lesson 24 Object Oriented Programming

24.1 The concept of Object-Oriented Programming

In this lesson, you will learn the basic concept of object-oriented programming. In order for a programming language to qualify as an object-oriented programming language, it must have three core technologies namely encapsulation, inheritance and polymorphism. These three terms are explained below:

24.1.1 Encapsulation

Encapsulation refers to the creation of self-contained modules that bind processing functions to the data. These user-defined data types are called classes. Each class contains data as well as a set of methods which manipulate the data. The data components of a class are called instance variables and one instance of a class is an object. For example, in a library system, a class could be a member, and John and Sharon could be two instances (two objects) of the library class.

24.1.2 Inheritance

Classes are created according to hierarchies, and inheritance allows the structure and methods in one class to be passed down the hierarchy. That means less programming is required when adding functions to complex systems. If a step is added at the bottom of a hierarchy, then only the processing and data associated with that unique step needs to be added. Everything else about that step is inherited.

24.1.3 Polymorphism

Object-oriented programming allows procedures about objects to be created whose exact type is not known until runtime. For example, a screen cursor may change its shape from an arrow to a line depending on the program mode. The routine to move the cursor on the screen in response to mouse movement would be written for “cursor,” and polymorphism allows that cursor to take on whatever shape is required at runtime. It also allows new shapes to be easily integrated.Visual Basic 2013 allows users to write programs that break down into modules. These modules represent the real-world objects and are knows as classes or types.

An object can be created out of a class and it is known as an instance of the class. A class can also comprise subclass. For example, the apple tree is a subclass of the plant class and the apple in your backyard is an instance of the apple tree class. Another example is student class is a subclass of the human class while your son John is an instance of the student class.

24.2 Some Examples

A class consists of data members as well as methods. In Visual Basic 2013, the program structure to define a Human class can be written as follows:

Public Class Human
'Data Members
Private Name As String
Private Birthdate As String
Private Gender As String
Private Age As Integer
'Methods

Overridable Sub ShowInfo( )
 MessageBox.Show(Name)
 MessageBox.Show(Birthdate)
 MessageBox.Show(Gender)
 MessageBox.Show(Age)

End Sub
End Class

Another Example:

Public Class Car
'Data Members
Private Brand As String
Private Model As String
Private Year Made As String
Private Capacity As Integer

'Methods

Overridable Sub ShowInfo( )
 MessageBox.Show(Brand)
 MessageBox.Show(Model)
 MessageBox.Show(Year Made)
 MessageBox.Show(Capacity)

End Sub
End Class

Let’s look at one example of how to create a class. The following example shows you how to create a class that can calculate your BMI (Body Mass Index).

To create a class, start Visual Basic 2013 as usual and choose Windows Applications. In the Visual Basic 2013 IDE, click on Project on the menu bar and select Add Class, the Add New Item dialog appears, as shown in Figure 24.1

The default class Class1.vb will appear as a new tab in a code window. Rename the class as MyClass.vb. Rename the form as MyFirstClass.vb. Now, in the MyClass.vb window, create a new class MyClass1 and enter the following code.

Public Class MyClass1
Public Function BMI(ByVal height As Single, ByVal weight As Single)
 BMI = Format((weight) / (height ^ 2), "0.00")
End Function
End Class

Now you have created a class (an object) called MyClass1 with a method known as BMI. In order to use the BMI class,  insert a button into the form and click on the button to enter the following code:

Private Sub BtnBMI_Click(sender As Object, e As EventArgs) Handles BtnBMI.Click
Dim MyObject As Object
Dim h, w As Single
 MyObject = New MyClass1()
 h = InputBox("What is your height in meter")
 w = InputBox("What is your weight in kg")
 MessageBox.Show(MyObject.BMI(h, w))
End Sub

When you run this program and click the button, the user will be presented with two input boxes to enter his or her height and weight subsequently and the value of BMI will be shown in a pop-up message box, as shown in the figures below: