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設計模式(3)-策略模式(Strategy)

編輯：C++入門知識

【描述】策略模式本質上利用的是面向對象的多態特性，構建者不必自身包含實現的邏輯，而是根據需要利用其他對象中的算法。

【UML圖】

圖1 UML圖

(1) 定義了一個Methods基類，包含一個策略的接口MethodsInterface。

(2) 定義了MethodsStrategyA、MethodsStrategyB兩種策略。

(3) 定義了一個策略構建者Context，包含ContextInterface方法。

【示例代碼】

methods.h

[html]
#ifndef METHODS_H
#define METHODS_H

class Methods
{
public:
Methods();

public:
virtual void MethodsInterface();
};

#endif // METHODS_H
#ifndef METHODS_H
#define METHODS_H

class Methods
{
public:
Methods();

public:
virtual void MethodsInterface();
};

#endif // METHODS_H

methods.cpp

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#include <QDebug>
#include "methods.h"

Methods::Methods()
{
qDebug()<<"construct Methods";
}

void Methods::MethodsInterface()
{
qDebug()<<"Methods::MethodsInterface";
}
#include <QDebug>
#include "methods.h"

Methods::Methods()
{
qDebug()<<"construct Methods";
}

void Methods::MethodsInterface()
{
qDebug()<<"Methods::MethodsInterface";
}

methodstrategya.h

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#ifndef METHODSTRATEGYA_H
#define METHODSTRATEGYA_H

#include "methods.h"

class MethodStrategyA : public Methods
{
public:
MethodStrategyA();

public:
void MethodsInterface();
};

#endif // METHODSTRATEGYA_H
#ifndef METHODSTRATEGYA_H
#define METHODSTRATEGYA_H

#include "methods.h"

class MethodStrategyA : public Methods
{
public:
MethodStrategyA();

public:
void MethodsInterface();
};

#endif // METHODSTRATEGYA_H

methodstrategya.cpp

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#include <QDebug>
#include "methodstrategya.h"

MethodStrategyA::MethodStrategyA()
{
qDebug()<<"construct MethodStrategyA";
}

void MethodStrategyA::MethodsInterface()
{
qDebug()<<"Strategy A";
}
#include <QDebug>
#include "methodstrategya.h"

MethodStrategyA::MethodStrategyA()
{
qDebug()<<"construct MethodStrategyA";
}

void MethodStrategyA::MethodsInterface()
{
qDebug()<<"Strategy A";
}

methodstrategyb.h

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#ifndef METHODSTRATEGYB_H
#define METHODSTRATEGYB_H

#include "methods.h"

class MethodStrategyB : public Methods
{
public:
MethodStrategyB();

public:
void MethodsInterface();
};

#endif // METHODSTRATEGYB_H
#ifndef METHODSTRATEGYB_H
#define METHODSTRATEGYB_H

#include "methods.h"

class MethodStrategyB : public Methods
{
public:
MethodStrategyB();

public:
void MethodsInterface();
};

#endif // METHODSTRATEGYB_H

methodstrategyb.cpp

[html] ‘
#include <QDebug>
#include "methodstrategyb.h"

MethodStrategyB::MethodStrategyB()
{
qDebug()<<"construct MethodStrategyB";
}

void MethodStrategyB::MethodsInterface()
{
qDebug()<<"Strategy B";
}
#include <QDebug>
#include "methodstrategyb.h"

MethodStrategyB::MethodStrategyB()
{
qDebug()<<"construct MethodStrategyB";
}

void MethodStrategyB::MethodsInterface()
{
qDebug()<<"Strategy B";
}

context.h

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#ifndef CONTEXT_H
#define CONTEXT_H

#include "methods.h"

class Context
{
public:
    Context(Methods *method);

private:
    Methods *_method;

public:
    void ContextInterface();
};

#endif // CONTEXT_H
#ifndef CONTEXT_H
#define CONTEXT_H

#include "methods.h"

class Context
{
public:
Context(Methods *method);

private:
Methods *_method;

public:
void ContextInterface();
};

#endif // CONTEXT_H

context.cpp

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#include "context.h"

Context::Context(Methods *method)
{
_method = method;
}

void Context::ContextInterface()
{
_method->MethodsInterface();
}
#include "context.h"

