一、什么是策略模式？

1. 应用场景：定义一系列的算法，把它们一个个封装起来，并且使它们可相互替换。本模式使得算法可独立于使用它的客户而变化。（官方解释）
2. 特点：最终执行的结果是固定的，但是执行的过程和逻辑不一致；
3. 归类：行为型模式

二、实战演练

1. 事例： 商城里面购买东西不同的优惠算法，商城有满80减20，满100减30的活动
2. 创建结果类,PayState.java

package com.demo.strategy;
public class PayState {
    private String name;
    private String activeName;
    private double totalAmount;
    private double discount;
    private double giveAmount;
    public PayState(String name,double totalAmount,double giveAmount,String activeName,double discount){
        this.name = name;
        this.totalAmount=totalAmount;
        this.giveAmount=giveAmount;
        this.activeName=activeName;
        this.discount=discount;
    }
    public String toString(){
        StringBuffer stringBuffer=new StringBuffer();
        stringBuffer.append("用户："+this.name+"\n");
        stringBuffer.append("商品总金额："+this.totalAmount+"\n");
        stringBuffer.append("收到金额："+this.giveAmount+"\n");
        stringBuffer.append("满足活动："+this.activeName+"\n");
        stringBuffer.append("优惠："+ this.discount +"\n");
        stringBuffer.append("找零："+ (this.giveAmount-this.totalAmount + this.discount) +"\n");
        return stringBuffer.toString();
    }
}

1. 创建策略接口 Pay.java

package com.demo.strategy.active;
import com.demo.strategy.PayState;
public interface Pay {
    /**
     *
     * @param name          用户
     * @param totalAmount   商品总金额
     * @param giveAmount    收到的金额
     * @return
     */
    public PayState payAmount(String name, double totalAmount, double giveAmount);
}

1. 创建不同的活动策略实现<br />

Active0.java

package com.demo.strategy.active;
import com.demo.strategy.PayState;
/**
 * 无活动
 */
public class Active0 implements Pay{
    @Override
    public PayState payAmount(String name, double totalAmount, double giveAmount) {
        return new PayState(name,totalAmount,giveAmount,"无",0);
    }
}

Active80.java

package com.demo.strategy.active;
import com.demo.strategy.PayState;
/**
 * 活动满80减20
 */
public class Active80 implements Pay{
    @Override
    public PayState payAmount(String name, double totalAmount, double giveAmount) {
        return new PayState(name,totalAmount,giveAmount,"活动满80减20",20);
    }
}

Active100.java

package com.demo.strategy.active;
import com.demo.strategy.PayState;
/**
 * 活动满100减30
 */
public class Active100 implements Pay{
    @Override
    public PayState payAmount(String name, double totalAmount, double giveAmount) {
        return new PayState(name,totalAmount,giveAmount,"活动满100减30",30);
    }
}

5. 定义活动枚举类管理 ActiveEnum.java

package com.demo.strategy.active;
/**
 * 活动枚举类
 */
public enum ActiveEnum {
    ACTIVE_0(new Active0()),
    ACTIVE_80(new Active80()),
    ACTIVE_100(new Active100());
    private Pay pay;
    ActiveEnum(Pay pay){
        this.pay = pay;
    }
    public Pay get(){ return  this.pay;}
}

5. 创建订单类 Order.java

package com.demo.strategy;
import com.demo.strategy.active.ActiveEnum;
public class Order {
    private String name;
    private double totalAmount;
    private double giveAmount;
    public Order(String name,double totalAmount,double giveAmount){
        this.name=name;
        this.totalAmount = totalAmount;
        this.giveAmount=giveAmount;
    }
    public PayState pay(ActiveEnum ActiveEnum){
        return ActiveEnum.get().payAmount(this.name,this.totalAmount,this.giveAmount);
    }
}

5. 最后创建测试类 PayTest.java

package com.demo.strategy;
import com.demo.strategy.active.ActiveEnum;
public class PayTest {
    public static void main(String[] args) {
        String name="zhangsan";
        double totalAmount=120;
        double giveAmount=130;
        Order order=new Order(name,totalAmount,giveAmount);
        //金额的逻辑判断没写，活动按照满足
        System.out.println(order.pay(ActiveEnum.ACTIVE_100));
    }
}

5. 结果

用户：zhangsan
商品总金额：120.0
收到金额：130.0
满足活动：活动满100减30
优惠：30.0
找零：40.0