前言：

重構發生的時機應該在什麼時候呢？

正確：一邊寫著程式碼，當完成之後，馬上想著是否能夠進行重構。

錯誤：寫完程式碼之後，等以後有時間去重構。

即重構本來就不是一件應該特別撥出時間做的事情，重構應該隨時隨地進行。

書上有這麼一句話，我非常喜歡：懶惰是程式設計師的一種美德。正如有句話說，不要讓你的“勤奮”毀掉了你，有著異曲同工之妙。《重構改善既有程式碼的設計》的作者說，“我是個很懶惰的程式設計師，我的懶惰表現形式之一就是：我總是記不住自己寫過的程式碼”，“我不是個偉大的程式設計師，我只是有著一些優秀習慣的好程式設計師”。

什麼是重構：

在不改變程式碼外部行為的前提下，對程式碼做出修改，以改程序序的內部結構。本質上說，重構是在程式碼寫好之後改進它的設計（注意：

舉個栗子：（在筆試的時候經常會有類似重構的例子）

場景：一個影片出租店的程式，計算每一位顧客的消費金額並列印詳單。操作者告訴程式：顧客租了哪些影片、租期多長，程式便根據租賃時間和影片單算出費用。影片分為三類：普通片、兒童片和新片。除了計算費用，還要為常客計算積分，積分會根據租片種類是否為新片而有不同。

未重構前的程式碼如下：

public class Customer {
//Movie只是一個簡單的純資料類
    private static 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 arg){
            _priceCode = arg;
        }
        
        public String getTitle(){
            return _title;
        }
    }
    //Rental表示某個租客租了一部電影
    private static 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;
        }
    }

    private String _name;
    private Vector _rentals = new Vector();
    
    public Customer(String name){
        _name = name;
    }
    public void addRental(Rental arg){
        _rentals.addElement(arg);
    }
    public String getName(){
        return _name;
    }
    //以下程式碼是需要重構的主題部分 
    public String statement(){
        double totalAmount = 0;
        int frequentRenterPoints = 0;
        Enumeration rentals = _rentals.elements();
        String result = "Rental Record for " + getName() + "\n";
        while(rentals.hasMoreElements()){
            double thisAmount = 0;
            Rental each = (Rental) 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" +
                    String.valueOf(thisAmount)  + "\n";
            totalAmount += thisAmount;
        }
        
        //add footer lines
        result += "Amount owed is " + String.valueOf(totalAmount) + "\n";
        result += "You earned " + String.valueOf(frequentRenterPoints) + 
                " frequent renter points";
        return result;
    }
}
 

分析：

首先，我一看到switch程式碼，就要看到分支的情況，這裡case只有三個分支型別：Movie.REGULAR，Movie.NEW_RELEASE，Movie.CHILDRENS，如果Movie中新增了一種型別此時case就不支援，因此，至少要新增一個default分支，並且throw 自定義找不到該值的異常，這樣程式更嚴謹。對該例重構的原則如下：

（1）對於switch case 這種語句，我習慣用策略列舉進行重構（具體用法參見：Effective Java 中文版 第2版P128-P136），通過列舉中列舉型別來分型別，對於不同型別處理具體策略採用定義一個抽象方法。

（2）根據高內聚、低耦合的原則，如果某個業務只是跟某個類相關，則將這個業務移動到該類中進行處理。

public class CustomerV2 {
    private static class RentalV2{
        private MovieV2 _movie;
        private int _daysRented;
        
        public RentalV2(MovieV2 movie, int daysRented){
            _movie = movie;
            _daysRented = daysRented;
        }
        public int getDaysRented(){
            return _daysRented;
        }
        public MovieV2 getMovie(){
            return _movie;
        }
        
        public int getRenterPointers(){
            if(_movie.equals(MovieV2.NEW_RELEASE) && _daysRented > 1)
                return 2;
            return 1;
        }
    }
    //通過策略列舉
    private static enum MovieV2{
        CHILDRENS(2) {
            @Override
            public double getAmount(int daysRented) {
                double amount = 0;
                amount += 2;
                if(daysRented > 2){
                    amount += (daysRented -2) * 1.5;
                }
                return amount;
            }
        },
        REGULAR(0) {
            @Override
            public double getAmount(int daysRented) {
                double amount = 0;
                amount += daysRented * 3;
                return amount;
            }
        },
        NEW_RELEASE(1) {
            @Override
            public double getAmount(int daysRented) {
                double amount = 0;
                if(daysRented > 3)
                    amount += (daysRented -3) * 1.5;
                return amount;
            }
        };
        
        private String _title;
        private final int _priceCode;
        MovieV2(int priceCode){
            this._priceCode = priceCode;
        }
        
        public abstract double getAmount(int daysRented);
    }
    
    private String _name;
    private Vector _rentals = new Vector();
    
    public CustomerV2(String name){
        _name = name;
    }
    public void addRental(RentalV2 arg){
        _rentals.addElement(arg);
    }
    public String getName(){
        return _name;
    }
    //計算amount和fequent分開
    public String statement(){
        double totalAmount = getTotalAmounts();
        int frequentRenterPointers = getFrequentRenterPointers();
        return getReturnData(totalAmount, frequentRenterPointers);
    }
    
    private double getTotalAmounts(){
        Enumeration rentals = _rentals.elements();
        double totalAmount = 0;
        while(rentals.hasMoreElements()){
            RentalV2 rental = (RentalV2) rentals.nextElement();
            MovieV2 movie = rental.getMovie();
            double amount = movie.getAmount(rental.getDaysRented());
            totalAmount += amount;
        }
        return totalAmount;
    }
    
    private int getFrequentRenterPointers(){
        Enumeration rentals = _rentals.elements();
        int frequentRenterPointers = 0;
        while(rentals.hasMoreElements()){
            RentalV2 rental = (RentalV2) rentals.nextElement();
            frequentRenterPointers += rental.getRenterPointers();
        }
        return frequentRenterPointers;
    }
    
    private String getReturnData(double totalAmount, int frequentRenterPointers){
        String result = "Rental Record for " + getName() + "\n";
        result += "Amount owed is " + String.valueOf(totalAmount) + "\n";
        result += "You earned " + String.valueOf(frequentRenterPointers) + 
                " frequent renter points";
        return result;
    }
    
}