设计模式最简Demo：策略模式 + 简单工厂模式

1. 程序需求

商场收费系统，需要支持正常收费，打折收费，现金返利，积分等计费方式。

2. 简单工厂模式

2.1 UML

2.2 Java实现

Cash

package simple_factory_pattern.market;public abstract class Cash {    public double originCash; //原价    public abstract double acceptCash();}

CashNormal

package simple_factory_pattern.market;public class CashNormal extends Cash {    @Override    public double acceptCash() {        return originCash;    }}

CashRebate

package simple_factory_pattern.market;public class CashRebate extends Cash {    private double rebate;    public CashRebate(double rebate) {        this.rebate = rebate;    }    @Override    public double acceptCash() {        return originCash * rebate;    }}

CashReturn

package simple_factory_pattern.market;public class CashReturn extends Cash {    private double condition;    private double ret;    public CashReturn(double condition, double ret) {        this.condition = condition;        this.ret = ret;    }    @Override    public double acceptCash() {        if (originCash > condition){            return originCash - (Math.floor(originCash / condition) * ret);        }        return originCash;    }}

CashAcceptFactory

package simple_factory_pattern.market;public class CashAcceptFactory {    public static Cash createCash(String select){        Cash cash = null;        switch (select){            case "原价支付":                cash = new CashNormal();                break;            case "打8折":                cash = new CashRebate(0.8);                break;            case "满300减100":                cash = new CashReturn(300, 100);                break;        }        return cash;    }}

Test

package simple_factory_pattern.market;public class Test {    public static void main(String[] args) {        Cash cash = CashAcceptFactory.createCash("打8折");        cash.originCash = 500;        System.out.println(cash.acceptCash());    }}

3. 结合策略模式

简单工厂模式虽然也能解决这个问题， 但这个模式只是解决对象的创建问题， 而且由于工厂本身包括了所有的收费方式， 商场是可能经常性地更改打折额度和返利额度， 每次维护或扩展收费方式都要改动这个工厂， 以致代码需重新编译部署， 这真的是很糟糕的处理方式。我们尝试使用策略模式加简单工厂模式对上述代码进行优化。

增加CashContext

package strategy_pattern.market;import simple_factory_pattern.market.Cash;import simple_factory_pattern.market.CashNormal;import simple_factory_pattern.market.CashRebate;import simple_factory_pattern.market.CashReturn;public class CashContext {    private Cash cash = null;    public CashContext(String type, double money) {        switch (type){            case "原价支付":                cash = new CashNormal();                break;            case "打8折":                cash = new CashRebate(0.8);                break;            case "满300减100":                cash = new CashReturn(300, 100);                break;        }        cash.originCash = money;    }    public double acceptCash(){        return cash.acceptCash();    }}

Test

package strategy_pattern.market;public class Test {    public static void main(String[] args) {        CashContext cashContext = new CashContext("打8折", 500);        System.out.println(cashContext.acceptCash());    }}

通过上面的改造，计费算法和客户端完全分离，客户端甚至不需要知道Cash类的存在。

4. 总结

策略模式是一种定义一系列算法的方法， 从概念上来看，所有这些算法完成的都是相同的工作，只是实现不同， 它可以以相同的方式 调用所有的算法，减少了各种算法类与使用算法类之间的耦合。