STARTING OUT WITH
Visual Basic 2008
FOURTH EDITION
Tony Gaddis
Haywood Community College
Kip Irvine
Florida International University
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 1
Chapter
12
Classes, Exceptions,
Collections, and
Scrollable Controls
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Introduction
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Classes
 Abstract Data Types
 Objects, Properties, Methods
Exceptions
Collections
Object Browser
Scrollable Controls
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 3
12.1
Classes and Objects
Classes Are Program Structures
That Define Abstract Data Types
and Are Used to Create Objects
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Abstract Data Types
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An abstract data type (ADT) is a data type
created by a programmer
ADTs are important in computer science and
object-oriented programming
An abstraction is a model of something that
includes only its general characteristics
Dog is an abstraction
 Defines a general type of animal but not a
specific breed, color, or size
 A dog is like a data type
 A specific dog is an instance of the data type
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 5
Classes
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A class is a program structure that defines an
abstract data type
 Must create the class first
 Then can create instances of the class
Class instances share common attributes
VB forms and controls are classes
 Each control in the toolbox represents a class
 Placing a button
on a form creates
an instance, or
object, of the class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 6
Class Properties, Methods, & Events
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Programs communicate with an object using the
properties and methods of the class
Class properties example: Buttons have Location,
Text, and Name properties
Class methods example: The Focus method
functions identically for every single button
Class event procedures: Each button in a form
has a different click event procedure
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 7
Object Oriented Design
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The challenge is to design classes that effectively
cooperate and communicate
Analyze application requirements to determine
ADTs that best implement the specifications
Classes are fundamental building blocks
 Typically represent nouns of some type
A well-designed class may outlive the application
 Other uses for the class may be found
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 8
Object Oriented Design Example
Specifications:
We need to keep a list of students that lets us track the
courses they have completed. Each student has a transcript
that contains all information about his or her completed
courses. At the end of each semester, we will calculate the
grade point average of each student. At times, users will
search for a particular course taken by a student.
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Nouns from the specification above typically
become classes in the program design
Verbs such as calculate GPA and search become
methods of those classes
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 9
OOD Class Characteristics
Class
Attributes (properties)
Operations (methods)
Student
LastName, FirstName,
IdNumber
Display, Input
StudentList AllStudents, Count
Add, Remove,
FindStudent
Course
Semester, Name,
Grade,Credits
Display, Input
Transcript
CourseList, Count
Display, Search,
CalcGradeAvg
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 10
Interface and Implementation
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Class interface is the portion of the class visible
to the application programmer
 Made available by creating properties,
methods, and events that are public
Class implementation is the portion of the class
hidden from client programs
 Kept hidden by designating member variables,
properties, & methods as private
Hiding of data and procedures inside a class is
referred to as encapsulation
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 11
12.2
Creating a Class
To Create a Class in Visual Basic,
You Create a Class Declaration
The Class Declaration Specifies the Member
Variables, Properties, Methods, and Events
That Belong to the Class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Class Declaration
Public Class Student
MemberDeclarations
End Class
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Student is the name of the class
Examples of MemberDeclarations
are presented in the following slides
To create a new class:
 Click Add New Item button on toolbar
 Select Class from Add New Item dialog box
 Provide a name for the class and click Add
 Adds a new, empty class file (.vb) to project
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 13
Member Variables
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A variable declared inside a class declaration
Syntax:
AccessSpecifer VariableName As DataType
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AccessSpecifier may be Public or Private
Example:
Public Class Student
Public strLastName As String
Public strFirstName As String
Public strId As String
End Class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
‘Holds last name
‘Holds first name
‘Holds ID number
Slide 12- 14
Creating an Instance of a Class
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A two step process creates an instance of a class
Declare a variable whose type is the class
Dim freshman As Student
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Create instance of the class with New keyword
and assign the instance to the variable
freshman = New Student()
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freshman defined here as an object variable
Can accomplish both steps in one statement
Dim freshman As New Student()
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 15
Accessing Members
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Can work with Public member variables of a class
object in code using this syntax:
objectVariable.memberVariable

