Chapter 12 Classes, Collections and Scrollable Controls

1

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

2
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

3
Classes

• A class is a program structure that defines an
  abstract data type
• First create the class
• Then create instances of the class
• Class instances share common attributes
• VB forms and controls are classes
• Each control in the toolbox represents a class
• Placing a buttonon a form creates an instance,
  or object, of the class

4
Class Properties, Methods, 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

5
Object Oriented Design

• The challenge is to design classes that
  effectively cooperate and communicate
• Analyze application requirements to determine
  ADTs that best implement the specifications
• Classes are fundamental building blocks
• Typically represent nouns of some type
• A well-designed class may outlive the application
• Other uses for the class may be found

6
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 typically
  become classes in the program design
• Verbs such as calculate GPA and search become
  methods of those classes

7
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
8
Interface and Implementation

• Class interface is the portion of the class
  visible to the application programmer
• Made available by creating properties, methods,
  and events that are public
• Class implementation is the portion of the class
  hidden from client programs
• Kept hidden by designating member variables,
  properties, methods as private
• Hiding of data and procedures inside a class is
  referred to as encapsulation

9
Class Declaration
Public Class Student MemberDeclarations End Class

• Student is the name of the class
• Examples of MemberDeclarations are presented in
  the following slides
• To create a new class
• Click 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

10
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 'Holds last name Public strFirstName As
String 'Holds first name Public strId As String
'Holds ID number End Class
11
Creating an Instance of a Class

• A two step process creates an instance of a class
  
• Declare a variable whose type is the class
• Create instance of the 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()
12
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 public member variables
  strFirstName, strLastName, and strID
• Can store values in member variables with

objectVariable.memberVariable
freshman.strFirstName "Joy" freshman.strLastName
"Robinson" freshman.strId "23G794"
13
Property Procedure

• A property procedure is a function that defines a
  property
• Controls access to property values
• Procedure has two sections Get and Set
• Get section executes when value is retrieved
• Set section executes when value is stored
• Properties almost always declared Public to allow
  access from outside the class
• Set section often provides data validation logic

14
Property Procedure Syntax
Public Property PropertyName() As
DataType Get Statements End
Get Set(ParameterDeclaration) Statements End
Set End Property
15
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 gt 0.0 And value lt 100.0
Then sngTestAvg value Else MessageBox
.Show( _ "Invalid test average.",
"Error") End If End Set End Property End
Class
16
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 gt 0.0 And
value lt 100.0 Then sngTestAvg
value Else MessageBox.Show( _ "Invalid test
average.", "Error") End If End Set
17
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
• Add ReadOnly keyword after access specifier
• This causes the propertyName to be read-only --
  not settable from outside of the class

Public ReadOnly Property PropertyName() As
DataType Get Statements End Get End Property
18
Read-Only Property Example
' TestGrade property procedure ReadOnly Property
TestGrade() As Char Get If sngTestAverage gt
90 return "A"c Else If sngTestAverage gt
80 return "B"c Else If sngTestAverage gt
70 return "C"c Else If sngTestAverage gt
60 return "D"c Else return "F"c End If End
Get End Property
19
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
• Object is a candidate for garbage collection when
  it is no longer referenced by any object variable
• The garbage collector monitors for and
  automatically destroys objects no longer needed

freshman Nothing
20
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

21
Going Out of Scope, Example
Sub CreateStudent() Dim sophomore As
Student sophomore New Student() sophomore.Firs
tName "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.)
22
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
23
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
24
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
25
Objects As Procedure Arguments

• Can use object variables as arguments to a
  procedure or function
• Example student object s as an argument
• Pass object variable with the procedure call

Sub DisplayStudentGrade(ByVal s As Student) '
Displays a students grade. MessageBox.Show("The
grade for " _ s.FirstName " " s.LastName
_ " is " s.TestGrade.ToString) End Sub
DisplayStudentGrade(freshman)
26
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
• Values of object properties may be changed
• The original object variable may not be assigned
  to a different object

27
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 first
name.") s.LastName InputBox("Enter last
name.") s.IdNumber InputBox("Enter ID
number.") s.TestAvg CSng(InputBox("Enter test
average.")) Return s End Function
28
Class Methods

• In addition to properties, a class may also
  contain Sub procedures and functions
• Methods are Sub procedures and functions 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

29
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 sngTestAvg As Single 'Holds test
avg (...Property procedures omitted...) ' Clear
method Public Sub Clear() strFirstName
String.Empty strLastName String.Empty strId
String.Empty sngTestAvg 0.0 End Sub End
Class
30
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

31
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 sngTestAvg As Single 'Holds test
avg ' Constructor Public Sub
New() strFirstName "(unknown)" strLastName
"(unknown)" strId "(unknown)" sngTestAvg
0.0 End Sub The rest of this class is
omitted. End Class
32
Finalizers

• VB provides a class method named Finalize
• Called automatically just before garbage
  collector removes an instance of a class
• Select Finalize from method name drop-down list
  to let Visual Basic create the following template
• Add your code following MyBase.Finalize()