Context::Context(Methods *method)
{
_method = method;
}

void Context::ContextInterface()
{
_method->MethodsInterface();
}

main.cpp

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#include <QDebug>
#include "context.h"
#include "methods.h"
#include "methodstrategya.h"
#include "methodstrategyb.h"

int main()
{
    Context *context;

    context = new Context(new MethodStrategyA);
    context->ContextInterface();

    context = new Context(new MethodStrategyB);
    context->ContextInterface();

    return 0;
}
#include <QDebug>
#include "context.h"
#include "methods.h"
#include "methodstrategya.h"
#include "methodstrategyb.h"

int main()
{
Context *context;

context = new Context(new MethodStrategyA);
context->ContextInterface();

context = new Context(new MethodStrategyB);
context->ContextInterface();

return 0;
}

【運行結果】

[html]
construct Methods
construct MethodStrategyA
Strategy A
construct Methods
construct MethodStrategyB
Strategy B
construct Methods
construct MethodStrategyA
Strategy A
construct Methods
construct MethodStrategyB
Strategy B

【結果分析】

利用多態特性，實現了不同策略的調用，由於是測試代碼，本身很簡單，省略了析構函數。

【實例剖析】

*本例來源於《重構-改善既有代碼的設計》第1章。原例采用Java語言編寫，本例利用Qt C++進行了改寫。

看下面一個關於電影租賃計價的例子。

movie.h

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#ifndef MOVIE_H
#define MOVIE_H

#include <QString>
enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2};

class Movie
{
public:
    Movie();
    Movie(QString title, int priceCode);

private:
    QString _title;
    int _priceCode;

public:
    QString getTitle();
    int getPriceCode();
    void setPriceCode(int arg);
};

#endif // MOVIE_H
#ifndef MOVIE_H
#define MOVIE_H

#include <QString>
enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2};

class Movie
{
public:
Movie();
Movie(QString title, int priceCode);

private:
QString _title;
int _priceCode;

public:
    QString getTitle();
    int getPriceCode();
    void setPriceCode(int arg);
};

#endif // MOVIE_H

movie.cpp

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#include <QDebug>
#include "movie.h"

Movie::Movie()
{
    qDebug()<<"construct Movie::Movie()";
}

Movie::Movie(QString title, int priceCode)
{
    qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode);
    _title = title;
    _priceCode = priceCode;
}

QString Movie::getTitle()
{
    return _title;
}

int Movie::getPriceCode()
{
    return _priceCode;
}

void Movie::setPriceCode(int arg)
{
    _priceCode = arg;
}
#include <QDebug>
#include "movie.h"

Movie::Movie()
{
qDebug()<<"construct Movie::Movie()";
}

Movie::Movie(QString title, int priceCode)
{
    qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode);
    _title = title;
    _priceCode = priceCode;
}

QString Movie::getTitle()
{
return _title;
}

int Movie::getPriceCode()
{
return _priceCode;
}

void Movie::setPriceCode(int arg)
{
_priceCode = arg;
}

rental.h

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#ifndef RENTAL_H
#define RENTAL_H

#include "movie.h"

class Rental
{
public:
    Rental(Movie movie, int daysRented);

private:
    Movie _movie;
    int _daysRented;

public:
    Movie getMovie();
    int getDaysRented();
    double getCharge(int daysRented);
};

#endif // RENTAL_H
#ifndef RENTAL_H
#define RENTAL_H

#include "movie.h"

class Rental
{
public:
Rental(Movie movie, int daysRented);

private:
Movie _movie;
int _daysRented;

public:
    Movie getMovie();
    int getDaysRented();
    double getCharge(int daysRented);
};

#endif // RENTAL_H

rental.cpp

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#include <QDebug>
#include "rental.h"

Rental::Rental(Movie movie, int daysRented)
{
    qDebug()<<"construct Rental";
    _movie = movie;
    _daysRented = daysRented;
}

Movie Rental::getMovie()
{
    return _movie;
}

int Rental::getDaysRented()
{
    return _daysRented;
}

double Rental::getCharge(int daysRented)
{
    double result = 0;
    qDebug()<<getMovie().getPriceCode();
    switch(getMovie().getPriceCode())
    {
    case REGULAR:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1.5;
        }
        break;
    case NEW_RELEASE:
        result += daysRented*3;
        break;
    case DISCOUNT:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1;
        }
        break;
    }
    qDebug()<<result;
    return result;
}
#include <QDebug>
#include "rental.h"

Rental::Rental(Movie movie, int daysRented)
{
    qDebug()<<"construct Rental";
    _movie = movie;
    _daysRented = daysRented;
}

