After deriving a class, it becomes available and you
can use it just as you would any other class. Here is an example:
Module Exercise
Public Function Main() As Integer
Dim ball As Sphere = New Sphere
Return 0
End Function
End Module
When a class is based on another class, all public members of the parent class are made
available to the derived class that can use them as necessary. While other
methods and classes can also use the public members of a class, the
difference is that the derived class can call the public members of the
parent as if they belonged to the derived class. That is, the child class
doesn't have to "qualify" the public members of the parent class
when these public members are used in the body of the derived class. This
is illustrated in the following program:
Code File: Circle.vbPublic Class Circle
Public Radius As Double
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
Public Class Sphere
Inherits Circle
Public Sub ShowCharacteristics()
' Because Sphere is based on Circle, you can access
' any public member(s) of Circle without qualifying it(them)
Radius = 35.84
Console.WriteLine("Circle Characteristics")
Console.WriteLine("=======================================================")
Console.WriteLine("Radius: {0}", Radius)
Console.WriteLine("Diameter: {0}", CalculateDiameter())
Console.WriteLine("Circumference: {0}", CalculateCircumference())
Console.WriteLine("=======================================================")
End Sub
End Class
Code File: Exercise.vbModule Exercise
Public Function Main() As Integer
Dim ball As Sphere = New Sphere
ball.ShowCharacteristics()
Return 0
End Function
End Module
This would produce:
Sphere Characteristics Radius: 35.84 Diameter: 71.68 Circumference: 225.1891712
Based on the relationship between a child class and
its parent, you can use Me in the child to access the public members of
the parent class. Here is an example:
Public Class Circle
Public Radius As Double
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
Public Class Sphere
Inherits Circle
Public Sub ShowCharacteristics()
' Because Sphere is based on Circle, you can access
' any public member(s) of Circle without qualifying it(them)
Me.Radius = 35.84
Console.WriteLine("Circle Characteristics")
Console.WriteLine("=======================================================")
Console.WriteLine("Radius: {0}", Me.Radius)
Console.WriteLine("Diameter: {0}", Me.CalculateDiameter())
Console.WriteLine("Circumference: {0}", Me.CalculateCircumference())
Console.WriteLine("=======================================================")
End Sub
End Class
We mentioned that, when deriving a new class based on
an existing one, all public members of the parent class are accessible to
its child classes. In fact, those public members are also available to all
other non-child classes and external procedures of the parent class. On
the other hand, when creating a class, if you think that a certain member
would not need to be accessed outside of the class, you should make it
private by starting it with the Private keyword. Here is an
example:
Public Class Circle
Public Radius As Double
Private Function ShowDescription()
Return "A circle is a round geometric shape constructed " & _
"so that all considered points of the shape are " & _
"at an equal distance from a common point called " & _
"the center. Also, two equally opposite points from " & _
"the center are at the exact same dictance from that center."
End Function
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
After creating a class, its private members are not
accessible to clients of the class, not even to the children of that
class. If you attempt to access a private member of a class outside of
that class, you would receive an error. That what would happen with the
following program:
Public Class Circle
Public Radius As Double
Private Function ShowDescription()
Return "A circle is a round geometric shape constructed " & _
"so that all considered points of the shape are " & _
"at an equal distance from a common point called " & _
"the center. Also, two equally opposite points from " & _
"the center are at the exact same dictance from that center."
End Function
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
Public Class Sphere
Inherits Circle
Public Sub ShowCharacteristics()
' Because Sphere is based on Circle, you can access
' any public member(s) of Circle without qualifying it(them)
Me.Radius = 35.84
Console.WriteLine("Circle Characteristics")
Console.WriteLine("=======================================================")
Console.WriteLine("Description: {0}", ShowDescription())
Console.WriteLine("Radius: {0}", Me.Radius)
Console.WriteLine("Diameter: {0}", Me.CalculateDiameter())
Console.WriteLine("Circumference: {0}", Me.CalculateCircumference())
Console.WriteLine("=======================================================")
End Sub
End Class
When creating a class that would be used as the base
of other classes, in some cases, you may want to create a special
relationship among a class and its eventual children. For example, you may
want to create some members of the parent class that only its derived
class can access. These types of members must be created with the Protected
keyword. Here is an example:
Public Class Circle
Protected Radius As Double
Private Function ShowDescription()
Return "A circle is a round geometric shape constructed " & _
"so that all considered points of the shape are " & _
"at an equal distance from a common point called " & _
"the center. Also, two equally opposite points from " & _
"the center are at the exact same dictance from that center."
End Function
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
Not just the member variables but you can also create
methods as protected as long as you intend it only for the class and its
children. Remember that a protected member cannot be accessed by the
clients of the class, only by the class itself and its children.
When accessing the protected members of a class from
its children, you can use Me to locate those members: Me gives you
access to non-Shared public and protected members of both the parent(s)
and its class. Here is an example:
Public Class Circle
Protected Radius As Double
Protected Function ShowDescription()
Return "A circle is a round geometric shape constructed " & _
"so that all considered points of the shape are " & _
"at an equal distance from a common point called " & _
"the center. Also, two equally opposite points from " & _
"the center are at the exact same dictance from that center."
End Function
Public Function CalculateDiameter() As Double
Return Radius * 2
End Function
Public Function CalculateCircumference() As Double
Return CalculateDiameter() * 3.14159
End Function
End Class
Public Class Sphere
Inherits Circle
Public Sub ShowCharacteristics()
' Because Sphere is based on Circle, you can access
' any public member(s) of Circle without qualifying it(them)
Me.Radius = 35.84
Console.WriteLine("Circle Characteristics")
Console.WriteLine("=======================================================")
Console.WriteLine("Description: {0}", Me.ShowDescription())
Console.WriteLine("-=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=-")
Console.WriteLine("Radius: {0}", Me.Radius)
Console.WriteLine("Diameter: {0}", Me.CalculateDiameter())
Console.WriteLine("Circumference: {0}", Me.CalculateCircumference())
Console.WriteLine("=======================================================")
End Sub
End Class
|
Inheritance
Subscribe to:
Post Comments (Atom)
Related Scripting
No comments:
Post a Comment