For example:
 If freshman references a Student class object
 And Student class has public member
variables strFirstName, strLastName, and strID
 Can store values in member variables with
freshman.strFirstName = "Joy"
freshman.strLastName = "Robinson"
freshman.strId = "23G794"
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 16
Property Procedure
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A property procedure is a function that defines a
property
Controls access to property values
Procedure has two sections: Get and Set
 Get code executes when value is retrieved
 Set code executes when value is stored
Properties almost always declared Public to allow
access from outside the class
Set code often provides data validation logic
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 17
Property Procedure Syntax
Public Property PropertyName() As DataType
Get
Statements
End Get
Set(ParameterDeclaration)
Statements
End Set
End Property
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Slide 12- 18
Property Procedure Example
Public Class Student
' Member variables
Private sngTestAvg As Single
Public Property TestAverage() As Single
Get
Return sngTestAvg
End Get
Set(ByVal value As Single)
If value >= 0.0 And value <= 100.0 Then
sngTestAvg = value
Else
MessageBox.Show( _
"Invalid test average.", "Error")
End If
End Set
End Property
End Class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 19
Setting and Validating a Property
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TestAverage property is set as shown:
Dim freshman as New Student()
freshman.TestAverage = 82.3
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Passes 82.3 into value parameter of Set
 If in the range 0.0 to 100.0, value is stored
 If outside the range, message box displayed
instead of value being stored
Set(ByVal value As Single)
If value >= 0.0 And value <= 100.0 Then
sngTestAvg = value
Else
MessageBox.Show( _
"Invalid test average.", "Error")
End If
End Set
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 20
Read-Only Properties
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Useful at times to make a property read-only
Allows access to property values but cannot
change these values from outside the class
Add ReadOnly keyword after access specifier
Public ReadOnly Property PropertyName() As DataType
Get
Statements
End Get
End Property

This causes the propertyName to be read-only -not settable from outside of the class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 21
Read-Only Property Example
' TestGrade property procedure
ReadOnly Property TestGrade() As Char
Get
If sngTestAverage >= 90
return "A“c
Else If sngTestAverage >= 80
return "B“c
Else If sngTestAverage >= 70
return "C“c
Else If sngTestAverage >= 60
return "D“c
Else
return "F“c
End If
End Get
End Property
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 22
Object Removal & Garbage Collection
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Memory space is consumed when objects are
instantiated
Objects no longer needed should be removed
Set object variable to Nothing so it no longer
references the object
freshman = Nothing
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Object is a candidate for garbage collection when
it is no longer referenced by any object variable
The garbage collector monitors for and
automatically destroys objects no longer needed
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 23
Going Out of Scope
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An object variable instantiated within a procedure
is local to that procedure
An object goes out of scope when
 Referenced only by local variables and
 The procedure ends
Object removed once it goes out of scope
An object instantiated in a procedure and
assigned to a global variable is not removed
 Reference remains when procedure ends
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 24
Going Out of Scope, Example
Sub CreateStudent()
Dim sophomore As Student
sophomore = New Student()
sophomore.FirstName = "Travis"
sophomore.LastName = "Barnes"
sophomore.IdNumber = "17H495"
sophomore.TestAverage = 94.7
g_studentVar = sophomore
End Sub
With this statement, sophomore will not go out of scope.
Without this statement, it will go out of scope when the
procedure ends. (g_studentVar is a module-level variable.)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 25
Comparing Object Variables
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Multiple variables can reference the same object
Dim collegeStudent As Student
Dim transferStudent As Student
collegeStudent = New Student()
transferStudent = collegeStudent
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Can test if two variables refer to same object
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Must use the Is operator
The = operator cannot be used to test for this
If collegeStudent Is transferStudent Then
' Perform some action
End If
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 26
IsNot & Nothing Object Comparisons
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Use the IsNot operator to determine that two
variables do not reference the same object
If collegeStudent IsNot transferStudent Then
' Perform some action
End If
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Use the special value Nothing to determine if a
variable has no object reference
If collegeStudent Is Nothing Then
' Perform some action
End If
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 27
Creating an Array of Objects
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Can create an entire array of object variables
 Declare an array whose type is a class
 Instantiate an object for each element
' Declare the array
Dim mathStudents(9) As Student
Dim i As Integer
For i = 0 To 9
' Assign each element to an object
mathStudents(i) = New Student()
Next i
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 28
Objects As Procedure Arguments

