Oct 2002 91.3913 R McFadyen 1

Recall UML Class Diagram

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:

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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

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Law of Demeter

What changes are needed to the current solution in order to adapt to the changes in the model (see next), and still follow the LoD?

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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..*

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Façade

&

Observer (Publish-subscribe)

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Facade P. 368+

Main Entry: fa·cade Variant(s): also fa·çade /f&-'säd/Function: nounEtymology: French façade, from Italian facciata, from faccia face, from (assumed) Vulgar Latin faciaDate: circa 16811 : the front of a building; also : any face of a building given special architectural treatment <a museum's east facade>2 : a false, superficial, or artificial appearance or effect

From www.m-w.com

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Facade P. 368+

Problem: There are a set of classes, a subsystem, that you need to interact with for some purpose, but you don’t want to create dependencies on this subsystem.

Solution: Create a class that wraps this subsystem. The wrapper will define an interface that hides the details (classes, methods) of the subsystem.

The façade is a “front-end” object that defines a single point of entry to the subsystem’s services.

Showing the classes as a package (a subsytem)

A façade,

a wrapper

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Facade P. 368+

Text example.

NextGen POS is a system to be sold to many customers.

Customers will want to customize NextGen POS

Example: Payment rules may vary for gift certificates. How are gift certificates to be handled:

•One per customer per purchase?

•No change is returned - another gift certificate issued?

To allow flexibility, it is desired to reduce the impact of changes to the rest of the system

To reduce the impact, the Façade pattern is applied

The subsytem will be hidden behind a single object

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Sale objects, in the Domain package, access the POS Rule Engine via the façade in the POSRuleEngine package

if ( POSRuleEngineFacade.getInstance().isInvalid(…) ...

On page 370:Public class Sale …

ASIDE:

on Page 371 - mention of the Singleton pattern

Figure 23.19

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if ( POSRuleEngineFacade.getInstance().isInvalid(…) ...

On page 370:Public class Sale …

ASIDE:

on Page 371 - mention of the Singleton pattern

Singleton is a pattern whereby one instance of an object of some class is created/allowed. The class will have a static method that returns the singleton.

Façades are normally accessed via the Singleton pattern.

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Facade

:Sale :POSRuleEngineFacade

isInvalid (…)

[valid] add (…)

:

makeLineItem()

:SalesLineItem

.

.

.

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Example: (from Design Patterns Explained by Shalloway & Trott)

Suppose a client object must deal with Databases, Models, and Elements. The client must first open the Database, then get a Model, then queries the Model for Elements, and finally gets Elements.

I.e.

Client

Database

Model

Element

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Example: (from Design Patterns Explained by Shalloway & Trott)

It may be easier to create a Database Façade that can be used by clients

I.e.

ClientA

Database

Model

Element

Db FacadeClientB

ClientC

To use the database, one only needs to become aware of DbFacade, and learn how to use it.

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When to use Facade

(from Design Patterns Explained by Shalloway & Trott)

•To create a simpler interface: number of methods, number of objects one has to deal with

•The system being hidden may have an older, more complex interface

•The cost of developing the façade, and of developers learning the new interface may be less than learning the old one

•You may only be needing a subset of the functionality

•You may want to augment the functionality of the system

•To facilitate tracking system usage – by forcing all requests to go through a Façade class, one can easily track the usage.

•To facilitate subsystem replacement – only one class is affected, the Façade class. This is the motivation in Larman’s example.

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Observer P. 368+

Also known as Publish-Subscribe

Applied in order to implement the Model-View Separation principle (see pages 471+)

•model and view are separated

•non-GUI objects are not directly coupled to GUI components

•domain objects are not directly coupled to window objects

•same as Model-View-Controller (MVC) principle that came from Smalltalk

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Observer

Problem: There are many objects (subscribers) needing to know of the state changes, or events, of another object (publisher), and we want to keep the coupling low.

Solution: Define a subscriber or listener interface that is implemented by the subscribers.

Situations:

Text example: A user interface object, a window, needs to be informed when a domain object changes

In some distributed meta-data environments, replicas need to be notified when the source changes

Alarm systems need notification of alarms being triggered

...

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Figure 23.20 the display must reflect the correct total

There is a requirement for a window to be updated whenever the total value of the sale changes

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Observer

Text example.

There is a requirement for a window to be updated whenever the total of the sale changes

A subscriber interface, PropertyListener, is defined.

SaleFrame1 is defined to inherit the PropertyListener interface. This will allow SaleFrame1 to be alerted of changes in the value of the sale total

A reference to the Sale is passed to SaleFrame1 when SaleFrame1 is initialized. This allows SaleFrame1 to subscribe to the Sale instance

The Sale only knows of objects that subscribe to it; it does not know what class of object they are - so, coupling is kept low.

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Figure 23.21 The Observer Pattern in a DCD

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Figure 23.22 a window subscribing

When a SaleFrame1 (the subscriber) is initialized, it subscribes to the Sale (the publisher)

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Figure 23.23 a sale publishing a change

A Sale receives a message changing its state. The Sale invokes its method, publishPropertyEvent, which will in turn notify any subscribers of the change

Note the activations for the sale

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Figure 23.24 a window receiving notification

The window receives notification of the state change and modifies its display appropriately

Notice that this is a continuation from the previous sequence diagram