Pemrograman VisualMinggu …3… Page 1 MINGGU Ke Tiga Pemrograman Visual Pokok Bahasan: Class,...

Minggu …3…

Pemrograman Visual

MINGGU Ke Tiga

Pemrograman VisualPemrograman Visual

Pokok Bahasan:Class, Objects, Methods and Instance Variable

Tujuan Instruksional Khusus:Mahasiswa dapat menjelaskan dan mengaplikasikan konsep class dan object pada Visual Basic 2008

Referensi:Deitel Deitel, Visual Basic 2008 (2009), How to Program, Prentice Hall. Chapter 4

Pemrograman Visual Minggu …3…

Agenda• Declare class with a method and

instantiating an object of class• Declare a method with parameter• Instance variable and properties• Value Types and Reference Types• Initializing Objects with constructors• Validating data with set accessors in

properties


Declare class with a method and instantiating an object of class

• Adding a class to a Visual Basic Project– Right click the project name in the solution

explorer and select add > class from the menu that appears

• Example of Class GradeBook


Declare class with a method and instantiating an object of class (Continued …)

• Using Class GradeBook

– You’ll now use class GradeBook in an apllication. A Visual Basic project that contains only class GradeBook is not an application that can be exceuted, because GradeBook does not contain main. If you try to compile such a project, you’ll get an error message like:

‘Sub Main’ was not found in ‘GradeBook.GradeBookTest’

• So How we can fixed the Problem???

– We must declare a separate class or module that contains main, or we must place main in class GradeBook


Example Using Class GradeBook


Declare a Method With Parameter

• What is a Parameter ???

• Example parameter of Class GradeBook


Example execution Class GradeBook using parameter


Instance Variables and Properties• In Previous discussion, we declare all of an

application’s variable in the main method. Variables declared in the body of a particular method are known as local variables and can be used only in that method. When a method terminates, the values of its local variables are lost.

• So How we can fixed the Problem???– We must create attributes exist before a method is called on

the object, while the method is executing and after the method completes execution.


Example using instance variables and properties


Example execution Class GradeBook using instance variables and properties


Value Type and Reference Type• Data type in Visual Basic are divided into two categories –

value types and reference type

Dim count as Integer = 7

A variable (count) of a value type (Integer)

Contains a value (7) of that type


Value Type and Reference Type (Continued …)

• A variable of a reference type (sometimes called a reference) contains the address of a location in memory where an object is stored

• Reference type instance variables are initialized by default to the value nothing

• A client of an object must use a reference to the object to invoke (i.e.,call) the object’s methods and access the object’s properties

Is a object


Initializing Objects with Constructors

• Each class you declare can provide a constructor that can be used to initialize an object of the class when the object is created

• Keyword new calls the class’s constructor to perform the initialization

• By default, the compiler provides a default constructor with no parameter in any class that does not explicitly include a constructor

• When you declare a class, you can provide your own constructor to specify custom initialization for objects of your class


Initializing Objects with Constructors (Continued …)

• A constructor must have the name New• Constructor declarations cannot return values• A class’s instance variables are initialized to their

default values if no constructor is provided


Validating Data with Set Accessors in Properties

• Properties can be used to keep an object in a consistent state, that is a state in which the object’s instance variables contains valid data

• A class’s set accessors cannot return values indicating a failed attempt to assign invalid data to objects of the class

• In Visual basic, it is possible to declare the get and set accessors of a property with different access modifiers. In this case, one of the accessors must have the same access as the property and the other must be more restrictive than the property. Private is more restrictive than public.


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