1. Chapters 9, 10 – Object-Oriented Programming: Inheritance
Outline Base Classes and Derived Classes, Is-a Relationship, and Inheritance Protected Members Method Overriding Constructors in Derived Classes (*)Finalizers in Derived Classes Substitutability Derived-Class-Object to Base-Class-Object Conversion Polymorphism Abstract Classes (MustInherit) Abstract Methods and Properties (MustOverride) NotInheritable Classes and NotOverridable Methods Interfaces (*) Delegates

2. Base classes and derived classes
inherits the features of
is a (special case of / kind of)
Fig. 9.1 Some simple inheritance examples.

3. Base Classes and Derived Classes
CommunityMember
Employee
Student
Alumnus
Faculty
Staff
Administrator
Teacher
Fig. 9.2 Inheritance hierarchy for university CCommunityMembers.

4. Base Classes and Derived Classes
CShape
CTwoDimensionalShape
CThreeDimensionalShape
CCircle
CSquare
CTriangle
CSphere
CCube
CCylinder
Fig. 9.3 Portion of a CShape class hierarchy.

5. accessible by derived classes but not by other classes
1 ' Fig. 9.9: Point2.vb
2 ' CPoint2 class contains an x-y coordinate pair as Protected data. 3
4 Public Class CPoint2
' implicitly Inherits Object 6
' point coordinate
Protected mX, mY As Integer 9
' default constructor
Public Sub New() 12
' implicit call to Object constructor occurs here
X = 0
Y = 0
End Sub ' New 17
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer) 21
' implicit call to Object constructor occurs here
X = xValue
Y = yValue
End Sub ' New 26
' property X
Public Property X() As Integer 29
Get
Return mX
End Get 33
accessible by derived classes but not by other classes

6. needed to override the method with the same name inherited from object
Set(ByVal xValue As Integer)
mX = xValue ' no need for validation
End Set 37
End Property ' X 39
' property Y
Public Property Y() As Integer 42
Get
Return mY
End Get 46
Set(ByVal yValue As Integer)
mY = yValue ' no need for validation
End Set 50
End Property ' Y 52
' return String representation of CPoint2
Public Overrides Function ToString() As String
Return "[" & mX & ", " & mY & "]"
End Function ' ToString 57
58 End Class ' CPoint2
needed to override the method with the same name inherited from object

7. 1 ' Fig. 9.10: Circle3.vb
2 ' CCircle3 class that inherits from class CPoint2. 3
4 Public Class CCircle3
Inherits CPoint2 ' CCircle3 Inherits from class CPoint2 6
Private mRadius As Double ' CCircle3's radius 8
' default constructor
Public Sub New() 11
' implicit call to CPoint constructor occurs here
Radius = 0
End Sub ' New 15
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer, ByVal radiusValue As Double) 19
' implicit call to CPoint2 constructor occurs here
mX = xValue
mY = yValue
Radius = radiusValue
End Sub ' New 25
' property Radius
Public Property Radius() As Double 28
Get
Return mRadius
End Get 32
Set(ByVal radiusValue As Double) 34
Circle3.vb
The keyword Inherits indicates inheritance CCircle3 inherits all members of class CPoint2 except constructors
Protected members of the base class may be accessed by the derived class (private members cannot), but not by client classes

8. Function ToString overrides function ToString in class CPoint’s
If radiusValue > 0
mRadius = radiusValue
End If 38
End Set 40
End Property ' Radius 42
' calculate CCircle3 diameter
Public Function Diameter() As Double
Return mRadius * 2
End Function ' Diameter 47
' calculate CCircle3 circumference
Public Function Circumference() As Double
Return Math.PI * Diameter()
End Function ' Circumference 52
' calculate CCircle3 area
Public Overridable Function Area() As Double
Return Math.PI * mRadius ^ 2
End Function ' Area 57
' return String representation of CCircle3
Public Overrides Function ToString() As String
Return "Center = " & "[" & mX & ", " & mY & "]" & _
"; Radius = " & mRadius
End Function ' ToString 63
64 End Class ' CCircle3
CCircle.vb
Function ToString overrides function ToString in class CPoint’s