Protected Overrides Sub Finalize() MyBase.Finaliz
e() End Sub
33
Overload Method Definitions Overloading is the
concept of defining multiple methods in the a
class using the same procedure name however, the
parameter list of the procedures must be
different. Example 1 - ToString method of the
Decimal class Dim amount As Decimal
32.3 amountTextBox.Text s.ToString() 'displays
32.3 amountTextBox.Text s.ToString("C2")
'displays 32.30 The first ToString() has no
parameter the second ToString("C2") has one
String parameter.
34
Overload Method Definitions (Cont'd) Example 2
Overloaded Constructors Public Sub New()
'default constructor _month
String.Empty _day String.Empty End
Sub Public Sub New(ByVal monthNum As
String, ByVal _ dayNum As String)
Month monthNum Day dayNum End Sub
35
Overload Method Definitions (Cont'd) To overload
methods other than constructors, you must use the
keyword Overloads. Public Overloads Function
CalculateGross(ByVal salary As _ Decimal) As
Decimal Return salary / 24D End
Function   Public Overloads Function
CalculateGross(ByVal hours As _ Decimal, ByVal
rate As Decimal) As Decimal Return hours
rate End Function
36
Using the Output Window

• Usually shown at bottom of Visual Studio display
• If not appearing, click View, Other Windows, then
  Output to display
• A valuable debugging tool
• Messages about an application displayed here
• Can add your own messages to Output window using
  Debug.WriteLine method
• Causes the expression Output to appear in the
  Output window
• Insert following code in form Load event to enable

Debug.WriteLine(Output)
Debug.Listeners.Add(New ConsoleTraceListener())
37
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 collections 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

38
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 methods and
  properties for use with individual collections

39
Adding Items to a Collection
Object.Add(Item , Key , Before ,After)

• Add is a method of the Collection class
• Object is a variable that refers to a collection
• Item can be an object, variable, or value that is
  added to the collection
• Key is a unique value optionally used to identify
  a member of the collection
• Before or After optionally specifies where a new
  item should be placed in the collection
• Default is to insert at the end of the collection

40
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)
41
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
42
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 collections Item method
• Item is the default method so it can be omitted
• Get value at index 3 of names collection by
• names.Item(3) or- names(3)

Object.Item(Index)
Object(Index)
43
IndexOutOfRange Exception

• If an invalid index is encountered, an index out
  of range exception will occur
• Should use Try-Catch to trap such messages
• Ctype casts collection object to Customer object

Try Dim cust as Customer 'Get collection index
from user input Dim index as Integer
CInt(txtIndex.Text) 'Locate the customer in the
collection cust CType(customers.Item(index),
Customer) Catch ex as IndexOutOfRangeException Me
ssageBox.Show(ex.Message) End Try
44
The Count Property

• The Count property of a collection gives the
  number of current items in the collection
• Note that, unlike an array, a collection index is
  not zero based
• First item in a collection found at index 1
• Following code adds collection items to a list box

Dim intX As Integer For intX 1 To
names.Count lstNames.Items.Add(names(intX).ToStri
ng()) Next intX
45
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 a member with a matching key
• 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)
46
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 intIndex as Integer Dim s as Student For
intIndex 1 to studentCollection.Count s
Ctype(studentCollection.Item(intIndex),
Student) MessageBox.Show(s.LastName) Next
47
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 intNum as Integer intNum CType(numbers(1),
Integer) intNum 0
Dim s as Student s CType(studentCollection.Item(
"49812"), Student) s.LastName "Griffin"
48
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
  ForNext
• Also no need to compare to Count property

Dim s As Student For Each s In studentCollection
MessageBox.Show(s.LastName) Next s
49
Removing Members of a Collection
Object.Remove(Expression)

• Remove is a method of the Collection class
• Object refers to 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

50
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 gt 0 and
_ intIndex lt studentCollection.Count
Then studentCollection.Remove(intIndex) End If
51
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

52
Student Collection Application Forms

• Select student ID numberfrom list box to see
  detailinformation for the student
• Click Remove Student button remove an
  instanceof the Student class
• Click Add Student button tocreate a new instance
  of the Student class
• Add Student form

53
Using the Object Browser

• A dialog box with information about objects used
  in a project
• Allows you to examine
• Information about forms used in a project
• Classes created for the project
• Other components used by VB in the project
• Tutorial 12-3 uses the Object Browser to examine
  the Student Collection project

54
Object Browser, Example
Student class selected Class members shown
55
Visual Appearance and Usage

• HScrollBar and VScrollBar look like normal
  scroll bars
• TrackBar has an arrow pointeras the slider with
  tick marks
• 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

56
Scrollable Control Properties

• Minimum the bars 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 a
  mouse click on an arrow at the end
• TickFrequency - for TrackBar only, the number of
  units between tick marks
• With min0 and max1000, if Tick Frequency is
  100, 10 tick marks are shown on the bar

57
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

58
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

59
Base and Derived Classes

• The Base Class is a general-purpose 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

60
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(Passenge
  rs)
• Public property for miles per gallon(MilesPerGall
  on)
• This class holds general data about a vehicle
• Can create more specialized classes from the
  Vehicle class

61
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
62
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
63
Overriding Properties and Methods

• Sometimes a base class property procedure or
  method must work differently 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

64
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

65
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
66
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 gt 1
And value lt 2 Then MyBase.Passengers
value Else MessageBox.Show("Passengers must
be 1 or 2", _ "Error") End If End Set End
Property
67
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
68
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

69
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 "
intPassengers.ToString _ " MPG "
sngMPG.ToString Return str End Function
70
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

71
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