Chapter 1: Classes

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Chapter 1 Classes
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Overview

• Introduction
• Value Types and Reference Types
• Object-Oriented Design
• Creating Classes
• Properties
• Constructors
• Multi-Tier Applications
• Composition Relationships
• Nested (Inner) Classes
• Using Text Files

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1.1 Introduction

• Object-Oriented Programming (OOP)
• A class is a pattern or blueprint
• defines group of related objects with common
  characteristics
• identifies a specific data type
• Attributes and behaviors

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1.2 Value Types and Reference Types

• Value type
• variable that contains its own data
• primitives, such as Integer, Single, Decimal
• Reference type
• contains a reference to another memory location
• objects and arrays, including strings

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Value Type

• Dim mCount As Integer 25

Dim temp As Integer mCount
temp 40 'mCount still equals 25
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Reference Type

• Dim S1 As Student

S1 New Student S1.Name "Fred Smith"
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Assigning Reference Types

• Dim tests() As Integer 80, 95, 88, 76, 54
• Dim scores() As Integer tests

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Life and the Garbage Collector

• Lifetime of an object is determined by...
• whether or not references to it still exist
• Object with no references is unreachable
• Unreachable object can be reclaimed by the
  garbage collector

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Namespaces

• A namespace groups one or more classes under a
  common name
• Avoids collisions between identical class names
• Good for
• large applications
• groups of programmers
• Every VB project has a root namespace

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1.3 Object-Oriented Design

• Object-Oriented Analysis
• detailed specification of the problem
• find classes that reflect the application domain
• Object-Oriented Design
• create classes that cooperate and communicate

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Design Steps

• Find the classes
• physical entities in the application domain
• control structures
• Describe the classes
• attributes characteristics (properties)
• operations actions the class may perform
  (methods)

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Interface and Implementation

• Interface
• visible portion of a class
• Implementation
• hidden from client programs
• private members
• encapsulation principle

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Inheritance

• A derived class inherits characteristics of an
  existing class
• GradStudent and StudentEmployee are derived
  classes

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Clients and Servers

• A server produces information consumed by client
• Client creates a reference to the server and
  sends requests via method calls
• Server sends responses via method return values

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1.4 Creating Classes

• Class declaration syntax
• Public Class classname
• member-declarations
• End Class
• Example
• Public Class Student
• Private id As String
• Public New( ) ...
• End Class

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Hands-On Tutorial

• Title Student Class
• Program name Student1
• Creates a Student class
• Retrieves and displays values of member variables
  (fields)
• Output

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Methods

• A method is a procedure declared inside a class
• sub procedure
• function procedure
• Example
• Public Class Student
• . . .
• Private Sub Print()
• MessageBox.Show(mID ", " mName _
• ", " mAge)
• End Sub
• End Class

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Hands-On Tutorial

• Adding the GetGradeAverage method
• Program name Student1
• Public mCreditsEarned As Single
• Public mTotalGradePoints As Single
• Public Function GetGradeAverage() As Single
• Return mTotalGradePoints / mCreditsEarned
• End Function

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Public and Private Member Access

• Public member
• accessible to entire program
• Private member
• invisible outside the enclosing class
• Fields should be private
• Methods can be public or private

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1.5 Properties

• Property procedures
• act like variables to clients
• Visual Studio .NET creates a template
• General format
• Public Property name() As type
• Get
• End Get
• Set(ByVal Value As type)
• End Set
• End Property

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Age Property Example

• Public Property Age() As Integer
• Get
• Return mAge
• End Get
• Set(ByVal Value As Integer)
• If Value gt 1 And Value lt 130 Then
• mAge Value
• End If
• End Set
• End Property

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ReadOnly Properties

• Can be read, but not modified by clients
• Use ReadOnly qualifier
• Example
• Public ReadOnly Property GradePointAverage() As
  Single
• Get
• If mCreditsEarned ltgt 0 Then
• Return mTotalGradePoints / mCreditsEarned
• Else
• Return 0.0
• End If
• End Get
• End Property

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Hands-On Tutorial

• Title Adding Properties to the Student Class
• Program name Student2
• Adds the following properties
• ID
• Name
• Age
• CreditsEarned
• GradeAverage

