy {
	public double apply(double amount) { return amount * 0.8; }
}

class Checkout {
	private DiscountStrategy discount;
	Checkout(DiscountStrategy d) { this.discount = d; }
	double total(double amount) { return discount.apply(amount); }
}