1. Object-Oriented Programming: Classes and Objects

2. Multitier Applications  Classes are used 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.

3. Three-tier Model  Most popular implementation

4. "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  Methods = Eat, Bake, or Crumble  Events = Cookie crumbling and informing you

5. Object-Oriented Terminology  Encapsulation  Inheritance  Polymorphism

6. Encapsulation  Combination of characteristics of an object along with its behavior in "one package"  Cannot make object do anything it does not 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.

7. Inheritance (1 of 2)  Ability to create a new class from an existing class  Original class is called Base Class, Superclass, or Parent Class.  Inherited class is called Subclass, Derived Class, or Child Class.  For example, each form created is inherited from the existing Form class.  Purpose of inheritance is reusability.

8. Inheritance Example  Base/Parent Class  Person  Derived/Child Class  Employee  Customer  Student The derived classes inherit from the base class.

9. Polymorphism  Methods having identical names, but different implementations  Radio button, check boxes, and list boxes all have a Select method—the Select method operates appropriately for its class.  Overloading — Several argument lists for calling the method  Example: MessageBox.Show method  Overriding — Refers to a method that has the same name as its base class  Method in subclass takes precedence.

10. Designing Your Own Class  Analyze characteristics needed by new objects.  Characteristics or properties are defined as variables.  Define the properties as variables in the class module.  Analyze behaviors needed by new objects.  Behaviors are methods.  Define the methods as sub procedures or functions.

11. Creating Properties in a Class  Define variables inside the Class module by declaring them as Private — these store the value of the properties of the class.  Do not make Public, since that would violate encapsulation (each object should be in charge of its own data).

12. Property Procedures  Properties in a class are accessed with accessor methods in a property procedure.  Name used for property procedure is the name of the property seen by the outside world.  Set accessor method.  Uses Value keyword to refer to incoming value for property  Assigns a value to the property  Get Statement uses the value keyword to refer to the incoming value of the property.  Must assign a return value to the procedure name or use a Return Statement to return a value.

13. Property Procedure General Form {Private | Protected } ClassVariable As DataType [Public] Property PropertyName( ) As DataType Get Return ClassVariable [|PropertyName = ClassVariable] End Get Set (ByVal Value As DataType) [statements, such As validation] ClassVariable = Value End Set End Property

14. Read-Only Properties  In some instances, a value for a property can be retrieved by an object but not changed.  A property can be written to create a read-only property.  Create a read-only property by using the ReadOnly modifier. ' The property procedure for a read-only property. ReadOnly Property TotalPay() As Decimal Get Return TotalPayDecimal End Get End Property

15. Write-Only Properties  At times, a property can be assigned by an object, but not retrieved.  Create a property block that contains only a Set to create a write-only property. ' Private module-level variable to hold the property value. Private PriceDecimal As Decimal Public WriteOnly Property Price() As Decimal Set(ByVal value As Decimal) If value >= 0 Then PriceDecimal = value End If End Set End Property

16. Constructors and Destructors  Constructor  Method that automatically executes when an object is instantiated  Constructor must be public and is named New.  Ideal location for an initialization tasks such as setting the initial values of variable and properties  Destructor  Method that automatically executes when an object is destroyed  Rarely needed in.NET classes (automatic garbage collection)

17. Overloading the Constructor  Overloading means that two methods have the same name but a different list of arguments (the signature).  Create by giving the same name to multiple procedures in a class module, each with a different argument list.

18. Parameterized Constructor  Constructor that requires arguments  Allows arguments to be passed when creating an object

20. Value Types and Reference Types  Data types are divided into two categories—value types and reference types.  A variable of a value type (such as Integer) contains a single value of that type.  Dim CountInteger as Integer = 7  A reference type variable is initialized by default to the value Nothing if you do not initialize it in its declaration to refer to an object.  When you attempt to use a variable that contains Nothing to interact with an object, you’ll receive a NullReferenceException.  Dim Employee as Person

21. Class Scope  A class’s instance variables, properties and methods have class scope.  Within this scope, a class’s members are accessible to all of the class’s other members and can be referenced simply by name.  Outside a class’s scope, class members cannot be referenced directly by name.  Those class members that are visible (such as Public members) can be accessed through a variable that refers to an object of the class (for example, time.Hour).

22. Object Initializers  Object initializers use the With keyword to allow you to create an object and initialize its properties in the same statement.  This is useful when a class does not provide an appropriate constructor to meet your needs.  To use object initializers, you follow the object creation expression with the With keyword and an object initializer list—a comma-separated list in curly braces ({ }) of properties and their values as in the following statements:  Dim timeObject1 As New Time() With {.Minute = 33,.Second = 12} Dim timeObject2 As New Time() With {.Minute = 45}  Each property name must be preceded by the dot separator (.) and can appear only once in the object-initializer list.

23. Auto-Implemented Properties  For properties that do not have any additional logic in their Set and Get accessors, there’s a feature—called auto-implemented properties—that allows you to write one line of code and have the compiler to generate the property’s code for you.  Public Property Hour As Integer  The compiler would then generate a Private instance variable of type Integer named _Hour and the following property code  Public Property Hour As Integer Get Return _Hour End Get Set(value As Integer) _Hour = value End Set End Property

24. Using Me to Access the Current Object  Every object can access itself through its Me reference.  Public Sub New(hour As Integer, minute As Integer, second As Integer) Me.hour = hour ' initialize instance variable hour Me.minute = minute ' initialize instance variable minute Me.second = second ' initialize instance variable second End Sub ' New  The identifiers qualified with “Me.” represent the instance variables.  The unqualified identifiers represent the constructor’s parameters.

25. Garbage Collection  Every object you create uses various system resources, including the memory that holds the object itself.  These resources must be returned to the system when they’re no longer needed to avoid resource leaks.  The Common Language Runtime (CLR) performs automatic garbage collection to reclaim the memory occupied by objects that are no longer in use.  When there are no more references to an object, it’s marked for garbage collection by the CLR.

26. Garbage Collection  Resources like memory that are allocated and reclaimed by the CLR are known as managed resources.  Other types of resource leaks can occur.  For example, an app may open a file on disk to modify the file’s contents.  If the app does not close the file, other apps may not be allowed to use the file until the app that opened the file finishes.  Resources like files that you must manage are known as unmanaged resources (because they’re not managed by the CLR).  Such resources are typically scarce and should be released as soon as they’re no longer needed by a program.

27. Instance Variables versus Shared Variables  Instance variables or properties  Separate memory location for each instance of the object  Shared variables or properties  Single variable that is available for ALL objects of a class  Can be accessed without instantiating an object of the class  When creating, use the Shared keyword to create.  Shared properties can be set to read-only so that their values can be retrieved but not set directly.