public interface PaymentStrategy {
    /**
     * 支付
     *
     * @param amount
     */
    void pay(double amount);
}

public class WechatPayment implements PaymentStrategy {
    private String username;
    private String password;
    public WechatPayment(String username, String password) {
        this.username = username;
        this.password = password;
    }
    /**
     * 支付
     *
     * @param amount
     */
    @Override
    public void pay(double amount) {
        System.out.println("微信支付:" + amount + "元");
    }
}
public class AliPayment implements PaymentStrategy{
    private String username;
    private String password;
    public AliPayment(String username, String password) {
        this.username = username;
        this.password = password;
    }
    /**
     * 支付
     *
     * @param amount
     */
    @Override
    public void pay(double amount) {
        System.out.println("支付寶支付:" + amount + "元");
    }
}

public class ShoppingCart {
    private List<Item> itemList;
    private PaymentStrategy paymentStrategy;
    public ShoppingCart() {
        itemList = new ArrayList<>();
    }
    public void setPaymentStrategy(PaymentStrategy paymentStrategy) {
        this.paymentStrategy = paymentStrategy;
    }
    public void addItem(Item item){
        itemList.add(item);
    }
    public void removeItem(Item item){
        itemList.remove(item);
    }
    public double calculateTotal(){
        return itemList.stream().mapToDouble(Item::getPrice).sum();
    }
    public void pay(){
        paymentStrategy.pay(calculateTotal());
    }
}

@Data
public class Item {
    private String name;
    private double price;
    public Item(String name, double price) {
        this.name = name;
        this.price = price;
    }
}

public class Demo {
    public static void main(String[] args) {
        ShoppingCart shoppingCart = new ShoppingCart();
        Item item1 = new Item("item1",10);
        Item item2 = new Item("item2",10);
        shoppingCart.addItem(item1);
        shoppingCart.addItem(item2);
        shoppingCart.setPaymentStrategy(new AliPayment("aliPayUsername","aliPayPassword"));
        shoppingCart.pay();
        shoppingCart.setPaymentStrategy(new WechatPayment("wechatUsername","wechatPassword"));
        shoppingCart.pay();
    }
}