请编写一个策略模式的实际应用案例代码，并解释其工作原理。

// 策略接口
interface PaymentStrategy {
    void pay(int amount);
}

// 具体策略类：支付宝支付
class AlipayStrategy implements PaymentStrategy {
    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Alipay.");
    }
}

// 具体策略类：微信支付
class WeChatPayStrategy implements PaymentStrategy {
    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using WeChat Pay.");
    }
}

// 具体策略类：信用卡支付
class CreditCardPayStrategy implements PaymentStrategy {
    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Credit Card.");
    }
}

// 上下文类：购物车
class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // 设置支付策略
    public void setPaymentStrategy(PaymentStrategy paymentStrategy) {
        this.paymentStrategy = paymentStrategy;
    }

    // 执行支付
    public void checkout(int amount) {
        paymentStrategy.pay(amount);
    }
}

// 测试代码
public class StrategyPatternExample {
    public static void main(String[] args) {
        // 创建购物车
        ShoppingCart cart = new ShoppingCart();

        // 用户选择支付宝支付
        cart.setPaymentStrategy(new AlipayStrategy());
        cart.checkout(100);  // 输出：Paid 100 using Alipay.

        // 用户选择微信支付
        cart.setPaymentStrategy(new WeChatPayStrategy());
        cart.checkout(200);  // 输出：Paid 200 using WeChat Pay.

        // 用户选择信用卡支付
        cart.setPaymentStrategy(new CreditCardPayStrategy());
        cart.checkout(300);  // 输出：Paid 300 using Credit Card.
    }
}