Movie Rental::getMovie()
{
return _movie;
}

int Rental::getDaysRented()
{
return _daysRented;
}

double Rental::getCharge(int daysRented)
{
    double result = 0;
    qDebug()<<getMovie().getPriceCode();
    switch(getMovie().getPriceCode())
    {
    case REGULAR:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1.5;
        }
        break;
    case NEW_RELEASE:
        result += daysRented*3;
        break;
    case DISCOUNT:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1;
        }
        break;
    }
    qDebug()<<result;
    return result;
}

main.cpp

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#include "movie.h"
#include "rental.h"

int main()
{
    int daysRented = 30;
    Movie movie("Book1",NEW_RELEASE);
    Rental rental(movie,daysRented);
    movie.setPriceCode(NEW_RELEASE);
    rental.getCharge(daysRented);
    return 0;
}
#include "movie.h"
#include "rental.h"

int main()
{
    int daysRented = 30;
    Movie movie("Book1",NEW_RELEASE);
    Rental rental(movie,daysRented);
    movie.setPriceCode(NEW_RELEASE);
    rental.getCharge(daysRented);
    return 0;
}

運行結果如下：

[html]
"construct Movie::Movie(Book1,1)"
construct Movie::Movie()
construct Rental
1
90
"construct Movie::Movie(Book1,1)"
construct Movie::Movie()
construct Rental
1
90
示例調用了getCharge，並返回了priceCode相應的計價。相對於《重構》一書最原始代碼，這段代碼經過了一定的重構處理。

先看下述代碼，

[html]
double Rental::getCharge(int daysRented)
{
    double result = 0;
    qDebug()<<getMovie().getPriceCode();
    switch(getMovie().getPriceCode())
    {
    case REGULAR:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1.5;
        }
        break;
    case NEW_RELEASE:
        result += daysRented*3;
        break;
    case DISCOUNT:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1;
        }
        break;
    }
    qDebug()<<result;
    return result;
}
double Rental::getCharge(int daysRented)
{
    double result = 0;
    qDebug()<<getMovie().getPriceCode();
    switch(getMovie().getPriceCode())
    {
    case REGULAR:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1.5;
        }
        break;
    case NEW_RELEASE:
        result += daysRented*3;
        break;
    case DISCOUNT:
        result +=2;
        if(daysRented>2)
        {
            result += (daysRented-2)*1;
        }
        break;
    }
    qDebug()<<result;
    return result;
}
這段代碼，有一些不好的地方：

(1) 這段代碼通過priceCode判斷電影的租金，采用了switch-case語言，實質上還是面向過程的范疇，代碼可重用性很低；

(2) 如果情形較多，switch-case會變得十分龐大。

有什麼辦法能去掉switch-case嗎？下面通過引入策略模式實現這一目的。

先看UML圖，

圖2

(1) 引入了一個Price基類，提供了getCharge()接口供子類實現；

(2) 將switch-case語句中的3種情形抽取為3個Price子類；

(3) Movie調用了Price提供的接口。

【代碼清單】

price.h

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#ifndef PRICE_H
#define PRICE_H

class Price
{
public:
    Price();

private:
    int _priceCode;

public:
    int getPriceCode();
    void setPriceCode(int arg);
    virtual double getCharge(int daysRented);
};

#endif // PRICE_H
#ifndef PRICE_H
#define PRICE_H

class Price
{
public:
Price();

private:
int _priceCode;

public:
    int getPriceCode();
    void setPriceCode(int arg);
    virtual double getCharge(int daysRented);
};

#endif // PRICE_H

price.cpp

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#include <QDebug>
#include "price.h"

Price::Price()
{
    qDebug()<<"construct Price";
    _priceCode = 0;
}

int Price::getPriceCode()
{
    qDebug()<<_priceCode;
    return _priceCode;
}

void Price::setPriceCode(int arg)
{
    _priceCode = arg;
}

double Price::getCharge(int daysRented)
{
    qDebug()<<"Price::getCharge";
    return daysRented;
}
#include <QDebug>
#include "price.h"

Price::Price()
{
qDebug()<<"construct Price";
_priceCode = 0;
}

int Price::getPriceCode()
{
qDebug()<<_priceCode;
return _priceCode;
}

void Price::setPriceCode(int arg)
{
_priceCode = arg;
}

double Price::getCharge(int daysRented)
{
qDebug()<<"Price::getCharge";
return daysRented;
}

regularprice.h

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#ifndef REGULARPRICE_H
#define REGULARPRICE_H

#include "price.h"

class RegularPrice : public Price
{
public:
RegularPrice();

public:
double getCharge(int daysRented);
};

