策略模式

1 概念

1 定义

在策略模式定义了一系列算法或策略，并将每个算法封装在独立的类中，使得它们可以互相替换。通过使用策略模式，可以在运行时根据需要选择不同的算法，而不需要修改客户端代码。

类型：行为型

2 应用场景

• 一个系统需要动态在几种策略中选择一种
• 系统很多类，区别仅仅是行为不同

3 优缺点

优点：

• 符合开闭原则
• 避免大量使用if…else, switch..case
• 提高算法的保密性和安全性

缺点：

• 客户端必须知道所有的策略类，并自行决定使用哪个类
• 产生很多策略类

2 代码实现

场景：慕课网活动促销，有多种方式：满减，返现，立减等，双11和618都采取不同的策略。

public interface PromotionStrategy {
    void doPromotion();
}

public class EmptyPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("无促销活动");
    }
}

public class LiJianPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("立减促销,课程的价格直接减去配置的价格");
    }
}

public class FanXianPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("返现促销,返回的金额存放到慕课网用户的余额中");
    }
}

// 促销活动
public class PromotionActivity {
  // 促销策略
    private PromotionStrategy promotionStrategy;

    public PromotionActivity(PromotionStrategy promotionStrategy) {
        this.promotionStrategy = promotionStrategy;
    }

  // 执行促销
    public void executePromotionStrategy(){
        promotionStrategy.doPromotion();
    }
}

public class Test {
    public static void main(String[] args) {
        PromotionActivity promotionActivity618 = new PromotionActivity(new LiJianPromotionStrategy());
        PromotionActivity promotionActivity1111 = new PromotionActivity(new FanXianPromotionStrategy());

        promotionActivity618.executePromotionStrategy();
        promotionActivity1111.executePromotionStrategy();
    }
}

输出：

立减促销,课程的价格直接减去配置的价格
返现促销,返回的金额存放到慕课网用户的余额中

代码演进

如果需要根据传过来的key，去选择对应的策略，使用共厂模式演进。。

public class PromotionStrategyFactory {
    private static Map<String,PromotionStrategy> PROMOTION_STRATEGY_MAP = new HashMap<String, PromotionStrategy>();
    static {
        PROMOTION_STRATEGY_MAP.put(PromotionKey.LIJIAN,new LiJianPromotionStrategy());
        PROMOTION_STRATEGY_MAP.put(PromotionKey.FANXIAN,new FanXianPromotionStrategy());
    }

    // 避免每次进来都需要new
    private static final PromotionStrategy NON_PROMOTION = new EmptyPromotionStrategy();

    private PromotionStrategyFactory(){

    }

    public static PromotionStrategy getPromotionStrategy(String promotionKey){
        PromotionStrategy promotionStrategy = PROMOTION_STRATEGY_MAP.get(promotionKey);
        return promotionStrategy == null ? NON_PROMOTION : promotionStrategy;
    }

    private interface PromotionKey{
        String LIJIAN = "LIJIAN";
        String FANXIAN = "FANXIAN";
    }
}

public class Test {
    public static void main(String[] args) {
        String promotionKey = "LIJIAN";
        PromotionActivity promotionActivity = new PromotionActivity(PromotionStrategyFactory.getPromotionStrategy(promotionKey));
        promotionActivity.executePromotionStrategy();
    }
}

输出：

立减促销,课程的价格直接减去配置的价格