10/5/2008
Objectives
Applied
• Given the specifications for an application that uses classes with
any of the members presented in this chapter, develop the
application and its classes.
Chapter 11
• Use class diagrams, the Class View window, and the Class Details
window to review, delete, and start the members of the classes in
a solution.
solution
How to create
and
d use classes
l
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 1
Murach’s Visual Basic 2008, C11
Slide 2
© 2008, Mike Murach & Associates, Inc.
The architecture of a three-layer application
Objectives (continued)
Knowledge
• List and describe the three layers of a three-layered application.
• Describe these members of a class: constructor, method, field, and
property.
Presentation
layer
Main Windows
form class
Other form
classes
Middle layer
Business
B
i
classes
Other supportt
Oth
classes
Database
classes
Database
• Describe the concept of encapsulation.
• Explain
p
how instantiation works.
• Describe the concept of overloading a method.
• Explain what a shared member is.
• Describe the difference between a class and a structure.
Database layer
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 3
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
The members of a Product class
The members of a Product class (continued)
Properties
Constructors
Code
Description
Price
Method
GetDisplayText(sep)
Murach’s Visual Basic 2008, C11
Description
A String type that contains a product code.
A String type that contains a product
description.
A Decimal type that contains the product’s
price.
Description
Returns a string that contains the code,
description, and price in a displayable
format. The sep parameter is a string that’s
used to separate the elements.
© 2008, Mike Murach & Associates, Inc.
New()
Slide 4
Description
Creates a Product object with default values.
New(code, description, price)
Creates a Product object using the specified
code, description, and price values.
Slide 5
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 6
1
10/5/2008
Types of class members
Types of class members (continued)
Class member Description
Property
Represents a data value associated with an object
instance.
Method
An operation that can be performed by an object.
Constructor
A special type of method that’s executed when an
object is instantiated.
Delegate
A special type of object that
that’ss used to wire an event
to a method.
Event
A signal that notifies other objects that something
noteworthy has occurred.
Field
A variable that’s declared at the class level.
Constant
A constant.
Class member Description
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 =.
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 7
Murach’s Visual Basic 2008, C11
Class and object concepts
The Product class
• An object is a self-contained unit that has properties, methods,
and other members.
Public Class Product
© 2008, Mike Murach & Associates, Inc.
Slide 8
Private m_Code As String
Private m_Description As String
Private m_Price As Decimal
• A class contains the code that defines the members of an object.
• An object is an instance of a class, and the process of creating an
object is called instantiation.
Public Sub New()
• Encapsulation is one of the fundamental concepts of objectoriented pprogramming.
g
g It lets yyou control the data and operations
p
within a class that are exposed to other classes.
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
• The data of a class is typically encapsulated within a class using
data hiding. In addition, the code that performs operations within
the class is encapsulated so it can be changed without changing
the way other classes use it.
• Although a class can have many different types of members, most
of the classes you create will have just properties, methods, and
constructors.
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 9
The Product class (continued)
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 10
The Product class (continued)
Public Property Code() As String
Get
Return m_Code
End Get
Set(ByVal value As String)
m_Code = value
End Set
End Property
Public Property Price() As Decimal
Get
Return m_Price
End Get
Set(ByVal value As Decimal)
m_Price = value
End Set
End Property
Public Property Description() As String
Get
Return m_Description
End Get
Set(ByVal value As String)
m_Description = value
End Set
End Property
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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 11
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 12
2
10/5/2008
Two Product objects that have been instantiated
from the Product class
product1
Code=CS08
Description=
Murach’s C# 2008
Price=54.50
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Code that creates the two Product object instances
Dim product1, product2 As Product
product1 = New Product("CS08", "Murach's C# 2008", 54.5D)
product2 = _
New Product("VB08", "Murach's Visual Basic 2008", 54.5D)
product2
Code=VB08
Description=Murach’s
Visual Basic 2008
Price=54.50
Another way to create the object instances
product1 = New Product With {.Code = "CS08", _
.Description = "Murach's C#
# 2008
2008", .Price = 54.5D}
product2 = New Product With {.Code = "VB08", _
.Description = "Murach's Visual Basic 2008", _
.Price = 54.5D}
Slide 13
Code that checks if an object variable refers to an
object
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 14
The dialog box for adding a class
Dim product As Product
.
.
If Product IsNot Nothing Then...
Code that disassociates an object from an object
variable
Dim product As Product
product = New Product("VB08", _
"Murach's Visual Basic 2008", 54.5D)
.
.
product = Nothing
The starting code for the new class
Public Class Product
End Class
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 15
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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 16
A version of the Product 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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 17
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 18
3
10/5/2008
The Product class with public fields (continued)
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
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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 19
A read/write property
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 20
A statement that sets a property value
Public Property Code() As String
Get
Return m_Code
End Get
Set(ByVal value As String)
m_Code = value
End Set
End Property
product.Code = txtProductCode.Text
A statement that gets a property value
Dim code As String = product.Code
A read-only
yp
property
p y
Public ReadOnly Property DiscountAmount() As Decimal
Get
m_DiscountAmount = Subtotal * DiscountPercent
Return m_DiscountAmount
End Get
End Property
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 21
A method that accepts a parameter
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 22
Two statements that call the method
Public Function GetDisplayText(ByVal sep As String) _
As String
Dim text As String = Code & sep _
& FormatCurrency(Price) _
& sep & Description
Return text
End Function
lblProduct.Text = product.GetDisplayText(vbTab)
lblProduct.Text = product.GetDisplayText()
How the IntelliSense feature lists overloaded
methods
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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 23
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 24
4
10/5/2008
A constructor with no parameters
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
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.Description
Me.Price = price
End Sub
Statements that call the constructors
Dim product1 As New Product
Dim product2 As New Product("VB08", _
"Murach's Visual Basic 2008", 54.5D)
Dim product3 As New Product(txtCode.Text)
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 25
© 2008, Mike Murach & Associates, Inc.
