COPYRIGHT 2010: Dr. David Scanlan, CSUS OBJECTIVES: How to write classes How to create an object from a class using the "New" keyword. How to communicate

COPYRIGHT 2010: Dr. David Scanlan, CSUS

OBJECTIVES:• How to write classes • How to create an object from a class using the "New" keyword.• How to communicate with an object from a Form.• Students will enter a simple program's code, execute it, and trace its execution with the debugger• Classes with and without inheritance.

OOP: Creating Object Oriented Programs

COPYRIGHT 2010: Dr. David Scanlan, CSUS OOP: Creating Object Oriented Programs

At least three times, read these notes; and write small OOP programs. If you do these things you will survive and enjoy OOP.

YOU MUST LEARN THESE SLIDES.



Designing Your Own Classes.

Using Your Own Classesto create Objects.


You are about to view a very simple OOP program which is not very realistic because it is so simple. It illustrates:

1. Writing a CLASS.2. Instantiating (creating) an OBJECT from the CLASS.3. Using the OBJECT in a short program.

Major reasons for using OOP are to create stable and reliable code that is easy to reuse.


This simple program calculates sales tax by:1. Sending the total sales amount (100.00) to an object from Form1.2. Next, the object calculates the sales tax (7.50) and sends it back to Form1.

(The next slides show how all this works.)3. The object’s code was created (instantiated) by the statement below:

Dim objCalculateSalesTax As New clsCalculateSalesTax

Form1

Object name Class name

OBJECT’S CODE100.00

7.50


To create (instantiate) an object from a class use the keyword:

New.

Dim objObjectName As New clsClassName

After executing the above statement, objObjectName will become an object created from the class called clsClassName.

New

New

New

New

NewNew

New

NewNewNew

New

New

New

New

New

New



What the statement above does as it is being executed:1. First, the underlined words make an empty location in memory for an object

with a type of data called clsClassName. We can get to that location usingthe word objObjectName.

Note: This first step is exactly the same as declaring an ordinary variable such as: Dim decPayRate As Decimal; Dim strLastName As String


4 GB memory4 GB memory

This empty location is reserved for an objectand it is accessed using the name objObjectName.

Empty location


4 GB memory4 GB memory


Dim objObjectName As New clsClassNameWhat the statement above does as it is being executed (Continued):2. Second, the word New takes code from a file within your project called

clsClassName.vb and makes a copy of it and places the copy in the locationcalled objObjectName. Note: When you write the code for clsClassName, the file will be automatically created and placed on the disk as

part of your Project. In the slides to follow, you will write the code for a class that will be placed in a file within your project.


objObjectName

Object’s codeThe file called clsClassName.vbcontains the Class.

New makes a copy of the class code from theclass file within your project and places that copy into the empty location called objObjectName.

The object can now be used.

A copy of clsClassName is sent to the location called objObjectName when

New is executed.


The next three slides will give you a very quick introduction to creating and using an OBJECT. Assumethe Sales Tax Calculator application below:

QUICK AND DIRTY INTRODUCTION TOCREATING AND USING AN OBJECT


This statement creates a location in memory and puts the object (objCalculateSalesTax) into it.

This statement will assign (SET) the object’sproperty (TotalSales) to 100.00.

This statement will use the method (ComputeSalesTax) of the object (objCalculateSalesTax) to compute the salestax. The sales tax value is then displayed in the label.

Property Method

HOW TO CREATE THE OBJECT AND USE IT.

Form1 code

COPYRIGHT 2010: Dr. David Scanlan, CSUS O

bjec

t’s c

ode:

obj

Cal

cula

teS

ales

Tax

Property name

Method name

How the object code works (see numbers left of code below):1. The object has a name: objCalculateSalesTax.2. A module-level variable (mdecTotalSales) is declared.3. TotalSales is the Property procedure which will give this object its only property.4. When TotalSales (Agrument) is assigned a value (100.00) in Form1’s code, it passes

ByValue 100.00 to Value (Parameter) which then assigns 100.00 to mdecTotalSales. 5. ComputSalesTax is a Method (function) which calculates the sales tax using

mdecTotalSales.

