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。
