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Chapter 10 – Object-Oriented Programming: Polymorphism Outline 10.1 Introduction 10.2 Derived-Class-Object to Base-Class-Object Conversion 10.3 Type Fields and switch Statements 10.4 Polymorphism Examples 10.5 Abstract Classes and Methods 10.6 Case Study: Inheriting Interface and Implementation 10.7 sealed Classes and Methods 10.8 Case Study: Payroll System Using Polymorphism 10.9 Case Study: Creating and Using Interfaces 10.10 Delegates 10.11 Operator Overloading

10.1 Introduction Polymorphism allows programmers to write: Programs that handle a wide variety of related classes in a generic manner Systems that are easily extensible

10.2 Derived-Class-Object to Base-Class-Object Conversion Class hierarchies Can assign derived-class objects to base-class references Can explicitly cast between types in a class hierarchy An object of a derived-class can be treated as an object of its base-class Array of base-class references that refer to objects of many derived-class types Base-class object is NOT an object of any of its derived classes

Point.cs 1 // Fig. 10.1: Point.cs 2 // Point class represents an x-y coordinate pair. 3 4 using System; 5 6 // Point class definition implicitly inherits from Object 7 public class Point 8 { 9 // point coordinate 10 private int x, y; 11 12 // default constructor 13 public Point() 14 { 15 // implicit call to Object constructor occurs here 16 } 17 18 // constructor 19 public Point( int xValue, int yValue ) 20 { 21 // implicit call to Object constructor occurs here 22 X = xValue; 23 Y = yValue; 24 } 25 26 // property X 27 public int X 28 { 29 get 30 { 31 return x; 32 } 33 Definition of class Point

Point.cs 34 set 35 { 36 x = value ; // no need for validation 37 } 38 39 } // end property X 40 41 // property Y 42 public int Y 43 { 44 get 45 { 46 return y; 47 } 48 49 set 50 { 51 y = value ; // no need for validation 52 } 53 54 } // end property Y 55 56 // return string representation of Point 57 public override string ToString() 58 { 59 return "[" + X + ", " + Y + "]" ; 60 } 61 62 } // end class Point

Circle.cs 1 // Fig. 10.2: Circle.cs 2 // Circle class that inherits from class Point. 3 4 using System; 5 6 // Circle class definition inherits from Point 7 public class Circle : Point 8 { 9 private double radius; // circle's radius 10 11 // default constructor 12 public Circle() 13 { 14 // implicit call to Point constructor occurs here 15 } 16 17 // constructor 18 public Circle ( int xValue, int yValue, double radiusValue ) 19 : base ( xValue, yValue ) 20 { 21 Radius = radiusValue; 22 } 23 24 // property Radius 25 public double Radius 26 { 27 get 28 { 29 return radius; 30 } 31 Definition of class Circle which inherits from class Point

Circle.cs 32 set 33 { 34 if ( value >= 0 ) // validate radius 35 radius = value ; 36 } 37 38 } // end property Radius 39 40 // calculate Circle diameter 41 public double Diameter() 42 { 43 return Radius * 2 ; 44 } 45 46 // calculate Circle circumference 47 public double Circumference() 48 { 49 return Math. PI * Diameter(); 50 } 51 52 // calculate Circle area 53 public virtual double Area() 54 { 55 return Math. PI * Math.Pow( Radius, 2 ); 56 } 57 58 // return string representation of Circle 59 public override string ToString() 60 { 61 return "Center = " + base .ToString() + 62 "; Radius = " + Radius; 63 } 64 65 } // end class Circle

PointCircleTest.cs 1 // Fig. 10.3: PointCircleTest.cs 2 // Demonstrating inheritance and polymorphism. 3 4 using System; 5 using System.Windows.Forms; 6 7 // PointCircleTest class definition 8 class PointCircleTest 9 { 10 // main entry point for application. 11 static void Main( string [] args ) 12 { 13 Point point1 = new Point( 30 , 50 ); 14 Circle circle1 = new Circle( 120 , 89 , 2.7 ); 15 16 string output = "Point point1: " + point1.ToString() + 17 "\nCircle circle1: " + circle1.ToString(); 18 19 // use 'is a' relationship to assign 20 // Circle circle1 to Point reference 21 Point point2 = circle1; 22 23 output += "\n\nCCircle circle1 (via point2): " + 24 point2.ToString(); 25 26 // downcast (cast base-class reference to derived-class 27 // data type) point2 to Circle circle2 28 Circle circle2 = ( Circle ) point2; 29 30 output += "\n\nCircle circle1 (via circle2): " + 31 circle2.ToString(); 32 33 output += "\nArea of circle1 (via circle2): " + 34 circle2.Area().ToString( "F" ); 35 Create a Point object Create a Circle object Assign a Point reference to reference a Circle object Assign a Circle reference to reference a Point object (downcast) Use base-class reference to access a derived-class object Use derived-class reference to access a base-class object

PointCircleTest.cs Program Output 36 // attempt to assign point1 object to Circle reference 37 if ( point1 is Circle ) 38 { 39 circle2 = ( Circle ) point1; 40 output += "\n\ncast successful" ; 41 } 42 else 43 { 44 output += "\n\npoint1 does not refer to a Circle" ; 45 } 46 47 MessageBox.Show( output, 48 "Demonstrating the 'is a' relationship" ); 49 50 } // end method Main 51 52 } // end class PointCircleTest Test if point1 references a Circle object – it cannot, because of the downcasting on line 28

10.3 Type Fields and switch Statements Using switch to determine the type of an object Distinguish between object, perform appropriate action depending on object type Potential problems using switch Programmer may forget to include a type test Programmer may forget to test all possible cases When new types are added, programmer may forget to modify all relevant switch structures Every addition or deletion of a class requires modification of every switch statement in the system; tracking this is time consuming and error prone

10.4 Polymorphism Examples Quadrilateral base-class Rectangle derived-class Square derived-class Parallelogram derived-class Trapezoid derived-class Method perimeter might need to be invoked on all classes With a Quadrilateral reference, C# polymorphically chooses the correct overriding method in the derived-class from which the object is instantiated