Can use object variables as arguments to a
procedure or function
 Example: student object s as an argument
Sub DisplayStudentGrade(ByVal s As Student)
' Displays a student’s grade.
MessageBox.Show("The grade for " & _
s.FirstName & " " & s.LastName & _
" is " & s.TestGrade.ToString)
End Sub

Pass object variable with the procedure call
DisplayStudentGrade(freshman)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 29
Objects Passed ByVal and ByRef
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If argument is declared using ByRef
 Values of object properties may be changed
 The original object variable may be assigned to
a different object
If argument is declared using ByVal
 Values of object properties may be changed
 The original object variable may not be
assigned to a different object
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 30
Functions Can Return Objects
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Example below instantiates a student object
Prompts for and sets its property values
Then returns the instantiated object
Dim freshman As Student = GetStudent()
…
Function GetStudent() As Student
Dim s As New Student()
s.FirstName = InputBox("Enter first name.")
s.LastName = InputBox("Enter last name.")
s.IdNumber = InputBox("Enter ID number.")
s.TestAvg = CSng(InputBox("Enter test average."))
Return s
End Function
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 31
Class Methods
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In addition to properties, a class may also contain
Sub procedures and functions
Methods are Sub procedures and functions
defined in a class
Typically operate on data stored in the class
The following slide shows a Clear method for the
Student class
 Method called with freshman.Clear()

Method clears member data in the Student
class object referenced by freshman
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 32
Clear Method for Student Class
Public Class Student
' Member variables
Private strLastName As String
Private strFirstName As String
Private strId As String
Private sngTestAvg As Single
(...Property procedures omitted...)
'Holds
'Holds
'Holds
'Holds
last name
first name
ID number
test avg
' Clear method
Public Sub Clear()
strFirstName = String.Empty
strLastName = String.Empty
strId = String.Empty
sngTestAvg = 0.0
End Sub
End Class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 33
Constructors
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A constructor is a method called automatically
when an instance of the class is created
Think of constructors as initialization routines
Useful for initializing member variables or
performing other startup operations
To create a constructor, simply create a Sub
procedure named New within the class
Next slide shows a Student class constructor
 The statement freshman = New Student()
 Creates an instance of the Student class
 Executes constructor to initialize properties of
the Student object referenced by freshman
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 34
Constructor Example
Public Class Student
' Member variables
Private strLastName As String
Private strFirstName As String
Private strId As String
Private sngTestAvg As Single
'Holds
'Holds
'Holds
'Holds
last name
first name
ID number
test avg
' Constructor
Public Sub New()
strFirstName = "(unknown)"
strLastName = "(unknown)"
strId = "(unknown)"
sngTestAvg = 0.0
End Sub
(The rest of this class is omitted.)
End Class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 35
Finalizers
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VB provides a class method named Finalize
Called automatically just before garbage collector
removes an instance of a class
Select Finalize from method name drop-down list
to let Visual Basic create the following template
Protected Overrides Sub Finalize()
MyBase.Finalize()
End Sub

