策略设计模式示例？

Question

Following is stretagy design pattern example take from here . First of all we will create the interface for our strategy, in our case to pay the amount passed as argument.

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"));
    }
 
}

I want to ask what is use of strategy pattern here?Once we have created a strategy in main.We have access to Strategy class now.We can directly call pay() method from there?Why do we need interface, all which does is call a method?

Answer 1

1. I want to ask what is use of strategy pattern here?

The answer is the user who has the ShoppingCart ( ShoppingCart cart = new ShoppingCart(); )

2. We can directly call pay() method from there?

I don't know exactly you mean

3. Why do we need interface, all which does is call a method?

We need the interface PaymentStrategy because we need use Polymorphism to implement many way to pay (paypal or credit card), let's change the source a bit and you can see clearer:

 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(); } }