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308 Object-Based Programming Chapter 8 ences of class Date. The Employee constructor (lines 16 27) takes eight arguments (first, last, birthMonth, birthDay, birthYear, hireMonth, hireDay and hireYear). Line 24 passes arguments birthMonth, birthDay and birth- Yearto the Dateconstructor to create the birthDateobject. Similarly, line 25 passes arguments hireMonth, hireDayand hireYearto the Dateconstructor to create the hireDateobject. Method ToEmployeeString(lines 30 35) returns a stringcontaining the name of the Employee and the string representations of the Employee s birthDate and hireDate. Class CompositionTest (Fig. 8.10) runs the application with method Main. Lines 13 14 instantiate an Employeeobject and lines 16 17 display the string representation of the Employee to the user. 1 // Fig. 8.9: Employee.cs 2 // Employee class definition encapsulates employee’s first name, 3 // last name, birth date and hire date. 4 5 using System; 6 7 // Employee class definition 8 public class Employee 9 { 10 private string firstName; 11 private string lastName; 12 private Date birthDate; // reference to a Date object 13 private Date hireDate; // reference to a Date object 14 15 // constructor initializes name, birth date and hire date 16 public Employee( string first, string last, 17 int birthMonth, int birthDay, int birthYear, 18 int hireMonth, int hireDay, int hireYear ) 19 { 20 firstName = first; 21 lastName = last; 22 23 // create new Date objects 24 birthDate = new Date( birthMonth, birthDay, birthYear ); 25 hireDate = new Date( hireMonth, hireDay, hireYear ); 26 27 } // end Employee constructor 28 29 // convert Employee to String format 30 public string ToEmployeeString() 31 { 32 return lastName + “, ” + firstName + 33 ” Hired: ” + hireDate.ToDateString() + 34 ” Birthday: ” + birthDate.ToDateString(); 35 } 36 37 } // end class Employee Fig. 8.9 Fig. 8.9Fig. 8.FiFi9g. 8.9g. 8.9Employeeclass encapsulates employee name, birthday and hire date.
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Chapter 8 Object-Based Programming 307 18 // validate month 19 if ( theMonth > 0 && theMonth <= 12 ) 20 month = theMonth; 21 22 else 23 { 24 month = 1; 25 Console.WriteLine( 26 "Month {0} invalid. Set to month 1.", theMonth ); 27 } 28 29 year = theYear; // could validate year 30 day = CheckDay( theDay ); // validate day 31 32 } // end Date constructor 33 34 // utility method confirms proper day value 35 // based on month and year 36 private int CheckDay( int testDay ) 37 { 38 int[] daysPerMonth = 39 { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; 40 41 // check if day in range for month 42 if ( testDay > 0 && testDay <= daysPerMonth[ month ] ) 43 return testDay; 44 45 // check for leap year 46 if ( month == 2 && testDay == 29 && 47 ( year % 400 == 0 || 48 ( year % 4 == 0 && year % 100 !=0 ) ) ) 49 return testDay; 50 51 Console.WriteLine( 52 "Day {0} invalid. Set to day 1.", testDay ); 53 54 return 1; // leave object in consistent state 55 } 56 57 // return date string as month/day/year 58 public string ToDateString() 59 { 60 return month + "/" + day + "/" + year; 61 } 62 63 } // end class Date Fig. 8.8 Fig. 8.8Fig. 8.FiFi8g. 8.8g. 8.8Dateclass encapsulates day, month and year information. (Part 2 of 2.) Class Employee (Fig. 8.9) encapsulates information relating to an employee s birthday and hire date (lines 10 13) using instance variables firstName, lastName, birthDateand hireDate. Members birthDateand hireDateare references to Date objects, each of which contains instance variables month, day and year. In this example, class Employeeis composed of two references of type stringand two refer
