• 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

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  • 原文地址:https://www.cnblogs.com/mess4u/p/3867307.html
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