策略模式

定义一系列算法，将每个算法都封装起来，并且它们可以互换。策略模式是一种对象行为模式。

一个行为触发业务逻辑计算，行为可能根据其属性使用不同的策略进行执行

比如订单事件进来之后进行活动的计算，新用户执行策略NewUserRewardStrategyA，老用户执行策略OldUserRewardStrategyA，可通过扩充RewardStrategy的子类来扩充不同属性用户的奖励计算策略

/**
 * 抽象一个策略
 * @author: nanxi
 * @date: 2021/7/28
 * @time: 2:51 下午
 */
public abstract class RewardStrategy {
    public abstract long reward(long userId);

    public void rewardAndSettlement(long userId, long rewardMoney) {
        // todo 实现
        System.out.println("userId: " + userId + " rewardMoney: " + rewardMoney);
    }
}

// 老用户的奖励策略
public class OldUserRewardStrategyA extends RewardStrategy {
    @Override
    public long reward(long userId) {
        return 200;
    }
}

// 新用户的奖励策略
public class NewUserRewardStrategyA extends RewardStrategy {
    @Override
    public long reward(long userId) {
        return 500;
    }
}

// 使用策略模式执行策略
public class RewardContext {
    // 奖励策略
    private RewardStrategy rewardStrategy;

    public RewardContext(RewardStrategy rewardStrategy) {
        this.rewardStrategy = rewardStrategy;
    }

    public void doStrategy(long userId) {
        long rewardMoney = rewardStrategy.reward(userId);
        rewardStrategy.rewardAndSettlement(userId, rewardMoney);
    }
}
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// 工厂类抽象类：用来生产奖励策略
public abstract class StrategyFactory<T> {
    abstract RewardStrategy createRewardStrategy(Class<T> c);
}

// 工厂生产类
public class RewardStrategyFactory extends StrategyFactory {
    // 根据传入的不同策略的class生成对应的实例
    @Override
    RewardStrategy createRewardStrategy(Class c) {
        RewardStrategy product = null;
        try {
            product = (RewardStrategy) Class.forName(c.getName()).newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return product;
    }
}

public class TestMain {
    /**
     * 主流程：创建工厂 -> 使用工厂创建具体的奖励策略 -> 使用封装的策略，实现对策略的访问和执行
     * @param args
     */
    public static void main(String[] args) {
        // 创建工厂
        RewardStrategyFactory factory = new RewardStrategyFactory();
        // todo 根据场景确定一个具体的策略实现类：NewUserRewardStrategyA
        // 使用工厂创建一个具体的奖励策略
        RewardStrategy newUserReward = factory.createRewardStrategy(NewUserRewardStrategyA.class);

        // 执行封装的策略，实现对策略的访问和执行
        RewardContext rewardContext = new RewardContext(newUserReward);
        rewardContext.doStrategy(1);
    }
}