10.4 Polymorphism Examples SpaceObject base-class – contains method DrawYourself Martian derived-class (implements DrawYourself ) Venutian derived-class (implements DrawYourself ) Plutonian derived-class (implements DrawYourself ) SpaceShip derived-class (implements DrawYourself ) A screen-manager program may contain a SpaceObject array of references to objects of various classes that derive from SpaceObject To refresh the screen, the screen-manager calls DrawYourself on each object in the array The program polymorphically calls the appropriate version of DrawYourself on each object, based on the type of that object

10.5 Abstract Classes and Methods Abstract classes Cannot be instantiated Used as base classes Class definitions are not complete – derived classes must define the missing pieces Can contain abstract methods and/or abstract properties Have no implementation Derived classes must override inherited abstract methods and properties to enable instantiation

10.5 Abstract Classes and Methods Abstract classes are used to provide appropriate base classes from which other classes may inherit (concrete classes) Abstract base classes are too generic to define real objects To define an abstract class, use keyword abstract in the declaration To declare a method or property abstract, use keyword abstract in the declaration; abstract methods and properties have no implementation

10.5 Abstract Classes and Methods Concrete classes use the keyword override to provide implementations for all the abstract methods and properties of the base-class Any class with an abstract method or property must be declared abstract Even though abstract classes cannot be instantiated, we can use abstract class references to refer to instances of any concrete class derived from the abstract class

10.6 Case Study: Inheriting Interface and Implementation Abstract base class Shape Concrete virtual method Area (default return value is 0) Concrete virtual method Volume (default return value is 0) Abstract read-only property Name Class Point2 inherits from Shape Overrides property Name (required) Does NOT override methods Area and Volume Class Circle2 inherits from Point2 Overrides property Name Overrides method Area , but not Volume Class Cylinder2 inherits from Circle2 Overrides property Name Overrides methods Area and Volume

Shape.cs 1 // Fig. 10.4: Shape.cs 2 // Demonstrate a shape hierarchy using an abstract base class. 3 using System; 4 5 public abstract class Shape 6 { 7 // return Shape's area 8 public virtual double Area() 9 { 10 return 0 ; 11 } 12 13 // return Shape's volume 14 public virtual double Volume() 15 { 16 return 0 ; 17 } 18 19 // return Shape's name 20 public abstract string Name 21 { 22 get ; 23 } 24 } Declaration of abstract class Shape Declaration of virtual methods Area and Volume with default implementations Declaration of read-only abstract property Name; implementing classes will have to provide an implementation for this property

Point2.cs 1 // Fig. 10.5: Point2.cs 2 // Point2 inherits from abstract class Shape and represents 3 // an x-y coordinate pair. 4 using System; 5 6 // Point2 inherits from abstract class Shape 7 public class Point2 : Shape 8 { 9 private int x, y; // Point2 coordinates 10 11 // default constructor 12 public Point2() 13 { 14 // implicit call to Object constructor occurs here 15 } 16 17 // constructor 18 public Point2( int xValue, int yValue ) 19 { 20 X = xValue; 21 Y = yValue; 22 } 23 24 // property X 25 public int X 26 { 27 get 28 { 29 return x; 30 } 31 32 set 33 { 34 x = value ; // no validation needed 35 } Class Point2 inherits from class Shape

Point2.cs 36 } 37 38 // property Y 39 public int Y 40 { 41 get 42 { 43 return y; 44 } 45 46 set 47 { 48 y = value ; // no validation needed 49 } 50 } 51 52 // return string representation of Point2 object 53 public override string ToString() 54 { 55 return "[" + X + ", " + Y + "]" ; 56 } 57 58 // implement abstract property Name of class Shape 59 public override string Name 60 { 61 get 62 { 63 return "Point2" ; 64 } 65 } 66 67 } // end class Point2 Point2’s implementation of the read-only Name property

Circle2.cs 1 // Fig. 10.6: Circle2.cs 2 // Circle2 inherits from class Point2 and overrides key members. 3 using System; 4 5 // Circle2 inherits from class Point2 6 public class Circle2 : Point2 7 { 8 private double radius; // Circle2 radius 9 10 // default constructor 11 public Circle2() 12 { 13 // implicit call to Point2 constructor occurs here 14 } 15 16 // constructor 17 public Circle2( int xValue, int yValue, double radiusValue ) 18 : base ( xValue, yValue ) 19 { 20 Radius = radiusValue; 21 } 22 23 // property Radius 24 public double Radius 25 { 26 get 27 { 28 return radius; 29 } 30 Definition of class Circle2 which inherits from class Point2

Circle2.cs 31 set 32 { 33 // ensure non-negative radius value 34 if ( value >= 0 ) 35 radius = value ; 36 } 37 } 38 39 // calculate Circle2 diameter 40 public double Diameter() 41 { 42 return Radius * 2 ; 43 } 44 45 // calculate Circle2 circumference 46 public double Circumference() 47 { 48 return Math. PI * Diameter(); 49 } 50 51 // calculate Circle2 area 52 public override double Area() 53 { 54 return Math. PI * Math.Pow( Radius, 2 ); 55 } 56 57 // return string representation of Circle2 object 58 public override string ToString() 59 { 60 return "Center = " + base .ToString() + 61 "; Radius = " + Radius; 62 } Override the Area method (defined in class Shape)

Circle2.cs 63 64 // override property Name from class Point2 65 public override string Name 66 { 67 get 68 { 69 return "Circle2" ; 70 } 71 } 72 73 } // end class Circle2 Override the read-only Name property

Cylinder2.cs 1 // Fig. 10.7: Cylinder2.cs 2 // Cylinder2 inherits from class Circle2 and overrides key members. 3 using System; 4 5 // Cylinder2 inherits from class Circle2 6 public class Cylinder2 : Circle2 7 { 8 private double height; // Cylinder2 height 9 10 // default constructor 11 public Cylinder2() 12 { 13 // implicit call to Circle2 constructor occurs here 14 } 15 16 // constructor 17 public Cylinder2( int xValue, int yValue, double radiusValue, 18 double heightValue ) : base ( xValue, yValue, radiusValue ) 19 { 20 Height = heightValue; 21 } 22 23 // property Height 24 public double Height 25 { 26 get 27 { 28 return height; 29 } 30 31 set 32 { 33 // ensure non-negative height value 34 if ( value >= 0 ) 35 height = value ; Class Cylinder2 derives from Circle2