9. CCircle.vb 1 ' Fig. 9.11: CircleTest3.vb 2 ' Testing class CCircle3. 3
4 Imports System.Windows.Forms 5
6 Module modCircleTest3 7
Sub Main()
Dim circle As CCircle3
Dim output As String 11
circle = New CCircle3(37, 43, 2.5) ' instantiate CCircle3 13
' get CCircle3's initial x-y coordinates and radius
output = "X coordinate is " & circle.X & vbCrLf & _
"Y coordinate is " & circle.Y & vbCrLf & "Radius is " & _
circle.Radius 18
' set CCircle3's x-y coordinates and radius to new values
circle.X = 2
circle.Y = 2
circle.Radius = 4.25 23
' display CCircle3's String representation
output &= vbCrLf & vbCrLf & _
"The new location and radius of circle are " & _
vbCrLf & circle.ToString() & vbCrLf 28
' display CCircle3's diameter
output &= "Diameter is " & _
String.Format("{0:F}", circle.Diameter()) & vbCrLf 32
CCircle.vb

10. CircleTest.vb 33 ' display CCircle3's circumference
output &= "Circumference is " & _
String.Format("{0:F}", circle.Circumference()) & vbCrLf 36
' display CCircle3's area
output &= "Area is " & String.Format("{0:F}", circle.Area()) 39
MessageBox.Show(output, "Demonstrating Class CCircle3")
End Sub ' Main 42
43 End Module ' modCircleTest3
CircleTest.vb

11. Constructors and (*)Finalizers in Derived Classes
Constructors in Derived Classes
Base-class
Base-class constructors are not inherited by derived classes
Each base-class constructor initializes the base-class instance variables that the derived-class object inherits
* Finalizers in Derived Classes
Derived Classes
Executing the finalizer method should free all resources acquired by the object before the garbage collector reclaims memory for that object
Keyword MyBase is used to invoke the finalizer of the base class (in the end)

12. MyBase.New invokes the CPoint constructor explicitly
1 ' Fig. 9.12: Circle4.vb
2 ' CCircle4 class that inherits from class CPoint. 3
4 Public Class CCircle4
Inherits CPoint ' CCircle4 Inherits from class CPoint 6
Private mRadius As Double 8
' default constructor
Public Sub New() 11
' implicit call to CPoint constructor occurs here
Radius = 0
End Sub ' New 15
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer, ByVal radiusValue As Double) 19
' use MyBase reference to CPoint constructor explicitly
MyBase.New(xValue, yValue)
Radius = radiusValue
End Sub ' New 24
' property Radius
Public Property Radius() As Double 27
Get
Return mRadius
End Get 31
Set(ByVal radiusValue As Double) 33
Circle4.vb
MyBase.New invokes the CPoint constructor explicitly

13. Circle4.vb 34 If radiusValue > 0 35 mRadius = radiusValue 36 End If37
End Set 39
End Property ' Radius 41
' calculate CCircle diameter
Public Function Diameter() As Double
Return mRadius * 2
End Function ' Diameter 46
' calculate CCircle4 circumference
Public Function Circumference() As Double
Return Math.PI * Diameter()
End Function ' Circumference 51
' calculate CCircle4 area
Public Overridable Function Area() As Double
Return Math.PI * mRadius ^ 2
End Function ' Area 56
' return String representation of CCircle4
Public Overrides Function ToString() As String 59
' use MyBase reference to return CPoint String representation
Return "Center= " & MyBase.ToString() & _
"; Radius = " & mRadius
End Function ' ToString 64
65 End Class ' CCircle4
Circle4.vb

