9/21/2010. Based on Chapter 2 in Advanced Programming Using Visual Basic.NET by Bradley and Millspaugh

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Sharlene Dorcas Hodge
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1 Building Multitier Programs with Classes Based on Chapter 2 in Advanced Programming Using Visual Basic.NET by Bradley and Millspaugh The Object-Oriented Oriented (OOP) Development Approach Large production projects are created by teams Each team works on a part as a separate building block for the app In OOP development (and in Visual Basic.NET) the building blocks are classes Classes consist of code Instantiated classes are objects 2 Class A class is a blueprint for an object Blueprint includes properties Attributes Characteristics Blueprint includes methods Functions Operations Behaviors 3 1

2 OOP Approach Provides Features that Help create Reusable Classes Abstraction Encapsulation Inheritance Polymorphism Reusable means create the class modules once, use them in many programs look for patterns Abstraction A model of an object that determines Characteristics properties Behaviors methods Abstraction Reality? 5 Encapsulation Combine characteristics of an object along with its behaviors into one package Data hiding Properties and procedures are hidden Programmer controls exposure of properties and methods available to other objects Encapsulated functionality communicates through messages 6 2

3 Inheritance The ability to create a new class from an existing class The existing (original) class is called the base, abstract, or parent class or the superclass Abstract classes cannot be instantiated The inherited class is called the derived or child class, or subclass A derived class has an is a relationship with its base class 7 Abstract Classes Derived classes can call functions shared from abstract class Derived class methods override methods of the same name in the base class An abstract class is a class created strictly for the purpose of inheritance 8 Inheritance Base class Superclass parent derived class Subclass child Public Class frmmain Inherits System.Windows.Forms.Form Object-Oriented Programming 9 3

Multiple Subclasses Can Inherit From a Base Class Base class Superclass parent derived class Subclass child 10 Polymorphism Ability to take on many shapes or forms Methods with identical names but

4 Multiple Subclasses Can Inherit From a Base Class Base class Superclass parent derived class Subclass child 10 Polymorphism Ability to take on many shapes or forms Methods with identical names but different implementations, depending on the situation The Select method is different for radio buttons, check boxes, and list boxes A SpaceShip game object has different behavior from a JetLiner game object, but both are derived from an Aircraft base class Polymorphism A single class can have more than one method with the same name but a different argument list The method is said to be overloaded When an overloaded method is called, the argument type determines which version of the method to use 4

5 You Have Observed Reusable Code Modules Example Microsoft Spell Checker Others 13 Multitier Applications Write components that can be replaced or modified without replacing or modifying other components Plug-in in new components Applications designed from components that work together. Each component performs a service. Each service provides part of the necessary actions. 14 Multitier Applications Three-tier applications are popular Presentation tier Business tier Data tier 15 5

6 Multitier Applications User Services Business Services Data Services User Interface Forms Controls Business Objects Validation Calculations Data Retrieval Data storage Menus Business logic Business rules 16 Multitier Applications Presentation tier the user interface You can change the method of delivery without changing the processing 17 Multitier Applications Business tier classes that handle or manipulate the data Calculations Validation to enforce business rules Data tier retrieving and storing the data in a database 18 6

7 Create Your Own Class Instance Variables are used to hold characteristics (properties) of an object for use within the class Properties are created with Property procedures Sub procedures or function procedures are used to define behaviors (methods) 19 Creating Properties in a Class Declare all instance variables in a class as Private (encapsulation) Private variable values are only available to procedures in the class Reveal variable values as needed by exposing them as a property of the class 20 Private InstanceVariable As DataType Declare at module level [Public] Property PropertyName As DataType Get PropertyName = InstanceVariable or Return InstanceVariable End Get Set(ByVal Value As DataType) [statements, such as validation] InstanceVariable = Value End Set End Property General Form of a Property Procedure 7

8 Property Can Be Read-Only Private mdecpay As Decimal Declare at module level ReadOnly Property Pay() As Decimal Get Return mdecpay End Get THERE IS NO SET End Property 22 Properties Can Be Write-Only Private mstrpassword As String Declare at the module level WriteOnly Property Password(ByVal Value as String) THERE IS NO GET Set mstrpassword = Value End Set End Property 23 Constructors and Destructors A constructor is a method that automatically executes when an object is instantiated. A procedure named New A destructor is a method that automatically executes when an object is destroyed. A procedure named Dispose overrides the Dispose method of the base class or Use the base class Dispose method 24 8

9 Overloading the Constructor Sub New() Empty argument list End Sub Two methods have the same name but a different list of arguments Sub New(ByVal dhours As Decimal, ByVal drate As Decimal) Constructor that passes arguments Assign incoming values to properties mdechours = dhours mdecrate = drate End Sub A signature is the argument list of a method or procedure 25 Calling a Parameterized Constructor A constructor that requires arguments Instantiate an object in the WebForm Class Try Dim payobject As New Payroll(Cdec(txtHours.Text),Cdec(txtRate.Text)) Catch Err As Exception MessageBox.Show( Enter the hours and rate., Payroll ) End Try 26 Two Tier Application Presentation Tier User Interface Forms Controls Menus Business Services Tier Business Objects Validation Calculations Business logic Business rules 27 9

10 Building Multitier Programs with Classes Based on Chapter 2 in Advanced Programming Using Visual Basic.NET by Bradley and Millspaugh 10

Advanced Programming Using Visual Basic 2008

Building Multitier Programs with Classes Advanced Programming Using Visual Basic 2008 The OOP Development Approach OOP = Object Oriented Programming Large production projects are created by teams Each