Add your code following MyBase.Finalize()
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 36
Using the Output Window
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Usually shown at bottom of Visual Studio display
If not appearing, click View, Other Windows, then
Output to display
A valuable debugging tool
Messages about an application displayed here
Can add your own messages to Output window
using Debug.WriteLine method
Debug.WriteLine(Output)
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Causes the expression Output to appear in the
Output window
Insert following code in form Load event to enable
Debug.Listeners.Add(New ConsoleTraceListener())
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Slide 12- 37
Create a Class
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Tutorial 12-1 demonstrates code needed to
create the Student class
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Slide 12- 38
12.3
Collections
A Collection Holds a Group of Items
It Automatically Expands and Shrinks in Size
to Accommodate the Items Added to It and
Allows Items to Be Stored With Associated Key
Values, Which May Be Used in Searches
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Collections
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A collection is similar to an array
A single unit that contains several items
Can access items in a collection by numeric index
A collection’s indices begin at one, not zero
Collections automatically expand and shrink as
items are added and removed
The items stored in a collection do not have to be
of the same type
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 40
A Collection is a Class
Dim customers As Collection
customers = New Collection()
' Or alternatively
Dim customers As New Collection()
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New collections are instantiations of the
Collection Class
The Collection Class provides methods and
properties for use with individual collections
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 41
Adding Items to a Collection
Object.Add(Item [, Key] [, Before] [,After])
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Add is a method of the Collection class
Object is a variable that refers to a collection
Item can be an object, variable, or value that is
added to the collection
Key is a unique value optionally used to identify
a member of the collection
Before or After optionally specifies where a new
item should be placed in the collection
Default is to insert at the end of the collection
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 42
Uses of Before and After

Add custData with key "Smith" before the item
with key "Thomas“
customers.Add(custData, "Smith", "Thomas")

Add custData with key "Smith" after the item with
key "Reece“
customers.Add(custData, "Smith",, "Reece")

Add custData after 3rd item in collection
customers.Add(custData,,,3)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 43
Add Method Exceptions

An exception can occur when adding to a
collection so Try-Catch should be used
 Cannot add member with same key as another
member of the collection
 If a key or index is specified for a Before or
After, the value must exist
Try
customers.Add(custData, "Smith")
Catch ex as ArgumentException
MessageBox.Show(ex.Message)
End Try
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 44
Accessing Item by Their Indices
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Can access an item in a collection using an index
value
Index value can be used in two ways:
 Using the collection’s Item method
Object.Item(Index)
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Item is the default method so it can be omitted
Object(Index)
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Get value at index 3 of names collection by:
names.Item(3) –or- names(3)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 45
IndexOutOfRange Exception
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If an invalid index is encountered, an index out of
range exception will occur
Should use Try-Catch to trap such messages
Ctype casts collection object to Customer object
Try
Dim cust as Customer
‘Get collection index from user input
Dim index as Integer = CInt(txtIndex.Text)
‘Locate the customer in the collection
cust = CType(customers.Item(index), Customer)
Catch ex as IndexOutOfRangeException
MessageBox.Show(ex.Message)
End Try
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 46
The Count Property
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The Count property of a collection gives the
number of current items in the collection
Note that, unlike an array, a collection index is not
zero based
 First item in a collection found at index 1
Following code adds collection items to a list box
Dim intX As Integer
For intX = 1 To names.Count
lstNames.Items.Add(names(intX).ToString())
Next intX
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 47
Searching for an Item by Key Value
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Item method can be used to retrieve an item with
a specific index
Object.Item(Expression)
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If Expression is a string, it is used as a key to
search for a member with a matching key
If Expression is numeric, it is used as an index
value for the item
If no item is found (via key or index), an
exception occurs
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 48
Retrieving Item Examples

Find studentCollection item with key 49812
 If Option Strict on, must cast result to Student
Dim s as Student
s = CType(studentCollection.Item(“49812”), Student)

Retrieve all members by index and display
LastName property in a message box
Dim intIndex as Integer
Dim s as Student
For intIndex = 1 to studentCollection.Count
s = Ctype(studentCollection.Item(intIndex), Student)
MessageBox.Show(s.LastName)
Next intIndex
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 49
Using References Versus Copies

When an Item in a collection is a fundamental VB
data type, only a copy is retrieved
 This code does not change the item at index 1
Dim intNum as Integer
intNum = CType(numbers(1), Integer)
intNum = 0

The Item in this collection is an object so:
 A reference is returned instead of a copy
 LastName of object in collection is changed
Dim s as Student
s = CType(studentCollection.Item("49812"), Student)
s.LastName = "Griffin"
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 50
For Each Loop with a Collection

