Object-oriented programming

In this part of the Visual Basic tutorial, we will talk about object oriented programming in Visual Basic.

There are three widely used programming paradigms there. Procedural programming, functional programming and object-oriented programming. Visual Basic supports both procedural and object-oriented programming.

Object-oriented programming (OOP) is a programming paradigm that uses objects and their interactions to design applications and computer programs. (Wikipedia)

There are some basic programming concepts in OOP:

The abstraction is simplifying complex reality by modeling classes appropriate to the problem. The polymorphism is the process of using an operator or function in different ways for different data input. The encapsulation hides the implementation details of a class from other objects. The inheritance is a way to form new classes using classes that have already been defined.

Objects

Objects are basic building blocks of a Visual Basic OOP program. An object is a combination of data and methods. In a OOP program, we create objects. These objects communicate together through methods. Each object can receive messages, send messages and process data.

There are two steps in creating an object. First, we create a class. A class is a template for an object. It is a blueprint, which describes the state and behavior that the objects of the class all share. A class can be used to create many objects. Objects created at runtime from a class are called instances of that particular class.

Option Strict On


Module Example

    Class Being
    
    End Class

    Sub Main()
        
        Dim b as New Being
        Console.WriteLine(b.ToString())      

    End Sub
    
End Module

In our first example, we create a simple object.

Class Being

End Class

This is a simple class definition. The body of the template is empty. It does not have any data or methods.

Dim b as New Being

We create a new instance of the Being class. For this we have the New keyword. The b variable is the handle to the created object.

Console.WriteLine(b.ToString())

The ToString() method of the object gives some basic description of the object.

$ ./object.exe 
Example+Being

We don't get much info, since the class definition was empty. We get the object class name and the module name, where the instance of this object was created.

Object attributes

Object attributes is the data bundled in an instance of a class. The object attributes are called instance variables or member fields. An instance variable is a variable defined in a class, for which each object in the class has a separate copy.

Option Strict On


Module Example

    Class Person

        Public Name As String
    
    End Class

    Sub Main()
        
        Dim p1 as New Person
        p1.Name = "Jane"

        Dim p2 as New Person
        p2.Name = "Beky"

        Console.WriteLine(p1.Name)
        Console.WriteLine(p2.Name)      

    End Sub
    
End Module

In the above Visual Basic code, we have a Person class with one member field.

Class Person
    Public Name As String
End Class

We declare a Name member field. The Public keyword specifies that the member field will be accessible outside the Class End Class block.

Dim p1 as New Person
p1.Name = "Jane"

We create an instance of the Person class. And set the Name variable to "Jane". We use the dot operator to access the attributes of objects.

Dim p2 as New Person
p2.Name = "Beky"

We create another instance of the Person class. Here we set the variable to "Beky".

Console.WriteLine(p1.Name)
Console.WriteLine(p2.Name)

We print the contents of the variables to the console.

$ ./person.exe 
Jane
Beky

We see the output of the program. Each instance of the Person class has a separate copy of the Name member field.

Methods

Methods are functions/procedures defined inside the body of a class. They are used to perform operations with the attributes of our objects. Methods are essential in encapsulation concept of the OOP paradigm. For example, we might have a Connect method in our AccessDatabase class. We need not to be informed how exactly Connect connects to the database. We only know that it is used to connect to a database. This is essential in dividing responsibilities in programming, especially in large applications.

Option Strict On


Module Example

    Class Circle

        Public Radius As Integer

        Public Sub SetRadius(ByVal Radius As Integer)
            Me.Radius = Radius
        End Sub

        Public Function Area() As Double
            Return Me.Radius * Me.Radius * Math.PI
        End Function
    
    End Class

    Sub Main()
        
        Dim c As New Circle
        c.SetRadius(5)

        Console.WriteLine(c.Area())

    End Sub
    
End Module

In the code example, we have a Circle class. We define two methods.

