策略模式详解
一、模式定义
策略模式(Strategy Pattern)属于行为型模式,它定义了一系列算法,并将每个算法封装起来,使它们可以相互替换。
二、核心结构
1. 抽象策略接口
public interface Strategy {void executeAlgorithm();
}
2. 具体策略实现
public class ConcreteStrategyA implements Strategy {public void executeAlgorithm() {System.out.println("执行策略A的算法逻辑");// 具体算法实现...}
}public class ConcreteStrategyB implements Strategy {public void executeAlgorithm() {System.out.println("执行策略B的算法逻辑");// 具体算法实现...}
}
3. 上下文类
public class Context {private Strategy strategy;public Context(Strategy strategy) {this.strategy = strategy;}public void setStrategy(Strategy strategy) {this.strategy = strategy;}public void executeStrategy() {strategy.executeAlgorithm();}
}
三、完整示例:电商促销系统
1. 促销策略接口
public interface PromotionStrategy {double applyPromotion(double originalPrice);
}
2. 具体促销策略
// 无促销
public class NoPromotion implements PromotionStrategy {public double applyPromotion(double originalPrice) {return originalPrice;}
}// 折扣促销
public class DiscountPromotion implements PromotionStrategy {private double discountRate;public DiscountPromotion(double discountRate) {this.discountRate = discountRate;}public double applyPromotion(double originalPrice) {return originalPrice * discountRate;}
}// 满减促销
public class FullReductionPromotion implements PromotionStrategy {private double fullAmount;private double reductionAmount;public FullReductionPromotion(double fullAmount, double reductionAmount) {this.fullAmount = fullAmount;this.reductionAmount = reductionAmount;}public double applyPromotion(double originalPrice) {return originalPrice >= fullAmount ? originalPrice - reductionAmount : originalPrice;}
}
3. 订单类(上下文)
public class Order {private double amount;private PromotionStrategy promotionStrategy;public Order(double amount) {this.amount = amount;this.promotionStrategy = new NoPromotion();}public void setPromotionStrategy(PromotionStrategy strategy) {this.promotionStrategy = strategy;}public double calculateFinalPrice() {return promotionStrategy.applyPromotion(amount);}
}
4. 客户端使用
public class Client {public static void main(String[] args) {Order order = new Order(1000);// 默认无促销System.out.println("原价: " + order.calculateFinalPrice());// 应用8折促销order.setPromotionStrategy(new DiscountPromotion(0.8));System.out.println("8折后: " + order.calculateFinalPrice());// 应用满1000减200order.setPromotionStrategy(new FullReductionPromotion(1000, 200));System.out.println("满减后: " + order.calculateFinalPrice());}
}
四、高级应用:策略工厂
1. 策略工厂类
public class PromotionStrategyFactory {private static Map<String, PromotionStrategy> strategies = new HashMap<>();static {strategies.put("NO_PROMOTION", new NoPromotion());strategies.put("DISCOUNT_8", new DiscountPromotion(0.8));strategies.put("FULL_1000_REDUCE_200", new FullReductionPromotion(1000, 200));}public static PromotionStrategy getStrategy(String strategyKey) {PromotionStrategy strategy = strategies.get(strategyKey);if(strategy == null) {throw new IllegalArgumentException("无效的策略类型");}return strategy;}
}
2. 改进的订单类
public class ImprovedOrder {private double amount;private PromotionStrategy promotionStrategy;public ImprovedOrder(double amount, String strategyKey) {this.amount = amount;this.promotionStrategy = PromotionStrategyFactory.getStrategy(strategyKey);}public double calculateFinalPrice() {return promotionStrategy.applyPromotion(amount);}
}
五、模式优势
- 符合
开闭原则
:新增策略无需修改现有代码 - 避免多重条件判断语句
- 提高算法的复用性和灵活性
- 算法可以自由切换
六、适用场景
- 系统需要在多种算法间动态切换
- 存在多个条件分支的复杂条件语句
- 需要隔离算法实现细节
- 算法需要自由组合的场景
七、注意事项
- 客户端必须了解所有策略的区别
- 会增加策略类的数量
- 策略对象会增大系统开销
- 适合算法经常变化的场景
八、与其他模式的关系
- 与
状态模式
:策略模式改变对象行为,状态模式改变对象状态 - 与
工厂模式
:常组合使用,策略工厂管理策略实例 - 与
模板方法模式
:都是封装算法,策略用组合,模板用继承
九、最佳实践
- 将策略定义为轻量级的无状态对象
- 使用枚举或工厂管理策略实例
- 考虑使用Lambda表达式简化策略实现(Java8+)
- 为策略接口提供合理的默认实现
- 保持策略接口的简洁性
十、完整示例代码结构
src/
├── main/
│ ├── java/
│ │ ├── strategy/
│ │ │ ├── PromotionStrategy.java
│ │ │ ├── NoPromotion.java
│ │ │ ├── DiscountPromotion.java
│ │ │ ├── FullReductionPromotion.java
│ │ │ ├── PromotionStrategyFactory.java
│ │ │ ├── Order.java
│ │ │ └── Client.java