1. Chapter 23 – Data Structures and Collections
Outline 23.1 Introduction 23.2   Self-Referential Classes 23.3   Linked Lists 23.4   Stacks 23.5   Queues 23.6   Trees   Binary Search Tree of Integer Values   Binary Search Tree of IComparable Objects 23.7   Collection Classes   Class Array   Class ArrayList   Class Stack   Class Hashtable

2. Dynamic Data Structures
23.1 Introduction
Dynamic Data Structures
Can grow and shrink at execution time:
Linked Lists
Stacks
Queues
Binary trees
Classes, Inheritance, and Composition help to create and package the data structures to enhance reusability and maintainability

3. 23.2 Self-Referential Classes
Contains a reference member referring to a class object of the same class type
Self-Referential Object
Usually referred to as a link, because it can be used to “tie” together an object of the same type
Can be linked together to form useful data structures
Nothing reference
Usually defines the end of a data structure

4. 23.2 Self-Referential Classes
Dynamic Memory Allocation
The ability of a program to obtain additional memory, to hold new variables
Also releases unneeded memory at execution time
Visual Basic performs automatic garbage collection
Keyword New
Essential to dynamic memory allocation
Dynamically allocates memory for a new object

5. Private instance variables mData and mNextNode
1 ' Fig : Node.vb
2 ' Self-referential Node class. 3
4 Class CNode
Private mData As Integer
Private mNextNode As CNode 7
Public Sub New(ByVal dataValue As Integer)
' constructor body
End Sub ' New 11
' Property Data
Public Property Data() As Integer 14
Get
' get body
End Get 18
Set(ByVal dataValue As Integer)
' set body
End Set 22
End Property ' Data 24
' Property NextNode
Public Property NextNode As CNode 27
Get
' get next node
End Get 31
Set(ByVal nodeValue As CNode)
' set next node
End Set 35
23.2 Self-Referential Classes
Private instance variables mData and mNextNode
Node.vb Define CNode constructor Cnode property for mData Cnode property for mNextNode
Property for mData, named Data
Property for mNextNode, named NextNode

6. 36 End Property ' NextNode 37 38 End Class 'CNode
Node.vb

7. 23.3 Linked Lists
Nodes
A linked list is a linear collection of self-referential class objects, called nodes
Nodes are connected by reference links
A linked list is accessed via a reference to the first node
Each other node is referenced by the current node’s link reference
The link reference of the last node is set to Nothing
A node can contain data of any type
Data is stored in a linked list dynamically

8. Advantages of linked lists
Appropriate to use a linked list when the number of elements in the data structure are unknown
Linked lists become full only when the system has insufficient memory to satisfy dynamic storage allocation requests
A linked list can be maintained in sorted order very easily

9. 23.3 Linked Lists 15 10
Fig Self-referential class objects linked together. H D …… Q
firstNode
lastNode
Fig Linked-list graphical representation.

10. 1 ' Fig : ListNodes.vb
2 ' Class to represent one node in a CList. 3
4 Public Class CListNode
Private mData As Object
Private mNextNode As CListNode 7
' create CListNode with dataValue in list
Public Sub New(ByVal dataValue As Object)
MyClass.New(dataValue, Nothing)
End Sub ' New 12
' create CListNode with dataValue and nextNodeValue in list
Public Sub New(ByVal dataValue As Object, _
ByVal nextNodeValue As Object) 16
mData = dataValue
mNextNode = nextNodeValue
End Sub ' New 20
' property Data
Public ReadOnly Property Data() As Object 23
Get
Return mData
End Get 27
End Property ' Data 29
' property mNext
Public Property NextNode() As CListNode 32
Get
Return mNextNode
End Get
ListNodes.vb Define CListNode constructor that takes one argument constructor that takes two arguments Data property NextNode property
A Clist accesses the CListNode member variables via it’s properties Data,
And NextNode

11. ListNodes.vb 36 37 Set(ByVal value As CListNode) 38 mNextNode = value
End Set 40
End Property ' NextNode 42
43 End Class ' CListNode
ListNodes.vb

12. The constructors for Clist initialize both references to Nothing
1 ' Fig : List.vb
2 ' Class CList definition. 3
4 Public Class CList
Private firstNode As CListNode
Private lastNode As CListNode
Private name As String 8
' construct empty List with specified name
Public Sub New(ByVal listName As String)
name = listName
firstNode = Nothing
lastNode = Nothing
End Sub ' New 15
' construct empty List with "list" as its name
Public Sub New()
MyClass.New("list")
End Sub ' New 20
' insert object at front of List
Public Sub InsertAtFront(ByVal insertItem As Object) 23
SyncLock (Me) ' ensure thread safe 25
' if this list is empty, create node
If IsEmpty() Then
lastNode = New CListNode(insertItem)
firstNode = lastNode
Else ' create node and insert before first node
firstNode = New CListNode(insertItem, firstNode)
End If 33
End SyncLock 35
Private member variable firstNode, a reference to the first CListNode in a CList
The constructors for Clist initialize both references to Nothing
List.vb Define CList constructor constructor which sets the name member variable method InsertAtFront
Private member variable lastNode, a reference to the last CListNode in a CList
Methods InsertAtFront, InsertAtBack, RemoveFromFront, and RemoveFromBack, are the primary methods of the CList
Method InsertAtFront places a new node at the front of the list
Call IsEmpty to determine whether the list is empty
Each method uses a SyncLock block to ensure that Clist objects are multithread safe when used in a multithread program

13. List.vb method InsertAtBack method RemoveFromFront
If the list is empty, set both firstNode and lastNode to refer to a new CListNode initialized with object insertItem
End Sub ' InsertAtFront 37
' insert object at end of List
Public Sub InsertAtBack(ByVal insertItem As Object) 40
SyncLock (Me) ' ensure thread safety 42
' if list is empty create node and set firstNode
If IsEmpty() Then
lastNode = New CListNode(insertItem)
firstNode = lastNode
Else ' create node and insert after last node
lastNode.NextNode = New CListNode(insertItem)
lastNode = lastNode.NextNode
End If 51
End SyncLock 53
End Sub ' InsertAtBack 55
' remove first node from list
Public Function RemoveFromFront() As Object 58
SyncLock (Me) ' ensure thread safety
Dim removeItem As Object = Nothing 61
' throw exception if removing node from empty list
If IsEmpty() Then
Throw New EmptyListException(name)
End If 66
removeItem = firstNode.Data ' retrieve data 68
List.vb method InsertAtBack method RemoveFromFront
Method InsertAtBack places a new node at the back of the list
If the list is not empty, thread the new node into the list by setting lastNode and lastNode.NextNode to refer to a new CListNode object initialized with object insertItem
Method RemoveFromFront removes the front node of the list and returns a reference to the removed data. The method throws a EmptyListException if the program tries to remove a node from an empty list

