今天继续总结《重构》这本书中的一个重构手法，Introduce Null Object。写这个手法是因为它确实很巧妙，在实际编程中经常会遇到这种情况，前人总结出来了这么一个经典的手法，当然还有由此手法扩展更普遍更经典的手法--Special Case。
刚入行的时候，听“老人”给我讲，书是要越读越薄的。当时没什么感受，觉得“老人”在故弄玄虚。工作4年多来，发现看过不少书，烂熟于心的却是最初入门的那一本--郭天祥C51。买的是一本影印盗版书（在大学里嘛，那年大二），不停的翻，书都翻烂了。现在那本书也不知道去哪了，但C51的所有内容都清清楚楚了。学习是为了什么，无非是为了精进，为以后做准备嘛。
扯远了，接下来半年的时间里，争取有时间就写写读书笔记吧。近几年看了好几本技术书籍，有些还看了好几遍。写下来的内容都是为了把学过的知识捋一遍。
开始今天的内容吧，这个重构手法在一定的应用场景下很有用。先来看看其应用场景。
重构前的主体代码：
package com.nelson.io;
public class Site {
private Customer _customer;
public Customer getCustomer()
{
return _customer;
}
public void setCustomer(Customer cus)
{
_customer = cus;
}
}
class Customer
{
String _name;
BillingPlan _billingplan;
PaymentHistory _paymentHistory;
public String getName() {return _name;}
public BillingPlan getPlan() {
return _billingplan;
}
public PaymentHistory getHistroy(){
return _paymentHistory;
}
public Customer(){
}
public Customer(String name,BillingPlan bill,PaymentHistory pay){
_name = name;
_billingplan = bill;
_paymentHistory = pay;
}
}
class BillingPlan
{
private int basicpay;
private int extrapay;
public BillingPlan(){
setBasicpay(0);
setExtrapay(0);
}
public BillingPlan(int pay)
{
setBasicpay(pay);
setExtrapay(0);
}
public BillingPlan(int basic,int extra)
{
basicpay = basic;
extrapay = extra;
}
static BillingPlan basic()
{
return new BillingPlan(100); //最低消费100元
}
public int getTotalExpand()
{
return basicpay+extrapay;
}
int getExtrapay() {
return extrapay;
}
void setExtrapay(int extrapay) {
this.extrapay = extrapay;
}
int getBasicpay() {
return basicpay;
}
void setBasicpay(int basicpay) {
this.basicpay = basicpay;
}
}
class PaymentHistory
{
public int getWeeksDelinquentInLastYear;
public PaymentHistory()
{
getWeeksDelinquentInLastYear = 0;
}
}重构前的应用代码：
package com.nelson.io;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello Java!");
//不正常的客户
Site site = new Site(); //这样默认Site中的_customer = null
Customer cus1 = site.getCustomer();
String strName;
if(cus1 == null) strName = "occupant"; //顾客名字暂时叫做occupant
else strName = cus1.getName(); //获取用户的名字
System.out.println("Current Customer1: "+strName);
BillingPlan plan1;
if(cus1 == null) plan1 = BillingPlan.basic();
else plan1 = cus1.getPlan();
System.out.println("Total Expand:"+plan1.getTotalExpand());
//////////////////////////////////////////////////////////
//正常的客户
BillingPlan plan2 = new BillingPlan(100,19);
PaymentHistory history2 = new PaymentHistory();
Customer cus= new Customer("xiaoming",plan2,history2);
site.setCustomer(cus);
Customer cus2 = site.getCustomer();
if(cus2 == null) strName = "occupant"; //顾客名字暂时叫做occupant
else strName = cus2.getName(); //获取用户的名字
System.out.println("Current Customer2: "+strName);
if(cus2 == null) plan1 = BillingPlan.basic();
else plan1 = cus2.getPlan();
System.out.println("Total Expand:"+plan1.getTotalExpand());
}
}在这里的重构要解决的问题是，应用代码中会不断的查询Site中的customer对象是否为空，这个应用的关键也在于允许customer=null的情况（这种情况存在不算错，而且必须应对这种情况）。Introduce Null Object手法就是去掉这里的重复查询是否为空的代码。
重构后的主体代码：
package com.nelson.io;
public class Site {
private Customer _customer;
public Customer getCustomer()
{
return (_customer==null)?Customer.newNull():_customer;
}
public void setCustomer(Customer cus)
{
_customer = cus;
