Visual Basic Programming - Lesson 24: Class Abstraction

	Class Abstraction

An Abstract Class

Introduction

It should appear to us clearly by now that inheritance is useful by allowing us to use an object that was already created but "upgrade" it with new features that were not available when the original object was born. In most cases, when creating a class, you may not thing that other classes would be inherited from it. In fact, this will usually not be your concern: you simply create a class and use it as needed.

In some other cases, rather as you build your experience with Visual Basic. You may create a class that, although useful at the time, you may already think of other classes that would be based on it. This means that, at the time you are creating such a class, you would already keep inheritance in mind. Visual Basic provides various features that can assist you with creating and implementing class with different goals in mind.

Abstracting a Class

Imagine that, when creating a class, you already find that it is too or highly generalized to serve one particular purpose. As an example, imagine you start creating a class you intend to use to process calculations for a rectangle, or a square, or a parallelogram, or even a triangle:


Rectangle	Square	Parallelogram	Triangle

You start thinking that the same class could be used for different types of geometric shapes. In this case, instead of creating a different class for each shape, you can create a generalized class that these shapes can be based on. Unfortunately, these shapes, although each characterized as geometric, don't have much in common; otherwise they would not be different. One of their common characteristics is that each has a name. While the parallelogram and the triangle have a base, the rectangle and the square don't explicitly have one. Also, neither the perimeter nor the area of these shapes are calculated the same. Still, as long as you find at least one characteristic that these objects have, you can create a class that would share. In other words, you can create a class that features one or more characteristics that these objects have. Then each object would customize its behavior(s) based on its particular characteristics. This is the basis of abstraction.

A class is referred to as abstract when it is only used to lay a foundation for other classes. In the Microsoft Visual Basic language, to create an abstract class, you must precede its Class keyword with MustInherit. Here is an example:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral

End Class

After creating a class and marking it as MustInherit, you can add one or more members to it, as done in the previous examples we used so far. Here is an example:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral
    Public Function ShowDescription() As String
        Return "Geometric Shape"
    End Function
End Class

If you create a class and mark it as MustInherit, it is considered incomplete. Because of that, although you can declare a variable of that type, you cannot initialize its instance using the New operator. Consider the following example:

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim quad As Quadrilateral = New Quadrilateral

        Console.WriteLine("Description = {0}", quad.Description)

        Return 0
    End Function

End Module

This would produce an error because you cannot use New to instantiate a MustInherit class. This means that, before using a MustInherit class, you must derive a class from it.

Overriding Members of an Abstract Class

The main idea for creating a MustInherit class is to lay a foundation that other classes can exploit. When creating the members of such a class, you can prepare them to be overridden. You have the option of creating overridable and non-overridable members. You will make the decision based on your requirements.

Sometimes when creating a particular member, you may intend all derived classes to implement their own version of the member. You must clearly indicate that any class that wants to inherit from the MustInherit class must (also) override a particular member. Such a member must be marked with the MustOverride keyword. To do this, when creating the member, precede its type with the MustOverride keyword. Here is an example:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral
    Public MustOverride Property Area() As Double

End Class

In the same way, you can add as many members as necessary. You will mark as MustOverride those of your choice and you can create others without MustOverride. Here are examples:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral
    Public Function ShowDescription() As String
        Return "Geometric Shape"
    End Function

    Public MustOverride Property Area() As Double
End Class

After creating a MustInherit class, you can inherit new classes from it using the Inherits keyword we saw in the previous lessons. Here is an example:

Public Class Square
    Inherits Quadrilateral

End Class

When deriving a class from a MustInherit, the first rule you must observe is that each member of the abstract that was marked as MustOverride must be overridden. Based on this, in our Square class, you must (at least) implement the Area property. Here is an example:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral
    Public Function ShowDescription() As String
        Return "Geometric Shape"
    End Function

