VB2008从入门到精通(PDF格式英文版)-第101章
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If TypeOf（obj） Is HashCodeExample Then
If obj。GetHashCode（） = Me。GetHashCode（） Then
Return True
End If
End If
Return False
End Function
Because the rule for GetHashCode（） is that two object instances with identical hash…code
values must return the same value； it makes sense to implement Equals（） using GetHashCode（）。
However； what started out as a good idea turns out to be a bad idea； as the following illustrates：
Dim s1 As String = 〃Hello〃
Dim s2 As String = 〃World〃
Dim x1 As Integer = 17 * 17 ＋ s1。GetHashCode（）
Dim x2 As Integer = 17 * 17 ＋ s2。GetHashCode（）
Dim h1 As HashCodeExample = New HashCodeExample （x2 * 37； s1）
Dim h2 As HashCodeExample = New HashCodeExample （x1 * 37； s2）
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CH A PT E R 1 0 ■ L E A R N I N G A B O U T P E R S IS T E N CE 283
Dim ht As Hashtable = New Hashtable（）
ht。Add（h1； Nothing）
ht。Add（h2； Nothing）
This shows that having two objects with pletely different states results in the same
hash…code value and generates an exception because Equals（） has been implemented incor
rectly。 In the implementation of Hashtable； when an added object collides with another already
existing object； an equality test is made。 If the equality test returns True； then the exception
is generated because Hashtable does not allow you to add an object with the same state as
another object。
The solution is not to fix the GetHashCode（） method； but rather to modify the Equals（） method：
Public Overrides Function Equals（ByVal obj As Object） As Boolean
If TypeOf （obj） Is HashcodeExample Then
If obj。GetHashCode（） Me。GetHashCode（） Then
Return False
End If
Dim toTest As HashcodeExample =
DirectCast（obj； HashcodeExample）
If toTest。value = Me。value Then
If toTest。buffer = Me。buffer Then
Return True
End If
End If
End If
Return False
End Function
The logic of the modified Equals（） method is to first test if both types are identical。 If not；
then False is returned。 Next； test if GetHashCode（） returns unequal values。 GetHashCode（） will
always return different values for objects that have different data members。 If the hash…code
values are equal； then es the hard work of individually testing each data member for equality。
The hard work is delegated as the last step； because any object that reaches that point will
probably be identical； but you need to be 100％ certain。
The Important Stuff to Remember
In this chapter； you learned how to process a stream of data using the console。 Here are the
main items to remember：
o When data is moved from one medium to another； it is streamed。
o There are two major types of streams： text and binary。
o Text streams are universal and can be read by all puters。
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284 CH AP T E R 1 0 ■ L E A R N I N G A B OU T P E R S IS TE N CE
o Binary streams are specific to the program and sometimes to the processor。 Imagine the
situation of having to decipher a C＋＋ data stream generated by a PowerPC chip。 Most
likely； the numbers that you read will be wrong because of the way that Intel or AMD
chips store their numbers。 Generally speaking； with binary streams； you will be conversing
with two implementations。 If not； use text streams。
o When streaming data； it is best to customize as little as possible。 Doing so will pli
cate your program； and potentially introduce errors where none should exist。
o It is important that you understand the concept of marshaling and the fact that each
medium will have a different representation of the type。 A large part of your programming
day will involve moving data from one stream to another。
Some Things for You to Do
The following are some exercises to apply what you learned in this chapter。
1。 In the implementation of TextProcessor； the display help routine was not very helpful。
Fix the implementation。
2。 There were no testing routines for TextProcessor。 Devise some realistic tests。 By realistic
tests； I mean tests that don’t just focus on the class library and consider the application
tested。 Focus on plete application tests。
3。 Having implemented the display help routine； think about whether or not the imple
mentation is correct。 The class Bootstrap is a general class that uses an IProcessor
instance； which means that different console applications will process different data。
Thus； writing a general help output might work； but it will not help in resolving problems。
Fix the console application TextProcessor and ReaderWriter project so that the help
message is both specific and general。
4。 In the Bootstrap class； when the output was redirected to a file （as indicated by the …out
argument）； there was no check on whether or not the file exists。 Extend the Bootstrap
class to include an additional mand…line argument that verifies if it is fine to overwrite
the output file if it exists。 If an output file does exist and there is no explicit overwriting；
generate an error and stop processing。
5。 The code in the final solution for IProcessor。Process（） has been identified as being hard
to maintain because the code to check for duplicate dates is scattered throughout the
method。 Rewrite the method implementation so that the code is logical and maintainable。
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C H A P T E R 1 1
■ ■ ■
Learning About Generics
Chapter 9 explained how to use lists； delegates； and lambda expressions。 In that chapter； you
also saw an example of generics when using lists to manage a collection of object instances。
The main focus of this chapter is generics and how to use them in a black box context
（the code doesn’t know the specifics of the generics parameter types）。 The secondary
focus is an implementation of lambda expressions using a spreadsheet。 The idea is to get you
well versed in generics and lambda expressions； which you will likely use in your own
production code； so that that there will be no surprises in your projects。
Why Use Generics？
Here’s a surprise for you： there is no imperative need for generics； so you could skip this
chapter and read the next one； right？ Wrong。 I could just as easily have said there is no need for
Visual Basic properties； nor any other Visual Basic construct that enriches your programming
abilities。 The reasons we have Visual Basic properties and generics are programming
elegance and expressiveness。
To understand what I am trying to get at； consider this sentence：
Ducks walk flat feet quack loud
Reading the sentence； you get an idea of what is being said； but you are not pletely
sure。 Visual Basic without generics is like thi
