重构(一) -- 一个入门实例

本文主要参考《重构：改善既有代码的设计》

什么是重构

重构是在不改变代码外在行为的前提下，对代码做出修改，以改进程序的内部结构的一个过程。

实例

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 arg) {
        _priceCode = arg;
    }

    public String getTitle() {
        return _title;
    }


    /**
     * 第十步将getCharge和getFrequentRenterPoint移动到movie,因为case
     */
    double tenthGetCharge(int dayRented) {
        double result = 0;
        switch (getPriceCode()) {
            case Movie.REGULAR:
                result += 2;
                if (dayRented > 2) {
                    result += (dayRented - 2) * 1.5;
                }
                break;
            case Movie.NEW_RELEASE:
                result += dayRented * 3;
                break;
            case Movie.CHILDRENS:
                result += 1.5;
                if (dayRented > 3) {
                    result += (dayRented - 3) * 1.5;
                }
                break;
        }
        return result;
    }

    int tenthGetFrequentRenterPoints(int dayRented) {
        if (getPriceCode() == Movie.NEW_RELEASE && dayRented > 1) {
            return 2;
        }
        return 1;
    }


    /**
     * state设计模式
     */
    private Price _price;

    public Movie(String title, Price price) {
        _title = title;
        _price = price;
    }

    double getCharge(int daysRented) {
        return _price.getCharge(daysRented);
    }

    int getFrequentRenterPoints(int daysRented) {
        return _price.getFrequentRenterPoints(daysRented);
    }
}

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;
    }

    /**
     * 第三步将计算金额代码从Customer迁到Rental
     * @return
     */
    double thirdGetCharge() {
        double result = 0;
        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;
    }

    /**
     * 第七步将积分政策拆分到Rental
     */
    int seventhGetFrequentRenterPoints() {
        if (getMovie().getPriceCode() == Movie.NEW_RELEASE && getDaysRented() > 1) {
            return 2;
        }
        return 1;
    }

    /**
     * 第十步将getCharge和getFrequentRenterPoint移动到movie,因为case
     */
    double tenthGetCharge() {
        return _movie.tenthGetCharge(_daysRented);
    }

    int tenthGetFrequentRenterPoints() {
        return _movie.tenthGetFrequentRenterPoints(_daysRented);
    }
}

public class Customer {
    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;
    }

    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;
    }

    /**
     * 第一步：将switch方法抽成firstStepAmountFor方法
     */
    public String firstStepStatement() {
        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();
            thisAmount = firstStepAmountFor(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" + 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;
    }

    private double firstStepAmountFor(Rental each) {
        double thisAmount = 0;
        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;
    }

    /**
     * 第二步：更新正确的名称
     */
    public String secondStepStatement() {
        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();
            thisAmount = secondStepAmountFor(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" + 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;
    }

    private double secondStepAmountFor(Rental aRental) {
        double thisAmount = 0;
        switch (aRental.getMovie().getPriceCode()) {
            case Movie.REGULAR:
                thisAmount += 2;
                if (aRental.getDaysRented() > 2) {
                    thisAmount += (aRental.getDaysRented() - 2) * 1.5;
                }
                break;
            case Movie.NEW_RELEASE:
                thisAmount += aRental.getDaysRented() * 3;
                break;
            case Movie.CHILDRENS:
                thisAmount += 1.5;
                if (aRental.getDaysRented() > 3) {
                    thisAmount += (aRental.getDaysRented() - 3) * 1.5;
                }
                break;
        }
        return thisAmount;
    }

    /**
     * 第四步：将方法中代码改成aRental方法调用
     */
    public String fourthStepStatement() {
        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();
            thisAmount = fourthStepAmountFor(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" + 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;
    }

    private double fourthStepAmountFor(Rental aRental) {
        return aRental.tenthGetCharge();
    }

    /**
     * 第五步：将方法中代码改成aRental方法调用
     */
    public String fifthStepStatement() {
        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();
            thisAmount = each.tenthGetCharge();

            // 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;
    }

    /**
     * 第六步：将方法里面thisAmount变量去除
     */
    public String sixthStepStatement() {
        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();

            // 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 += each.tenthGetCharge();
        }
        //add footer lines
        result += "Amount owed is " + String.valueOf(totalAmount) + "\n";
        result += "You earned " + String.valueOf(frequentRenterPoints) + "frequent renter points";
        return result;
    }

    /**
     * 第七步将积分政策拆分到Rental
     */
    public String seventhStepStatement() {
        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();

            frequentRenterPoints += each.seventhGetFrequentRenterPoints();
            //show figures for this rental
            result += "\t" + each.getMovie().getTitle() + "\t" + String.valueOf(thisAmount) + "\n";
            totalAmount += each.tenthGetCharge();
        }
        //add footer lines
        result += "Amount owed is " + String.valueOf(totalAmount) + "\n";
        result += "You earned " + String.valueOf(frequentRenterPoints) + "frequent renter points";
        return result;
    }

    /**
     * 第八步将charge和积分拆分，但是循环增加一次
     */
    private double getTotalCharge() {
        double result = 0;
        Enumeration rentals = _rentals.elements();
        while (rentals.hasMoreElements()) {
            Rental each = (Rental)rentals.nextElement();
            result += each.tenthGetCharge();
        }
        return result;
    }

    private int getTotalFrequentRenterPoints() {
        int result = 0;
        Enumeration rentals = _rentals.elements();
        while (rentals.hasMoreElements()) {
            Rental each = (Rental)rentals.nextElement();
            result += each.seventhGetFrequentRenterPoints();
        }
        return result;
    }

}

public abstract class Price {

    public abstract double getCharge(int daysRented);

    int getFrequentRenterPoints(int daysRented) {
        return 1;
    }
}

public class RegularPrice extends Price {

    @Override
    public double getCharge(int daysRented) {
        double result = 2;
        if (daysRented > 2) {
            result += (daysRented -2) * 1.5;
        }
        return result;
    }
}

public class ChildrensPrice extends Price {

    @Override
    public double getCharge(int daysRented) {
        double result = 1.5;
        if (daysRented > 3) {
            result += (daysRented - 3) * 1.5;
        }
        return result;
    }
}

public class NewReleasePrice extends Price {

    @Override
    public double getCharge(int daysRented) {
        return daysRented * 3;
    }

    int getFrequentRenterPoints(int daysRented) {
        return daysRented > 1 ? 2 :1;
    }
}

上面很简单就是一个Custom租用不同Movie时用Rental类计算费用的代码，重构步骤分为两大类，第一类是代码解耦放到合适的类中并已标出步骤顺序，第二类是引入state模式解耦，看完第一章之后就已经能体会到重构的巨大潜力。

总结与思考

1:重构需要有度
不是任何时候都需要重构，比如业务代码非常少且简单，后续也不会有新的代码需要加入，不一定需要去引入设计模式等重构方案去重构，譬如上面有个循环拆分成两个循环的情况作者也表示需要斟酌，所以重构需要思考，不可盲目重构，一开始编程时就有一个好的方案。
2:重构需要逐步稳步进行
正如作者进行的步骤一样，重构应以保持系统功能稳定为前提，逐步推进并实施，并可采用一些已验证的方法或准则去重构，保证重构的同时不影响系统稳定性。

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