1. Chapter 8 – Object-Based Programming
Outline Classes as User Defined Types Value-Types and Reference-Types 8.4   Controlling Access to Members 8.5   Constructors 8.6   Overloaded Constructors 8.7   Properties 8.8   Composition: Objects as Instance Variables of Other Classes 8.9   Using the Me Reference 8.10   Garbage Collection 8.11   Shared Class Members 8.12   Const and ReadOnly Members 8.13   Data Abstraction and Information Hiding 8.15   Namespaces and Assemblies

2. Classes as User Defined Types (1) (JPF)
May be used to represent a user defined type
Example: Time data type
Syntactically much similar to a module
Data and operations on the data are defined together
Instance variables
Represent the data (internal data structure) of an instance of the type
Example: hour, minutes and seconds
Instance methods (functions and procedures)
Represent operations on instances of the type
Example: change the time (procedure), convert to string (function)

3. Classes as User Defined Types (2) (JPF)
Encapsulation (private and public)
Encapsulation is a fundamental principle of object orientation
Instance variables are usually declared private, so that clients of the class do not have direct access to them (can only have indirect access to them via public methods)
Clients of a class do not need to know its internal data structure
Advantage: integrity of the data (example: 0<=hour<=23, ...) is completely controlled within the class
Advantage: the internal data structure may be changed (example: time as seconds elapsed since a reference point), without affecting the clients of the class (as long as the syntax and semantics of public methods is not changed)
Special methods named New (constructors) take care of initializing the instance variables of newly allocated instances (object) of the class
Auxiliary internal methods should also be declared private

4. Classes as User Defined Types (3) (JPF)
Objects and variables
Instances of a class are called objects
An object stores the values of the instance variables
A variable of class type holds a reference to an object and not the data (or object) itself
This is different from what happens with a primitive type
Dim i as Integer = 123
Dim t as CTime = New CTime(10,30, 0) i
123 t1
a variable of type CTime
an object of type CTime

5. Value-Types and Reference-Types (JPF)
Primitive types such as Integer are value-types
Variables of a value-type contain data
Comparison and assignment act on values
Reference types
Classes (user defined or defined in the .Net class libraries) are reference-types
Variable of a reference-type contain references to objects (or Nothing when no object is referenced)
Objects are created with New
New returns a reference to a newly allocated object
The result of New is usually assigned to a variable for later use
Comparison and assignment act on references

6. Value-Types and Reference-Types (JPF)
Example
Dim t1 as CTime = New CTime(10,30, 0)
Dim t2 as CTime = New CTime(10,30, 0)
Dim t3 as Ctime = t1
if t1 = t ' dá FALSE
if t1 = t3 ' dá TRUE t3 t1 t2

7. Class CTime inherits existing pieces of class Object. CTime.vb
1 ' Fig. 8.1: CTime.vb
2 ' Represents time in 24-hour format. 3
4 Class CTime
Inherits Object 6
' declare Integer instance values for hour, minute and second
Private mHour As Integer '
Private mMinute As Integer '
Private mSecond As Integer ' 11
' Method New is the CTime constructor method, which initializes
' instance variables to zero
Public Sub New()
SetTime(0, 0, 0)
End Sub ' New 17
' set new time value using universal time;
' perform validity checks on data;
' set invalid values to zero
Public Sub SetTime(ByVal hourValue As Integer, _
ByVal minuteValue As Integer, ByVal secondValue As Integer) 23
' check if hour is between 0 and 23, then set hour
If (hourValue >= 0 AndAlso hourValue < 24) Then
mHour = hourValue
Else
mHour = 0
End If 30
Class CTime inherits existing pieces of class Object.
CTime.vb
Introduces a New constructor and constructors do not return values

8. 31 ' check if minute is between 0 and 59, then set minute
If (minuteValue >= 0 AndAlso minuteValue < 60) Then
mMinute = minuteValue
Else
mMinute = 0
End If 37
' check if second is between 0 and 59, then set second
If (secondValue >= 0 AndAlso secondValue < 60) Then
mSecond = secondValue
Else
mSecond = 0
End If 44
End Sub ' SetTime 46
' convert String to universal-time format
Public Function ToUniversalString() As String
Return String.Format("{0}:{1:D2}:{2:D2}", _
mHour, mMinute, mSecond)
End Function ' ToUniversalString 52
' convert to String in standard-time format
Public Function ToStandardString() As String
Dim suffix As String = " PM"
Dim format As String = "{0}:{1:D2}:{2:D2}"
Dim standardHour As Integer 58
' determine whether time is AM or PM
If mHour < 12 Then
suffix = " AM"
End If 63
Error checks input values for variables hourValue, minuteValue and secondValue
CTime.vb
Argument 0 takes default value zero Arguments 1 and 2 take decimal number format

