1. March 200692.3913 R. McFadyen1 Principle of Least Knowledge – page 265 Principle: talk only to your immediate friends Also called Law of Demeter (LoD) If objects traverse long object structure paths and send messages to distant, indirect (stranger) objects, the system is fragile with respect to changes in the object structures - a common point of instability in systems. LoD helps us avoid creating such designs See page 267 in Head First Patterns

2. March 200692.3913 R. McFadyen2 Law of Demeter Also called Don’t Talk to Strangers Each class should only use a limited set of other classes: only units “closely” related to the current unit. “Each class should only talk (send messages) to its friends.” “Don’t talk to strangers.”

3. March 200692.3913 R. McFadyen3 Law of Demeter FRIENDS

4. March 200692.3913 R. McFadyen4 Don’t Talk to Strangers PaymentRegisterSale getTenderedAmount() paymentAmount() endSale() enterItem() makePayment()... becomeComplete() makeLineItem() makePayment() getTotal() getPayment... The class diagram shows that Register knows about Sale, and Sale knows about Payments that have been made towards it a method to get a payment Suppose Register needs to find out the amount of the payment

5. March 200692.3913 R. McFadyen5 Don’t Talk to Strangers Assume: Register has a paymentAmount method which returns the current amount tendered for the payment Sale has a method, getPayment, which returns the Payment instance associated with the Sale Consider: In order to return the payment amount, we could have a paymentAmount method in Register such as: public void paymentAmount() { Payment payment = sale.getPayment() Money amount = payment. getTenderedAmount () }

6. March 200692.3913 R. McFadyen6 Don’t Talk to Strangers :Payment :Register:Sale The previous has messages: Register will have a dependency on Payment This increases the coupling in our system getPayment() getTenderedAmount ()

7. March 200692.3913 R. McFadyen7 Don’t Talk to Strangers :Payment :Register:Sale If getPayment() in Sale would invoke getTenderedAmount() in Payment, and return the payment amount, then we can de- couple Register from Payment make the solution more robust, less sensitive to changes, less coupling Register will get the payment amount it is after, but it won’t know how it was obtained Objects are only sending messages to their friends getTenderedAmount () getPayment()

8. March 200692.3913 R. McFadyen8 Law of Demeter presentation: www.ccs.neu.edu/research/demeter/talks/frameworks/ubs/LoD.ppt Karl J. Lieberherr; Northeastern University other resources www.ccs.neu.edu/research/demeter/ www.ccs.neu.edu/home/lieber/LoD.html Article on Information hiding www.computer.org/certification/beta/McConnell_Missing.html Aside – if you have an interest

9. March 200692.3913 R. McFadyen9 Example: Applying LoD as system changes BusRoute BusStopList BusStop BusList BusPersonList Person passengers buses busStops waiting 0..*

10. March 200692.3913 R. McFadyen10 BusRoute BusStopList BusStop BusList BusPersonList Person passengers buses busStops waiting 0..* Find all persons waiting at any bus stop on a bus route Collaborating classes:

11. March 200692.3913 R. McFadyen11 class BusRoute { BusStopList busstops; void printWaitingPassengers () { busstops->printWaitingPassengers (); } } class BusStopList { BusStop stops[]; void printWaitingPassengers () { for (int i = 0; i < stops.length; i++) stops[i].printWaitingPassengers (); } Applying Law of Demeter - Partial Java Solution

12. March 200692.3913 R. McFadyen12 class BusStop { PersonList waiting; void printWaitingPassengers () { waiting.print (); } } class PersonList { Person people[]; void print () { for (int i = 0; i < people.length; i++) people[i].print (); } } class Person { String name; void print () { System.stdout.println (name); } } Applying Law of Demeter - Partial Java Solution

13. March 200692.3913 R. McFadyen13 BusRoute BusStopList BusStop BusList BusPersonList Person passengers buses busStops waiting 0..* Suppose the class model is modified to incorporate Villages. VillageList Village villages 0..* What software changes are needed and still adhere to LoD?