8. Visibility and Class Diagrams

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Visibility and Class Diagrams

|Visibility

and Class

Diagrams

MTI8305 - Software Modeling 2

Design Model: Determining Visibility

� Objectives

� Identify four kinds of visibility

� Design to establish visibility

� Illustrate kinds of visibility in the UML notation

� Visibility between Objects

� Visibility is the ability of one object to see or have reference toanother. The designs created for the system events (enterItem inFigure 11.1) illustrate messages between objects.

� For a sender object to send a message to a receiver object, thesender must be visible to the receiver – the sender must havesome kind of reference or pointer to receiver object.

� When creating a design of interacting objects, it is necessary toensure that the visibility is present to support messageinteraction.

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Visibility between Objects

: RegisterenterItem

(itemID, quantity)

: ProductCatalog

spec := getSpecification( itemID )

{public void enterItem( itemID, qty ){ ... spec = catalog.getSpecification(itemID) ...}}

class Register{ ... private ProductCatalog catalog; ...}

Figure 11.1: Visibility from the Register to ProductCatalog is required.

For example, the getSpecification message sent from a Register to a

ProductCatalog implies that the ProductCatalog instance is visible to

the Register instance as shown in Figure 11.1 below:

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Diagrams


Visibility

� Visibility is the ability to an object to “see” or have a reference to anotherobject.

� It is related to issue of scope: Is one resource (e.g. instance) withinscope of another? Four common ways that visibility can be achieved fromobject A to object B are:� Attribute Visibility – B is an attribute of A.� Parameter Visibility – B is a parameter of a method of A.� Local Visibility – B is a (non-parameter) local object in a method of A.� Global Visibility – B is in some way globally visible.

� The motivation to consider visibility is:� For an object A to send a message to an object B, B must be visible to

A.

� For example, to create an interaction diagram in which a message issent from a Register instance to a ProductCatalog instance, theRegister must have visibility to the ProductCatalog.

� A typical visibility solution is that a reference to the ProductCataloginstance is maintained as an attribute of the Register.

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1. Attribute Visibility

� Attribute visibility from A to B exists when B is an attribute

of A, a common visibility in object-oriented systems.

� It is a relatively permanent visibility because it persists as

long as A and B exist.

� To illustrate, in a Java class definition, a Register instance

may have attribute visibility to a ProductCatalog, since it is

an attribute (Java instance variable) of the Register

public class Register {

…

private ProductCatalog catalog {

….

}

}

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Diagrams


: RegisterenterItem

(itemID, quantity)

: ProductCatalog


{public void enterItem(itemID, qty){ ... spec = catalog.getSpecification(itemID) ...}}

class Register{ ... private ProductCatalog catalog; ...}

Figure 11.2: Attribute visibility

This visibility is required because in the enterItemdiagram in Figure 11.2, a Register needs to send the

getSpecification message to a ProductCatalog

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2. Parameter Visibility

� Parameter visibility from A to B exists when B is passedas a parameter to a method of A and second mostcommon visibility in object-oriented systems afterattribute visibility.

� It is a relatively temporary visibility because it persistsonly within the scope of the method.

� It is common to transform parameter visibility intoattribute visibility.

� To illustrate, when the makeLineItem message is sent to aSale instance, a ProductSpecification instance is passedas a parameter.

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2: makeLineItem(spec, qty)enterItem(id, qty)

2: spec := getSpecification(id)2.1: create(spec, qty)

:Register :Sale

:ProductCatalog

sl : SalesLineItem// initializing method (e.g., a Java constructor){SalesLineItem(ProductSpecification spec, int qty){...productSpec = spec; // parameter to attribute visibility...}}

Figure 11.3: Parameter Visibility

Within the scope of makeLineItem method, the Sale has

parameter visibility to a ProductSpecification as in Figure

11.3.

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3. Local Visibility

� Local visibility from A to B exists when declared as a local object

within a method of A, a third most common visibility after

parameter visibility in object-oriented systems.

� It is relatively temporary visibility because is persists only withinthe scope of the method.

� As with parameter visibility, it is common to transform locallydeclared local visibility into attribute visibility.

� Two common means by which local visibility is achieved are

� Create a new local instance and assign it to a local variable.

� Assign the returning object from a method invocation to a local

variable.

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: RegisterenterItem

(itemID, quantity)

: ProductCatalog


{enterItem(id, qty){...// local visibility via assignment of returning objectProductSpecification spec = catalog.getSpecification(id);...}}

Figure 11.4: Parameter to attribute visibility

An example of second variation (assigning the returningAn example of second variation (assigning the returningobject to a local variable) can be found in the enterItemmethod of class Register as shown Figure 11.4.

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4. Global Visibility

� Global visibility from A to B exists when B is global to A. It is arelatively permanent visibility because it persists as long as Aand B exist, a least common visibility in object-orientedsystems.

� One way to achieve visibility is to assign an instance to aglobal variable possible in C++.

� The preferred method to achieve global visibility is to use theSingleton pattern. In software engineering, the singletonpattern is a design pattern that is used to restrict instantiationof a class to one object.

� This is useful when exactly one object is needed to coordinateactions across the system. Sometimes it is generalized tosystems that operate more efficiently when only one or a fewobjects exist.

