110J1

1
Inheritance

• Ability to define a new class from an existing
  class
• Purpose of Inheritance is reusability
• For example, each form created is inherited from
  the existing Form class
• Original class is called Base Class, Superclass,
  or Parent Class
• Inherited class is called Subclass, Derived
  Class, or Child Class

2
Inheritance (cont.)

• Examine 1st line of code for a form in the Editor

Inherited Class, Derived Class, Subclass, Child
Class
Public Class Form1 Inherits System.Windows.Forms.
Form
Base Class, Superclass, Parent Class
3
Aggregation-Has a

• Sometimes a class refers to other objects that
  are essential components of the class and dont
  have an independent existence
• Public Class Car
• Properties, pieces of the car
• Dim mtheEngine As Engine
• Dim mTransmission As Transmission
• more code
• End Class
• Still a has-a relationship.
• A Transmission is not a Car, it is just part of a
  Car

4
Abstraction is-a

• Classes can be related in hierarchical ways
• Animals cats, dogs, birds
• Vehicles road vehicles car, truck, bus
  aircraft helicopter, jet, blimp
• Relationship is not the same as composition
  (has-a)
• A cat is-a animal
• Often, classes related this way have some
  behavior or state in common

5
Inheritance Example

• Base Class
• Person
• Derived Classes
• Employee
• Customer
• Student

6
Inheritance Implemented

• New class can
• Be based on another class (base class)
• Inherit the properties and methods (but not
  constructors) of the base class, which can be
• One of the VB existing classes
• Your own class
• Designate Inheritance by adding the Inherits
  statement referencing the base class

7
Polymorphism Overriding and Overloading

• Different classes of objects may have behaviors
  that are named the same but are implemented
  differently (Overriding)
• An object of a certain class might have several
  behaviors associated that are named the same but
  have different signatures (different type-number
  of arguments) and different implementation.(Overlo
  ading)

8
Polymorphism Implemented

• Overloading
• Argument type-number (signature) determines which
  version of a method is used
• Example MessageBox.Show method
• Overriding
• Refers to a class that has the same method name
  as its base class
• Method in subclass takes precedence

9
Overloading Implemented

• Overloading means that two or more methods have
  the same name but a different list of arguments.
• Create it by giving the same name to multiple
  procedures in your class module, each with a
  different argument list and a different
  implementation.

10
Overriding Methods

• Methods created in subclass with the same name
  and the same argument list as the methods in the
  base class.
• Subclass will use the method in its own class
  rather than that in the base class
• To override a method
• Declare the base class method with the
  Overridable keyword
• Declare the subclass method with the Overrides
  keyword

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Example Representing Animals

• Base Class
• Animal
• Derived Classes
• Dog
• Cat
• Cow

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Example Representing Animals

• Generic Animal
• Public Class Animal
• properties, module level variables
• .
• Return the noise this animal makes
• Public Overridable Function noise( ) As String
• Return "?
• End Function
• End Class

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Specific Animals

• Cats
• Public Class Cat
• Inherits Animal
• properties, module level variables
• .
• Return the noise cats makes
• Public Overrides Function noise( ) As String
• Return Meow
• End Function
• End Class

• Dogs
• Public Class Dog
• Inherits Animal
• properties, module level variables
• .
• Return the noise a dog makes
• Public Overrides Function noise( ) As String
• Return Woof
• End Function
• End Class

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MyBase Keyword
MyBase is commonly used to access base methods
that are overridden in a derived class.
Public Class Animal Dim mstrMessage As
String Property Property message() As
String code . End Property Public
Overridable Function noise() As
String Me.mstrMessage "the noise is "
End Function End Class
Public Class Dog Inherits Animal Public
Overrides Function noise() As String
MyBase.noise() MyBase takes all existing
code in the base class Return
(Me.message "Woof") End Function End Class
15
Creating a Base Class Strictly for Inheritance

• Classes can be created strictly for inheritance
  and are never instantiated
• Subclasses are created and instantiated which
  inherit the base class properties and methods
• For such a base class include the MustInherit
  modifier on the class declaration

16
Object Browser

• Use it to view the names, properties, methods,
  events and constants of VB objects, your own
  objects, and objects available from other
  applications
• How to access it
• Tab in Editor Window
• View menu, Other Window, Object Browser

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Opening Object Browser from Toolbar
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Object Browser
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Examining VB Classes
Members of System.Windows.Forms.MessageBox Class
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Examining VB Classes (cont.)
Display the MessageBoxButtons Constants