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or methods in a shared method。
Getting back to the Node declaration； the shared data member RootNodes is used to define
a root for the search tree。 As when instantiating a type； there is a constructor for the shared type
that is called before the first time it is needed。 The shared constructor is like the previously
defined constructor； except the Public keyword is replaced with Shared。 For the search tree
case； it is used to initialize the tree and state。
We now have a plete definition of the Node class； with the following source code。 Take
a moment to look it over and fit the pieces together。
Public Class Node
Shared Sub New（）
Dim montreal As Node = New Node（〃Montreal〃； 0； 0）
Dim newyork As Node = New Node（〃New York〃； 0； …3）
Dim miami As Node = New Node（〃Miami〃； …1； …11）
Dim toronto As Node = New Node（〃Toronto〃； …4； …1）
Dim houston As Node = New Node（〃Houston〃； …10； …9）
Dim losangeles As Node = New Node（〃Los Angeles〃； …17； …6）
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98 CH AP T E R 4 ■ L E A R N IN G AB OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S
Dim seattle As Node = New Node（〃Seattle〃； …16； …1）
montreal。Connections = New Node（） ｛newyork； toronto； losangeles｝
newyork。Connections = New Node（） ｛montreal； houston； miami｝
miami。Connections = New Node（） ｛toronto； houston； newyork｝
toronto。Connections = New Node（） ｛miami； seattle； montreal｝
houston。Connections = New Node（） ｛miami； seattle； newyork｝
seattle。Connections = New Node（） ｛toronto； houston； losangeles｝
losangeles。Connections = New Node（） ｛montreal； seattle； houston｝
Node。RootNodes = New Node（） ｛montreal； newyork； miami； toronto； houston； _
losangeles； seattle｝
End Sub
Public Sub New（ByVal city As String； ByVal X As Double； ByVal Y As Double）
Me。CityName = city
Me。X = X
Me。Y = Y
Me。Connections = Nothing
End Sub
Public CityName As String
Public Connections As Node（）
Public Shared RootNodes As Node（）
Public X As Double
Public Y As Double
End Class
Defining the Algorithm Test
The Node type is a self…contained type； meaning that the algorithm does not need to instantiate
the tree structure （the Node constructor does it for you）。 This is an example of good design；
because if you had to add more cities； the only changes required would be to Node itself。 Any
search algorithm that uses the Node type does not need to be changed。
■Note When you have the ability to create code that localizes changes without affecting other pieces of
code； it is called decoupling code。 You want to write code that is decoupled from other code； so that when
changes are made in one piece of code； other pieces of code continue functioning。 As you will experience
when developing code； decoupling of code is a daily struggle。
For illustrative purposes； let’s try a first stab at the search algorithm and see where things
take us。 We could start by defining the search class or start by defining the test that will test the
search class。 Let’s define the test first； because it allows us to figure out what shape the search
class should take：
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CH AP T E R 4 ■ L E A R N I N G A B OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S 99
Public Sub TestSearch（）
SearchAlgorithm。DepthFirstFindRoute（〃Montreal〃； 〃Seattle〃）
End Sub
In the test； the search algorithm is called directly using
SearchAlgorithm。DepthFirstFindRoute（）。 Here； SearchAlgorithm is the name of the class；
and DepthFirstFindRoute（） is the name of the method。 The class name implies that this class
will contain all search algorithm implementations。 This is wrong； because most likely； each
search algorithm will require multiple methods； leading to a very large and plicated
SearchAlgorithm class。 If this is the case； then maintaining the SearchAlgorithm class will
bee a nightmare。
A better solution would be to identify a single class as being a single implementation of a
search algorithm。 Then for each class； we can define a mon method identifier that is used
to find the route between two points。 Doing this results in the following modified test：
Public Sub TestSearch（）
DepthFirstSearch。FindRoute（〃Montreal〃； 〃Seattle〃）
End Sub
Now the test implies that the class DepthFirstSearch has a shared method FindRoute（）。
This is acceptable； and if you were to implement BreadthFirstSearch； the naming would be
BreadthFirstSearch。FindRoute（）。 However； there is another problem； which relates to multiple
users being able to use the algorithm during the execution of a program。 Going back to the
push…to…talk feature of a cell phone； the method FindRoute（） is shared and thus a shared
resource。 If multiple users do use this algorithm； they will share the resource。 This could be
problematic if you are storing temporary data in the data members of the DepthFirstSearch
class。 Using a shared method could corrupt your found search path。
The more appropriate solution is to define the method FindRoute（） as a non…shared
method； implying that DepthFirstSearch must be instantiated before we can call FindRoute（）。
We should modify the test again as follows：
Public Sub TestSearch（）
Di