#endif // REGULARPRICE_H
#ifndef REGULARPRICE_H
#define REGULARPRICE_H

#include "price.h"

class RegularPrice : public Price
{
public:
RegularPrice();

public:
double getCharge(int daysRented);
};

#endif // REGULARPRICE_H

regularprice.cpp

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#include <QDebug>
#include "regularprice.h"

RegularPrice::RegularPrice()
{
    qDebug()<<"construct RegularPrice";
}

double RegularPrice::getCharge(int daysRented)
{
    qDebug()<<"RegularPrice::getCharge";
    double result = 2;
    if(daysRented>2)
    {
        result += (daysRented-2)*1.5;
    }
    return result;
}
#include <QDebug>
#include "regularprice.h"

RegularPrice::RegularPrice()
{
qDebug()<<"construct RegularPrice";
}

double RegularPrice::getCharge(int daysRented)
{
    qDebug()<<"RegularPrice::getCharge";
    double result = 2;
    if(daysRented>2)
    {
        result += (daysRented-2)*1.5;
    }
    return result;
}

newreleaseprice.h

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#ifndef NEWRELEASEPRICE_H
#define NEWRELEASEPRICE_H

#include "price.h"

class NewReleasePrice : public Price
{
public:
NewReleasePrice();

public:
double getCharge(int daysRented);
};

#endif // NEWRELEASEPRICE_H
#ifndef NEWRELEASEPRICE_H
#define NEWRELEASEPRICE_H

#include "price.h"

class NewReleasePrice : public Price
{
public:
NewReleasePrice();

public:
double getCharge(int daysRented);
};

#endif // NEWRELEASEPRICE_H

newreleaseprice.cpp

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#include <QDebug>
#include "newreleaseprice.h"

NewReleasePrice::NewReleasePrice()
{
    qDebug()<<"construct NewReleasePrice";
}

double NewReleasePrice::getCharge(int daysRented)
{
    qDebug()<<"NewReleasePrice::getCharge";
    return daysRented*3;;
}
#include <QDebug>
#include "newreleaseprice.h"

NewReleasePrice::NewReleasePrice()
{
qDebug()<<"construct NewReleasePrice";
}

double NewReleasePrice::getCharge(int daysRented)
{
qDebug()<<"NewReleasePrice::getCharge";
return daysRented*3;;
}

discountprice.h

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#ifndef DISCOUNTPRICE_H
#define DISCOUNTPRICE_H

#include "price.h"

class DiscountPrice : public Price
{
public:
DiscountPrice();

public:
double getCharge(int daysRented);
};

#endif // DISCOUNTPRICE_H
#ifndef DISCOUNTPRICE_H
#define DISCOUNTPRICE_H

#include "price.h"

class DiscountPrice : public Price
{
public:
DiscountPrice();

public:
double getCharge(int daysRented);
};

#endif // DISCOUNTPRICE_H

discountprice.cpp

[html]
#include <QDebug>
#include "discountprice.h"

DiscountPrice::DiscountPrice()
{
    qDebug()<<"construct DiscountPrice";
}

double DiscountPrice::getCharge(int daysRented)
{
    qDebug()<<"DiscountPrice::getCharge";
    double result = 2;
    if(daysRented>2)
    {
        result += daysRented;
    }

    return result;
}
#include <QDebug>
#include "discountprice.h"

DiscountPrice::DiscountPrice()
{
qDebug()<<"construct DiscountPrice";
}

double DiscountPrice::getCharge(int daysRented)
{
    qDebug()<<"DiscountPrice::getCharge";
    double result = 2;
    if(daysRented>2)
    {
        result += daysRented;
    }

return result;
}

movie.h

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#ifndef MOVIE_H
#define MOVIE_H

#include <QString>
#include "price.h"
enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2};

class Movie
{
public:
    Movie();
    Movie(QString title, int priceCode);
    ~Movie();

private:
    QString _title;
    int _priceCode;
    Price *_price;

public:
    QString getTitle();
    int getPriceCode();
    void setPriceCode(int arg, Price *_price);
    double getCharge(int daysRented);
};

#endif // MOVIE_H
#ifndef MOVIE_H
#define MOVIE_H

#include <QString>
#include "price.h"
enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2};

class Movie
{
public:
    Movie();
    Movie(QString title, int priceCode);
    ~Movie();

private:
    QString _title;
    int _priceCode;
    Price *_price;

public:
    QString getTitle();
    int getPriceCode();
    void setPriceCode(int arg, Price *_price);
    double getCharge(int daysRented);
};