1

2

3

4

HOW THE OBJECT WORKS

5

ArgumentParameter

Note: The parameter “Value” could have been given any name, but “Value” is standard.

COPYRIGHT 2010: Dr. David Scanlan, CSUS HOW THE OBJECT WORKS WITH THE FORM1 CODE

100.00

7.50

1. In Form1’s code, the property TotalSales is assigned 100.00 from txtTotalSales.Text.2. This 100.00 is passed to the TotalSales Property in the object and 7.50 is returned using

the method ComputeSalesTax.

Object createdfrom the class.

Form1 code thatcreates the objectfrom the classand then uses the object.

Form1 Code

Object’s Code


Lab Exercise

1. Pass out the code for this program.2. Have students enter and run the code.3. Have students single step through the program and watch the flow of control from Form1 code to the Object's code and back again.4. Students must understand this code COMPLETELY.

If we do not do this Lab Exercise in class, you need to do it on your own.

COPYRIGHT 2010: Dr. David Scanlan, CSUS INSTRUCTIONS ON HOW TO CREATE AND USE A CLASS

Steps:1. Create a project. Use any meaningful name you want.2. Create a GUI for a program that inputs a decimal value and displays a tax

amount in a currency format:

Lab Exercise


Steps:3. Enter this code within the button-click code.

INSTRUCTIONS ON HOW TO CREATE AND USE A CLASS


Steps:4. Select the project name and right-click on it.



Steps:The screen should now look like this.5. Select Add, then Class.



Steps:The screen should now look like this. 6. Select Class below and type in the class name. This example uses

clsCalculateSalesTax.vb.

Don’t forget to press “Add”



Steps:7. Your screen should now look like the one below.8. Add the class code here. (See the next slide for the class code.)



Steps:9. After entering the class’s code below, your class should look like this.



Steps:10. The solution explorer should now look like this. Your class has been added to your project and is now a file within your project.

Your class is here as a“.vb” file.



Steps:11. Congratulations. You have just written your first class.12. Next, run your project. It should work.13. Don’t forget to save your project.14. Using the interactive debugger, single step thru the code and watch

the values and the flow of program.

You must understand this code to be able to do the next programming assignment.



Now, what have you just done?Answers: You have coded a class called clsCalculateSalesTax and have

saved the code in a file called clsCalculateSalesTax.vb in yourproject.

SUMMARY

NOTE: mdecTotalSales is a module-level variable. It can also be called a member variable as it is a member of the class clsCalculateSalesTax andcan be used anywhere within the class. Note that the variable is declaredin the typical module-level section (declaration section) of the code.


Now, what have you just done (continued)?Answers:

(1) You have created the object (objCalculateSalesTax) from the class(clsCalculateSalesTax) by using the file called clsCalculateSalesTax.vb, and

(2) assigned 100.00 to the property TotalSales, and(3) used the method called ComputeSalesTax. This method returned the value

7.50 which was displayed in lblSalesTax.Text.Note: see numbers 1,2,3 below that correspond to the above numbers.

SUMMARY

12

3


The next slides will give you a more detailed look at the previous example.

AN IN-DEPTH LOOK


What this OBJECT (objCalculateSalesTax) does:1. Receives a value (100.00) in the object's property: "TotalSales"2. Calculates the sales tax (7.50) using the object's method:

"ComputeSalesTax".3. Next, the object sends the sales tax value (7.50) back to Form1.

Sends 100.00

Sends back 7.50

THE OBJECT IS CODE IN MEMORY

objCalculateSalesTax

TotalSales

ComputeSalesTax

OBJECT Name

Property Name

Method Name

THIS FORM1 IS CODE IN MEMORY

This CLASS DIAGRAM represents the codedobject.

The code in Form1 creates and uses the object.

