Section 11.1. Classes and Objects

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11.1. Classes and Objects

noun A word used to denote or name a person, place, thing, quality, or act.

verb That part of speech that expresses existence, action, or occurrence.

adjective Any of a class of words used to modify a noun or other substantive by limiting, qualifying, or specifying.

The American Heritage Dictionary of the English Language

"A good rule of thumb for object-oriented programming is that classes are the nouns in your analysis of the problem. The methods in your object correspond to verbs that the noun does. The properties are the adjectives that describe the noun."

Gary Cornell & David Jezak

Practical experience in the financial, scientific, engineering, and software design industries has revealed some difficulties with traditional program design methodologies. As programs grow in size and become more complex, and as the number of programmers working on the same project increases, the number of dependencies and interrelationships throughout the code increases exponentially. A small change made by one programmer in one place may have many effects, both intended and unintended, in many other places. The effects of this change may ripple throughout the entire program, requiring the rewriting of a great deal of code along the way.

A partial solution to this problem is "data hiding" where, within a unit, as much implementation detail as possible is hidden. Data hiding is an important principle underlying object-oriented programming. An object is an encapsulation of data and procedures that act on the data. The only things of concern to a programmer using an object are the tasks that the object can perform and the parameters used by these tasks. The details of the data structures and procedures are hidden within the object.

Two types of objects will be of concern to us: control objects and code objects. Examples of control objects are text boxes, list boxes, buttons, and all the other controls that can be created from the Toolbox. So far, most of our programs have contained a single class block beginning with a line, such as "Public Class frmName" and ending with the line "End Class." A code object is a specific instance of a user-defined type, called a class, which is defined similarly to a structure, but in a separate class block of the form

Class ClassName   statements End Class

Each class block is delineated in the Code window by an elongated left bracket appearing to the left of the block. Both control objects and class objects have properties and respond to methods. The main differences are that control objects are predefined and have physical manifestations, whereas the programmer must create the class blocks for code objects. In this section, when we use the word "object" without a qualifier, we mean "code object."

Whenever you double-click on the TextBox icon in the Toolbox, a new text box is created. Although each text box is a separate entity, they all have the same properties and methods. Each text box is said to be an instance of the class TextBox. In some sense, the TextBox icon in the Toolbox is a template or blueprint for creating text boxes. When you look at the Properties window for a text box, the dropdown list box at the top of the window reads something like "TextBox1 System.Windows.Forms.TextBox." TextBox1 is the name of the control object and it is said to be an instance of the class "TextBox." You can't set properties or invoke methods of the TextBox class; you can only set properties or invoke methods of the specific text boxes that are instances of the class. The analogy is often made between a class and a cookie cutter. The cookie cutter is used to create cookies that you can eat, but you can't eat the cookie cutter.

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Object-oriented programs are populated with objects that hold data, have properties, respond to methods, and raise events. (The generation of events will be discussed in the next section.) Six examples are as follows:

In a professor's program to assign and display semester grades, a student object might hold a single student's name, social security number, midterm grade, and final exam grade. A CalcSemGrade method might calculate the student's semester grade. Events might be raised when improper data are passed to the object.
In a payroll program, an employee object might hold an employee's name, hourly wage, and hours worked. A CalculatePay method would tell the object to calculate the wages for the current pay period.
In a checking account program, a check register object might have methods that record and total the checks written during a certain month, a deposit slip object might record and total the deposits made during a certain month, and an account object might keep a running total of the balance in the account. The account object would raise an event to alert the bank when the balance gets too low.
In a bookstore inventory program, a textbook object might hold the name, author, quantity in stock, and wholesale price of an individual textbook. A CalculateRetailPrice method might instruct the textbook object to calculate the selling price of the textbook. An event could be triggered when the book goes out of stock.
In a game program, an airplane object might hold the location of an airplane. At any time, the program could tell the object to display the airplane at its current location or to drop a bomb. An event can be triggered each time a bomb is released so that the program can determine if anything was hit.
In a card game program, a card object might hold the denomination and suit of a specific card. An IdentifyCard method might return a string such as "Ace of Spades." A deck of cards object might consist of an array of card objects and a ShuffleDeck method that thoroughly shuffles the deck. A Shuffling event might indicate the progress of the shuffle.

