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设计模式基础概念（行为模式）：策略模式

news 2025/8/15 5:54:00

概述

策略模式是一种行为设计模式，它能让你定义一系列算法，并将每种算法分别放入独立的类中，以使算法的对象能够相互替换。

主要目的是通过定义相似的算法，替换if else 语句写法，并且可以随时相互替换

结构

在这里插入图片描述

示例

策略模式在 Java 代码中很常见。它经常在各种框架中使用，能在不扩展类的情况下向用户提供改变其行为的方式。

javax.servlet.http.HttpServlet： service()方法，还有所有接受 HttpServletRequest和 HttpServletResponse对象作为参数的 doXXX()方法。

识别方法：策略模式可以通过允许嵌套对象完成实际工作的方法，以及允许将该对象替换为不同对象的设置器来识别。

伪代码实现

strategies

策略（strategies）的定义：所有具体策略的通用接口，它声明了一个上下文用于执行策略的方法

public interface PayStrategy {boolean pay(int paymentAmount);void collectPaymentDetails();
}

Concrete Strategies

具体策略（Concrete Strategies）：实现上下文所用算法的各种不同变体。

PayByPayPal: 使用 PayPal 支付

public class PayByPayPal implements PayStrategy {@Overridepublic void collectPaymentDetails() {// todo}@Overridepublic boolean pay(int paymentAmount) {// todo}

PayByCreditCard: 使用信用卡支付

public class PayByCreditCard implements PayStrategy {@Overridepublic void collectPaymentDetails() {// todo}@Overridepublic boolean pay(int paymentAmount) {// todo}

Context

上下文（Context）：维护指向具体策略的引用，且仅通过策略接口与该对象进行交流。

public class Order {private int totalCost = 0;private boolean isClosed = false;// 提供一个计算的接口供客户端使用。public void processOrder(PayStrategy strategy) {strategy.collectPaymentDetails();// Here we could collect and store payment data from the strategy.}}

Client

客户端（Client）会创建一个特定策略对象并将其传递给上下文。

上下文则会提供一个设置器以便客户端在运行时替换相关联的策略。

public class Client {private static Map<Integer, Integer> priceOnProducts = new HashMap<>();private static BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));private static Order order = new Order();private static PayStrategy strategy;static {priceOnProducts.put(1, 2200);priceOnProducts.put(2, 1850);priceOnProducts.put(3, 1100);priceOnProducts.put(4, 890);}public static void main(String[] args) throws IOException {while (!order.isClosed()) {int cost;String continueChoice;do {System.out.print("Please, select a product:" + "\n" +"1 - Mother board" + "\n" +"2 - CPU" + "\n" +"3 - HDD" + "\n" +"4 - Memory" + "\n");int choice = Integer.parseInt(reader.readLine());cost = priceOnProducts.get(choice);System.out.print("Count: ");int count = Integer.parseInt(reader.readLine());order.setTotalCost(cost * count);System.out.print("Do you wish to continue selecting products? Y/N: ");continueChoice = reader.readLine();} while (continueChoice.equalsIgnoreCase("Y"));if (strategy == null) {System.out.println("Please, select a payment method:" + "\n" +"1 - PalPay" + "\n" +"2 - Credit Card");String paymentMethod = reader.readLine();// Client creates different strategies based on input from user,// application configuration, etc.if (paymentMethod.equals("1")) {strategy = new PayByPayPal();} else {strategy = new PayByCreditCard();}}// Order object delegates gathering payment data to strategy object,// since only strategies know what data they need to process a// payment.order.processOrder(strategy);System.out.print("Pay " + order.getTotalCost() + " units or Continue shopping? P/C: ");String proceed = reader.readLine();if (proceed.equalsIgnoreCase("P")) {// Finally, strategy handles the payment.if (strategy.pay(order.getTotalCost())) {System.out.println("Payment has been successful.");} else {System.out.println("FAIL! Please, check your data.");}order.setClosed();}}}
}