AN IN-DEPTH LOOK



TotalSales

ComputeSalesTax

OBJECT

FORM1

Computer's Memory 4GB RAM

These two units of code communicate in the computer’s memory using:1. Function procedures, and/or2. General Procedures.

In this program we used only Function Procedures as your will see on the next slide. >>>>

100.00

7.50

4GB RAM

CodeCode

AN IN-DEPTH LOOK


Computer's Memory 4GB RAM

These two units of code communicate using:1. Function Procedures, or2. General Procedures

In this program we only used Function Procedures.

4GB RAMFORM1'S CODE

OBJECT CODE100.00

7.50

AN IN-DEPTH LOOK



TotalSales

ComputeSalesTax

OBJECT Name

This code creates and uses the object.This code is the object.

The OBJECT'S code and the Form's code communicate

THE OBJECT THE FORM1 CODE THAT USES THE OBJECT

Method Name

Property Name

What to look for in this slide:1. Note the places where the object's name is used.2. Note the places where the object's property name is used.3. Note the places where the object's method name is used.

Note: The actual code for the object is not shown below. Only a Class Diagram is shown.

AN IN-DEPTH LOOK


Class Name

"New" keywordcreates object from the classname.

Object

PropertyMethod

How the code below in Form1 creates the object andcommunicates with the object code.

Note: The object's code is not show here.

Object

AN IN-DEPTH LOOK


Class Name

"New" keywordcreates object from the classname.

Object

Property Method

What the code from Form1 below does:1. Creates the object “objCalculateSalesTax" using the statement below:

Dim objCalculateSalesTax As New clsCalculateSalesTax'The data type of the object “objCalculateSalesTax" is the class “clsCalculateSalesTax"

2. The "TotalSales" property in the object is Set using the statement below:objCalculateSalesTax.TotalSales = CDec(txtTotalSales.Text)

3. The “ComputeSalesTax" method returns the sales tax using the statement below"lblSalesTax.Text = FormatCurrency(objCalculateSalesTax.ComputeSalesTax)

AN IN-DEPTH LOOK


Coding the Class

Name ofthe Class

Module-level variable name.

Property Name

This Property procedure allows for:

1. Set: assigning a value to a property.

Method procedure:1. This is a function procedure that returns a value through its name: ComputeSalesTax.

Method Name

AN IN-DEPTH LOOK


Coding the Class

What does "Set" do?Set assigns the Value "100.00" to mdecTotalSales so mdecTotalSales can be used in the Method "ComputeSalesTax".

Remember, mdecTotalSales is a module-level variable and can be used anywhere in the Class.

100.00

AN IN-DEPTH LOOK


Coding the Class

What does "Get" do?

Get would do NOTHING in this program. Get, when used, allows the program that accesses the object to get the current value of a property.

clsCalculateSalesTax

TotalSales = mdecTotalSales

AN IN-DEPTH LOOK


Coding the Class

Using Set without Get

WriteOnly• If you wish to use Set without Get, you must place WriteOnly here.

WriteOnly• If you wish to use Set without Get you must use WriteOnly.

• Using WriteOnly causes the Property Procedure to be a write only procedure. That is, you can only Set a property. You cannot Get (or read) a property from TotalSales.

100.00

AN IN-DEPTH LOOK


Coding the Class

What do module-level variables do?Module-level variables• These variables can be used in ANY procedure within the class.• The “m” means module. You may think of these variables as module-level variables, as they are recognized anywhere within the Class.

• In this program, "mdecTotalSales" allows for communication between the two procedures below:1. The Property Procedure2. The Function Procedure.

AN IN-DEPTH LOOK

Note: This module-level variable is often calleda member variable, because it is a member of the class and can be used anywhere within the class.


Property

Method

Form1 Code

Class code

100.00

7.50

Co

mm

un

icat

ion

AN IN-DEPTH LOOK

100.00

7.50

COPYRIGHT 2010: Dr. David Scanlan, CSUS AN IN-DEPTH LOOK

This slide and the next one illustrate how to use SETand GET with a very simple program.