14. List.vb method RemoveFromBack
' reset firstNode and lastNode references
If firstNode Is lastNode Then
firstNode = Nothing
lastNode = Nothing
Else
firstNode = firstNode.NextNode
End If 76
Return removeItem ' return removed item 78
End SyncLock 80
End Function ' RemoveFromFront 82
' remove last node from CList
Public Function RemoveFromBack() As Object 85
SyncLock (Me) ' ensure thread safe
Dim removeItem As Object = Nothing 88
' throw exception if removing node from empty list
If IsEmpty() Then
Throw New EmptyListException(name)
End If 93
removeItem = lastNode.Data ' retrieve data 95
' reset firstNode and last node references
If firstNode Is lastNode Then
lastNode = Nothing
firstNode = lastNode
Else
Dim current As CListNode = firstNode
102
Method RemoveFromBack removes the last node of the list and returns a reference to the removed data. The method throws a EmptyListException if the program tries to remove a node from an empty list
List.vb method RemoveFromBack

15. List.vb method IsEmpty Print
' loop while current node is not lastNode
While (Not (current.NextNode Is lastNode))
current = current.NextNode ' move to next node
End While
107
' current is new lastNode
lastNode = current
current.NextNode = Nothing
End If
112
Return removeItem ' return removed data
114
End SyncLock
116
End Function ' RemoveFromBack
118
' return true if list is empty
Public Function IsEmpty() As Boolean
121
SyncLock (Me)
123
If firstNode Is Nothing Then
Return True
Else
Return False
End If
129
End SyncLock
131
End Function ' IsEmpty
133
' output List contents
Public Overridable Sub Print()
136
List.vb method IsEmpty Print
Method IsEmpty is a predicate method that determines whether the list is empty, i.e. whether the reference to the first node of the list is empty

16. SyncLock (Me)
138
If IsEmpty() Then
Console.WriteLine("Empty " & name)
141
Return
End If
144
Console.Write("The " & name & " is: ")
146
Dim current As CListNode = firstNode
148
' output current node data while not at end of list
While Not current Is Nothing
Console.Write(current.Data & " ")
current = current.NextNode
End While
154
Console.WriteLine(vbCrLf)
156
End SyncLock
158
End Sub ' Print
160
161 End Class ' CList
Creates CListNode reference current and initializes it with firstnode. While current is not Nothing, there are still items in the list, so the method prints current.Data and then assigns current.NextNode to current
List.vb writes to console window writes data from current node to console
Method Print first determines whether the list is empty, if so Print displays a String consisting of “Empty” and the list’s name and then returns control to the calling method. Otherwise, Print outputs the data in the list.

17. EmptyListException.vb Define EmptyListException constructor
1 ' Fig : EmptyListException.vb
2 ' Class EmptyListException definition. 3
4 Public Class EmptyListException
Inherits ApplicationException 6
Public Sub New(ByVal name As String)
MyBase.New("The " & name & " is empty")
End Sub ' New 10
11 End Class ' EmptyListException
EmptyListException.vb Define EmptyListException constructor

18. ListTest.vb Define modListTest main
1 ' Fig : ListTest.vb
2 ' Testing class CList. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary 6
7 Module modListTest 8
Sub Main()
Dim list As CList = New CList() ' create CList container 11
' create data to store in CList
Dim aBoolean As Boolean = True
Dim aCharacter As Char = "$"c
Dim anInteger As Integer = 34567
Dim aString As String = "hello" 17
' use CList insert methods
list.InsertAtFront(aBoolean) ' insert Boolean at front
list.Print() 21
list.InsertAtFront(aCharacter) ' insert Char at front
list.Print() 24
list.InsertAtBack(anInteger) ' insert Integer at back
list.Print() 27
list.InsertAtBack(aString) ' insert String at back
list.Print() 30
' use CList remove methods
Dim removedObject As Object 33
' remove data from list and print after each removal
Try
ListTest.vb Define modListTest main

19. ListTest.vb 36 37 ' remove object from front of list
removedObject = list.RemoveFromFront()
Console.WriteLine(Convert.ToString(removedObject) & _
" removed") 41
list.Print() 43
' remove object from front of list
removedObject = list.RemoveFromFront()
Console.WriteLine(Convert.ToString(removedObject) & _
" removed") 48
list.Print() 50
' remove object from back of list
removedObject = list.RemoveFromBack()
Console.WriteLine(Convert.ToString(removedObject) & _
" removed") 55
list.Print() 57
' remove object from back of list
removedObject = list.RemoveFromBack()
Console.WriteLine(Convert.ToString(removedObject) & _
" removed") 62
list.Print() 64
' Catch exception if list is empty
Catch emptyListException As EmptyListException
Console.Error.WriteLine(vbCrLf & _
Convert.ToString(emptyListException)) 69
End Try
ListTest.vb

20. 71 72 End Sub ' Main 73 74 End Module ' modListTest
ListTest.vb
The list is: True
The list is: $ True
The list is: $ True 34567
The list is: $ True hello
$ removed
The list is: True hello
True removed
The list is: hello
hello removed
The list is: 34567
34567 removed
Empty list

21. 23.3 Linked Lists 11 7 12
new ListNode
(a)
firstNode
(b)
Fig InsertAtFront graphical representation.

22. 23.3 Linked Lists
(a) 7 12
New ListNode 11 5
firstNode
lastNode
(b)
Fig InsertAtBack graphical representation.

23. 23.3 Linked Lists
(a) 7 12
lastNode 11 5
firstNode
(b)
removeItem
Fig RemoveFromFront graphical representation.

24. 23.3 Linked Lists
(a) 7 12
lastNode 11 5
firstNode
(b)
current
removeItem
Fig RemoveFromBack graphical representation.

25. 23.4 Stacks
Stack
New nodes can be added and removed only at the top
Last-in, first-out (LIFO) data structure
Bottom of stack indicated by the link member in the bottom node of the stack set to Nothing
Constrained version of a linked list
Push
Adds a new node to the top of the stack
Pop
Removes a node from the top of the stack
Returns the data from the popped node

26. Class CstackInheritance inherits from class CList
1 ' Fig: 23.12: StackInheritance.vb
2 ' Implementing a stack by inheriting from class CList. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary 6
7 ' class CStackInheritance inherits class CList
8 Public Class CStackInheritance
Inherits CList 10
' pass name "stack" to CList constructor
Public Sub New()
MyBase.New("stack")
End Sub ' New 15
' place dataValue at top of stack by inserting dataValue at
' front of linked list
Public Sub Push(ByVal dataValue As Object)
MyBase.InsertAtFront(dataValue)
End Sub ' Push 21
' remove item from top of stack by removing item at front of
' linked list
Public Function Pop() As Object
Return MyBase.RemoveFromFront()
End Function ' Pop 27
28 End Class ' CStackInheritance
Class CstackInheritance inherits from class CList
StackInheritance.vb inherits from class Clist constructor Push method Pop method
Method Push calls InsertAtFront
Method Pop calls RemoveFromFront