9. 64 ' convert from universal-time format to standard-time format
If (mHour = 12 OrElse mHour = 0) Then
standardHour = 12
Else
standardHour = mHour Mod 12
End If 70
Return String.Format(format, standardHour, mMinute, _
mSecond) & suffix
End Function ' ToStandardString 74
75 End Class ' CTime
CTime.vb

10. TimeTest.vb 1 ' Fig. 8.2: TimeTest.vb 2 ' Demonstrating class CTime. 3
4 Imports System.Windows.Forms ' para não ter de fazer System.Windows.Forms.MessageBox 5
6 Module modTimeTest 7
Sub Main()
Dim time As CTime = New CTime() ' call CTime constructor
Dim output As String 11
output = "The initial universal times is: " & _
time.ToUniversalString() & vbCrLf & _
"The initial standard time is: " & _
time.ToStandardString() 16
time.SetTime(13, 27, 6) ' set time with valid settings 18
output &= vbCrLf & vbCrLf & _
"Universal time after setTime is: " & _
time.ToUniversalString() & vbCrLf & _
"Standard time after setTime is: " & _
time.ToStandardString() 24
time.SetTime(99, 99, 99) ' set time with invalid settings 26
output &= vbCrLf & vbCrLf & _
"After attempting invalid settings: " & vbCrLf & _
"Universal time: " & time.ToUniversalString() & _
vbCrLf & "Standard time: " & time.ToStandardString() 31
MessageBox.Show(output, "Testing Class CTime")
End Sub ' Main 34
35 End Module ' modTimeTest
TimeTest.vb

11. TimeTest.vb

12. 8.4 Controlling Access to Members
Public versus Private
Control access to a class’s instance variables and methods
Public
Serve primarily to present interfaces of a class
Private
Holds clients private data safely
Get and set functions
Have ability to access private data
Class members that are visible can be accessed only through a “handle” (ObjectReferenceName.memberName)

13. Cannot access a Private variable
1 ' Fig. 8.3: RestrictedAccess.vb
2 ' Demonstrate error from attempt to access Private class member. 3
4 Module modRestrictedAccess 5
Sub Main()
Dim time As New CTime() 8
time.mHour = 7 ' error
End Sub ' Main 11
12 End Module ' modRestrictedAccess
RestrictedAccess.vb
Cannot access a Private variable

14. 8.5 Initializing Class Objects: Constructors
Initializing Constructors
False for Booleans and Nothing for references
If an instance variable is not initialized the compiler will assign a default value
Form of declarations
Dim ObjectReference As New ClassName(arguments)
Programs without default constructor are provided with one by the compiler

15. 8.6 Using Overloaded Constructors
Must have different numbers and/or types and/or orders of parameters

16. 1 ' Fig. 8.4: CTime2.vb
2 ' Represents time and contains overloaded constructors. 3
4 Class CTime2
Inherits Object 6
' declare Integers for hour, minute and second
Private mHour As Integer '
Private mMinute As Integer '
Private mSecond As Integer ' 11
' constructor initializes each variable to zero and
' ensures that each CTime2 object starts in consistent state
Public Sub New()
SetTime()
End Sub ' New 17
' CTime2 constructor: hour supplied;
' minute and second default to 0
Public Sub New(ByVal hourValue As Integer)
SetTime(hourValue)
End Sub ' New 23
' CTime2 constructor: hour and minute supplied;
' second defaulted to 0
Public Sub New(ByVal hourValue As Integer, _
ByVal minuteValue As Integer) 28
SetTime(hourValue, minuteValue)
End Sub ' New 31
' CTime2 constructor: hour, minute and second supplied
Public Sub New(ByVal hourValue As Integer, _
ByVal minuteValue As Integer, ByVal secondValue As Integer) 35
CTime2.vb

17. CTime2.vb 36 SetTime(hourValue, minuteValue, secondValue)
End Sub ' New 38
' CTime2 constructor: another CTime2 object supplied
Public Sub New(ByVal timeValue As CTime2)
SetTime(timeValue.mHour, timeValue.mMinute, timeValue.mSecond)
End Sub ' New 43
' set new time value using universal time;
' perform validity checks on data;
' set invalid values to zero
Public Sub SetTime(Optional ByVal hourValue As Integer = 0, _
Optional ByVal minuteValue As Integer = 0, _
Optional ByVal secondValue As Integer = 0) 50
' perform validity checks on hour, then set hour
If (hourValue >= 0 AndAlso hourValue < 24) Then
mHour = hourValue
Else
mHour = 0
End If 57
' perform validity checks on minute, then set minute
If (minuteValue >= 0 AndAlso minuteValue < 60) Then
mMinute = minuteValue
Else
mMinute = 0
End If 64
' perform validity checks on second, then set second
If (secondValue >= 0 AndAlso secondValue < 60) Then
mSecond = secondValue
Else
mSecond = 0
End If
CTime2.vb

