Chapter 12 Classes, Collections and Scrollable Controls

Topics

•Classes•Abstract Data Types•Objects, Properties, Methods

•Exceptions•Collections•Object Browser•Scrollable Controls

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

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 button

on a form creates an instance, or object, of the class

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

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

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

OOD Class Characteristics

Class Attributes (properties) Operations (methods)

Student LastName, FirstName, Display, Input IdNumber

StudentList AllStudents, Count Add, Remove, FindStudent

Course Semester, Name, Display, InputGrade,Credits

Transcript CourseList, Count Display, Search,CalcGradeAvg

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

Class Declaration

• 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

Public Class StudentMemberDeclarations

End Class

Member Variables

• A variable declared inside a class declaration• Syntax:

• AccessSpecifier may be Public or Private• Example:

AccessSpecifer VariableName As DataType

Public Class StudentPublic strLastName As String 'Holds last namePublic strFirstName As String 'Holds first namePublic strId As String 'Holds ID number

End Class

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 New Student()

freshman = New Student()

Dim freshman As Student

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

freshman.strFirstName = "Joy"freshman.strLastName = "Robinson"freshman.strId = "23G794"

objectVariable.memberVariable

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

Property Procedure Syntax

Public Property PropertyName() As DataTypeGet

StatementsEnd GetSet(ParameterDeclaration)

StatementsEnd Set

End Property

Property Procedure Example

Public Class Student' Member variablesPrivate sngTestAvg As Single

Public Property TestAverage() As SingleGet

Return sngTestAvgEnd GetSet(ByVal value As Single)

If value >= 0.0 And value <= 100.0 ThensngTestAvg = value

ElseMessageBox.Show( _

"Invalid test average.", "Error")End If

End SetEnd Property

End Class

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

Set(ByVal value As Single)If value >= 0.0 And value <= 100.0 Then

sngTestAvg = valueElse

MessageBox.Show( _"Invalid test average.", "Error")

End IfEnd Set

Dim freshman as New Student()freshman.TestAverage = 82.3

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 DataTypeGet

StatementsEnd Get

End Property

Read-Only Property Example

' TestGrade property procedureReadOnly Property TestGrade() As Char

GetIf sngTestAverage >= 90

return "A"cElse If sngTestAverage >= 80

return "B"cElse If sngTestAverage >= 70

return "C"cElse If sngTestAverage >= 60

return "D"cElse

return "F"cEnd If

End GetEnd Property

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

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

Going Out of Scope, Example

Sub CreateStudent()Dim sophomore As Studentsophomore = New Student()sophomore.FirstName = "Travis"sophomore.LastName = "Barnes"sophomore.IdNumber = "17H495"sophomore.TestAverage = 94.7g_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.)

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 StudentDim transferStudent As StudentcollegeStudent = New Student()transferStudent = collegeStudent

If collegeStudent Is transferStudent Then' Perform some action

End If

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

• Can create an entire array of object variables– Declare an array whose type is a class

– Instantiate an object for each element

Creating an Array of Objects

' Declare the arrayDim mathStudents(9) As StudentDim i As IntegerFor i = 0 To 9

' Assign each element to an objectmathStudents(i) = New Student()

Next i

• 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

Objects As Procedure Arguments

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)

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

Functions Can Return Objects

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

• Example below instantiates a student object• Prompts for and sets its property values• Then returns the instantiated object

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

Clear Method for Student Class

Public Class Student' Member variablesPrivate strLastName As String 'Holds last namePrivate strFirstName As String 'Holds first namePrivate strId As String 'Holds ID numberPrivate sngTestAvg As Single 'Holds test avg

(...Property procedures omitted...)

