1. Classes, Exceptions, Collections, and Scrollable Controls
Chapter 12
Classes, Exceptions, Collections,
and Scrollable Controls

2. Chapter 12 Introduction

3. Chapter 12 Topics Classes Exceptions Collections Object Browser
Abstract Data Types
Objects, Properties, Methods
Exceptions
Collections
Object Browser
Scrollable Controls

4. Section 12.1 Classes and Objects
Classes Are Program Structures That Define Abstract Data Types and Are Used to Create Objects

5. Abstract Data Types
An abstract data type (ADT) is a data type created by a programmer
ADTs are important in computer science and object-oriented programming
An abstraction is a model of something that includes only its general characteristics
Dog is an abstraction
Defines a general type of animal but not a specific breed, color, or size
A dog is like a data type
A specific dog is an instance of the data type

6. Classes
A class is a program structure that defines an abstract data type
Must create the class first
Then can create instances of the class
Class instances share common attributes
VB forms and controls are classes
Each control in the toolbox, is its own class
When you place a button on a form you’re creating an instance of the button class
An instance is also called an object

7. Properties, Methods, and Events
Programs communicate with an object using the properties and methods of the class
Class properties example: Buttons have Location, Text, and Name properties
Class methods example: The Focus method functions identically for every single button
Class event procedures: Each button in a form has a different click event procedure

8. Object Oriented Design
The challenge is to design classes that effectively cooperate and communicate
Analyze program specifications to determine ADTs that best implement the specifications
Classes are fundamental building blocks
Typically represent nouns of some type

9. Object Oriented Design Example
Specifications:
We need to keep a list of students that lets us track the courses they have completed. Each student has a transcript that contains all information about his or her completed courses. At the end of each semester, we will calculate the grade point average of each student . At times, users will search for a particular course taken by a student.
Nouns from the specification above usually become classes in the program design
Verbs such as calculate GPA and search become methods of those classes

10. OOD Class Characteristics
Class Attributes (properties) Operations (methods)
Student LastName, FirstName, Display, Input
IdNumber
StudentList AllStudents, Count Add, Remove,
FindStudent
Course Semester, Name, Display, Input
Grade,Credits
Transcript CourseList, Count Display, Search,
CalcGradeAvg

11. Interface and Implementation
Class interface is the portion of the class visible to the application programmer
Class implementation is the portion of the class hidden from client programs
Private member variables
Private properties
Private methods
Hiding of all data and procedures inside a class is referred to as encapsulation

12. Section 12.2 Creating a Class
To Create a Class in Visual Basic, You Create a Class Declaration
The Class Declaration Specifies the Member Variables, Properties, Methods, and Events That Belong to the Class

13. Class Declaration ClassName is the name of the class
Public Class ClassName
MemberDeclarations
End Class
ClassName is the name of the class
Examples of MemberDeclarations are presented in the following slides
To create a new class:
Clicking Add New Item button on toolbar
Select Class from Add New Item dialog box
Provide a name for the class and click Add
Adds a new, empty class file (.vb) to project

14. Member Variables A variable declared inside a class declaration
Syntax:
AccessSpecifier may be Public or Private
Example:
AccessSpecifer VariableName As DataType
Public Class Student
Public strLastName As String ‘Student last name
Public strFirstName As String ‘Student first name
Public strId As String ‘Student ID number
End Class

15. Creating an Instance of a Class
A two step process creates a class object
Declare a variable whose type is the class
Create instance of class with New keyword and assign the instance to the variable
freshman defined here as an object variable
Can accomplish both steps in one statement
Dim freshman As Student
freshman = New Student()
Dim freshman As New Student()

16. Accessing Members
Can work with Public member variables of a class object in code using this syntax:
For example:
If freshman references a Student class object
And Student class has member variables strFirstName, strLastName, and strID
Can store values in member variables with
object.memberVariable
freshman.strFirstName = "Joy"
freshman.strLastName = "Robinson"
freshman.strId = "23G794"

17. Property Procedure
A property procedure is a function that behaves like a property
Controls access to property values
Procedure has two sections: Get and Set
Get code executes when value is retrieved
Set code executes when value is stored
Properties almost always declared Public to allow access from outside the class
Set code often provides data validation logic

18. Property Procedure Syntax
Public Property PropertyName() As DataType
Get
Statements
End Get
Set(ParameterDeclaration)
End Set
End Property