Public Radius As Integer

We have one member field. It is the Radius of the circle. The Public keyword is an access specifier. It tells that the variable is fully accessible from the outside world.

Public Sub SetRadius(ByVal Radius As Integer)
    Me.Radius = Radius
End Sub

This is the SetRadius() method. It is a normal Visual Basic procedure. The Me variable is a special variable, which we use to access the member fields from methods.

Public Function Area() As Double
    Return Me.Radius * Me.Radius * Math.PI
End Function

The Area() method returns the area of a circle. The Math.PI is a built-in constant.

$ ./circle.exe 
78.5398163397448

Running the example.

Access modifiers

Access modifiers set the visibility of methods and member fields. Visual Basic has five access modifiers: Public, Protected, Private, Friend, and ProtectedFriend. Public members can be accessed from anywhere. Protected members can be accessed only within the class itself and by inherited and parent classes. Friend members may be accessed from within the same assembly (exe or DLL). ProtectedFriend is a union of protected and friend modifiers.

Access modifiers protect data against accidental modifications. They make the programs more robust.

Option Strict On


Module Example

    Class Person

        Public Name As String
        Private Age As Byte

        Public Function GetAge() As Byte
            Return Me.Age
        End Function

        Public Sub SetAge(ByVal Age As Byte)
            Me.Age = Age
        End Sub
    
    End Class

    Sub Main()
        
        Dim p as New Person
        p.Name = "Jane"

        p.setAge(17)

        Console.WriteLine("{0} is {1} years old", _
           p.Name, p.GetAge)

    End Sub
    
End Module

In the above program, we have two member fields. One is declared Public, the other Private.

Public Function GetAge() As Byte
    Return Me.Age
End Function

If a member field is Private, the only way to access it is via methods. If we want to modify an attribute outside the class, the method must be declared Public. This is an important aspect of data protection.

Public Sub SetAge(ByVal Age As Byte)
    Me.Age = Age
End Sub

The SetAge() method enables us to change the private Age variable from outside of the class definition.

Dim p as New Person
p.Name = "Jane"

We create a new instance of the Person class. Because the Name attribute is Public, we can access it directly. However, this is not recommended.

p.setAge(17)

The SetAge() method modifies the Age member field. It cannot be accessed or modified directly, because it is declared Private.

Console.WriteLine("{0} is {1} years old", _
    p.Name, p.GetAge)

Finally, we access both members to build a string.

$ ./modifiers.exe 
Jane is 17 years old

Running the example.

Option Strict On


Module Example

    Class Base

        Public Name As String = "Base"
        Protected Id As Integer = 5323
        Private IsDefined As Boolean = True
    
    End Class

    Class Derived 
        Inherits Base
    
        Public Sub Info()
            Console.WriteLine("This is Derived Class")
            Console.WriteLine("Members inherited:")
            Console.WriteLine(Me.Name)
            Console.WriteLine(Me.Id)
            'Console.WriteLine(Me.IsDefined)
        End Sub

    End Class

    Sub Main()
        
        Dim drv As Derived = New Derived
        drv.Info()
     
    End Sub
    
End Module

In the preceding program, we have a Derived class, which inherits from the Base class. The Base class has three member fields. All with different access modifiers. The IsDefined member is not inherited. The Private modifier prevents this.

Class Derived 
    Inherits Base

The class Derived inherits from the Base class.

Console.WriteLine(Me.Name)
Console.WriteLine(Me.Id)
'Console.WriteLine(Me.IsDefined)

The Public and the Protected members are inherited by the Derived class. They can be accessed. The Private member is not inherited. The line accessing the member field is commented. If we uncommented the line, it would not compile.

$ ./protected.exe 
This is Derived Class
Members inherited:
Base
5323

Running the program, we receive this output. The Public and Protected members are inherited, the Private member is not.

Method overloading

Method overloading allows the creation of several methods with the same name which differ from each other in the type of the input.