27. StackTest.vb main objects to Push on the stack
1 ' Fig : StackTest.vb
2 ' Testing stack implementations. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary
6 Imports StackInheritanceLibrary 7
8 ' demonstrates functionality of stack implementations
9 Module modStackInheritanceTest 10
Sub Main()
Dim stack As CStackInheritance = New CStackInheritance() 13
' create objects to store in stack
Dim aBoolean As Boolean = True
Dim aCharacter As Char = Convert.ToChar("$")
Dim anInteger As Integer = 34567
Dim aString As String = "hello" 19
' use method Push to add items to stack
stack.Push(aBoolean) ' add Boolean
stack.Print() 23
stack.Push(aCharacter) ' add Char
stack.Print() 26
stack.Push(anInteger) ' add Integer
stack.Print() 29
stack.Push(aString) ' add String
stack.Print() 32
' use method Pop to remove items from stack
Dim removedObject As Object = Nothing 35
Defines four objects that will be pushed onto the stack and popped off the stack
StackTest.vb main objects to Push on the stack
These lines push the objects onto the stack

28. StackTest.vb Pop the objects off the stack
' remove items from stack
Try 38
' pop item and output removed item
While True
removedObject = stack.Pop()
Console.WriteLine(removedObject & " popped")
stack.Print()
End While 45
' catch exception if Pop was called while stack empty
Catch emptyListException As EmptyListException
Console.Error.WriteLine(emptyListException.StackTrace)
End Try 50
End Sub ' Main 52
53 End Module ' modStackTest
StackTest.vb Pop the objects off the stack
An infinite While loop pops the elements from the stack. The program uses method Print, inherited from class Clist, to output the contents of the stack after each operation.
The stack is: True
The stack is: $ True
The stack is: $ True
The stack is: hello $ True
hello popped
34567 popped

29. StackTest.vb $ popped The stack is: True True popped Empty stack
True popped
Empty stack
at LinkedListLibrary.CList.RemoveFromFront() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_04\LinkedListLibrary\List.vb:line 64
at StackInheritanceLibrary.CStackInheritance.Pop() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_12\StackInheritanceLibrary\StackInheritance.vb:line 25
at StackInheritanceTest.modStackInheritance.Main() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_13\StackTest\StackTest.vb:line 41
StackTest.vb

30. StackComposition.vb constructor Push method Pop method IsEmpty Print
1 ' Fig : StackComposition.vb
2 ' StackComposition definition with composed CList object. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary 6
7 ' class CStackComposition encapsulates CList's capabilities
8 Public Class CStackComposition
Private stack As CList 10
' construct empty stack
Public Sub New()
stack = New CList("stack")
End Sub ' New 15
' add object to stack
Public Sub Push(ByVal dataValue As Object)
stack.InsertAtFront(dataValue)
End Sub ' Push 20
' remove object from stack
Public Function Pop() As Object
Return stack.RemoveFromFront()
End Function ' Pop 25
' determine whether stack is empty
Public Function IsEmpty() As Boolean
Return stack.IsEmpty()
End Function ' IsEmpty 30
' output stack content
Public Sub Print()
stack.Print()
End Sub ' Print
Composition enables us to hide the methods of class Clist that should not appear in our Stack’s Public interface by providing Public interface methods only to the required Clist methods
StackComposition.vb constructor Push method Pop method IsEmpty Print

31. 35 36 End Class ' CStackComposition
StackComposition.vb

32. 23.5 Queues Queue Similar to a supermarket checkout line
First-in, first-out (FIFO) data structure
Nodes are removed only from the head
Nodes are inserted only at the tail
The insert and remove operations are known as enqueue and dequeue
Useful in computing
Print spooling, information packets in networks, file server requests

33. Inherits the Clist class
1 ' Fig : QueueInheritance.vb
2 ' Implementing a queue by inheriting from class CList. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary 6
7 ' class CQueueInheritance inherits from class CList
8 Public Class CQueueInheritance
Inherits CList 10
' pass name "queue" to CList constructor
Public Sub New()
MyBase.New("queue")
End Sub 15
' place dataValue at end of queue by inserting dataValue at end
' of linked list
Public Sub Enqueue(ByVal dataValue As Object)
MyBase.InsertAtBack(dataValue)
End Sub ' Enqueue 21
' remove item from front of queue by removing item at front of
' linked list
Public Function Dequeue() As Object
Return MyBase.RemoveFromFront()
End Function ' Dequeue 27
28 End Class ' CQueueInheritance
Inherits the Clist class
QueueInheritance.vb inherits from Clist constructor method Enqueue method Dequeue
Method Enqueue calls the InsertAtBack method of it’s base-class
Method Dequeue calls the RemoveFromFront method of it’s base-class

34. Objects are created and then added to the queue
1 ' Fig : QueueTest.vb
2 ' Testing queue implementation. 3
4 ' Deitel namespaces
5 Imports LinkedListLibrary
6 Imports QueueInheritanceLibrary 7
8 ' demonstrate queue functionality
9 Module modQueueTest 10
Sub Main()
Dim queue As CQueueInheritance = New CQueueInheritance() 13
' create data to store in queue
Dim aBoolean As Boolean = True
Dim aCharacter As Char = Convert.ToChar("$")
Dim anInteger As Integer = 34567
Dim aString As String = "hello" 19
' use method Enqueue to add items to queue
queue.Enqueue(aBoolean) ' add Boolean
queue.Print() 23
queue.Enqueue(aCharacter) ' add Char
queue.Print() 26
queue.Enqueue(anInteger) ' add Integer
queue.Print() 29
queue.Enqueue(aString) ' add String
queue.Print() 32
' use method Dequeue to remove items from queue
Dim removedObject As Object = Nothing 35
QueueTest.vb main objects to be added to the queue insert the objects to the queue
Objects are created and then added to the queue

35. QueueTest.vb removing the objects from the queue
' remove items from queue
Try 38
' dequeue item and output removed item
While True
removedObject = queue.Dequeue()
Console.WriteLine(removedObject & " dequeue")
queue.Print()
End While 45
' if exception occurs, print stack trace
Catch emptyListException As EmptyListException
Console.Error.WriteLine(emptyListException.StackTrace)
End Try 50
End Sub ' Main 52
53 End Module ' modQueueTest
QueueTest.vb removing the objects from the queue
An infinite While loop dequeues the elements from the queue until no objects are left
The queue is: True
The queue is: True $
The queue is: True $ 34567
The queue is: True $ hello
True dequeue
The queue is: $ hello
$ dequeue
The queue is: hello

36. QueueTest.vb 34567 dequeue The queue is: hello hello dequeue
hello dequeue
Empty queue
at LinkedListLibrary.CList.RemoveFromFront() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_04\LinkedListLibrary\List.vb:line 64
at QueueInheritanceLibrary.CQueueInheritance.Dequeue() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_15\QueueInheritanceLibrary\QueueInheritance.vb:line 25
at QueueTest.modQueueInheritanceTest.Main() in C:\books\2001\vbhtp2\ch23\Examples\Fig23_16\QueueTest\QueueTest.vb:
line 41
QueueTest.vb

37. 23.6 Trees Tree Binary trees
A nonlinear, two-dimensional data structure
nodes contain two or more links
all other data structures we have discussed only contain one
Binary trees
all nodes contain two links
none, one, or both of which may be Nothing
The root node is the first node in a tree.
Each link in the root node refers to a child
The left child is the first node in the left subtree, and the right child is the first node in the right subtree
A node with no children is called a leaf node

38. 23.6 Trees B A D C
Fig Binary tree graphical representation.

