2006 Pearson Education, Inc. All rights reserved. 1 9 9 Classes and Objects: A Deeper Look.

1

2006 Pearson Education, Inc. All rights reserved.

99

Classes and Objects: A Deeper Look

2


Instead of this absurd division into sexes, they ought to class people as static and dynamic.

— Evelyn Waugh

Is it a world to hide virtues in?— William Shakespeare

But what, to serve our private ends,Forbids the cheating of our friends?

— Charles Churchill

3


Don’t be “consistent,” but be simply true.

— Oliver Wendell Holmes, Jr.

This above all: to thine own self be true.— William Shakespeare

4


OBJECTIVES

In this chapter you will learn: Encapsulation and data hiding. The concepts of data abstraction and abstract data

types (ADTs). To use keyword this. To use indexers to access members of a class. To use static variables and methods. To use readonly fields. To take advantage of C#’s memory management

features. How to create a class library. When to use the internal access modifier.

5


9.1 Introduction

9.2 Time Class Case Study

9.3 Controlling Access to Members

9.4 Referring to the Current Object’s Members with the this Reference

9.5 Indexers

9.6 Time Class Case Study: Overloaded Constructors

9.7 Default and Parameterless Constructors

9.8 Composition

9.9 Garbage Collection and Destructors

9.10 static Class Members

9.11 readonly Instance Variables

9.12 Software Reusability

9.13 Data Abstraction and Encapsulation

9.14 Time Class Case Study: Creating Class Libraries

9.15 internal Access

9.16 Class View and Object Browser

9.17 (Optional) Software Engineering Case Study: Starting to Program the Classes of the ATM System

9.18 Wrap-Up

6


1 // Fig. 9.1: Time1.cs

2 // Time1 class declaration maintains the time in 24-hour format.

3 public class Time1

4 {

5 private int hour; // 0 - 23

6 private int minute; // 0 - 59

7 private int second; // 0 - 59

8

9 // set a new time value using universal time; ensure that

10 // the data remains consistent by setting invalid values to zero

11 public void SetTime( int h, int m, int s )

12 {

13 hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); // validate hour

14 minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); // validate minute

15 second = ( ( s >= 0 && s < 60 ) ? s : 0 ); // validate second

16 } // end method SetTime

17

18 // convert to string in universal-time format (HH:MM:SS)

19 public string ToUniversalString()

20 {

21 return string.Format( "{0:D2}:{1:D2}:{2:D2}",

22 hour, minute, second );

23 } // end method ToUniversalString

Outline

Time1.cs

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24

25 // convert to string in standard-time format (H:MM:SS AM or PM)

26 public override string ToString()

27 {

28 return string.Format( "{0}:{1:D2}:{2:D2} {3}",

29 ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ),

30 minute, second, ( hour < 12 ? "AM" : "PM" ) );

31 } // end method ToString

32 } // end class Time1

Outline

Time1.cs

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8


Software Engineering Observation 9.1

Methods and properties that modify the values of private variables should verify that the intended new values are proper. If they are not, they should place the private variables in an appropriate consistent state.

9


1 // Fig. 9.2: Time1Test.cs

2 // Time1 object used in an application.

3 using System;

4

5 public class Time1Test

6 {

7 public static void Main( string[] args )

8 {

9 // create and initialize a Time1 object

10 Time1 time = new Time1(); // invokes Time1 constructor

11

12 // output string representations of the time

13 Console.Write( "The initial universal time is: " );

14 Console.WriteLine( time.ToUniversalString() );

15 Console.Write( "The initial standard time is: " );

16 Console.WriteLine( time.ToString() );

17 Console.WriteLine(); // output a blank line

18

19 // change time and output updated time

20 time.SetTime( 13, 27, 6 );

21 Console.Write( "Universal time after SetTime is: " );


23 Console.Write( "Standard time after SetTime is: " );



Outline

Time1Test.cs

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26

27 // set time with invalid values; output updated time

28 time.SetTime( 99, 99, 99 );

29 Console.WriteLine( "After attempting invalid settings:" );

30 Console.Write( "Universal time: " );


32 Console.Write( "Standard time: " );


34 } // end Main

35 } // end class Time1Test The initial universal time is: 00:00:00 The initial standard time is: 12:00:00 AM Universal time after SetTime is: 13:27:06 Standard time after SetTime is: 1:27:06 PM After attempting invalid settings: Universal time: 00:00:00 Standard time: 12:00:00 AM

Outline

Time1Test.cs

(2 of 2)

11



Classes simplify programming because the client can use only the public members exposed by the class. Such members are usually client oriented rather than implementation oriented. Clients are neither aware of, nor involved in, a class’s implementation. Clients generally care about what the class does but not how the class does it. (Clients do, of course, care that the class operates correctly and efficiently.)