' Clear methodPublic Sub Clear()

strFirstName = String.EmptystrLastName = String.EmptystrId = String.EmptysngTestAvg = 0.0

End SubEnd Class

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

Constructor ExamplePublic Class Student

' Member variablesPrivate strLastName As String 'Holds last namePrivate strFirstName As String 'Holds first namePrivate strId As String 'Holds ID numberPrivate sngTestAvg As Single 'Holds test avg

' ConstructorPublic Sub New()

strFirstName = "(unknown)"strLastName = "(unknown)"strId = "(unknown)"sngTestAvg = 0.0

End Sub

‘ The rest of this class is omitted.End Class

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.Finalize()End Sub

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 = 32.3amountTextBox.Text = s.ToString() 'displays 32.3amountTextBox.Text = s.ToString("C2") 'displays $32.30

The first ToString() has no parameter; the second ToString("C2") has one String parameter.

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

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

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())

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

A Collection is a Class

• New collections are instantiations of the Collection Class

• The Collection Class provides methods and properties for use with individual collections

Dim customers As Collectioncustomers = New Collection()

' Or alternatively

Dim customers As New Collection()

Adding Items to a Collection

• 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

Object.Add(Item [, Key] [, Before] [,After])

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)

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 existTry

customers.Add(custData, "Smith")Catch ex as ArgumentException

MessageBox.Show(ex.Message)End Try

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

– 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(Index)

Object.Item(Index)

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

TryDim cust as Customer'Get collection index from user inputDim index as Integer = CInt(txtIndex.Text)'Locate the customer in the collectioncust = CType(customers.Item(index), Customer)

Catch ex as IndexOutOfRangeExceptionMessageBox.Show(ex.Message)

End Try

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 IntegerFor intX = 1 To names.Count

lstNames.Items.Add(names(intX).ToString())Next intX

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)

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 Students = CType(studentCollection.Item("49812"), Student)

Dim intIndex as IntegerDim s as StudentFor intIndex = 1 to studentCollection.Count

s = Ctype(studentCollection.Item(intIndex), Student)MessageBox.Show(s.LastName)

Next

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 s as Students = CType(studentCollection.Item("49812"), Student)s.LastName = "Griffin"

Dim intNum as IntegerintNum = CType(numbers(1), Integer)intNum = 0

For Each Loop with a Collection

Dim s As StudentFor Each s In studentCollection

MessageBox.Show(s.LastName)Next s

• 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

Removing Members of a Collection

• 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

Object.Remove(Expression)

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) ThenstudentCollection.Remove("49812")

End If

Dim intIndex As Integer = 7If intIndex > 0 and _

intIndex <= studentCollection.Count ThenstudentCollection.Remove(intIndex)

End If

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

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

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

Object Browser, Example

Student class selected Class members shown

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

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 a

mouse 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

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

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

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

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

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 TruckInherits Vehicle' Other new properties' Additional methods

End Class

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 = 2pickUp.MilesPerGallon = 18pickUp.MaxCargoWeight = 2000Pickup.FourWheelDrive = True

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

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

Overridable Base Class Property

Public Overridable Property Passengers() As IntegerGet

Return intPassengersEnd GetSet(ByVal value As Integer)

intPassengers = valueEnd Set

End Property

• Overridable keyword added to base class property procedure

Overridden Derived Class Property

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 = valueElse MessageBox.Show("Passengers must be 1 or 2", _

"Error")End If

End SetEnd Property

• Overrides keyword and new logic added to derived class property procedure

• Overriding a method is similar to a property• Specify Overridable and Overrides keywords• An overridable base class method

• An overriding derived class method

Overriding Methods

Public Overridable Sub ProcedureName()

Public Overridable Function ProcedureName() As DataType

Public Overrides Sub ProcedureName()

Public Overrides Function ProcedureName() As DataType

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

• Object class ToString method is Overridable• Vehicle class might override the ToString method as

shown below

ToString Override Example

' Overriden ToString methodPublic Overrides Function ToString() As String

' Return a string representation' of a vehicle.Dim str As String

str = "Passengers: " & intPassengers.ToString & _" MPG: " & sngMPG.ToString

Return strEnd Function

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

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