The most important object-oriented term is class. A class is a template from which objects are created. The class specifies the properties and methods that will be common to all objects that are instances of that class. Classes are formulated in class blocks. An object, which is an instance of a class, can be created in a program with a pair of statements of the form

Dim objectName As className objectName = New className(arg1, arg2, ...)

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The first of these two lines of code declares what type of object the variable will refer to. The actual object does not exist until it is created with the New keyword, as done in the second line. This is known as creating an instance of an object and is where an object is actually created from its corresponding class. After this second line of code executes, the object is then ready for use. The first line can appear either in the Declarations section of a program (to declare a class-level variable) or inside a procedure (to declare a local variable). The instantiation line can only appear in a procedure; however, any object variable can be instantiated when declared (as either class-level or local) by using the single line

Dim objectName As New className(arg1, arg2, ...)

In a program, properties, methods, and events of the object are accessed with statements of the form shown in the following table:

Task	Statement
Assign a value to a property	`objectName.propertyName = value`
Assign the value of a property to a variable	`varName = objectName.propertyName`
Carry out a method	`objectName.methodName(arg1, ...)`
Raise an event	`RaiseEvent eventName`

The program in Example 1 uses a class named Student to calculate and display a student's semester grade. The information stored by an object of the type Student consists of a student's name, Social Security number, and grades on two exams (midterm and final). This data is stored in variables declared with the statements

Private m_name As String          'Name Private m_ssn As String           'Social security number Private m_midterm As Double       'Numerical grade on midterm exam Private m_final As Double         'Numerical grade on final exam

The word Private guarantees that the variables cannot be accessed directly from outside the object. In object-oriented programming terminology, these variables are called member variables (or instance variables). We will follow the common convention of beginning the name of each member variable with the prefix "m_". Each of these variables is used to hold the value of a property. However, instead of being accessed directly, each member variable is accessed indirectly by a property block. For instance, the following property block consists of a Get property procedure to retrieve (or read) the value of the Name property and a Set property procedure to assign (or write) the value of the Name property:

Public Property Name() As String   Get     Return m_name   End Get   Set(ByVal value As String)     m_name = value   End Set End Property

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In a property block, additional code can be added after the Get and Set statements to validate the data before they are returned or stored. The word Public allows the property to be accessed from outside the code for the Student class block. For instance, the Name property can be accessed by code in the form's class block. On the other hand, since the member variables were declared as Private, they cannot be accessed directly from code in the form's block. They can only be accessed through Property procedures that allow values to be checked and perhaps modified. Also, a Property procedure is able to take other steps necessitated by a change in the value of a member variable.

A property block needn't contain both Get and Set property procedures. For instance, the block

Public WriteOnly Property MidGrade() As Double   Set(ByVal value As double)     m_midterm = value   End Set End Property

specifies the MidGrade property as "write only." This property could be specified to be "read only" with the block

Public ReadOnly Property MidGrade() As Double   Get     Return m_midTerm   End Get End Property

Methods are constructed with Sub or Function procedures. A Function procedure is used when the method returns a value; otherwise a Sub procedure will suffice. For instance, the method CalcSemGrade, which is used to calculate a student's semester grade, is created as follows:

Function CalcSemGrade() As String   Dim grade As Double   grade = (m_midterm + m_final) / 2   grade = Math.Round(grade)  'Round the grade.   Select Case grade     Case Is >= 90       Return "A"     Case Is >= 80       Return "B"     Case Is >= 70       Return "C"     Case Is >= 60       Return "D"     Case Else       Return "F"   End Select End Function

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An object of the type Student is declared in the code for the form with a pair of statements such as

Dim pupil As Student   'Declare pupil as object of type Student pupil = New Student()  'Create an instance of Student

After these two statements are executed, properties and methods can be utilized with statements such as

pupil.Name = "Adams, Al"              'Assign a value to m_name txtBox.Text = pupil.Name              'Display the student's name lstBox.Items.Add(pupil.CalcSemGrade)  'Display semester grade.

The first statement calls the Set property procedure for the Name property, the second statement calls the Get property procedure for the Name property, and the third statement calls the method CalcSemGrade.

