Java Reference
In-Depth Information
Introducing Responsibility (Chapter 7)
SOLUTION 7.1
Some problems with the given diagram follow:
•
The
Rocket.thrust()
method returns a
Rocket
instead of a number or a physical
quantity.
•
The
LiquidRocket
class has a
getLocation()
method, although nothing in the
diagram or in the problem domain suggests that we model rockets as having a
location. Even if we did, there is no reason for liquid-fueled rockets to have a location,
whereas other
Rocket
objects do not.
•
The
isLiquid()
method may be an acceptable alternative to using the
instanceof
operator, but then we'd expect the superclass to also have an
isLiquid()
method that would return
false
.
•
CheapRockets
is plural, although class names are conventionally singular.
•
We could model cheapness with attributes alone, so there is no justification for
creating a class just for cheap rockets. The
CheapRockets
class also introduces a
factoring that conflicts with factoring the rocket model as liquid or solid. For example,
it is not clear how to model a cheap liquid rocket.
•
The
CheapRockets
class implements
Runnable
, although this interface has
nothing to do with cheap rocket objects from the problem domain.
•
The model shows that
Firework
is a subclass of
LiquidRocket
, declaring that all
fireworks are liquid rockets, which is false.
•
The model shows a direct relation between reservations and types of firework,
although no such relation exists in the problem domain.
•
The
Reservation
class has its own copy of
city
, which it should get by delegation
to a
Location
object.
•
A
CheapRocket
is composed of
Runnable
objects, which is simply bizarre.
SOLUTION 7.2
The value of this challenge is not to get the right answer but rather to exercise your thinking
about what makes up a good class. Consider whether your definition addresses the following
points.
•
A nuts-and-bolts description of a class is, "A class is a named collection of fields that
hold data values and methods that operate on those values" (Flanagan 1999b, p. 61).
•
A class establishes a collection of fields: that is, it defines the attributes of an object.
The attribute types are other classes, primitive data types, such as
boolean
and
int
,
or interfaces.
•
A class designer should be able to justify how a class's attributes are related.
•
The name of a class should reflect the meaning of the class both as a collection of
attributes and with respect to the class's behavior.
•
A class must support all the behaviors it defines, as well as all those in superclasses,
and all methods in interfaces that the class implements. (A decision to
not
support
a superclass or an interface method is occasionally justifiable.)
•
A class should have a justifiable relationship to its superclass.
