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More Object-Oriented Design
Simple Program Design
Third Edition
A Step-by-Step Approach
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Objectives
• To introduce the concept of multiple
classes, polymorphism and method
overriding in object-oriented design
• To describe relationships between classes
• To develop an object-oriented solution to
a complex problem
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Object-Oriented Design with
Multiple Classes
• The advantage of object-oriented
programming languages is their usefulness in
constructing large programs
• In designing programs that use multiple
classes, we need to consider not only the
individual class design, but the relationships
between the classes and therefore between
the objects that are instantiated from those
classes
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Notations
• Three popular methods for representing OO
design were developed by James Rumbaugh,
Grady Booch, and Ivar Jacobson
• A fourth notation called UML, standing for Unified
Modeling Langauge, has emerged from the work
of Raumbaugh, Booch, and Jacobson
• The notation in this chapter is based on a
simplified UML standard
• UML allows a designer to represent the
relationship between classes, as well as between
objects
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Relationships Between Classes
• When more than one class is used in a program,
there can be one of three possible relationships
between any two of the classes:
1. The simplest relationship is between two classes that
are independent of each other in the program but
perhaps able to use the services each provides
2. A class may be related to another class by inheritance
3. A class may need another class as part of itself in order
to be able to function at all
• Refer to the figure on page 194 of the textbook for
an example of using UML notation
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Polymorphism
• Polymorphism, meaning many-shaped,
describes the use of methods of the same
name for a variety of purposes
• See the diagram on page 194 of the textbook
as an example of polymorphism
• Method overriding occurs when a parent
class provides a method, but the inheriting
child class defines its own version of that
method
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Programming Example with
Multiple Classes
• Example 12.1 Calculate employee’s pay
• Create a program that will read employee
data from a file and, for each employee,
compute the employee’s pay, which
should then be printed together with the
input data
• Refer to the figures and steps and stages
for the remainder of this example on
pages 195 to 209 of the textbook
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Summary
• Most object-oriented programs need more
than one class
• Classes can be related to each other through
use, by inheritance, or by composition
• When classes are related by inheritance, all
subclasses or child classes inherit the
attributes and methods of the parent class,
and supplement them with attributes and
methods needed by the subtype
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Summary
• Through polymorphism, several methods may
have the same name
• Using method overriding, a child class may
substitute the parent class version of a method
with its own specific version
• There are four steps in analyzing a multiple class
problem:
– identify the objects and the method to be performed
– determine the relationship between the objects
– design the algorithms for the methods using structured design
– develop the mainline algorithm