Advanced Classes

1
Advanced Classes
2
Overview

• Value Types
• Objects and Reflection
• Interfaces
• Inheritance
• Collections
• Visual Inheritance
• Delegates

3
Value Types
4
Characteristics of Value Types

• assignment operator copies contents
• Dim N As Integer, M As Integer
• M N
• compared using relational operators (lt, gt, etc.)
• If M lt N Then ...
• New operator not required when creating instances
• Faster program execution

5
Enums

• System.Enum is the base class for enumerated
  types
• inherits from System.Object
• Limited set of integral values
• Example
• Enum AccountType
• Checking 100
• Savings 101
• Trading 102
• Annuity 103
• End Enum

6
Assigning Values to Enums

• Declare variable
• Dim acct As AccountType
• Assign value using Enum type as qualifier
• acct AccountType.Checking
• Cannot assign equivalent integers
• acct 101 'Error!

7
Assigning Values to Enums

• GetType returns a System.Type object
• Dim acct As AccountType
• acct AccountType.Checking
• Dim T as Type acct.GetType()
• GetName returns enum variable's value
• AccountType.GetName( T, acct )

See the Objects.Enums program
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Objects and Reflection
9
Reflection

• Finding out about objects at runtime
• get its type (its class)
• get list of members
• enabling class for advanced VB.NET features
• Runtime Type Identification
• TypeOf operator returns boolean value
• Dim myAcct As New Account
• TypeOf (myAcct) Is Account 'equals True

10
Type Class

• Represents type declarations of classes,
  interfaces, arrays, value types, and enumerations
• System.Object.GetType method
• Function GetType( ) As System.Type
• System.Type methods
• FullName returns fully qualified class name
• ToString returns fully qualified class name

11
System.Object Class

• Ultimate superclass (cosmic superclass)
• base class for all other classes
• Methods
• Function Equals( Object ) As Boolean
• Shared Function ReferenceEquals( Object, Object )
  As Boolean
• Function ToString( ) As String
• Function GetType( ) As Type
• Function GetHashCode( ) As Integer
• Function MemberwiseClone( ) As Object

12
Overriding ToString

• Example from the Account class
• Must use the Overrides qualifier
• Public Overrides Function ToString() As String
• Return "AccountId " AccountId _
• ", Balance " FormatCurrency(Balance)
• End Function

13
Comparing Objects

• Objects cannot be compared using , lt, gt, etc.
• Objects can be compared by Equals method
• Dim a1 As New Account, a2 As New Account
• If a1.Equals( a2 ) Then ...
• Object.Compare just compares object references
• i.e. their addresses
• For example
• a2 a1 'copy the reference
• If a1.Equals( a2 ) Then ... 'equals True

14
Comparing Objects

• ReferenceEquals works the same as Equals
• Dim a1 As New Account, a2 As New Account
• If ReferenceEquals( a1, a2 ) Then ... 'False
• a2 a1
• If ReferenceEquals( a1, a2 ) Then ... 'True

15
Overriding Object.Equals

• To compare the states of two objects, your class
  must override Object.Equals
• Simple contract
• comparing an object to Null returns False
• objects related by inheritance are never equal
• compare the values in one or more fields

16
Account.Equals Example

• Arbitrary decision compare the account ID
  values
• Public Overrides Overloads _
• Function Equals(ByVal obj As Object) As Boolean
• If obj Is Nothing Then Return False
• If Not obj.GetType Is Me.GetType Then Return
  False
• Return AccountId CType(obj, Account).AccountId
• End Function

17
Account.Equals Example

• Calling Equals
• Dim a1 As New Account(1001, 50.25D)
• Dim a2 As New Account(1001, 400D)
• Dim a3 As New Account(2002, 50.25D)
• Dim nv As Account Nothing
• WriteLine(a1.Equals(nv)) 'False
• WriteLine(a1.Equals(a2)) 'True
• WriteLine(a1.Equals(a3)) 'False

18
Interfaces
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Defining an Interface

• Like a class, but not implemented directly
• All members are implicitly public
• Specifies methods, properties, types, events
• Interface keyword
• no implementations, just prototypes no fields
• Example IPayable
• Interface IPayable
• Function CalculateTax() As Decimal
• ReadOnly Property NetPay() As Decimal
• End Interface

20
Implementing an Interface

• Class must use the Implements keyword
• Must implement all interface methods and
  properties
• if you plan to create class instances
• Example
• Class Employee
• Implements IPayable
• Public Function CalculateTax() As Decimal _
• Implements IPayable.CalculateTax
• ...
• End Function
• Public ReadOnly Property NetPay() As Decimal _
• Implements IPayable.NetPay
• ...
• End Property
• End Class