14. CircleTest4.vb 1 ' Fig. 9.13: CircleTest4.vb
2 ' Testing class CCircle4. 3
4 Imports System.Windows.Forms 5
6 Module modCircleTest4 7
Sub Main()
Dim circle As CCircle4
Dim output As String 11
circle = New CCircle4(37, 43, 2.5) ' instantiate CCircle4 13
' get CCircle4's initial x-y coordinates and radius
output = "X coordinate is " & circle.X & vbCrLf & _
"Y coordinate is " & circle.Y & vbCrLf & "Radius is " & _
circle.Radius 18
' set CCircle4's x-y coordinates and radius to new values
circle.X = 2
circle.Y = 2
circle.Radius = 4.25 23
' display CCircle4's String representation
output &= vbCrLf & vbCrLf & _
"The new location and radius of circle are " & _
vbCrLf & circle.ToString() & vbCrLf 28
' display CCircle4's diameter
output &= "Diameter is " & _
String.Format("{0:F}", circle.Diameter()) & vbCrLf 32
CircleTest4.vb

15. CircleTest4.vb 33 ' display CCircle4's circumference
output &= "Circumference is " & _
String.Format("{0:F}", circle.Circumference()) & vbCrLf 36
' display CCircle4's area
output &= "Area is " & String.Format("{0:F}", circle.Area()) 39
MessageBox.Show(output, "Demonstrating Class CCircle4")
End Sub ' Main 42
43 End Module ' modCircleTest4
CircleTest4.vb

16. The opposite is not true
Substitutability
An object of a derived class can be treated as an object of its base class
An object of a derived class may be passed where an object of its base class is expected
The opposite is not true
A base-class object is not an object of any of its derived classes
Assigning a base-class reference to a derived-class reference causes InvalidCastException

17. Derived-Class-Object to Base-Class-Object Conversion
Also called "upcast" (up in the inheritance tree)
Always possible
Performed implicitly
A consequence of substitutability

18. Polymorphism
Dim ref_derived = new DerivedClassName()
Dim ref_base as BaseClassName
ref_base = ref_derived
ref_base.MethodOverridenInDerivedClass()
Executes the method implemented in the derived class (the class of the object referenced by ref_derived)
The implementation used depends on the object referenced and not on the type of the variable that holds the reference

19. 1 ' Fig. 10.1: Point.vb
2 ' CPoint class represents an x-y coordinate pair. 3
4 Public Class CPoint 5
' point coordinate
Private mX, mY As Integer 8
' default constructor
Public Sub New() 11
' implicit call to Object constructor occurs here
X = 0
Y = 0
End Sub ' New 16
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer) 20
' implicit call to Object constructor occurs here
X = xValue
Y = yValue
End Sub ' New 25
' property X
Public Property X() As Integer 28
Get
Return mX
End Get 32
Set(ByVal xValue As Integer)
mX = xValue ' no need for validation
End Set
Point.vb

20. Point.vb 36 37 End Property ' X 38 39 ' property Y
Public Property Y() As Integer 41
Get
Return mY
End Get 45
Set(ByVal yValue As Integer)
mY = yValue ' no need for validation
End Set 49
End Property ' Y 51
' return String representation of CPoint
Public Overrides Function ToString() As String
Return "[" & mX & ", " & mY & "]"
End Function ' ToString 56
57 End Class ' CPoint
Point.vb

21. Circle.vb 1 ' Fig. 10.2: Circle.vb
2 ' CCircle class that inherits from class CPoint. 3
4 Public Class CCircle
Inherits CPoint ' CCircle Inherits from class CPoint 6
Private mRadius As Double 8
' default constructor
Public Sub New() 11
' implicit call to CPoint constructor occurs here
Radius = 0
End Sub ' New 15
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer, ByVal radiusValue As Double) 19
' use MyBase reference to CPoint constructor explicitly
MyBase.New(xValue, yValue)
Radius = radiusValue
End Sub ' New 24
' property Radius
Public Property Radius() As Double 27
Get
Return mRadius
End Get 31
Set(ByVal radiusValue As Double) 33
Circle.vb

