重构技巧 引入Null对象

描述：有两个类，学生类和导师类，学生的导师类可能不存在，因此在获取学生导师名字等信息时都要先判断导师名字是否为空。重构后通过一个空导师类来处理导师为空的相应逻辑。

Before

 # introduce null object

 class Master:
     def __init__(self):
         pass

     def setValue(self, name, major, age):
         self.name = name
         self.major = major
         self.age = age

     def getName(self):
         return self.name

     def getMajor(self):
         return self.major

     def getAge(self):
         return str(self.age)

 class Student:
     def __init__(self, name):
         self.master = None

     def setMaster(self, masterName):
         self.master = masterName

     def getMaster(self):
         return self.master

 def getMasterNameByStudent(student):
     master = student.getMaster()
     if master is not None:
         print master.getName()
     else:
         print "Not exist"

 def getMasterMajorByStudent(student):
     master = student.getMaster()
     if master is not None:
         print master.getMajor()
     else:
         print "Not exist"

 def getMasterAgeByStudent(student):
     master = student.getMaster()
     if master is not None:
         print master.getAge()
     else:
         print "Not exist"

After

 # introduce null object

 class Master:
     def __init__(self):
         pass

     def setValue(self, name, major, age):
         self.name = name
         self.major = major
         self.age = age

     def isNull(self):
         return False

     def getName(self):
         return self.name

     def getMajor(self):
         return self.major

     def getAge(self):
         return str(self.age)

 class NoneMaster(Master):
     def isNull(self):
         return True

     def getName(self):
         return "Not exist"

     def getMajor(self):
         return "Not exist"

     def getAge(self):
         return "Not exist"

 class Student:
     def __init__(self, name):
         self.master = None

     def setMaster(self, master):
         self.master = master

     def getMaster(self):
         if self.master is None:
             return NoneMaster()
         else:
             return self.master

 def getMasterNameByStudent(student):
     master = student.getMaster()
     print master.getName()

 def getMasterMajorByStudent(student):
     master = student.getMaster()
     print master.getMajor()

 def getMasterAgeByStudent(student):
     master = student.getMaster()
     print master.getAge()

测试：

 master_1 = Master()
 master_1.setValue("zhang liang", "computer", 40)

 foo_1 = Student("wang gang")
 foo_1.setMaster(master_1)

 foo_2 = Student("li ming") 

 getMasterNameByStudent(foo_1)
 getMasterNameByStudent(foo_2)
 print

 getMasterMajorByStudent(foo_1)
 getMasterMajorByStudent(foo_2)
 print

 getMasterAgeByStudent(foo_1)
 getMasterAgeByStudent(foo_2)

参考：重构 Martin Fowler 人民邮电出版社