19. Property Procedure Example
Public Class Student
' Member variables
Private sngTestAvg As Single
Public Property TestAverage() As Single
Get
Return sngTestAvg
End Get
Set(ByVal value As Single)
If value >= 0.0 And value <= Then
sngTestAvg = value
Else
MessageBox.Show( _
"Invalid test average.", "Error")
End If
End Set
End Property
End Class

20. Setting and Validating a Property
TestAverage property is set as shown:
Passes 82.3 into value parameter of Set
If in the range 0.0 to 100.0, value is stored
If outside the range, message box displayed instead of value being stored
Dim freshman as New Student()
freshman.TestAverage = 82.3
Set(ByVal value As Single)
If value >= 0.0 And value <= Then
sngTestAvg = value
Else
MessageBox.Show( _
"Invalid test average.", "Error")
End If
End Set

21. Read-Only Properties Useful at times to make a property read-only
Allows access to property values but cannot change these values from outside the class
Use the ReadOnly keyword instead of Public
This causes the propertyName to be read-only -- not settable from outside of the class
ReadOnly Property PropertyName() As DataType
Get
Statements
End Get
End Property

22. Read-Only Property Example
' TestGrade property procedure
ReadOnly Property TestGrade() As Char
Get
If sngTestAverage >= 90
return "A"
Else If sngTestAverage >= 80
return "B"
Else If sngTestAverage >= 70
return "C"
Else If sngTestAverage >= 60
return "D"
Else
return "F"
End If
End Get
End Property

23. Object Removal & Garbage Collection
Memory space is consumed when objects are instantiated
Objects no longer needed should be removed
Set object variable to Nothing so it no longer references the object
Variable is a candidate for garbage collection when it no longer references an object
The garbage collector runs periodically destroying objects no longer needed
freshman = Nothing

24. Going Out of Scope
An object variable instantiated within a procedure is local to that procedure
An object goes out of scope when
Referenced only by local variables and
The procedure ends
Object removed once it goes out of scope
An object instantiated in a procedure and assigned to a global variable is not removed
Reference remains when procedure ends

25. Going Out of Scope, Example
Sub CreateStudent()
Dim sophomore As Student
sophomore = New Student()
sophomore.FirstName = "Travis"
sophomore.LastName = "Barnes"
sophomore.IdNumber = "17H495"
sophomore.TestAverage = 94.7
g_studentVar = sophomore
End Sub
With this statement, sophomore will not go out of scope.
Without this statement, it will go out of scope when the procedure ends. (g_studentVar is a module-level variable.)

26. Comparing Object Variables
Multiple variables can reference the same object
Can test if two variables refer to same object
Must use the Is operator
The = operator cannot be used to test for this
Dim collegeStudent As Student
Dim transferStudent As Student
collegeStudent = New Student()
transferStudent = collegeStudent
If collegeStudent Is transferStudent Then
' Perform some action
End If

27. IsNot & Nothing Object Comparisons
Use the IsNot operator to determine that two variables do not reference the same object
Use the special value Nothing to determine if a variable has no object reference
If collegeStudent IsNot transferStudent Then
' Perform some action
End If
If collegeStudent Is Nothing Then
' Perform some action
End If

28. Creating an Array of Objects
Can create an entire array of object variables
Declare an array whose type is a class
Instantiate an object for each element
' Declare the array
Dim mathStudents(9) As Student
Dim i As Integer
For i = 0 To 9
' Assign each element to an object
mathStudents(i) = New Student()
Next i

29. Objects As Procedure Arguments
Can use object variables as arguments to a sub or function
Example: student object s as an argument
Pass object variable with the procedure call
Sub DisplayStudentGrade(ByVal s As Student)
' Displays a student’s grade.
MessageBox.Show("The grade for " & _
s.FirstName & " " & s.LastName & _
" is " & s.TestGrade.ToString)
End Sub
DisplayStudentGrade(freshman)

30. Objects Passed ByVal and ByRef
If argument is declared using ByRef
Values of object properties may be changed
The original object variable may be assigned to a different object
If argument is declared using ByVal
The original object variable may not be assigned to a different object