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306 Object-Based Programming Chapter 8 the student object when to calculate the GPA. The client code simply should use the GPA property. The client should not be aware of the underlying implementation. 8.8 Composition: Objects References as Instance Variables ofOther Classes In many situations, referencing existing objects is more convenient than rewriting the objects code for new classes in new projects. Suppose we were to implement an Alarm- Clock object that needs to know when to sound its alarm. Referencing an existing Time object (like those from earlier examples in this chapter) is easier than writing a new Time object. The use of references to objects of preexisting classes as members of new objects is called composition (or aggregation). Software Engineering Observation 8.16 One form of software reuse is composition, in which a class has as members references to objects of other classes. The application of Fig. 8.8, Fig. 8.9 and Fig. 8.10 demonstrates composition. The program contains three classes. Class Date(Fig. 8.8) encapsulates information relating to a specific date. Class Employee(Fig. 8.9) encapsulates the name of the employee and two Date objects representing the Employee s birthday and hire date. Class CompositionTest (Fig. 8.10) creates an object of class Employeeto demonstrate composition. Class Datedeclares intinstance variables month, dayand year(lines 9 11). Lines 16 32 define the constructor, which receives values for month, dayand yearas arguments and assigns these values to the instance variables after ensuring that the values are in a consistent state. Note that lines 25 26 print an error message if the constructor receives an invalid month value. Ordinarily, rather than printing error messages, a constructor would throw an exception. We discuss exceptions in Chapter 11, Exception Handling. Method ToDateString( lines 58 61) returns the string representation of a Date. 1 // Fig. 8.8: Date.cs 2 // Date class definition encapsulates month, day and year. 3 4 using System; 5 6 // Date class definition 7 public class Date 8 { 9 private int month; // 1-12 10 private int day; // 1-31 based on month 11 private int year; // any year 12 13 // constructor confirms proper value for month; 14 // call method CheckDay to confirm proper 15 // value for day 16 public Date( int theMonth, int theDay, int theYear ) 17 { Fig. 8.8 Fig. 8.8Fig. 8.FiFi8g. 8.8g. 8.8Dateclass encapsulates day, month and year information. (Part 1 of 2.)
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Chapter 8 Object-Based Programming 305 Fig. 8.7 Fig. 8.7Fig. 8.FiFi7g. 8.7g.8.7Properties demonstration for class Time3. (Part 5 of 5.) Line 34 declares Time3reference time. Line 41 in the constructor creates an object of class Time3and assigns it to time. The GUI contains three text fields in which the user can input values for the Time3 object s hour, minute and second variables, respectively. Next to each text field is a button the user can click to set the value of a particular Time3 property. Lines 66 90 declare three event-handling methods for the buttons Clickevents. Each event handler alters the values a Time3property (Hour, Minuteor Second). The GUI also contains a button that enables the user to increment the second value by 1. Using the Time3object s properties, method addButton_Click(lines 93 108) determines and sets the new time. For example, 23:59:59 becomes 00:00:00 when the user presses the button. Each modification of the time results in a call to UpdateDisplay, which uses the Time3properties to display the hour, minuteand secondvalues, and also displays the universal-and standard-time representations. Properties are not limited to accessing privatedata properties also can be used to calculate values associated with an object. One example of this would be a student object with a property representing the student s GPA (called GPA). Programmers can either provide code that calculates the student s GPA in the getaccessor for this property, or they can simply return a privatevariable containing the GPA, called gpa. (The value in this variable will need to be calculated in some other way, such as using a CalculateGPAmethod.) The programmer can use either technique, but we recommend using a property that calculates the GPA. Remember that client code should not be required to tell
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304 Object-Based Programming Chapter 8 Fig. 8.7 Fig. 8.7Fig. 8.FiFi7g. 8.7g.8.7Properties demonstration for class Time3. (Part 4 of 5.)
