1. Enterprise Development Using Visual Basic 6.0 Autumn 2002 Tirgul #5

2. Objectives Understanding objects Class Modules Properties Get/Let
Defining Types
Collections
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3. Object Classes Objects contains: Data Behavior Controls are objects
data (properties)
behavior(Functions and events)
All objects of the same type has the same data and behavior
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4. The Class Module
A class is a template: it defines the data and behavior of the objects that belong to it.
An instance is a variable associated with an object (a specific instance of a class).
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5. timeToCrewel(Dist as integer) as integer setSpeed(Speed as integer)
The Snail Abstraction
timeToCrewel(Dist as integer) as integer
age
goToSleep()
IsSleep
setSpeed(Speed as integer)
Color
Gender
Properties
Functions
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6. All Objects has the same properties and behavior
Multiple instances of a class
Class Car
Color as Colur
Speed as Double
Size as Double
Color = red
Speed = 200
Size = 100.5
Color = Blue
Speed = 120
Size = 160.7
Color = Yellow
Speed = 90
Size = 150.6
All Objects has the same properties and behavior
But not same values
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7. Property Methods Provide access to the instance variables
can assign or retrieve the value of an instance variable
property
A named attribute of an object. Define object characteristics such as size, color, and screen location, or the state of an object, such as enabled or disabled.
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8. Property Let Assign a value to a class property Syntax: Optionally:
Use ByVal retain the value of the parameter
Dim sName as String
Property Let Name(ByVal newName As String)
sName = newName
End Property
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9. Property Get Gets a value of a property Syntax: Dim sName as String
Property Get Name() as String
Name = sName
End Property
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10. Class Initialization Important for defaults
Will be called each new Instance
Private Sub Class_Initialize()
CTN = “ ”
age = 0
fName = “”
End Sub
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11. Behavior Functions Defines the behavior of the object
Done same as form functions/Subs
Public will be used by the user
Private will be hidden for the user
Public function getSalary(code as integer) as double
getSalary = getSalaryByCode(code)
End function
Private function getSalaryByCode(code as integer) as double
getSalaryByCode = rsSalary.Field(code).value
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12. Class can use class
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13. Declaring an Object Variable
You need to define the object and create new instance.
Could be done in the same line but better to separate
Set: Assign an object reference to variable
Dim myStudent as new cStudents
Dim myStudent as cStudents …
Set myStudent = new cStudents
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14. Collections A very useful object to hold objects
Unlike List, can hold Objects
Functions
Add – Adding objects
Item(index) – retrieve the Item in the index
Count – Number of items
Remove(Index) – remove an Item in the index
Dim Emp1 as new cEmployee
Dim Emp2 as new cEmployee
Dim Factory As New Collection
Factory.Add Emp1
Factory.Add Emp2
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15. Item Method Retrieves a specific Item from the collection
For index = 1 To Factory.Count
Debug.Print Factory.Item(index).Name
Next index
Property of the item
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16. Programmer defined Data Types
Type is a degenerate class
In a form (private) or module(Public)
Useful to hold data in a logic group
Referenced as a base type (no New)
Public Type Point
x as Integer
Y as Integer
End Type
Dim myPoint as Point
myPoint.x = 1
myPoint.y = 2 4 3 2 1 1 2 3 4
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17. Source Examples prjEmployee – Simple class use
prgStudent – Properties use
prgRoll_Call - Class_Initialize, Container class
prgOrderItem – with reserved word
prgBankAccount – Class functions
prgArithmeticSeries
Lec3LinkList – List example
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