12



Interfaces change less frequently than implementations. When an implementation changes, implementation-dependent code must change accordingly. Hiding the implementation reduces the possibility that other application parts will become dependent on class-implementation details.

13


Common Programming Error 9.1

An attempt by a method that is not a member of a class to access a private member of that class is a compilation error.

14


1 // Fig. 9.3: MemberAccessTest.cs

2 // Private members of class Time1 are not accessible.

3 public class MemberAccessTest

4 {


6 {

7 Time1 time = new Time1(); // create and initialize Time1 object

8

9 time.hour = 7; // error: hour has private access in Time1

10 time.minute = 15; // error: minute has private access in Time1

11 time.second = 30; // error: second has private access in Time1

12 } // end Main

13 } // end class MemberAccessTest

Outline

MemberAccessTest.cs

15


1 // Fig. 9.4: ThisTest.cs

2 // this used implicitly and explicitly to refer to members of an object.

3 using System;

4

5 public class ThisTest

6 {


8 {

9 SimpleTime time = new SimpleTime( 15, 30, 19 );

10 Console.WriteLine( time.BuildString() );

11 } // end Main

12 } // end class ThisTest

13

14 // class SimpleTime demonstrates the "this" reference

15 public class SimpleTime

16 {

17 private int hour; // 0-23

18 private int minute; // 0-59

19 private int second; // 0-59

20

21 // if the constructor uses parameter names identical to

22 // instance variable names the "this" reference is

23 // required to distinguish between names

24 public SimpleTime( int hour, int minute, int second )

25 {

26 this.hour = hour; // set "this" object's hour instance variable

27 this.minute = minute; // set "this" object's minute

28 this.second = second; // set "this" object's second

29 } // end SimpleTime constructor

Outline

ThisTest.cs

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30

31 // use explicit and implicit "this" to call ToUniversalString

32 public string BuildString()

33 {

34 return string.Format( "{0,24}: {1}\n{2,24}: {3}",

35 "this.ToUniversalString()", this.ToUniversalString(),

36 "ToUniversalString()", ToUniversalString() );

37 } // end method BuildString

38



41 {

42 // "this" is not required here to access instance variables,

43 // because method does not have local variables with same

44 // names as instance variables


46 this.hour, this.minute, this.second );


48 } // end class SimpleTime this.ToUniversalString(): 15:30:19 ToUniversalString(): 15:30:19

Outline

ThisTest.cs

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17



It is often a logic error when a method contains a parameter or local variable that has the same name as an instance variable of the class. In such a case, use reference this if you wish to access the instance variable of the class—otherwise, the method parameter or local variable will be referenced.

18


Error-Prevention Tip 9.1

Avoid method parameter names or local variable names that conflict with field names. This helps prevent subtle, hard-to-locate bugs.

19


Performance Tip 9.1

C# conserves memory by maintaining only one copy of each method per class—this method is invoked by every object of the class. Each object, on the other hand, has its own copy of the class’s instance variables (i.e., non-static variables). Each method of the class implicitly uses the this reference to determine the specific object of the class to manipulate.

20



Declaring indexers as static is a syntax error.

21


1 // Fig. 9.5: Box.cs

2 // Box class definition represents a box with length,

3 // width and height dimensions with indexers.

