Microsoft VB 2005: Reloaded, Advanced

1
Microsoft VB 2005 Reloaded, Advanced

• Chapter 4
• Object Orientation Inheritance and Polymorphism

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Objectives

• Explain how Visual Basic predefined classes use
  inheritance
• Create overloaded methods in a class
• Override methods in a base class with methods in
  a derived class

3
Objectives (continued)

• Create Visual Basic classes that have inheritance
  relationships
• Create abstract classes and methods in an
  application
• Use overtyped variables through polymorphism
• Use the Class Designer to create and modify
  applications

4
Inheritance among Visual Basic Predefined Classes

• Inheritance
• Special relationship between two classes
• Base class gives the derived class direct access
  to all of its Public (not Private) members
• Idea behind inheritance
• Promote code reuse

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An Inheritance Hierarchy for Predefined .NET
Framework Classes

• Inheritance hierarchy
• Treelike structure showing how classes inherit
  the members of other classes
• MarshalByRefObject
• Required for objects in one application domain to
  communicate with objects in another application
  domain using object references

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An Inheritance Hierarchy for Predefined .NET
Framework Classes (continued)
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The Is-A Class Relationship

• Inheritance is called an is-a relationship
• Instance of a derived class is a (or rather, is
  an) instance of any of its base classes

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Inheritance among Programmer-Defined Classes

• Inheritance can prove equally helpful when you
  are creating your own classes within applications

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Creating an Inheritance Relationship

• When you create a new Windows Application project
  in Visual Studio 2005
• The form that appears uses inheritance

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Creating an Inheritance Relationship (continued)
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Creating an Inheritance Relationship (continued)
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Creating an Inheritance Relationship (continued)
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More About Access Modifiers

• Access modifiers
• Public
• No restrictions on access to a class member
• Private
• Class members can be directly accessed only
  within the class in which the members are
  declared
• Friend
• Class member can be directly accessed from
  anywhere in the application in which it is
  defined
• Intra-application access

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More About Access Modifiers (continued)

• Access modifiers (continued)
• Protected
• Member is directly accessible within the
  declaring class and classes that inherit from the
  declaring class
• Protected Friend
• Grants access anywhere within an application and
  within any derived classes
• Principle of least privilege
• Elements in a computing environment should be
  granted only information and resources that are
  immediately necessary

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More About Access Modifiers (continued)
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Inheritance Overloading and Overriding

• Objective
• Understand how inheritance relates to the topics
  of overloading and overriding methods in Visual
  Basic

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Overloaded Methods in the Same Class

• Overloaded method
• One of two or more methods in the same class or
  inheritance hierarchy with the same name
• But an otherwise different method signature
• Method signature
• Combination of method name and its parameter list
• If two methods have the same name but different
  signatures
• They must have different parameter lists

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Overloaded Methods in the Same Class (continued)
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Overloaded Methods in the Same Class (continued)
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Overloaded Methods in the Same Class (continued)
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Overloaded Methods in a Derived Class

• Within an inheritance hierarchy of classes
• An overloaded method in a derived class must use
  the keyword Overloads in its declaration

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Overriding Methods

• Overridden method
• A method in a base class with exactly the same
  signature as a method in a derived class
• Method in derived class has precedence
• Will be used by objects instantiated from the
  derived class
• Objects instantiated from the base class will use
  the method in the base class
• Example
• ToString() method

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Overriding Methods (continued)
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Using Inheritance in Applications

• Objective
• Implement inheritance in an application

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Planning the College Personnel Application Using
UML
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Implementing the College Personnel Application
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Implementing the College Personnel Application
(continued)
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Abstract Classes and Methods

• Abstract classes and methods
• Makes program code more organized and secure

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Abstract Classes

• Abstract class
• Class with data members and/or method members
  from which objects cannot be instantiated
• Container for common sets of data members and
  method members
• That other object-defining classes can inherit
  and implement
• Concrete class
• Class from which objects can be instantiated
• Declaring an abstract class example
• Public MustInherit Class Person

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Abstract Methods

• Abstract method
• Declared in a base class but has no body or
  implementation in that base class
• Must be implemented in all derived classes
• Implementations can be different in each class
• Declaring abstract methods
• Use keyword MustOverride in base class
• Use keyword Overrides in derived class

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Polymorphism and Overtyping

• Polymorphism literally means many forms
• Polymorphism
• Overloading and overriding are both examples of
  polymorphism
• Overtyping is another example of polymorphism
• Overtyping
• Variable with the data type of a base class can
  refer to a variable with the data type of a
  derived class

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Visual Studio 2005 Class Designer

• Class Designer
• Visual design environment for the Common Language
  Runtime
• Allows you to design a complete OO application
  using drag-and-drop techniques
• Also provides a visual representation of an OO
  application by creating a class diagram
• Makes it easier for you to edit existing code and
  communicate your application design to others

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Adding a Class Diagram to an Existing Application
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Adding a Class Diagram to an Existing Application
(continued)
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Adding a Class Diagram to an Existing Application
(continued)
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Adding a Class Diagram to an Existing Application
(continued)
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Adding a Class Diagram to an Existing Application
(continued)
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Adding a Class Diagram to an Existing Application
(continued)
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Adding a Class with the Class Designer
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Adding a Class with the Class Designer (continued)
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Adding a Class with the Class Designer (continued)
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Making Final Modifications to the Application

• After using Class Designer
• Programmer still needs to provide details of
  writing the code necessary to make application
  work as planned

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Summary

• Inheritance
• Special relationship between two classes in which
  the base class gives to the derived class direct
  access to all of its Public (not Private) data
  members and method members
• Called an is-a relationship between classes
• Inheritance hierarchy
• Treelike structure showing how classes inherit
  the members of other classes
• Create an inheritance relationship using the
  keyword Inherits

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Summary (continued)

• Access modifiers Public, Private, Friend,
  Protected, Protected Friend
• An overloaded method is one of two or more
  methods in the same class or inheritance
  hierarchy with the same name but different
  signatures
• An overridden method is a method in a base class
  with same signature as a method in a derived
  class
• Adding classes to an existing applications
• An abstract class is a class from which objects
  cannot be instantiated

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Summary (continued)

• An abstract method
• Method that is declared in a base class but that
  has no body or implementation in that base class
• Polymorphism usually implies the ability of a
  variable with the data type of a base class to
  refer to a variable with the data type of a
  derived class
• Visual Studio 2005 Class Designer
• Allows you to create a visual representation of
  an object-oriented application