Cylinder2.cs 36 } 37 } 38 39 // calculate Cylinder2 area 40 public override double Area() 41 { 42 return 2 * base.Area() + base .Circumference() * Height; 43 } 44 45 // calculate Cylinder2 volume 46 public override double Volume() 47 { 48 return base .Area() * Height; 49 } 50 51 // return string representation of Circle2 object 52 public override string ToString() 53 { 54 return base .ToString() + "; Height = " + Height; 55 } 56 57 // override property Name from class Circle2 58 public override string Name 59 { 60 get 61 { 62 return "Cylinder2" ; 63 } 64 } 65 66 } // end class Cylinder2 Override Area implementation of class Circle2 Override read-only property Name Override Volume implementation of class Shape

AbstractShapesTest.cs 1 // Fig. 10.8: AbstractShapesTest.cs 2 // Demonstrates polymorphism in Point-Circle-Cylinder hierarchy. 3 using System; 4 using System.Windows.Forms; 5 6 public class AbstractShapesTest 7 { 8 public static void Main( string [] args ) 9 { 10 // instantiate Point2, Circle2 and Cylinder2 objects 11 Point2 point = new Point2( 7 , 11 ); 12 Circle2 circle = new Circle2( 22 , 8 , 3.5 ); 13 Cylinder2 cylinder = new Cylinder2( 10 , 10 , 3.3 , 10 ); 14 15 // create empty array of Shape base-class references 16 Shape[] arrayOfShapes = new Shape[ 3 ]; 17 18 // arrayOfShapes[ 0 ] refers to Point2 object 19 arrayOfShapes[ 0 ] = point; 20 21 // arrayOfShapes[ 1 ] refers to Circle2 object 22 arrayOfShapes[ 1 ] = circle; 23 24 // arrayOfShapes[ 1 ] refers to Cylinder2 object 25 arrayOfShapes[ 2 ] = cylinder; 26 27 string output = point.Name + ": " + point + "\n" + 28 circle.Name + ": " + circle + "\n" + 29 cylinder.Name + ": " + cylinder; 30 Create an array of Shape objects Assign a Shape reference to reference a Point2 object Assign a Shape reference to reference a Circle2 object Assign a Shape reference to reference a Cylinder2 object

AbstractShapesTest.cs Program Output 31 // display Name, Area and Volume for each object 32 // in arrayOfShapes polymorphically 33 foreach ( Shape shape in arrayOfShapes ) 34 { 35 output += "\n\n" + shape.Name + ": " + shape + 36 "\nArea = " + shape.Area().ToString( "F" ) + 37 "\nVolume = " + shape.Volume().ToString( "F" ); 38 } 39 40 MessageBox.Show( output, "Demonstrating Polymorphism" ); 41 } 42 } Use a foreach loop to access every element of the array Rely on polymorphism to call appropriate version of methods

10.7 sealed Classes and Methods sealed is a keyword in C# sealed methods cannot be overridden in a derived class Methods that are declared static and private , are implicitly sealed sealed classes cannot have any derived-classes Creating sealed classes can allow some runtime optimizations e.g., virtual method calls can be transformed into non- virtual method calls

10.8 Case Study: Payroll System Using Polymorphism Base-class Employee abstract abstract method Earnings Classes that derive from Employee Boss CommissionWorker PieceWorker HourlyWorker All derived-classes implement method Earnings Driver program uses Employee references to refer to instances of derived-classes Polymorphism calls the correct version of Earnings

Employee.cs 1 // Fig. 10.9: Employee.cs 2 // Abstract base class for company employees. 3 using System; 4 5 public abstract class Employee 6 { 7 private string firstName; 8 private string lastName; 9 10 // constructor 11 public Employee( string firstNameValue, 12 string lastNameValue ) 13 { 14 FirstName = firstNameValue; 15 LastName = lastNameValue; 16 } 17 18 // property FirstName 19 public string FirstName 20 { 21 get 22 { 23 return firstName; 24 } 25 26 set 27 { 28 firstName = value ; 29 } 30 } 31 Definition of abstract class Employee

Employee.cs 32 // property LastName 33 public string LastName 34 { 35 get 36 { 37 return lastName; 38 } 39 40 set 41 { 42 lastName = value ; 43 } 44 } 45 46 // return string representation of Employee 47 public override string ToString() 48 { 49 return FirstName + " " + LastName; 50 } 51 52 // abstract method that must be implemented for each derived 53 // class of Employee to calculate specific earnings 54 public abstract decimal Earnings(); 55 56 } // end class Employee Declaration of abstract class Earnings – implementation must be provided by all derived classes

Boss.cs 1 // Fig. 10.10: Boss.cs 2 // Boss class derived from Employee. 3 using System; 4 5 public class Boss : Employee 6 { 7 private decimal salary; // Boss's salary 8 9 // constructor 10 public Boss( string firstNameValue, string lastNameValue, 11 decimal salaryValue) 12 : base ( firstNameValue, lastNameValue ) 13 { 14 WeeklySalary = salaryValue; 15 } 16 17 // property WeeklySalary 18 public decimal WeeklySalary 19 { 20 get 21 { 22 return salary; 23 } 24 25 set 26 { 27 // ensure positive salary value 28 if ( value > 0 ) 29 salary = value ; 30 } 31 } 32 Definition of class Boss – derives from Employee

Boss.cs 33 // override base-class method to calculate Boss's earnings 34 public override decimal Earnings() 35 { 36 return WeeklySalary; 37 } 38 39 // return string representation of Boss 40 public override string ToString() 41 { 42 return "Boss: " + base .ToString(); 43 } 44 } Implementation of method Earnings (required by classes deriving from Employee)

