Chapter 6
OOP: Creating Object-
Oriented Programs
Programming In
Visual Basic .NET

Object-Oriented (OO) Program
•
Objects
– Consist of one or more data values which define the state or properties of the object
Class
Message
Data
– Encapsulated by a set of functions (methods) that can be applied to that object
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Object-Oriented (OO) Program
•
Class
– Defines:
• Characteristics of the data contained by objects of the class
• Functions that can be applied to the objects of the class
Class
Data Data Data
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"Cookie Analogy"
•
Class = Cookie cutter
• Instantiate = Making a cookie using the cookie cutter
• Instance = Newly made cookie
• Properties of the Instance may have different values
– Icing property can be True or False
–
Flavor property could be Lemon or Chocolate
Data Data Data
Object
6- 4
Object Object
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"Cookie Analogy" (continued)
•
Methods = Eat, Bake, or Crumble
• Events = Cookie crumbling all by itself and informing you
6- 5
Distinction between method and an event is somewhat fuzzy
© 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

Object-Oriented Terminology
•
Encapsulation
• Inheritance
• Polymorphism
• Reusable Classes
• Multitier Applications
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Encapsulation
•
Combination of characteristics of an object along with its behavior in "one package"
•
Cannot make object do anything it does not already
"know" how to do
•
Cannot make up new properties, methods, or events
•
Sometimes referred to as data hiding; an object can expose only those data elements and procedures that it wishes
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Encapsulation
Class
Message
Data
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Inheritance
•
Ability to create a new class from an existing class
– Original class is called Base Class, Superclass, or
Parent Class
–
Inherited class is called Subclass, Derived Class, or
Child Class
•
For example, each form created is inherited from the existing Form class
•
Purpose of inheritance is reusability
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Inheritance (continued)
•
Examine first line of code for a form in the Editor
Inherited Class: Subclass, Derived Class, Child Class
Public Class Form1
Inherits System.Windows.Forms.Form
6- 10
Original Class: Base Class, Superclass, Parent Class
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Inheritance Example
•
Base Class
– Person
• Subclasses
– Employee
– Customer
–
Student
Properties
Person
-Name
-Address
-Phone
Employee Customer Student
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Polymorphism
•
Methods having identical names but different implementations
•
Overloading
–
Several argument lists for calling the method
– Example : MessageBox.Show method
• Overriding
– Refers to a class that has the same method name as its base class
– Method in subclass takes precedence
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Reusability
•
Big advantage of OOP over traditional programming
• New classes created can be used in multiple projects
• Each object created from the class can have its own properties
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Multitier Applications
•
Classes are used to create multitier applications
• Each of the functions of a multitier application can be coded in a separate component and stored and run on different machines
•
Goal is to create components that can be combined and replaced
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Three-tier Model
•
Most popular implementation
Presentation Tier Business Tier
User Interface
Forms
Controls
Menus
Business Objects
Validation
Calculations
Business Logic
Business Rules
Data Tier
Data Retrieval
Data Storage
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Instantiating An Object
•
Creating a new object based on a class
• Create an instance of the class by using the New keyword
• General Form
6- 16
New className( )
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New Keyword Examples
Dim arialFont As Font = New Font ("Arial", 12) messageLabel.Font = arialFont
OR messageLabel.Font = New Font ("Arial", 12)
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Specifying a Namespace
•
In your projects, you have noticed the Inherits clause when VB creates a new form class
6- 18
Name of the Class
Public Class Form1
Inherits System.Windows.Forms.Form
Namespace
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Specifying a Namespace
(continued)
• Entire namespace is not needed for any classes in the namespaces that are automatically included in a Windows
Forms project which include
–
System
– System.Windows.Forms
–
System.Drawing
• When referring to a class in a different namespace
–
Write out the entire namespace or
– Add an Imports statement at the top of the code to specify the namespace
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Designing Your Own Class
•
Analyze characteristics needed by new objects
– Characteristics or properties are defined as variables
– Define the properties as variables in the class module
• Analyze behaviors needed by new objects
– Behaviors are methods
–
Define the methods as sub procedures or functions
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Creating Properties in a Class
•
Define variables inside the Class module by declaring them as Private
•
Do not make Public, that would violate encapsulation
(each object should be in charge of its own data)
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Property Procedures
• Properties in a class are accessed with accessor methods in a property procedure
•
Name used for property procedure is the name of the property seen by the outside world
• Set Statement
–
Uses Value keyword to refer to incoming value for property
– Assigns a value to the property
•
Get Statement
– Retrieves a property value
–
Must assign a return value to the procedure name
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Property Procedure General Form
Private ClassVariable As DataType
[Public] Property PropertyName( ) As DataType
Get
Return ClassVariable [PropertyName = ClassVariable]
End Get
Set (ByVal Value As DataType )
[ statements, such As validation ]
ClassVariable = Value
End Set
End Property
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Read-Only Properties
•
In some instances a value for a property should only be retrieved by an object and not changed
–
Create a read-only property by using the ReadOnly modifier
–
Write only the Get portion of the property procedure
6- 24
[Public] ReadOnly Property PropertyName( ) As DataType
© 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

