More about classes and objects

1. More about classes and objects Classes in Visual Basic.NET

2. 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

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

4. 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

5. 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

6. 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

7. 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

8. 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()

9. 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

10. 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