See Interfaces.IPayableExample
21
Interface Parameters

• Functions can declare interface type parameters
• Sub ProcessPayroll(ByVal P As IPayable)
• WriteLine("Tax " FormatCurrency(P.Calcula
  teTax()))
• WriteLine("Net Pay " FormatCurrency(P.NetPay)
  )
• End Sub
• Pass objects as arguments if they implement the
  interface
• Dim emp As New Employee("Jones, Dan", 35000D)
• ProcessPayroll(emp)
• Output
• Tax 134.62
• Net Pay 1,211.54

22
IComparable Interface

• Standard interface type in .NET
• Used by collection classes
• Implement in your class if objects will be
  inserted in collections
• Definition
• Interface IComparable
• Function CompareTo(ByVal obj As Object) As
  Integer
• End Interface

23
Behavior of CompareTo

• A.CompareTo(null) returns a positive integer.
• A System.ArgumentException is thrown if A and B
  are different types.
• If A is less than B, A.CompareTo(B) returns a
  negative integer.
• If A is equal to B, A.CompareTo(B) returns zero.
• If A is greater than B, A.CompareTo(B) returns a
  positive integer.

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Employee.CompareTo (version 1)

• Basic version
• Class Employee
• Implements IComparable
• Public Function CompareTo(ByVal obj As Object) As
  Integer _
• Implements IComparable.CompareTo
• Dim emp2 As Employee CType(obj, Employee)
• Return CInt(mSalary - emp2.mSalary)
• End Function
• ...(etc.)
• End Class

25
Employee.CompareTo (version 2)

• Why is this version better?
• Public Function CompareTo(ByVal obj As Object) As
  Integer _
• Implements IComparable.CompareTo
• If obj Is Nothing Then Return 1
• If Not Me.GetType() Is obj.GetType() Then _
• Throw New ArgumentException
• Dim emp2 As Employee CType(obj, Employee)
• Return CInt(mSalary - emp2.mSalary)
• End Function

See Interfaces.EmployeeCompare
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Sorting an Array of Employees

• Sort executes correctly because Employee
  implements IComparable
• Dim staff(3) As Employee
• staff(0) New Employee("Jones, Dan", 65000D)
• staff(1) New Employee("Ramirez, Julio", 45000D)
• staff(2) New Employee("Bond, Barry", 55000D)
• staff(3) New Employee("Chong, Gary", 75000D)
• System.Array.Sort(staff)

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IComparer Interface

• Declaration
• Interface IComparer
• Function Compare(ByVal x As Object, _
• ByVal y As Object) As Integer
• End Interface
• Class defining the collection elements does not
  directly implement IComparer

28
Windmill Example

• RpmComparator compares the RPMs of windmills
• Structure Windmill
• Public rpm As Integer
• End Structure
• Class RpmComparator
• Implements IComparer
• Public Function Compare(ByVal x As Object, _
• ByVal y As Object) As Integer _
• Implements IComparer.Compare
• Return CType(x, Windmill).rpm - CType(y,
  Windmill).rpm
• End Function
• End Class

29
Using RpmComparator

• RpmComparator compares the RPMs of windmills
• The following code builds and sorts the array in
  ascending order by by rpm speed
• Dim wind(4) As Windmill
• wind(0).rpm 50
• wind(1).rpm 45
• wind(2).rpm 72
• wind(3).rpm 55
• Array.Sort(wind, New RpmComparator)

See Interfaces.Comparators
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Inheritance
31
Defining Inheritance

• Ability of a class to specialize the properties,
  methods, and events of another class
• Inherits keyword
• base class, derived class
• superclass, subclass
• Desktop VB .NET forms use inheritance
• Public class frmMain
• Inherits System.Windows.Forms.Form

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Access Modifiers
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Person Class Example

• Class Person
• Public Sub Display()
• WriteLine("Person " mName)
• End Sub
• Friend Property Name() As String
• Get
• Return mName
• End Get
• Set(ByVal Value As String)
• mName Value
• End Set
• End Property
• Protected ID As Integer
• Private mName As String
• End Class

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Inheriting from the Person Class

• Class Hero
• Inherits Person
• Sub TestAccess()
• Display() 'ok Public
• Name "Sam Jones" 'ok Friend
• ID 12345 'ok Protected
• mName "Joe Smith" 'error Private
• End Sub
• End Class

See Inheritance.Heroes_0
35
Heroes and Villains
UML diagram
36
Person, Hero, Villain