Can use a For Each loop to read members of a
collection
 Eliminates the counter variable required to use
a For…Next
 Also no need to compare to Count property
Dim s As Student
For Each s In studentCollection
MessageBox.Show(s.LastName)
Next s
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 51
Removing Members of a Collection
Object.Remove(Expression)
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Remove is a method of the Collection class
Object refers to collection Member removed from
Expression can be
 Numeric and interpreted as an index
 Or a string and interpreted as a key value
 Exception thrown if Expression not found
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 52
Removing Member Examples

Verify “49812” is a key value in collection, then
remove member with this key value
If studentCollection.Contains(“49812”) Then
studentCollection.Remove("49812")
End If

Verify index location 7 exists in collection, then
remove member at this index location
Dim intIndex As Integer = 7
If intIndex > 0 and _
intIndex <= studentCollection.Count Then
studentCollection.Remove(intIndex)
End If
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 53
Working with Collections
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Since a collection is an instance of a class
 Procedures accept collections as arguments
 Functions can return a collection
 Follow same guidelines as any class object
Parallel collections work like parallel arrays
 Can use index to relate parallel collections just
as we did with arrays
 Or can use key values to relate collections
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Slide 12- 54
12.4
The Student Collection
Application
Create an Application that Uses a
Collection of Student Objects
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Student Collection Application Forms
 Select student ID number
from list box to see detail
information for the student
 Click Remove Student
button remove an instance
of the Student class
 Click Add Student button to
create a new instance of the
Student class
 Add Student form
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 56
12.5
The Object Browser
The Object Browser Is a Dialog
Box That Allows You to Browse All
Classes and Components Available
to Your Project
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Using the Object Browser
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
A dialog box with information about objects used
in a project
Allows you to examine
 Information about forms used in a project
 Classes created for the project
 Other components used by VB in the project
Tutorial 12-3 uses the Object Browser to examine
the Student Collection project
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Slide 12- 58
Object Browser, Example
Student class selected
Class members shown
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Slide 12- 59
12.6
Scroll Bars and Track Bars
The HScrollBar, VScrollBar, and
TrackBar Controls Provide a
Graphical Way to Adjust a Number
Within a Range of Values
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Visual Appearance and Usage
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HScrollBar and VScrollBar
look like normal scroll bars
TrackBar has an arrow pointer
as the slider with tick marks
Scrollable controls hold
integers in their Value property
 Position of slider corresponds to Value
 Move scroll bar to increase or decrease Value
 Right increases, left decreases horizontal bar
 Up increases, down decreases vertical bar
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 61
Scrollable Control Properties
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Minimum – the bar’s lowest possible value
Maximum – the bar's highest possible value
Value – the bar's value at the current position
LargeChange – change in the Value property with
a mouse click on/near the slider
SmallChange – change in the Value property for
a mouse click on an arrow at the end
TickFrequency - for TrackBar only, the number of
units between tick marks

With min=0 and max=1000, if Tick Frequency is 100,
10 tick marks are shown on the bar
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 62
Coding for Scrollable Controls
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Any change to the position of a scroll bar
generates a Scroll event
 Allows program to react to a shift in scroll bar
Standard prefixes for these controls
 Horizontal scroll bar is hsb
 Vertical scroll bar is vsb
 TrackBar is tb
Tutorial 12-4 demonstrates how to set up Scroll
events and use of these controls
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 63
12.7
Introduction to Inhertance
Inheritance Allows a New Class to be
Based on an Existing Class
The New Class Inherits the Accessible Member
Variables, Methods, and Properties of the Class on
Which It Is Based
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Why Inheritance?
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Inheritance allows new classes to derive their
characteristics from existing classes
The Student class may have several types of
students such as
 GraduateStudent
 ExchangeStudent
 StudentEmployee
These can become new classes and share all the
characteristics of the Student class
Each new class would then add specialized
characteristics that differentiate them
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 65
Base and Derived Classes
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The Base Class is a general-purpose class that
other classes may be based on
A Derived Class is based on the base class and
inherits characteristics from it
Can think of the base class as a parent and the
derived class as a child
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 66
The Vehicle Class (Base Class)
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Consider a Vehicle class with the following:
 Private variable for number of passengers
 Private variable for miles per gallon
 Public property for number of passengers
(Passengers)
 Public property for miles per gallon
(MilesPerGallon)
This class holds general data about a vehicle
Can create more specialized classes from the
Vehicle class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 67
The Truck Class (Derived Class)
Public Class Truck
Inherits Vehicle
' Other new properties
' Additional methods
End Class

