设计模式-策略（Strategy）模式

主要角色

• 策略
• 具体策略

职责

具体算法解决具体的问题，策略模式可以整体替换算法的实现，使用不同的算法解决同一个问题。

类图

类关系示意图

代码

public interface PromotionStrategy {
    void doPromotion();
}
---
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");
    }
}
---
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 ManJianPromotionStrategy implements PromotionStrategy{
    @Override
    public void doPromotion() {
        System.out.println("满减促销,满200-20元");
    }
}
---
public class PromotionActivity {
    private PromotionStrategy promotionStrategy;

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

    public void executePromotionStrategy(){
        promotionStrategy.doPromotion();
    }
}
---
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());
        PROMOTION_STRATEGY_MAP.put(PromotionKey.MANJIAN,new ManJianPromotionStrategy());
    }

    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";
        String MANJIAN = "MANJIAN";
    }
}

使用

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

技巧

• 策略模式非常适合替代一长串的 if else。
• 策略模式简单的理解就是，要实现数组排序的功能，具体的实现有很多种，这时候就可以用策略模式。