x0041 lect8 pol morebook vn 0563

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Advanced Programming
Lecture 7
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


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// Fig. 10.23: Interfaces2Test.cs
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// Demonstrating polymorphism with interfaces in
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// Point-Circle-Cylinder hierarchy.
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using System.Windows.Forms;
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public class Interfaces2Test
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{
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public static void Main( string[] args )

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{
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// instantiate Point3, Circle3 and Cylinder3
objects
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Point3 point = new Point3( 7, 11 );
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Circle3 circle = new Circle3( 22, 8, 3.5 );
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Cylinder3 cylinder = new Cylinder3( 10, 10,
3.3, 10 );
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// create array of IShape references
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IShape[] arrayOfShapes = new IShape[ 3 ];
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// arrayOfShapes[ 0 ] references Point3
object
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arrayOfShapes[ 0 ] = point;
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foreach ( IShape shape in arrayOfShapes )

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{
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output += "\n\n" + shape.Name + ":\nArea
= " +
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shape.Area() + "\nVolume = " +
shape.Volume();
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}
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MessageBox.Show( output, "Demonstrating
Polymorphism" );
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}
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}