Chapter 6, Part 2

1
CIS159

• Chapter 6, Part 2
• Constructors
• Ch06_2.ppt

2
Overview of Topics

• Class vs Object
• ReadOnly Properties
• Instance vs. Shared
• Constructors and Destructors

3
Class and Objects

• A class is a definition of a data type.
• A class definition includes members variables and
  methods.
• An object is a variable declared using a class
  definition as the data type.
• A class is the definition, and an object is an
  instance of the class.

4
DateMDY Class

• Notation
• - private
• public
• protected

5
DateMDY Definition

• Public Class DateMDY
• Private intMonth, intDay, intYear As Integer
• Property Month( ) As Integer
• Get
• Month intMonth
• End Get
• Set (ByVal Value As Integer) Value required
  keyword
• intMonth Value
• End Set
• End Property
• need property procedures for Day and Year
• Public Function getDate( ) As String getDate
  CStr(intMonth) / CStr(intDay)
  / CStr(intYear)End Function
• End Class

6
Public vs Private Members

• Public variables can be referenced and altered in
  procedures that declare an object using the class
  definition.
• Public methods can be called directly in
  procedures that declare an object using the class
  definition.
• Public is the default if not specified.
• Private variables can only be referenced and
  altered by methods defined inside of the class.
• Private methods can only be called by methods
  defined inside of the class.
• We must provide Property Procedures for private
  variables that allow programmers to set and get
  the values stored in the private variables.

7
ReadOnly Properties

• This is used for variables that are maintained by
  the class, but may need to provide the value to
  the program at some time.
• This could be used on running totals, counts, or
  constants used in the class.
• The getDate function could be converted to a
  ReadOnly Property.
• Use the keyword ReadOnly on the property
  procedure to only create the Get portion.

8
Defining ReadOnly

• Public Class DateMDY
• Need to return the date for all instances
  of DateMDY.Convert getDate to Property Date.
• ReadOnly Property Date ( ) As String
• Get
• Date CStr(intMonth) /
  CStr(intDay) / CStr(intYear)
• End Get
• No Set because it is read only.End Property
• End Class

9
Using ReadOnly Property Procedures

• Dim bday As New DateMDY
• bday.Month 10 use property procedure names
• bday.Day 31 Automatically calls Set
  procedure
• bday.Year 1980
• txtDate.Text bday.Date( ) Calls the Get
  procedure for Date

10
Instance vs. Shared Variables

• Variables defined in a class are normally
  considered Instance variables.
• Each object created would have separate memory
  allocations for their own variables.
• A shared variable is a single variable for all
  objects.
• It can be used to keep a running total, count, or
  it could be a constant.
• Shared members can be accessed without
  instantiating an object of the class (use
  className.member).
• Property methods that reference a shared variable
  must also be declared as shared.
• Methods that are declared as shared, must only
  reference class variables that are shared and/or
  local variables.

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Defining Shared

• Public Class DateMDY
• The current date is needed for all
  instances of DateMDYWe would need to assign the
  system date to these variables
• Private Shared intCurrMonth, intCurrDay,
  intCurrYear As Integer
• Shared ReadOnly Property CurrentDate( ) As
  String
• Get
• CurrentDate CStr(intCurrMonth)
  / CStr(intCurrDay) /
  CStr(intCurrYear)
• End Get
• No Set because it is read only.End Property
• End Class

12
Using Shared Variables

• Dim bday As New DateMDY
• Dim dueDate As New DateMDY
• lblCurrDate.Text bday.CurrentDate Both
  have the same value
• lblCurrDate.Text dueDate.CurrentDate
• Shared methods can also be called using class
  name.
• lblCurrDate.Text DateMDY.CurrentDate
• Need to assign values first

13
Passing Objects to Functions

• Individual variables can be passed to functions
  using Get procedures FormatNumber(bday.Month)
• Entire objects can be passed to functionsSub
  add30Days(ByRef dueDate As DateMDY)
• The reference (address) is passed to the
  function, so the local object actually points to
  the location of the original object.
• Any changes performed in the function will effect
  the original object.
• In a call-by-value the changes will be local to
  the function.

14
Constructor Characteristics

• A special method of the class.
• The name of the Sub is New ( ).
• Can NOT return a value.
• Primarily used to initialize member variables.
• Automatically called when an object of that class
  type is declared.
• Must be Public.