Write-Only Properties
•
At times a property can be assigned by an object but not retrieved
–
Create a property block that contains only a Set to create a write-only property
6- 25
[Public] WriteOnly Property PropertyName( ) As DataType
© 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

Class Methods
•
Create methods by coding public procedures within a class
• Methods declared with the Private keyword are available only within the class
•
Methods declared with the Public keyword are available to external objects
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Constructors and Destructors
•
Constructor
– Method that automatically executes when an object is instantiated
–
Constructor must be public and is named New
• Destructor
– Method that automatically executes when an object is destroyed
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Overloading Methods
•
Overloading means that two methods have the same name but a different list of arguments (the signature)
•
Create by giving the same name to multiple procedures in your class module, each with a different argument list
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Parameterized Constructor
•
Constructor that requires arguments
• Allows arguments to be passed when creating an object
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Create a New Class
•
Project, Add Class
• In Add New Item dialog box, choose
Class
•
Name the Class
• Define the Class properties
• To allow access from outside the class, add property procedures
• Code the methods
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Creating a New Object
Using a Class
•
Similar to creating a new tool for the toolbox but not yet creating an instance of the class
•
Declare a variable for the new object
•
Then, instantiate the object using the New keyword
Private aBookSale As BookSale aBookSale = New BookSale( )
Or
Dim aBookSale As New Booksale( )
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Creating a New Object
Using a Class (continued)
•
If object variable is needed in multiple procedures, delcare the object at class level
•
Instantiate the object
–
Only when(if) it is needed
– Inside a Try/Catch block for error handling (Try/Catch block must be inside a procedure)
• Pass values for the arguments at instantiation when using a parameterized constructor
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Instance Variables versus Shared
Variables
•
Instance variables or properties
– Separate memory location for each instance of the object
•
Shared variables or properties
– Single variable that is available for ALL objects of a class
– Can be accessed without instantiating an object of the class
– Use the Shared keyword to create
6- 33
Shared Methods can also be created
© 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

Garbage Collection
•
Feature of .NET Common Language Runtime (CLR) that cleans up unused components
•
Periodically checks for unreferenced objects and releases all memory and system resources used by the objects
•
Microsoft recommends depending on Garbage Collection rather than Finalize procedures
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Inheritance
•
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
• Use the Inherits statement following the class header and prior to any comments
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Overriding Methods
•
Methods with the same name and the same argument list as the base class
•
Derived class (subclass) will use the new method rather than the method in the base class
•
To override a method
–
Declare the original method with the Overridable keyword
–
Declare the new method with the Overrides keyword
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Creating a Base Class Strictly for
Inheritance
•
Classes can be created strictly for inheritance by two or more similar classes and are never instantiated
•
For a base class that you intend to inherit from, include the
MustInherit modifier on the class declaration
•
In each base class method that must be overridden, include the MustOverride modifier and no code in the base class method
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Inheriting Form Classes
•
Many projects require several forms
• Create a base form and inherit the visual interface to new forms
•
Base form inherits from System.Windows.Forms.Form
• New form inherits from Base form
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Creating Inherited Form Class
•
Project menu, Add Windows Form
• Modify the Inherits Statement to inherit from base form using project name as the namespace
•
OR
• Project menu, Add Inherited Form
• In dialog select name of base form
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Coding for Events of an Inherited
Class
•
Copy procedure from base class into derived class and make modifications
•
An alternate way is to use the inherited event handler in the derived class
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Managing Multiclass Projects
•
VB projects are automatically assigned a namespace which defaults to the name of the project
•
Add an existing class to a project by copying the file into the project folder and then adding the file to the project
–
Project/Add Existing Item
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Displaying Values on a Different
Form
•
Refer to controls on another form by using the identifier for the form instance
•
General Syntax
•
Example
FormInstance.ControlName.Property
Dim aSummaryForm As New summaryForm( ) aSummaryForm.SalesLabelTotal.Text = aBookSale.SalesTotal.ToString( " C " )
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Using the 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
•
To Display
–
Click on Tab in Editor Window
–
Object Browser toolbar button
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Object Browser
Objects list
Browse list
Find
Symbol Members list
Namespace icons
Constants icon
Class icon
6- 44
Method icon
Property icon
Description Pane
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Examining VB Classes
Members of
System.Windows.Forms.MessageBox
Class
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Examining VB Classes (continued)
Display the
MessageBoxButtons Constants
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