设计模式——策略模式

简单介绍
一个例子

策略模式是设计模式里面比较简单的设计模式，其特点简单又实用，并且可以让你的代码看起来高大上，维护代码时还方便扩张

多重条件语句不易维护，而使用策略模式可以避免使用多重条件语句，如 if…else 语句、switch…case 语句

简单介绍

策略模式（Strategy Pattern）是一种行为设计模式，它在运行时选择算法或行为。它定义了一系列的算法，并将每个算法封装起来，使它们可以相互替换。这样，算法的变化可以独立于使用算法的客户，这么说会有些抽象，下面会举个例子的

主要组成部分：

1，策略接口（Strategy Interface）：定义了所有支持的算法的公共操作
2，具体策略类（Concrete Strategy Classes）：实现了策略接口，提供了具体的算法实现
3，上下文（Context）：持有一个策略对象的引用。使用策略对象来执行特定的算法

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一个例子

我们举个例子来说明策略模式的必要性，现在有一个需求：我们需要做一个旅游平台，现在需要提供机票、火车票、酒店、门票等产品的下单、查询、退款接口，因为这些产品的行为都是类似的，所以我们不打算为他们分别提供接口，而是收口到一个 contrllor 中，我们先来看看使用 if else 去处理，会发生什么：

public class TravelService {@Resourceprivate FlightOrderService flightOrderService;@Resourceprivate TrainOrderService trainOrderService;@Resourceprivate HotelOrdeService hotelOrdeService;@Resourceprivate TicketOrdeService ticketOrdeService;public String placeOrder(String productType, Order order) {if ("flight".equals(productType)) {return flightOrderService.processFlightOrder(order);} else if ("train".equals(productType)) {return trainOrderService.processTrainOrder(order);} else if ("hotel".equals(productType)) {return hotelOrdeService.processHotelOrder(order);} else if ("ticket".equals(productType)) {return ticketOrdeService.processTicketOrder(order);} else {return "Unsupported product type";}}public String queryOrder(String productType, String orderId) {if ("flight".equals(productType)) {return flightOrderService.queryFlightOrder(orderId);} else if ("train".equals(productType)) {return trainOrderService.queryTrainOrder(orderId);} else if ("hotel".equals(productType)) {return hotelOrdeService.queryHotelOrder(orderId);} else if ("ticket".equals(productType)) {return ticketOrdeService.queryTicketOrder(orderId);} else {return "Unsupported product type";}}public String refundOrder(String productType, String orderId) {if ("flight".equals(productType)) {return flightOrderService.refundFlightOrder(orderId);} else if ("train".equals(productType)) {return trainOrderService.refundTrainOrder(orderId);} else if ("hotel".equals(productType)) {return hotelOrdeService.refundHotelOrder(orderId);} else if ("ticket".equals(productType)) {return ticketOrdeService.refundTicketOrder(orderId);} else {return "Unsupported product type";}}
}

可以看到上面代码有如下问题：

1，扩展性有限：如果我想要再引入动物园票啥的业务，我需要再额外写 else if（这里举的例子比较简单，改动起来比较简单，但是在实际的工程中，需要改动的地方太多了）
2，过多的 if else，会导致代码的甬余

改用策略模式后，我们再看看：

先提供一个抽象的策略接口，定义订单行为：

public interface OrderStrategy {String placeOrder(Order order);String queryOrder(String orderId);String refundOrder(String orderId);
}

写四个类实现订单接口：

public class FlightOrderStrategy implements OrderStrategy {@Overridepublic String placeOrder(Order order) {return "Flight order processed: " + order;}@Overridepublic String queryOrder(String orderId) {return "Flight order queried: " + orderId;}@Overridepublic String refundOrder(String orderId) {return "Flight order refunded: " + orderId;}
}public class TrainOrderStrategy implements OrderStrategy {@Overridepublic String placeOrder(Order order) {return "Train order processed: " + order;}@Overridepublic String queryOrder(String orderId) {return "Train order queried: " + orderId;}@Overridepublic String refundOrder(String orderId) {return "Train order refunded: " + orderId;}
}public class HotelOrderStrategy implements OrderStrategy {@Overridepublic String placeOrder(Order order) {return "Hotel order processed: " + order;}@Overridepublic String queryOrder(String orderId) {return "Hotel order queried: " + orderId;}@Overridepublic String refundOrder(String orderId) {return "Hotel order refunded: " + orderId;}
}public class TicketOrderStrategy implements OrderStrategy {@Overridepublic String placeOrder(Order order) {return "Ticket order processed: " + order;}@Overridepublic String queryOrder(String orderId) {return "Ticket order queried: " + orderId;}@Overridepublic String refundOrder(String orderId) {return "Ticket order refunded: " + orderId;}
}

上下文类：

public class TravelService {private OrderStrategy strategy;public TravelController(OrderStrategy strategy) {this.strategy = strategy;}public void setStrategy(OrderStrategy strategy) {this.strategy = strategy;}public String placeOrder(Order order) {return strategy.placeOrder(order);}public String queryOrder(String orderId) {return strategy.queryOrder(orderId);}public String refundOrder(String orderId) {return strategy.refundOrder(orderId);}
}

客户端代码

public class Client {public static void main(String[] args) {TravelController controller = new TravelController(new FlightOrderStrategy());Order order = new Order("12345", "Flight to Paris");System.out.println(controller.placeOrder(order)); // 输出: Flight order processed: Order{id='12345', description='Flight to Paris'}System.out.println(controller.queryOrder("12345")); // 输出: Flight order queried: 12345System.out.println(controller.refundOrder("12345")); // 输出: Flight order refunded: 12345// 动态更换策略controller.setStrategy(new HotelOrderStrategy());Order hotelOrder = new Order("67890", "Hotel in New York");System.out.println(controller.placeOrder(hotelOrder)); // 输出: Hotel order processed: Order{id='67890', description='Hotel in New York'}System.out.println(controller.queryOrder("67890")); // 输出: Hotel order queried: 67890System.out.println(controller.refundOrder("67890")); // 输出: Hotel order refunded: 67890}
}