CommisionWorker.cs 1 // Fig. 10.11: CommisionWorker.cs 2 // CommissionWorker class derived from Employee 3 using System; 4 5 public class CommissionWorker : Employee 6 { 7 private decimal salary; // base weekly salary 8 private decimal commission; // amount paid per item sold 9 private int quantity; // total items sold 10 11 // constructor 12 public CommissionWorker( string firstNameValue, 13 string lastNameValue, decimal salaryValue, 14 decimal commissionValue, int quantityValue ) 15 : base ( firstNameValue, lastNameValue ) 16 { 17 WeeklySalary = salaryValue; 18 Commission = commissionValue; 19 Quantity = quantityValue; 20 } 21 22 // property WeeklySalary 23 public decimal WeeklySalary 24 { 25 get 26 { 27 return salary; 28 } 29 Definition of class CommissionWorker – derives from Employee

CommisionWorker.cs 30 set 31 { 32 // ensure non-negative salary value 33 if ( value > 0 ) 34 salary = value ; 35 } 36 } 37 38 // property Commission 39 public decimal Commission 40 { 41 get 42 { 43 return commission; 44 } 45 46 set 47 { 48 // ensure non-negative commission value 49 if ( value > 0 ) 50 commission = value ; 51 } 52 } 53 54 // property Quantity 55 public int Quantity 56 { 57 get 58 { 59 return quantity; 60 } 61

CommisionWorker.cs 62 set 63 { 64 // ensure non-negative quantity value 65 if ( value > 0 ) 66 quantity = value ; 67 } 68 } 69 70 // override base-class method to calculate 71 // CommissionWorker's earnings 72 public override decimal Earnings() 73 { 74 return WeeklySalary + Commission * Quantity; 75 } 76 77 // return string representation of CommissionWorker 78 public override string ToString() 79 { 80 return "CommissionWorker: " + base .ToString(); 81 } 82 83 } // end class CommissionWorker Implementation of method Earnings (required by classes deriving from Employee)

PieceWorker.cs 1 // Fig. 10.12: PieceWorker.cs 2 // PieceWorker class derived from Employee. 3 using System; 4 5 public class PieceWorker : Employee 6 { 7 private decimal wagePerPiece; // wage per piece produced 8 private int quantity; // quantity of pieces produced 9 10 // constructor 11 public PieceWorker( string firstNameValue, 12 string lastNameValue, decimal wagePerPieceValue, 13 int quantityValue ) 14 : base ( firstNameValue, lastNameValue ) 15 { 16 WagePerPiece = wagePerPieceValue; 17 Quantity = quantityValue; 18 } 19 20 // property WagePerPiece 21 public decimal WagePerPiece 22 { 23 get 24 { 25 return wagePerPiece; 26 } 27 28 set 29 { 30 if ( value > 0 ) 31 wagePerPiece = value ; 32 } 33 } 34 Definition of class PieceWorker – derives from Employee

PieceWorker.cs 35 // property Quantity 36 public int Quantity 37 { 38 get 39 { 40 return quantity; 41 } 42 43 set 44 { 45 if ( value > 0 ) 46 quantity = value ; 47 } 48 } 49 50 // override base-class method to calculate 51 // PieceWorker's earnings 52 public override decimal Earnings() 53 { 54 return Quantity * WagePerPiece; 55 } 56 57 // return string representation of PieceWorker 58 public override string ToString() 59 { 60 return "PieceWorker: " + base .ToString(); 61 } 62 } Implementation of method Earnings (required by classes deriving from Employee)

HourlyWorker.cs 1 // Fig. 10.13: HourlyWorker.cs 2 // HourlyWorker class derived from Employee. 3 using System; 4 5 public class HourlyWorker : Employee 6 { 7 private decimal wage; // wage per hour of work 8 private double hoursWorked; // hours worked during week 9 10 // constructor 11 public HourlyWorker( string firstNameValue, string LastNameValue, 12 decimal wageValue, double hoursWorkedValue ) 13 : base ( firstNameValue, LastNameValue ) 14 { 15 Wage = wageValue; 16 HoursWorked = hoursWorkedValue; 17 } 18 19 // property Wage 20 public decimal Wage 21 { 22 get 23 { 24 return wage; 25 } 26 27 set 28 { 29 // ensure non-negative wage value 30 if ( value > 0 ) 31 wage = value ; 32 } 33 } 34 Definition of class HourlyWorker – derives from Employee

HourlyWorker.cs 35 // property HoursWorked 36 public double HoursWorked 37 { 38 get 39 { 40 return hoursWorked; 41 } 42 43 set 44 { 45 // ensure non-negative hoursWorked value 46 if ( value > 0 ) 47 hoursWorked = value ; 48 } 49 } 50 51 // override base-class method to calculate 52 // HourlyWorker earnings 53 public override decimal Earnings() 54 { 55 // compensate for overtime (paid "time-and-a-half") 56 if ( HoursWorked <= 40 ) 57 { 58 return Wage * Convert.ToDecimal( HoursWorked ); 59 } 60 61 else 62 { 63 // calculate base and overtime pay 64 decimal basePay = Wage * Convert.ToDecimal( 40 ); 65 decimal overtimePay = Wage * 1.5M * 66 Convert.ToDecimal( HoursWorked - 40 ); 67 Implementation of method Earnings (required by classes deriving from Employee)

HourlyWorker.cs 68 return basePay + overtimePay; 69 } 70 } 71 72 // return string representation of HourlyWorker 73 public override string ToString() 74 { 75 return "HourlyWorker: " + base .ToString(); 76 } 77 }

EmployeesTest.cs 1 // Fig. 10.14: EmployeesTest.cs 2 // Demonstrates polymorphism by displaying earnings 3 // for various Employee types. 4 using System; 5 using System.Windows.Forms; 6 7 public class EmployeesTest 8 { 9 public static void Main( string [] args ) 10 { 11 Boss boss = new Boss( "John" , "Smith" , 800 ); 12 13 CommissionWorker commissionWorker = 14 new CommissionWorker( "Sue" , "Jones" , 400 , 3 , 150 ); 15 16 PieceWorker pieceWorker = new PieceWorker( "Bob" , "Lewis" , 17 Convert.ToDecimal( 2.5 ), 200 ); 18 19 HourlyWorker hourlyWorker = new HourlyWorker( "Karen" , 20 "Price", Convert.ToDecimal( 13.75 ), 50 ); 21 22 Employee employee = boss; 23 24 string output = GetString( employee ) + boss + " earned " + 25 boss.Earnings().ToString( "C" ) + "\n\n" ; 26 27 employee = commissionWorker; 28 29 output += GetString( employee ) + commissionWorker + 30 " earned " + 31 commissionWorker.Earnings().ToString( "C" ) + "\n\n" ; 32 33 employee = pieceWorker; 34 Assign Employee reference to reference a Boss object Use method GetString to polymorphically obtain salary information. Then use the original Boss reference to obtain the information

