《重构 改善既有代码的设计》读书笔记1

首先我们看重构之前的，共有三个类，Customer，Movie，Rental。全都是自己写的数据类。

// Rental: 表示某个顾客租了一部影片
public class Rental 
{
    private Movie _movie;
    private int _daysRented;
    
    public Rental(Movie movie, int daysRented)
    {
        _movie = movie;
        _daysRented = daysRented;
    }
    
    public int getDaysRented()
    {
        return _daysRented;
    }
    
    public Movie getMovie()
    {
        return _movie;
    }
}

// Movie: 纯数据类，代表一个影片
public class Movie 
{
    public static final int CHILDRENS = 2;
    public static final int REGULAR = 0;
    public static final int NEW_RELEASE = 1;
    
    private String _title;
    private int _priceCode;
    
    public Movie(String title, int priceCode)
    {
        _title = title;
        _priceCode = priceCode;
    }
    
    public int getPriceCode()
    {
        return _priceCode;
    }
    
    public void setPriceCode(int priceCode)
    {
        _priceCode = priceCode;
    }
    
    public String getTitle()
    {
        return _title;
    }
}

// Customer: 顾客
public class Customer
{
    private String _name;
    private Vector<Rental> _rentals = new Vector<Rental>();
    
    public Customer(String name)
    {
        _name = name;
    }
    
    public void addRental(Rental rental)
    {
        _rentals.add(rental);
    }
    
    public String getName()
    {
        return _name;
    }
    
    public String statement()
    {
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            double thisAmount = 0;
            Rental each = rentals.nextElement();
            
            // determine amounts for each line
            switch(each.getMovie().getPriceCode())
            {
            case Movie.REGULAR:
                thisAmount += 2;
                if(each.getDaysRented() > 2)
                {
                    thisAmount += (each.getDaysRented() - 2) * 1.5;
                }
                break;
                
            case Movie.NEW_RELEASE:
                thisAmount += each.getDaysRented() * 3;
                break;
                
            case Movie.CHILDRENS:
                thisAmount += 1.5;
                if(each.getDaysRented() > 3)
                {
                    thisAmount += (each.getDaysRented() - 3) * 1.5;
                }
                break;
            }
            
            // add frequent renter points
            frequentRenterPoints++;
            // add bonus for a two day new release rental
            if((each.getMovie().getPriceCode() == Movie.NEW_RELEASE)
                    && each.getDaysRented() > 1)
            {
                frequentRenterPoints++;
            }
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + thisAmount + "\n";
            totalAmount += thisAmount;
        }
        
        // add footer lines
        result += "Amount owed is" + totalAmount + "\n";
        result += "You earned " + frequentRenterPoints + "frequent renter points";
        return result;
    }
}

我们可以看到，Customer中的statement()函数过于复杂，这时我们就开始想要重构了。

首先我们看到，很明显的就是statement()中的switch语句，我们应该把它放到单独的一个函数中，所以我们要对switch处进行重构。

第一次重构：提取函数后的Customer类

// Customer: 顾客
public class Customer
{
    private String _name;
    private Vector<Rental> _rentals = new Vector<Rental>();
    
    public Customer(String name)
    {
        _name = name;
    }
    
    public void addRental(Rental rental)
    {
        _rentals.add(rental);
    }
    
    public String getName()
    {
        return _name;
    }
    
    public String statement()
    {
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            double thisAmount = 0;
            Rental each = rentals.nextElement();
            
            thisAmount += amountFor(each);
            
            // add frequent renter points
            frequentRenterPoints++;
            // add bonus for a two day new release rental
            if((each.getMovie().getPriceCode() == Movie.NEW_RELEASE)
                    && each.getDaysRented() > 1)
            {
                frequentRenterPoints++;
            }
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + thisAmount + "\n";
            totalAmount += thisAmount;
        }
        
        // add footer lines
        result += "Amount owed is" + totalAmount + "\n";
        result += "You earned " + frequentRenterPoints + "frequent renter points";
        return result;
    }
    
    private double amountFor(Rental each)
    {
        double thisAmount = 0;
        // determine amounts for each line
        switch(each.getMovie().getPriceCode())
        {
        case Movie.REGULAR:
            thisAmount += 2;
            if(each.getDaysRented() > 2)
            {
                thisAmount += (each.getDaysRented() - 2) * 1.5;
            }
            break;
            
        case Movie.NEW_RELEASE:
            thisAmount += each.getDaysRented() * 3;
            break;
            
        case Movie.CHILDRENS:
            thisAmount += 1.5;
            if(each.getDaysRented() > 3)
            {
                thisAmount += (each.getDaysRented() - 3) * 1.5;
            }
            break;
        }
        
        return thisAmount;
    }
}

之后我们会发现amountFor()中的一些变量的名字不是很好，所以我们进行重命名

    private double amountFor(Rental aRental)
    {
        double result = 0;
        // determine amounts for each line
        switch(aRental.getMovie().getPriceCode())
        {
        case Movie.REGULAR:
            result += 2;
            if(aRental.getDaysRented() > 2)
            {
                result += (aRental.getDaysRented() - 2) * 1.5;
            }
            break;
            
        case Movie.NEW_RELEASE:
            result += aRental.getDaysRented() * 3;
            break;
            
        case Movie.CHILDRENS:
            result += 1.5;
            if(aRental.getDaysRented() > 3)
            {
                result += (aRental.getDaysRented() - 3) * 1.5;
            }
            break;
        }
        
        return result;
    }

之后我们发现，amountFor()在Customer类中，可是却没有使用来自Customer类的信息，而是使用了来自Rental类的信息，所以我们需要考虑将amountFor()搬移到Rental类中