Example 1. (This item is displayed on pages 556 - 558 in the print version)

The following program uses the class Student to calculate and display a student's semester grade:

Object	Property	Setting
frmGrades	Text	Semester Grade
lblName	Text	Name:
txtName
lblSSN	Text	SSN:
mtxtSSN	Mask	000-00-000
lblMidterm	Text	Midterm:
txtMidterm
lblFinal	Text	Final:
txtFinal
btnEnter	Text	&Enter Information
btnDisplay	Text	&Display Grade
btnQuit	Text	&Quit
lstGrades

Public Class frmGrades   Dim pupil As Student  'pupil is an object of class Student   Private Sub btnEnter_Click(...) Handles btnEnter.Click     pupil = New Student()     'Create an instance of Student.     'Read the values stored in the text boxes.     pupil.Name = txtName.Text     pupil.SocSecNum = mtxtSSN.Text     pupil.MidGrade = CDbl(txtMidterm.Text)     pupil.Final = CDbl(txtFinal.Text)     'Clear text boxes and list box     txtName.Clear()     mtxtSSN.Clear()     txtMidterm.Clear()     txtFinal.Clear() 
[Page 557]    lstGrades.Items.Clear()     'Notify user that grades for the student have been recorded.     lstGrades.Items.Add("Student Recorded.")   End Sub   Private Sub btnDisplay_Click(...) Handles btnDisplay.Click     Dim fmtStr As String = "{0,-20}{1,-15}{2,-4}"     lstGrades.Items.Clear()     lstGrades.Items.Add(String.Format(fmtStr, pupil.Name, _                       pupil.SocSecNum, pupil.CalcSemGrade))   End Sub   Private Sub btnQuit_Click(...) Handles btnQuit.Click     End   End Sub End Class Class Student   Private m_name As String     'Name   Private m_ssn As String      'Social security number   Private m_midterm As Double  'Numerical grade on midterm exam   Private m_final As Double    'Numerical grade on final exam   Public Property Name() As String     Get       Return m_name     End Get     Set(ByVal value As String)       m_name = value     End Set   End Property   Public Property SocSecNum() As String     Get       Return m_ssn     End Get     Set(ByVal value As String)       m_ssn = value     End Set   End Property   Public WriteOnly Property MidGrade() As Double     Set(ByVal value As Double)       m_midterm = value     End Set   End Property   Public WriteOnly Property Final() AsDouble     Set(ByVal value As Double)       m_final = value     End Set   End Property 
[Page 558]  Function CalcSemGrade() As String     Dim grade As Double     grade = (m_midterm + m_final) / 2     grade = Math.Round(grade)  'Round the grade.     Select Case grade       Case Is  >= 90         Return "A"       Case Is  >= 80         Return "B"       Case Is  >= 70         Return "C"       Case Is  >= 60         Return "D"       Case Else         Return "F"     End Select   End Function End Class

[Run, enter the data for a student (such as "Adams, Al", "123-45-6789", "82", "87"), press the Enter Information button to send the data to the object, and press the Display Grade button to display the student's name, Social Security number, and semester grade.]

In summary, the following six steps are used to create a class:

1.	Identify a thing in your program that is to become an object.
2.	Determine the properties and methods that you would like the object to have. (As a rule of thumb, properties should access data, and methods should perform operations.)
3.	A class will serve as a template for the object. The code for the class is placed in a class block of the form Class ClassName statements End Class

4.	For each of the properties in Step 2, declare a private member variable with a statement of the form Private m_variableName As DataType [Page 559]Member variables can be preceded with the keyword Public, which allows direct access to the member variables from the code in the form. However, this is considered poor programming practice. By using Set property procedures to update the data, we can enforce constraints and carry out validation.
5.	For each of the member variables in Step 4, create a Property block with Get and/or Set procedures to retrieve and assign values of the variable. The general forms of the procedures are Public Property PropertyName() As DataType Get (Possibly additional code.) Return m_variableName End Get Set(ByVal value As DataType) (Possibly additional code.) m_variableName = value End Set End Property In the Get or Set code, additional code can be added to prevent the object from storing or returning invalid or corrupted data. For example, an If... Then block could be added to only allow valid Social Security numbers, alerting the user in the event of an invalid number.
6.	For each method in Step 2, create a Sub procedure or Function procedure to carry out the task.

Example 2.