EmployeesTest.cs Program Output 35 output += GetString( employee ) + pieceWorker + 36 " earned " + pieceWorker.Earnings().ToString( "C" ) + 37 "\n\n" ; 38 39 employee = hourlyWorker; 40 41 output += GetString( employee ) + hourlyWorker + 42 " earned " + hourlyWorker.Earnings().ToString( "C" ) + 43 "\n\n" ; 44 45 MessageBox.Show( output, "Demonstrating Polymorphism" , 46 MessageBoxButtons. OK , MessageBoxIcon. Information ); 47 48 } // end method Main 49 50 // return string that contains Employee information 51 public static string GetString( Employee worker ) 52 { 53 return worker.ToString() + " earned " + 54 worker.Earnings().ToString( "C" ) + "\n" ; 55 } 56 } Definition of method GetString, which takes as an argument an Employee object. Polymorphically call the method of the appropriate derived class

10.9 Case Study: Creating and Using Interfaces Interfaces specify the public services (methods and properties) that classes must implement Interfaces provide no default implementations vs. abstract classes which may provide some default implementations Interfaces are used to “bring together” or relate disparate objects that relate to one another only through the interface

10.9 Case Study: Creating and Using Interfaces Interfaces are defined using keyword interface Use inheritance notation to specify a class implements an interface ( ClassName : InterfaceName ) Classes may implement more then one interface (a comma separated list of interfaces) Classes that implement an interface, must provide implementations for every method and property in the interface definition Example: interface IAge that returns information about an object’s age Can be used by classes for people, cars, trees (all have an age)

IAge.cs 1 // Fig. 10.15: IAge.cs 2 // Interface IAge declares property for setting and getting age. 3 4 public interface IAge 5 { 6 int Age { get ; } 7 string Name { get ; } 8 } Definition of interface IAge Classes implementing this interface will have to define read-only properties Age and Name

Person.cs 1 // Fig. 10.16: Person.cs 2 // Class Person has a birthday. 3 using System; 4 5 public class Person : IAge 6 { 7 private string firstName; 8 private string lastName; 9 private int yearBorn; 10 11 // constructor 12 public Person( string firstNameValue, string lastNameValue, 13 int yearBornValue ) 14 { 15 firstName = firstNameValue; 16 lastName = lastNameValue; 17 18 if ( yearBornValue > 0 && yearBornValue <= DateTime.Now.Year ) 19 yearBorn = yearBornValue; 20 else 21 yearBorn = DateTime.Now.Year; 22 } 23 24 // property Age implementation of interface IAge 25 public int Age 26 { 27 get 28 { 29 return DateTime.Now.Year - yearBorn; 30 } 31 } 32 Class Person implements the IAge interface Definition of Age property (required)

Person.cs 33 // property Name implementation of interface IAge 34 public string Name 35 { 36 get 37 { 38 return firstName + " " + lastName; 39 } 40 } 41 42 } // end class Person Definition of Name property (required)

Tree.cs 1 // Fig. 10.17: Tree.cs 2 // Class Tree contains number of rings corresponding to its age. 3 using System; 4 5 public class Tree : IAge 6 { 7 private int rings; // number of rings in tree trunk 8 9 // constructor 10 public Tree( int yearPlanted ) 11 { 12 // count number of rings in Tree 13 rings = DateTime.Now.Year - yearPlanted; 14 } 15 16 // increment rings 17 public void AddRing() 18 { 19 rings++; 20 } 21 22 // property Age implementation of interface IAge 23 public int Age 24 { 25 get 26 { 27 return rings; 28 } 29 } Class Tree implements the IAge interface Implementation of Age property ( required)

Tree.cs 30 31 // property Name implementation of interface IAge 32 public string Name 33 { 34 get 35 { 36 return "Tree" ; 37 } 38 } 39 40 } // end class Tree Definition of Name property (required)

InterfacesTest.cs 1 // Fig. 10.18: InterfacesTest.cs 2 // Demonstrating polymorphism with interfaces. 3 using System.Windows.Forms; 4 5 public class InterfacesTest 6 { 7 public static void Main( string [] args ) 8 { 9 Tree tree = new Tree( 1978 ); 10 Person person = new Person( "Bob" , "Jones" , 1971 ); 11 12 // create array of IAge references 13 IAge[] iAgeArray = new IAge[ 2 ]; 14 15 // iAgeArray[ 0 ] refers to Tree object polymorphically 16 iAgeArray[ 0 ] = tree; 17 18 // iAgeArray[ 1 ] refers to Person object polymorphically 19 iAgeArray[ 1 ] = person; 20 21 // display tree information 22 string output = tree + ": " + tree.Name + "\nAge is " + 23 tree.Age + "\n\n" ; 24 25 // display person information 26 output += person + ": " + person.Name + "\nAge is: " 27 + person.Age + "\n\n" ; 28 Create array of IAge references Assign an IAge reference to reference a Tree object Assign an IAge reference to reference a Person object

InterfacesTest.cs Program Output 29 // display name and age for each IAge object in iAgeArray 30 foreach ( IAge ageReference in iAgeArray ) 31 { 32 output += ageReference.Name + ": Age is " + 33 ageReference.Age + "\n" ; 34 } 35 36 MessageBox.Show( output, "Demonstrating Polymorphism" ); 37 38 } // end method Main 39 40 } // end class InterfacesTest Use foreach loop to access each element of the array Use polymorphism to call the property of the appropriate class

IShape.cs 1 // Fig. 10.19: IShape.cs 2 // Interface IShape for Point, Circle, Cylinder Hierarchy. 3 4 public interface IShape 5 { 6 // classes that implement IShape must implement these methods 7 // and this property 8 double Area(); 9 double Volume(); 10 string Name { get ; } 11 } Definition of IShape interface Classes implementing the interface must define methods Area and Volume (each of which take no arguments and return a double) and a read-only string property Name