31. Functions Can Return Objects
Example below instantiates a student object
Prompts for and sets its property values
Then returns the instantiated object
Dim freshman As Student = GetStudent() …
Function GetStudent() As Student
Dim s As New Student()
s.FirstName = InputBox("Enter the first name.")
s.LastName = InputBox("Enter the last name.")
s.IdNumber = InputBox("Enter the ID number.")
s.TestAverage = InputBox("Enter the test average.")
Return s
End Function

32. Class Methods
In addition to properties, a class may also contain sub and function procedures
Methods are procedures defined in a class
Typically operate on data stored in the class
The following slide shows a Clear method for the Student class
Method called with freshman.Clear()
Method clears member data in the Student class object referenced by freshman

33. Clear Method for Student Class
Public Class Student
' Member variables
Private strLastName As String 'Holds last name
Private strFirstName As String 'Holds first name
Private strId As String 'Holds ID number
Private sngTestAverage As Single 'Holds test avg
(...Property procedures omitted...)
' Clear method
Public Sub Clear()
strFirstName = String.Empty
strLastName = String.Empty
strId = String.Empty
sngTestAverage = 0.0
End Sub
End Class

34. Constructors
A constructor is a method called automatically when an instance of the class is created
Think of constructors as initialization routines
Useful for initializing member variables or performing other startup operations
To create a constructor, simply create a Sub procedure named New within the class
Next slide shows a Student class constructor
The statement freshman = New Student()
Creates an instance of the Student class
Executes constructor to initialize properties of the Student object referenced by freshman

35. Constructor Example Public Class Student ' Member variables
Private strLastName As String 'Holds last name
Private strFirstName As String 'Holds first name
Private strId As String 'Holds ID number
Private sngTestAverage As Single 'Holds test avg
' Constructor
Public Sub New()
strFirstName = "(unknown)"
strLastName = "(unknown)"
strId = "(unknown)"
testAvg = 0.0
End Sub
(The rest of this class is omitted.)
End Class

36. Create a Class
Tutorial 12-1 demonstrates code needed to set up for the Student class

37. A Collection Holds a Group of Items
Section 12.3 Collections
A Collection Holds a Group of Items
It Automatically Expands and Shrinks in Size to Accommodate the Items Added to It, and Allows Items to Be Stored With Associated Key Values, Which May Be Used in Searches

38. Collections A collection is similar to an array
A single unit that contains several items
Can access items in a collection by numeric index
A collection’s indices begin at one, not zero
Collections automatically expand and shrink as items are added and removed
The items stored in a collection do not have to be of the same type

39. A Collection is a Class
Dim customers As Collection
customers = New Collection()
' Or alternatively
Dim customers As New Collection()
New collections are instantiations of the Collection Class
The Collection Class provides various methods and properties for use with individual collections

40. Adding Items to a Collection
Object.Add(Item [, Key] [, Before] [,After])
Add is a method of the Collection class
Object is the collection that Item is added to
Item can be an object, variable, or value
Key is a unique value optionally used to identify a member of the collection
Before or After is used to specify where a new item should be placed in the collection
Default is to insert at the end of the collection

41. Uses of Before and After
Add custData with key "Smith" before the item with key "Thomas“
Add custData with key "Smith" after the item with key "Reece“
Add custData after 3rd item in collection
customers.Add(custData, "Smith", "Thomas")
customers.Add(custData, "Smith",, "Reece")
customers.Add(custData,,,3)

42. Add Method Exceptions
An exception can occur when adding to a collection so Try-Catch should be used
Cannot add member with same key as another member of the collection
If a key or index is specified for a Before or After, the value must exist
Try
customers.Add(custData, "Smith")
Catch ex as ArgumentException
MessageBox.Show(ex.Message)
End Try

43. Accessing Item by Their Indices
Can access an item in a collection using an index value
Index value can be used in two ways:
Using the collection’s Item method
Or specify the index as is done with an array
Get value at index 3 of names collection by:
names.Item(3) –or- names(3)
Object.Item(Index)
Object(Index)

44. IndexOutOfRange Exception
If an invalid index is encountered, an index out of range exception will occur
Should use Try-Catch to trap such messages
Dim custData as Customer
Try
custData = Ctype(customers.Item(5), Customer)
Catch ex as IndexOutOfRangeException
MessageBox.Show(ex.Message)
End Try

45. The Count Property
The Count property of a collection gives the number of current items in the collection
Dim intX As Integer
For intX = 1 To names.Count
lstNames.Items.Add(names(intX).ToString())
Next intX