The following modification of the program in Example 1 calculates semester grades for students who have registered on a "Pass/Fail" basis. We create a new class, named PFStudent, with the same member variables and property procedures as the class Student. The only change needed in the class block occurs in the CalcSemGrade method. The new code for this method is

 Function CalcSemGrade() As String   Dim grade As Double   grade = (m_midterm + m_final) / 2   grade = Math.Round(grade) 'Round the grade.   If grade >= 60 Then       Return "Pass"   Else       Return "Fail"   End If End Function

The only change needed in the form's code is to replace the two occurrences of Student with PFStudent. When the program is run with the same input as in Example 1, the output will be

Adams, Al 123-45-6789 Pass

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Object Constructors

Each class has a special method called a constructor that is always invoked as an object is instantiated. The constructor takes zero or more arguments, and the code inside the procedure block performs any tasks needed for initializing an object. It is often used to set default values for member variables and to create other objects associated with this object. The first line of the constructor for a class has the form

 Public Sub New(ByVal par1 As dataType1, ByVal par2 As dataType2, ...)

The graphical program in Example 3 illustrates the use of a constructor to specify the size and placement of a circle. This task involves "pixels," which are a unit of screen measurement. (The width of the screen is about 1000 pixels.) The settings for the Top, Left, Height, and Width properties of a control are given in pixels. For instance, the statements

picBox.Width = 85         'About 1 inch picBox.Height = 128        'About 1.5 inch picBox.Top = 170           'About 2 inches picBox.Left = 425          'About 5 inches

set the size of the picture box control as 1" by 1.5" and place the control 2 inches from the top of the form and 5 inches from the left side of the form. Refer to Section 9.2 for a discussion of drawing graphics in a picture box.

Example 3. (This item is displayed on pages 560 - 562 in the print version)

The following program contains a circle object. The object keeps track of the location and diameter of the circle. (The location is specified by two numbers, called the coordinates, giving the distance from the left side and top of the picture box. Distances and the diameter are measured in pixels.) A Show method displays the circle and a Move method adds 20 pixels to each coordinate of the circle. Initially, the (unseen) circle is located at the upper-left corner of the picture box and has a diameter of 40. The form has a button captioned "Move and Show Circle" that invokes both methods. Notice that the Xcoord, Ycoord, and Diameter properties, rather than the member variables, appear in the methods.

Object	Property	Setting
frmCircle	Text	Moving Circle
btnMove	Text	Move and Show Circle
btnQuit	Text	Quit
picCircle

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Public Class frmCircle   Dim round As New Circle()   Private Sub btnMove_Click(...) Handles btnMove.Click     round.Move(20)     round.Show(picCircle.CreateGraphics)   End Sub   Private Sub btnQuit_Click(...) Handles btnQuit.Click     End   End Sub End Class Class Circle   Private m_x As Integer  'Dist from left side of picture box to circle   Private m_y As Integer  'Distance from top of picture box to the circle   Private m_d As Integer  'Diameter of circle   Public Sub New()     'Set the initial location of the circle to the upper left     'corner of the picture box, and set its diameter to 40.     Xcoord = 0     Ycoord = 0     Diameter = 40   End Sub   Public Property Xcoord() As Integer     Get       Return m_x     End Get     Set(ByVal value As Integer)       m_x = value     End Set   End Property   Public Property  Ycoord() As Integer     Get       Return m_y     End Get     Set(ByVal value As Integer)       m_y = value     End Set   End Property   Public Property Diameter() As Integer     Get       Return m_d     End Get     Set(ByVal value AsInteger)       m_d = value     End Set   End Property 
[Page 562]  Sub Show(ByRef g As Graphics)     'Draw a circle with the given graphics context.     g.DrawEllipse(Pens.Black, Xcoord, Ycoord, Diameter, Diameter)   End Sub   Sub Move(ByVal distance As Integer)     Xcoord += distance     Ycoord += distance   End Sub End Class

[Run, and press the button ten times.]

