重构改善既有代码的设计（一）

买了一本《重构改善既有代码的设计》，感觉味道不错，逗比来晒晒。

初品重构实例，就遇到两个没用过的类Vector、Enumeration。在这里做个知识的补充，Vector和ArrayList都是继承自AbstractList，ArrayList会比Vector快，他是非同步的，如果设计涉及到多线程安全，还是用Vector比较好一些 。Vector的相关使用方法参照API调用即可，下面来看Vector内部的一段代码：

/**
* Returns an enumeration of the components of this vector. The
* returned {@code Enumeration} object will generate all items in
* this vector. The first item generated is the item at index {@code 0},
* then the item at index {@code 1}, and so on.
*
* @return  an enumeration of the components of this vector
* @see     Iterator
*/
public Enumeration<E> elements() {
return new Enumeration<E>() {
int count = 0;

public boolean hasMoreElements() {
return count < elementCount;
}

public E nextElement() {
synchronized (Vector.this) {
if (count < elementCount) {
return elementData(count++);
}
}
throw new NoSuchElementException("Vector Enumeration");
}
};
}

这里的Enumeration接口源码如下：

public interface Enumeration<E> {
/**
* Tests if this enumeration contains more elements.
*
* @return  <code>true</code> if and only if this enumeration object
*           contains at least one more element to provide;
*          <code>false</code> otherwise.
*/
boolean hasMoreElements();

/**
* Returns the next element of this enumeration if this enumeration
* object has at least one more element to provide.
*
* @return     the next element of this enumeration.
* @exception  NoSuchElementException  if no more elements exist.
*/
E nextElement();
}

两个方法分别是：1.枚举对象中是否还含有元素，如果返回true，则表示还含有至少一个的元素。2.枚举对象还含有元素，该方法得到对象中的下一个元素。

现在来看看需求：影片出租店，计算每位顾客的消费金额和明细，操作者告诉顾客租了那些影片、租期、和根据租期影片类型计算出费用和积分，影片类型分为普通片、儿童片、新片。下面是重构前相关代码：

/**
*电影
**/
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) {
super();
this._title = _title;
this._priceCode = _priceCode;
}

public String getTitle() {
return _title;
}

public void setTitle(String _title) {
this._title = _title;
}

public int getPriceCode() {
return _priceCode;
}

public void setPriceCode(int _priceCode) {
this._priceCode = _priceCode;
}

}

/**
*租赁
**/
public class Rental {

private Movie _movie;
private int _daysRented;

public Rental(Movie _movie, int _daysRented) {
super();
this._movie = _movie;
this._daysRented = _daysRented;
}

public Movie getMovie() {
return _movie;
}
public void setMovie(Movie _movie) {
this._movie = _movie;
}
public int getDaysRented() {
return _daysRented;
}
public void setDaysRented(int _daysRented) {
this._daysRented = _daysRented;
}

}

/**
* 顾客
**/
public class Customer {

private String _name;

private Vector<Rental> _rentals = new Vector<>();

public Customer(String _name) {
super();
this._name = _name;

}

public void addRental(Rental arg){
_rentals.add(arg);
}

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 = (Rental)rentals.nextElement();

switch (each.getMovie().getPriceCode()) {
case Movie.CHILDRENS:

thisAmount += 1.5;
if(each.getDaysRented()>3){
thisAmount += (each.getDaysRented()-3)*1.5;
}
break;
case Movie.NEW_RELEASE:

thisAmount += each.getDaysRented()*3;
break;
case Movie.REGULAR:

thisAmount += 2;
if(each.getDaysRented()>2){
thisAmount += (each.getDaysRented()-2)*1.5;
}
break;

default:
break;
}

frequentRenterPoints++;

if(each.getMovie().getPriceCode()==Movie.NEW_RELEASE && each.getDaysRented()>1)frequentRenterPoints++;

result += "\t"+each.getMovie().getTitle()+"\t"+String.valueOf(thisAmount)+"\n";
totalAmount +=thisAmount;