46. Storing Objects in a Collection
Storing an object, without a key
Storing an object, with a key
Dim studentCollection As New Collection()
studentCollection.Add(studentData)
studentCollection.Add(studentData, _
studentData.IdNumber)

47. Searching for an Item by Key Value
Item method can be used to retrieve an item with a specific index
If Expression is a string, it is used as a key to search for the matching member
If Expression is numeric, it is used as an index value for the item
If no item is found (via key or index), an exception occurs
Object.Item(Expression)

48. Retrieving Item Examples
Find studentCollection item with key 49812
If Option Strict on, must cast result to Student
Retrieve all members by index and display LastName property in a message box
Dim s as Student
s = Ctype(studentCollection.Item(“49812”), Student)
Dim i as Integer
Dim s as Student
For I = 1 to studentCollection.Count
s = Ctype(studentCollection.Item(i), Student)
MessageBox.Show(s.LastName)
Next i

49. Using References Versus Copies
When an Item in a collection is a fundamental VB data type, only a copy is retrieved
This code does not change the item at index 1
The Item in this collection is an object so:
A reference is returned instead of a copy
LastName of object in collection is changed
Dim n as Integer
n = CType(numbers(1), Integer)
n = 0
Dim s as Student
s = CType(studentCollection.Item("49812"), Student)
s.LastName = "Griffin"

50. For Each Loop with a Collection
Can use a For Each loop to read members of a collection
Eliminates the counter variable required to use a For…Next
Also no need to compare to Count property
Dim s As Student
For Each s In studentCollection
MessageBox.Show(s.LastName)
Next s

51. Removing Members of a Collection
Object.Remove(Expression)
Remove is a method of the Collection class
Object is collection Member removed from
Expression can be
Numeric and interpreted as an index
Or a string and interpreted as a key value
Exception thrown if Expression not found

52. Removing Member Examples
Verify “49812” is a key value in collection, then remove member with this key value
Verify index location 7 exists in collection, then remove member at this index location
If studentCollection.Contains(“49812”) Then
studentCollection.Remove("49812")
End If
Dim intIndex As Integer = 7
If intIndex > 0 _
and intIndex <=studentCollection.Count Then
studentCollection.Remove(intIndex)
End If

53. Working with Collections
Since a collection is an instance of a class
Procedures accept collections as arguments
Functions can return a collection
Follow same guidelines as any class object
Parallel collections work like parallel arrays
Can use index to relate parallel collections just as we did with arrays
Or can use key values to relate collections

54. Section 12.4 The Student Collection Application
Create an Application that Uses a Collection of Student Objects

55. Section 12.5 The Object Browser
The Object Browser Is a Dialog Box That Allows You to Browse All Classes and Components That Are Available to Your Project

56. Working with Collections
A dialog box with information about objects
Allows you to examine
Information about forms in your project
Classes you’ve created
Other components used by VB in your project
Tutorial 12-3 uses the Object Browser to examine the Student Collection project

57. Object Browser, Example

58. Section 12.6 Scroll Bars and Track Bars
The HScrollBar, VScrollBar, and TrackBar Controls Provide a Graphical Way to Adjust a Number Within a Range of Values

59. Visual Appearance and Usage
The HScrollBar and VScrollBar look like normal scroll bars
The TrackBar has an arrow pointer as the slider and has tick marks along its length
Scrollable controls hold integers in their Value property
Position of slider corresponds to Value
Move scroll bar to increase or decrease Value
Right increases, left decreases horizontal bar
Up increases, down decreases vertical bar

60. Scrollable Control Properties
Minimum – the bar’s lowest possible value
Maximum – the bar's highest possible value
Value – the bar's value at the current position
LargeChange – change in the Value property with a mouse click on/near the slider
SmallChange – change in the Value property for amouse click on an arrow at the end
TickFrequency - for TrackBar only, the number of units between tick marks
With min=0 and max=1000, if Tick Frequency is 100, 10 tick marks are shown on the bar

61. Coding for Scrollable Controls
Any change to the position of a scroll bar generates a Scroll event
Allows program to react to a shift in scroll bar
Standard prefixes for these controls
Horizontal scroll bar is hsb
Vertical scroll bar is vsb
TrackBar is tb
Tutorial 12-4 demonstrates how to set up Scroll events and use of these controls

