Object-Oriented Programming: Classes and Objects

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 • 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] PropertyPropertyName( ) 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. 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 SetEnd Property

23. 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.

24. 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.

25. 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.