Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 10 Creating Classes and Objects
Objectives After studying this chapter, you should be able to: Define a class Instantiate an object from a class that you define Add Property procedures to a class Include data validation in a class Create default and parameterized constructors Microsoft Visual Basic 2005: Reloaded, Second Edition 2
Objectives (continued) Include methods in a class Overload the methods in a class Create a derived class using inheritance Microsoft Visual Basic 2005: Reloaded, Second Edition 3
Classes and Objects Object-oriented programs are based on objects that are instantiated (created) from classes Properties: attributes that describe the object Methods: behaviors that allow the object to perform tasks A class encapsulates properties and methods VB.Net has many built-in classes You can define your own classes Microsoft Visual Basic 2005: Reloaded, Second Edition 4
Defining a Class Class statement: defines a class Use Pascal casing for the class name Define attributes and behaviors of the class within the class Code editor automatically adds the Class statement Microsoft Visual Basic 2005: Reloaded, Second Edition 5
Defining a Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 6
Defining a Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 7
Defining a Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 8
Defining a Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 9
Defining a Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 10
Example 1 Using a Class that Contains Public Variables Only Microsoft Visual Basic 2005: Reloaded, Second Edition 11
Using a Class that Contains Public Variables Only (continued) Any class variable declared with Public keyword can be accessed by any application that contains an instance of the class Use Pascal case for Public variables in a class Microsoft Visual Basic 2005: Reloaded, Second Edition 12
Using a Class that Contains Public Variables Only (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 13
Using a Class that Contains Public Variables Only (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 14
Using a Class that Contains Public Variables Only (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 15
Using a Class that Contains Public Variables Only (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 16
Using a Class that Contains Public Variables Only (continued) Access an object s attributes using the dot operator: objectvariable.attribute Disadvantages of Public variables in a class: The class cannot control the values assigned to the variables Violates the concept of OOP encapsulation Microsoft Visual Basic 2005: Reloaded, Second Edition 17
Example 2 Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods Microsoft Visual Basic 2005: Reloaded, Second Edition 18
Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued) Class variables declared with Private keyword: Can only be used by the class Are hidden from the rest of the application Names should start with underscore _ Application can change class variable values only by using the class s methods Microsoft Visual Basic 2005: Reloaded, Second Edition 19
Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued) Property procedure: A Public method in the class for manipulating a class variable Exposes a Private class variable as a Property for use by the application Microsoft Visual Basic 2005: Reloaded, Second Edition 20
Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 21
Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued) Get block: contains code to allow an application to retrieve the contents of the variable exposed as a Property Set block: allows an application to assign a value to the variable exposed as a Property ReadOnly keyword: makes a property readable but not settable (no Set block) WriteOnly keyword: makes a property settable but not readable (no Get block) Microsoft Visual Basic 2005: Reloaded, Second Edition 22
Constructors Constructor: A method whose instructions are processed automatically when an object is instantiated from a class Purpose is to initialize the class s Private variables Method name must be New May or may not have parameters Default constructor: a constructor with no parameters Microsoft Visual Basic 2005: Reloaded, Second Edition 23
Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 24
Methods Other than Constructors Methods in a class can be Sub or Function procedures Use Pascal casing for method names Microsoft Visual Basic 2005: Reloaded, Second Edition 25
Methods Other than Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 26
Methods Other than Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 27
Methods Other than Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 28
Methods Other than Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 29
Example 3 Using a Class that Contains Two Constructors Microsoft Visual Basic 2005: Reloaded, Second Edition 30
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 31
Using a Class that Contains Two Constructors (continued) Parameterized constructor: a constructor method that contains parameters Method signature: method name and parameter list Best practices: Parameterized constructor should set the values of the class variables using the class s Property procedures to take advantage of any validation code in the Property procedures Microsoft Visual Basic 2005: Reloaded, Second Edition 32
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 33
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 34
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 35
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 36
Using a Class that Contains Two Constructors (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 37
Example 4 Using a Class that Contains Overloaded Methods Microsoft Visual Basic 2005: Reloaded, Second Edition 38
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 39
Using a Class that Contains Overloaded Methods (continued) Overloaded methods: two or more methods that have the same name but different parameters Constructors can be overloaded To overload a non-constructor method, use the Overloads keyword in the method declaration Many of VB s built-in methods are overloaded, as shown in the Intellisense feature: Microsoft Visual Basic 2005: Reloaded, Second Edition 40
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 41
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 42
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 43
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 44
Using a Class that Contains Overloaded Methods (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 45
Example 5 Using a Base Class and a Derived Class Inheritance: one class can be created from another class Base class: the original class Derived class: the new class created from the base class Inherits keyword: specifies the base class Microsoft Visual Basic 2005: Reloaded, Second Edition 46
Using a Base Class and a Derived Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 47
Using a Base Class and a Derived Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 48
Using a Base Class and a Derived Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 49
Using a Base Class and a Derived Class (continued) Derived class contains all of the attributes and behaviors of the base class Derived class may also contain its own attributes and behaviors Overridable keyword: indicates the base class method can be overridden by the derived class Overrides keyword: indicates that a method in the derived class overrides the method with the same name in the base class Microsoft Visual Basic 2005: Reloaded, Second Edition 50
Using a Base Class and a Derived Class (continued) MyBase keyword: refers to the base class MyBase.New: tells the computer to process the code in the base class s constructor Microsoft Visual Basic 2005: Reloaded, Second Edition 51
Using a Base Class and a Derived Class (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition 52
Programming Tutorial Microsoft Visual Basic 2005: Reloaded, Second Edition 53
Programming Example Microsoft Visual Basic 2005: Reloaded, Second Edition 54
Summary Objects are instantiated from classes Classes encapsulate their attributes and behaviors Class Public data members and methods are exposed to any application that creates an object from the class; class Private members are not Class Public properties allow an application to manipulate private class data members ReadOnly keyword allows a property to be retrieved but not changed Microsoft Visual Basic 2005: Reloaded, Second Edition 55
Summary (continued) WriteOnly allows a property to be changed but not retrieved Property Get block implements retrieving the property value Property Set block implements changing the property value Constructor: a sub procedure named New that is processed when an object is created from the class Default constructor: has no parameters Microsoft Visual Basic 2005: Reloaded, Second Edition 56
Summary (continued) Derived class: a new class created from another class (the base class) Overridable keyword: indicates that a method in the base class can be overridden in the derived class Overrides keyword: indicates that a method in the derived class overrides a method of the same name in the base class Inherits clause: creates a derived class MyBase keyword: refers to the base class Microsoft Visual Basic 2005: Reloaded, Second Edition 57