场景

老大：

我们的收银台项目要新增一个功能。在结账的时候，可能会打折，打几折都有可能，有的是满几百就减免多少，你来实现下这个功能。

我：

好的

我心想：

说白了，就是对总钱数进行运算得到一个新的结果。只是怎么运算的事情。不同情况下采用不同的运算方式，又可以称不同的策略。

代码如下：

策略接口

package StrategyPattern;
public interface IStrategy {
    double cal(Double total);
}

打折策略

package StrategyPattern;
public class StrategyDiscount implements IStrategy {
    /*
     * 折扣
     */
    private int discount;
    public StrategyDiscount(int discount) {
        this.discount = discount;
    }
    public double cal(Double total) {
        return total * discount * 0.01;
    }
}

正常无优惠

package StrategyPattern;
public class StrategyNormal implements IStrategy {
    public double cal(Double total) {
        return total;
    }
}

返利

package StrategyPattern;
public class StrategyReturn implements IStrategy {
    private double condition;
    private double returnNum;
    public StrategyReturn(double condition, double returnNum) {
        this.condition = condition;
        this.returnNum = returnNum;
    }
    public double cal(Double total) {
        if (total >= condition)
            return total - Math.floor(total / condition) * returnNum;
        else
            return total;
    }
}

有一个context来决定使用哪个策略

package StrategyPattern;
public class StrategyContext {
    private IStrategy strategy;
    public StrategyContext(String type) throws Exception {
        switch (type) {
        case "normal":
            strategy = new StrategyNormal();
            break;
        case "discount":
            strategy = new StrategyDiscount(1);
            break;
        case "return":
            strategy = new StrategyReturn(300, 100);
            break;
        default:
            throw new Exception("没有合适的策略");
        }
    }
    public double cal(Double total) {
        return strategy.cal(total);
    }
}

客户端直接调用 context 即可。

package StrategyPattern;
public class Program {
    public static void main(String[] args) throws Exception {
        String type = "打折";
        double total = 100;
        StrategyContext context = new StrategyContext(type);
        double result =  context.cal(total);
    }
}

类图

客户端直接调用 StrategyContext，而其又根据不同的条件采取不同的策略。