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Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 1
STARTING OUT WITH
Visual Basic 2008FOURTH EDITION
Tony GaddisHaywood Community College
Kip IrvineFlorida International University
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Chapter
Classes, Exceptions, Collections, and Scrollable Controls
12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 3
Introduction
Classes Abstract Data Types Objects, Properties, Methods
Exceptions Collections Object Browser Scrollable Controls
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Classes and Objects12.1
Classes Are Program Structures That Define Abstract Data Types and Are Used to Create Objects
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 represents a class Placing a button
on a form creates an instance, or object, of the class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 7
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 8
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 typically become classes in the program design
Verbs such as calculate GPA and search become methods of those classes
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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, InputGrade,Credits
Transcript CourseList, Count Display, Search,CalcGradeAvg
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 11
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Creating a Class12.2
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 13
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 14
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 15
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 17
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 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 18
Property Procedure Syntax
Public Property PropertyName() As DataTypeGet
StatementsEnd GetSet(ParameterDeclaration)
StatementsEnd Set
End Property
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 19
Property Procedure ExamplePublic 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 storedSet(ByVal value As Single)
If value >= 0.0 And value <= 100.0 ThensngTestAvg = value
ElseMessageBox.Show( _"Invalid test average.", "Error")
End IfEnd Set
Dim freshman as New Student()freshman.TestAverage = 82.3
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 22
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 25
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.)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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 StudentDim transferStudent As StudentcollegeStudent = New Student()transferStudent = collegeStudent
If collegeStudent Is transferStudent Then' Perform some action
End If
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 28
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 29
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)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
Values of object properties may be changed The original object variable may not be
assigned to a different object
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 31
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 32
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 33
Clear Method for Student ClassPublic 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 35
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 36
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 37
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())
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 38
Create a Class Tutorial 12-1 demonstrates code needed to
create the Student class
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Collections12.3
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 40
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 41
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()
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 42
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])
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 43
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)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 44
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
Trycustomers.Add(custData, "Smith")
Catch ex as ArgumentExceptionMessageBox.Show(ex.Message)
End Try
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 45
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)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 46
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 47
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 48
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)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 49
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 intIndex
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 50
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 changedDim s as Students = CType(studentCollection.Item("49812"), Student)s.LastName = "Griffin"
Dim intNum as IntegerintNum = CType(numbers(1), Integer)intNum = 0
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 51
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 52
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)
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 53
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 54
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
The Student Collection Application12.4
Create an Application that Uses a Collection of Student Objects
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Student Collection Application Forms
Slide 12- 56
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
The Object Browser12.5
The Object Browser Is a Dialog Box That Allows You to Browse All
Classes and Components Available to Your Project
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 58
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 59
Object Browser, Example
Student class selected Class members shown
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Scroll Bars and Track Bars12.6
The HScrollBar, VScrollBar, and TrackBar Controls Provide a
Graphical Way to Adjust a Number Within a Range of Values
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 61
Visual Appearance and Usage HScrollBar and VScrollBar
look like normal scroll bars TrackBar has an arrow pointer
as 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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 62
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 63
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Introduction to Inhertance12.7
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 65
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 66
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 67
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 68
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 69
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 70
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 71
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 72
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 73
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Overriding a method is similar to a property Specify Overridable and Overrides keywords An overridable base class method
An overriding derived class method
Slide 12- 74
Overriding Methods
Public Overridable Sub ProcedureName()
Public Overridable Function ProcedureName() As DataType
Public Overrides Sub ProcedureName()
Public Overrides Function ProcedureName() As DataType
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 75
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Object class ToString method is Overridable Vehicle class might override the ToString method
as shown below
Slide 12- 76
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 77
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
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 12- 78
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