1. 110-I 1 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

2. 110-I 2 So Far... Since Chapter 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

3. 110-I 3 The class is the blueprint for the object. It describes –The data contained in the object (properties) – The actions that the object can perform (methods, event procedures) An object is an instance of a class. What is a Class?

4. 110-I 4 Defining a class does not create an object, only a definition of what that type of object looks like and behaves. Example: the Button class. You can have many buttons in your project (many instances of the Button class). Each button will have its own properties; different text, different name, different code for the event procedures. A Class vs. an Object (1)

5. 110-I 5 A Class vs. an Object (2) 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 Every button on the form is an Instance of the Button Class Defining your own Class is like creating a new tool for the Toolbox

6. 110-I 6 "Cookie Analogy" Class = Cookie cutter Instantiate = Making a cookie using the cookie cutter Instance = Newly made cookie Properties of the Instance may have different values –Icing property can be True or False –Flavor property could be Lemon or Chocolate

7. 110-I 7 "Cookie Analogy" (cont.) Methods = Bake Events = Cookie crumbling when you step on it accidentally

8. 110-I 8 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 The details of the implementation of the class are not relevant (they are hidden) to the user of the class.

9. 110-I 9 Reusability The main purpose behind OOP and Inheritance in particular New classes created with Class Module can be used in multiple projects Each object created from the class can have its own properties

10. 110-I 10 Class Design - Analyze: Characteristics of your new objects –Characteristics will be properties –Define the properties as module level variables in the class module Behaviors of your new objects –Behaviors will be methods –Define the methods as sub procedures and functions in the class module

11. 110-I 11 Create a New Class Add Class Module to your project 1. Go to New Item dialog, choose Class 2. Name the Class 3. Define the Class properties 4.Code the methods STEPS:

12. 110-I 12 Properties of a Class Define variables inside the Class Module by declaring them as Private at module level. Do not make them Public-that would violate Encapsulation (each object should be in charge of its own data) Private mintPatientNum as Integer Private mdtmDate as DateTime Private mstrLastName as String

13. 110-I 13 Assign Values to Properties Write special property procedures to –Pass the values to the class module –Return values from the class module Name used for property procedure is the name of the property seen by the outside world Property Get –Retrieves property values from a class –Like a function must return a value Property Set –Sets or assigns values to properties

14. 110-I 14 Property Procedure General Form Private ClassVariable As DataType [Public] Property PropertyName As DataType Get Return ClassVariable End Get Set (ByVal Value As DataType) [statements, such as validation] ClassVariable = Value End Set End Property

15. 110-I 15 Read-Only Properties In some instances a value for a property should only be retrieved by an object and not changed –Create a read-only property by using the ReadOnly modifier –Write only a Get portion of the property procedure [Public] ReadOnly Property PropertyName As DataType

16. 110-I 16 Code a Method Create methods by adding sub procedures and functions for the behaviors to the class module (just like we have up to now). The sub procedures and functions should be defined as Public unless there are going to be used for implementation purposes.

17. 110-I 17 Analyze the characteristics (properties) and behaviors (methods) of your object Write Property Procedures to assign and retrieve the properties of the class It allows us to declare all module variables (properties) private Write Sub Procedures and Functions for the behaviors of the class Summary:Defining a Class(1)

18. 110-I 18 Writing Property Procedures (they are public by default) Private mstrLastName As String Property LastName( ) As String Get Return mstrLastName End Get Set (ByVal Value As String) mstrLastName= Value End Set End Property Declaration of property Property Procedure Returns value of property Sets value of property (needs argument) Summary:Defining a Class(2)

19. 110-I 19 Read Only Property Procedures Example: Private mstrLastName As String ReadOnly Property LastName( ) As String Get Return mstrLastName End Get End Property Declaration of property Property Procedure Returns value of property Summary:Defining a Class(3)

20. 110-I 20 BookSale class Three private instance variables - mstrTitle - mintQuantity - mdecPrice Three public property Procedures –Title –Quantity –Price One public method –ExtendedPrice The class diagram is incomplete (e.g., no type for the instance variables, no information about the method)

21. 110-I 21 Public Class BookSale ‘ Class Name: BookSale Private mstrTitle As String Private mintQuantity As Integer Private mdecPrice As Decimal ‘Property Procedures Property Title( ) As String Get Return mstrTitle End Get Set (ByVal Value As String) mstrTitle= Value End Set End Property A complete class code (1)

22. 110-I 22 Property Quantity( ) As Integer Get Return mintQuantity End Get Set (ByVal Value As Integer) mintQuantity= Value End Set End Property Property Price( ) As Decimal Get Return mdecPrice End Get Set (ByVal Value As Decimal) mdecPrice= Value End Set End Property A complete class code (2)

23. 110-I 23 ‘write the methods Public Function ExtendedPrice() As Decimal Return CDec(mintQuantity* mdecPrice) End Function End Class A complete class code (3)

24. 110-I 24 Creating a New Object Using a Class Steps Declare a variable for the new object with datatype of the class Then, instantiate the object using the New keyword

25. 110-I 25 Instantiating An Object Creating a new object based on a class Create an instance of the class by using the New keyword and specify the class New className ( )

26. 110-I 26 Examples of Instantiating An Object Dim fntMyFont = New Font ("Arial", 12) lblMsg.Font = fntMyFont OR lblMsg.Font = New Font ("Arial", 12)

