Lecture Set 11
Creating and Using Classes
Part B – Class Features – Constructors,
Methods, Fields, Properties, Shared Data
Objectives
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Describe these members of a class: constructor,
method, field, and property
Explain how instantiation works
Describe the concept of overloading a method
Explain what a shared member is
Common Class Elements
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(repeated)
Classes contain other elements:
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Procedures (methods) – functions and subs – operations
that can be performed by an object.
Constructors – special type of method that’s executed when
an object is instantiated
Data stores (including properties)
 Other forms – objects of the form data type
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Numerous objects of types defined by other classes
Variables (of primitive types)
Events (and Event handlers -- not always tied to forms) –
an event is a signal that notifies other objects that
something noteworthy has occurred
Common Class Elements
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User defined data types – structures, enumerations,
complex structured types (arrays of structures, lists or
arrays of objects, etc)
Delegate - A special type of object that’s used to wire an
event to a method
Data Stores
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(continued - repeated)
Property - Represents a data value associated with an object
instance
Field - A variable that’s declared at the class level
Constant
- A constant associated with the class
Default property - A special type of property that is used by
default if a property name isn’t specified. It must include a
parameter, which is typically used to access individual items
within a class that represents a collection of objects.
Operator - A special type of method that’s performed for a
Visual Basic operator such as + or =.
How Instantiation Works
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The process of creating an object from a
class is called instantiation
You can create many instances of a single
class
A class defines a reference type
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The variable that is created for the object of a
class thus contains a reference to the memory
location where the object is stored and not the
object itself
When an object is instantiated the appropriate
constructor is executed
Fields
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Fields are class variables that are global to
(accessible by) all of the executable
components of a class
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Usually declared as private and are not
accessible outside the class
May also be declared as public or protected and
may also be declared using the friend modifier
Fields
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(continued)
The desirable goal in using classes is to
encapsulate and protect class data stores from
external reference except through methods of
the class
Declaring fields as public defeats the purpose of
using classes
If accessing some class data stores from outside
the class is considered cumbersome, we can
access them through the use of properties,
which simplifies the access mechanism we have
to use while still protecting the data
Examples of Field Declarations
Private m_quantity As Integer
' A private field
Public Price As Decimal
' A public field
Public ReadOnly Limit As Integer = 90
' A public read-only field
A Class that Uses Public Fields (Instead of Properties)
Public Class Product
Public Code As String
Public Description As String
Public Price As Decimal
Public Sub New()
End Sub
Public Sub New(ByVal code As String, _
ByVal description As String, _
ByVal price As Decimal)
Me.Code = code
Me.Description = description
Me.Price = price
End Sub
Shared versus Instance Data
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Class data can be instance data or shared
data
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Shared data is declared with the Shared
keyword
 One copy exists no matter the number of
class instances
One copy of instance data exists for each class
instance
How are shared data useful? 
A Note on Designing with Shared Data
NOTE: A shared function operates within the context of shared data – it can access
any shared data in a class, but it cannot access instance members
How is the shared counter NextHuman used below?
Public Class Human
'*** instance members
Private ID As Integer
Public Function GetInfo() As String
Return "Human #" & CStr(ID)
End Function
'*** shared members
Private Shared NextHuman As Integer = 1
Public Shared Function GetNextHuman() As Human
Dim temp As New Human()
temp.ID = NextHuman
NextHuman += 1
Return temp
End Function
End Class
'*** factory method
The Product Class with Public Fields (continued)
Public Function GetDisplayText(ByVal sep As String) _
As String
Dim text As String = Code & sep _
& FormatCurrency(Price) _
& sep & Description
Return text
End Function
End Class
Using Properties to Access Private Data

If we wish to provide more convenient access
to the private or protected data of a class, we
can do so through the use of public
properties
The Syntax for Coding a Public Property
Public [ReadOnly|WriteOnly] Property PropertyName As type
[Get
[statements]
{PropertyName = propertyvalue} | {Return propertyvalue}
End Get]
[Set(ByVal variableName As type)
[statements]
propertyvalue = newvalue
End Set]
End Property
Different Types of Properties
A read/write property
Public Property Code() As String
Get
Return m_Code
End Get
Set(ByVal value As String)
m_Code = value
End Set
End Property
A read-only property
Public ReadOnly Property DiscountAmount() As Decimal
Get
m_DiscountAmount = Subtotal * DiscountPercent
Return m_DiscountAmount
End Get
End Property
Accessing Properties
A statement that sets a property value
product.Code = txtProductCode.Text
A statement that gets a property value
Dim code As String = product.Code
Methods
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Classes contain subs and functions
The public subs and functions are called
methods
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There also can be private subs and functions
I call these methods as well (although the book
does not seem to)
Why might we want a private method?
