In this page ,I will explain the polymorphism. And I will explain the merit , demerit and usage of it.
About the merit, first of all, about the point that "If" statements are not needed , I will explain it from the module cohesion and coupling in detail. There are cases that the polymorphism must be used ( for example, event processing ), and I will explain it.
About the usage of the polymorphism, there are the design pattern of GoF , and I will explain it.
The polymorphism is a mechanism that when the method of object is called , movement of if is changed by the type of it.
For the time being , please read the following class diagram and sample source code. If you execute this sample source code, please pay attention to your Excel VBA vertion.
The version is need 6 or more. If your Excel Version is 97 or your Excel is mac edtion , you cannot execute it.
Figure 1 - The class diagram of the polymorphism sample
Class0 ( Class Module )
Option Explicit
Public Sub printClassName()
End
Sub
Class1 ( Class Module )
Option Explicit
Implements Class0
Private Sub Class0_printClassName()
Debug.Print "Class1"
End
Sub
Class2 ( Class Module )
Option Explicit
Implements Class0
Private Sub Class0_printClassName()
Debug.Print "Class2"
End
Sub
Class3 ( Class Module )
Option Explicit
Implements Class0
Private Sub Class0_printClassName()
Debug.Print "Class3"
End
Sub
Module1 ( Standard Module )
Option Explicit
Public Sub
main()
Dim objs(1 To 3) As
Class0
Dim i As Integer
Set objs(1) = New
Class1
Set objs(2) = New
Class2
Set objs(3) = New Class3
For i = LBound(objs) To
UBound(objs)
Call
objs(i).printClassName
Next
End Sub
Execution result ( Immediate Window )
Class1
Class2
Class3
At the standard module, the "Call objs(i).printClassName" called the "printClassName" method without considering contents of the "objs" array.
And , the execution result ( result of main method at the standard module ) is according to definition of the "printClassName" method that is defined at each class module.
I will explain the point that "If" statment is not needed.
(It is difficult to explain. Because , in the VBA , if you use the Object type , you can realize the same.)
If the polymorphism is not used in the above-mentioned source code ( standard module ) , you must program the following in the "For" statement.
If
TypeName(objs(i)) = "Class1" Then
'...
Else If TypeName(objs(i)) =
"Class2" Then
'...
Else If TypeName(objs(i)) = "Class3"
Then
'...
End If
If this program is written , the readable of source code becomes worse. And the cost and time of the test of the program becomes more increased. ( If you don't know the test , please read the White-box testing.)
Then , I want you to remember the module cohesion, and I think that this is the Logical cohesion.
( I think that the cohesion of the Module1 become worse. )
And , the point that the Module1 is coupled the Class1 , Class2 and Class3 that is a sub-class of the Class0 is bad.
When a sub-class of the Class0 become increase, if there is only one place that "if" statement is used , you can maintain the program. But , if there are 1,000 place , I think that you cannot maintain it. ( I think that you will leak the maintenance. )
The summary of the merit :
1. The source code using the Class0 become decreased.
2. The cost and time of the test become decreased.
3. The module cohision become better , and the module coupling become lower.
Next , I will explain the demerit.
First of all , the Class0 becomes needed. In short , the line of the source code become increased.
Next . I want you to think the following case.
Afterwards, you hit upon that the Class4 is needed.
You design the Class4 , then you hit upon that the "printClassName" method of the Class4 needs arguments 1. But it of the Class0 doesn't have arguments. What do you do ? If you increase arguments of the "printClassName" method of the Class0 , you must maintain the Class1, Class2 and Class3. And If the method is used by 1,000 place, the cost and time of the maintenance become very more.
Like this , afterwards , I offen hit opon the insufficiency of the arguments of the override method and the inconsistency of the return value type. This is really ...
Of course, I securely design in order not to become so. But for that reason , the design time become more increased. If there are 1,000 place using the "printClassMethod" , I think that you mast surely use the polymorphism. But if there is one plase , I think that you should not use 2.
The following Table 1 is the summary of the merit and demerit of the polymorphism.
Table 1 - The summary of the merit and demerit of the polymorphism
The merit of the polymorphism
The demerit of the polymorphism
1. You can decrease the source code of the class using the interface.
2. You can decrease the "If" statements. and you can decrease the cost and time of the test of the class using the interface.
3. You can design the higher cohesion class using the interface. And you can design lower coupling.
4. The class using the interface is not needed to modify when the number of the sub-class is increased.
1. The cost and time is increased by designing and programming the interface.
2. If the design of the interface is bad , you cannot correspond when the number of the sub-class is increased.
3. the cost and time is increased because of the evasion of 2.
If the polymorphism is not used, there are the things that can not be realized. In this case, you must use the polymorphism.
The case is a case that you design the framework and a case that you use the framework.
For example , the event processing ( Generally, this is programed by the Observer pattern . But in the VBA (note. VBA version is 6 or more) , this is programed by the "Event" statement. ) and so on.
About the usage of the polymorphism , in the design pattern of the GoF , I will introduce the pattern that the polymorphism is related. There are only 23 pattern in the design pattern of the GoF , but as there are a lot of pattern that the polymorphism is related , I cannot introduce everything. I'm sorry.
Observer pattern
This pattern is used for the event processing design of GUI ( for example , java.awt package ) and so on.
The interface is defined in the framework, and the class realizing the interface is defined in the side using the framework.
Strategy pattern
When there are some strategy , and you want to switch those, this pattern is used.
For example, please think about the encryption processing.
There are some algorithms of the encryption. For example, there are the DES , AES and so on . But These becomes impossible to use some time becuase of broken.
If the algorithm that you use is broken, and if the algorithm is easy to change , the modification become decreased.
For example, java.security package. I think that the package use some other pattern too.
State pattern
When you want to switch the behavior of the method for the state of the object , you should use this pattern.
In this pattern, one state is defined as one class.
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