27. 110-I 27 Best Practices You may declare and instantiate an object at the same time but it is not the best practice Should declare the variable separately in the Declarations section Instantiate the object –Only when(if) it is needed –Inside a Try/Catch block for error handling (Try/Catch block must be inside a procedure)

28. 110-I 28 Example using our class Public Class frmBookSale : Inherits System.Windows.Forms.Form Private mBookSale As BookSale ‘declare module level variable Private Sub mnuFileCalculateSale(..) Try mBookSale = New BookSale() 'instantiate the object 'Set the properties of the booksale object mBookSale.Title = txtTitle.Text mBookSale.Quantity = CInt(txtQuantity.Text) mBookSale.Price = CDec(txtPrice.Text) ‘Display the book title and calculate and format the result lblTitle.Text=mBookSale.Title lblExtendedPrice.Text = FormatCurrency(mBookSale.ExtendedPrice()) Catch 'display message if there is an error MessageBox.Show("Please make sure your entries are correct”) End Try End Sub End Class Calling the object method Calling “Set” property procedures Creating the object Calling “Get” property procedure

29. 110-I 29 Instance Members And Shared Members Members include both properties and methods A shared member has ONE copy for ALL objects of the class No need to instantiate an object to access the shared members (property or method), just use the Class Name ex. Color.Red A instance member has ONE copy for EACH object or instance of the class Class Name Property Name Use the Shared Keyword to create a Shared Member

30. 110-I 30 Instance versus Shared Members Instance Members –Separate memory location for each instance of the object Shared Members –Single memory location that is available for ALL objects of a class –Can be accessed without instantiating an object of the class –Use the Shared keyword to create

31. 110-I 31 Creating Shared Members Use the Shared Keyword to create a Shared Member Example. Private Shared mdecSalesTax As Decimal Shared ReadOnly Property SalesTax( ) As Decimal Get Return mdecSalesTax End Get End Property Make shared properties Read-Only, so their values can be retrieved but not set directly [Public|Private] Shared Function FunctionName (ArgumentList) _ As Datatype

32. 110-I 32 The KeyWord Me An instance method of a class is always called using an instance of the class (an object), Object.Method(Arguments); The instance object is not part of the actual parameter list. However, it is still passed to the method implicitly. It can't appear in the formal parameter list To refer to the object used to make the call, use the keyword Me.

33. 110-I 33 Using The KeyWord Me An example(1) In an instance method, access the object implicitly passed to that method with the me keyword. Public class SomeClass private var As Integer Public Sub changeVar(n As Integer) Me.var = n ‘OK, but superfluous End Sub End Class Could also do Public Sub changeVar(var As Integer) Me.var = var End Sub var is local to changeVar. Since the name is the same as the instance variable var, we need to use Me to refer to the instance variable var (potentially confusing). The local variable shadows the instance variable.

34. 110-I 34 Using The KeyWord Me An example(2) In an instance method, any reference to an instance variable or any call to another instance method done without an explicit reference uses the implicit object(me). Public class Bar Public Sub foo1() ‘ code End Sub Public Sub foo2() foo1(); ‘call the instance method foo1 ‘ of the implicit object. ‘ same as Me.foo1(); End Sub End Class

35. 110-I 35 Constructors and Destructors Constructor –Method that automatically executes when an object is instantiated –Create by writing a Public Sub New procedure Destructor –Method that automatically executes when an object is destroyed –Create by writing a Finalize procedure –Usage discouraged by Microsoft

36. 110-I 36 Overloading Overloading means that 2 or more methods have the same name but a different list of arguments (the signature) Create by giving the same name to multiple procedures in your class module, each with a different argument list

37. 110-I 37 Parameterized Constructor Constructor that requires arguments Arguments are passed when an object is instantiated Can be used to assign initial property values

38. 110-I 38 More on Constructors Sub New() ‘Constructor with empty argument List ‘Initialize Instance Variables End Sub Sub New( Arguments) ‘Constructor with argument List ‘Use Arguments to initialize instance variables End Sub

39. 110-I 39 Example Using a Constructors Sub New( ByVal Title As String, ByVal Quantity As Integer, ByVal Price As Decimal) ‘Use Arguments to initialize instance variables Me.Title=Title Me.Quantity=Quantity Me.Price=Price End Sub Call to the Set Property Procedure Set the value using the argument

40. 110-I 40 References and assignment (1) Consider Public Sub SomeSales() ‘ create two objects mBookSale1 = New BookSale() mBookSale2 = New BookSale() ‘ Set title, quantity and price mBookSale1.Quantity=5 mBookSale2.Quantity=10 mBookSale1.Price=2 mBookSale2.Price=3 mBookSale2= mBookSale1 ‘ Calculate the cost of each sale and display it lblDisplay1.Text= mBookSale1.ExtendedPrice() lblDisplay2.Text= mBookSale2.ExtendedPrice() End Sub What happens?

41. 110-I 41 References and assignment (2) Before executing mBookSale2 = mBookSale1 mBookSale1:BookSale mBookSale2:BookSale  mBookSale1 = mBookSale2 :BookSale mBookSale2:BookSale X  The object that mBookSale2 referred to can't be accessed anymore. The memory it uses is freed by the garbage collector.  The sale price is the same in both labels Note: if 2 or more references refer to the same object, drop the object reference in the UML notation mBookSale1 mBookSale2 mBookSale1 mBookSale2

42. 110-I 42 Garbage Collection Feature of.NET Common Language Runtime (CLR) that cleans up unused components Periodically checks for un-referenced objects and releases all memory and system resources used by the objects Microsoft recommends depending on Garbage Collection rather than Finalize procedures