39. 23.6 Trees 47 25 77 11 7 17 65 93 68 43 31 44
Fig A binary search tree containing 12 values.

40. Class CTreeNode is a self-referential class containing three Private data members. Two of type CTreeNode, mLeftNode and mRightNode, and one of type Integer, mData
1 ' Fig : TreeNode.vb
2 ' Class CTreeNode represents a node in a CTree. 3
4 Public Class CTreeNode
Private mLeftNode As CTreeNode
Private mData As Integer
Private mRightNode As CTreeNode 8
' initialize data and make that a leaf node
Public Sub New(ByVal nodeData As Integer)
mData = nodeData
mRightNode = Nothing ' node has no children
LeftNode = Nothing ' node has no children
End Sub ' New 15
' property LeftNode
Public Property LeftNode() As CTreeNode 18
Get
Return mLeftNode
End Get 22
Set(ByVal value As CTreeNode)
mLeftNode = value
End Set 26
End Property ' LeftNode 28
' property Data
Public Property Data() As Integer 31
Get
Return mData
End Get 35
Constructor initializes references mLeftNode and mRightNode to Nothing, initially making every CTreeNode a leaf node
TreeNode.vb Private member variables of CTreeNode constructor Property LeftNode Property Data
Properties LeftNode, Data, and RightNode provide access to a CTreeNode’s Private data members.

41. TreeNode.vb Property RightNode method Insert
Set(ByVal value As Integer)
mData = value
End Set 39
End Property ' Data 41
' property RightNode
Public Property RightNode() As CTreeNode 44
Get
Return mRightNode
End Get 48
Set(ByVal value As CTreeNode)
mRightNode = value
End Set 52
End Property ' RightNode 54
' insert node into tree
Public Sub Insert(ByVal insertValue As Integer) 57
' insert in left subtree
If insertValue < mData Then 60
' insert new CTreeNode
If mLeftNode Is Nothing Then
LeftNode = New CTreeNode(insertValue) 64
' continue traversing left subtree
Else
LeftNode.Insert(insertValue)
End If 69
TreeNode.vb Property RightNode method Insert
Method Insert recursively determines the location for the new node in the tree and inserts the node at that location. It compares the value to insert with the mData value in the root node. Then it determines whether it should be inserted in the left subtree or the right subtree depending on it’s value compared to the other nodes in the tree.

42. TreeNode.vb 70 ' insert in right subtree
ElseIf insertValue > mData Then 72
' insert new CTreeNode
If RightNode Is Nothing Then
RightNode = New CTreeNode(insertValue) 76
' continue traversing right subtree
Else
RightNode.Insert(insertValue)
End If 81
End If 83
End Sub ' Insert 85
86 End Class ' CTreeNode
TreeNode.vb

43. Tree.vb constructor InsertNode method PreorderTraversal method
Class CTree manipulates objects of class CTreeNode. It contains a Private root node, a reference to the root node of the tree.
1 ' Fig : Tree.vb
2 ' Class CTree is a tree containing CTreeNodes. 3
4 Public Class CTree
Private root As CTreeNode 6
' construct an empty CTree of integers
Public Sub New()
root = Nothing
End Sub ' New 11
' insert new node in binary search tree
Public Sub InsertNode(ByVal insertValue As Integer) 14
SyncLock (Me) 16
' if node does not exist, create node
If root Is Nothing Then
root = New CTreeNode(insertValue)
Else ' otherwise insert node into tree
root.Insert(insertValue)
End If 23
End SyncLock 25
End Sub ' InsertNode 27
' begin preorder traversal
Public Sub PreorderTraversal() 30
SyncLock (Me)
PreorderHelper(root)
End SyncLock 34
End Sub ' PreOrderTraversal
Tree.vb constructor InsertNode method PreorderTraversal method
The CTree constructor initializes root to Nothing to indicate that the tree is initially empty
Method InsertNode inserts a node in the tree. It calls CTreeNode method Insert.
Methods InorderTraversal, PreorderTraversal and PostorderTraversal call helper methods InorderHelper, PreorderHelper and PostorderHelper, respectively, to traverse the tree and print the node values
The PreorderTraversal method processes the value in each node as the node is visited. After it processes the values in the left subtree, then the values in the right subtree.

44. 36
' recursive method to perform preorder traversal
Private Sub PreorderHelper(ByVal node As CTreeNode) 39
If node Is Nothing Then
Return
End If 43
' output node data
Console.Write(node.Data & " ") 46
' traverse left subtree
PreorderHelper(node.LeftNode) 49
' traverse right subtree
PreorderHelper(node.RightNode) 52
End Sub ' PreorderHelper 54
' begin inorder traversal
Public Sub InorderTraversal() 57
SyncLock (Me)
InorderHelper(root)
End SyncLock 61
End Sub ' InorderTraversal 63
' recursive method to perform inorder traversal
Private Sub InorderHelper(ByVal node As CTreeNode) 66
If node Is Nothing Then
Return
End If 70
Tree.vb helper method PreorderHelper method InorderTraversal helper method InorderHelper
The helper methods in class CTree allow the programmer to start the traversal without first obtaining a reference to the root node first. Methods InorderTraversal, PreorderTraversal and PostorderTraversal simply take the Private reference root and pass it to the appropriate helper method to initiate the traversal of the tree
The InorderTraversal method does not process the value in a node until the values in that node’s left subtree are processed

45. Tree.vb method PostorderTraversal helper method PostorderHelper
' traverse left subtree
InorderHelper(node.LeftNode) 73
' output node data
Console.Write(node.Data & " ") 76
' traverse right subtree
InorderHelper(node.RightNode) 79
End Sub ' InorderHelper 81
' begin postorder traversal
Public Sub PostorderTraversal() 84
SyncLock (Me)
PostorderHelper(root)
End SyncLock 88
End Sub ' PostorderTraversal 90
' recursive method to perform postorder traversal
Private Sub PostorderHelper(ByVal node As CTreeNode) 93
If node Is Nothing Then
Return
End If 97
' traverse left subtree
PostorderHelper(node.LeftNode)
100
' traverse right subtree
PostorderHelper(node.RightNode)
103
The PostorderTraversal method processes the value in each node after the values of all that node’s children are processed
Tree.vb method PostorderTraversal helper method PostorderHelper

46. Tree.vb 104 ' output node data 105 Console.Write(node.Data & " ") 106
End Sub ' PostorderHelper
108
109 End Class ' CTree
Tree.vb