22. Circle.vb 34 If radiusValue >= 0 ' mRadius must be nonnegative
mRadius = radiusValue
End If 37
End Set 39
End Property ' Radius 41
' calculate CCircle diameter
Public Function Diameter() As Double
Return mRadius * 2
End Function ' Diameter 46
' calculate CCircle circumference
Public Function Circumference() As Double
Return Math.PI * Diameter()
End Function ' Circumference 51
' calculate CCircle area
Public Overridable Function Area() As Double
Return Math.PI * mRadius ^ 2
End Function ' Area 56
' return String representation of CCircle
Public Overrides Function ToString() As String 59
' use MyBase reference to return CCircle String representation
Return "Center= " & MyBase.ToString() & _
"; Radius = " & mRadius
End Function ' ToString 64
65 End Class ' CCircle
Circle.vb

23. Test.vb polymorphism 1 ' Fig. 10.3: Test.vb
2 ' Demonstrating inheritance and polymorphism. 3
4 Imports System.Windows.Forms 5
6 Class CTest 7
' demonstrate "is a" relationship
Shared Sub Main()
Dim output As String
Dim point1, point2 As CPoint
Dim circle1, circle2 As CCircle 13
point1 = New CPoint(30, 50)
circle1 = New CCircle(120, 89, 2.7) 16
output = "CPoint point1: " & point1.ToString() & _
vbCrLf & "CCircle circle1: " & circle1.ToString() 19
' use is-a relationship to assign CCircle to CPoint reference
point2 = circle1 22
output &= vbCrLf & vbCrLf & _
"CCircle circle1 (via point2): " & point2.ToString() 25
' downcast (cast base-class reference to derived-class
' data type) point2 to circle2
circle2 = CType(point2, CCircle) ' allowed only via cast 29
output &= vbCrLf & vbCrLf & _
"CCircle circle1 (via circle2): " & circle2.ToString() 32
output &= vbCrLf & "Area of circle1 (via circle2): " & _
String.Format("{0:F}", circle2.Area()) 35
Test.vb
polymorphism

24. Test.vb polymorphism 36 ' assign CPoint object to CCircle reference
If (TypeOf point1 Is CCircle) Then
circle2 = CType(point1, CCircle)
output &= vbCrLf & vbCrLf & "cast successful"
Else
output &= vbCrLf & vbCrLf & _
"point1 does not refer to a CCircle"
End If 44
MessageBox.Show(output, _
"Demonstrating the 'is a' relationship")
End Sub ' Main 48
49 End Class ' CTest
Test.vb
polymorphism

25. Abstract Classes, Methods and Properties
Cannot be instantiated ( New AbstractClassName )
A class is made abstract by using keyword MustInherit
Normally contains one or more abstract methods or properties
Keyword MustOverride declares a method or property as abstract
MustOverride methods and properties do not provide implementations

26. Abstract property (no implementation)
1 ' Fig. 10.4: Shape.vb
2 ' Demonstrate a shape hierarchy using MustInherit class. 3
4 Imports System.Windows.Forms 5
6 Public MustInherit Class CShape 7
' return shape area
Public Overridable Function Area() As Double
Return 0
End Function ' Area 12
' return shape volume
Public Overridable Function Volume() As Double
Return 0
End Function ' Volume 17
' overridable method that should return shape name
Public MustOverride ReadOnly Property Name() As String 20
21 End Class ' CShape
Shape.vb
Abstract class
Keyword Overridable must accompany every method in abstract class
Abstract property (no implementation)

27. 1 ' Fig. 10.5: Point2.vb
2 ' CPoint2 class represents an x-y coordinate pair. 3
4 Public Class CPoint2
Inherits CShape ' CPoint2 inherits from MustInherit class CShape 6
' point coordinate
Private mX, mY As Integer 9
' default constructor
Public Sub New() 12
' implicit call to Object constructor occurs here
X = 0
Y = 0
End Sub ' New 17
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer) 21
' implicit call to Object constructor occurs here
X = xValue
Y = yValue
End Sub ' New 26
' property X
Public Property X() As Integer 29
Get
Return mX
End Get 33
Point2.vb