    Public MustOverride ReadOnly Property Area() As Double
End Class

Public Class Square
    Inherits Quadrilateral

    Public Overrides ReadOnly Property Area() As Double
        Get
            Return 0
        End Get
    End Property
End Class

In the derived class, as a new one, you can add new members as you judge them necessary. Here are examples:

File: Quadrilateral.vb

Public MustInherit Class Quadrilateral
    Public Function ShowDescription() As String
        Return "Geometric Shape"
    End Function

    Public MustOverride ReadOnly Property Area() As Double
End Class

Public Class Square
    Inherits Quadrilateral

    Public sd As Double

    Public Sub New()
        sd = 0
    End Sub

    Public Sub New(ByVal side As Double)
        sd = side
    End Sub

    Public Property Side() As Double
        Get
            Return sd
        End Get
        Set(ByVal Value As Double)
            If Value <= 0 Then
                sd = 0
            Else
                sd = Value
            End If
        End Set
    End Property

    Public Overrides ReadOnly Property Area() As Double
        Get
            Return sd * sd
        End Get
    End Property
End Class

After deriving a new class from a MustInherit class, you can declare a variable of it and instantiate it using the New operator. Here is an example:

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim sqr As Square = New Square

        sqr.Side = 35.75

        Console.WriteLine(" -=- Square Characteristics -=-")
        Console.WriteLine("Description: {0}", sqr.ShowDescription)
        Console.WriteLine("Side:   {0}", sqr.Side)
        Console.WriteLine("Area:   {0}", sqr.Area)

        Return 0
    End Function

End Module

This would produce:

-=- Square Characteristics -=-
Description: Geometric Shape
Side:   35.75
Area:   1278.0625

As mentioned earlier, after creating a MustInherit class, you can declare a variable of it but you cannot instantiate it using the New operator. Consider the following example:

Module Exercise

    Public Function Main() As Integer
        Dim sqr As Quadrilateral
        sqr = New Square

        Return 0
    End Function

End Module

These declaration and instantiation are legal but the (tri) variable gives you access only to members that are present in the parent class. This means that this declaration gives you access to the ShowDescription() method and the Area property of the Quadrilateral class. This is illustrated in the following:

Module Exercise

    Public Function Main() As Integer
        Dim sqr As Quadrilateral
        sqr = New Square

        Console.WriteLine(" -=- Square Characteristics -=-")
        Console.WriteLine("Description: {0}", sqr.ShowDescription)
        Console.WriteLine("Area:        {0}", sqr.Area)

        Return 0
    End Function

End Module

This would produce:

-=- Square Characteristics -=-
Description: Geometric Shape
Area:   0

When instantiating a class derived from a MustInherit class, if you want to access its members, you must apply its name to the New operator as we saw in the last example of the previous section. Here is an example:

Module Exercise

    Public Function Main() As Integer
        Dim sqr As Square = New Square(42.68)

        Console.WriteLine(" -=- Square Characteristics -=-")
        Console.WriteLine("Description: {0}", sqr.ShowDescription)
        Console.WriteLine("Side:   {0}", sqr.Side)
        Console.WriteLine("Area:   {0}", sqr.Area)

        Return 0
    End Function

End Module

This would produce:

-=- Square Characteristics -=-
Description: Geometric Shape
Side:   42.68
Area:   1821.5824

Classes and Procedures

Introduction

In Lesson 5, we saw that a procedure was an assignment that complemented a program. We also saw that there were two types of procedures: functions and sub routines. These concepts of sub procedures and functions also apply to classes. This means that a procedure that process a class, it can be passed a class as argument, and it can return a class.

