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

 1 # introduce null object
 2 
 3 class Master:
 4     def __init__(self):
 5         pass
 6 
 7     def setValue(self, name, major, age):
 8         self.name = name
 9         self.major = major
10         self.age = age
11 
12     def getName(self):
13         return self.name
14 
15     def getMajor(self):
16         return self.major
17 
18     def getAge(self):
19         return str(self.age)
20 
21 
22 class Student:
23     def __init__(self, name):
24         self.master = None
25 
26     def setMaster(self, masterName):
27         self.master = masterName
28 
29     def getMaster(self):
30         return self.master
31 
32 def getMasterNameByStudent(student):
33     master = student.getMaster()
34     if master is not None:
35         print master.getName()
36     else:
37         print "Not exist"
38 
39 def getMasterMajorByStudent(student):
40     master = student.getMaster()
41     if master is not None:
42         print master.getMajor()
43     else:
44         print "Not exist"
45 
46 def getMasterAgeByStudent(student):
47     master = student.getMaster()
48     if master is not None:
49         print master.getAge()
50     else:
51         print "Not exist"

After

 1 # introduce null object
 2 
 3 class Master:
 4     def __init__(self):
 5         pass
 6 
 7     def setValue(self, name, major, age):
 8         self.name = name
 9         self.major = major
10         self.age = age
11 
12     def isNull(self):
13         return False
14 
15     def getName(self):
16         return self.name
17 
18     def getMajor(self):
19         return self.major
20 
21     def getAge(self):
22         return str(self.age)
23 
24 
25 class NoneMaster(Master):
26     def isNull(self):
27         return True
28 
29     def getName(self):
30         return "Not exist"
31 
32     def getMajor(self):
33         return "Not exist"
34 
35     def getAge(self):
36         return "Not exist"
37 
38 class Student:
39     def __init__(self, name):
40         self.master = None
41 
42     def setMaster(self, master):
43         self.master = master
44 
45     def getMaster(self):
46         if self.master is None:
47             return NoneMaster()
48         else:
49             return self.master
50 
51 def getMasterNameByStudent(student):
52     master = student.getMaster()
53     print master.getName()
54 
55 def getMasterMajorByStudent(student):
56     master = student.getMaster()
57     print master.getMajor()
58 
59 def getMasterAgeByStudent(student):
60     master = student.getMaster()
61     print master.getAge()

测试：

 1 master_1 = Master()
 2 master_1.setValue("zhang liang", "computer", 40)
 3 
 4 foo_1 = Student("wang gang")
 5 foo_1.setMaster(master_1)
 6 
 7 foo_2 = Student("li ming") 
 8 
 9 getMasterNameByStudent(foo_1)
10 getMasterNameByStudent(foo_2)
11 print
12 
13 getMasterMajorByStudent(foo_1)
14 getMasterMajorByStudent(foo_2)
15 print
16 
17 getMasterAgeByStudent(foo_1)
18 getMasterAgeByStudent(foo_2)

参考：重构 Martin Fowler 人民邮电出版社

重构技巧 引入Null对象

重构技巧引入Null对象