28. Point2.vb 34 Set(ByVal xValue As Integer)
mX = xValue ' no need for validation
End Set 37
End Property ' X 39
' property Y
Public Property Y() As Integer 42
Get
Return mY
End Get 46
Set(ByVal yValue As Integer)
mY = yValue ' no need for validation
End Set 50
End Property ' Y 52
' return String representation of CPoint2
Public Overrides Function ToString() As String
Return "[" & mX & ", " & mY & "]"
End Function ' ToString 57
' implement MustOverride property of class CShape
Public Overrides ReadOnly Property Name() As String 60
Get
Return "CPoint2"
End Get 64
End Property ' Name 66
67 End Class ' CPoint2
Point2.vb

29. Circle2.vb 1 ' Fig. 10.6: Circle2.vb
2 ' CCircle2 class inherits from CPoint2 and overrides key members. 3
4 Public Class CCircle2
Inherits CPoint2 ' CCircle2 Inherits from class CPoint2 6
Private mRadius As Double 8
' default constructor
Public Sub New() 11
' implicit call to CPoint2 constructor occurs here
Radius = 0
End Sub ' New 15
' constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer, ByVal radiusValue As Double) 19
' use MyBase reference to CPoint2 constructor explicitly
MyBase.New(xValue, yValue)
Radius = radiusValue
End Sub ' New 24
' property Radius
Public Property Radius() As Double 27
Get
Return mRadius
End Get 31
Set(ByVal radiusValue As Double) 33
If radiusValue >= 0 ' mRadius must be nonnegative
mRadius = radiusValue
Circle2.vb

30. Circle2.vb 36 End If 37 38 End Set 39 40 End Property ' Radius 41
' calculate CCircle2 diameter
Public Function Diameter() As Double
Return mRadius * 2
End Function ' Diameter 46
' calculate CCircle2 circumference
Public Function Circumference() As Double
Return Math.PI * Diameter()
End Function ' Circumference 51
' calculate CCircle2 area
Public Overrides Function Area() As Double
Return Math.PI * mRadius ^ 2
End Function ' Area 56
' return String representation of CCircle2
Public Overrides Function ToString() As String 59
' use MyBase to return CCircle2 String representation
Return "Center = " & MyBase.ToString() & _
"; Radius = " & mRadius
End Function ' ToString 64
' override property Name from class CPoint2
Public Overrides ReadOnly Property Name() As String 67
Get
Return "CCircle2"
End Get
Circle2.vb

31. 71 72 End Property ' Name 73 74 End Class ' CCircle2
Circle2.vb

32. Cylinder2.vb 1 ' Fig. 10.7: Cylinder2.vb
2 ' CCylinder2 inherits from CCircle2 and overrides key members. 3
4 Public Class CCylinder2
Inherits CCircle2 ' CCylinder2 inherits from class CCircle2 6
Protected mHeight As Double 8
' default constructor
Public Sub New() 11
' implicit call to CCircle2 constructor occurs here
Height = 0
End Sub ' New 15
' four-argument constructor
Public Sub New(ByVal xValue As Integer, _
ByVal yValue As Integer, ByVal radiusValue As Double, _
ByVal heightValue As Double) 20
' explicit call to CCircle2 constructor
MyBase.New(xValue, yValue, radiusValue)
Height = heightValue ' set CCylinder2 height
End Sub ' New 25
' property Height
Public Property Height() As Double 28
Get
Return mHeight
End Get 32
' set CCylinder2 height if argument value is positive
Set(ByVal heightValue As Double) 35
Cylinder2.vb

