Java Reference
In-Depth Information
LoggingApptCalendar lapptc = new LoggingApptCalendar();
lapptc.addAppts(new Appt[] {new Appt(), new Appt(), new
Appt()});
If you also add a
System.out.println(msg);
method call to
Logger
's
log(String msg)
method,tooutputthismethod'sargumenttostandardoutput,you
will discover that
log()
outputs a total of six messages; each of the expected three
messages (one per
Appt
object) is duplicated.
When
LoggingApptCalendar
's
addAppts()
method is called, it first calls
Logger.log()
for each
Appt
instance in the
appts
array that is passed to
ad-
dAppts()
. This method then calls
ApptCalendar
's
addAppts()
method via
super.addAppts(appts);
.
ApptCalendar
's
addAppts()
methodcalls
LoggingApptCalendar
'sover-
riding
addAppt()
method for each
Appt
instance in its
appts
array argument.
addAppt()
executes
Logger.log(appt.toString());
tologits
appt
argu-
ment's string representation, and you end up with three additional logged messages.
If you did not override the
addAppts()
method, this problem would go away.
However, the subclass would be tied to an implementation detail:
ApptCalendar
's
addAppts()
method calls
addAppt()
.
Itisnotagoodideatorelyonanimplementationdetailwhenthedetailisnotdocu-
mented. (I previously stated that you do not have access to
ApptCalendar
's source
code.) When a detail is not documented, it can change in a new version of the class.
Becauseabaseclasschangecanbreakasubclass,thisproblemisknownasthe
fragile
base class problem
.Arelatedcauseoffragilitythatalsohastodowithoverridingmeth-
ods occurs when new methods are added to a superclass in a subsequent release.
Forexample,supposeanewversionofthelibraryintroducesanew
public void
addAppt(Appt appt, boolean unique)
method into the
ApptCalendar
class.Thismethodaddsthe
appt
instancetothecalendarwhen
unique
isfalse,and,
when
unique
istrue,itaddsthe
appt
instanceonlyifithasnotpreviouslybeenad-
ded.
Becausethismethodhasbeenaddedafterthe
LoggingApptCalendar
classwas
created,
LoggingApptCalendar
does not override the new
addAppt()
method
with a call to
Logger.log()
. As a result,
Appt
instances passed to the new
ad-
dAppt()
method are not logged.
Hereisanotherproblem:Youintroduceamethodintothesubclassthatisnotalsoin
thesuperclass.Anewversionofthesuperclasspresentsanewmethodthatmatchesthe