Slide 26
A class that contains shared members
Default values for instance variables
Data type
All numeric types
Boolean
Char
Date
Object
Murach’s Visual Basic 2008, C11
Public Class Validator
Default value
0 (zero)
False
Null (binary 0)
01/01/01 00:00:00
Null reference
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
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 27
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 28
The Product Maintenance form
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(txtDescription)
Validator.IsPresent(txtPrice) Then
isValidData = True
Else
isValidData = False
End If
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
The New Product form
Slide 29
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 30
5
10/5/2008
The Tag property settings for the text boxes
on the New Product form
The Product class
Control
txtCode
txtDescription
txtPrice
Code
Property
Tag property setting
Code
Description
Price
Description
Price
Method
GetDisplayText(sep)
Constructor
New()
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.
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 31
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
The ProductDB class
The Validator class
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.
Property
N t
Note
IsPresent(textBox)
s ese t(te t o )
• 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.
Title
Shared method
IsInt32(textBox)
IsDecimal(textBox)
Slide 32
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 w
wass eentered
e ed into
o thee text
e box.
bo .
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.
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 33
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
© 2008, Mike Murach & Associates, Inc.
© 2008, Mike Murach & Associates, Inc.
Slide 34
The Product Maintenance form (continued)
The code for the Product Maintenance form
Murach’s Visual Basic 2008, C11
Murach’s Visual Basic 2008, C11
Slide 35
Private Sub btnAdd_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnAdd.Click
Dim newProductForm As New frmNewProduct
newProductForm.ShowDialog()
If newProductForm.Product IsNot Nothing Then
products.Add(newProductForm.Product)
ProductDB.SaveProducts(products)
Me.FillProductListBox()
End If
End Sub
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 36
6
10/5/2008
The Product Maintenance form (continued)
The Product Maintenance form (continued)
Private Sub btnDelete_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnDelete.Click
Dim i As Integer = lstProducts.SelectedIndex
If i <> -1 Then
Dim product As Product = products(i)
Dim message As String = _
"Are you sure you want to delete " _
& product.Description & "?"
Dim button As DialogResult = _
MessageBox.Show(message, _
"Confirm Delete", MessageBoxButtons.YesNo)
If button = DialogResult.Yes Then
products.Remove(product)
ProductDB.SaveProducts(products)
Me.FillProductListBox()
End If
End If
End Sub
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 37
Private Sub btnExit_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnExit.Click
Me.Close()
End Sub
End Class
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 38
The New Product form (continued)
The code for the New Product form
Private Sub btnCancel_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnCancel.Click
Me.Close()
End Sub
Public Class frmNewProduct
Public Product As Product
Private Sub btnSave_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnSave.Click
If IsValidData() Then
Product = New Product(txtCode.Text, _
txtDescription.Text, CDec(txtPrice.Text))
Me.Close()
End If
End Sub
End Class
Private Function IsValidData() As Boolean
Return Validator.IsPresent(txtCode) AndAlso _
Validator.IsPresent(txtDescription) AndAlso _
Validator.IsPresent(txtPrice) AndAlso _
Validator.IsDecimal(txtPrice)
End Function
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 39
The code for the Validator class
Slide 40
Else
Return True
End If
End Function
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 IsDecimal( _
ByVal textBox As TextBox) As Boolean
Try
Convert.ToDecimal(textBox.Text)
Return True
Catch ex As FormatException
MessageBox.Show(textBox.Tag.ToString _
& " must be a decimal value.", Title)
textBox.Select(0, textBox.Text.Length)
Return False
End Try
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
© 2008, Mike Murach & Associates, Inc.
© 2008, Mike Murach & Associates, Inc.
The Validator class (continued)
Public Class Validator
Murach’s Visual Basic 2008, C11
Murach’s Visual Basic 2008, C11
End Function
End Class
Slide 41
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 42
7
10/5/2008
The Class View window* for the Product
Maintenance application
A class diagram* that shows two of the classes in
the project
*Not available in Visual Basic 2008 Express Edition
*Not available in Visual Basic 2008 Express Edition
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 43
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 44
A Product structure
The basic syntax for creating a structure
Public Structure StructureName
member declarations
End Structure
Public Structure Product
Public Code As String
Public Description As String
Public Price As Decimal
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
Public Function GetDisplayText( _
ByVal sep As String) As String
Return Code & sep & FormatCurrency(Price) _
& sep & Description
End Function
End Structure
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 45
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 46
Code that declares a variable as a structure type
and assigns values to it
Dim p As Product
' Create an instance of the Product structure.
p.Code = "VB08"
' Assign values to each instance variable.
p.Description = "Murach's Visual Basic 2008"
p.Price = 54.5D
Dim message As String = p.GetDisplayText(vbCrLf)
' Call a method.
Code that uses the structure’s constructor
Dim p As New Product("VB08", _
"Murach's Visual Basic 2008", 54.5D)
Murach’s Visual Basic 2008, C11
© 2008, Mike Murach & Associates, Inc.
Slide 47
8