33. 36 If heightValue >= 0 Then ' mHeight must be nonnegative
mHeight = heightValue
End If 39
End Set 41
End Property ' Height 43
' override method Area to calculate CCylinder2 surface area
Public Overrides Function Area() As Double
Return 2 * MyBase.Area + MyBase.Circumference * mHeight
End Function ' Area 48
' calculate CCylinder2 volume
Public Overrides Function Volume() As Double
Return MyBase.Area * mHeight
End Function ' Volume 53
' convert CCylinder2 to String
Public Overrides Function ToString() As String
Return MyBase.ToString() & "; Height = " & mHeight
End Function ' ToString 58
' override property Name from class CCircle2
Public Overrides ReadOnly Property Name() As String 61
Get
Return "CCylinder2"
End Get 65
End Property ' Name 67
68 End Class ' CCylinder2
Cylinder2.vb

34. 1 ' Fig. 10.8: Test2.vb
2 ' Demonstrate polymorphism in Point-Circle-Cylinder hierarchy. 3
4 Imports System.Windows.Forms 5
6 Class CTest2 7
Shared Sub Main() 9
' instantiate CPoint2, CCircle2 and CCylinder2 objects
Dim point As New CPoint2(7, 11)
Dim circle As New CCircle2(22, 8, 3.5)
Dim cylinder As New CCylinder2(10, 10, 3.3, 10) 14
' instantiate array of base-class references
Dim arrayOfShapes As CShape() = New CShape(2){} 17
' arrayOfShapes(0) refers to CPoint2 object
arrayOfShapes(0) = point 20
' arrayOfShapes(1) refers to CCircle2 object
arrayOfShapes(1) = circle 23
' arrayOfShapes(2) refers to CCylinder2 object
arrayOfShapes(2) = cylinder 26
Dim output As String = point.Name & ": " & _
point.ToString() & vbCrLf & circle.Name & ": " & _
circle.ToString() & vbCrLf & cylinder.Name & _
": " & cylinder.ToString() 31
Dim shape As CShape 33
Test2.vb

35. Test2.vb 34 ' display name, area and volume for each object in
' arrayOfShapes polymorphically
For Each shape In arrayOfShapes
output &= vbCrLf & vbCrLf & shape.Name & ": " & _
shape.ToString() & vbCrLf & "Area = " & _
String.Format("{0:F}", shape.Area) & vbCrLf & _
"Volume = " & String.Format("{0:F}", shape.Volume)
Next 42
MessageBox.Show(output, "Demonstrating Polymorphism")
End Sub ' Main 45
46 End Class ' CTest2
Test2.vb

36. NottInheritable Classes and NotOverridable Methods and Properties
Cannot be inherited by other classes
Keyword NottInheritable
NotOverridable Methods and Properties
Cannot be overridden in derived classes
Keyword NotOverridable

37. Interfaces An interface may be seen as a fully abstract class
Interface definition begins with keyword Interface
Contains a list of public and abstract methods and properties
Interfaces are implemented by classes, that is, the properties and methods specified in the interface must be implemented by classes that implement the interface
A class may implement multiple interfaces (but can only inherit from a single base class)
Many examples in the .Net class libraries

38. 1 ' Fig : IAge.vb
2 ' Interface IAge declares property for setting and getting age. 3
4 Public Interface IAge 5
' classes that implement IAge must define these properties
ReadOnly Property Age() As Integer
ReadOnly Property Name() As String 9
10 End Interface ' IAge
IAge.vb

39. 1 ' Fig : Person.vb
2 ' Class CPerson has a birthday. 3
4 Public Class CPerson
Implements IAge 6
Private mYearBorn As Integer
Private mFirstName As String
Private mLastName As String 10
' constructor receives first name, last name and birth date
Public Sub New(ByVal firstNameValue As String, _
ByVal lastNameValue As String, _
ByVal yearBornValue As Integer) 15
' implicit call to Object constructor
mFirstName = firstNameValue
mLastName = lastNameValue 19
' validate year
If (yearBornValue > 0 AndAlso _
yearBornValue <= Date.Now.Year) 23
mYearBorn = yearBornValue
Else
mYearBorn = Date.Now.Year
End If 28
End Sub ' New 30
' property Age implementation of interface IAge
ReadOnly Property Age() As Integer _
Implements IAge.Age 34
Keyword Implements indicates that class CPerson implements interface IAge
Person.vb
Implementation of Age