When the line that instantiates an object contains arguments, the values of these arguments are passed to the object's New procedure. For instance, in Example 3, the first line typed in the form's code can be changed to

Dim round As New Circle(0, 0, 40)

and the New procedure for the Circle class can be changed to

Public Sub New(ByVal x As Integer, ByVal y As Integer, ByVal d As               Integer)   'Set the initial location of the circle to x pixels from   'the left side and y pixels from the top of the picture box.   'Set the diameter to d pixels.   Xcoord = x   Ycoord = y   Diameter = d End Sub

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Practice Problems 11.1

Which of the following analogies is out of place?

class : object
sewing pattern : garment
blueprint : house
programmer : program
cookie cutter : cookie

In Example 1, suppose that the first four lines of the event procedure btnEnter_Click are replaced with

 Private Sub btnEnter_Click(...) Handles btnEnter.Click   'Create an instance of Student.   Dim ssn As String = "123-45-6789"    'Social Security Number   pupil = New Student(ssn)

Create a New procedure and revise the SocSecNum property block for the Student class to be consistent with the last line in the preceding code.

Exercises 11.1

Exercises 1 through 14 refer to the class Student. When applicable, assume that pupil is an instance of the class.

1.	What will be the effect if the MidGrade property block is changed to the following? Public WriteOnly Property MidGrade() As Double Set(ByVal value As Double) Select Case midterm Case Is < 0 m_midterm = 0 Case Is > 100 m_midterm = 100 Case Else m_midterm = value End Select End Set End Property
2.	What will be the effect if the MidGrade property block is changed to the following? Public WriteOnly Property MidGrade() As Double Set(ByVal value As Double) m_midterm = value + 10 End Set End Property
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3.	Modify the class block for Student so that the following statement will display the student's midterm grade: lstGrades.Items.Add(pupil.MidGrade)
4.	Modify the class block for Student so that the student's semester average can be displayed with a statement of the form lstGrades.Items.Add(pupil.Average)
5.	In the class block for Student, why can't the third line of the CalcSemGrade method be written grade = (MidGrade + Final) / 2
6.	Write code for the class block that sets the two grades to 10 whenever an instance of the class is created.
7.	What is the effect of adding the following code to the class block: Public Sub New() m_ssn = "999-99-9999" End Sub In Exercises 8 through 14, determine the error in the given form code.
8.	Dim scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click Dim firstName as String scholar.Name = "Warren" firstName = scholar.Name End Sub
9.	Dim scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click Dim nom as String scholar = Student() scholar.Name = "Peace, Warren" nom = scholar.Name End Sub
10.	Dim scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click Dim nom as String scholar = New Student() m_name = "Peace, Warren" nom = scholar.Name End Sub
11.	Dim scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click Dim nom As String scholar = New Student() [Page 565] scholar.Name = "Peace, Warren" nom = m_name End Sub
12.	Dim scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click Dim grade As String scholar = New Student() scholar.CalcSemGrade = "A" grade = scholar.CalcSemGrade() End Sub
13.	Dim pupil, scholar As Student Private Sub btnGo_Click(...) Handles btnGo.Click scholar = New Student() pupil = New Student() scholar.MidGrade = 89 pupil.MidGrade = scholar.MidGrade lstGrades.Items.Add(pupil.MidGrade) End Sub
14.	Dim scholar As Student scholar = New Student() Private Sub btnGo_Click(...) Handles btnGo.Click scholar.Name = "Transmission, Manuel" End Sub
15.	In the following program, determine the output displayed in the list box when the button is pressed: Public Class frmCountry Dim nation As New Country("Canada", "Ottawa") Private Sub btndisplay_Click(...) Handles btnDisplay.Click nation.Population = 31 lstBox.Items.Add("Country: " & nation.Name) lstBox.Items.Add("Capital: " & nation.Capital) lstBox.Items.Add("Pop: " & nation.Population & " million") End Sub End Class Class Country Private m_name As String Private m_capital As String Private m_population As Double Sub New(ByVal name As String, ByVal capital As String) m_name = name m_capital = capital End Sub [Page 566] Public ReadOnly Property Name() As String Get Return m_name End Get End Property Public ReadOnly Property Capital() As String Get Return m_capital End Get End Property Public Property Population() As Double Get Return m_population End Get Set(ByVal value As Double) m_population = value End Set End Property End Class

Exercises 16 through 18 refer to the class Circle.

