Mediator
Matt G. Ellis
Overview
► Intent
► Motivation
► Mediators
in GUI applications
► Mediators and Relational Integrity
► Conclusion
► Questions
Intent
► Define
an object that encapsulates how a
set of objects interact. Mediator promotes
loose coupling by keeping objects from
referring to each other explicitly, and it lets
you very their interaction independently
Motivation
► Object
Oriented design encourages
distribution of behavior among objects
► However, Good design is thwarted by every
object referencing every other object
► Changing systems behavior becomes
difficult
► Helps to prevent classes from becoming
“thick”
Mediator versus Façade
► Façade
pattern help refractor FlightPanel
from Oozinoz
► Refactoring can only go so far, complex
applications still might need complex code
even after applying Façade pattern
Mediators at Oozinoz
► Chemicals
for fireworks
kept in tubs
► Robots move most of
the tubs from machine
to machine
► However, humans can
override the system
FlightPanel_1
► Many
methods exist to
lazy-initialize variables
► Rest control event
handling logic
Challenge 1
► Refactor
PlaceATub_1 into two classes, introducing
a new PlaceATubMediator that receives the events
of the GUI
Challenge 1
► Refactor
PlaceATub_1
into two classes,
introducing a new
PlaceATubMediator
that receives the
events of the GUI
Relational Integrity
► If
Object A points to Object B then…
► Object B points to Object A
► A more rigorous definition can be found in
Metsker, page 108
Relational Integrity and Java
► Two
Major Problems
 Objects forget
previous values
 No built in support
for Relational
Integrity
Tub
Machine
T305 StarPress-2402
T308 StarPress-2402
T377 ShellAssembler-2301
T379 ShellAssembler-2301
T389 ShellAssembler-2301
T001 Fuser-2102
T002 Fuser-2102
Model
Tub
Machine
T305 StarPress-2402
T308 StarPress-2402
T377 ShellAssembler-2301
T379 ShellAssembler-2301
T389 ShellAssembler-2301
T001 Fuser-2102
T002 Fuser-2102
Challenge 2
► Suppose
we have this code:
//tell tub about machine, and machine about tub
t.setMachine(m);
m.addTub(t);
► What
happens when t is tub T308 and m is
Fuser-2101?
Challenge 2
Challenge 2
Challenge 2
Challenge 2
► Really
Bad Things…
► Two machines think
they have tub T308 in
them
► This can’t happen in
the real world, why
should it happen at
Oozinoz?
► Mediators can help
Mediators for Relational Integrity
► Pull
all relational information into a mediator
outside both classes
► Have both tubs and machines have a
reference to this mediator
► Use a Map to store these key/value pairs
Mediators for Relational Integrity
► getMachine
is simple, since t is the key of the
map, HashMap makes it easy to get the value.
Mediators for Relational Integrity
► Somewhat
same.
more complex, but the intent is the
Mediators for Relational Integrity
► The
most trivial method of all. Relational Integrity
is maintained by the internal structure of the Map
Challenge 3
► Write
the code for the Tub methods: getMachine()
and setMachine()
Conclusions
► Mediators
provide loose coupling creating a
“pluggable” system
 Changing a mediator can change how
applications deal with events
► Mediators
often found in GUIs
 Swing’s event framework nudges the use of
mediators, but they can be in the same class
► Mediators
also help to provide relational
integrity between objects
Questions