40. Implementation of Name
Get
Return Date.Now.Year - mYearBorn
End Get 38
End Property ' Age 40
' property Name implementation of interface IAge
ReadOnly Property Name() As String _
Implements IAge.Name 44
Get
Return mFirstName & " " & mLastName
End Get 48
End Property ' Name 50
51 End Class ' CPerson
Person.vb
Implementation of Name

41. Definition of Age and Name
1 ' Fig : Tree.vb
2 ' Class CTree contains number of rings corresponding to age. 3
4 Public Class CTree
Implements IAge 6
Private mRings As Integer 8
' constructor receives planting date
Public Sub New(ByVal yearPlanted As Integer) 11
' implicit call to Object constructor
mRings = Date.Now.Year - yearPlanted
End Sub ' New 15
' increment mRings
Public Sub AddRing()
mRings += 1
End Sub ' AddRing 20
' property Age
ReadOnly Property Age() As Integer _
Implements IAge.Age 24
Get
Return mRings
End Get 28
End Property ' Age 30
' property Name implementation of interface IAge
ReadOnly Property Name() As String _
Implements IAge.Name 34
Tree.vb
Definition of Age and Name

42. Tree.vb 35 Get 36 Return "Tree" 37 End Get 38 39 End Property ' Name40
41 End Class ' CTree
Tree.vb

43. Test.vb 1 ' Fig. 10.18: Test.vb 2 ' Demonstrate polymorphism. 3
4 Imports System.Windows.Forms 5
6 Class CTest 7
Shared Sub Main() 9
' instantiate CTree and CPerson objects
Dim tree As New CTree(1976)
Dim person As New CPerson("Bob", "Jones", 1983) 13
' instantiate array of interface references
Dim iAgeArray As IAge() = New IAge(1){} 16
' iAgeArray(0) references CTree object
iAgeArray(0) = tree 19
' iAgeArray(1) references CPerson object
iAgeArray(1) = person 22
' display tree information
Dim output As String = tree.ToString() & ": " & _
tree.Name & vbCrLf & "Age is " & tree.Age & vbCrLf & _
vbCrLf 27
' display person information
output &= person.ToString() & ": " & _
person.Name & vbCrLf & "Age is " & person.Age & _
vbCrLf 32
Dim ageReference As IAge 34
Test.vb

44. Test.vb 35 ' display name and age for each IAge object in iAgeArray
For Each ageReference In iAgeArray
output &= vbCrLf & ageReference.Name & ": " & _
"Age is " & ageReference.Age
Next 40
MessageBox.Show(output, "Demonstrating Polymorphism")
End Sub ' Main 43
44 End Class ' CTest
Test.vb

45. Are classes that encapsulate a set of references to methods
* Delegates
Are classes that encapsulate a set of references to methods
A delegate object that contains method references can be passed to another method
Singlecast delegates
Delegates containing a single method and they are created and derived from class Delegate
Multicast delegates
Delegates containing multiple methods and they are derived from class MulticastDelegate
AddressOf
Creates a delegate instance enclosing a reference to that method

46. Declaration of a delegate Comparator
1 ' Fig : DelegateBubbleSort.vb
2 ' Uses delegates to sort random numbers (ascending or descending). 3
4 Public Class CDelegateBubbleSort 5
' delegate definition
Public Delegate Function Comparator( _
ByVal element1 As Integer, _
ByVal element2 As Integer) As Boolean 10
' sort array depending on comparator
Public Sub SortArray(ByVal array As Integer(), _
ByVal Compare As Comparator) 14
Dim i, pass As Integer 16
For pass = 0 To array.GetUpperBound(0) 18
' comparison inner loop
For i = 0 To array.GetUpperBound(0) - 1 21
If Compare(array(i), array(i + 1)) Then
Swap(array(i), array(i + 1))
End If 25
Next ' inner loop 27
Next ' outer loop 29
End Sub ' SortArray 31
' swap two elements
Private Sub Swap(ByRef firstElement As Integer, _
ByRef secondElement As Integer) 35
Declaration of a delegate Comparator
DelegateBubbleSort.vb

