PRJ566: PROJECT PLANNING AND MANAGEMENT Class Diagrams.

PRJ566: PROJECT PLANNING AND MANAGEMENT

Class Diagrams

Class Design Analysis objects are things in the real

world, e.g invoice, registration form, sales person

Design objects are logical constructs which may map to objects in the real world but which find their existence only in code

Transformation Is the process of turning real world domain

abstraction into an object in the model.

Every significant object in the domain analysis is a candidate for being transformed into one or more design objects (Noun Filter)

Class Diagrams

A class diagram shows the existence of classes and their relationships in the logical view of a system

UML modeling elements in class diagrams Classes and their structure and behavior Association, aggregation, and inheritance

relationships Multiplicity and navigation indicators

Copyright © 1997 by Rational Software Corporation

Relationships Relationships provide a pathway for communication

between objects Sequence and/or collaboration diagrams are

examined to determine what links between objects need to exist to accomplish the behavior -- if two objects need to “talk” there must be a link between them

Types of relationships are: Association Aggregation


Relationships An association is a bi-directional connection

between classes An association is shown as a line connecting the related

classes An aggregation is a stronger form of relationship

where the relationship is between a whole and its parts An aggregation is shown as a line connecting the related

classes with a diamond next to the class representing the whole


Registration Manager

Math 101: Course

3: add student(joe)

RegistrationManager

Course

Finding Association Relationships

Relationships are discovered by examining interaction diagrams If two objects must “talk” there must be a

pathway for communication


Multiplicity and Navigation

Multiplicity defines how many objects participate in a relationships Multiplicity is the number of instances of one

class related to ONE instance of the other class For each association and aggregation, there are

two multiplicity decisions to make: one for each end of the relationship

Aggregations

An aggregation is a special kind of association the composition of a set of parts—a whole-

part relationship a “has a” or “consists of” relationship

Aggregations

Examples: An invoice has invoice items A company has contacts

Aggregate Class

Component Class

Aggregation Symbol

Invoice

InvoiceItem1..n

1

Company

ContactPerson

1..n

11

1..n1..n

1

Proxy Is an object in the design which represents

key aspects of a real world entity Also called a surrogate

First create proxy for all the actors in the system, e.g. customer, guest

Then for many of the real world objects, e.g, invoice, receipt, reservation

AggregationsEssential property The aggregate (Invoice) acts as a proxy for its

parts (InvoiceItem) This means the aggregate takes on operations

which are then propagated to the individual parts. (These operations don’t necessarily cause a change in the aggregate itself.)

Here adding a ContactPerson does not change the proxy (Company).

: UCControl : Company : ContactPerson

1: addCompanyContact(companyID, contactInfo)

2: addContact(contactInfo)

Here adding an InvoiceItem does change the proxy (Invoice). Its totals are updated.

: UCControl : Invoice : InvoiceItem

1: addToInvoice(invoiceID, itemID, qty, price)

2: add(itemID, qty, price)

3: updateTotals()

AggregationsAggregation vs Association In contrast to the association, the involved

classes do not maintain an equal relationship but the aggregate class assumes a special role for delegation of responsibility and leadership

Multiplicity in Aggregation Generally a 1:M relationship

The aggregate will be the 1 The parts will be the many 1…n

Aggregation Can describe a relationship where the parts

are existence-dependent on the whole. If the whole is deleted, the individual parts will

be deleted. If an individual part is deleted, the aggregate will

survive.

CompositionComposition is a strict form of aggregation. The life of the parts is subordinate to the

whole. The parts are generated together with the

aggregate or later. Parts are destroyed before the whole

Composition The component parts depend on the aggregate

for existence—delete the aggregate and the components disappear (this is called a cascading delete)

CompositionWhich of these might be composition? An invoice has invoice items A company has contacts A catalogue has catalogue items An MSWord document style has a font, a

language, a paragraph control, and so on…

When we delete Invoice, does InvoiceItem disappear? Yes! It has no meaning without Invoice.

To indicate a composition, the diamond is coloured black.

