VB2008从入门到精通(PDF格式英文版)-第89章
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You could also use SortedDictionary； but that implies the elements within the list are sorted。
To add values to the dictionary； use the Add（） method：
dictionary。Add（〃List〃； lst）
dictionary。Add（〃List To Be Added〃； lstToBeAdded）
When working with IDictionary objects； you might want to know whether or not a key is
available。 The following code is used to verify if a key exists。
If dictionary。ContainsKey（〃List〃） Then
。 。 。
End If
If you want to iterate the keys； use this form：
For Each key As String In dictionary。Keys
Next
Iterate the values as follows：
For Each value As Object In dictionary。Values
Next
Using a Stack
A Stack is a special list that behaves like a stack of paper on a table。 When you add three items
on the Stack； the last one added to the Stack is the first one off the Stack。 Here is an example of
using a Stack：
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C HA P TE R 9 ■ L E AR N I N G A B O U T L I ST S； DE L E G AT E S ； AN D L A M B D A E X PR E SSI O N S 247
Dim stack As Stack（Of String） = New Stack（Of String）（）
stack。Push（〃first〃）
stack。Push（〃second〃）
stack。Push（〃third〃）
Dim popped As String = stack。Pop（）
If popped。pareTo（〃third〃） = 0 Then
' This is what we expect
End If
The code demonstrates using the Push（） method to push items on the stack and the Pop（）
method to remove items from the stack。 Remember that Push（） is an explicit addition； and
Pop（） an explicit removal （though a call to Pop（） returns the object removed from the stack so
you can do something with it； as shown in the code）。
If you want to know what is on the top of the stack； use Peek（）； which acts like Pop（）； except
it does not remove the item from the list。
Using a Queue
A Queue is another special type of list that behaves like a queue that you would encounter at
ticket counter。 As people start queuing； the first person to be served is the one at the front of the
line。 Here is an example of using a Queue：
Dim queue As Queue（Of String） = New Queue（Of String）（）
queue。Enqueue（〃first〃）
queue。Enqueue（〃second〃）
queue。Enqueue（〃third〃）
Dim dequeued As String = queue。Dequeue（）
If dequeued = 〃first〃 Then
' This is what we expect
End If
The Important Stuff to Remember
In this chapter； you learned about using delegates； lambda expressions； extension methods；
and lists。 The main items to remember are as follows：
o You are using Visual Basic 2008； and thus you should use the generics…based
collection classes。
o There are many different types of lists。 The main types are the simple object collection；
key/value collection； stack； and queue。
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248 CH AP T E R 9 ■ L E A R N IN G AB OU T L I ST S； D E L E G A T E S； A N D L A M B DA E X P R E S SI ON S
o generics…based classes are type…safe and have better performance than old…style
collections。
o Delegates help you define a generic method…calling mechanism without needing to
implement an interface。
o Delegates can be shared methods； instance methods； or module methods。 The only
important aspect to the method is to make sure the method signature matches the
delegate declaration。
o Lambda expressions are a specialized form of delegate method that enable you to write
deferred execution code。 The advantage of deferred execution is that the code can contain
a state whenever it is executed。
Some Things for You to Do
The following are some things for you to do to start applying your budding knowledge of soft
ware engineering to improving the code base。
1。 Collection classes before Visual Basic 2005 allowed you to mix types。 With Visual Basic
2005 and later； the generics classes do not allow you to mix types。 Provide a
solution where you could mix types with Visual Basic 2005 and later collections。
2。 Create a list that contains the numbers 1 to 20。 Remove the numbers； 15； 10； and 3 to 7。
3。 Create a list with an object that is defined as follows：
Class MyType
Public Value as String
End Class
4。 Add ten elements to the list； and then sort the list alphabetically from A to Z。 Hint： look
at the method Sort（） and implement a custom Iparer（Of ）。 As part of this exercise；
you need to investigate and figure out how to use Iparer（Of ）。 My suggestion is to
search the MSDN and Code Project web sites。
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C H A P T E R 1 0
■ ■ ■
Learning About Persistence
Your programs will probably need to read and/or write data to some type of storage device。
That storage device might be a hard disk； USB drive； or even the network。 The key concept is
that you are taking information from memory and transferring it to some other location。 Later；
you will retrieve that information and use it to execute some task。
Taking data from memory and transferring it to another place is referred to as persistence。
Most examples of persistence involve creating an object； and then saving that object via a file
to the hard disk。 However； reading and writing an object is not just saving data to the hard disk；
even though that is often the result。 Reading and writing data to the hard disk is about reading
and writing to data streams。 This chapter focuses on the process of reading and writing data
to streams。
This chapter’s example is a set of applications for a lottery…prediction system。 You’ll see
how streams are generic concepts that can apply to files； the console； or even the network。
Throughout this book； the examples use console applications to test some code； and this
chapter’s example also includes console applications。 Although Visual Basic is typically used as
a GUI development tool； the point of using the console is to help you understand the Visual
Basic programming language。 One of the biggest criticisms of Visual Basic developers has been
that they can’t develop like real developers。 This book is intended for real developers who happen
to use Visual Basic。
Organizing the Lottery…Prediction System
Let’s say we want to predict the next set of lottery numbers。 We have a program that saves the
numbers drawn； and each week； we run a program that retrieves the drawn numbers and predicts
the next set of numbers。 Many will argue that lottery numbers are random and thus cannot be
predicted。 But that doesn’t mean that we can’t write a program to generate the probabilities；
and that usually entails knowing which numbers have been drawn previously。
The lottery…prediction example involves three applications： TextProcessor； which is used
to read a messed…up lottery number file that will be cleaned up； Binary2Text； which converts a
binary stream into text； and Text2Binary； which converts a text stream into binary。 Five projects
are defined for these applications：
o Binary2Text： A console program that is used to convert a binary lottery ticket stream into
a text stream。
o Lot