Declared as:
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Truck class derived from Vehicle class
 Inherits all non-private methods, properties,
and variables of Vehicle class
Truck class defines two properties of its own
 MaxCargoWeight – holds top cargo weight
 FourWheelDrive – indicates if truck is 4WD
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Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 68
Instantiating the Truck Class
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Instantiated as:
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Values stored in MaxCargoWeight and
FourWheelDrive properties
 Properties declared explicitly by Truck class
Values also stored in MilesPerGallon and
Passengers properties
 Properties inherited from Vehicle class
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Dim pickUp as New Truck()
pickUp.Passengers = 2
pickUp.MilesPerGallon = 18
pickUp.MaxCargoWeight = 2000
Pickup.FourWheelDrive = True
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 69
Overriding Properties and Methods
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Sometimes a base class property procedure or
method must work differently for a derived class
 Can override base class method or property
 Must write the method or property as desired in
the derived class using same name
When an object of the derived class accesses the
property or calls the method
 VB uses overridden version in derived class
 Version in base class is not used
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 70
Property Override Example
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Vehicle class has no restriction on number of
passengers
But may wish to restrict the Truck class to two
passengers at most
Can override Vehicle class Passengers property
by:
 Coding Passengers property in derived class
 Specify Overridable in base class property
 Specify Overrides in derived class property
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 71
Overridable Base Class Property
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Overridable keyword added to base class
property procedure
Public Overridable Property Passengers() As Integer
Get
Return intPassengers
End Get
Set(ByVal value As Integer)
intPassengers = value
End Set
End Property
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 72
Overridden Derived Class Property
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Overrides keyword and new logic added to
derived class property procedure
Public Overrides Property Passengers() As Integer
Get
Return MyBase.Passengers
End Get
Set(ByVal value As Integer)
If value >= 1 And value <= 2 Then
MyBase.Passengers = value
Else
MessageBox.Show("Passengers must be 1 or 2", _
"Error")
End If
End Set
End Property
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 73
Overriding Methods
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Overriding a method is similar to a property
Specify Overridable and Overrides keywords
An overridable base class method
Public Overridable Sub ProcedureName()
Public Overridable Function ProcedureName() As DataType

An overriding derived class method
Public Overrides Sub ProcedureName()
Public Overrides Function ProcedureName() As DataType
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 74
Overriding the ToString Method
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Every programmer created class is derived from
a built-in class named Object
Object class has a method named ToString which
returns a fully-qualified class name
Method can be overridden to return a string
representation of data stored in an object
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 75
ToString Override Example
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Object class ToString method is Overridable
Vehicle class might override the ToString method
as shown below
' Overriden ToString method
Public Overrides Function ToString() As String
' Return a string representation
' of a vehicle.
Dim str As String
str = "Passengers: " & intPassengers.ToString & _
" MPG: " & sngMPG.ToString
Return str
End Function
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 76
Base & Derived Class Constructors
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A constructor (named New) may be defined for
both the base class and a derived class
When a new object of the derived class is
created, both constructors are executed
 The constructor of the base class will be called
first
 Then the constructor of the derived class will
be called
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 77
Protected Members
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In addition to Private and Public, the access
specifier may be Protected
 Protected base class members are treated as
public to classes derived from this base
 Protected base class members are treated as
private to classes not derived from this base

Tutorial 12-5 provides an opportunity to work with
base and derived classes
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Slide 12- 78