策略设计模式示例？

public abstract class DiscountStrategy {
    public abstract double calculateDiscount(double price);
}

public class FixedDiscountStrategy extends DiscountStrategy {
    private double discount;

    public FixedDiscountStrategy(double discount) {
        this.discount = discount;
    }

    @Override
    public double calculateDiscount(double price) {
        return price - discount;
    }
}

public class PercentageDiscountStrategy extends DiscountStrategy {
    private double percentage;

    public PercentageDiscountStrategy(double percentage) {
        this.percentage = percentage;
    }

    @Override
    public double calculateDiscount(double price) {
        return price * (1 - percentage);
    }
}

public class ShoppingCart {
    private DiscountStrategy discountStrategy;

    public void setDiscountStrategy(DiscountStrategy discountStrategy) {
        this.discountStrategy = discountStrategy;
    }

    public double calculateTotalPrice(double price) {
        double discountPrice = discountStrategy.calculateDiscount(price);
        // 其他计算逻辑...
        return discountPrice;
    }
}

public class Main {
    public static void main(String[] args) {
        ShoppingCart cart = new ShoppingCart();

        // 使用固定折扣策略
        DiscountStrategy fixedDiscountStrategy = new FixedDiscountStrategy(10);
        cart.setDiscountStrategy(fixedDiscountStrategy);
        double totalPrice = cart.calculateTotalPrice(100);
        System.out.println("Total price with fixed discount: " + totalPrice);

        // 使用百分比折扣策略
        DiscountStrategy percentageDiscountStrategy = new PercentageDiscountStrategy(0.2);
        cart.setDiscountStrategy(percentageDiscountStrategy);
        totalPrice = cart.calculateTotalPrice(100);
        System.out.println("Total price with percentage discount: " + totalPrice);
    }
}