What is method overloading good for? The Qt4 library gives a nice example for the usage. The QPainter class has three methods to draw a rectangle. Their name is drawRect() and their parameters differ. One takes a reference to a floating point rectangle object, another takes a reference to an integer rectangle object and the last one takes four parameters, x, y, width, height. If the C++ language, which is the language in which Qt is developed, didn't have method overloading, the creators of the library would have to name the methods like drawRectRectF(), drawRectRect(), drawRectXYWH(). The solution with method overloading is more elegant.

Option Strict On


Module Example

    Class Sum

        Public Function GetSum() As Integer
            Return 0
        End Function

        Public Function GetSum(ByVal x As Integer) As Integer
            Return x
        End Function

        Public Function GetSum(ByVal x As Integer, _
            ByVal y As Integer) As Integer
            Return x + y
        End Function
    
    End Class

    Sub Main()
        
        Dim s As Sum = New Sum
        
        Console.WriteLine(s.getSum())
        Console.WriteLine(s.getSum(20))
        Console.WriteLine(s.getSum(20, 30))

    End Sub
    
End Module

We have three methods called GetSum(). They differ in input parameters.

Public Function GetSum(ByVal x As Integer) As Integer
    Return x
End Function

This one takes one parameter.

Console.WriteLine(s.getSum())
Console.WriteLine(s.getSum(20))
Console.WriteLine(s.getSum(20, 30))

We call all three methods.

$ ./overloading.exe 
0
20
50

And this is what we get, when we run the example.

The constructor

A constructor is a special kind of a method. It is automatically called, when the object is created. The purpose of the constructor is to initiate the state of the object. The name of the constructor in Visual Basic is New. The constructors are methods, so they can be overloaded too.

Option Strict On


Module Example

    Class Being
    
        Sub New()
            Console.WriteLine("Being is being created")
        End Sub
        
        Sub New(ByVal name As String)
            Console.WriteLine("Being {0} is created", name)
        End Sub
    
    End Class
    
    Sub Main()
        
        Dim b As New Being
        Dim t As New Being("Tom")
                
    End Sub
    
End Module

We have a Being class. This class has two constructors. The first one does not take parameters, the second one takes one parameter.

Sub New(ByVal name As String)
    Console.WriteLine("Being {0} is created", name)
End Sub

This constructor takes one String parameter.

Dim b As New Being

An instance of the Being class is created. This time the constructor without a parameter is called upon object creation.

$ ./constructor.exe 
Being is being created
Being Tom is created

This is the output of the program.

In the next example, we initiate data members of the class. Initiation of variables is a typical job for constructors.

Option Strict On


Module Example

    Class MyFriend
    
        Private Born As Date
        Private Name As String
        
        Sub New(ByVal Name As String, ByVal Born As Date)
            Me.Name = Name
            Me.Born = Born
        End Sub

        Public Sub GetInfo()
            Console.WriteLine("{0} was born on {1}", _
                Me.Name, Me.Born.ToShortDateString)
        End Sub
    
    End Class
    
    Sub Main()
    
        Dim name As String = "Lenka"    
        Dim born As Date = #5/3/1990#
    
        Dim fr As MyFriend = New MyFriend(name, born)
        fr.GetInfo()
        
    End Sub
    
End Module

We have a Friend class with data members and methods.

Private Born As Date
Private Name As String

We have two variables in the class definition. The Private keyword is an access modifier. It is a form of encapsulation. The Private keyword is the most restrictive modifier. It allows only the object in question to access the variable. No descendants, no other objects.

Sub New(ByVal Name As String, ByVal Born As Date)
    Me.Name = Name
    Me.Born = Born
End Sub

In the constructor, we initiate the two data members. The Me variable is a handler used to reference the object variables.

Dim fr As MyFriend = New MyFriend(name, born)
fr.GetInfo()

We create a Friend object with two arguments. Then we call the GetInfo() method of the object.