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Shared Variables

• Contains data that is shared among all class
  instances
• Also called a shared field
• Program example SharedMembers
• Usually private
• Example
• Private Shared mInstanceCount As Integer 0

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Shared Properties

• Sets/returns value of shared variable
• Usually public
• Example
• Public Shared ReadOnly Property InstanceCount() _
• As Integer
• Get
• Return mInstanceCount
• End Get
• End Property

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1.6 Constructors

• Methods that run automatically when an instance
  of a class is created
• Always named New
• Can be overloaded
• each must have different parameter list
• Default constructor example
• Public Sub New()
• mID "000000000"
• mName "(unknown)"
• End Sub

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Overloading the Constructor

• Public Sub New(ByVal id As String, ByVal name As
  String, _
• ByVal age As Integer)
• mID id
• mName name
• mAge age
• End Sub
• 'Sample call
• Dim s2 As New Student("222222222", "Julio
  Gonzalez", 23)

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Optional Constructor Parameters

• Optional keyword lets you avoid overloading the
  constructor
• You must supply default arguments
• Public Sub New( _
• Optional ByVal id As String "000000000", _
• Optional ByVal name As String "(unknown)", _
• Optional ByVal age As Integer 0)
• mID id
• mName name
• mAge age
• End Sub

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Hands-On Tutorial

• Title Adding a Parameterized Constructor to the
  Student Class
• Student3 program

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1.7 Multi-Tier Applications

• Multi-tier application model
• Design Patterns (1995) by Erich Gamma, Richard
  Helm, and others
• Three layers, or tiers
• presentation
• business
• data

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Three Tiers (Layers)

• Presentation
• Interacts with the user, provides all
  input-output
• Business
• Contains processing rules and logic, including
  behaviors and attributes belonging to application
  entities
• Data
• Information storage and retrieval system, such as
  Database connections, recordsets, text files, and
  XML data

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Hands-On Tutorial

• Title Bank Teller Application
• Program BankTeller
• Presentation tier
• frmTeller class
• Business/Data tier
• Account class

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Hands-On Tutorial

• Title Bank Teller Transaction Log
• Program BankTeller2
• Presentation tier
• frmTeller class
• Business tier
• Account class
• Data tier
• TransactionLog class
• logs all transactions to a file

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1.8 Composition Relationships Between Classes

• Created when one class contains fields that are
  instances of other classes
• Also known as aggregation, or containing
  relationship

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Employee Information Example

• clsEmployee class contains objects of the
  following types
• clsPersonalInfo
• clsAddress
• clsSalaryItem
• clsProject

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Using UML to Describe Classes
UML stands for Universal Modeling Language
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Overriding ToString

• Standard way of displaying the state of an object
• Example
• Public Overrides Function ToString() As String
• Return mStreet ", " _
• mCity ", " _
• mState ", " _
• mZip ", " mPhone
• End Function

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Hands-On Tutorial

• Title Employee Information
• Program EmployeeInfo
• Uses the clsEmployee, clsPersonalInfo,
  clsAddress, clsSalaryItem, and clsProject classes

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1.9 Optional Topic Nested (Inner) Classes

• A nested class is declared inside an existing
  class
• Good way of hiding a class
• Prevents name clashes with other classes having
  the same name
• Class NetworkConnection
• Class Address
• Private mURL(3) As Integer
• End Class
• End Class

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1.10 Optional Topic Using Text Files

• Advantages
• save and retrieve information without the
  overhead of a database
• platform independent
• edit with any text editor
• Required Imports
• System.IO
• System.IO.File

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Text File Examples - 1

• Find out if a text file exists
• If Exists("input.txt") Then ...
• Open a text file for reading
• Dim inFile As StreamReader OpenText("input.txt")
  
• Create a new text file
• Dim outFile As StreamWriter CreateText("output.t
  xt")
• Append to an existing text file
• Dim outFile As StreamWriter AppendText("output.t
  xt")

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Text File Examples - 2

• Read a line from a text file
• Dim temp As String
• temp inFile.ReadLine()
• Check for end of file
• While inFile.Peek ltgt -1
• temp inFile.ReadLine()
• End While
• Write a line to a text file
• Dim temp As String "Some data"
• outFile.WriteLine(temp)
• Close a StreamReader or StreamWriter
• inFile.Close()

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The End