18. CTime2.vb 71 72 End Sub ' SetTime 73
' convert String to universal-time format
Public Function ToUniversalString() As String
Return String.Format("{0}:{1:D2}:{2:D2}", _
mHour, mMinute, mSecond)
End Function ' ToUniversalString 79
' convert to String in standard-time format
Public Function ToStandardString() As String
Dim suffix As String = " PM"
Dim format As String = "{0}:{1:D2}:{2:D2}"
Dim standardHour As Integer 85
' determine whether time is AM or PM
If mHour < 12 Then
suffix = " AM"
End If 90
' convert from universal-time format to standard-time format
If (mHour = 12 OrElse mHour = 0) Then
standardHour = 12
Else
standardHour = mHour Mod 12
End If 97
Return String.Format(format, standardHour, mMinute, _
mSecond) & suffix
End Function ' ToStandardString
101
102 End Class ' CTime2
CTime2.vb

19. TimeTest2.vb 1 ' Fig. 8.5: TimeTest2.vb
2 ' Demonstrates overloading constructors. 3
4 Imports System.Windows.Forms 5
6 Module modTimeTest2 7
Sub Main() 9
' use overloaded constructors
Dim time1 As New CTime2()
Dim time2 As New CTime2(2)
Dim time3 As New CTime2(21, 34)
Dim time4 As New CTime2(12, 25, 42)
Dim time5 As New CTime2(27, 74, 99)
Dim time6 As New CTime2(time4) ' use time4 as initial value 17
Const SPACING As Integer = 13 ' spacing between output text 19
' invoke time1 methods
Dim output As String = "Constructed with: " & vbCrLf & _
" time1: all arguments defaulted" & vbCrLf & _
Space(SPACING) & time1.ToUniversalString() & _
vbCrLf & Space(SPACING) & time1.ToStandardString() 25
' invoke time2 methods
output &= vbCrLf & _
" time2: hour specified; minute and second defaulted" & _
vbCrLf & Space(SPACING) & _
time2.ToUniversalString() & vbCrLf & Space(SPACING) & _
time2.ToStandardString() 32
TimeTest2.vb

20. TimeTest2.vb 33 ' invoke time3 methods 34 output &= vbCrLf & _
" time3: hour and minute specified; second defaulted" & _
vbCrLf & Space(SPACING) & time3.ToUniversalString() & _
vbCrLf & Space(SPACING) & time3.ToStandardString() 38
' invoke time4 methods
output &= vbCrLf & _
" time4: hour, minute and second specified" & _
vbCrLf & Space(SPACING) & time4.ToUniversalString() & _
vbCrLf & Space(SPACING) & time4.ToStandardString() 44
' invoke time5 methods
output &= vbCrLf & _
" time5: hour, minute and second specified" & _
vbCrLf & Space(SPACING) & time5.ToUniversalString() & _
vbCrLf & Space(SPACING) & time5.ToStandardString() 50
' invoke time6 methods
output &= vbCrLf & _
" time6: Time2 object time4 specified" & vbCrLf & _
Space(SPACING) & time6.ToUniversalString() & _
vbCrLf & Space(SPACING) & time6.ToStandardString() 56
MessageBox.Show(output, _
"Demonstrating Overloaded Constructor")
End Sub ' Main 60
61 End Module ' modTimeTest2
TimeTest2.vb

21. TimeTest2.vb

22. 8.7 Properties Private and Public Get accessor Set accessor
In Visual Basic instance variables as private does not guarantee
data integrity
Set accessor
Cannot return values indicating a failed attempt to assign invalid data to objects of the class
Control the setting of instance variables to valid values
Get and Set accessors are not required
A property with only Get accessor is called ReadOnly
A property with only Set accessor is called WriteOnly

