策略设计模式示例？

Question

以下是来自here 的策略设计模式示例。 首先，我们将为我们的策略创建接口，在我们的例子中，支付作为参数传递的金额。

public interface PaymentStrategy {
 
    public void pay(int amount);
}

public class CreditCardStrategy implements PaymentStrategy {
 
    private String name;
    private String cardNumber;
    private String cvv;
    private String dateOfExpiry;
 
    public CreditCardStrategy(String nm, String ccNum, String cvv, String expiryDate){
        this.name=nm;
        this.cardNumber=ccNum;
        this.cvv=cvv;
        this.dateOfExpiry=expiryDate;
    }
    @Override
    public void pay(int amount) {
        System.out.println(amount +" paid with credit/debit card");
    }
 
}



 public class PaypalStrategy implements PaymentStrategy {
     
        private String emailId;
        private String password;
     
        public PaypalStrategy(String email, String pwd){
            this.emailId=email;
            this.password=pwd;
        }
     
        @Override
        public void pay(int amount) {
            System.out.println(amount + " paid using Paypal.");
        }
     
    }


public class Item {
 
    private String upcCode;
    private int price;
 
    public Item(String upc, int cost){
        this.upcCode=upc;
        this.price=cost;
    }
 
    public String getUpcCode() {
        return upcCode;
    }
 
    public int getPrice() {
        return price;
    }
 
}

    public class ShoppingCart {
     
   

 //List of items
    List<Item> items;
 
    public ShoppingCart(){
        this.items=new ArrayList<Item>();
    }
 
    public void addItem(Item item){
        this.items.add(item);
    }
 
    public void removeItem(Item item){
        this.items.remove(item);
    }
 
    public int calculateTotal(){
        int sum = 0;
        for(Item item : items){
            sum += item.getPrice();
        }
        return sum;
    }
 
    public void pay(PaymentStrategy paymentMethod){
        int amount = calculateTotal();
        paymentMethod.pay(amount);
    }
}



public class ShoppingCartTest {
 
    public static void main(String[] args) {
        ShoppingCart cart = new ShoppingCart();
 
        Item item1 = new Item("1234",10);
        Item item2 = new Item("5678",40);
 
        cart.addItem(item1);
        cart.addItem(item2);
 
        //pay by paypal
        cart.pay(new PaypalStrategy("myemail@example.com", "mypwd"));
 
        //pay by credit card
        cart.pay(new CreditCardStrategy("Pankaj Kumar", "1234567890123456", "786", "12/15"));
    }
 
}

我想问这里策略模式有什么用？一旦我们在main中创建了策略。我们现在可以访问Strategy类。我们可以直接从那里调用pay()方法？为什么我们需要接口，所有哪个是调用方法？

Answer 1

1.我想问这里策略模式有什么用？

答案是拥有 ShoppingCart (ShoppingCart cart = new ShoppingCart();) 的用户

2.我们可以直接从那里调用pay()方法吗？

我不知道你的意思

3. 为什么我们需要接口，它所做的只是调用一个方法？

我们需要PaymentStrategy接口，因为我们需要使用Polymorphism来实现多种支付方式（paypal或信用卡），让我们稍微改变一下来源，你会看得更清楚：

public class ShoppingCart {
    // other functions

    public void pay(PaymentStrategy paymentMethod, int amount){
        paymentMethod.pay(amount);
    }

    public void payWithStrategy() {
        int amount = calculateTotal();
        if (amount > 10000) { // because your credit card limit is 10000$ so you must use paypal
            pay(new PaypalStrategy("myemail@example.com", "mypwd"), amount);
        }
        else { // you really like credit card so when the money lower than the card limit, you always choose it
            pay(new CreditCardStrategy("Pankaj Kumar", "1234567890123456", "786", "12/15"), amount);
        }
    }
}



public class ShoppingCartTest {

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

        Item item1 = new Item("1234",10);
        Item item2 = new Item("5678",40);

        cart.addItem(item1);
        cart.addItem(item2);

        cart.payWithStrategy();
    }

}