策略模式

1.定义

策略模式又叫政策模式，它是将定义的算法家族，封装起来，使他们之间可以互相替换，从事算法的变化不影响使用算法的用户，属于行为型模式。
策略模式就是面向对象的继承和多态机制，让一种行为在不同的场景下有不同的实现

2.策略模式的应用场景

1.不同薪资纳税比例不一样
2.在线支付 可以选择多种支付工具
策略模式可以解决在多中类似的算法情况下，使用if...else或switch...case所带来的复杂性和臃肿：
1.针对同一类型问题，有多中处理方式，每一种都能独立解决问题
2.算法需要自由切换的场景
3.需要屏蔽算法规则的场景

3.策略模式的三种角色

1.上下文角色（context）：用来操作策略模式的上下文场景，屏蔽高层模块对策略、算法的直接访问封装可能存在的变化
2.抽象策略模式：规定策略或算法的行为
3.具体策略角色：具体的策略或算法实现

4.demo

不同的优惠策略

/**
 * create by yufeng on 2021/7/5 17:00
 */
public interface IPromotionStrategy {
    void doPromotion();
}
/**
 * create by yufeng on 2021/7/5 17:13
 */
public class CashbackStrategy implements IPromotionStrategy {
    public void doPromotion() {
        System.out.println("返现，直接打款到支付宝账号");
    }
}
/**
 * create by yufeng on 2021/7/5 17:15
 */
public class CouponStrategy implements IPromotionStrategy {
    public void doPromotion() {
        System.out.println("使用优惠券抵扣");
    }
}
/**
 * create by yufeng on 2021/7/5 17:20
 */
public class EmptyStrategy implements IPromotionStrategy {
    public void doPromotion() {
        System.out.println("无优惠");
    }
}
/**
 * create by yufeng on 2021/7/5 17:22
 */
public class GroupbuyStrategy implements IPromotionStrategy {
    public void doPromotion() {
        System.out.println("5人成团，可以优惠");
    }
}
/**
 * create by yufeng on 2021/7/5 17:30
 */
public class PromotionStrategyFacory {

    private static Map PROMOTIONS = new HashMap();

    static {
        PROMOTIONS.put(PromotionKey.COUPON,new CouponStrategy());
        PROMOTIONS.put(PromotionKey.CASHBACK,new CashbackStrategy());
        PROMOTIONS.put(PromotionKey.GROUPBUY,new GroupbuyStrategy());
    }

    private static final IPromotionStrategy EMPTY = new EmptyStrategy();

    private PromotionStrategyFacory(){}

    public static IPromotionStrategy getPromotionStrategy(String promotionKey){
        IPromotionStrategy strategy = PROMOTIONS.get(promotionKey);
        return strategy == null ? EMPTY : strategy;
    }
    private interface PromotionKey{
        String COUPON = "COUPON";
        String CASHBACK = "CASHBACK";
        String GROUPBUY = "GROUPBUY";
    }

    public static  Set getPromotionKeys(){
        return PROMOTIONS.keySet();
    }
}
/**
 * create by yufeng on 2021/7/5 17:32
 */
public class Test {
    public static void main(String[] args) {

        String promotionKey = "COUPON";

        IPromotionStrategy promotionStrategy = PromotionStrategyFacory.getPromotionStrategy(promotionKey);
        promotionStrategy.doPromotion();
    }
}
执行结果：
    使用优惠券抵扣

支付场景

/**
 * create by yufeng on 2021/7/5 18:13
 */
public class MsgResult {
    private int code;
    private Object data;
    private String msg;

    public MsgResult(int code, String msg, Object data) {
        this.code = code;
        this.data = data;
        this.msg = msg;
    }

    @Override
    public String toString() {
        return "MsgResult{" +
                "code=" + code +
                ", data=" + data +
                ", msg='" + msg + '\'' +
                '}';
    }
}
/**
 * create by yufeng on 2021/7/5 18:13
 */
public abstract class Payment {

    public abstract String getName();

    //通用逻辑放到抽象类里面实现
    public MsgResult pay(String uid, double amount){
        //余额是否足够
        if(queryBalance(uid) < amount){
            return new MsgResult(500,"支付失败","余额不足");
        }
        return new MsgResult(200,"支付成功","支付金额" + amount);
    }

    protected abstract double queryBalance(String uid);
}
/**
 * create by yufeng on 2021/7/5 18:13
 */
public class AliPay extends Payment {
    public String getName() {
        return "支付宝";
    }

    protected double queryBalance(String uid) {
        return 900;
    }
}

/**
 * create by yufeng on 2021/7/5 18:13
 */
public class WechatPay extends Payment {
    public String getName() {
        return "微信支付";
    }

    protected double queryBalance(String uid) {
        return 263;
    }
}
/**
 * create by yufeng on 2021/7/5 18:13
 */
public class UnionPay extends Payment {
    public String getName() {
        return "银联支付";
    }

    protected double queryBalance(String uid) {
        return 120;
    }
}

/**
 * create by yufeng on 2021/7/5 18:13
 */
public class PayStrategy {
    public static  final String ALI_PAY = "AliPay";
    public static  final String JD_PAY = "JdPay";
    public static  final String WECHAT_PAY = "WechatPay";
    public static  final String UNION_PAY = "UnionPay";
    public static  final String DEFAULT_PAY = ALI_PAY;

    private static Map strategy = new HashMap();

    static {
        strategy.put(ALI_PAY,new AliPay());
        strategy.put(JD_PAY,new JDPay());
        strategy.put(WECHAT_PAY,new WechatPay());
        strategy.put(UNION_PAY,new UnionPay());
    }

    public static Payment get(String payKey){
        if(!strategy.containsKey(payKey)){
            return strategy.get(DEFAULT_PAY);
        }
        return strategy.get(payKey);
    }
}
/**
 * create by yufeng on 2021/7/5 18:13
 */
public class Order {
    private String uid;
    private String orderId;
    private double amount;

    public Order(String uid, String orderId, double amount) {
        this.uid = uid;
        this.orderId = orderId;
        this.amount = amount;
    }

    public MsgResult pay(){
        return pay(PayStrategy.DEFAULT_PAY);
    }

    public MsgResult pay(String payKey){
        Payment payment = PayStrategy.get(payKey);
        System.out.println("欢迎使用" + payment.getName());
        System.out.println("本次交易金额为" + amount + "，开始扣款");
        return payment.pay(uid,amount);
    }
}
/**
 * create by yufeng on 2021/7/5 18:13
 */
public class Test {
    public static void main(String[] args) {
        Order order = new Order("1","2020031401000323",324.5);
        System.out.println(order.pay(PayStrategy.UNION_PAY));
    }
}

类图：

image.png

5.优缺点

优点：
符合开闭原则
避免过多if...else switc..case
使用策略模式提高了算法的保密性和安全性
缺点：
客户端必须知道所有策略，并自行决定用哪个策略
代码中会产生非常多的策略类，增加维护难度

6.源码地址

策略模式源码地址