在这里我们同时改了一下函数名：amountFor() -> getCharge()

新的Rental：

// Rental: 表示某个顾客租了一部影片
public class Rental 
{
    private Movie _movie;
    private int _daysRented;
    
    public Rental(Movie movie, int daysRented)
    {
        _movie = movie;
        _daysRented = daysRented;
    }
    
    public int getDaysRented()
    {
        return _daysRented;
    }
    
    public Movie getMovie()
    {
        return _movie;
    }
    
    public double getCharge()
    {
        double result = 0;
        // determine amounts for each line
        switch(getMovie().getPriceCode())
        {
        case Movie.REGULAR:
            result += 2;
            if(getDaysRented() > 2)
            {
                result += (getDaysRented() - 2) * 1.5;
            }
            break;
            
        case Movie.NEW_RELEASE:
            result += getDaysRented() * 3;
            break;
            
        case Movie.CHILDRENS:
            result += 1.5;
            if(getDaysRented() > 3)
            {
                result += (getDaysRented() - 3) * 1.5;
            }
            break;
        }
        
        return result;
    }
}

新的Customer中的statement：

public String statement()
    {
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            double thisAmount = 0;
            Rental each = rentals.nextElement();
            
            thisAmount += each.getCharge();
            
            // add frequent renter points
            frequentRenterPoints++;
            // add bonus for a two day new release rental
            if((each.getMovie().getPriceCode() == Movie.NEW_RELEASE)
                    && each.getDaysRented() > 1)
            {
                frequentRenterPoints++;
            }
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + thisAmount + "\n";
            totalAmount += thisAmount;
        }
        
        // add footer lines
        result += "Amount owed is" + totalAmount + "\n";
        result += "You earned " + frequentRenterPoints + "frequent renter points";
        return result;
    }

之后我们发现，stateme()中的thisAmount显得有点多余了，接受each.getCharge()的执行结果，然后就不会有任何修改，所以我们运用Replace Temp With Query把thisAmount去掉

新的statement()：

public String statement()
    {
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            Rental each = rentals.nextElement();
            
            // add frequent renter points
            frequentRenterPoints++;
            // add bonus for a two day new release rental
            if((each.getMovie().getPriceCode() == Movie.NEW_RELEASE)
                    && each.getDaysRented() > 1)
            {
                frequentRenterPoints++;
            }
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + each.getCharge() + "\n";
            totalAmount += each.getCharge();
        }
        
        // add footer lines
        result += "Amount owed is" + totalAmount + "\n";
        result += "You earned " + frequentRenterPoints + "frequent renter points";
        return result;
    }

下一步对“常客积分计算”做类似的处理，积分的计算视影片的种类而有所不同不过不像收费规则(getCharge())有那么多变化，我们将常客积分计算的任务交给Rental：

Rental中新增加的函数：

    public int getFrequentRentalPoints()
    {
        int frequentRenterPoints = 0;
        frequentRenterPoints++;
        if(getMovie().getPriceCode() == Movie.NEW_RELEASE
                && getDaysRented() > 1)
        {
            frequentRenterPoints++;
        }
        
        return frequentRenterPoints;
    }

Customer的statement函数：

public String statement()
    {
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            Rental each = rentals.nextElement();
            
            frequentRenterPoints += each.getFrequentRentalPoints();
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + each.getCharge() + "\n";
            totalAmount += each.getCharge();
        }
        
        // add footer lines
        result += "Amount owed is" + totalAmount + "\n";
        result += "You earned " + frequentRenterPoints + "frequent renter points";
        return result;
    }

之后我们需要去除临时变量：

Customer中的statement函数以及新增加的getTotalCharge和getTotalFrequentRenterPoints函数

    public String statement()
    {
        Enumeration<Rental> rentals = _rentals.elements();
        
        String result = "Rental record for " + getName() + "\n";
        
        while(rentals.hasMoreElements())
        {
            Rental each = rentals.nextElement();
            
            // show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t"
                    + each.getCharge() + "\n";
        }
        
        // add footer lines
        result += "Amount owed is" + getTotalCharge() + "\n";
        result += "You earned " + getTotalFrequentRenterPoints() + "frequent renter points";
        return result;
    }
    
    private int getTotalFrequentRenterPoints()
    {
        int result = 0;
        for(int n=0; n<_rentals.size(); n++)
        {
            result += _rentals.get(n).getFrequentRentalPoints();
        }
        return result;
    }
    
    private double getTotalCharge()
    {
        double result = 0;
        for(int n=0; n<_rentals.size(); n++)
        {
            result += _rentals.get(n).getCharge();
        }
        
        return result;
    }