}

result += "Amount owed is " + String.valueOf(totalAmount) + "\n";

result += "You earned " + String.valueOf(frequentRenterPoints) + " frequent renter points ";

return result;
}
}

下面是测试代码和结果：

public class Test {

public static void main(String[] args) {
Customer mCustomer=new Customer("Mery Jing");
mCustomer.addRental(new Rental(new Movie("烈日灼心", Movie.NEW_RELEASE), 5));
mCustomer.addRental(new Rental(new Movie("武动乾坤", Movie.CHILDRENS), 2));
mCustomer.addRental(new Rental(new Movie("无极天下", Movie.REGULAR), 1));

System.out.println(mCustomer.statement());
}
}

测试结果：
Rental Record for Mery Jing
烈日灼心    15.0
武动乾坤    1.5
无极天下    2.0
Amount owed is 18.5
You earned 4 frequent renter points

当你发现自己的程序需要添加一个特性，而代码结构无法使你很容易的达成目的，这时候你就要先重构那个程序，重构的第一步永远都相同：搭建一个可靠地测试环境，并拥有检测自身的能力。就上面的代码来看statement（）方法内容过长，处理事情过多，代码块应该越小越好，一个方法一个功能，这样容易管理移动，下面开始第一次重构（Eclipse：alt+shift+M抽取方法快捷键）：

double thisAmount=0;
Rental each = (Rental)rentals.nextElement();
thisAmount = amountFor(each, thisAmount);
frequentRenterPoints++;
//.............略.........................

/**
* 计算价格
* @param each
* @param result >变量的命名要具有可读性，要让人能轻易读懂，重命名快捷键（eclipse）：alt+shift+R
* @return result
*/
private double amountFor(Rental each, double result) {
switch (each.getMovie().getPriceCode()) {
case Movie.CHILDRENS:

result += 1.5;
if(each.getDaysRented()>3){
result += (each.getDaysRented()-3)*1.5;
}
break;
case Movie.NEW_RELEASE:

result += each.getDaysRented()*3;
break;
case Movie.REGULAR:

result += 2;
if(each.getDaysRented()>2){
result += (each.getDaysRented()-2)*1.5;
}
break;

default:
break;
}
return result;
}

租赁影片计算价格并没有与顾客关联，而是用到了Rental ，根据面向对象的知识，这里把把该方法提取到Rental 类里面，并重命名为getCharge():

double thisAmount=0;
Rental each = (Rental)rentals.nextElement();
thisAmount = each.getCharge();
frequentRenterPoints++;
//.............略.........................

/**
* 计算价格
* 变量的命名要具有可读性，要让人能轻易读懂，重命名快捷键（eclipse）：alt+shift+R
* @return result
*/
public double getCharge() {
double result=0;
switch (getMovie().getPriceCode()) {
case Movie.CHILDRENS:

result += 1.5;
if(getDaysRented()>3){
result += (getDaysRented()-3)*1.5;
}
break;
case Movie.NEW_RELEASE:

result += getDaysRented()*3;
break;
case Movie.REGULAR:

result += 2;
if(getDaysRented()>2){
result += (getDaysRented()-2)*1.5;
}
break;

default:
break;
}
return result;
}

做完以上修改，回过头来再看statement（）方法，就会发现thisAmout都点点多余了，这里可以直接通过方法获取了，修改如下：

while(rentals.hasMoreElements()){

Rental each = (Rental)rentals.nextElement();

frequentRenterPoints++;

if(each.getMovie().getPriceCode()==Movie.NEW_RELEASE && each.getDaysRented()>1)frequentRenterPoints++;

result += "\t"+each.getMovie().getTitle()+"\t"+String.valueOf(each.getCharge())+"\n";
totalAmount +=each.getCharge();

}

对于积分的计算，也没有与顾客关联上，这一和计算价格同理，抽取方法到Rental，statement()方法字节调用即可。这里代码就不贴了，以上内容基本来自《重构改善既有代码的设计》，个别内容本人添加。虽然原书内容重构还在继续，本人已然明了。欲知该例子重构更多内容自己买书吧，逗比倍懒写不动啦啦。