Chapter 6 OOP: Creating Object-Oriented Programs

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Chapter 6OOP Creating Object-Oriented Programs

• Programming In
• Visual Basic.NET

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Object Terminology Review

• Object - like a noun, a thing
• Buttons, Text Boxes, Labels
• Customer, Book, Library
• Properties - like an adjective, characteristics
  of object
• Text, ForeColor, Checked, Visible, Enabled,
  Price, Quantity

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Object Terminology Review

• Methods - like a verb, an action or behavior,
  something the object can do or have done to it
• ShowDialog, Focus, Clear, ToUpper,
  ToLower,Extendprice, CheckOut
• Events - object response to user action or other
  events
• Click, Enter, Activate

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Thus Far . . .

• Since Chapter 1 we have been using objects
• Up until now the classes for all objects used
  have been predefined
• We have created new objects for these classes by
  using the controls in the Toolbox
• VB allows programmers to create their own object
  types by creating a Class Module

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Class and Instance

• When we add a button object from the button tool
  in the toolbox to the form we are creating an
  Instance of the Button Class
• The button object is an Instance of the Button
  Class
• Every button on the form is an Instance
• Defining your own Class is like creating a new
  tool for the Toolbox

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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

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"Cookie Analogy" (cont.)

• Methods Eat, Bake, or Crumble
• Events Cookie crumbling all by itself and
  informing you

Methods and Events are often difficult to
distinguish!
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Encapsulation

• Combination of characteristics/properties/attribut
  es of an object along with its
  behavior/methods/events in "one package
• Cannot make object do anything it doesn't already
  "know" how to do
• Cannot make up new properties, methods, or
  events
• Sometimes referred to as data hidingan object
  can expose only those data elements and
  procedures that it wishes

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Inheritance

• Ability to create 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

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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
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Inheritance Example

• Base Class
• Person
• Subclasses
• Employee
• Customer
• Student

Dept
Balance
GPA
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Polymorphism

• Different classes of objects may have behaviors
  that are named the same but are implemented
  differently
• Programmers can request an action without knowing
  exactly what kind of object they have or exactly
  how it will carry out the action

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Polymorphism Implemented

• Overloading
• Argument type 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

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Reusability

• The main purpose behind OOP and Inheritance in
  particular
• New classes created with Class Module can be used
  in multiple projects
• Each object created from the class can have its
  own properties

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Multitier (n-tier) Applications

• Common use of classes is 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 common implementation of multitier

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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 and specify the class
• General Form

New className ( )
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Examples of InstantiatingAn Object
Dim fntMyFont New Font ("Arial",
12) lblMsg.Font fntMyFont OR lblMsg.Font
New Font ("Arial", 12) NEW is a constructor
method
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Specifying a Namespace

• In your projects, you have noticed the Inherits
  clause when VB creates a new form class

Name of the Class
Public Class Form1 Inherits System.Windows.Forms.
Form
Namespace
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Namespace (cont.)

• 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 classes in a different
  namespace
• Write out the entire namespace
• Add an Imports Statement to include the namespace

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lblMessage.Font NEW System.Drawing.Font(Arial,
12) Or IMPORTS System.Drawing lblMessage.Font
NEW Font(Arial,12)
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Class Design - Analyze

• Characteristics of your new objects
• Characteristics will be properties
• Define the properties as variables in the class
  module
• Behaviors of your new objects
• Behaviors will be methods
• Define the methods as sub procedures and
  functions in the class module

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Create a New Class

• Project, Add Class Module
• Add New Item dialog, choose Class
• Name the Class
• Define the Class properties
• Code the methods

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Properties of 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)
• Private mintPatientNum as Integer
• Private mdtmDate as Date
• Private mstrLastName as String

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Assign Values to Properties

• Write special property procedures to
• Pass the values to the class module
• Return values from the class module
• Name used for property procedure is the name of
  the property seen by the outside world
• Property Get
• Retrieves property values from a class
• Like a function must return a value
• Property Set
• Sets or assigns values to properties

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Property Procedure General Form
Private ClassVariable As DataType Public
Property PropertyName As DataType Get PropertyN
ame ClassVariable End Get Set (ByVal Value
As DataType) statements, such as
validation ClassVariable Value End Set End
Property
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Property Procedure General Form
Private mintQtyOrdered As Integer Public
Property QuantityOrdered As Integer Get
QuantityOrdered mintQtyOrdered End Get Set
(ByVal Value As DataType) statements, such as
validation mintQtyOrdered 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 a Get portion of the property procedure

Public ReadOnly Property PropertyName As
DataType
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Code a Method

• Create methods by adding sub procedures and
  functions for the behaviors to the class module
• The sub procedures and functions should be
  defined as Public

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Create Regions

• Regions of code allow sections of code to be
  hidden in the same way that the Editor hides
  Windows generated code
• To add a region
• Include Region Statement followed by a string
  literal giving the region's name
• End Region tag will be added automatically
• Write code between the 2 statements

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Creating a New Object Using a Class

• Similar to creating a new tool for the toolbox
  but not yet instantiating
• Declare a variable for the new object with
  datatype of the class
• Then, instantiate the object using the New keyword

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Best Practices

• You may declare and instantiate an object at the
  same time but not best practice
• Should declare the variable separately in the
  Declarations section
• 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)

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Instance versus Shared Variables

• Instance variables or properties
• Separate memory location for each instance of the
  object
• Shared variables or properties
• Single memory location 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

Shared Methods can also be created
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Shared Properties

• ReadOnly
• Otherwise violates encapsulation

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Constructors and Destructors

• Constructor
• Method that automatically executes when an object
  is instantiated
• Create by writing a Public Sub New procedure
• Destructor
• Method that automatically executes when an
  object is destroyed
• Create by writing a Finalize procedure
• Usage discouraged by Microsoft

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Overloading

• Overloading means that 2 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
• Can be used to assign initial property values
• If parameterizing NEW()
• Must have a NEW() with no parameters

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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 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

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Overriding Methods

• Methods created in subclass with the same name
  and the same argument list
• 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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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

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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
• Subclass from inherits from Base form

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Creating Inherited Form Class

• Project menu, Add Windows Form
• Modify the Inherits Statement to inherit from
  bass form using project name as the namespace
• OR
• Project menu, Add Inherited Form
• In dialog select name of Base form

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Creating Inherited Form Class

• Cannot Delete inherited controls
• Make visible false
• Overriding inherited events
• Cant double click control
• Copy and Paste
• In Code window
• Select Overrides
• Select event

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Referencing Values on a Different Form

• Use the identifier for the other form's instance
  to refer to controls on different form
• General Syntax
• Example

FormInstance.ControlName.Property
Dim frmSumInstance as New frmSum(
) frmSumInstance.lblTotal.TextFormatCurrency(mBoo
kSale.mdecTotal)
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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
• Accessed
• 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