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c)
A
Car
class has an
is-a
relationship with the
SteeringWheel
and
Brakes
classes.
d)
When a subclass redefines a superclass method by using the same signature, the subclass
is said to overload that superclass method.
Answers to Self-Review Exercises
9.1
a) Inheritance. b)
public
and
protected
. c)
is-a
or inheritance. d)
has-a
or composition.
e) hierarchical. f)
public
. g) constructor. h)
super
.
9.2
a) True. b) False. A
has-a
relationship is implemented via composition. An
is-a
relationship
is implemented via inheritance. c) False. This is an example of a
has-a
relationship. Class
Car
has an
is-a
relationship with class
Vehicle
. d) False. This is known as overriding, not overloading—an
overloaded method has the same name, but a different signature.
Exercises
9.3
(Using Composition Rather Than Inheritance)
Many programs written with inheritance
could be written with composition instead, and vice versa. Rewrite class
BasePlusCommissionEm-
ployee
(Fig.9.11) of the
CommissionEmployee
-
BasePlusCommissionEmployee
hierarchy to use
composition rather than inheritance.
9.4
(Software Reuse)
Discuss the ways in which inheritance promotes software reuse, saves time
during program development and helps prevent errors.
9.5
(
Student
Inheritance Hierarchy)
Draw an inheritance hierarchy for students at a university
similar to the hierarchy shown in Fig. 9.2. Use
Student
as the superclass of the hierarchy, then ex-
tend
Student
with classes
UndergraduateStudent
and
GraduateStudent
. Continue to extend the hi-
erarchy as deep (i.e., as many levels) as possible. For example,
Freshman
,
Sophomore
,
Junior
and
Senior
might extend
UndergraduateStudent
, and
DoctoralStudent
and
MastersStudent
might be
subclasses of
GraduateStudent
. After drawing the hierarchy, discuss the relationships that exist be-
tween the classes. [
Note:
You do not need to write any code for this exercise.]
9.6
(
Shape
Inheritance Hierarchy)
The world of shapes is much richer than the shapes included
in the inheritance hierarchy of Fig. 9.3. Write down all the shapes you can think of—both two-di-
mensional and three-dimensional—and form them into a more complete
Shape
hierarchy with as
many levels as possible. Your hierarchy should have class
Shape
at the top. Classes
TwoDimension-
alShape
and
ThreeDimensionalShape
should extend
Shape
. Add additional subclasses, such as
Quad-
rilateral
and
Sphere
, at their correct locations in the hierarchy as necessary.
9.7
(
protected
vs.
private
)
Some programmers prefer not to use
protected
access, because
they believe it breaks the encapsulation of the superclass. Discuss the relative merits of using
pro-
tected
access vs. using
private
access in superclasses.
9.8
(
Quadrilateral
Inheritance Hierarchy)
Write an inheritance hierarchy for classes
Quadri-
lateral
,
Trapezoid
,
Parallelogram
,
Rectangle
and
Square
. Use
Quadrilateral
as the superclass
of the hierarchy. Create and use a
Point
class to represent the points in each shape. Make the hier-
archy as deep (i.e., as many levels) as possible. Specify the instance variables and methods for each
class. The
private
instance variables of
Quadrilateral
should be the
x-y
coordinate pairs for the
four endpoints of the
Quadrilateral
. Write a program that instantiates objects of your classes and
outputs each object's area (except
Quadrilateral
).
9.9
(What Does Each Code Snippet Do?)
a)
Assume that the following method call is located in an overridden
earnings
method in
a subclass:
super
.earnings()