Point3.cs 1 // Fig. 10.20: Point3.cs 2 // Point3 implements interface IShape and represents 3 // an x-y coordinate pair. 4 using System; 5 6 // Point3 implements IShape 7 public class Point3 : IShape 8 { 9 private int x, y; // Point3 coordinates 10 11 // default constructor 12 public Point3() 13 { 14 // implicit call to Object constructor occurs here 15 } 16 17 // constructor 18 public Point3( int xValue, int yValue ) 19 { 20 X = xValue; 21 Y = yValue; 22 } 23 24 // property X 25 public int X 26 { 27 get 28 { 29 return x; 30 } 31 Class Point3 implements the IShape interface

Point3.cs 32 set 33 { 34 x = value ; 35 } 36 } 37 38 // property Y 39 public int Y 40 { 41 get 42 { 43 return y; 44 } 45 46 set 47 { 48 y = value ; 49 } 50 } 51 52 // return string representation of Point3 object 53 public override string ToString() 54 { 55 return "[" + X + ", " + Y + "]" ; 56 } 57 58 // implement interface IShape method Area 59 public virtual double Area() 60 { 61 return 0 ; 62 } 63 Implementation of IShape method Area (required), declared virtual to allow deriving classes to override

Point3.cs 64 // implement interface IShape method Volume 65 public virtual double Volume() 66 { 67 return 0 ; 68 } 69 70 // implement property Name of IShape 71 public virtual string Name 72 { 73 get 74 { 75 return "Point3" ; 76 } 77 } 78 79 } // end class Point3 Implementation of IShape method Volume (required), declared virtual to allow deriving classes to override Implementation of IShape property Name (required), declared virtual to allow deriving classes to override

Circle3.cs 1 // Fig. 10.21: Circle3.cs 2 // Circle3 inherits from class Point3 and overrides key members. 3 using System; 4 5 // Circle3 inherits from class Point3 6 public class Circle3 : Point3 7 { 8 private double radius; // Circle3 radius 9 10 // default constructor 11 public Circle3() 12 { 13 // implicit call to Point3 constructor occurs here 14 } 15 16 // constructor 17 public Circle3( int xValue, int yValue, double radiusValue ) 18 : base ( xValue, yValue ) 19 { 20 Radius = radiusValue; 21 } 22 23 // property Radius 24 public double Radius 25 { 26 get 27 { 28 return radius; 29 } 30 Definition of class Circle3 which inherits from Point3

Circle3.cs 31 set 32 { 33 // ensure non-negative Radius value 34 if ( value >= 0 ) 35 radius = value ; 36 } 37 } 38 39 // calculate Circle3 diameter 40 public double Diameter() 41 { 42 return Radius * 2 ; 43 } 44 45 // calculate Circle3 circumference 46 public double Circumference() 47 { 48 return Math. PI * Diameter(); 49 } 50 51 // calculate Circle3 area 52 public override double Area() 53 { 54 return Math. PI * Math.Pow( Radius, 2 ); 55 } 56 57 // return string representation of Circle3 object 58 public override string ToString() 59 { 60 return "Center = " + base .ToString() + 61 "; Radius = " + Radius; 62 } 63 Override the Point3 implementation of Area

Circle3.cs 64 // override property Name from class Point3 65 public override string Name 66 { 67 get 68 { 69 return "Circle3" ; 70 } 71 } 72 73 } // end class Circle3 Override the Point3 implementation of Name

Cylinder3.cs 1 // Fig. 10.22: Cylinder3.cs 2 // Cylinder3 inherits from class Circle2 and overrides key members. 3 using System; 4 5 // Cylinder3 inherits from class Circle3 6 public class Cylinder3 : Circle3 7 { 8 private double height; // Cylinder3 height 9 10 // default constructor 11 public Cylinder3() 12 { 13 // implicit call to Circle3 constructor occurs here 14 } 15 16 // constructor 17 public Cylinder3( int xValue, int yValue, double radiusValue, 18 double heightValue ) : base ( xValue, yValue, radiusValue ) 19 { 20 Height = heightValue; 21 } 22 23 // property Height 24 public double Height 25 { 26 get 27 { 28 return height; 29 } 30 Declaration of class Cylinder3 which inherits from class Circle3

Cylinder3.cs 31 set 32 { 33 // ensure non-negative Height value 34 if ( value >= 0 ) 35 height = value ; 36 } 37 } 38 39 // calculate Cylinder3 area 40 public override double Area() 41 { 42 return 2 * base .Area() + base .Circumference() * Height; 43 } 44 45 // calculate Cylinder3 volume 46 public override double Volume() 47 { 48 return base .Area() * Height; 49 } 50 51 // return string representation of Cylinder3 object 52 public override string ToString() 53 { 54 return "Center = " + base .ToString() + 55 "; Height = " + Height; 56 } 57 Override the Circle3 implementation of Area Override the Point3 implementation of Volume

Cylinder3.cs 58 // override property Name from class Circle3 59 public override string Name 60 { 61 get 62 { 63 return "Cylinder3" ; 64 } 65 } 66 67 } // end class Cylinder3 Override the Circle3 implementation of Name

Interfaces2Test.cs 1 // Fig. 10.23: Interfaces2Test.cs 2 // Demonstrating polymorphism with interfaces in 3 // Point-Circle-Cylinder hierarchy. 4 5 using System.Windows.Forms; 6 7 public class Interfaces2Test 8 { 9 public static void Main( string [] args ) 10 { 11 // instantiate Point3, Circle3 and Cylinder3 objects 12 Point3 point = new Point3( 7 , 11 ); 13 Circle3 circle = new Circle3( 22 , 8 , 3.5 ); 14 Cylinder3 cylinder = new Cylinder3( 10 , 10 , 3.3 , 10 ); 15 16 // create array of IShape references 17 IShape[] arrayOfShapes = new IShape[ 3 ]; 18 19 // arrayOfShapes[ 0 ] references Point3 object 20 arrayOfShapes[ 0 ] = point; 21 22 // arrayOfShapes[ 1 ] references Circle3 object 23 arrayOfShapes[ 1 ] = circle; 24 25 // arrayOfShapes[ 2 ] references Cylinder3 object 26 arrayOfShapes[ 2 ] = cylinder; 27 28 string output = point.Name + ": " + point + "\n" + 29 circle.Name + ": " + circle + "\n" + 30 cylinder.Name + ": " + cylinder; 31 Create an array of IShape references Assign an IShape reference to reference a Point3 object Assign an IShape reference to reference a Circle3 object Assign an IShape reference to reference a Cylinder3 object