Slide 1/2

Construct this GUI and enter the code on the next slide.Run the program and step thru it by setting breakpoints.

COPYRIGHT 2010: Dr. David Scanlan, CSUS AN IN-DEPTH LOOK

Slide 2/2

Enter the following code for the Form1 code and the class code.Run the program and step thru it by setting breakpoints.


There are two ways to create an object but only one will be covered in these notes. I think the one-step method is the easiest method to understand.


Two ways to create an object

Dim objObjectName As clsClassNameobjObjectName = NEW clsClassName

One-statement method

Two-statement method

The two-statement method will not be covered in these notes.

1. Dim objObjectName As …. .clsClassNamemakes a location for the object called

objObjectName with a clsClassNametype of data.

2. New creates the object and places it intothe location called objObjectName

AN IN-DEPTH LOOK


INHERITANCEAs previously mentioned, a major goal of OOP is reusable code.

OOP allows for this through inheritance; that is, previously written classes can be used again as part of a new application.

In the next slides we will use inheritance by writing a new class that will inherit the single property and single method of our previously

written “clsCalculateSalesTax" class. (Note: We will change the name of our base class from “clsCalculateSalesTax” to

just “clsSalesTax.” This will make the code easier to read and clearer.)


Slide topic: Two Class Diagrams showing inheritance

Inheritance

clsSalesTax

TotalSales

ComputeSalesTax()

CLASS Name

Property Name

Method Name

Base Class

CLASS Name

What gets inherited from clsSalesTax?clsSalesTaxAndSurtax inherits:

1. TotalSales2. ComputeSalesTax()

User interface

Our user interface can now use one property and three methods using the class clsSalesTaxAndSurtax:

One property and three methods:1. clsSalesTaxAndSurtax.TotalSales

1. clsSalesTaxAndSurtax.ComputeSalesTax()2. clsSalesTaxAndSurtax.ComputeSurtax()3. clsSalesTaxAndSurtax.ComputeTotalTax()

clsSalesTaxAndSurtax

ComputeSurtax()ComputeTotalTax()

Derived Class

No Property

Methods Names


Slide topic: What "seems" to be the affect of this inheritance.

Inheritance

clsSalesTax

TotalSales

ComputeSalesTax()

Base Class



Derived ClassclsSalesTaxAndSurtax

TotalSales

ComputSalesTax()ComputeSurtax()ComputeTotalTax()

Derived Class

Although the Derived Class still only has two methods, it acts like it has one property and three methods because we can now use these:1. clsSalesTaxAndSurtax.TotalSales

1. clsSalesTaxAndSurtax.ComputeSalesTax()2. clsSalesTaxAndSurtax.ComputeSurtax()3. clsSalesTaxAndSurtax.ComputeTotalTax()

IMPORTANT: clsSalesTaxAndSurtax does NOT contain "TotalSales" or "ComputeSalesTax()".

It only seems that way.

The derived class reaches up for the properties and methods of the base class.


Slide topic: Code for the base class "clsSalesTax"

Inheritance

clsSalesTax

TotalSales

ComputeSalesTax()

Base Class

The Code for the Base Class

You must use Get so that the Derived Class can Get the value of TotalSales when the Derived Class inherits the Property "TotalSales".


Slide topic: Code for the derived class "clsSalesTaxAndSurtax"

Inheritance

The Code for the Derived Class



Derived Class IMPORTANT KEYWORD

Because of inheritance,clsSalesTaxAndSurtax can use:1. TotalSales2. ComputeSalesTax()These two are Inherited from the Base Class.


Slide topic: Code for the derived class "clsSalesTaxAndSurtax"

Inheritance

The Code for the Derived ClassNote: When objSalesTaxAndSurtax is created from clsSalesTaxAndSurtax (Derived Class),objSalesTax (Base Class) is also created.Thus, Dim objSalesTaxAndSurtax As New clsSalesTaxAndSurtax will cause the creation of these two objects in memory.1. objSalesTaxAndSurtax2. objSalesTax Derived Class Code

IMPORTANT KEYWORD


Slide topic: Code for the User Interface.

