More about classes and objects Classes in Visual Basic.NET

We have seen... Basics of OOP terminology Object Class Method Overloading Overriding Basics of OOP concepts Abstraction Polymorphism Encapsulation Inheritance How to draw class diagrams

Shape, Circle and Rectangle Shape x, y : Integer draw() : String Circle r : Integer Rectangle h, w : Integer

Creating classes in VB.NET Classes are written in files, in a similar way than forms. They are added using the Add class... item in the Project menu. Involves... setting up the variables implementing a constructor implementing the methods required

Code for Shape Public Class Shape Protected x, y As Integer Public Sub New() x = 12 y = 30 End Sub Public Function draw() draw = "I am a nice shape" End Function End Class

Abstract classes Cannot be instantiated Only classes that extend them can be instantiated Used as holders for their subclasses VB.NET uses keyword "MustInherit" Public MustInherit Class Shape

Creating subclasses Two options: New file (through Add class...) Same file Subclasses might hold variables of their own VB.NET uses keyword "Inherits" Public Class Circle Inherits Shape

Overriding methods A subclass can redefine methods that appear in its parent Circle can give a new implementation to inherited method draw() VB.NET uses the keyword "Overrides" Public Overrides Function draw()

Abstract methods Sometimes the parent class just "mentions" a method, without providing an implementation VB.NET uses keyword "MustOverride" Any class with at least one abstract method must be declared abstract It is a good idea to make the method draw() in Shape abstract

Abstract Shape Public MustInherit Class Shape Protected x, y As Integer Public Sub New() x = 12 y = 42 End Sub Public MustOverride Function draw() End Class