./constructor2.exe 
Lenka was born on 5/3/1990

Class constants

Visual Basic enables to create class constants. These constants do not belong to a concrete object. They belong to the class. By convention, constants are written in uppercase letters.

Option Strict On


Module Example

    Class Math

        Public Const PI As Double = 3.14159265359
    
    End Class

    Sub Main()
         Console.WriteLine(Math.PI)    
    End Sub
    
End Module

We have a Math class with a PI constant.

Public Const PI As Double = 3.14159265359

The Const keyword is used to define a constant.

$ ./classconstant.exe 
3.14159265359

Running the example.

The ToString() method

Each object has a ToString() method. It returns a human-readable representation of the object. The default implementation returns the fully qualified name of the type of the Object. Note that when we call the Console.WriteLine() method with an object as a parameter, the ToString() is being called.

Option Strict On


Module Example

    Class Being
    
        Public Overrides Function ToString As String    
            Return "This is Being Class"
        End Function
    
    End Class

    Sub Main()
        
        Dim b as New Being
        Dim o As New Object

        Console.WriteLine(o.ToString())
        Console.WriteLine(b.ToString())     
        Console.WriteLine(b) 

    End Sub
    
End Module

We have a Being class in which we override the default implementation of the ToString() method.

Public Overrides Function ToString As String    
    Return "This is Being Class"
End Function

Each class created inherits from the base Object. The ToString() method belongs to this Object class. We use the Overrides keyword to inform that we are overriding a method.

Dim b as New Being
Dim o As New Object

We create two objects. One custom defined and one built-in.

Console.WriteLine(o.ToString())
Console.WriteLine(b.ToString())  

We call the ToString() method on these two objects.

Console.WriteLine(b) 

As we have specified earlier, calling the Console.WriteLine() on the object will call its ToString() method.

$ ./override.exe 
System.Object
This is Being Class
This is Being Class

This is what we get, when we run the script.

Inheritance

The inheritance is a way to form new classes using classes that have already been defined. The newly formed classes are called derived classes, the classes that we derive from are called base classes. Important benefits of inheritance are code reuse and reduction of complexity of a program. The derived classes (descendants) override or extend the functionality of base classes (ancestors).

Option Strict On


Module Example

    Class Being
        Sub New()
            Console.WriteLine("Being is created")
        End Sub
    End Class
    
    Class Human 
        Inherits Being
    
        Sub New()
            Console.WriteLine("Human is created")
        End Sub
    
    End Class
    
    Sub Main()
        
        Dim h As New Human
        
    End Sub
    
End Module

In this program, we have two classes. A base Being class and a derived Human class. The derived class inherits from the base class.

Class Human 
    Inherits Being

In Visual Basic, we use the Inherits keyword to create inheritance relations.

Dim h As New Human

We instantiate the derived Human class.

$ ./inheritance.exe 
Being is created
Human is created

We can see that both constructors were called. First, the constructor of the base class is called, then the constructor of the derived class.

A more complex example follows.

Option Strict On


Module Example

    Class Being
    
        Dim Shared Count As Integer = 0
    
        Sub New()
            Count = Count + 1
            Console.WriteLine("Being is created")
        End Sub
        
        Sub GetCount()
            Console.WriteLine("There are {0} Beings", Count)
        End Sub
        
    End Class
    
    Class Human 
        Inherits Being
    
        Sub New()
            Console.WriteLine("Human is created")
        End Sub
    
    End Class
    
    Class Animal 
        Inherits Being
        
        Sub New
            Console.WriteLine("Animal is created")
        End Sub
        
    End Class
    
    Class Dog
        Inherits Animal
        
        Sub New()
            Console.WriteLine("Dog is created")
        End Sub
        
    End Class
    

    Sub Main()
        
        Dim h As New Human
        Dim d As New Dog
        d.GetCount()
        
    End Sub
    
End Module

We have four classes. The inheritance hierarchy is more complicated. The Human and the Animal classes inherit from the Being class. And the Dog class inherits directly from the Animal class and indirectly from the Being class. We also introduce a concept of a Shared variable.