Interfaces2Test.cs Program Output 32 foreach ( IShape shape in arrayOfShapes ) 33 { 34 output += "\n\n" + shape.Name + ":\nArea = " + 35 shape.Area() + "\nVolume = " + shape.Volume(); 36 } 37 38 MessageBox.Show( output, "Demonstrating Polymorphism" ); 39 } 40 } Use polymorphism to call the appropriate class’s method or property

10.10 Delegates Sometimes useful to pass methods as arguments to other methods Delegates are sets of references to methods Delegate objects can be passed to methods; methods can then invoke the methods the delegate objects refer to Delegates that contain one method are known as singlecast delegates and are created or derived from class Delegate Delegates that contain multiple methods aare known as multicast delegates and are created or derived from class MulticastDelegate

10.10 Delegates Delegates must be declared before use A delegate declaration specifies the parameters and return type of the methods the delegate can refer to Methods who can be referred to by a delegate, must have the same signature as the delegate Delegate instances can then be created to refer to a method(s) Once a delegate instance is created, the method it refers to can be invoked

DelegateBubbleSort.cs 1 // Fig. 10.24: DelegateBubbleSort.cs 2 // Demonstrating delegates for sorting numbers. 3 4 public class DelegateBubbleSort 5 { 6 public delegate bool Comparator( int element1, 7 int element2 ); 8 9 // sort array using Comparator delegate 10 public static void SortArray( int [] array, 11 Comparator Compare ) 12 { 13 for ( int pass = 0 ; pass < array.Length; pass++ ) 14 15 for ( int i = 0 ; i < array.Length - 1 ; i++ ) 16 17 if ( Compare( array[ i ], array [ i + 1 ] ) ) 18 Swap( ref array[ i ], ref array[ i + 1 ] ); 19 } 20 21 // swap two elements 22 private static void Swap( ref int firstElement, 23 ref int secondElement ) 24 { 25 int hold = firstElement; 26 firstElement = secondElement; 27 secondElement = hold; 28 } 29 } Delegate Comparator definition declaration; defines a delegate to a method that takes two integer parameters and returns a boolean Method SortArray which takes an integer array and a Comparator delegate Call delegate method to compare array elements Method Swap, swaps the two arguments (passed by reference)

BubbleSortForm.cs 1 // Fig. 10.25: BubbleSortForm.cs 2 // Demonstrates bubble sort using delegates to determine 3 // the sort order. 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; 8 using System.Windows.Forms; 9 10 public class BubbleSortForm : System.Windows.Forms.Form 11 { 12 private System.Windows.Forms.TextBox originalTextBox; 13 private System.Windows.Forms.TextBox sortedTextBox; 14 private System.Windows.Forms.Button createButton; 15 private System.Windows.Forms.Button ascendingButton; 16 private System.Windows.Forms.Button descendingButton; 17 private System.Windows.Forms.Label originalLabel; 18 private System.Windows.Forms.Label sortedLabel; 19 20 private int [] elementArray = new int [ 10 ]; 21 22 // create randomly generated set of numbers to sort 23 private void createButton_Click( object sender, 24 System.EventArgs e ) 25 { 26 // clear TextBoxes 27 originalTextBox.Clear(); 28 sortedTextBox.Clear(); 29 30 // create random-number generator 31 Random randomNumber = new Random(); 32

BubbleSortForm.cs 33 // populate elementArray with random integers 34 for ( int i = 0 ; i < elementArray.Length; i++ ) 35 { 36 elementArray[ i ] = randomNumber.Next( 100 ); 37 originalTextBox.Text += elementArray[ i ] + "\r\n" ; 38 } 39 } 40 41 // delegate implementation for ascending sort 42 private bool SortAscending( int element1, int element2 ) 43 { 44 return element1 > element2; 45 } 46 47 // sort randomly generated numbers in ascending order 48 private void ascendingButton_Click( object sender, 49 System.EventArgs e ) 50 { 51 // sort array, passing delegate for SortAscending 52 DelegateBubbleSort.SortArray( elementArray, 53 new DelegateBubbleSort.Comparator( 54 SortAscending ) ); 55 56 DisplayResults(); 57 } 58 59 // delegate implementation for descending sort 60 private bool SortDescending( int element1, int element2 ) 61 { 62 return element1 < element2; 63 } 64 Method SortAscending returns true if the first argument is larger then the second; returns false otherwise Method SortDescending returns true if the first argument is smaller then the second; returns false otherwise To sort in ascending order, send a delegate for the SortAscending method to method SortArray

BubbleSortForm.cs 65 // sort randomly generating numbers in descending order 66 private void descendingButton_Click( object sender, 67 System.EventArgs e ) 68 { 69 // sort array, passing delegate for SortDescending 70 DelegateBubbleSort.SortArray( elementArray, 71 new DelegateBubbleSort.Comparator( 72 SortDescending ) ); 73 74 DisplayResults(); 75 } 76 77 // display the sorted array in sortedTextBox 78 private void DisplayResults() 79 { 80 sortedTextBox.Clear(); 81 82 foreach ( int element in elementArray ) 83 sortedTextBox.Text += element + "\r\n" ; 84 } 85 86 // main entry point for application 87 public static void Main( string [] args ) 88 { 89 Application.Run( new BubbleSortForm() ); 90 } 91 } To sort in descending order, send a delegate to the SortDescending method to method SortArray

BubbleSortForm.cs Program Output

10.11 Operator Overloading C# contains many operators (such as + and - ) that are defined for some primitive types It is often useful to use operators with user-defined types (e.g., a complex number class) Operator notation may often be more intuitive then method calls C# allows programmers to overload operators to make them sensitive to the context in which they are used

10.11 Operator Overloading Methods define the actions to be taken for the overloaded operator They are in the form: public static ReturnType operator operator-to-be-overloaded( arguments ) These methods must be declared public and static The return type is the type returned as the result of evaluating the operation The keyword operator follows the return type to specify that this method defines an operator overload The last piece of information is the operator to be overloaded (such as +, -, *, etc.) If the operator is unary, one argument must be specified, if the operator is binary, then two, etc.