23. 1 ' Fig. 8.6: CTime3.vb
2 ' Represents time in 24-hour format and contains properties. 3
4 Class CTime3
Inherits Object 6
' declare Integers for hour, minute and second
Private mHour As Integer
Private mMinute As Integer
Private mSecond As Integer 11
' CTime3 constructor: initialize each instance variable to zero
' and ensure that each CTime3 object starts in consistent state
Public Sub New()
SetTime(0, 0, 0)
End Sub ' New 17
' CTime3 constructor:
' hour supplied, minute and second defaulted to 0
Public Sub New(ByVal hourValue As Integer)
SetTime(hourValue, 0, 0)
End Sub ' New 23
' CTime3 constructor:
' hour and minute supplied; second defaulted to 0
Public Sub New(ByVal hourValue As Integer, _
ByVal minuteValue As Integer) 28
SetTime(hourValue, minuteValue, 0)
End Sub ' New 31
' CTime3 constructor: hour, minute and second supplied
Public Sub New(ByVal hourValue As Integer, _
ByVal minuteValue As Integer, ByVal secondValue As Integer) 35
CTime3.vb

24. CTime3.vb 36 SetTime(hourValue, minuteValue, secondValue)
End Sub ' New 38
' CTime3 constructor: another CTime3 object supplied
Public Sub New(ByVal timeValue As CTime3)
SetTime(timeValue.mHour, timeValue.mMinute, _
timeValue.mSecond)
End Sub ' New 44
' set new time value using universal time;
' uses properties to perform validity checks on data
Public Sub SetTime(ByVal hourValue As Integer, _
ByVal minuteValue As Integer, ByVal secondValue As Integer) 49
Hour = hourValue ' looks
Minute = minuteValue ' dangerous
Second = secondValue ' but it is correct
End Sub ' SetTime 54
' property Hour
Public Property Hour() As Integer 57
' return mHour value
Get
Return mHour
End Get 62
' set mHour value
Set(ByVal value As Integer) 65
If (value >= 0 AndAlso value < 24) Then
mHour = value
Else
mHour = 0
End If
CTime3.vb

25. CTime3.vb 71 72 End Set 73 74 End Property ' Hour 75
' property Minute
Public Property Minute() As Integer 78
' return mMinute value
Get
Return mMinute
End Get 83
' set mMinute value
Set(ByVal value As Integer) 86
If (value >= 0 AndAlso value < 60) Then
mMinute = value
Else
mMinute = 0
End If 92
End Set 94
End Property ' Minute 96
' property Second
Public Property Second() As Integer 99
' return mSecond value
Get
Return mSecond
End Get
104
CTime3.vb

26. CTime3.vb 105 ' set mSecond value 106 Set(ByVal value As Integer) 107
If (value >= 0 AndAlso value < 60) Then
mSecond = value
Else
mSecond = 0
End If
113
End Set
115
End Property ' Second
117
' convert String to universal-time format
Public Function ToUniversalString() As String
Return String.Format("{0}:{1:D2}:{2:D2}", _
mHour, mMinute, mSecond)
End Function ' ToUniversalString
123
' convert to String in standard-time format
Public Function ToStandardString() As String
Dim suffix As String = " PM"
Dim format As String = "{0}:{1:D2}:{2:D2}"
Dim standardHour As Integer
129
' determine whether time is AM or PM
If mHour < 12 Then
suffix = " AM"
End If
134
CTime3.vb

27. 135 ' convert from universal-time format to standard-time format
If (mHour = 12 OrElse mHour = 0) Then
standardHour = 12
Else
standardHour = mHour Mod 12
End If
141
Return String.Format(format, standardHour, mMinute, _
mSecond) & suffix
End Function ' ToStandardString
145
146 End Class ' CTime3
CTime3.vb

28. TimeTest3.vb 1 ' Fig. 8.7: TimeTest3.vb 2 ' Demonstrates Properties. 3
4 Imports System.Windows.Forms 5
6 Class FrmTimeTest3
Inherits Form 8
' Label and TextBox for hour
Friend WithEvents lblSetHour As Label
Friend WithEvents txtSetHour As TextBox 12
' Label and TextBox for minute
Friend WithEvents lblSetMinute As Label
Friend WithEvents txtSetMinute As TextBox 16
' Label and TextBox for second
Friend WithEvents lblSetSecond As Label
Friend WithEvents txtSetSecond As TextBox 20
' Labels for outputting time
Friend WithEvents lblOutput1 As Label
Friend WithEvents lblOutput2 As Label 24
' Button for adding one second to time
Friend WithEvents cmdAddSecond As Button 27
Dim time As New CTime3() 29
' Visual Studio .NET generated code 31
TimeTest3.vb

