February 2005 92.3913 R. McFadyen 1

Polymorphism

Indirection

Pure Fabrication

Protected Variations (Law of Demeter)

More GRASP Patterns

February 2005 92.3913 R. McFadyen 2

Protected Variations

Problem: How do we design systems so that changes in its elements do not have an unfavourable impact on other elements?

Solution: Identify points of predicted variation/instability and assign responsibilities to create a stable interface around them

Example:

Law of Demeter (LoD)

Special case of this pattern.

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

February 2005 92.3913 R. McFadyen 3

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.”

February 2005 92.3913 R. McFadyen 4

Law of Demeter

FRIENDS

February 2005 92.3913 R. McFadyen 5

Don’t Talk to Strangers

PaymentRegister Sale

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

add a method to get a payment

Suppose Register needs to find out the amount of the payment

February 2005 92.3913 R. McFadyen 6

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()

}

A little different from the text’s example

February 2005 92.3913 R. McFadyen 7

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()

February 2005 92.3913 R. McFadyen 8

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 - see Parnas’ concept of information hiding on

Objects are only sending messages to their friends

getTenderedAmount()

getPayment()

February 2005 92.3913 R. McFadyen 9

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

February 2005 92.3913 R. McFadyen 10

Example: Applying LoD as system changes

BusRoute BusStopList

BusStopBusList

Bus PersonList

Person

passengers

buses

busStops

waiting

0..*

0..*

0..*

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BusRoute BusStopList

BusStopBusList

Bus PersonList

Person

passengers

buses

busStops

waiting

0..*

0..*

0..*

Find all persons waiting at any bus stop on a bus route

Collaborating classes:

February 2005 92.3913 R. McFadyen 12

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

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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

February 2005 92.3913 R. McFadyen 14

BusRoute BusStopList

BusStopBusList

Bus PersonList

Person

passengers

busesbusStops

waiting

0..*

0..*

0..*

Suppose the class model is modified to incorporate Villages.

VillageList

Village

villages

0..*

What software changes are needed and still adhere to LoD?