advanced object- oriented programming programming right from the start with visual basic.net 1/e 14

AdvancedObject-

OrientedProgramming

Programming Right from the Start with Visual Basic .NET 1/e

14

Crews/Murphy – Programming Right From the Start with Visual Basic.NET 1/e – ©2004 Prentice Hall 2

Objectives

• Understand the differences between classes and objects

• Understand object terminology, including encapsulation, inheritance, and polymorphism

• Know how to create your own classes


Objectives (cont.)

• Know how to write constructors with multiple parameter lists

• Develop applications that use classes and objects


14-1 Writing Objects

• VB .NET is an object-oriented language.

• Forms are objects.

• Controls are objects.

• Controls are objects that have GUI properties and methods.


Classes and Objects

• An object is a combination of data and actions that can be treated as a unit.

• A class is the structure of an object, a blueprint that describes the properties (data) and methods (actions) of an object.

• An object is created from a class.


14-2 Object-Oriented Terminology

• A language is considered to be object-oriented if it supports three main features:– Encapsulation– Inheritance– Polymorphism


Encapsulation

• Encapsulation refers to grouping related properties and methods so they can be treated as a single unit or object.

• Encapsulation also refers to protecting the inner contents of an object from being damaged or incorrectly referenced by external code.


Encapsulation (cont.)

• One of the basic rules of encapsulation is that class data should be modified or retrieved only through property procedures.

• Limiting how external code interacts with the object allows for later modification without risk of compatibility problems.


Encapsulation (cont.)

• Encapsulation allows you to control how the data and procedures are used.

• You should declare internal details of a class as Private to prevent them from being used outside your class; this technique is called data hiding.


Inheritance

• Inheritance describes the ability to create new classes based on an existing class.

• The existing class is called the base class, and the new class derived from the base class is called the derived class.


Inheritance (cont.)

• The derived class inherits all the properties, methods, and events of the base class and can be customized with additional properties and methods.

• Inheritance takes code reuse to a whole new level.


Inheritance (cont.)


Polymorphism• Polymorphism is the ability for objects

from different classes to respond appropriately to identical method names or operators.

• Polymorphism is essential to object-oriented programming because it allows you to use shared names, and the system will apply the appropriate code for the particular object.


14-3 Creating YourOwn Classes

• A class definition consists of fields, properties, and methods.

• A field is a variable in the class and is usually private.

• A property is a programming construct that typically provides the interface to a field in a class.


14-3 Creating YourOwn Classes (cont.)

• A method is a function or a sub procedure within a class.

• The class definition also may contain constructor methods that are called when a new object is instantiated from the class.


Fields

• Fields provide storage for the data in an object and are treated just like variables.

• For this text, all field values will be declared Private and all field names will begin with “F”.– [Public|Private] fieldname As datatype


Properties

• Private fields of a class cannot be accessed by external code.

• If you want an object’s field data to be read or changed, you should include property procedures in the class definition.

• The Get property procedure typically retrieves a Private field.


Properties (cont.)

• The Set property procedure typically assigns a new value to a Private field.

• Some fields are intended to be read-only, meaning external code can view the value of the field, but cannot change its value.


Methods

• Methods are procedures defined within a class.

• Methods have access to all data within the object – even Private data.[Private|Public] Sub procedurename([parameters])

[statements]

End Sub


Constructors

• A constructor is a special method that executes during the creation of an object.

• All constructor methods are procedures named New.– Sub New ([parameters])

[Statements]

End Sub


Constructors (cont.)

• When you define a class derived from another class, the first line of a constructor is typically a call to the constructor of the base class.

• The base class is referenced by using the keyword MyBase.– MyBase.New()


14-4 The Complete TMilitaryTime Class Definition


14-5 The Complete TPerson and TStudent Class Definitions


Chapter Summary

• VB. NET is an object-oriented language that supports encapsulation, inheritance, and polymorphism.

• Encapsulation refers to grouping related properties and methods so they can be treated as a single unit or object.

• Inheritance describes the ability to create new classes based on an existing class.


Chapter Summary (cont.)

• Polymorphism is the ability for objects from different classes to respond appropriately to identical method names or operators.

• A class definition consists of fields, properties, and methods.

• A constructor is a special method that executes during the creation of an object.

AdvancedObject-

OrientedProgramming

Programming Right from the Start with Visual Basic .NET 1/e

14

advanced object- oriented programming programming right from the start with visual basic.net 1/e 14

Documents

prentice hall

class data

base class

derived class

existing class

encapsulation cont

properties data

inheritance cont