29. TimeTest3.vb 32 ' update time display 33 Private Sub UpdateDisplay()
lblOutput1.Text = "Hour: " & time.Hour & "; Minute: " & _
time.Minute & "; Second: " & time.Second 36
lblOutput2.Text = "Standard time is: " & _
time.ToStandardString & "; Universal Time is: " _
& time.ToUniversalString()
End Sub ' UpdateDisplay 41
' invoked when user presses Add Second button
Protected Sub cmdAddSecond_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdAddSecond.Click 46
' add one second
time.Second = (time.Second + 1) Mod 60
txtSetSecond.Text = time.Second 50
' add one minute if 60 seconds have passed
If time.Second = 0 Then
time.Minute = (time.Minute + 1) Mod 60
txtSetMinute.Text = time.Minute 55
' add one hour if 60 minutes have passed
If time.Minute = 0 Then
time.Hour = (time.Hour + 1) Mod 24
txtSetHour.Text = time.Hour
End If 61
End If 63
UpdateDisplay()
End Sub ' cmdAddSecond_Click 66
TimeTest3.vb

30. TimeTest3.vb 67 ' handle event when txtSetHour's text changes
Protected Sub txtSetHour_TextChanged(ByVal sender As _
System.Object, ByVal e As System.EventArgs) _
Handles txtSetHour.TextChanged 71
time.Hour = Convert.ToInt32(txtSetHour.Text)
UpdateDisplay()
End Sub ' txtSetHour_TextChanged 75
' handle event when txtSetMinute's text changes
Protected Sub txtSetMinute_TextChanged(ByVal sender As _
System.Object, ByVal e As System.EventArgs) _
Handles txtSetMinute.TextChanged 80
time.Minute = Convert.ToInt32(txtSetMinute.Text)
UpdateDisplay()
End Sub ' txtSetMinute_TextChanged 84
' handle event when txtSetSecond's text changes
Protected Sub txtSetSecond_TextChanged(ByVal sender _
As System.Object, ByVal e As System.EventArgs) _
Handles txtSetSecond.TextChanged 89
time.Second = Convert.ToInt32(txtSetSecond.Text)
UpdateDisplay()
End Sub ' txtSetSecond_TextChanged 93
94 End Class ' FrmTimeTest3
TimeTest3.vb

31. TimeTest3.vb

32. 8.8 Composition: Objects as Instance Variables of Other Classes
Referencing Existing Objects
Software Reuse:
A form of composition is software reuse

33. CDay.vb 1 ' Fig. 8.8: CDay.vb 2 ' Encapsulates month, day and year. 3
4 Imports System.Windows.Forms 5
6 Class CDay
Inherits Object 8
Private mMonth As Integer ' 1-12
Private mDay As Integer ' 1-31 based on month
Private mYear As Integer ' any year 12
' constructor confirms proper value for month, then calls
' method CheckDay to confirm proper value for day
Public Sub New(ByVal monthValue As Integer, _
ByVal dayValue As Integer, ByVal yearValue As Integer) 17
' ensure month value is valid
If (monthValue > 0 AndAlso monthValue <= 12) Then
mMonth = monthValue
Else
mMonth = 1 23
' inform user of error
Dim errorMessage As String = _
"Month invalid. Set to month 1." 27
MessageBox.Show(errorMessage, "", _
MessageBoxButtons.OK, MessageBoxIcon.Error)
End If 31
mYear = yearValue
mDay = CheckDay(dayValue) ' validate day 34
End Sub ' New
CDay.vb

34. CDay.vb 36 37 ' confirm proper day value based on month and year
Private Function CheckDay(ByVal testDayValue As Integer) _
As Integer 40
Dim daysPerMonth() As Integer = _
{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31} 43
If (testDayValue > 0 AndAlso _
testDayValue <= daysPerMonth(mMonth)) Then 46
Return testDayValue
End If 49
' check for leap year in February
If (mMonth = 2 AndAlso testDayValue = 29 AndAlso _
mYear Mod 400 = 0 OrElse mYear Mod 4 = 0 AndAlso _
mYear Mod 100 <> 0) Then 54
Return testDayValue
Else 57
' inform user of error
Dim errorMessage As String = _
"day " & testDayValue & "invalid. Set to day 1. " 61
MessageBox.Show(errorMessage, "", _
MessageBoxButtons.OK, MessageBoxIcon.Error) 64
Return 1 ' leave object in consistent state
End If 67
End Function ' CheckDay 69
CDay.vb

35. CDay.vb 70 ' create string containing month/day/year format
Public Function ToStandardString() As String
Return mMonth & "/" & mDay & "/" & mYear
End Function ' ToStandardString 74
75 End Class ' CDay
CDay.vb