47. TreeTest.vb 1 ' Fig. 23.21: TreeTest.vb
2 ' This program tests class CTree. 3
4 ' Deitel namespaces
5 Imports BinaryTreeLibrary 6
7 Module modTreeTest 8
' test class CTree
Sub Main()
Dim tree As CTree = New CTree()
Dim insertValue As Integer
Dim i As Integer 14
Console.WriteLine("Inserting Values: ")
Dim randomNumber As Random = New Random() 17
' insert 10 random integers from 0-99 in tree
For i = 1 To 10
insertValue = randomNumber.Next(100)
Console.Write(insertValue & " ")
tree.InsertNode(insertValue)
Next 24
' perform preorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & "Preorder Traversal")
tree.PreOrderTraversal() 28
' perform inorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & "Inorder Traversal")
tree.InOrderTraversal() 32
TreeTest.vb

48. TreeTest.vb 33 ' perform postorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & "Postorder Traversal")
tree.PostOrderTraversal() 36
Console.WriteLine()
End Sub ' Main 39
40 End Module ' modTreeTest
TreeTest.vb
Inserting Values:
Preorder Traversal
Inorder Traversal
Postorder Traversal

49. 23.6 Trees 27 13 42 6 33 48 17
Fig A binary search tree.

50. Here we take advantage of Visual Basic’s polymorphic capabilities and we implement classes CTreeNode and CTree, which now manipulates objects that implement the interface IComparable
1 ' Fig : TreeNode2.vb
2 ' Class CTreeNode uses IComparable objects for objects 3
4 Public Class CTreeNode
Private mLeftNode As CTreeNode
Private mData As IComparable
Private mRightNode As CTreeNode 8
' initialize data and make this a leaf node
Public Sub New(ByVal nodeData As IComparable)
mData = nodeData
mRightNode = Nothing ' node has no children
LeftNode = Nothing ' node has no children
End Sub ' New 15
' property LeftNode
Public Property LeftNode() As CTreeNode 18
Get
Return mLeftNode
End Get 22
Set(ByVal value As CTreeNode)
mLeftNode = value
End Set 26
End Property ' LeftNode 28
' property Data
Public Property Data() As IComparable 31
Get
Return mData
End Get 35
TreeNode2.vb Private member variables constructor Property LeftNode Property Data
Instead of being a binary search tree of type Integer, it is now of type IComparable data

51. 36 Set(ByVal value As IComparable)
mData = value
End Set 39
End Property ' Data 41
' property RightNode
Public Property RightNode() As CTreeNode 44
Get
Return mRightNode
End Get 48
Set(ByVal value As CTreeNode)
mRightNode = value
End Set 52
End Property ' RightNode 54
' insert node into tree
Public Sub Insert(ByVal insertValue As IComparable) 57
'insert in left subtree
If insertValue.CompareTo(mData) < 0 Then 60
' insert new TreeNode
If mLeftNode Is Nothing Then
LeftNode = New CTreeNode(insertValue) 64
' continue traversing left subtree
Else
LeftNode.Insert(insertValue)
End If 69
Instead of being a binary search tree of type Integer, it is now of type IComparable data
TreeNode2.vb Property RightNode Insert Method call to the Interfaces CompareTo method
This line now compares Icomparable objects using the interfaces method CompareTo

52. TreeNode2.vb 70 ' insert in right subtree
ElseIf insertValue.CompareTo(mData) Then 72
' insert new TreeNode
If RightNode Is Nothing Then
RightNode = New CTreeNode(insertValue) 76
' continue traversing right subtree
Else
RightNode.Insert(insertValue)
End If 81
End If 83
End Sub ' Insert 85
86 End Class ' CTreeNode
TreeNode2.vb

53. 1 ' Fig : Tree2.vb
2 ' Class CTree contains nodes with IComparable data 3
4 Public Class CTree
Private root As CTreeNode 6
' construct an empty CTree of integers
Public Sub New()
root = Nothing
End Sub ' New 11
' insert new node in binary search tree
Public Sub InsertNode(ByVal insertValue As IComparable) 14
SyncLock (Me) 16
' if node does not exist, create one
If root Is Nothing Then
root = New CTreeNode(insertValue)
Else ' otherwise insert node in tree
root.Insert(insertValue)
End If 23
End SyncLock 25
End Sub ' InsertNode 27
' begin preorder traversal
Public Sub PreorderTraversal() 30
SyncLock (Me)
PreorderHelper(root)
End SyncLock 34
End Sub ' PreorderTraversal
Tree2.vb constructor method InsertNode calls the Insert method of class CTreeNode method PreorderTraversal
Instead of being a binary search tree of type Integer, it is now of type IComparable data

54. 36
' recursive method to perform preorder traversal
Private Sub PreorderHelper(ByVal node As CTreeNode) 39
If node Is Nothing Then
Return
End If 43
' output node data
Console.Write(Convert.ToString(node.Data) & " ") 46
' traverse left subtree
PreOrderHelper(node.LeftNode) 49
' traverse right subtree
PreOrderHelper(node.RightNode) 52
End Sub ' PreOrderHelper 54
' begin inorder traversal
Public Sub InorderTraversal() 57
SyncLock (Me)
InorderHelper(root)
End SyncLock 61
End Sub ' InorderTraversal 63
' recursive method to perform inorder traversal
Private Sub InorderHelper(ByVal node As CTreeNode) 66
If node Is Nothing Then
Return
End If 70
Tree2.vb helper method PreorderHelper method InorderTraversal helper method InorderHelper

55. Tree2.vb method PostorderTraversal helper method PostorderHelper
' traverse left subtree
InorderHelper(node.LeftNode) 73
' output node data
Console.Write(Convert.ToString(node.Data) & " ") 76
' traverse right subtree
InorderHelper(node.RightNode) 79
End Sub ' InorderHelper 81
' begin postorder traversal
Public Sub PostorderTraversal() 84
SyncLock (Me)
PostOrderHelper(root)
End SyncLock 88
End Sub ' PostorderTraversal 90
' recursive method to perform postorder traversal
Private Sub PostorderHelper(ByVal node As CTreeNode) 93
If node Is Nothing Then
Return
End If 97
' traverse left subtree
PostorderHelper(node.LeftNode)
100
' traverse right subtree
PostorderHelper(node.RightNode)
103
' output node data
Console.Write(Convert.ToString(node.Data) & " ")
Tree2.vb method PostorderTraversal helper method PostorderHelper