As described in Lesson 8, to create a sub procedure, type the Sub keyword followed by a name, followed by parentheses and an end of line. To indicate the end of a sub procedure, you must type End Sub. Therefore, the syntax of a sub procedure is:

Sub ProcedureName()

End Sub

Between the Sub and the End Sub lines, you can declare the necessary variables and they can be of regular types or based on classes. Here is an example:

Module Exercise
    Private Sub ShowCharacteristics()
        Dim reg As RegularTriangle = New RegularTriangle(35.28, 26.44)

    End Sub

    Public Function Main() As Integer
        

        Return 0
    End Function

End Module

After declaring a variable based on a class, you can regularly use it as we have done with objects in the Main() function so far. Here is an example:

File: Exercise.vb

Module Exercise
    Private Sub ShowCharacteristics()
        Dim reg As RegularTriangle = New RegularTriangle(35.28, 26.44)

        Console.WriteLine("Shape Type:    {0}", reg.Type)
        Console.WriteLine("Triangle Type: {0}", reg.Name)
        Console.WriteLine("=-= Characteristics =-=")
        Console.WriteLine("Base:   {0}", reg.Base)
        Console.WriteLine("Height: {0}", reg.Height)
        Console.WriteLine("Area:   {0}", reg.CalculateArea)
    End Sub

    Public Function Main() As Integer
        ShowCharacteristics()

        Return 0
    End Function

End Module

In the same way, you can declare as many class variables as you see fit in a procedure.

Returning an Object

So far, as we have learned since Lesson, we know that a function can be used to return a value. In the same way, you can create a function that returns an object of a class. When creating such a function, set its type as that of the class it would return. The formula to follow is still:

Function FunctionName() As ClassName
    
End Function

In the body of the class, which is the section between the Function and the End Function lines, you can perform any assignment you judge necessary. For example you can declare local variables. Before exiting the function, you must make sure it returns a value based on its As type. You can do this using the Return keyword followed by the value to return. Here is an example:

Private Function CreateTriangle() As RegularTriangle
        Dim regTri As RegularTriangle
        Dim base As Double, height As Double

        Console.WriteLine("Enter the measurements of the triangle")
        Console.Write("Base:   ")
        base = CDbl(Console.ReadLine())
        Console.Write("Height: ")
        height = CDbl(Console.ReadLine())

        regTri = New RegularTriangle(base, height)
        Return regTri
End Function

After defining the function, since it returns a value, when calling it, you can assign it to a variable of the type that it returns. Here is an example:

Module Exercise
    Private Function CreateTriangle() As RegularTriangle
        Dim regTri As RegularTriangle
        Dim base As Double, height As Double

        Console.WriteLine("Enter the measurements of the triangle")
        Console.Write("Base:   ")
        base = CDbl(Console.ReadLine())
        Console.Write("Height: ")
        height = CDbl(Console.ReadLine())

        regTri = New RegularTriangle(base, height)
        Return regTri
    End Function

    Private Sub ShowCharacteristics()
        Dim reg As RegularTriangle = New RegularTriangle

        reg = CreateTriangle()

        Console.WriteLine("Shape Type:    {0}", reg.Type)
        Console.WriteLine("Triangle Type: {0}", reg.Name)
        Console.WriteLine("=-= Characteristics =-=")
        Console.WriteLine("Base:   {0}", reg.Base)
        Console.WriteLine("Height: {0}", reg.Height)
        Console.WriteLine("Area:   {0}", reg.CalculateArea)
    End Sub

    Public Function Main() As Integer
        ShowCharacteristics

        Return 0
    End Function

End Module

Here is a test of the above code:

Enter the measurements of the triangle
Base:   18.44
Height: 12.62

Shape Type:    Triangle
Triangle Type: Regular
=-= Characteristics =-=
Base:   18.44
Height: 12.62
Area:   116.3564

Passing a Class as Argument

Like a regular variable, a class can be passed as argument to a procedure. When a procedure receives such an argument, it can process it as necessary. All the rules we reviewed for regular variables apply to a class, as long as you keep in mind that an object has members that you may need to be aware of. When calling the procedure, make sure you pass it a value argument based on the class it passed to it. Here is an example of a class passed as argument:

Module Exercise
    Private Function CreateTriangle() As RegularTriangle
        Dim regTri As RegularTriangle
        Dim base As Double, height As Double