36. CEmployee.vb 1 ' Fig. 8.9: CEmployee.vb
2 ' Represent employee name, birthday and hire date. 3
4 Class CEmployee
Inherits Object 6
Private mFirstName As String
Private mLastName As String
Private mBirthDate As CDay ' member object reference
Private mHireDate As CDay ' member object reference 11
' CEmployee constructor
Public Sub New(ByVal firstNameValue As String, _
ByVal lastNameValue As String, _
ByVal birthMonthValue As Integer, _
ByVal birthDayValue As Integer, _
ByVal birthYearValue As Integer, _
ByVal hireMonthValue As Integer, _
ByVal hireDayValue As Integer, _
ByVal hireYearValue As Integer) 21
mFirstName = firstNameValue
mLastName = lastNameValue 24
' create CDay instance for employee birthday
mBirthDate = New CDay(birthMonthValue, birthDayValue, _
birthYearValue) 28
' create CDay instance for employee hire date
mHireDate = New CDay(hireMonthValue, hireDayValue, _
hireYearValue)
End Sub ' New 33
CEmployee.vb

37. 34 ' return employee information as standard-format String
Public Function ToStandardString() As String
Return mLastName & ", " & mFirstName & " Hired: " _
& mHireDate.ToStandardString() & " Birthday: " & _
mBirthDate.ToStandardString()
End Function ' ToStandardString 40
41 End Class ' CEmployee
CEmployee.vb

38. CompositionTest.vb 1 ' Fig. 8.10: CompositionTest.vb
2 ' Demonstrate an object with member object reference. 3
4 Imports System.Windows.Forms 5
6 Module modCompositionTest 7
Sub Main()
Dim employee As New CEmployee( _
"Bob", "Jones", 7, 24, 1949, 3, 12, 1988) 11
MessageBox.Show(employee.ToStandardString(), _
"Testing Class Employee")
End Sub ' Main 15
16 End Module ' modCompositionTest
CompositionTest.vb

39. 8.9 Using the Me Reference Me Reference
Every object can access a reference to itself using a Me reference.
Me explicitly
Me implicitly
The explicit use of the Me reference can increase program clarity where Me is optional

40. Reference Me can refer to any instance variable explicitly
1 ' Fig. 8.11: CTime4.vb
2 ' Encapsulate time using Me reference. 3
4 Class CTime4
Private mHour, mMinute, mSecond As Integer 6
' CTime4 constructor
Public Sub New(ByVal mHour As Integer, _
ByVal mMinute As Integer, ByVal mSecond As Integer) 10
Me.mHour = mHour
Me.mMinute = mMinute
Me.mSecond = mSecond
End Sub ' New 15
' create String using Me and implicit references
Public Function BuildString() As String
Return "Me.ToUniversalString(): " & Me.ToUniversalString() _
& vbCrLf & "ToUniversalString(): " & ToUniversalString()
End Function ' BuildString 21
' convert to String in standard-time format
Public Function ToUniversalString() As String
Return String.Format("{0:D2}:{1:D2}:{2:D2}", _
mHour, mMinute, mSecond)
End Function ' ToUniversalString 27
28 End Class ' CTime4
CTime4.vb
Reference Me can refer to any instance variable explicitly
Uses reference Me explicitly
Uses reference Me implicitly

41. MeTest.vb 1 ' Fig. 8.12: MeTest.vb 2 ' Demonstrates Me reference. 3
4 Imports System.Windows.Forms 5
6 Module modMeTest 7
Sub Main()
Dim time As New CTime4(12, 30, 19) 10
MessageBox.Show(time.BuildString(), _
"Demonstrating the 'Me' Reference")
End Sub ' Main 14
15 End Module ' modMeTest
MeTest.vb

42. * Finalizer methods
Finalizer method performs termination housekeeping on that object just before the garbage collector reclaims the object's memory.

43. Accessed with ClassName.SharedMemberName
8.11 Shared Class Members
Shared Class Variable
Contains only one copy of this variable in memory
When a single copy of the data will suffice, use Shared class variables to save storage.
Shared class variables are not the same as global variables because Shared class variables have class scope
Shared Class Method
have no Me reference
Accessed with ClassName.SharedMemberName

44. Initializes mCount to 0 by default
1 ' Fig. 8.13: CEmployee2.vb
2 ' Class CEmployee2 uses Shared variable. 3
4 Class CEmployee2
Inherits Object 6
Private mFirstName As String
Private mLastName As String 9
' number of objects in memory
Private Shared mCount As Integer 12
' CEmployee2 constructor
Public Sub New(ByVal firstNameValue As String, _
ByVal lastNameValue As String) 16
mFirstName = firstNameValue
mLastName = lastNameValue 19
mCount += 1 ' increment shared count of employees
Console.WriteLine _
("Employee object constructor: " & mFirstName & _
" " & mLastName)
End Sub ' New 25
' finalizer method decrements Shared count of employees
Protected Overrides Sub Finalize()
mCount -= 1 ' decrement mCount, resulting in one fewer object
Console.WriteLine _
("Employee object finalizer: " & mFirstName & _
" " & mLastName & "; count = " & mCount)
End Sub ' Finalize 33
CEmployee2.vb
Initializes mCount to 0 by default
CEmployee2 constructor increments mCount Finalize method decrements mCount

