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定义:策略模式(Strategy Pattern)又叫也叫政策模式(Policy Pattern),他是将定义的算法家族,分别封装起来,让它们之间可以互相替换,从而让算法的变化不会影响到使用算法的用户.

可以避免多重分支的if…else…和switch语句

属于行为型模式

适用场景

1.假如系统中有很多类,而他们的区别仅仅在于他们的行为不同.

2.一个系统需要动态地在几种算法中选择一种.

3.需要屏蔽算法规则

优点

1.策略模式符合开闭原则

2.避免使用多重条件转移语句,如if…else…语句,switch语句

3.使用策略模式可以提高算法的保密性和安全性

缺点

1.客户端必须知道所有的策略,并且自行决定使用哪一个策略类

2.代码中会产生非常多策略类,增加维护难度.

例子

一般用法

public interface IStrategy {
       void algorithm();}

public class Context {
       private IStrategy mStrategy;    public Context(IStrategy mStrategy){
           this.mStrategy = mStrategy;    }    public void algorithm(){
           this.mStrategy.algorithm();    }}

public class ConcreteStrategyA implements IStrategy{
       @Override    public void algorithm() {
           System.out.println("Strategy A");    }}

public class ConcreteStrategyB implements IStrategy{
       @Override    public void algorithm() {
           System.out.println("Strategy B");    }}

public class Test {
       public static void main(String[] args) {
           IStrategy strategy = new ConcreteStrategyA();        Context context = new Context(strategy);        context.algorithm();        context = new Context(new ConcreteStrategyB());        context.algorithm();    }}

具体例子

public interface IPromotionStrategy {
       void doPromotion();}

public class PromotionActivity {
       private IPromotionStrategy strategy;    public PromotionActivity(IPromotionStrategy strategy) {
           this.strategy = strategy;    }    public void execute(){
           this.strategy.doPromotion();    }}

public class CashbackStrategy implements IPromotionStrategy{
       @Override    public void doPromotion() {
           System.out.println("使用返现策略");    }}

public class CouponStrategy implements IPromotionStrategy {
       @Override    public void doPromotion() {
           System.out.println("使用优惠卷");    }}

public class EmptyStrategy implements IPromotionStrategy{
       @Override    public void doPromotion() {
           System.out.println("无优惠");    }}

public class GroupBuyStrategy implements IPromotionStrategy{
       @Override    public void doPromotion() {
           System.out.println("3人成团,可以优惠");    }}

public class PromotionStrategyFactory {
       private static Map
   
     PROMOTIONS = new HashMap<>();    private PromotionStrategyFactory(){
       }    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();    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();    }}
    
   

public class Test {
       public static void main(String[] args) {
           PromotionStrategyFactory.getPromotionKeys();        String promotionKey = "COUPON";        IPromotionStrategy promotionStrategy = PromotionStrategyFactory.getPromotionStrategy(promotionKey);        promotionStrategy.doPromotion();    }}

支付方式使用策略模式

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);}

public class AliPay extends Payment{
       public String getName(){
           return "支付宝";    }    @Override    protected double queryBalance(String uid) {
           return 999;    }}

public class JDPay extends Payment{
       public String getName(){
           return "京东支付";    }    @Override    protected double queryBalance(String uid) {
           return 988;    }}

public class UnionPay extends Payment{
       public String getName(){
           return "银联支付";    }    @Override    protected double queryBalance(String uid) {
           return 1000;    }}

public class WechatPay extends Payment {
       @Override    public String getName() {
           return "微信支付";    }    @Override    protected double queryBalance(String uid) {
           return 0;    }}

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 = "UnionNPay";    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);    }}
   

public class MsgResult {
       private int code;    private Object data;    private String msg;    public MsgResult(int code, Object data, String msg) {
           this.code = code;        this.data = data;        this.msg = msg;    }    public int getCode() {
           return code;    }    public void setCode(int code) {
           this.code = code;    }    public String getMsg() {
           return msg;    }    public void setMsg(String msg) {
           this.msg = msg;    }    public Object getData() {
           return data;    }    public void setData(Object data) {
           this.data = data;    }    @Override    public String toString() {
           return "MsgResult{" +                "code=" + code +                ", msg='" + msg + '\'' +                ", data=" + data +                '}';    }}

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(null);    }    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);    }}

*/public class Test {
       public static void main(String[] args) {
           Order order = new Order("1","2020092709341212",521);        System.out.println(order.pay(PayStrategy.JD_PAY));    }}

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