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Chapter 8 Object-Based Programming 303 60 displayLabel2.Text = “Standard time: ” + 61 time.ToStandardString() + “nUniversal time: ” + 62 time.ToUniversalString(); 63 } 64 65 // set Hour property when hourButton pressed 66 private void hourButton_Click( 67 object sender, System.EventArgs e ) 68 { 69 time.Hour = Int32.Parse( hourTextBox.Text ); 70 hourTextBox.Text = “”; 71 UpdateDisplay(); 72 } 73 74 // set Minute property when minuteButton pressed 75 private void minuteButton_Click( 76 object sender, System.EventArgs e ) 77 { 78 time.Minute = Int32.Parse( minuteTextBox.Text ); 79 minuteTextBox.Text = “”; 80 UpdateDisplay(); 81 } 82 83 // set Second property when secondButton pressed 84 private void secondButton_Click( 85 object sender, System.EventArgs e ) 86 { 87 time.Second = Int32.Parse( secondTextBox.Text ); 88 secondTextBox.Text = “”; 89 UpdateDisplay(); 90 } 91 92 // add one to Second when addButton pressed 93 private void addButton_Click( 94 object sender, System.EventArgs e ) 95 { 96 time.Second = ( time.Second + 1 ) % 60; 97 98 if ( time.Second == 0 ) 99 { 100 time.Minute = ( time.Minute + 1 ) % 60; 101 102 if ( time.Minute == 0 ) 103 time.Hour = ( time.Hour + 1 ) % 24; 104 } 105 106 UpdateDisplay(); 107 108 } // end method addButton_Click 109 110 } // end class TimeTest3 Fig. 8.7 Fig. 8.7Fig. 8.FiFi7g. 8.7g.8.7Properties demonstration for class Time3. (Part 3 of 5.)
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302 Object-Based Programming Chapter 8 8 using System.Windows.Forms; 9 using System.Data; 10 11 // TimeTest3 class definition 12 public class TimeTest3 : System.Windows.Forms.Form 13 { 14 private System.Windows.Forms.Label hourLabel; 15 private System.Windows.Forms.TextBox hourTextBox; 16 private System.Windows.Forms.Button hourButton; 17 18 private System.Windows.Forms.Label minuteLabel; 19 private System.Windows.Forms.TextBox minuteTextBox; 20 private System.Windows.Forms.Button minuteButton; 21 22 private System.Windows.Forms.Label secondLabel; 23 private System.Windows.Forms.TextBox secondTextBox; 24 private System.Windows.Forms.Button secondButton; 25 26 private System.Windows.Forms.Button addButton; 27 28 private System.Windows.Forms.Label displayLabel1; 29 private System.Windows.Forms.Label displayLabel2; 30 31 // required designer variable 32 private System.ComponentModel.Container components = null; 33 34 private Time3 time; 35 36 public TimeTest3() 37 { 38 // Required for Windows Form Designer support 39 InitializeComponent(); 40 41 time = new Time3(); 42 UpdateDisplay(); 43 } 44 45 // Visual Studio .NET generated code 46 47 // main entry point for application 48 [STAThread] 49 static void Main() 50 { 51 Application.Run( new TimeTest3() ); 52 } 53 54 // update display labels 55 public void UpdateDisplay() 56 { 57 displayLabel1.Text = “Hour: ” + time.Hour + 58 “; Minute: ” + time.Minute + 59 “; Second: ” + time.Second; Fig. 8.7 Fig. 8.7Fig. 8.FiFi7g. 8.7g.8.7Properties demonstration for class Time3. (Part 2 of 5.)
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Chapter 8 Object-Based Programming 301 Lines 57 69, 72 84 and 87 99 define Time3properties Hour, Minuteand Second, respectively. Each property begins with a declaration line that includes the property s access modifier (public), type (int) and name (Hour, Minuteor Second). The body of each property contains get and set accessors, which are declared using the reserved words getand set. The getaccessor declarations are on lines 59 62, 74 77 and 89 92. These accessors return the hour, minuteand secondinstance variable values that objects request. The setaccessors are declared on lines 64 67, 79 82 and 94 97. The body of each setaccessor performs the same conditional statement that was previously performed by method SetTimeto set the hour, minuteor second. Method SetTime(lines 48 54) now uses properties Hour, Minuteand Secondto ensure that instance variables hour, minute and second have valid values. After we define a property, we can use it in the same way that we use a variable. We assign values to properties using the = (assignment) operator. When this assignment occurs, the code in the setaccessor for that property executes. The reserved word valuerepresents the argument to the setaccessor. Similarly, methods ToUniversalString(102 106) and ToStandardString (109 114) now use properties Hour, Minute and Second to obtain the values of instance variables hour, minuteand second. Referencing the property executes the getaccessor for that property. When we use set and get accessor methods throughout the constructors and other methods of class Time3, we minimize the changes that we must make to the class definition in the event that we alter the data representation from hour, minute and second to another representation (such as total elapsed seconds in the day). When such changes are made, we must provide only new set and get accessor bodies. Using this technique also enables programmers to change the implementation of a class without affecting the clients of that class (as long as all the publicmethods of the class still are called in the same way). Software Engineering Observation 8.15 Accessing private data through set and get accessors not only protects the instance variables from receiving invalid values, but also hides the internal representation of the instance variables from that class s clients. Thus, if representation of the data changes (typically, to reduce the amount of required storage or to improve performance), only the method implementations need to change the client implementations need not change, as long as the interface provided by the methods is preserved. Class TimeTest3 (Fig. 8.7) defines an application with a GUI for manipulating an object of class Time3. [Note: We do not show Visual Studio s Windows Form Designer generated code. Instead, line 45 provides a comment to indicate where the generated code appears in the source code file. You can view this code on the CD that accompanies this book.] 1 // Fig. 8.7: TimeTest3.cs 2 // Demonstrating Time3 properties Hour, Minute and Second. 3 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; Fig. 8.7 Fig. 8.7Fig. 8.FiFi7g. 8.7g.8.7Properties demonstration for class Time3. (Part 1 of 5.)
