Java Reference
In-Depth Information
3.11. Designing a Class to Be Extended
The
Attr
class is an example of a well-designed classit follows the design
private
and accessible only through accessor methods, thereby protect-
ing them from modifications that violate the class's contract. The
Attr
class presents a clean interface to users of the class and at the same
time decouples itself from those classes to allow its own implementation
to change in the future.
Given that
ColorAttr
extends
Attr
, should we have designed
Attr
differ-
ently to make it more suitable for extension? Should the
name
and
value
fields have been
protected
, instead of
private
, so that a subclass could
access them directly? Such decisions require careful thought and con-
sideration of both the benefits and consequences. Making the
Attr
fields
protected would not benefit a subclass because all of the actions that can
be performed on those fields can be done via the
public
methods that
Attr
provides. On the other hand, making the fields
protected
would pre-
vent any future modifications to the implementation of
Attr
because sub-
classes could depend on the existence and type of those fields, as well as
direct access to them. So in this case,
Attr
's current design is suited for
extension as well as for general use.
In our linked-list queue class,
SingleLinkQueue
, we did make the
head
and
tail
fields
protected
. In that case there was a great performance be-
nefit in allowing the subclass to directly access cells of the linked listit
would be impractical to implement the override of
add
in
PriorityQueue
if the only tools available to use were the original
add
and
remove
meth-
ods. The low-level nature of the
SingleLinkQueue
class also means that we
are not concerned about locking in implementation detailsit is after all a
linked-list implementation of a queue and that doesn't really leave much
scope for change. If we had written a more general queue class that just
happened to be using a linked-list implementation, then it would be a dif-
ferent story.
A non-final class has two interfaces. The
public
interface is for program-
mers
using
your class. The
protected
interface is for programmers
ex-