47. DelegateBubbleSort.vb 36 Dim hold As Integer 37 38 hold = firstElement
firstElement = secondElement
secondElement = hold
End Sub ' Swap 42
43 End Class ' CDelegateBubbleSort
DelegateBubbleSort.vb

48. FrmBubbleSort.vb 1 ' Fig. 10.25: FrmBubbleSort.vb
2 ' Create GUI that enables user to sort array. 3
4 Imports System.Windows.Forms 5
6 Public Class CFrmBubbleSort
Inherits Form 8
' TextBox that contains original list
Friend WithEvents txtOriginal As TextBox
Friend WithEvents lblOriginal As Label 12
' TextBox that contains sorted list
Friend WithEvents txtSorted As TextBox
Friend WithEvents lblSorted As Label 16
' Buttons for creating and sorting lists
Friend WithEvents cmdCreate As Button
Friend WithEvents cmdSortAscending As Button
Friend WithEvents cmdSortDescending As Button 21
' Windows Form Designer generate code 23
' reference to object containing delegate
Dim mBubbleSort As New CDelegateBubbleSort() 26
' original array with unsorted elements
Dim mElementArray As Integer() = New Integer(9){} 29
' delegate implementation sorts in asending order
Private Function SortAscending(ByVal element1 As Integer, _
ByVal element2 As Integer) As Boolean 33
Return element1 > element2
End Function ' SortAscending
FrmBubbleSort.vb

49. 36
' delegate implementation sorts in descending order
Private Function SortDescending(ByVal element1 As Integer, _
ByVal element2 As Integer) As Boolean 40
Return element1 < element2
End Function ' SortDescending 43
' creates random generated numbers
Private Sub cmdCreate_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles cmdCreate.Click 47
txtSorted.Clear() 49
Dim output As String
Dim randomNumber As Random = New Random()
Dim i As Integer 53
' create String with 10 random numbers
For i = 0 To mElementArray.GetUpperBound(0)
mElementArray(i) = randomNumber.Next(100)
output &= mElementArray(i) & vbCrLf
Next 59
txtOriginal.Text = output ' display numbers 61
' enable sort buttons
cmdSortAscending.Enabled = True
cmdSortDescending.Enabled = True
End Sub ' cmdCreate_Click 66
FrmBubbleSort.vb

50. FrmBubbleSort.vb 67 ' display array contents in specified TextBox
Private Sub DisplayResults() 69
Dim output As String
Dim i As Integer 72
' create string with sorted numbers
For i = 0 To mElementArray.GetUpperBound(0)
output &= mElementArray(i) & vbCrLf
Next 77
txtSorted.Text = output ' display numbers
End Sub ' DisplayResults 80
' sorts randomly generated numbers in ascending manner
Private Sub cmdSortAscending_Click(ByVal sender As _
System.Object, ByVal e As System.EventArgs) _
Handles cmdSortAscending.Click 85
' sort array
mBubbleSort.SortArray(mElementArray, AddressOf SortAscending) 88
DisplayResults() ' display results 90
cmdSortAscending.Enabled = False
cmdSortDescending.Enabled = True
End Sub ' cmdSortAscending_Click 94
' sorts randomly generated numbers in descending manner
Private Sub cmdSortDescending_Click(ByVal sender As _
System.Object, ByVal e As System.EventArgs) _
Handles cmdSortDescending.Click 99
' create sort object and sort array
mBubbleSort.SortArray(mElementArray, AddressOf SortDescending)
FrmBubbleSort.vb

51. FrmBubbleSort.vb 102 103 DisplayResults() ' display results 104
cmdSortDescending.Enabled = False
cmdSortAscending.Enabled = True
End Sub ' cmdSortDescending_Click
108
109 End Class ' CFrmBubbleSort
FrmBubbleSort.vb

52. FrmBubbleSort.vb