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300 Object-Based Programming Chapter 8 64 set 65 { 66 hour = ( ( value >= 0 &&value < 24 ) ? value : 0 ); 67 } 68 69 } // end property Hour 70 71 // property Minute 72 public int Minute 73 { 74 get 75 { 76 return minute; 77 } 78 79 set 80 { 81 minute = ( ( value >= 0 && value < 60 ) ? value : 0 ); 82 } 83 84 } // end property Minute 85 86 // property Second 87 public int Second 88 { 89 get 90 { 91 return second; 92 } 93 94 set 95 { 96 second = ( ( value >= 0 && value < 60 ) ? value : 0 ); 97 } 98 99 } // end property Second 100 101 // convert time to universal-time (24 hour) format string 102 public string ToUniversalString() 103 { 104 return String.Format( 105 "{0:D2}:{1:D2}:{2:D2}", Hour, Minute, Second ); 106 } 107 108 // convert time to standard-time (12 hour) format string 109 public string ToStandardString() 110 { 111 return String.Format( "{0}:{1:D2}:{2:D2} {3}", 112 ( ( Hour == 12 || Hour == 0 ) ? 12 : Hour % 12 ), 113 Minute, Second, ( Hour < 12 ? "AM" : "PM" ) ); 114 } 115 116 } // end class Time3 Fig. 8.6 Fig. 8.Fig.Fi 8.6g. 8.66Properties provide controlled access to an object s data. (Part 3 of 3.) Fig. 8.
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Chapter 8 Object-Based Programming 299 13 // Time3 constructor initializes instance variables to 14 // zero to set default time to midnight 15 public Time3() 16 { 17 SetTime( 0, 0, 0 ); 18 } 19 20 // Time3 constructor: hour supplied, minute and second 21 // defaulted to 0 22 public Time3( int hour ) 23 { 24 SetTime( hour, 0, 0 ); 25 } 26 27 // Time3 constructor: hour and minute supplied, second 28 // defaulted to 0 29 public Time3( int hour, int minute ) 30 { 31 SetTime( hour, minute, 0 ); 32 } 33 34 // Time3 constructor: hour, minute and second supplied 35 public Time3( int hour, int minute, int second ) 36 { 37 SetTime( hour, minute, second ); 38 } 39 40 // Time3 constructor: initialize using another Time3 object 41 public Time3( Time3 time ) 42 { 43 SetTime( time.Hour, time.Minute, time.Second ); 44 } 45 46 // Set new time value in 24-hour format. Perform validity 47 // checks on the data. Set invalid values to zero. 48 public void SetTime( 49 int hourValue, int minuteValue, int secondValue ) 50 { 51 Hour = hourValue; // invoke Hour property set 52 Minute = minuteValue; // invoke Minute property set 53 Second = secondValue; // invoke Second property set 54 } 55 56 // property Hour 57 public int Hour 58 { 59 get 60 { 61 return hour; 62 } 63 Fig. 8.6 Fig. 8.Fig.Fi 8.6g. 8.66Properties provide controlled access to an object s data. (Part 2 of 3.) Fig. 8.
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