4 public class Box

5 {

6 private string[] names = { "length", "width", "height" };

7 private double[] dimensions = new double[ 3 ];

8

9 // constructor

10 public Box( double length, double width, double height )

11 {

12 dimensions[ 0 ] = length;

13 dimensions[ 1 ] = width;

14 dimensions[ 2 ] = height;

15 }

Outline

Box.cs

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16

17 // indexer to access dimensions by integer index number

18 public double this[ int index ]

19 {

20 get

21 {

22 // validate index to get

23 if ( ( index < 0 ) || ( index >= dimensions.Length ) )

24 return -1;

25 else

26 return dimensions[ index ];

27 } // end get

28 set

29 {

30 if ( index >= 0 && index < dimensions.Length )

31 dimensions[ index ] = value;

32 } // end set

33 } // end numeric indexer

Outline

Box.cs

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34

35 // indexer to access dimensions by their string names

36 public double this[ string name ]

37 {

38 get

39 {

40 // locate element to get

41 int i = 0;

42 while ( ( i < names.Length ) &&

43 ( name.ToLower() != names[ i ] ) )

44 i++;

45

46 return ( i == names.Length ) ? -1 : dimensions[ i ];

47 } // end get

48 set

49 {

50 // locate element to set

51 int i = 0;

52 while ( ( i < names.Length ) &&

53 ( name.ToLower() != names[ i ] ) )

54 i++;

55

56 if ( i != names.Length )

57 dimensions[ i ] = value;

58 } // end set

59 } // end string indexer

60 } // end class Box

Outline

Box.cs

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1 // Fig. 9.6: BoxTest.cs

2 // Indexers provide access to a Box object's members.

3 using System;

4

5 public class BoxTest

6 {


8 {

9 // create a box

10 Box box = new Box( 30, 30, 30 );

11

12 // show dimensions with numeric indexers

13 Console.WriteLine( "Created a box with the dimensions:" );

14 Console.WriteLine( "box[ 0 ] = {0}", box[ 0 ] );



Outline

BoxTest.cs

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17

18 // set a dimension with the numeric indexer

19 Console.WriteLine( "\nSetting box[ 0 ] to 10...\n" );

20 box[ 0 ] = 10;

21

22 // set a dimension with the string indexer

23 Console.WriteLine( "Setting box[ \"width\" ] to 20...\n" );

24 box[ "width" ] = 20;

25

26 // show dimensions with string indexers

27 Console.WriteLine( "Now the box has the dimensions:" );

28 Console.WriteLine( "box[ \"length\" ] = {0}", box[ "length" ] );

29 Console.WriteLine( "box[ \"width\" ] = {0}", box[ "width" ] );

30 Console.WriteLine( "box[ \"height\" ] = {0}", box[ "height" ] );

31 } // end method Main

32 } // end class BoxTest Created a box with the dimensions: box[ 0 ] = 30 box[ 1 ] = 30 box[ 2 ] = 30 Setting box[ 0 ] to 10... Setting box[ "width" ] to 20... Now the box has the dimensions: box[ "length" ] = 10 box[ "width" ] = 20 box[ "height" ] = 30

Outline

BoxTest.cs

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1 // Fig. 9.7: Time2.cs

2 // Time2 class declaration with overloaded constructors.


4 {




8

9 // Time2 parameterless constructor: initializes each instance variable

10 // to zero; ensures that Time2 objects start in a consistent state

11 public Time2() : this( 0, 0, 0 ) { }

12

13 // Time2 constructor: hour supplied, minute and second defaulted to 0

14 public Time2( int h ) : this( h, 0, 0 ) { }

15

16 // Time2 constructor: hour and minute supplied, second defaulted to 0

17 public Time2( int h, int m ) : this( h, m, 0 ) { }

18

19 // Time2 constructor: hour, minute and second supplied

20 public Time2( int h, int m, int s )

21 {

22 SetTime( h, m, s ); // invoke SetTime to validate time

23 } // end Time2 three-parameter constructor

24

25 // Time2 constructor: another Time2 object supplied

26 public Time2( Time2 time )

27 : this( time.Hour, time.Minute, time.Second ) { }

Outline

Time2.cs

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27


28




32 {

33 Hour = h; // set the Hour property

34 Minute = m; // set the Minute property

35 Second = s; // set the Second property


37

38 // Properties for getting and setting

39 // property that gets and sets the hour

40 public int Hour

41 {

42 get

43 {

44 return hour;

45 } // end get

46 // make writing inaccessible outside the class

47 private set

48 {

49 hour = ( ( value >= 0 && value < 24 ) ? value : 0 );

50 } // end set

51 } // end property Hour

Outline

Time2.cs

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52

53 // property that gets and sets the minute

54 public int Minute

55 {

56 get

57 {

58 return minute;