56. 106 107 End Sub ' PostorderHelper 108 109 End Class ' CTree
Tree2.vb

57. TreeTest2.vb main 1 ' Fig. 23.25: TreeTest2.vb
2 ' This program tests class CTree. 3
4 ' Deitel namespaces
5 Imports BinaryTreeLibrary2 6
7 Module modTreeTest2 8
' test class CTree.
Sub Main()
Dim integerArray As Integer() = {8, 2, 4, 3, 1, 7, 5, 6}
Dim doubleArray As Double() = _
{8.8, 2.2, 4.4, 3.3, 1.1, 7.7, 5.5, 6.6} 14
Dim stringArray As String() = {"eight", "two", "four", _
"three", "one", "seven", "five", "six"} 17
' create Integer tree
Dim integerTree As CTree = New CTree()
PopulateTree(integerArray, integerTree, "integerTree")
TraverseTree(integerTree, "integerTree") 22
' create Double tree
Dim doubleTree As CTree = New CTree()
populateTree(doubleArray, doubleTree, "doubleTree")
TraverseTree(doubleTree, "doubleTree") 27
' create String tree
Dim stringTree As CTree = New CTree()
populateTree(stringArray, stringTree, "stringTree")
TraverseTree(stringTree, "stringTree") 32
End Sub ' Main 34
TreeTest2.vb main

58. TreeTest2.vb 35 ' populate tree with array elements
Public Sub PopulateTree(ByVal array As Array, _
ByVal tree As CTree, ByVal name As String) 38
Dim data As IComparable
Console.WriteLine(vbCrLf & "Inserting into " & name & ":") 41
For Each data In array
Console.Write(Convert.ToString(data) & " ")
tree.InsertNode(data)
Next 46
End Sub ' PopulateTree 48
' perform traversals
Public Sub TraverseTree(ByVal tree As CTree, _
ByVal treeType As String) 52
' perform preorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & _
"Preorder Traversal of " & treeType) 56
tree.PreorderTraversal() 58
' perform inorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & _
"Inorder Traversal of " & treeType) 62
tree.InorderTraversal() 64
' perform postorder traversal of tree
Console.WriteLine(vbCrLf & vbCrLf & _
"Postorder Traversal of " & treeType) 68
TreeTest2.vb

59. TreeTest2.vb 69 tree.PostorderTraversal() 70
Console.WriteLine(vbCrLf)
End Sub ' TraverseTree 73
74 End Module ' CTreeTest2
TreeTest2.vb
Inserting into integerTree:
Preorder Traversal of integerTree
Inorder Traversal of integerTree
Postorder Traversal of integerTree
Inserting into doubleTree:
Preorder Traversal of doubleTree
Inorder Traversal of doubleTree
Postorder Traversal of doubleTree

60. TreeTest2.vb Inserting into stringTree:
eight two four three one seven five six
Preorder Traversal of stringTree
eight two four five three one seven six
Inorder Traversal of stringTree
eight five four one seven six three two
Postorder Traversal of stringTree
five six seven one three four two eight
TreeTest2.vb

61. 23.7 Collection Classes Collection Classes Store collections of data
Each instance is known as a collection, which is a set of items
The programmer uses existing data structures without worrying about how the data structures are implemented
They represent a great example of code reuse
Allows for quicker coding, excellent performance, maximum execution speed, and minimize memory consumption

62. 1 ' Fig : UsingArray.vb
2 ' Using class Array to perform common array manipulations. 3
4 Imports System.Windows.Forms
5 Imports System.Collections 6
7 ' demonstrate algorithms of class Array
8 Public Class CUsingArray
Private integerValues As Integer() = {1, 2, 3, 4, 5, 6}
Private doubleValues As Double() = _
{8.4, 9.3, 0.2, 7.9, 3.4} 12
Private integerValuesCopy(6) As Integer
Private output As String 15
' build and display program output
Public Sub Start()
Dim result As Integer 19
output = "Initial Array Values:" & vbCrLf
PrintArray() ' output initial array contents 22
' sort doubleValues
Array.Sort(doubleValues) 25
' copy integerValues into integerValuesCopy
Array.Copy(integerValues, integerValuesCopy, _
integerValues.Length) 29
output &= vbCrLf & vbCrLf & _
"Array values after Sort and Copy:" & vbCrLf 32
PrintArray() ' output array contents
output &= vbCrLf & vbCrLf 35
UsingArray.vb Private member variables Start method call to Print method call to Sort method call to Copy method
Using SharedArray method Copy to copy elements from array integerArray into array intergerArrayCopy

63. UsingArray.vb PrintArray method
' search for value 5 in integerValues
result = Array.BinarySearch(integerValues, 5) 38
If result >= 0 Then
output &= "5 found at element " & result & _
" in integerValues"
Else
output &= "5 not found" & " in integerValues"
End If 45
output &= vbCrLf 47
' search for value 8763 in integerValues
result = Array.BinarySearch(integerValues, 8763) 50
If result >= 0 Then
output &= "8763 found at element " & _
result & " in integerValues"
Else
output &= "8763 was not found" & " in integerValues"
End If 57
MessageBox.Show(output, "Using Class Array", _
MessageBoxButtons.OK, MessageBoxIcon.Information) 60
End Sub ' Start 62
' append array output to output string
Private Sub PrintArray()
Dim doubleElement As Double
Dim integerElement As Integer 67
output &= "doubleValues: " 69
Invokes SharedArray method BinarySearch to perform binary searches on array integerValues. Method BinarySearch receives sorted array in which to search and the key for which to search
UsingArray.vb PrintArray method

64. UsingArray.vb main 70 ' output each element in array doubleValues
For Each doubleElement In doubleValues
output &= doubleElement & " "
Next 74
output &= vbCrLf & " integerValues: " 76
' output each element in array integerValues
For Each integerElement In integerValues
output &= integerElement & " "
Next 81
output &= vbCrLf & " integerValuesCopy: " 83
' output each element in array integerValuesCopy
For Each integerElement In integerValuesCopy
output &= integerElement & " "
Next 88
End Sub ' PrintArray 90
' main entry point for application
Shared Sub Main()
Dim application As CUsingArray = New CUsingArray()
application.Start()
End Sub ' Main 96
97 End Class ' CUsingArray
UsingArray.vb main

65. 23.7 Collection Classes

66. 23.7 Collection Classes
Fig ArrayList methods (partial list).

67. Creates an ArrayList with an initial capacity of one element
1 ' Fig : ArrayListTest.vb
2 ' Demonstrating class ArrayList functionality. 3
4 Imports System.Collections
5 Imports System.Text
6 Imports System.Windows.Forms 7
8 Public Class FrmArrayList
Inherits Form 10
' Buttons for invoking ArrayList functionality
Friend WithEvents cmdAdd As Button
Friend WithEvents cmdRemove As Button
Friend WithEvents cmdFirst As Button
Friend WithEvents cmdLast As Button
Friend WithEvents cmdIsEmpty As Button
Friend WithEvents cmdContains As Button
Friend WithEvents cmdLocation As Button
Friend WithEvents cmdTrim As Button
Friend WithEvents cmdStatistics As Button
Friend WithEvents cmdDisplay As Button 22
' TextBox for user input
Friend WithEvents txtInput As TextBox
Friend WithEvents lblEnter As Label 26
Friend WithEvents txtConsole As TextBox ' TextBox for output 28
' Visual Studio .NET generated code 30
' ArrayList for manipulating Strings
Private arrayList As ArrayList = New ArrayList(1) 33
FrmArrayList.vb
Creates an ArrayList with an initial capacity of one element