16.	Enhance the program in Example 3 so that the Get property procedures and the Set property procedures of the Xcoord and Ycoord properties are used by the form code.
17.	Modify Example 3 so that the circle originally has its location at the lower-right corner of the picture box and moves diagonally upward each time btnMove is pressed.
18.	Modify the form code of Example 3 so that each time btnMove is pressed, the distance moved (in pixels) is a randomly selected number from 0 to 40.
19.	Write the class block for a class called Square. The class should have three properties, Length, Perimeter, and Area, with their obvious meanings. When a value is assigned to one of the properties, the values of the other two should be recalculated automatically. When the following form code is executed, the numbers 5 and 20 should be displayed in the text boxes: Dim poly As Square Private Sub btnGo_Click(...) Handles btnGo.Click poly = New Square() poly.Area = 25 txtLength.Text = CStr(poly.Length) txtPerimeter.Text = CStr(poly.Perimeter) End Sub
20.	Modify the class Square in the previous exercise so that all squares will have lengths between 1 and 10. For instance, the statement `poly.Area = .5` should result in a square of length 1, and the statement `poly.Area = 200` should result in a square of length 10.
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21.	Write the class block for a class called PairOfDice. A Random object should be used to obtain the value for each die. When the following form code is executed, three numbers (such as 3, 4, and 7) should be displayed in the text boxes. Dim cubes As PairOfDice Private Sub btnGo_Click(...) Handles btnGo.Click cubes = New PairOfDice() cubes.Roll() txtOne.Text = CStr(cubes.Die1) txtTwo.Text = CStr(cubes.Die2) txtSum.Text = CStr(cubes.SumOfFaces) End Sub
22.	Write a program to toss a pair of dice 1000 times, and display the number of times that the sum of the two faces is 7. The program should use an instance of the class PairOfDice discussed in the previous exercise.
23.	Write the code for a class called College. The class should have properties Name, NumStudents, and NumFaculty. The method SFRatio should compute the studentfaculty ratio. When the following form code is executed, the number 18.7 should be displayed in the text box: Dim school As College Private Sub btnGo_Click(...) Handles btnGo.Click school = New College() school.Name = "University of Maryland, College Park" school.NumStudents = 30648 school.NumFaculty = 1638 txtBox.Text = FormatNumber(school.SFRatio, 1) End Sub
24.	Write a program that calculates an employee's pay for a week based on the hourly wage and the number of hours worked. All computations should be performed by an instance of the class Wages.
25.	Write a program to implement the cash register in Figure 11.1. The program should have a class called CashRegister that keeps track of the balance and allows deposits and withdrawals. The class should not permit a negative balance. Figure 11.1. Form for Exercise 25.
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26.	Write a program that calculates the average of up to 50 numbers input by the user and stored in an array. See Figure 11.2. The program should use a class named Statistics and have an AddNumber method that stores numbers into an array one at a time. The class should have a Count property that keeps track of the number of numbers stored and a method called Average that returns the average of the numbers. Figure 11.2. Form for Exercise 26.
27.	Write a program that a college bookstore can use to keep track of and determine the retail prices of textbooks. All computations should be performed by an instance of the class Textbook. The class should have properties Title, Author, Cost (wholesale cost), Quantity (number of copies in stock), and the method Price, that is, the retail price. Assuming that the bookstore marks up books by 25%, the Price should be 1.25 times the Cost.
28.	Write a program that calculates an employee's FICA tax, with all computations performed by an instance of a class FICA. The FICA tax has two components: the Social Security benefits tax, which in 2005 is 6.2 percent of the first $90,000 of earnings for the year, and the Medicare tax, which is 1.45 percent of earnings.
29.	Write a program that adds two fractions and displays their sum in reduced form. The program should use a Fraction class that stores the numerator and denominator of a fraction and has a Reduce method that divides each of the numerator and denominator by their greatest common divisor. Exercise 39 of Section 6.1 contains an algorithm for calculating the greatest common divisor of two numbers.

Solutions to Practice Problems 11.1

(d) A programmer is not a template for creating a program.

 Public Sub New(ByVal ssn As String)   'Assign the value of ssn to the member variable m_ssn.   m_ssn = ssn End Sub Public ReadOnly Property SocSecNum() As String   Get     Return m_ssn   End Get End Property

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Note: Since a student's Social Security number never changes, there is no need to have a Set property procedure for SocSecNum.