59 } // end get


61 private set

62 {

63 minute = ( ( value >= 0 && value < 60 ) ? value : 0 );

64 } // end set

65 } // end property Minute

66

67 // property that gets and sets the second

68 public int Second

69 {

70 get

71 {

72 return second;

73 } // end get


75 private set

76 {

77 second = ( ( value >= 0 && value < 60 ) ? value : 0 );

78 } // end set

79 } // end property Second

Outline

Time2.cs

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80



83 {

84 return string.Format(

85 "{0:D2}:{1:D2}:{2:D2}", Hour, Minute, Second );


87



90 {


92 ( ( Hour == 0 || Hour == 12 ) ? 12 : Hour % 12 ),

93 Minute, Second, ( Hour < 12 ? "AM" : "PM" ) );



Outline

Time2.cs

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30



A constructor can call methods of the class. Be aware that the instance variables might not yet be in a consistent state, because the constructor is in the process of initializing the object. Using instance variables before they have been initialized properly is a logic error.

31



When one object of a class has a reference to another object of the same class, the first object can access all the second object’s data and methods (including those that are private).

32



When implementing a method of a class, use the class’s properties to access the class’s private data. This simplifies code maintenance and reduces the likelihood of errors.

33


1 // Fig. 9.8: Time2Test.cs

2 // Overloaded constructors used to initialize Time2 objects.

3 using System;

4

5 public class Time2Test

6 {


8 {

9 Time2 t1 = new Time2(); // 00:00:00

10 Time2 t2 = new Time2( 2 ); // 02:00:00

11 Time2 t3 = new Time2( 21, 34 ); // 21:34:00

12 Time2 t4 = new Time2( 12, 25, 42 ); // 12:25:42

13 Time2 t5 = new Time2( 27, 74, 99 ); // 00:00:00

14 Time2 t6 = new Time2( t4 ); // 12:25:42

15

16 Console.WriteLine( "Constructed with:\n" );

17 Console.WriteLine( "t1: all arguments defaulted" );

18 Console.WriteLine( " {0}", t1.ToUniversalString() ); // 00:00:00

19 Console.WriteLine( " {0}\n", t1.ToString() ); // 12:00:00 AM

20

21 Console.WriteLine(

22 "t2: hour specified; minute and second defaulted" );



25

26 Console.WriteLine(

27 "t3: hour and minute specified; second defaulted" );


29 Console.WriteLine( " {0}\n", t3.ToString() ); // 9:34:00 PM

Outline

Time2Test.cs

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34


30

31 Console.WriteLine( "t4: hour, minute and second specified" );


33 Console.WriteLine( " {0}\n", t4.ToString() ); // 12:25:42 PM

34

35 Console.WriteLine( "t5: all invalid values specified" );



38

39 Console.WriteLine( "t6: Time2 object t4 specified" );


41 Console.WriteLine( " {0}", t6.ToString() ); // 12:25:42 PM

42 } // end Main

43 } // end class Time2Test

Outline

Time2Test.cs

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35


Constructed with: t1: all arguments defaulted 00:00:00 12:00:00 AM t2: hour specified; minute and second defaulted 02:00:00 2:00:00 AM t3: hour and minute specified; second defaulted 21:34:00 9:34:00 PM t4: hour, minute and second specified 12:25:42 12:25:42 PM t5: all invalid values specified 00:00:00 12:00:00 AM t6: Time2 object t4 specified 12:25:42 12:25:42 PM

Outline

Time2Test.cs

(3 of 3)

36



If a class has constructors, but none of the public constructors are parameterless constructors, and an application attempts to call a parameterless constructor to initialize an object of the class, a compilation error occurs. A constructor can be called with no arguments only if the class does not have any constructors (in which case the default constructor is called) or if the class has a public parameterless constructor.

37



Only constructors can have the same name as the class. Declaring a method, property or field with the same name as the class is a compilation error.

38



One form of software reuse is composition, in which a class has as members references to objects of other classes.

39


1 // Fig. 9.9: Date.cs

2 // Date class declaration.