• Class Person
• Public Name As String
• End Class
• Class Hero
• Inherits Person
• Public Ability As String
• End Class
• Class Villain
• Inherits Person
• Public BadDeeds As ArrayList
• End Class

See Inheritance.Heroes_1
37
Wizard

• Class Wizard
• Inherits Person
• Enum MagicSpecialty
• casts_spells
• casts_out_spirits
• vanishes
• speaks_in_tongues
• wisdom
• End Enum
• Public Specialty As MagicSpecialty
• End Class

38
Creating Instances

• A Hero has a name and ability
• Dim H As New Hero
• H.Name "Superman"
• H.Ability "Invincible"
• A Villain has a name, along with a list of bad
  deeds
• Dim V As New Villain
• V.Name "Evil Witch"
• V.BadDeeds.Add("Casts spells")
• V.BadDeeds.Add("Turns princes into frogs")
• A Wizard has a name and a specialty
• Dim W As New Wizard
• W.Name "Merlin"
• W.Specialty W.MagicSpecialty.wisdom

39
Inheritance with Constructors

• When constructing a derived object...
• default base class constructor is automatically
  called before the derived class constructor
  executes
• If the base constructor has parameters...
• you must call it explicitly, passing arguments

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Example

• Class Person
• Sub New(ByVal name As String)
• mName name
• End Sub
• Private mName As String
• End Class
• Class Hero
• Inherits Person
• Sub New(ByVal name As String, ByVal ability As
  String)
• MyBase.New(name)
• mAbility ability
• End Sub
• Private mAbility As String
• End Class

See Inheritance.Heroes_2
41
Assigning Object References

• Upward cast is automatic
• Dim P As Person
• Dim H As New Hero("Aquaman","Swims")
• P H
• Downward cast must be explicit
• Dim P As Person New Hero("Superman","Flies")
• Dim H As Hero CType(P, Hero)
• Invalid cast compiles, but throws an exception at
  runtime
• Dim P As New Person("Bob")
• Dim H As Hero CType(P, Hero)

42
Overriding and Overloading

• override a method
• replace a base class method with a derived class
  method having the same signature
• overload a method
• create a new method having the same name as an
  existing method in the same class or a base class
  
• new method must have a different signature

43
Modifiers
44
Overriding the foo( ) Method

• Class Base
• Overridable Sub foo()
• Console.WriteLine("foo in Base Class")
• End Sub
• End Class
• Class Derived Inherits Base
• Overrides Sub foo()
• WriteLine("foo in Derived Class")
• End Sub
• End Class

45
Calling foo( )

• Late binding principle Common Language Runtime
  looks up the object's type and executes the
  appropriate version of foo( )
• Calls Derived.foo
• Dim obj As New Derived
• obj.foo()
• Calls Derived.foo
• Dim obj As Base New Derived
• obj.foo()

See Inheritance.Overriding
46
Abstract Classes Methods

• Abstract Class
• declared with the MustInherit modifier
• You cannot create an instance of an abstract
  class
• Abstract Method
• declared with the MustOverride modifier
• prototype, with no implementation

47
Example

• MustInherit Class Base
• MustOverride Sub foo()
• End Class
• Class Derived Inherits Base
• Overrides Sub foo()
• WriteLine("foo in Derived Class")
• End Sub
• End Class

48
Abstract Employee Class

• (partial listing)
• MustInherit Class Employee
• Implements IComparable
• Sub New(ByVal empId As Integer, ByVal name As
  String)
• mEmpId empId
• mName name
• End Sub
• MustOverride ReadOnly Property GrossPay() As
  Decimal
• (etc.)

See Inheritance.AbstractEmployee
49
Polymorphism

• Webster "having, occurring, or assuming various
  forms, characters, or styles"
• OOP base type's ability to reference various
  derived types
• Dim emp As Employee New SalariedEmployee(1001,
  "Johnson, Cal", 57000)

50
Polymorphism

• DoCalculations method accepts any type of
  Employee
• Sub DoCalculations(ByVal emp As Employee)
• WriteLine(emp.ToString() " " _
• FormatCurrency(emp.GrossPay) " - " _
• FormatCurrency(emp.CalculateTax) " " _
• FormatCurrency(emp.NetPay))
• End Sub
• Dim X As SalariedEmployee, Y as HourlyEmployee
• DoCalculations( X )
• DoCalculations( Y )

See Inheritance.Polymorphism
51
Collections
52
.NET Collection Classes

• Powerful set of utility classes for holding
  objects
• Automatic search, sort, insert, delete operations
• Alternative to relational database
• hierarchical data representation

