Concepts of Programming Languages

526 Chapter 12 Support for Object-Oriented Programming

from another class is a derived class or subclass. A class from which the new

class is derived is its parent class or superclass. The subprograms that define

the operations on objects of a class are called methods. The calls to methods

are sometimes called messages. The entire collection of methods of an object

is called the message protocol, or message interface, of the object. Computa-

tions in an object-oriented program are specified by messages sent from objects

to other objects, or in some cases, to classes.

Passing a message is indeed different from calling a subprogram. A subpro-

gram typically processes data that is either passed by its caller as a parameter

or is accessed nonlocally or globally. A message is sent to an object is a request

to execute one of its methods. At least part of the data on which the method

is to operate is the object itself. Objects have methods that define processes

the object can perform on itself. Because the objects are of abstract data types,

these should be the only ways to manipulate the object. A subprogram defines

a process that it can perform on any data sent to it (or made available nonlo-

cally or globally).

As a simple example of inheritance, consider the following: Suppose we

have a class named Vehicles, which has variables for year, color, and make. A

natural specialization, or subclass, of this would be Truck, which could inherit

the variables from Vehicle, but would add variables for hauling capacity and

number of wheels. Figure 12.1 shows a simple diagram to indicate the rela-

tionship between the Vehicle class and the Truck class, in which the arrow

points to the parent class.

There are several ways a derived class can differ from its parent.^1 Following

are the most common differences between a parent class and its subclasses:

1. The parent class can define some of its variables or methods to have
   private access, which means they will not be visible in the subclass.


2. The subclass can add variables and/or methods to those inherited from
   the parent class.


3. The subclass can modify the behavior of one or more of its inherited
   methods. A modified method has the same name, and often the same
   protocol, as the one of which it is a modification.
   The new method is said to override the inherited method, which is then
   called an overridden method. The purpose of an overriding method is to


4. If a subclass does not differ from its parent, it obviously serves no purpose.

Figure 12.1

A simple example of
inheritance

Vehicle

Truck