62. Section 12.7 Introduction to Inhertance
Inheritance Allows a New Class to be Based on an Existing Class
The New Class Inherits the Accessible Member Variables, Methods, and Properties of the Class on Which It Is Based

63. Why Inheritance?
Inheritance allows new classes to derive their characteristics from existing classes
The Student class may have several types of students such as
GraduateStudent
ExchangeStudent
StudentEmployee
These can become new classes and share all the characteristics of the Student class
Each new class would then add specialized characteristics that differentiate them

64. Base and Derived Classes
The Base Class is a class that other classes may be based on
A Derived Class is based on the base class and inherits characteristics from it
Can think of the base class as a parent and the derived class as a child

65. The Vehicle Class (Base Class)
Consider a Vehicle class with the following:
Private variable for number of passengers
Private variable for miles per gallon
Public property for number of passengers (Passengers)
Public property for miles per gallon (MilesPerGallon)
This class holds general data about a vehicle
Can create more specialized classes from the Vehicle class

66. The Truck Class (Derived Class)
Declared as:
Truck class derived from Vehicle class
Inherits all non-private methods, properties, and variables of Vehicle class
Truck class defines two properties of its own
MaxCargoWeight – holds top cargo weight
FourWheelDrive – indicates if truck is 4WD
Public Class Truck
Inherits Vehicle
' Other new properties
' Additional methods
End Class

67. Instantiating the Truck Class
Instantiated as:
Values stored in MaxCargoWeight and FourWheelDrive properties
Properties declared explicitly by Truck class
Values also stored in MilesPerGallon and Passengers properties
Properties inherited from Vehicle class
Dim pickUp as New Truck()
pickUp.Passengers = 2
pickUp.MilesPerGallon = 18
pickUp.MaxCargoWeight = 2000
Pickup.FourWheelDrive = True

68. Overriding Properties and Methods
Sometimes a base class property procedure or method does not work for a derived class
Can override base class method or property
Must write the method or property as desired in the derived class using same name
When an object of the derived class accesses the property or calls the method
VB uses overridden version in derived class
Version in base class is not used

69. Property Override Example
Vehicle class has no restriction on number of passengers
But may wish to restrict the Truck class to two passengers at most
Can override Vehicle class Passengers property by:
Coding Passengers property in derived class
Specify Overridable in base class property
Specify Overrides in derived class property

70. Overridable Base Class Property
Overridable keyword added to base class property procedure
Public Overridable Property Passengers() As Integer
Get
Return intPassengers
End Get
Set(ByVal value As Integer)
intPassengers = value
End Set
End Property

71. Overridden Derived Class Property
Overrides keyword and new logic added to derived class property procedure
Public Overrides Property Passengers() As Integer
Get
Return MyBase.Passengers
End Get
Set(ByVal value As Integer)
If value >= 1 And value <= 2 Then
MyBase.Passengers = value
Else
MessageBox.Show("Passengers must be 1 or 2", _
"Error")
End If
End Set
End Property

72. Overriding Methods Overriding a method is similar to a property
Specify Overridable and Overrides keywords
An overridable base class method
An overriding derived class method
Public Overridable Sub ProcedureName()
Public Overridable Function ProcedureName() As DataType
Public Overrides Sub ProcedureName()
Public Overrides Function ProcedureName() As DataType

73. Overriding the ToString Method
Every programmer created class is derived from a built-in class named Object
Object class has a method named ToString which returns a fully-qualified class name
Method can be overridden to return a string representation of data stored in an object

74. ToString Override Example
Object class ToString method is Overridable
Vehicle class might override the ToString method as shown below
' Overriden ToString method
Public Overrides Function ToString() As String
' Return a string representation
' of a vehicle.
Dim str As String
str = "Passengers: " & numPassengers.ToString & _
" MPG: " & mpg.ToString
Return str
End Function

75. Base & Derived Class Constructors
A constructor (named New) may be defined for both the base class and a derived class
When a new object of the derived class is created, both constructors are executed
The constructor of the base class will be called first
Then the constructor of the derived class will be called

76. Protected Members
In addition to Private and Public, the access specifier may be Protected
Protected base class members are treated as public to classes derived from this base
Protected base class members are treated as private to classes not derived from this base
Tutorial 12-5 provides an opportunity to work with base and derived classes