53
System.Collections Namespace
54
HashTable Class

• Dictionary-type collection stores keys and their
  associated values
• optimized for quick storage and retrieval
• keys must be unique
• Implements IDictionary and ICollection interfaces
• ICollection
• Count property
• CopyTo method copies a collection into an Array

IDictionary
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IDictionary Interface
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Averages Example

• Create the Hashtable
• Dim averages As New Hashtable
• With averages
• .Add("Joe", 230.2)
• .Add("Dan", 330.2)
• .Add("Ann", 210.0)
• .Add("Bob", 430.2)
• .Add("Sam", 160.6)
• .Add("Jim", 200.7)
• End With

57
Averages Example

• Look up an entry
• Dim obj As Object averages.Item("Dan")
• Contains
• If averages.Contains("Bob") Then ...
• ContainsKey
• If averages.ContainsKey("Bob") Then ...

58
Averages Example

• Display the Keys collection
• Dim name As String
• For Each name In averages.Keys
• WriteLine(name)
• Next
• Display the Values collection
• Dim score As Single
• For Each score In averages.Values
• WriteLine(score)
• Next

59
Averages Example

• Display the Entries collection
• Dim entry As DictionaryEntry
• For Each entry In averages
• WriteLine(CStr(entry.Key) "--gt"
  CSng(entry.Value))
• Next

See Collections.HashTableEx
60
Calling ContainsValue

• Class defining values must override Equals( )
• Example user collection
• Dim users As New Hashtable
• users.Add("joe","xxabc2")
• users.Add("sam","ieshfds")
• ContainsValue
• If averages.ContainsValue("xxabc2") Then ...

61
SortedList Class

• Implements the IDictionary interface
• each entry is a DictionaryEntry object
• Maintains the keys in sorted order
• keys must be unique
• Example
• Dim list As New SortedList
• Dim emp As Employee
• emp New Employee(1001, "Jones, Dan")
• list.Add(emp.EmpId, emp)
• emp New Employee(3001, "Baker, Sam")

See Collections.SortedListEx
62
Visual Inheritance
63
Creating Derived Forms

• Visual Inheritance is the building of new forms
  from existing ones
• Useful Application
• create Master form with a standard appearance and
  control set
• custom forms can extend the Master form
• Creating a derived form in Visual Studio
• Right-click on project name select Add, select
  Add Inherited Form

64
Creating Derived Forms

• (continued)
• Select the base form in the Inheritance Picker

65
BaseForm and Login Classes

• Visual Inheritance Hands-on Example
• File VisualInherit

66
Scope of Controls

• Scope considerations
• make all controls in the form Protected
• Event handling
• if base class has no handler, you can create one
  in the derived class
• if base class has a handler, you have to do more
  work
• event handlers are never overridable
• event arrives at derived class first

67
Handling a Click Event
68
Delegates(optional topic)
69
Delegates

• Declares a method type (including signature)
• Delegate variables usually point to methods
• Declare a delegate
• Delegate Sub Sub_NoParms()
• Declare a method of the same type
• Sub Hello()
• System.Console.WriteLine("Hello there")
• End Sub

70
Delegates

• Assign the variable a method of the same type
• Dim deleg As Sub_NoParms
• deleg AddressOf Hello
• Invoke the method
• deleg.invoke()

71
Indirect Method Calls

• Declare a delegate and two matching methods
• Delegate Sub Sub_OneString(ByVal parm As String)
• Sub ShowColor(ByVal color As String)
• System.Console.WriteLine("Color " color)
• End Sub
• Sub ShowFruit(ByVal fruit As String)
• System.Console.WriteLine("Fruit " fruit)
• End Sub

72
Indirect Method Calls

• Create a method that calls any delegate
• Sub Call_a_proc(ByVal deleg As Sub_OneString, _
• ByVal parm As String)
• deleg.Invoke(parm)
• End Sub
• Call Call_a_proc
• Call_a_proc(AddressOf ShowColor, "Green")
• Call_a_proc(AddressOf ShowFruit, "Orange")
• Output
• Color Green
• Fruit Orange

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Key Terms

• abstract class
• abstract method
• base class
• collection
• comparator
• dictionary
• derived class
• downward cast
• enabling technology
• Enum
• Hashtable
• IComparable
• IComparer
• IDictionary

• interface
• overload
• override
• polymorphism
• reference type
• Reflection
• runtime type identification
• SortedList
• Structure
• TypeOf
• value equality
• value type
• visual inheritance

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The End