3 using System;

4

5 public class Date

6 {

7 private int month; // 1-12

8 private int day; // 1-31 based on month

9 private int year; // any year (could validate)

10

11 // constructor: use property Month to confirm proper value for month;

12 // use property Day to confirm proper value for day

13 public Date( int theMonth, int theDay, int theYear )

14 {

15 Month = theMonth; // validate month

16 Year = theYear; // could validate year

17 Day = theDay; // validate day

18 Console.WriteLine( "Date object constructor for date {0}", this );

19 } // end Date constructor

Outline

Date.cs

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40


20

21 // property that gets and sets the year

22 public int Year

23 {

24 get

25 {

26 return year;

27 } // end get

28 private set // make writing inaccessible outside the class

29 {

30 year = value; // could validate

31 } // end set

32 } // end property Year

Outline

Date.cs

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41


33

34 // property that gets and sets the month

35 public int Month

36 {

37 get

38 {

39 return month;

40 } // end get


42 {

43 if ( value > 0 && value <= 12 ) // validate month

44 month = value;

45 else // month is invalid

46 {

47 Console.WriteLine( "Invalid month ({0}) set to 1.", value );

48 month = 1; // maintain object in consistent state

49 } // end else

50 } // end set

51 } // end property Month

Outline

Date.cs

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42


52

53 // property that gets and sets the day

54 public int Day

55 {

56 get

57 {

58 return day;

59 } // end get


61 {

62 int[] daysPerMonth =

63 { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

64

65 // check if day in range for month

66 if ( value > 0 && value <= daysPerMonth[ Month ] )

67 day = value;

68 // check for leap year

69 else if ( Month == 2 && value == 29 &&

70 ( Year % 400 == 0 || ( Year % 4 == 0 && Year % 100 != 0 ) ) )

71 day = value;

72 else

73 {

74 Console.WriteLine( "Invalid day ({0}) set to 1.", value );

75 day = 1; // maintain object in consistent state

76 } // end else

77 } // end set

78 } // end property Day

Outline

Date.cs

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43


79

80 // return a string of the form month/day/year


82 {

83 return string.Format( "{0}/{1}/{2}", Month, Day, Year );


85 } // end class Date

Outline

Date.cs

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1 // Fig. 9.10: Employee.cs

2 // Employee class with references to other objects.

3 public class Employee

4 {

5 private string firstName;

6 private string lastName;

7 private Date birthDate;

8 private Date hireDate;

9

10 // constructor to initialize name, birth date and hire date

11 public Employee( string first, string last,

12 Date dateOfBirth, Date dateOfHire )

13 {

14 firstName = first;

15 lastName = last;

16 birthDate = dateOfBirth;

17 hireDate = dateOfHire;

18 } // end Employee constructor

19

20 // convert Employee to string format


22 {

23 return string.Format( "{0}, {1} Hired: {2} Birthday: {3}",

24 lastName, firstName, hireDate, birthDate );


26 } // end class Employee

Outline

Employee.cs

45


1 // Fig. 9.11: EmployeeTest.cs

2 // Composition demonstration.

3 using System;

4

5 public class EmployeeTest

6 {


8 {

9 Date birth = new Date( 7, 24, 1949 );

10 Date hire = new Date( 3, 12, 1988 );

11 Employee employee = new Employee( "Bob", "Blue", birth, hire );

12

13 Console.WriteLine( employee );

14 } // end Main

15 } // end class EmployeeTest Date object constructor for date 7/24/1949 Date object constructor for date 3/12/1988 Blue, Bob Hired: 3/12/1988 Birthday: 7/24/1949

Outline

EmployeeTest.cs

46



A class that uses system resources, such as files on disk, should provide a method to eventually release the resources. Many FCL classes provide Close or Dispose methods for this purpose.

47



Use a static variable when all objects of a class must use the same copy of the variable.

48



It is a compilation error to access or invoke a static member by referencing it through an instance of the class, like a non-static member.

49



Static variables and methods exist, and can be used, even if no objects of that class have been instantiated.

50


1 // Fig. 9.12: Employee.cs

2 // Static variable used to maintain a count of the number of

3 // Employee objects in memory.