}
}
class Customer implements Nullable
{
String _name;
BillingPlan _billingplan;
PaymentHistory _paymentHistory;
public String getName() {return _name;}
public BillingPlan getPlan() {
return _billingplan;
}
public PaymentHistory getHistroy(){
return _paymentHistory;
}
protected Customer(){
}
public Customer(String name,BillingPlan bill,PaymentHistory pay){
_name = name;
_billingplan = bill;
_paymentHistory = pay;
}
public static Customer newNull()
{
return new NullCustomer();
}
public boolean isNull() {
return false;
}
}
class NullCustomer extends Customer
{
public String getName() {return "occupant";}
public BillingPlan getPlan() {
return BillingPlan.basic();
}
public boolean isNull() {
return true;
}
}
class BillingPlan
{
private int basicpay;
private int extrapay;
public BillingPlan(){
setBasicpay(0);
setExtrapay(0);
}
public BillingPlan(int pay)
{
setBasicpay(pay);
setExtrapay(0);
}
public BillingPlan(int basic,int extra)
{
basicpay = basic;
extrapay = extra;
}
static BillingPlan basic()
{
return new BillingPlan(100); //最低消费100元
}
public int getTotalExpand()
{
return basicpay+extrapay;
}
int getExtrapay() {
return extrapay;
}
void setExtrapay(int extrapay) {
this.extrapay = extrapay;
}
int getBasicpay() {
return basicpay;
}
void setBasicpay(int basicpay) {
this.basicpay = basicpay;
}
}
class PaymentHistory
{
public int getWeeksDelinquentInLastYear;
public PaymentHistory()
{
getWeeksDelinquentInLastYear = 0;
}
}
interface Nullable
{
boolean isNull();
}重构后的测试代码：
package com.nelson.io;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello Java!");
//////////////////////////////////////////////////////////////////
//不正常的客户
Site site = new Site(); //这样默认Site中的_customer = null
Customer cus1 = site.getCustomer();
String strName = cus1.getName(); //获取用户的名字
System.out.println("Current Customer1: "+strName);
BillingPlan plan1 = cus1.getPlan();
System.out.println("Total Expand:"+plan1.getTotalExpand());
if(!cus1.isNull())
System.out.println("Customer1 is not null customer");
else
System.out.println("Customer1 is null customer");
System.out.println();
//////////////////////////////////////////////////////////
//正常的客户
BillingPlan plan2 = new BillingPlan(100,19);
PaymentHistory history2 = new PaymentHistory();
Customer cus= new Customer("xiaoming",plan2,history2);
site.setCustomer(cus);
Customer cus2 = site.getCustomer();
strName = cus2.getName(); //获取用户的名字
System.out.println("Current Customer2: "+strName);
cus2.getPlan();
System.out.println("Total Expand:"+plan1.getTotalExpand());
if(!cus2.isNull())
System.out.println("Customer2 is not null customer");
else
System.out.println("Customer2 is null customer");
}
}重构后的测试代码中，减少了对customer是否为null的判断。当测试代码中出现很多这样的判断时，此项重构价值得以体现。而且这样的好处还有，如果程序中碰到一个对象为null，不加判断去调用对象的函数，将造成程序崩溃。而将null的情况封装为Null Object后，将永不会出现这种异常。程序运行更安全。测试代码中的调用方只管使用方法就好了，保证不出错。注意Customer和NullCustomer中都实现了Nullable接口，意在告诉调用方有NullCustomer类存在。如果调用方实在想知道谁是NullCustomer谁是Customer，通过接口函数就可以知道了。
引申的Special Case模式讲的就是特例模式，当SIte中的Customer = null时也算一种特例，当Customer中的name = “”时也是一种特例，也可以定义UnknownCustomer。或者种种其他的特殊情况--程序运行过程中大部分情况都是正常的Customer，但偶尔出现NullCustomer或者UnknownCustomer，Null Object和Special Case 保证程序能处理“极端”情况。