1. OOP: Creating Object-Oriented Programs

2. VB & Object Oriented Programming Objects have properties, methods, and generate events Classes have been predefined up until now Text box class Form or command button class Languages such as Java and Smalltalk were designed to be OO from the beginning VB has evolved and modified to accommodate OOP VB.NET supports writing object oriented (OO) programs

3. 3 Object Terminology Review Object - like a noun, a thing Buttons, Text Boxes, Labels Properties - like an adjective, characteristics of object Text, ForeColor, Checked, Visible, Enabled Methods - like a verb, an action or behavior, something the object can do or have done to it ShowDialog, Focus, Clear, ToUpper, ToLower Events - object response to user action or other events Click, Enter, Activate

4. 4 Thus Far... Since day 1 we have been using objects Up until now the classes for all objects used have been predefined We have created new objects for these classes by using the controls in the Toolbox VB allows programmers to create their own object types by creating a Class Module

5. 5 Class and Instance When we add a button object from the button tool in the toolbox to the form we are creating an Instance of the Button Class The button object is an Instance of the Button Class Every button on the form is an Instance Defining your own Class is like creating a new tool for the Toolbox

6. Class Modules VB allows you to create your own new objects with a class module. User-created class modules have properties and methods. A tool such as the command button in a tool box is the object class.

7. Classes and Objects A command button on a form is an instance of the class—an object. Creating your own class is similar to adding a tool to the toolbox. A cookie cutter is a metaphor for a class. You create many cookies (objects) with the cutter (a class). Each cookie is a new instance of an object of the cookie cutter class.

8. Defining a New Class Defining your new class is like creating a new tool for the toolbox – the process does not create the object, only a definition of what the object looks like and how it will behave. You may then create as many instances of the class as you like e.g. your class may be employee, student, product etc

9. OOP Characteristics Object Oriented Programming (OOP) says that a true object oriented language has the following three characteristics; 1) encapsulation 2) polymorphism 3) inheritance

10. 10 Encapsulation Combination of characteristics of an object along with its behavior in "one package" Cannot make object do anything it doesn't already "know" how to do Cannot make up new properties, methods, or events Sometimes referred to as data hiding;an object can expose only those data elements and procedures that it wishes

11. 11 Polymorphism Different classes of objects may have behaviors that are named the same but are implemented differently Printer.print behaves differently from PictureBox.print, even though “print” is the method used for each.

12. 12 Inheritance Ability to create a new class from an existing class Purpose of Inheritance is reusability The new ones take on all the behaviors and properties of the parents from which they are cloned. In each form created is inherited from the existing Form class Original class is called Base Class, Superclass, or Parent Class Inherited class is called Subclass, Derived Class, or Child Class

13. 13 Example of Inheritance in VB.NET Examine 1st line of code for a form in the Editor (look at Visual Studio 2005 in the labs) Public Class Form1 Inherits System.Windows.Forms.Form Inherited Class, Derived Class Subclass, Child Class Base Class, Superclass, Parent Class

14. Hello World in VB.NET

15. 15 Goal of Inheritance Base Class Person Subclasses Employee Customer Student Person -Name -Address -Phone EmployeeStudentCustomer

16. VB & Object Oriented Programming Big advantage of OOP is the ability to reuse objects. Reusing objects saves time and effort New classes created with a Class Module can be used in multiple projects You can create three objects from the same class, yet you can set their properties differently.

17. Reusability Reusability means that you do not have to reinvent the wheel each time you create an application e.g. you can create a class for data access that can be reused. Reusing code that has previously been tested and debugged saves you time Organisations can create applications more rapidly

18. 18 Multitier Applications Common use of classes is to create multitier applications Each of the functions of a multitier application can be coded in a separate component and stored and run on different machines Goal is to create components that can be combined and replaced

19. 19 Three-tier Model Most common implementation of multitier Presentation TierBusiness TierData Tier User Interface Forms Controls Menus Business Objects Validation Calculations Business Logic Business Rates Data Retrieval Data Storage

20. this represents the class Textbox these represent objects— instances of the class Classes and Objects

21. Classes Design new behaviors and properties for classes you create You assign property values in your class You assign properties or reveal them with Property Get and Property Let procedures

22. Accessing the Values of a Class The Property Get procedure retrieves a property from a class, making it available to the outside world. The Property Let procedure assigns a value from the outside world to a property of the class. Property Let and Property Get are the only way to assign and retrieve property values for a class. Otherwise, there is no other “doorway” through which they can pass.