68. 34 ' add item to end of arrayList
Private Sub cmdAdd_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdAdd.Click 37
arrayList.Add(txtInput.Text)
txtConsole.Text = "Added to end: " & txtInput.Text
txtInput.Clear()
End Sub ' cmdAdd_Click 42
'remove specified item from arrayList
Private Sub cmdRemove_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdRemove.Click 46
arrayList.Remove(txtInput.Text)
txtConsole.Text = "Removed: " & txtInput.Text
txtInput.Clear()
End Sub ' cmdRemove_Click 51
' display first element
Private Sub cmdFirst_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdFirst.Click 55
' get first element
Try
txtConsole.Text = "First element: " & arrayList(0) 59
' show exception if no elements in arrayList
Catch outOfRange As ArgumentOutOfRangeException
txtConsole.Text = outOfRange.ToString()
End Try 64
End Sub ' cmdFirst_Click 66
When the user clicks Add, event handler cmdAdd_Click invokes method Add to append the String in the inputTextBox to the ArrayList
FrmArrayList.vb
When the user clicks Remove, event handler cmdRemove_Click invokes Remove to remove the String specified in the inputTextBox from the ArrayList
Event handlers cmdFirst_Click and cmdLast_Click use the ArrayList subscript operator to retrieve the first element and last element respectively.

69. ' display last element
Private Sub cmdLast_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdLast.Click 70
' get last element
Try
txtConsole.Text = "Last element: " & _
arrayList(arrayList.Count - 1) 75
' show exception if no elements in arrayList
Catch outOfRange As ArgumentOutOfRangeException
txtConsole.Text = outOfRange.ToString()
End Try 80
End Sub ' cmdLast_Click 82
' determine whether arrayList is empty
Private Sub cmdIsEmpty_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdIsEmpty.Click 86
If arrayList.Count = 0 Then
txtConsole.Text = "arrayList is empty"
Else
txtConsole.Text = "arrayList is not empty"
End If 92
End Sub ' cmdIsEmpty_Click 94
' determine whether arrayList contains specified object
Private Sub cmdContains_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdContains.Click 98
If arrayList.Contains(txtInput.Text) Then
txtConsole.Text = "arrayList contains " & _
txtInput.Text()
FrmArrayList.vb
Event handler cmdIsEmpty_Click uses ArrayList property Count to determine whether the ArrayList is empty
Event handler cmdContains_Click uses ArrayList method Contains to determine whether the object that Contains receives as an argument currently is in the ArrayList

70. When the user clicks Location, event handler cmdIsEmpty_Click invokes ArrayList method IndexOf to determine the index of a particular object in the ArrayList
Else
txtConsole.Text = txtInput.Text & " not found"
End If
105
End Sub ' cmdContains_Click
107
' determine location of specified object
Private Sub cmdLocation_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdLocation.Click
111
txtConsole.Text = "Element is at location " & _
arrayList.IndexOf(txtInput.Text)
End Sub ' cmdLocation_Click
115
' trim arrayList to current size
Private Sub cmdTrim_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdTrim.Click
119
arrayList.TrimToSize()
txtConsole.Text = "Vector trimmed to size"
End Sub ' cmdTrim_Click
123
' show arrayList current size and capacity
Private Sub cmdStatistics_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdStatistics.Click
127
txtConsole.Text = "Size = " & arrayList.Count & _
"; capacity = " & arrayList.Capacity
End Sub ' cmdStatistics_Click
131
' display contents of arrayList
Private Sub cmdDisplay_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdDisplay.Click
135
FrmArrayList.vb
When the user clicks Trim, event handler cmdTrim_Click invokes method TrimToSize to set the Capacity property so that it is equal to the Count property
When the user clicks Statistics, cmdStatistics_Click uses the Count and Capacity properties to display the current number of elements in the ArrayList and the maximum number of elements that can be stored without the allocation of more memory to the ArrayList.
When the user clicks Display, cmdDisplay_Click outputs the contents of the ArrayList

71. 136 Dim enumerator As IEnumerator = arrayList.GetEnumerator()
Dim buffer As StringBuilder = New StringBuilder()
138
While enumerator.MoveNext()
buffer.Append(enumerator.Current & " ")
End While
142
txtConsole.Text = buffer.ToString()
End Sub ' cmdDisplay_Click
145
146 End Class ' FrmArrayList
FrmArrayList.vb

72. 23.7 Collection Classes
FrmArrayList.vb

73. StackTest.vb constructor
1 ' Fig : StackTest.vb
2 ' Demonstrates class Stack functionality. 3
4 Imports System.Collections
5 Imports System.Text
6 Imports System.Windows.Forms 7
8 Public Class FrmStackTest
Inherits Form 10
' Buttons invoking Stack functionality
Friend WithEvents cmdPush As Button
Friend WithEvents cmdPop As Button
Friend WithEvents cmdPeek As Button
Friend WithEvents cmdIsEmpty As Button
Friend WithEvents cmdSearch As Button
Friend WithEvents cmdDisplay As Button 18
' TextBox receives input from user
Friend WithEvents txtInput As TextBox
Friend WithEvents lblStatus As Label
Friend WithEvents lblEnter As Label 23
Private stack As Stack 25
Public Sub New()
MyBase.New() 28
InitializeComponent() 30
stack = New Stack() ' create stack
End Sub ' New 33
' Visual Studio .NET generated code 35
StackTest.vb constructor
The FrmStackTest constructor creates a Stack with the default initial capacity of 10

74. 36 ' push element onto stack
Private Sub cmdPush_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdPush.Click 39
Stack.Push(txtInput.Text)
lblStatus.Text = "Pushed: " & txtInput.Text
End Sub ' cmdPush_Click 43
' pop element from stack
Private Sub cmdPop_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdPop.Click 47
' pop element
Try
lblStatus.Text = "Popped: " & stack.Pop() 51
' print message if stack is empty
Catch invalidOperation As InvalidOperationException
lblStatus.Text = invalidOperation.ToString()
End Try 56
End Sub ' cmdPop_Click 58
' peek at top element of stack
Private Sub cmdPeek_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdPeek.Click 62
' view top element
Try
lblStatus.Text = "Top: " & stack.Peek() 66
' print message if stack is empty
Catch invalidOperation As InvalidOperationException
lblStatus.Text = invalidOperation.ToString()
End Try
Event handler cmdPush_Click uses method Push to add a user-specified string to the Stack
StackTest.vb
Event handler cmdPop_Click calls method Pop to remove an object from the Stack
Event handler cmdPeek_Click calls method Peek to view the object on top of the Stack