Dim Shared Count As Integer = 0

We define a Shared variable. Shared members are members that are shared by all instances of a class. In other programming languages, they are called static members.

Sub New()
    Count = Count + 1
    Console.WriteLine("Being is created")
End Sub

Each time the Being class is instantiated, we increase the Count variable by one. This way we keep track of the number of instances created.

Class Animal 
    Inherits Being
...
Class Dog
    Inherits Animal
...

The Animal inherits from the Being and the Dog inherits from the Animal. Indirectly, the Dog inherits from the Being as well.

Dim h As New Human
Dim d As New Dog
d.GetCount

We create instances from the Human and from the Dog classes. We call the GetCount() method of the Dog object.

$ ./inheritance2.exe 
Being is created
Human is created
Being is created
Animal is created
Dog is created
There are 2 Beings

The Human object calls two constructors: the Dog object calls three constructors. There are two beings instantiated.

Abstract classes and methods

Abstract classes cannot be instantiated. If a class contains at least one abstract method, it must be declared abstract too. Abstract methods cannot be implemented, they merely declare the methods' signatures. When we inherit from an abstract class, all abstract methods must be implemented by the derived class. Furthermore, these methods must be declared with the same of less restricted visibility.

Unlike Interfaces, abstract classes may have methods with full implementation and may also have defined member fields. So abstract classes may provide a partial implementation. Programmers often put some common functionality into abstract classes. And these abstract classes are later subclassed to provide more specific implementation. For example, the Qt graphics library has a QAbstractButton, which is the abstract base class of button widgets, providing functionality common to buttons. Buttons Q3Button, QCheckBox, QPushButton, QRadioButton, and QToolButton inherit from this base abstract class.

Formally put, abstract classes are used to enforce a protocol. A protocol is a set of operations, which all implementing objects must support.

Option Strict On


Module Example

    MustInherit Class Drawing
        Protected x As Integer = 0
        Protected y As Integer = 0

        Public MustOverride Function Area() As Double

        Public Function GetCoordinates() As String
            Return String.Format("x: {0}, y: {1}", _
                Me.x, Me.y)
        End Function
    
    End Class

    Class Circle  
        Inherits Drawing

        Private Radius As Integer

        Sub New(ByVal x As Integer, ByVal y As Integer, _
            ByVal r As Integer)
            Me.x = x
            Me.y = y
            Me.Radius = r
        End Sub 

        Public Overrides Function Area() As Double
            Return Me.Radius * Me.Radius * Math.PI
        End Function

        Public Overrides Function ToString() As String
            Return String.Format("Circle, at x: {0}, y: {1}, radius: {2}", _
                Me.x, Me.y, Me.Radius)
        End Function

    End Class

    Sub Main()
        
        Dim c as New Circle(12, 45, 22)
       
        Console.WriteLine(c)
        Console.WriteLine("Area of circle: {0}", c.Area())
        Console.WriteLine(c.GetCoordinates())

    End Sub
    
End Module

We have an abstract base Drawing class. The class defines two member fields, defines one method and declares one method. One of the methods is abstract, the other one is fully implemented. The Drawing class is abstract, because we cannot draw it. We can draw a circle, a dot or a square. The Drawing class has some common functionality to the objects that we can draw.

MustInherit Class Drawing

In Visual Basic, we use the MustInherit keyword to define an abstract class.

Public MustOverride Function Area() As Double

An abstract method is preceded with a MustOverride keyword.

Class Circle  
    Inherits Drawing

A Circle is a subclass of the Drawing class. It must implement the abstract Area() method.

$ ./abstractclass.exe 
Circle, at x: 12, y: 45, radius: 22
Area of circle: 1520.53084433746
x: 12, y: 45

Output of the program.

This was the first part of the description of OOP in Visual Basic.