Chapter 12
Classes, Exceptions, Collections,
and Scrollable Controls
Starting Out with Visual Basic .NET 2nd Edition
12.1
Introduction
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Chapter 12 Topics
• Classes
• Abstract Data Types
• Objects, Properties, Methods
•
•
•
•
Exceptions
Collections
Object Browser
Scrollable Controls
Starting Out with Visual Basic .NET 2nd Edition
12.2
Classes and Objects
Classes Are Program Structures
That Define Abstract Data Types
and That Are Used to Create Objects
Starting Out with Visual Basic .NET 2nd Edition
Abstract Data Types
• An abstract data type (ADT), is a data type
created by a programmer
• ADTs are very important in computer
science and especially significant in objectoriented programming
• An abstraction is a general model of
something—a definition that includes only
the general characteristics of an object.
Starting Out with Visual Basic .NET 2nd Edition
Classes
• A class is a program structure that defines an
abstract data type
• First create a class then create instances of the class
• All class instances share common characteristics,
for example:
• Visual Basic .NET controls and forms are classes
• In the toolbox, each icon represents a class
• When you select the button tool from the toolbox and
place it on a form you are creating an instance of the
button class
• An instance is also called an object
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Class Properties, Methods,
and Event Procedures
• Class properties example: Buttons have
Location, Text, and Name properties
• Class methods example: Buttons have a
Focus method
• Class event procedures: Buttons can have a
click event procedure
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Object Oriented Design
• This involves taking the program
specifications and doing an analysis to
determine what ADTs would be best to use
to implement those specifications
• In general, the classes will be the nouns
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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.
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OOD Class Characteristics
Class
Attributes(properties) Operations (methods)
Student
LastName, FirstName,
IdNumber
Display, Input
StudentList AllStudents, Count
Add, Remove,
FindStudent
Course
Semester, Name,
Grade,Credits
Display, Input
Transcript
CourseList, Count
Display, Search,
CalcGradeAvg
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Interface and Implementation
• The class interface is the portion of the
class that is visible to the application
programmer
• The class implementation is the portion of a
class that is hidden from client programs; it
is created from private member variables,
private properties, and private methods
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12.4
Exceptions and
Error Handling
Exceptions Signal Errors or Unexpected Events That
Occur While an Application Is Running
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Run-Time Errors
• Examples:
• Attempted division by zero
• Attempting to open a non-existent file
• Visual Basic .NET will display an error
message
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Exceptions
• Run-time errors are caused by exceptions
• An exception is an event or condition that
happens unexpectedly, and causes the
application to halt
• These may be caught by an error handler
• The process of intercepting and responding
to exceptions is called error trapping
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Handling an Exception
Try
TryBlock
Catch
CatchBlock
End Try
• The TryBlock is code that
• is always executed
• may cause an exception
• The CatchBlock code is executed if there is
an exception in the associated TryBlock
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Try/Catch Examples
Try
result = num1 / num2
Catch
MessageBox.Show("An error has occurred.")
End Try
Dim inputFile As System.IO.StreamReader
Try
inputFile = System.IO.File.OpenText("names.txt")
Catch
MessageBox.Show("An error has occurred.")
End Try
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Some Conditions That Cause
Exceptions From the OpenText Method
• The file is not found
• The path does not exist
• The application does not have permission to
open the file
• An empty string, or the value Nothing, was
passed as an argument to the method
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Handling Specific Exceptions
Try
inputFile = System.IO.File.OpenText("globby.txt")
Catch ex As FileNotFoundException
MessageBox.Show("File not found.")
Catch ex As DirectoryNotFoundException
MessageBox.Show("Directory not found.")
End Try
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Using System-Defined Error Messages
Try
inputFile = System.IO.File.OpenText("globby.txt")
Catch ex As FileNotFoundException
' ex is the Exception Variable
MessageBox.Show(ex.Message)
Catch ex As DirectoryNotFoundException
' ex is the Exception Variable
MessageBox.Show(ex.Message)
End Try
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Generic Exception Type
'Exception' Usage
Function OpenFileForReading(ByVal filename As String) _
As System.IO.StreamReader
Dim fileObject As System.IO.StreamReader
Try
fileObject = System.IO.File.OpenText(filename)
Catch ex As Exception
' Generic Exception
MessageBox.Show(ex.Message, "Error")
fileObject = Nothing
End Try
Return fileObject
End Function
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The Try…Finally Statement
Try
TryBlock
Catch [ExceptionVar As ExceptionType]
CatchBlock
Finally
FinallyBlock
End Try
• The FinallyBlock is executed
• After the Try block
• After any Catch blocks have executed
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Try…Finally Example
Sub ReadEntireFile(ByVal filename As String, _
ByRef fileContents As String)
' Reads the entire contents of a file
' into a string.
Dim fileObject As System.IO.StreamReader
fileObject = OpenFileForReading(filename)
If Not (fileObject Is Nothing) Then
Try
fileContents = fileObject.ReadToEnd
Catch ex As Exception
MessageBox.Show(ex.Message)
fileContents = Nothing
Finally
fileObject.Close()
End Try
End If
End Sub
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12.8
Scroll Bars and Track Bars
The HScrollBar, VScrollBar, and TrackBar
Controls Provide a Graphical Way to Adjust a
Number Within a Range of Values
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Visual Appearance
• 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
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Bar Properties
•
•
•
•
Minimum - bar's minimum value
Maximum - bar's maximum value
Value - bar's current value
LargeChange - increment (decrement) for a mouse
click on or near the slider
• SmallChange - increment (decrement) for a mouse
clicl on an arrow at the end
• TickFrequency - (TrackBar only) distance
between the tick marks
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