Inheritance

Our user interface can now use one property and three methods using the class clsSalesTaxAndSurtax:Property

1. clsSalesTaxAndSurtax.TotalSalesMethods

1. clsSalesTaxAndSurtax.ComputeSalesTax2. clsSalesTaxAndSurtax.ComputeSurtax3. clsSalesTaxAndSurtax.ComputeTotalTax

.TotalSales

.ComputeSalesTax

.ComputeSurtax

.ComputeTotalTax

Three MethodsOne Property

The Code for the User Interface


Slide topic: Use obj because after instantiation, SalesTaxAndSurtax is an object.

Inheritance

Inherited from clsSalesTax1. clsSalesTaxAndSurtax.TotalSales2. clsSalesTaxAndSurtax.ComputeSalesTax

From clsSalesTaxAndSurtax3. clsSalesTaxAndSurtax.ComputeSurtax4. clsSalesTaxAndSurtax.ComputeTotalTax

The Code for the User Interface

Inherited from clsSalesTax1. objSalesTaxAndSurtax.TotalSales2. objSalesTaxAndSurtax.ComputeSalesTax

From clsSales AndTaxSurtax3. objSalesTaxAndSurtax.ComputeSurtax4. objSalesTaxAndSurtax.ComputeTotalTax

Use obj

BEFORE INSTANTIATON AFTER INSTANTIATON


Slide topic: Use obj because after instantiation, SalesTaxAndSurtax is an object.

Inheritance

The Code for the User InterfaceWe are using cls and obj to HELP you as a beginning program distinguish between a class and an object of the class in your code.

In the real world, it is not likely that the programmer will use cls and obj, but should.


Lab Exercise

1. Pass out the code for this program.See Directory: LABS-FALL 2007 for handout

2. Have students enter and run the code.3. Have students single step through the program and watch the flow of control from Form1 code to the Objects’ code and back again.4. Students must understand this code COMPLETELY.







The code on the following three slides is the same as the last three slides, but you will be able to copy and paste following code.


Public Class clsSalesTax Private mdecTotalSales As Decimal Public Property TotalSales() As Decimal Get TotalSales = mdecTotalSales End Get Set(ByVal Value As Decimal) mdecTotalSales = Value End Set End Property Public Function ComputeSalesTax() As Decimal Return mdecTotalSales * 0.075D End FunctionEnd Class

This code below can be copied into Visual Basic.NetCopy and Paste as needed.


Public Class clsSalesTaxAndSurtax Inherits clsSalesTax Public Function ComputeSurtax() As Decimal If TotalSales > 10000 Then Return TotalSales * 0.02D ElseIf TotalSales > 1000D Then Return TotalSales * 0.01D ElseIf TotalSales <= 1000D Then Return 0D End If End Function Public Function ComputeTotalTax() As Decimal Return ComputeSurtax() + ComputeSalesTax() End FunctionEnd Class




Private Sub btnCalculateSalesTax_Click(ByVal sender As System.Object, ByVal e As _ System.EventArgs) Handles btnCalculateSalesTax.Click Dim objSalesTaxAndSurtax As New clsSalesTaxAndSurtax objSalesTaxAndSurtax.TotalSales = CDec(txtTotalSales.Text) lblDisplaySalesTax.Text = FormatCurrency(objSalesTaxAndSurtax.ComputeSalesTax) lblDisplaySurtax.Text = FormatCurrency(objSalesTaxAndSurtax.ComputeSurtax) lblDisplayTotalTax.Text = FormatCurrency(objSalesTaxAndSurtax.ComputeTotalTax)End Sub


The End

Inheritance

Documents

COPYRIGHT 2010: Dr. David Scanlan, CSUS OBJECTIVES: How to write classes How to create an object from a class using the "New" keyword. How to communicate