Methods perform operations on the data
stores (attributes) of a class
Method Signatures and Overloading
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A method’s signature is defined by its name
and its unique combination of parameters
Note that the return value is not involved – it
is assumed to always be the same
We can write methods having the same name
but different sets of parameters 
This is known as overloading
Overloading provides multiple ways of
invoking a given method
Methods and Overloading
A method that accepts a parameter
Public Function GetDisplayText(ByVal sep As String) _
As String
Dim text As String = Code & sep _
& FormatCurrency(Price) _
& sep & Description
Return text
End Function
An overloaded version of the method that doesn’t
accept parameters
Public Function GetDisplayText() As String
Dim text As String = Code & ", " _
& FormatCurrency(Price) _
& ", " & Description
Return text
End Function
Overloading and Intellisense
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How does Intellisense handle overloaded
functions (when it shows you the parameter
list)
Two statements that call the method
lblProduct.Text = product.GetDisplayText(vbTab)
lblProduct.Text = product.GetDisplayText()
How the IntelliSense feature lists overloaded
methods
Constructors
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By default, VB will provide you with a default
constructor for each class you write
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When you use new to create a new object (an
instance of a class), the VB default constructor
assigns default values to all instance variables of
the new object
If this is not what you want, you can write your
own constructor(s) by taking advantage of
overloading
How does the compiler know which constructor
is being referenced when you use New applied
to a type (class) with multiple constructors
Overloading Constructors
A constructor with no parameters
Public Sub New()
End Sub
A constructor with three parameters
Public Sub New(ByVal code As String, _
ByVal description As String, _
ByVal price As Decimal)
Me.Code = code
Me.Description = description
Me.Price = price
End Sub
More on Constructors
A constructor with one parameter
Public Sub New(ByVal code As String)
Dim p As Product = ProductDB.GetProduct(code)
Me.Code = p.Code
Me.Description = p.Description
Me.Price = p.Price
End Sub
Statements that call the constructors
Dim product1 As New Product
Dim product2 As New Product("VB05", _
"Murach's Visual Basic 2005", 52.5D)
Dim product3 As New Product(txtCode.Text)
Default Values for Instance Variables
Data type
All numeric types
Boolean
Char
Date
Object
Default value
0 (zero)
False
Null (binary 0)
01/01/01 00:00:00
Null reference
Murach’s Visual Basic
© 2006, Mike Murach
2005, C11
Slide 24
A Class that Contains Shared Members (optional)
Public Class Validator
Private Shared m_Title As String = "Entry Error"
Public Shared Property Title() As String
Get
Return m_Title
End Get
Set(ByVal value As String)
m_Title = value
End Set
End Property
Public Shared Function IsPresent(ByVal textBox _
As TextBox) As Boolean
If textBox.Text = "" Then
MessageBox.Show(textBox.Tag.ToString _
& " is a required field.", Title)
textBox.Select()
Return False
Slide 25
A class that contains shared members (cont.)
Else
Return True
End If
End Function
End Class
Code that uses shared members
If Validator.IsPresent(txtCode) AndAlso _
Validator.IsPresent(txtDescription) AndAlso _
Validator.IsPresent(txtPrice) Then
isValidData = True
Else
isValidData = False
End If
Class Shared Members
(optional)
A class that contains shared members (cont.)
Else
Return True
End If
End Function
End Class
Code that uses shared members
If Validator.IsPresent(txtCode) AndAlso _
Validator.IsPresent(txtDescription) AndAlso _
Validator.IsPresent(txtPrice) Then
isValidData = True
Else
isValidData = False
End If
The Product Class
Property
Code
Description
Price
Method
GetDisplayText(sep)
Constructor
New()
(optional)
Description
A String variable; contains a product code.
A String variable; contains a description of
the product.
A Decimal variable; contains the product’s
price.
Description
Returns a string with the code, description,
and price separated by the sep string.
Description
Creates a Product object with default values.
New(code, description, price)
Creates a Product object using the specified
values.
The ProductDB Class (optional)
Method
Description
GetProducts()
A shared method that returns a List() of
Product objects from the Products file.
SaveProducts(list)
A shared method that writes the products in
the specified List() of Product objects to the
Products file.
Note
 You don’t need to know how the ProductDB class works. You just
need to know about its methods so you can use them in your code.
The Validator Class
Property
Title
Shared method
IsPresent(textBox)
IsInt32(textBox)
IsDecimal(textBox)
(optional)
Description
A shared string; contains the text that’s
displayed in the title bar of a dialog box for an
error message.
Returns a Boolean value that indicates
whether…
Data was entered into the text box.
An Integer type was entered into the text box.
A Decimal type was entered into the text box.
IsWithinRange(textBox, min, max)
The value entered into the text box is within
the specified range.
The Code for the Product Maintenance Form (optional)
Public Class frmProductMaint
Private products As List(Of Product)
Private Sub frmProductMaint_Load( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
products = ProductDB.GetProducts
Me.FillProductListBox()
End Sub
Private Sub FillProductListBox()
lstProducts.Items.Clear()
For Each p As Product In products
lstProducts.Items.Add(p.GetDisplayText(vbTab))
Next
End Sub