4 using System;

5

6 public class Employee

7 {

8 private string firstName;

9 private string lastName;

10 private static int count = 0; // number of objects in memory

11

12 // initialize employee, add 1 to static count and

13 // output string indicating that constructor was called

14 public Employee( string first, string last )

15 {

16 firstName = first;

17 lastName = last;

18 count++; // increment static count of employees

19 Console.WriteLine( "Employee constructor: {0} {1}; count = {2}",

20 FirstName, LastName, Count );

21 } // end Employee constructor

Outline

Employee.cs

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51


22

23 // subtract 1 from static count when the garbage collector

24 // calls destructor to clean up object;

25 // confirm that destructor was called

26 ~Employee()

27 {

28 count--; // decrement static count of employees

29 Console.WriteLine( "Employee destructor: {0} {1}; count = {2}",

30 FirstName, LastName, Count );

31 } // end destructor

32

33 // read-only property that gets the first name

34 public string FirstName

35 {

36 get

37 {

38 return firstName;

39 } // end get

40 } // end property FirstName

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Employee.cs

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52


41

42 // read-only property that gets the last name

43 public string LastName

44 {

45 get

46 {

47 return lastName;

48 } // end get

49 } // end property LastName

50

51 // read-only property that gets the employee count

52 public static int Count

53 {

54 get

55 {

56 return count;

57 } // end get

58 } // end property Count

59 } // end class Employee

Outline

Employee.cs

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53


1 // Fig. 9.13: EmployeeTest.cs

2 // Static member demonstration.

3 using System;

4

5 public class EmployeeTest

6 {


8 {

9 // show that count is 0 before creating Employees

10 Console.WriteLine( "Employees before instantiation: {0}",

11 Employee.Count );

12

13 // create two Employees; count should become 2

14 Employee e1 = new Employee( "Susan", "Baker" );

15 Employee e2 = new Employee( "Bob", "Blue" );

16

17 // show that count is 2 after creating two Employees

18 Console.WriteLine( "\nEmployees after instantiation: {0}",


Outline

EmployeeTest.cs

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54


20

21 // get names of Employees

22 Console.WriteLine( "\nEmployee 1: {0} {1}\nEmployee 2: {2} {3}\n",

23 e1.FirstName, e1.LastName,

24 e2.FirstName, e2.LastName );

25

26 // in this example, there is only one reference to each Employee,

27 // so the following statements cause the CLR to mark each

28 // Employee object as being eligible for destruction

29 e1 = null; // object e1 no longer needed

30 e2 = null; // object e2 no longer needed

31

32 GC.Collect(); // ask for garbage collection to occur now

33 // wait until the destructors

34 // finish writing to the console

35 GC.WaitForPendingFinalizers();

36

37 // show Employee count after calling garbage collector and

38 // waiting for all destructors to finish

39 Console.WriteLine( "\nEmployees after destruction: {0}",


41 } // end Main

42 } // end class EmployeeTest

Outline

EmployeeTest.cs

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55


Outline

EmployeeTest.cs

(3 of 3)

Employees before instantiation: 0 Employee constructor: Susan Baker; count = 1

Employee constructor: Bob Blue; count = 2 Employees after instantiation: 2

Employee 1: Susan Baker Employee 2: Bob Blue

Employee destructor: Bob Blue; count = 1 Employee destructor: Susan Baker; count = 0

Employees after destruction: 0

56



A compilation error occurs if a static method calls an instance (non-static) method in the same class by using only the method name. Similarly, a compilation error occurs if a static method attempts to access an instance variable in the same class by using only the variable name.

57



Referring to the this reference in a static method is a syntax error.

58



Declaring an instance variable as readonly helps enforce the principle of least privilege. If an instance variable should not be modified after the object is constructed, declare it to be readonly to prevent modification.

59



Attempting to modify a readonly instance variable anywhere but its declaration or the object’s constructors is a compilation error.

60


Error-Prevention Tip 9.2

Attempts to modify a readonly instance variable are caught at compilation time rather than causing execution-time errors. It is always preferable to get bugs out at compile time, if possible, rather than allowing them to slip through to execution time (where studies have found that repairing is often many times more costly).

61


1 // Fig. 9.14: Increment.cs

2 // readonly instance variable in a class.