ComplexNumber.cs 1 // Fig. 10.26: ComplexNumber.cs 2 // Class that overloads operators for adding, subtracting 3 // and multiplying complex numbers. 4 5 public class ComplexNumber 6 { 7 private int real; 8 private int imaginary; 9 10 // default constructor 11 public ComplexNumber() {} 12 13 // constructor 14 public ComplexNumber( int a, int b ) 15 { 16 Real = a; 17 Imaginary = b; 18 } 19 20 // return string representation of ComplexNumber 21 public override string ToString() 22 { 23 return "( " + real + 24 ( imaginary < 0 ? " - " + ( imaginary * -1 ) : 25 " + " + imaginary ) + "i )" ; 26 } 27 28 // property Real 29 public int Real 30 { 31 get 32 { 33 return real; 34 } 35 Class ComplexNumber definition Property Real provides access to the real part of the complex number

ComplexNumber.cs 36 set 37 { 38 real = value ; 39 } 40 41 } // end property Real 42 43 // property Imaginary 44 public int Imaginary 45 { 46 get 47 { 48 return imaginary; 49 } 50 51 set 52 { 53 imaginary = value ; 54 } 55 56 } // end property Imaginary 57 58 // overload the addition operator 59 public static ComplexNumber operator + ( 60 ComplexNumber x, ComplexNumber y ) 61 { 62 return new ComplexNumber( 63 x.Real + y.Real, x.Imaginary + y.Imaginary ); 64 } 65 Property Imaginary provides access to the imaginary part of a complex number Overload the addition (+) operator for ComplexNumbers.

ComplexNumber.cs 66 // provide alternative to overloaded + operator 67 // for addition 68 public static ComplexNumber Add( ComplexNumber x, 69 ComplexNumber y ) 70 { 71 return x + y; 72 } 73 74 // overload the subtraction operator 75 public static ComplexNumber operator - ( 76 ComplexNumber x, ComplexNumber y ) 77 { 78 return new ComplexNumber( 79 x.Real - y.Real, x.Imaginary - y.Imaginary ); 80 } 81 82 // provide alternative to overloaded - operator 83 // for subtraction 84 public static ComplexNumber Subtract( ComplexNumber x, 85 ComplexNumber y ) 86 { 87 return x - y; 88 } 89 90 // overload the multiplication operator 91 public static ComplexNumber operator * ( 92 ComplexNumber x, ComplexNumber y ) 93 { 94 return new ComplexNumber( 95 x.Real * y.Real - x.Imaginary * y.Imaginary, 96 x.Real * y.Imaginary + y.Real * x.Imaginary ); 97 } Method Add – provides an alternative to the addition operator Overload the subtraction (-) operator for ComplexNumbers Method Subtract – provides an alternative to the subtraction operator Overloads the multiplication (*) operator for ComplexNumbers

ComplexNumber.cs 98 99 // provide alternative to overloaded * operator 100 // for multiplication 101 public static ComplexNumber Multiply( ComplexNumber x, 102 ComplexNumber y ) 103 { 104 return x * y; 105 } 106 107 } // end class ComplexNumber Method Multiply – provides an alternative to the multiplication operator

OperatorOverloading.cs 1 // Fig 10.27: OperatorOverloading.cs 2 // An example that uses operator overloading 3 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; 8 using System.Windows.Forms; 9 using System.Data; 10 11 public class ComplexTest : System.Windows.Forms.Form 12 { 13 private System.Windows.Forms.Label realLabel; 14 private System.Windows.Forms.Label imaginaryLabel; 15 private System.Windows.Forms.Label statusLabel; 16 17 private System.Windows.Forms.TextBox realTextBox; 18 private System.Windows.Forms.TextBox imaginaryTextBox; 19 20 private System.Windows.Forms.Button firstButton; 21 private System.Windows.Forms.Button secondButton; 22 private System.Windows.Forms.Button addButton; 23 private System.Windows.Forms.Button subtractButton; 24 private System.Windows.Forms.Button multiplyButton; 25 26 private ComplexNumber x = new ComplexNumber(); 27 private ComplexNumber y = new ComplexNumber(); 28 29 [STAThread] 30 static void Main() 31 { 32 Application.Run( new ComplexTest() ); 33 } 34

OperatorOverloading.cs 35 private void firstButton_Click( 36 object sender, System.EventArgs e ) 37 { 38 x.Real = Int32.Parse( realTextBox.Text ); 39 x.Imaginary = Int32.Parse( imaginaryTextBox.Text ); 40 realTextBox.Clear(); 41 imaginaryTextBox.Clear(); 42 statusLabel.Text = "First Complex Number is: " + x; 43 } 44 45 private void secondButton_Click( 46 object sender, System.EventArgs e ) 47 { 48 y.Real = Int32.Parse( realTextBox.Text ); 49 y.Imaginary = Int32.Parse( imaginaryTextBox.Text ); 50 realTextBox.Clear(); 51 imaginaryTextBox.Clear(); 52 statusLabel.Text = "Second Complex Number is: " + y; 53 } 54 55 // add complex numbers 56 private void addButton_Click( object sender, System.EventArgs e ) 57 { 58 statusLabel.Text = x + " + " + y + " = " + ( x + y ); 59 } 60 61 // subtract complex numbers 62 private void subtractButton_Click( 63 object sender, System.EventArgs e ) 64 { 65 statusLabel.Text = x + " - " + y + " = " + ( x - y ); 66 } 67 Use addition operator to add two ComplexNumbers Use subtraction operator to subtract two ComplexNumbers

OperatorOverloading.cs Program Output 68 // multiply complex numbers 69 private void multiplyButton_Click( 70 object sender, System.EventArgs e ) 71 { 72 statusLabel.Text = x + " * " + y + " = " + ( x * y ); 73 } 74 75 } // end class ComplexTest Use multiplication operator to multiply two ComplexNumbers

OperatorOverloading.cs Program Output

Csphtp1 10

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