Visual Basic.NET How to Program (PDF)

Chapter 10 Object-Oriented Programming: Polymorphism 389
Alfred North Whitehead
Outline
10.1 Introduction
10.2 Derived-Class-Object to Base-Class-Object Conversion
10.3 Type Fields and Select Case Statements
10.4 Polymorphism Examples
10.5 Abstract Classes and Methods
10.6 Case Study: Inheriting Interface and Implementation
10.7 NotInheritable Classes and NotOverridable Methods
10.8 Case Study: Payroll System Using Polymorphism
10.9 Case Study: Creating and Using Interfaces
10.10 Delegates
Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises
10.1 Introduction
The previous chapter’s object-oriented programming (OOP) discussion focussed on one of its
key component technologies, inheritance. In this chapter, we continue our study of OOP polymorphism.
Both inheritance and polymorphism are crucial technologies in the development
of complex software. Polymorphism enables us to write programs that handle a wide variety
of related classes and facilitates adding new classes and capabilities to a system.
Using polymorphism, it is possible to design and implement systems that are easily
extensible. Programs can process objects of all classes in a class hierarchy generically as
objects of a common base class. Furthermore, a new class can be added with little or no
modification to the generic part of the program, as long as those new classes are part of the
inheritance hierarchy that the program generically processes. The only parts of a program
that must be altered to accommodate new classes are those program components that
require direct knowledge of the new classes that the programmer adds to the hierarchy. In
this chapter, we demonstrate two substantial class hierarchies and manipulate objects from
those hierarchies polymorphically.
10.2 Derived-Class-Object to Base-Class-Object Conversion
Section 9.4 created a point-circle class hierarchy, in which class CCircle inherited from
class CPoint. The programs that manipulated objects of these classes always used
CPoint references to refer to CPoint objects and CCircle references to refer to
CCircle objects. In this section, we discuss the relationship between classes in a hierarchy
that enables a program to assign derived-class objects to base-class references—a fundamental
part of programs that process objects polymorphically. This section also
discusses explicit casting between types in a class hierarchy.
An object of a derived class can be treated as an object of its base class. This enables
various interesting manipulations. For example, a program can create an array of base-class
references that refer to objects of many derived-class types. This is allowed despite the fact