3 public class Increment

4 {

5 // readonly instance variable (uninitialized)

6 private readonly int INCREMENT;

7 private int total = 0; // total of all increments

8

9 // constructor initializes readonly instance variable INCREMENT

10 public Increment( int incrementValue )

11 {

12 INCREMENT = incrementValue; // initialize readonly variable (once)

13 } // end Increment constructor

14

15 // add INCREMENT to total

16 public void AddIncrementToTotal()

17 {

18 total += INCREMENT;

19 } // end method AddIncrementToTotal

20

21 // return string representation of an Increment object's data


23 {

24 return string.Format( "total = {0}", total );


26 } // end class Increment

Outline

Increment.cs

62


1 // Fig. 9.15: IncrementTest.cs

2 // readonly instance variable initialized with a constructor argument.

3 using System;

4

5 public class IncrementTest

6 {


8 {

9 Increment incrementer = new Increment( 5 );

10

11 Console.WriteLine( "Before incrementing: {0}\n", incrementer );

12

13 for ( int i = 1; i <= 3; i++ )

14 {

15 incrementer.AddIncrementToTotal();

16 Console.WriteLine( "After increment {0}: {1}", i, incrementer );

17 } // end for

18 } // end Main

19 } // end class IncrementTest Before incrementing: total = 0 After increment 1: total = 5 After increment 2: total = 10 After increment 3: total = 15

Outline

IncrementTest.cs

63



If a readonly instance variable is initialized to a constant only in its declaration, it is not necessary to have a separate copy of the instance variable for every object of the class. The variable should be declared const instead. Constants declared with const are implicitly static, so there will only be one copy for the entire class.

64


Good Programming Practice 9.1

Avoid reinventing the wheel. Study the capabilities of the FCL. If the FCL contains a class that meets your application’s requirements, use that class rather than create your own.

65



Programmers create types through the class mechanism. New types can be designed to be as convenient to use as the simple types. This marks C# as an extensible language. Although the language is easy to extend via new types, the programmer cannot alter the base language itself.

66


1 // Fig. 9.16: Time1.cs

2 // Time1 class declaration in a namespace.

3 namespace Chapter09

4 {


6 {




10




14 {

15 hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); // validate hour

16 minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); // validate minute

17 second = ( ( s >= 0 && s < 60 ) ? s : 0 ); // validate second


Outline

Time1.cs

(1 of 2)

67


19



22 {


24 hour, minute, second );


26



29 {


31 ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ),

32 minute, second, ( hour < 12 ? "AM" : "PM" ) );



35 } // end namespace Chapter09

Outline

Time1.cs

(2 of 2)

68


Fig. 9.17 | Creating a Class Library Project.

69


Fig. 9.18 | Adding a Reference.

70


1 // Fig. 9.19: Time1NamespaceTest.cs

2 // Time1 object used in an application.

3 using Chapter09;

4 using System;

5

6 public class Time1NamespaceTest

7 {


9 {

10 // create and initialize a Time1 object

11 Time1 time = new Time1(); // calls Time1 constructor

12

13 // output string representations of the time

14 Console.Write( "The initial universal time is: " );


16 Console.Write( "The initial standard time is: " );



Outline

Time1NamespaceTest.cs

(1 of 2)

71


19

20 // change time and output updated time

21 time.SetTime( 13, 27, 6 );

22 Console.Write( "Universal time after SetTime is: " );


24 Console.Write( "Standard time after SetTime is: " );



27

28 // set time with invalid values; output updated time

29 time.SetTime( 99, 99, 99 );

30 Console.WriteLine( "After attempting invalid settings:" );

31 Console.Write( "Universal time: " );


33 Console.Write( "Standard time: " );


35 } // end Main

36 } // end class Time1NamespaceTest The initial universal time is: 00:00:00 The initial standard time is: 12:00:00 AM Universal time after SetTime is: 13:27:06 Standard time after SetTime is: 1:27:06 PM After attempting invalid settings: Universal time: 00:00:00 Standard time: 12:00:00 AM

Outline

Time1NamespaceTest.cs

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72


1 // Fig. 9.20: InternalAccessTest.cs

2 // Members declared internal in a class are accessible by other classes

3 // in the same assembly.