        Console.WriteLine("Enter the measurements of the triangle")
        Console.Write("Base:   ")
        base = CDbl(Console.ReadLine())
        Console.Write("Height: ")
        height = CDbl(Console.ReadLine())

        regTri = New RegularTriangle(base, height)
        Return regTri
    End Function

    Private Sub ShowCharacteristics(ByVal reg As RegularTriangle)
        Console.WriteLine("Shape Type:    {0}", reg.Type)
        Console.WriteLine("Triangle Type: {0}", reg.Name)
        Console.WriteLine("=-= Characteristics =-=")
        Console.WriteLine("Base:   {0}", reg.Base)
        Console.WriteLine("Height: {0}", reg.Height)
        Console.WriteLine("Area:   {0}", reg.CalculateArea)
    End Sub

    Public Function Main() As Integer
        Dim Angle3 = CreateTriangle()
        Console.WriteLine()
        ShowCharacteristics(Angle3)
        Console.WriteLine()

        Return 0
    End Function

End Module

Besides the function, in Lesson 5, we saw that, by passing an argument by reference, a sub procedure could return a value. This characteristic also applies to a class passed as argument. When passing the argument, precede it with the ByRef keyword. In the procedure, process the argument as you see fit, unless you choose not to. When calling the procedure, pass it a valid variable based on the type of the argument. Here is an example:

Module Exercise
    Private Sub CreateTriangle(ByRef regTri As RegularTriangle)
        Dim base As Double, height As Double

        Console.WriteLine("Enter the measurements of the triangle")
        Console.Write("Base:   ")
        base = CDbl(Console.ReadLine())
        Console.Write("Height: ")
        height = CDbl(Console.ReadLine())

        regTri = New RegularTriangle(base, height)
    End Sub

    Private Sub ShowCharacteristics(ByVal reg As RegularTriangle)
        Console.WriteLine("Shape Type:    {0}", reg.Type)
        Console.WriteLine("Triangle Type: {0}", reg.Name)
        Console.WriteLine("=-= Characteristics =-=")
        Console.WriteLine("Base:   {0}", reg.Base)
        Console.WriteLine("Height: {0}", reg.Height)
        Console.WriteLine("Area:   {0}", reg.CalculateArea)
    End Sub

    Public Function Main() As Integer
        Dim Angle3 As RegularTriangle = New RegularTriangle

        CreateTriangle(Angle3)
        Console.WriteLine()
        ShowCharacteristics(Angle3)
        Console.WriteLine()

        Return 0
    End Function

End Module

Here is an example of testing it:

Enter the measurements of the triangle
Base:   50.05
Height: 25.52

Shape Type:    Triangle
Triangle Type: Regular
=-= Characteristics =-=
Base:   50.05
Height: 25.52
Area:   638.638

Technique of Using Variables and Arguments

Static Variables

Like a regular variable, a class can be locally declared as static, using the Static keyword. If the value of such a locally declared variable changes, when the procedure ends, the value of the variable is changed and would be kept until the next call.

Optional Arguments

When passing an argument of a class, you can specify that it is not required. Such an argument is considered optional. To specify that an argument is optional, when creating its procedure, type the Optional keyword to the left of the argument's name and assign it the default value. All the other rules we reviewed for optional arguments are also applied here.

Procedure Overloading

If you want to create various procedures that takes a class argument, you can create a procedure with the same name but different signatures. This is referred to as overloading the procedure. When doing this, follow the same rules we reviewed for overloading a procedure: the must have the same name, they cannot have the same number of argument when the arguments at each position are of the same types.

Sealed Classes

Introduction

All of the classes we have used so far can serve as parents of other classes. This is the default behavior of a regular class: the ability to derive a new class from it. In some cases, you may not want any class to be derived from a particular class you are creating. Such a class is referred to as sealed.

A class is said to be sealed when you cannot inherit from it. If you try, you would receive an error. To create a sealed class in Microsoft Visual Basic, precede the name of the class with the NotInheritable keyword.