Invoice

InvoiceItem

1..n

1

1..n

1

When we delete Company, does ContactPerson disappear? Yes! It has no meaning without Company.

ContactPerson

Company

1..n

1

1..n

1

When about Catalogue and CatalogueItem?

If CatalogueItem belongs to only one catalogue and it is the intersection between that Catalogue and an Item then yes, it would disappear if Catalogue were deleted and we would have a composition relationship. But note the Item itself would not disappear!

Catalog

ItemCatalogItem

1..n

1

1..n

1

0..n 10..n 1

This sequence diagram shows the proxy relationship of Catalogue to CatalogueItem

: CatalogControl

: Catalog : CatalogItem : Item

1: initiateGetItem()

2: getItem()

4: initiateAddItem(catID, itemInfo)

3: *item list

5: addCatItem (catID, itemInfo)

6: addCatItem

This sequence diagram shows an example of the cascading delete that identifies Composition.

: CatalogControl

: Catalog : CatalogItem

1: initiateDeleteCat(catID)

2: deleteCat(catID)

AggregationsAggregation vs Association Useful in modeling—an aggregation shows

where there is higher binding between classes which in turn makes models easier to understand

Aggregation helps determine implementation (e.g. arrays, etc.)

When in doubt use an association


Inheritance

Inheritance is a relationships between a base class and its derived classes

There are two ways to find inheritance: Generalization Specialization

Common attributes, operations, and/or relationships are shown at the highest applicable level in the hierarchy

Inheritance Generalization

Identify similar structure and behaviour among classes

Specialization Identify a parent class--how can it be

“specialized” for use in other parts of the system?

Inheritance Inheritance is a “generalization” relationship The base class (parent class, super class)

carries a certain set of characteristics (attributes, operations, relationships)

The derived classes (child classes, sub classes) inherit those characteristics plus they have some of their own

Inheritance A child class “is a” special type of the more

general parent class E.g.

“A video is a type of library item” “A part-time student is a type of student” “A reserve item screen is a type of library

screen”

Why Inheritance?Why do we care about inheritance? Less duplication More reusability More standardization Less change impact Classes are more focused

Inheritance Structures

showing inheritance are called Inheritance Hierarchies

LibraryItem

itemIDitemTitleitemDescription

addItem()getItem()retireItem()

LibraryBook

ISBNauthorpublisher

LibraryCD

artistrecordingCompanynumberOfCDsnumberOfTracks

LibraryVideo

productionCompanynumberOfVideosvideoLength

How do you find inheritance? Look for classes that share many operations,

attributes (But be careful! Don’t create false inheritance!)

Look for actions or text (i.e. in specs) that imply that there are several types of objects in a class--investigate further.


Inheritance

GeneralizationThe capability to create base classes that

encapsulate structure and behaviour

common to several classes.

InheritanceGeneralization: Here we see similar

structure and behaviour among

classes

LibraryItem



LibraryBook

ISBNauthorpublisher

LibraryCD


LibraryVideo


InheritanceGeneralization: Now we create a parent class

LibraryItem

InheritanceGeneralization: Now we move all common

elements from children to parent and we draw the inheritance relationship

LibraryItem



LibraryBook

ISBNauthorpublisher

LibraryCD


LibraryVideo



Inheritance

SpecializationThe ability to create derived classes that represent

refinements to the base class—typically structure and behaviour are added to the new derived class.

Inheritance

In Rational Rose you need to set Parent attributes as PROTECTED instead of private

Inheritance

You will see the inherited operations and attributes only in the specification window not in class diagrams

Inheritance A critical aspect of public inheritance is that

it should model specialization/ generalization and nothing else.

Base Classes and Derived Classes Derived classes must know who their base

class is, and they depend on their base class.

Base classes should know nothing about their derived classes.

Class Design Build the inheritance hierarchy Map use cases to object interaction Delegate every responsibility of the system

to a specific object Fully understand the collaboration between

objects Create a blueprint for your implementation

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PRJ566: PROJECT PLANNING AND MANAGEMENT Class Diagrams.

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