45. Member mCount can be referenced without declaring a CEmployee2 object
' return first name
Public ReadOnly Property FirstName() As String 36
Get
Return mFirstName
End Get 40
End Property ' FirstName 42
' return last name
Public ReadOnly Property LastName() As String 45
Get
Return mLastName
End Get 49
End Property ' LastName 51
' property Count
Public ReadOnly Shared Property Count() As Integer 54
Get
Return mCount
End Get 58
End Property ' Count 60
61 End Class ' CEmployee2
CEmployee2.vb
Member mCount can be referenced without declaring a CEmployee2 object

46. Sets objects’ references to Nothing
1 ' Fig. 8.14: SharedTest.vb
2 ' Demonstrates Shared members. 3
4 Imports System.Windows.Forms 5
6 Module modSharedTest 7
Sub Main()
Dim output As String 10
Console.WriteLine("Employees before instantiation: " & _
CEmployee2.Count) 13
Dim employee1 As CEmployee2 = _
New CEmployee2("Susan", "Baker") 16
Dim employee2 As CEmployee2 = _
New CEmployee2("Bob", "Jones") 19
' output of employee2 after instantiation
Console.WriteLine(vbCrLf & _
"Employees after instantiation: " & vbCrLf & _
"via Employee.Count: " & CEmployee2.Count) 24
' display name of first and second employee
Console.WriteLine(vbCrLf & "Employees 1: " & _
employee1.FirstName & " " & employee1.LastName & _
vbCrLf & "Employee 2: " & employee2.FirstName & " " & _
employee2.LastName) 30
' mark employee1 and employee2 for garbage collection
employee1 = Nothing
employee2 = Nothing 34
SharedTest.vb
Declares two CEmployee2 objects and increments mCount by two
Sets objects’ references to Nothing

47. SharedTest.vb 35 System.GC.Collect() ' request garbage collection
End Sub ' Main 37
38 End Module ' modShared
SharedTest.vb
Employees before instantiation: 0
Employee object constructor: Susan Baker
Employee object constructor: Bob Jones
Employees after instantiation:
via Employee.Count: 2
Employees 1: Susan Baker
Employee 2: Bob Jones
Employee object finalizer: Bob Jones; count = 1
Employee object finalizer: Susan Baker; count = 0

48. 8.12 Const and ReadOnly Members
Const or ReadOnly
Const
A data member must be initialized in its declaration
Cannot be modified once initialized
ReadOnly
A data member can be initialized either in the class structure or in its declaration

49. CCircleConstants.vb 1 ' Fig. 8.15: CCircleConstants.vb
2 ' Encapsulate constants PI and radius. 3
4 Class CCircleConstants 5
' PI is constant data member
Public Const PI As Double = 8
' radius is uninitialized constant
Public ReadOnly RADIUS As Integer 11
' constructor of class CCircleConstants
Public Sub New(ByVal radiusValue As Integer)
RADIUS = radiusValue
End Sub ' New 16
17 End Class ' CCircleConstants
CCircleConstants.vb

50. ConstAndReadOnly.vb 1 ' Fig. 8.16: ConstAndReadOnly.vb
2 ' Demonstrates Const and ReadOnly members. 3
4 Imports System.Windows.Forms 5
6 Module modConstAndReadOnly 7
Sub Main()
Dim random As Random = New Random()
Dim circle As CCircleConstants = _
New CCircleConstants(random.Next(1, 20)) 12
Dim radius As String = Convert.ToString(circle.RADIUS) 14
Dim output As String = "Radius = " & radius & vbCrLf _
& "Circumference = " + String.Format("{0:N3}", _
circle.RADIUS * 2 * CCircleConstants.PI) 18
MessageBox.Show(output, "Circumference", _
MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub ' Main 22
23 End Module ' modConstAndReadOnly
ConstAndReadOnly.vb

51. 8.15 Namespaces and Assemblies
.NET Framework Class Library
.NET Framework:
Must be imported to a Visual Basic program by including a reference to those libraries
Namespaces:
Namespaces help minimize naming collisions by proving a convention for unique class names
By default, every executable file you create with Visual Basic .NET contains a namespace with the same name as your project. For example, if you define an object within a project named ListBoxProject, the executable file, ListBoxProject.exe, contains a namespace called ListBoxProject.
If you use the Imports statement without an alias, you can use all the names in that namespace without qualification provided they are unique to the project.

