策略模式：灵活切换算法的设计之道

策略模式的定义与核心思想

策略模式（Strategy Pattern）属于行为型设计模式，允许在运行时选择算法的行为。其核心思想是将算法封装成独立的类，使得它们可以互相替换，客户端代码无需关心具体实现细节。

策略模式的结构

策略模式包含三个关键角色：

Context（上下文）：持有策略接口的引用，负责调用具体策略。
Strategy（策略接口）：定义算法的公共接口，通常为抽象类或接口。
ConcreteStrategy（具体策略）：实现策略接口的具体算法类。

策略模式的实现示例

以下是一个简单的策略模式代码示例（Java）：

// 策略接口  
interface PaymentStrategy {  
    void pay(int amount);  
}  

// 具体策略：信用卡支付  
class CreditCardPayment implements PaymentStrategy {  
    public void pay(int amount) {  
        System.out.println("Paid " + amount + " via Credit Card.");  
    }  
}  

// 具体策略：支付宝支付  
class AlipayPayment implements PaymentStrategy {  
    public void pay(int amount) {  
        System.out.println("Paid " + amount + " via Alipay.");  
    }  
}  

// 上下文类  
class PaymentContext {  
    private PaymentStrategy strategy;  

    public PaymentContext(PaymentStrategy strategy) {  
        this.strategy = strategy;  
    }  

    public void executePayment(int amount) {  
        strategy.pay(amount);  
    }  
}  

// 客户端调用  
public class Client {  
    public static void main(String[] args) {  
        PaymentContext context = new PaymentContext(new CreditCardPayment());  
        context.executePayment(100);  

        context = new PaymentContext(new AlipayPayment());  
        context.executePayment(200);  
    }  
}

策略模式的优点

开闭原则：新增策略无需修改现有代码，只需扩展新类。
消除条件语句：避免大量if-else或switch-case逻辑。
灵活性：运行时动态切换算法，适合需要频繁变更策略的场景。

策略模式的适用场景

系统需要多种算法变体，且未来可能扩展。
算法逻辑复杂或需要隔离，避免污染客户端代码。
需要避免暴露算法细节给客户端。

策略模式与状态模式的区别

策略模式和状态模式结构相似，但意图不同：

策略模式：客户端主动选择策略，策略之间通常无关。
状态模式：状态转换由内部条件触发，状态之间可能存在关联。

实际应用案例

支付系统：支持多种支付方式（信用卡、支付宝、微信支付）。
排序算法：动态选择冒泡排序、快速排序等。
游戏AI：根据场景切换不同的行为策略（攻击、逃跑）。

总结

策略模式通过将算法封装为独立对象，提升了代码的可维护性和扩展性。其核心在于解耦策略的定义与使用，是应对算法多变场景的经典解决方案。

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