Introduce Null Object

　　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。