4 using System;

5

6 public class InternalAccessTest

7 {


9 {

10 InternalData internalData = new InternalData();

11

12 // output string representation of internalData

13 Console.WriteLine( "After instantiation:\n{0}", internalData );

14

15 // change internal access data in internalData

16 internalData.number = 77;

17 internalData.message = "Goodbye";

18

19 // output string representation of internalData

20 Console.WriteLine( "\nAfter changing values:\n{0}", internalData );

21 } // end Main

22 } // end class InternalAccessTest

Outline

InternalAccessTest.cs

(1 of 2)

73


23

24 // class with internal access instance variables

25 class InternalData

26 {

27 internal int number; // internal-access instance variable

28 internal string message; // internal-access instance variable

29

30 // constructor

31 public InternalData()

32 {

33 number = 0;

34 message = "Hello";

35 } // end InternalData constructor

36

37 // return InternalData object string representation


39 {

40 return string.Format(

41 "number: {0}; message: {1}", number, message );


43 } // end class InternalData After instantiation: number: 0; message: Hello After changing values: number: 77; message: Goodbye

Outline

InternalAccessTest.cs

(2 of 2)

74


Fig. 9.21 | Class View of class Time1 (Fig. 9.1) and class TimeTest (Fig. 9.2).

75


Fig. 9.22 | Object Browser for class Math.

76


Fig. 9.23 | Class diagram with visibility markers.

77


Fig. 9.24 | Class diagram with navigability arrows.

78


1 // Fig. 9.25: Withdrawal.cs

2 // Class Withdrawal represents an ATM withdrawal transaction

3 public class Withdrawal

4 {

5 // parameterless constructor

6 public Withdrawal()

7 {

8 // constructor body code

9 } // end constructor

10 } // end class Withdrawal

Outline

Withdrawal.cs

79





4 {

5 // attributes

6 private int accountNumber; // account to withdraw funds from

7 private decimal amount; // amount to withdraw from account

8



11 {




Outline

Withdrawal.cs

80


1 // Fig. 9.27: Withdrawal.cs 2 // Class Withdrawal represents an ATM withdrawal transaction


4 {

5 // attributes


7 private decimal amount; // amount to withdraw

8

9 // references to associated objects

10 private Screen screen; // ATM’s screen

11 private Keypad keypad; // ATM’s keypad

12 private CashDispenser cashDispenser; // ATM’s cash dispenser

13 private BankDatabase bankDatabase; // account information database

14



17 {




Outline

Withdrawal.cs

81


Outline

Account.cs




4 {

5 // attributes


7 private decimal amount; // amount to withdraw

8

9 // references to associated objects

10 private Screen screen; // ATM’s screen

11 private Keypad keypad; // ATM’s keypad

12 private CashDispenser cashDispenser; // ATM’s cash dispenser

13 private BankDatabase bankDatabase; // account information database

14



17 {



20

21 // operations

22 // perform transaction

23 public void Execute()

24 {

25 // Execute method body code

26 } // end method Execute


82



Many UML modeling tools can convert UML-based designs into C# code, considerably speeding up the implementation process. For more information on these “automatic” code generators, refer to the Web resources listed at the end of Section 3.10.

83


1 // Fig. 9.29: Account.cs

2 // Class Account represents a bank account.

3 public class Account

4 {

5 private int accountNumber; // account number

6 private int pin; // PIN for authentication

7 private decimal availableBalance; // available withdrawal amount

8 private decimal totalBalance; // funds available + pending deposit

9


11 public Account()

12 {



15

16 // validates user PIN

17 public bool ValidatePIN()

18 {

19 // ValidatePIN method body code

20 } // end method ValidatePIN

21

22 // read-only property that gets the available balance

23 public decimal AvailableBalance

24 {

25 get

26 {

27 // AvailableBalance get accessor body code

28 } // end get

29 } // end property AvailableBalance

Outline

Account.cs

(1 of 2)

84


Outline

Account.cs

(2 of 2)

30

31 // read-only property that gets the total balance

32 public decimal TotalBalance

33 {

34 get

35 {

36 // TotalBalance get accessor body code

37 } // end get

38 } // end property TotalBalance

39

40 // credits the account

41 public void Credit()

42 {

43 // Credit method body code

44 } // end method Credit

45

46 // debits the account

47 public void Debit()

48 {

49 // Debit method body code

50 } // end method Debit

51 } // end class Account

2006 Pearson Education, Inc. All rights reserved. 1 9 9 Classes and Objects: A Deeper Look.

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Transcript of 2006 Pearson Education, Inc. All rights reserved. 1 9 9 Classes and Objects: A Deeper Look.