23. Form of the two procedures: [Public] Property Get Procedurename ([optional arg list]) [As datatype] statements in the procedure Procedurename = Propertyname End Property [Public] Property Let Procedurename ([optional arg list])IncomingValue _ [As datatype] statements in the procedure Propertyname = Procedurename End Property Example Public Property Get LastName() As String LastName = mstrLastName End Property Property Procedures

24. Creating a New Class 1) Define a new class module 2) Define the class properties 3) Add property procedures 4) Create a method 5) Save the class module

25. Define a new class module: Open a new project Select Add Class Module from the Project menu Click the New tab and choose Class Module; click Open Change the class name to one of your choosing (CProduct in this case) new name class module selection New tab

26. Defining Class Properties Declare properties of a class with variables (called instance variables) Variables are private and placed in General Declarations section Add property Let and Get procedures for each instance variable Code a method, which is a class’ behavior

27. Defining Class Properties Define three class properties (instance variables): Private mcurPriceAs Currency Private mintQuantityAs Integer Private mstrDescription As String Only procedures within the class can access the properties defined as private To provide a “window” through which properties can be retrieved or set, you code special methods called property procedures. Add Property Procedures, select Add Procedure from the Tools menu, name the property by naming the Procedure in the Name text box, and select options Property and Public. The Quantity property of the new class is defined this way—with two property procedures (also called accessor methods)

28. instance variables property procedures for the mintQuantity instance variable

29. Creating a New Object Using a Class Creating a class module defines a new class Code that creates and uses objects is placed in form and code modules, whereas a class must be placed in a class module Create an object of a class with the New keyword—called instantiating an object The New keyword creates a new instance of a class when the object is first used

30. Dimensioning objects with the New keyword: Dim EmployeeAs New cEmployee Private mInventoryAs New Cinventory Next, create a form that instantiates an object of the class CProduct

31. Collections You can create your own or use collections built in to VB Each project has a Controls collection and a Forms collection You can use a For Each… Next statement to set properties of multiple controls

32. For Each/Next Statements There is a need to have a method of reference to each Element in an Array A For/Next Loop can be used, however, another useful looping construct is the For Each/Next Loop; the significant advantage of using the For Each/Next Loop,is that the Subscripts of the Array do not have to be manipulated For Each ElementName In ArrayName Statement(s) in Loop Next ElementName

33. VB automatically references each Element of the Array, assigns its value to ElementName, and makes one pass through the loop If the Array has n Elements, the loop will execute n times The Variable used for ElementName must be a Variant Data Type For Each ElementName In ArrayName Statement(s) in Loop Next ElementName

34. Examle: Using an array named stName, print each element of the array Dim vOneName As Variant For Each vOneName In stName Print vOneName‘Print one element of the array Next vOneName The For Each/Next loop will execute if the array has at least one element; All the statements within the body of the loop are executed for the each element in the array Furthermore, an Exit For Statement may be used within the loop to exit early, just as in a For/Next loop

35. Using a For Each… Next Loop Private Sub cmdExit_Click() ‘this procedure will unload all forms that are currently loaded ‘in the project Dim OneForm As Form For Each OneForm in Forms Unload OneForm Next End This is a very useful piece of code for quitting a project

36. Disabling Controls using a For Each…Next Loop Dim AnyControl As Control For Each AnyControl in frmMain.Controls AnyControl.Enabled = False Next Uses the controls collection to change a property of multiple controls

38. Collections A Collection Class holds references for a series of objects created from the same class or from different classes Actually a collection holds references to the objects You reference by Index Number or a Key Similar to list box and the associated items in the list box

39. Key for Collection Objects Key must be a string Can be used to reference individual objects in the collection Declare the Key as String at the module level of the Class module for the object (not the collection) Add Property Get and Let procedures

40. Create a collection by writing a new class module and then declaring an object variable (to create an object). Each object in the collection is known as an item. Each object has a position known as its index. A collection is declared with a Dim, Public, or Static statement just like any other object variable: Dim mProducts As New Collection VB always updates the Count property of a collection every time you add or remove an item from a collection. Collections

41. The Add method adds an item to the collection. mProducts.Add Prod1 The Remove method removes an item according to its index in the collection. mProducts.Remove 2 It is best to provide a unique key to each object in a collection, rather than rely upon an index (see remove example above). It is difficult to associate an index with a particular product. Using a key value makes locating an object much easier. Add a string in the CProduct class: Private mstrProductCode As String Use the string to add an object to a collection or remove it from a collection Collections

42. Using a Collection in a Form

43. Using the Object Browser Object Browser reveals names of objects, properties, methods, events, and VB constants View objects: View, Object Browser Object Browser is best way to look up an object’s available properties, methods, etc. Select an object and press F1 for help

44. ComboBox is selected object Method symbol Property symbol Event symbol Description