策略模式（Strategy Pattern）

策略模式是属于行为型模式之一。当我们对一个具体的作业有多个算法可以调用时，策略模式使客户端用户决定实际上运行实现那个算法。

如果使用过MessageDigest.getInstance(String algorithm)这个方法获取一个MessageDigest对象的朋友，你可知道，这个就是一个标准的策略模式。根据algorithm算法名称来知道要调用那个算法，最后得到得到MessageDigest对象的实例。下面看一个简单的实例，UML图如下所示：

我们简单的实现一个购物车和两种不同的支付方式，分别是信用卡支付和银联支付。< http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+tqjS5dK7uPay38LUxKPKvbXEvdO/2qGjPC9wPgo8cD48cHJlIGNsYXNzPQ=="brush:java;">public interface PaymentStrategy { public void pay(int amount); }
创建信用卡的实现类。

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");
          }
 
}

银联支付类。

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

接下来，购物车中需要放置商品，定义一个商品项目类。

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;
          }
 
}

import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;
 
public class ShoppingCart {
 
          //List of items
          List
  
    items;
 
          public ShoppingCart(){
                    this.items=new ArrayList
   
    (); } 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); } }
   
  

一切类都定义好了，最后，测试这个设计模式。

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 UnionPaymentStrategy("myemail@example.com", "mypwd"));
 
                    //pay by credit card
                    cart.pay(new CreditCardStrategy("Pankaj Kumar", "1234567890123456", "786", "12/15"));
          }
 
}

程序的输出结果：

50 paid using UnionPay.
50 paid with credit/debit card