75. Event handler cmdIsEmpty_Click determines whether the Stack is empty by comparing the Stack’s Count property to zero 71
End Sub ' cmdPeek_Click 73
' determine whether stack is empty
Private Sub cmdIsEmpty_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdIsEmpty.Click 77
If stack.Count = 0 Then
lblStatus.Text = "Stack is empty"
Else
lblStatus.Text = "Stack is not empty"
End If 83
End Sub ' cmdIsEmpty_Click 85
' determine whether specified element is on stack
Private Sub cmdSearch_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdSearch.Click 89
If stack.Contains(txtInput.Text) Then
lblStatus.Text = txtInput.Text & " found"
Else
lblStatus.Text = txtInput.Text & " not found"
End If 95
End Sub ' cmdSearch_Click 97
' display stack contents
Private Sub cmdDisplay_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdDisplay.Click
101
Dim enumerator As IEnumerator = stack.GetEnumerator()
Dim buffer As StringBuilder = New StringBuilder()
104
StackTest.vb
Event handler cmdSearch_Click uses Stack method Contains to determine whether the Stack contains the specified object specified as its argument
Event handler cmdDisplay_Click uses an IEnumerator to traverse the Stack and display its contents

76. StackTest.vb 105 While enumerator.MoveNext()
buffer.Append(enumerator.Current & " ")
End While
108
lblStatus.Text = buffer.ToString()
End Sub ' cmdDisplay_Click
111
112 End Class ' FrmStackTest
StackTest.vb

77. 23.7 Collection Classes
StackTest.vb

78. FrmHashTableTest.vb constructor
1 ' Fig : FrmHashTableTest.vb
2 ' Demonstrate class Hashtable functionality. 3
4 Imports System.Collections
5 Imports System.Text
6 Imports System.Windows.Forms 7
8 Public Class FrmHashTableTest
Inherits Form 10
' Buttons invoke Hashtable functionality
Friend WithEvents cmdAdd As Button
Friend WithEvents cmdGet As Button
Friend WithEvents cmdRemove As Button
Friend WithEvents cmdEmpty As Button
Friend WithEvents cmdContains As Button
Friend WithEvents cmdClear As Button
Friend WithEvents cmdListObjects As Button
Friend WithEvents cmdListKeys As Button 20
' TextBoxes enable user to input hashtable data
Friend WithEvents txtFirst As TextBox
Friend WithEvents txtLast As TextBox
Friend WithEvents txtConsole As TextBox 25
Friend WithEvents lblFirst As Label
Friend WithEvents lblLast As Label
Friend WithEvents lblStatus As Label 29
Private table As Hashtable 31
Public Sub New()
MyBase.New() 34
FrmHashTableTest.vb constructor

79. 35 'This call is required by the Windows Form Designer.
InitializeComponent() 37
table = New Hashtable() ' create Hashtable object
End Sub ' New 40
' Visual Studio .NET generated code 42
' add last name and CEmployee object to table
Private Sub cmdAdd_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdAdd.Click 46
Dim employee As New CEmployee(txtFirst.Text, txtLast.Text) 48
' add new key/value pair
Try
table.Add(txtLast.Text, employee)
lblStatus.Text = "Put: " & employee.ToString() 53
' if key does not exist or is in table, throw exception
Catch argumentException As ArgumentException
lblStatus.Text = argumentException.ToString()
End Try 58
End Sub ' cmdAdd_Click 60
' get object for given key
Private Sub cmdGet_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdGet.Click 64
Dim result As Object = table(txtLast.Text) 66
Event handler cmdAdd_Click reads the first name and last name of an employee from the user interface, creates an object of class CEmployee and adds the CEmployee to the HashTable
FrmHashTableTest.vb
Event handler cmdGet_Click retrieves the object associated with a specified key using the Hashtable‘s subscript operator

80. 67 If Not result Is Nothing Then
lblStatus.Text = "Get: " & result.ToString()
Else
lblStatus.Text = "Get: " & txtLast.Text & " not in table"
End If 72
End Sub ' cmdGet_Click 74
' remove key/value pair from table
Private Sub cmdRemove_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdRemove.Click 78
table.Remove(txtLast.Text)
lblStatus.Text = "Object Removed"
End Sub ' cmdRemove_Click 82
' determine whether table is empty
Private Sub cmdEmpty_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdEmpty.Click 86
lblStatus.Text = "Table is " 88
If table.Count = 0 Then
lblStatus.Text &= "empty"
Else
lblStatus.Text &= "not empty"
End If 94
End Sub ' cmdEmpty_Click 96
' determine whether table contains specified key
Private Sub cmdContains_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdContains.Click
100
Event handler cmdRemove_Click invokes Hashtable method Remove to delete a key and its associated object from the Hashtable
FrmHashTableTest.vb
Event handler cmdEmpty_Click uses Hashtable property Count to determine whether the Hashtable is empty
Event handler cmdContains_Click invokes Hashtable method ContainsKey to determine whether the Hashtable contains the specified key

81. Event handler cmdClear_Click invokes Hashtable method Clear to delete all Hashtable entries
lblStatus.Text = "Contains key: " & _
table.ContainsKey(txtLast.Text)
End Sub ' cmdContains_Click
104
' discard all table contents
Private Sub cmdClear_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdClear.Click
108
table.Clear()
lblStatus.Text = "Clear: Table is now empty"
End Sub ' cmdClear_Click
112
' display list of all objects in table
Private Sub cmdListObjects_Click( _
ByVal sender As System.Object, ByVal e As System.EventArgs) _
Handles cmdListObjects.Click
117
Dim enumerator As IDictionaryEnumerator = _
table.GetEnumerator()
120
Dim buffer As StringBuilder = New StringBuilder()
122
While enumerator.MoveNext()
buffer.Append(Convert.ToString(enumerator.Value) & _
vbCrLf)
End While
127
txtConsole.Text = buffer.ToString()
End Sub ' cmdListObjects_Click
130
' display list of keys in table
Private Sub cmdListKeys_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdListKeys.Click
134
FrmHashTableTest.vb
Event handler cmdListObjects_Click uses property Value of the enumerator to output the objects in the Hashtable
Event handler cmdListKeys_Click uses the Key property of the enumerator to output the keys in the Hashtable

82. FrmHashTableTest.vb 135 Dim enumerator As IDictionaryEnumerator = _
table.GetEnumerator()
137
Dim buffer As StringBuilder = New StringBuilder()
139
While enumerator.MoveNext()
buffer.Append(enumerator.Key & vbCrLf)
End While
143
txtConsole.Text = buffer.ToString()
End Sub ' cmdListKeys_Click
146
147 End Class ' FrmHashTableTest
FrmHashTableTest.vb

83. 23.7 Collection Classes
FrmHashTableTest.vb

84. 23.7 Collection Classes
FrmHashTableTest.vb

85. Employee.vb 1 ' Fig. 23.31: Employee.vb
2 ' Class CEmployee for use with HashTable. 3
4 Public Class CEmployee
Private firstName, lastName As String 6
Public Sub New(ByVal first As String, ByVal last As String)
firstName = first
lastName = last
End Sub ' New 11
' return Employee first and last names as String
Public Overrides Function ToString() As String
Return firstName & " " & lastName
End Function ' ToString 16
17 End Class ' CEmployee
Employee.vb