Abstraction and inheritance5.2
Two very fundamental parts of an object oriented language like VB.NET are abstraction and inheritance. They play a huge role in the whole of the .NET Framework and can be used to the full to improve program development on the whole - it is after all good practise to build reusable code, which is one of the key aims of both these technologies.
According to Wikipedia:
Abstraction is conceptual process by which general rules and concepts are derived from the usage and classification of specific examples
The general idea of abstraction in object oriented programming is to make development easier. It is a clever way of creating an class that does not have any purpose but to be customised. Here is an example:
Suppose there is an animal of an unknown type (it could be a cat, dog or any other animal) but then it is given some classification as a dog. It is clear that a dog differs from a cat, for instance, a cat does not bark, whereas a dog does.
In programming terms, the animal is the superclass (or root class) of dog and cat, but they do not share everything, for instance, barking. They do however, both have the ability to communicate, so the superclass animal can have this ability. However, there is no such thing as a plain animal, it has a classification for instance the type of animal.
This can be represented as a hierarchical tree:
Animal has some very basic commands such as walk, communicate, eat and so on, but the fact that an animal can be either a cat or a dog or any other animal, means that the animal does not have any specific way of performing these functionalities. This is solved by leaving this to the subclass. This is called abstraction.
To put this into perspective in VB.NET, a class which is the superclass will have some methods that can perform some tasks and some methods that need to be specified.
Public MustInherit Class Animal 'This variable has been specified and will be passed to all subclasses Dim minHeight As Integer Dim sleeping As Boolean Sub New() minHeight = 10 End Sub 'This method has been left empty giving the subclass power over what it does Public MustOverride Sub Communicate() Public Sub Sleep() 'Inverts the sleeping variable sleeping = Not sleeping End Sub Public Sub OutputHeight() MsgBox("My minimum height is " & minHeight) End Sub End Class
MustInherit is used to define that the class is abstract. Any methods, variables, structures and so on must be inherited.
This also means that this class cannot be instantiated on its own
Dim a As New Animal.
The other new keyword that appears is the
MustOverride before the
Sub keyword. This is used to determine that this method will not do anything on it's own
but it must be given code to perform something. The subclass is
the one that will define this.
Inheritance builds upon the fundamentals of abstraction. It is designed to transform one class into another. Inheritance however is not limited to abstract classes.
With abstract classes
In terms of abstract classes, inheritance is the way that they can be used. For the Animal class, the following sample demonstrates the inherited class.
Public Class Dog Inherits Animal Public Sub Bark() 'Place code for Bark here End Sub Public Overrides Sub Communicate() Bark() End Sub End Class
Most notably are the two new keywords
Inherits keyword is used
to define which class this class will inherit properties and
methods from, including variables and so on. The
is used to specify what a an abstract method (that's one in an
abstract class with
MustOverride in front of it)
will do. In this case, the
Communicate method is
being given code that will call the method
following sample instantiates the
Dim d As New Dog d.Bark() d.Communicate() d.OutputHeight() d.Sleep()
Bark will do the same thing. To conceal the
Bark method it can be declared
the access modifier.
With standard classes
Inheritance is not just limited to abstract classes. Standard
classes are also inheritable. The following example takes the
Dog class and creates a class for a labrador.
Public Class Labrador Inherits Dog Public Overrides Sub Communicate() MyBase.Communicate() MsgBox("I'm a labrador") End Sub End Class
MyBase keyword is used to refer to the
parent class of the class, that is in this case the
This means that it calls the
Communicate method within the
If this is taken a step further as shown in the following sample, the object gets deeper, but at the cost of speed (for each inherited class it will keep getting slower).
Public Class Jack Inherits Labrador Public Overrides Sub Communicate() MyBase.Communicate() MsgBox("My name is Jack") End Sub End Class
This will inherit from
Labrador which in turn inherits from
Dog which in turn inherits from
Classes from within the .NET Framework can also be inherited.
In the following case this class is inherited from the
RichTextBox control class.
Public Class AdvancedRTB Inherits System.Windows.Forms.RichTextBox Public Sub Print() 'Place code to print rich textbox here End Sub End Class
If a class has important methods that should remain public but
should not be able to be inherited, for instance in the case of
protecting a rich textbox control from others editing it,
inheritance must be prevented using the
This modifier will prevent access directly to methods and is
demonstrated below with the sample shown before.
Public NotInheritable Class AdvancedRTB Inherits System.Windows.Forms.RichTextBox Public Sub Print() 'Place code to print rich textbox here End Sub End Class
Why abstraction is useful
Abstraction is useful when there are methods that must be
implemented a certain way along side methods that can be implemented
another way. One example is with the
Animal class which
must be able to do things like
Sleep, which all animals do and could
be directly implemented into the class, but there are also methods
Communicate that can be performed in different
ways for each animal, so this is left to the class of animal such as
Cat to implement.
Abstraction leaves scope for individualisation rather than objectification.