52. CEmployee3.vb 1 ' Fig. 8.17: CEmployee3.vb
2 ' Class CEmployee3 uses Shared variable. 3
4 Public Class CEmployee3
Inherits Object 6
Private mFirstName As String
Private mLastName As String 9
' number of objects in memory
Private Shared mCount As Integer 12
' CEmployee3 constructor
Public Sub New(ByVal firstNameValue As String, _
ByVal lastNameValue As String) 16
mFirstName = firstNameValue
mLastName = lastNameValue 19
mCount += 1 ' increment shared count of employees
Console.WriteLine _
("Employee object constructor: " & mFirstName & _
" " & mLastName)
End Sub ' New 25
' finalizer method decrements Shared count of employees
Protected Overrides Sub Finalize()
mCount -= 1 ' decrement mCount, resulting in one fewer object
Console.WriteLine _
("Employee object finalizer: " & mFirstName & _
" " & mLastName & "; count = " & mCount)
End Sub ' Finalize 33
CEmployee3.vb

53. CEmployee3.vb 34 ' return first name
Public ReadOnly Property FirstName() As String 36
Get
Return mFirstName
End Get 40
End Property ' FirstName 42
' return last name
Public ReadOnly Property LastName() As String 45
Get
Return mLastName
End Get 49
End Property ' LastName 51
' property Count
Public ReadOnly Shared Property Count() As Integer 54
Get
Return mCount
End Get 58
End Property ' Count 60
61 End Class ' CEmployee3
CEmployee3.vb

54. 8.15 Namespaces
Fig Simple Class Library.

55. The module imports class CEmployee3
1 ' Fig. 8.19: AssemblyTest.vb
2 ' Demonstrates assembly files and namespaces. 3
4 Imports EmployeeLibrary ' contains class CEmployee3 5
6 Module modAssemblyTest 7
Public Sub Main()
Dim output As String 10
Console.WriteLine("Employees before instantiation: " & _
CEmployee3.Count) 13
Dim employee1 As CEmployee3 = _
New CEmployee3("Susan", "Baker") 16
Dim employee2 As CEmployee3 = _
New CEmployee3("Bob", "Jones") 19
' output of employee after instantiation
Console.WriteLine(vbCrLf & "Employees after instantiation:" _
& vbCrLf & "via Employee.Count: " & CEmployee3.Count) 23
' display name of first and second employee
Console.WriteLine(vbCrLf & "Employees 1: " & _
employee1.FirstName & " " & employee1.LastName & _
vbCrLf & "Employee 2: " & employee2.FirstName & " " & _
employee2.LastName) 29
' mark employee1 and employee2 for garbage collection
employee1 = Nothing
employee2 = Nothing 33
AssemblyTest.vb
The module imports class CEmployee3

56. AssemblyTest.vb 34 System.GC.Collect() ' request garbage collection
End Sub ' Main 36
37 End Module ' modAssemblyTest
AssemblyTest.vb
Employees before instantiation: 0
Employee object constructor: Susan Baker
Employee object constructor: Bob Jones
Employees after instantiation:
via Employee.Count: 2
Employees 1: Susan Baker
Employee 2: Bob Jones
Employee object finalizer: Bob Jones; count = 1
Employee object finalizer: Susan Baker; count = 0

57. * 8.16 Class View and Object Browser
Displays a project’s class members
To access this feature select VIEW>CLASS VIEW
(+) node called collapsed
(-) node is called expanded
Object Browser
Lists the Framework Class Library Available in Visual Basic
To access this feature, right click any Visual Basic class or method in the code editor and select Go To Definition
It illustrates the functionally provided by a specific object

58. 8.16 Class View and Object Browser
Fig Class View of Fig. 8.1 and Fig. 8.2.

59. 8.16 Class View and Object Browser
Fig Object Browser when user selects Object from CTime.vb,

60. 8.16 Class View and Object Browser
Fig Object Browser when user selects Object from CTime.vb,

61. Classes as Real World Concepts (JPF)
Examples in a banking application:
Customer
holds data about a customer (number, name, address, ...)
provides operations on customers (create, update the address, query accounts, ...)
Account
holds data about an account (number, owners, movements, balance, ...)
provides operations on accounts (create, deposit, withdrawal, query movements, query balance, query owners, ...)
Related with each other