#endif // MOVIE_H

movie.cpp

[html]
#include <QDebug>
#include "movie.h"
#include "regularprice.h"
#include "newreleaseprice.h"
#include "discountprice.h"

Movie::Movie()
{
    qDebug()<<"construct Movie::Movie()";

}

Movie::Movie(QString title, int priceCode)
{
    qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode);
    _price = new Price;
    _title = title;
    _priceCode = priceCode;
}

Movie::~Movie()
{
    delete _price;
}

QString Movie::getTitle()
{
    return _title;
}

int Movie::getPriceCode()
{
    return _price->getPriceCode();
}

void Movie::setPriceCode(int arg, Price *price)
{
    _price = price;
    _price->setPriceCode(arg);
}

double Movie::getCharge(int daysRented)
{
    return _price->getCharge(daysRented);
}
#include <QDebug>
#include "movie.h"
#include "regularprice.h"
#include "newreleaseprice.h"
#include "discountprice.h"

Movie::Movie()
{
qDebug()<<"construct Movie::Movie()";

}

Movie::Movie(QString title, int priceCode)
{
    qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode);
    _price = new Price;
    _title = title;
    _priceCode = priceCode;
}

Movie::~Movie()
{
delete _price;
}

QString Movie::getTitle()
{
return _title;
}

int Movie::getPriceCode()
{
return _price->getPriceCode();
}

void Movie::setPriceCode(int arg, Price *price)
{
_price = price;
_price->setPriceCode(arg);
}

double Movie::getCharge(int daysRented)
{
return _price->getCharge(daysRented);
}

rental.h

#include "movie.h"

class Rental
{
public:
Rental(Movie movie, int daysRented);

private:
Movie _movie;
int _daysRented;

public:
    Movie getMovie();
    int getDaysRented();
    double getCharge(int daysRented);
};

#endif // RENTAL_H

rental.cpp

Rental::Rental(Movie movie, int daysRented)
{
    qDebug()<<"construct Rental";
    _movie = movie;
    _daysRented = daysRented;
}

Movie Rental::getMovie()
{
return _movie;
}

int Rental::getDaysRented()
{
return _daysRented;
}

double Rental::getCharge(int daysRented)
{
return _movie.getCharge(daysRented);
}

main.cpp

[html]
#include <QDebug>
#include "movie.h"
#include "rental.h"
#include "regularprice.h"
#include "newreleaseprice.h"
#include "discountprice.h"

int main()
{
    int daysRented = 30;
    Movie movie("Book1",NEW_RELEASE);
    Rental rental(movie,daysRented);
    movie.setPriceCode(NEW_RELEASE, new NewReleasePrice);
    rental.getCharge(daysRented);
    return 0;
}
#include <QDebug>
#include "movie.h"
#include "rental.h"
#include "regularprice.h"
#include "newreleaseprice.h"
#include "discountprice.h"

int main()
{
    int daysRented = 30;
    Movie movie("Book1",NEW_RELEASE);
    Rental rental(movie,daysRented);
    movie.setPriceCode(NEW_RELEASE, new NewReleasePrice);
    rental.getCharge(daysRented);
    return 0;
}

【運行結果】

[html]
"construct Movie::Movie(Book1,1)"
construct Price
construct Movie::Movie()
construct Rental
construct Price
construct NewReleasePrice
NewReleasePrice::getCharge
"construct Movie::Movie(Book1,1)"
construct Price
construct Movie::Movie()
construct Rental
construct Price
construct NewReleasePrice
NewReleasePrice::getCharge

【結果分析】

[html]
movie.setPriceCode(NEW_RELEASE, new NewReleasePrice);
movie.setPriceCode(NEW_RELEASE, new NewReleasePrice);
傳入priceCode和Price子對象作為參數，從運行結果可以看出，調用了NewReleasePrice的getCharge方法。

應用策略模式後，Rental::getCharge中僅有一行代碼。

[html] view plaincopyprint?double Rental::getCharge(int daysRented)
{
return _movie.getCharge(daysRented);
}
double Rental::getCharge(int daysRented)
{
return _movie.getCharge(daysRented);
}
將原來的getCharge做了搬移處理，而Movie::getCharge也很簡短：

[html] view plaincopyprint?double Movie::getCharge(int daysRented)
{
return _price->getCharge(daysRented);
}
double Movie::getCharge(int daysRented)
{
return _price->getCharge(daysRented);
}

*應用策略模式後，消除了switch-case語句，函數更為簡潔。將Price單獨抽出，成為一個類組，提高了代碼的可重用性。

作者：tandesir