15
Default Constructor

• Default constructor has no parameters.
• Always define a default constructor, even if it
  will not be doing any initialization.
• If one is not defined,the compiler will generate
  one.
• Usually used to assign a default value. Public
  Sub New ( ) intMonth 1 assign default
  date intDay 1 intYear 1900 End Sub

16
Overloaded Constructors

• Constructors can be overloaded.
• Same method name but a different number or type
  of parameters.
• Initial values can be passed at declaration if
  there is a parameterized constructor
  defined.Sub New (ByVal m As Integer, _ ByVal
  d As Integer, _ ByVal y As
  Integer) intMonth m intDay d intYear
  yEnd Sub

17
Property Procedures and Constructors

• Use property procedures by using the property
  name, because any validation would be performed
  in the Set procedure.
• Sub New (ByVal m As Integer, _ ByVal d As
  Integer, _ ByVal y As Integer) Month
  m calls set procedure Day d Year yEnd
  Sub

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Keyword Me

• Sometimes we declare variables in class
  procedures using the same names as of the class
  variables or property names.
• Local variables override class variables and
  properties.
• Use Me to reference the class variables or
  properties.Sub New (ByVal Month As Integer,
  _ ByVal Day As Integer, _ ByVal Year As
  Integer) Me.Month Month calls set
  procedure Me.Day Day Me.Year YearEnd Sub

19
Date Property

• In the next two slides, you are presented with
  some questions, and you will need to recall what
  the Date property does, so it is presented here
  as a reminder.
• ReadOnly Property Date ( ) As String
• Get
• Date CStr(intMonth) /
  CStr(intDay) / CStr(intYear)
• End Get
• No Set because it is read only.End Property

20
Using Default Constructors

• Public Sub New ( ) intMonth 1 assign default
  date intDay 1 intYear 1900End Sub
• In the declaration below determine why the
  default constructor is called and what is
  returned by Date( ).
• Dim bday As New DateMDY
• txtDate.Text bday.Date( )

21
Using Overloaded Constructors

• Sub New (ByVal m As Integer, _ ByVal d As
  Integer, _ ByVal y As Integer) Month
  m calls set procedure Day d Year yEnd
  Sub
• In the declaration below determine why overloaded
  constructor is called and what is returned by
  Date( ).
• Dim payDate As New DateMDY(2, 26, 2002)
• txtDate.Text payDate.Date( )

22
Constructor Example

• Dim strName As String calls default
• Dim strName As New String (Marquez) calls
  overloaded
• Note must use New to access overloaded String
  constructor.

23
Destructor

• A method that is automatically called when object
  goes out of scope.
• Must be public.
• The name of Sub is Finalize( ).
• Can NOT return a value.
• Can be used to cleanup dynamic variables.
• It is implemented as an Override method.
• It is recommended that we do not declare one
  unless it is required, because the .Net Framework
  performs it own garbage collection (releases
  memory).

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Class Outline - Expanded

• Class Name
• Properties (variables)
• Operations (methods)
• Constructors
• Property Procedure (set and get)
• Operations (getDate, Date)
• Destructors

25
Data Validation in Property Procedures

• In property procedures data validation should
  usually be included.
• When invalid data is sent in it can be rejected
  by Throwing an exception.
• That means that the declaration of the object and
  usage of its property methods should be inside
  an Try/Catch block.
• In our assignments we will NOT be including data
  validation ?
• The textbook examples include if statements to
  validate, but dont do anything if data is
  invalid ?.

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Data Validation Definition

• Property Description As String
• Get
• Description mstrDescription
• End Get
• Set (ByVal Value As String) If Value Not
  Then
• mstrDescription Value Else
  Throw New System.Exception(Description is
  missing.)End Set
• End Property

27
Data Validation Usage

• Private Sub btnProcess()
• Try objBookSale New BookSale
• objBookSale.Description txtDescription.Test
  additional processing here
• Catch ex As Exception
• MessageBox.Show("Error " ControlChars.NewLine
  _ ex.Message) End Try
• End Sub

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Summary

• Class vs Object
• ReadOnly Properties
• Instance vs. Shared
• Constructors and Destructors

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Next

• Review BookSale class in textbook (p283-284).
• Shared, ReadOnly, Constructors
• Review VB8 Classes
• Complete clsCustomer and clsOrder
• Next Class
• Ch 6 Part 3 Inheritance
• Assign VB9 Inheritance