x0041 lect8 pol morebook vn 0563

1 Advanced Programming Lecture Inheritance 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 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 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 Case Study: Payroll System • 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 54 // Fig 10.23: Interfaces2Test.cs // Demonstrating polymorphism with interfaces in // Point-Circle-Cylinder hierarchy using System.Windows.Forms; public class Interfaces2Test { 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[ ]; 18 19 // arrayOfShapes[ ] references Point3 object 20 arrayOfShapes[ ] = point; 21 55 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 } ... to refer to instances of derived-classes • Polymorphism calls the correct version of Earnings 54 // Fig 10.23: Interfaces2Test.cs // Demonstrating polymorphism with interfaces in // Point-Circle-Cylinder... shape.Area() + "\nVolume = " + shape.Volume(); 36 } 37 38 MessageBox.Show( output, "Demonstrating Polymorphism" ); 39 } 40 }