In this chapter, we will describe two important and powerful features in objectoriented
programming—inheritance and polymorphism. The inheritance feature of
object-oriented programming was introduced in Chapter 1. We will provide a more
detailed explanation and examples of inheritance in this chapter.
The second major topic we cover in this chapter is polymorphism, another
indispensable feature in object-oriented programming, which allows programmers
to send the same message to objects from different classes. Consider the statement
account.computeMonthlyFee();
where account could be either a SavingsAccount or a CheckingAccount object. If
account is a SavingsAccount object, then the method computeMonthlyFee defined
for the SavingsAccount class is executed. Likewise, if account is a CheckingAccount
object, then the method computeMonthlyFee defined for the CheckingAccount
class is executed. Sending the same message therefore could result in executing
different methods. The message computeMonthlyFee is called a polymorphic message
because depending on the receiver object, different methods are executed.
Polymorphism helps us write code that is easy to modify and extend. We will
explain polymorphism in this chapter.
