Looking at the
Employee class in Listings 4, 5 and 6 again, you may wonder why we can use the
Write method of the
System.Console class without first instantiating a
System.Console object. This is because in OOP languages there is a special type of member called a static member. The term shared is also used in VB.NET.
Static means the member is available without having to instantiate an object. For example, the class called
SalaryLevel provides static fields only in Listing 7.
Listing 7: Static members in a class
Class SalaryLevel Public Shared Level1 As Decimal = 35000 Public Shared Level2 As Decimal = 40000 Public Shared Level3 As Decimal = 45000 Public Shared Level4 As Decimal = 50000 Public Shared Level5 As Decimal = 55000 Public Shared Level6 As Decimal = 60000 Public Shared Level7 As Decimal = 65000 Public Shared Level8 As Decimal = 70000 Public Shared Level9 As Decimal = 75000 Public Shared Level10 As Decimal = 80000 End Class
You can then use the class in your program, as illustrated in Listing 8.
Listing 8: Using a static member of a class
Imports System Class SalaryLevel Public Shared Level1 As Decimal = 35000 Public Shared Level2 As Decimal = 40000 Public Shared Level3 As Decimal = 45000 Public Shared Level4 As Decimal = 50000 Public Shared Level5 As Decimal = 55000 Public Shared Level6 As Decimal = 60000 Public Shared Level7 As Decimal = 65000 Public Shared Level8 As Decimal = 70000 Public Shared Level9 As Decimal = 75000 Public Shared Level10 As Decimal = 80000 End Class Class Employee Dim yearlyBonus As Decimal = 4000 Public Sub PrintSalary() ' print the salary to the Console, use the static field of SalaryLevel Console.Write(SalaryLevel.Level4) End Sub Public Shared Sub Main() Dim employee As Employee employee = New Employee() employee.PrintSalary() End Sub End Class
PrintSalary method of the
Employee class, we can use the
Level4 static field in the
SalaryLevel class without first creating an object of type
SalaryLevel. Members that are not static are called instance members.
A constructor is a special method that must be present in a class for the class to get instantiated. In VB.NET, this method is called
New. But, as you have seen, there is no
New method in the classes in the previous code. That's right. When there is no constructor present, VB.NET will create one for you. When you instantiate an object by using the
New keyword, the class's constructor is called. You can provide initialization code that is guaranteed to run when the object is instantiated.
If you write a constructor explicitly in your class, VB.NET won't create it anymore.
Inheritance is a feature that allows you to extend a class. If you need some functionality, you can create a brand new class. If part of the functionality you need has been provided in a class written by someone else, however, you can then write a new class that extends the original class. Your class is called the child class or derived class, and the original class is called the parent class or the base class. The process of extending a class is called extension. Sometimes, the term subclass or inherit is used to describe the act of extending a class. In VB.NET a class can only extend one parent class. Multiple class inheritance is not allowed in VB.NET.
Syntactically, extending a class is done using a semicolon after the class name, followed by the word
Inherits and the parent class name. For example, Listing 9 shows how we extend the class
Employee to create a new class called
Listing 9: Extending a class
Imports System Class Employee Dim salary As Decimal = 40000 Dim yearlyBonus As Decimal = 4000 Public Sub PrintSalary() ' print the salary to the Console Console.Write(salary) End Sub End Class Class Manager: Inherits Employee End Class
If the word
Inherits appears on the next line, you don't need the semicolon.
Class Manager Inherits Employee End Class
Now, you can instantiate a
Manager object and use members from the
The code is given in Listing 10.
//////QUESTION: IMPORTS SYSTEM WAS STUCK IN THE ABOVE GRAF. IT PROBABLY BELONGS AS TEH FIRST LINE OF CODE. CHECK WITH AU.//////////
Listing 10: Instantiating a Manager object
Imports System Class Employee Public salary As Decimal = 40000 Public yearlyBonus As Decimal = 4000 Public Sub PrintSalary() ' print the salary to the Console Console.Write(salary) End Sub End Class Class Manager: Inherits Employee End Class Module Module1 Public Sub Main() Dim manager As Manager manager = New Manager() manager.PrintSalary() End Sub End Module
The example in Listing 11 shows how we can extend the
Manager class by writing a new method called
Listing 11: Adding a method to the derived class
Class Manager: Inherits Employee Public Sub PrintBonus() Console.Write(yearlyBonus) End Sub End Class
Note that member accessibility restriction applies. For example, you can't make the
yearlyBonus field private, since this field is not accessible from the
Manager class. Therefore, trying to compile the code will result in an error.
Inheritance is a common practice. In fact, the .NET Framework class library consist of hierarchies of classes that are derived from other classes. For example, the
Button class in the
Windows.Forms namespace is a child class of
ButtonBase, which itself is a child of
Control. All classes will eventually have the
System.Object class as its root.
System.Object is called the root or the superclass in the .NET Framework class library.
The power of inheritance can be demonstrated by the code in Listing 12.
Listing 12: Extending System.Windows.Forms.Form
Public Class MyForm : Inherits System.Windows.Forms.Form End Class
This blank class declaration, when compiled and run, will display a Windows form. You can create a form without a single line of code. This is because
System.WIndows.Forms.Form. It inherits the functionality of the
You can prevent classes from being inherited by using the
NotInheritable keyword. For example, the code in Listing 13 is a class called
Calculator that cannot be inherited.
Listing 13: A non-inheritable class
NotInheritable Class Calculator End Class
Trying to extend this class will raise a compile error. Why would you want to make your class non- inheritable? In some cases, you may not want other people to be able to extend your class. Another reason is that an inheritable class produces faster code. You should use inheritable classes with caution, however, because noninheritance defeats the purpose of OOP. Only use it if you are 100% sure that you will not extend the class. This type of class is also called a final class in some other OOP languages.
Part 2 of this series can be found here
Return to the .NET DevCenter.
Microsoft, Keep copying from OOP please
2006-02-22 06:14:20 metramo [View]
about this article
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MS version of Java?
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oop explained in 3 pages...wow
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No, what is the point?
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