[Lesson 3]<< [CONTENTS] >> [Lesson 5]
The Concept of Object Oriented Programming
In preceding lessons, you have learned how to write simple VB programme code. However, you still don’t have much understanding about the underlying concepts surrounding VB2010, particularly the concept of object oriented programming.
Though VB2010 is very much similar to VB6 in terms of syntaxes and program structure, their underlying concepts are very different. The main difference is that VB2010 is an Object Oriented Programming Language. Though VB6 may have some OOP capabilities, it is not an object oriented programming language. An object oriented programming language has three core technologies namely encapsulation, inheritance and polymorphism. These three concepts are explained below:
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 member, and John and Sharon could be two instances (two objects) of the library class.
Inheritance
In object oriented programming, classes are created according to their hierarchies, and inheritance allows the structure and methods in one class to be passed down the hierarchy to another class. That means less programming is required when adding functions to complex systems, therefore save time and effort. 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. The ability to reuse existing objects is considered a major advantage of object oriented programming.
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 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.
VB6 is not an object oriented programming language because it does not have inheritance capabilities(although it can make use of some benefits of inheritance). However, VB2010 is a fully functional Object Oriented Programming Language, just like other OOP such as C++ and Java. It is different from the earlier versions of VB because it focuses more on the data itself while the previous versions focus more on the actions. Previous versions of VB are known as procedural or functional programming language. Some other procedural programming languages are C, Pascal and Fortran.
VB2010 allows users to write programs that break down into modules. These modules represents the real-world objects known as classes or types. An object can be created out of a class known as an instance of the class. A class can also comprise subclass. For example, 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.
A class consists of data members as well as methods. In VB2010, the program structure to define a Human class can be written as follows:
‘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
Let’s look at one example on 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 class, start VB2010 as usual and choose Windows Applications. In the VB2010 IDE, click on Project on the menu bar and select Add Class, the Add New Item dialog appears, as shown in the Figure below:
The default class Class1.vb will appear as a new tab with a code window. Rename the class as MyClass.vb. Rename the form as MyFirstClass.vb.
Now, in the MyClass.vb window, enter the follow code
Public Function BMI(ByVal height As Single, ByVal weight As Single)
BMI = Format((weight) / (height ^ 2), “0.00”)
End Function
Now you have created a class (an object) called MyClass 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:
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.