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String type； because it is treated like an object that has its own hash…code calculation
implementation。
Notice in the implementations of the Append（） methods how a calculation is performed
and then added to the data member _runningTotal。 The return value is a Me reference； so that
the methods can be chained together。 This allows a client to use the HashCodeAutomater class；
as the following GetHashCode（） implementation demonstrates：
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282 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
Class HashcodeExample
Public value As Integer
Public buffer As String
Public Sub New （ByVal val As Integer； ByVal buf As String）
value = val
buffer = buf
End Sub
Public Overrides Function GetHashCode（） As Integer
Return New HashCodeAutomater（）
。Append（value）
。Append（buffer）。toHashCode（）
End Function
End Class
The implementation of HashcodeExample has two data members： value and buffer。 The
two data members make up the class’s state。 Not all data members are used when calculating
a class instance’s hash…code value。 For example； if HashcodeExample had a data member that
referenced a database connection； it should not be used when calculating the hash code； because
the database connection is the type used to get the state and does not influence the state—it is
a means to an end。
Implementing Equals（）
Once the GetHashCode（） method has been implemented； the Equals（） method can be
implemented：
Public Overrides Function Equals（ByVal obj As Object） As Boolean
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 poss