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:A :B1: msg()

:C2: msg()

:D3: msg()

«association»

«parameter»

«local»

:E4: msg()

«global»

«association» is used forattribute visibility

Figure 11.5: Sample design class diagram

Illustration of visibility in UML notation to show the kind of visibility

in a collaboration diagram as shown in Figure 11.5.

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MTI8305 - Software Modeling13

Design Class Diagrams (DCD)

� Objectives

� Create design class diagrams(DCDs).

� Identify the classes, methods andassociations to show in a DCD.

� Creation of DCDs

� DCDs are usually created inparallel with interaction diagrams

� Example DCD

� Figure 11.6 illustrates a partialsoftware definition of the Registerand Sale classes.

� Besides to basic associations andattributes, the diagram is extendedto illustrate, for example, themethods of each class, attributetype information and attributevisibility and navigation betweenobjects.

Register

enterItem(...)

Sale

dateisComplete : Booleantime

makeLineItem(...)

Captures

Navigability

1 1

Three section box forclass definition.

methods; there are parameters, but unspecified type information

Figure 11.6: Sample design class diagram

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DCD and UP Terminology

� A design class diagram (DCD) illustrates the specifications forsoftware classes and interfaces (e.g. Java are related methodswith empty bodies) in an application.

� Typical information includes:� Classes, associations and attributes� Interfaces with their operations and constants� Methods� Attribute type information� Navigability� Dependencies

� In contrast to conceptual classes in the Domain Model, designclasses in the DCDs show definitions for software classes ratherthan real-world concepts.

� In UP it is common to speak of “design class diagrams”, that isshorter and implies “class diagrams in the Design Model”.

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Domain Model vs. Design Model Classes

� To reiterate, in UP Domain Model, a Sale does not represent a software definition; it is an abstraction of a real-world concept which we are interested in making a statement.

� By contrast, DCDs express for the software application - the definition of classes as software components.

� In these diagrams, a Salerepresents of software class in Figure 11.7.

Register

...

endSale()enterItem(...)makePayment(...)

Sale


makeLineItem(...)

Captures

RegisterSale


Captures

software class

1 1

11Domain Model

Design Model

Concept; conceptual class

Figure 11.7: Domain model vs.

Design Model Classes

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Creating a POS DCD

� The first step in creation of DCDs as part of the solution model is to identifythose classes that participate in the software solution.

� These can be found by scanning all the interaction diagrams and listing theclasses mentioned.

� For the POS application these are: Register, Sale, ProductCatalog,ProductSpecification, Store, SalesLineItem and Payment.

� The next step is to draw a class diagram for these classes and include theattributes previously identified in the Domain Model that are also used in thedesign as Figure 11.8.

R e g i s t e r

. . .

. . .

S a l e

d a t ei s C o m p l e t et i m e

. . .

S a l e s L i n e I t e m

q u a n t i t y

. . .

P r o d u c t C a t a l o g

. . .

P r o d u c t S p e c i f i c a t i o n

d e s c r i p t i o np r i c ei t e m I D

. . .

S t o r e

a d d r e s sn a m e

. . .

P a y m e n t

a m o u n t

. . .

Figure 11.8: Software classes in the application

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Adding Method Names

� Method names of each class can be identified by analyzing the interaction diagrams.

� For example, if message makeLineItem is sent to an instance of class Sale, then class Sale must define a makeLineItemmethod as in Figure 11.9.

� Generally, set of all messages sent to a class X across all interaction diagrams indicates the majority of methods that class X must define.

:Register :Sale2: makeLineItem(spec, qty)

Sale

...

makeLineItem(...)

Figure 11.9: Method names from

interaction diagrams

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Diagrams


Adding Method Names cont’d

� The inspection of POS yields allocation of methods in Figure 11.10.

� Method names issues include:

� Interpretation of the create message

� Depiction of accessing methods

� Interpretation of messages to multi-objects

� Language-dependentsyntax

SalesLineItem

quantity

getSubtotal()

ProductCatalog

...

getSpecification()

ProductSpecification

descriptionpriceitemID

...

Store

addressname

addSale(...)

Payment

amount

...

Register

...

endSale()enterItem(...)makeNewSale()makePayment(...) Sale

dateisCompletetime

becomeComplete()makeLineItem(...)makePayment(...)getTotal()

Figure 11.10: Methods in the application

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Adding More Type Information

� The types of attributes, method parameters andmethod return values may all be optionally be shown.

� The DCD should be created by considering theaudience:

� If it is being created in a CASE tool with automaticcode generation, full and exhaustive details arenecessary.

� If it is being created for software developers to read,exhaustive low-level detail may adversely affectnoise-to-value ratio.

� The amount of information shown depends on theintended audience as in Figure 11.11.

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Diagrams


Adding More Information

SalesLineItem

quantity : Integer

getSubtotal() : Money

ProductCatalog

...

getSpecification(id: ItemID) : ProductSpecification


description : Textprice : MoneyitemID : ItemID

...

Store

address : Addressname : Text

addSale(s : Sale)

Payment

amount : Money

...

Register

...

endSale()enterItem(id : ItemID, qty : Integer)makeNewSale()makePayment(cashTendered : Money)

Sale

date : DateisComplete : Booleantime : Time

becomeComplete()makeLineItem(spec : ProdSpecification , qty : Integer)makePayment(cashTendered : Money)getTotal() : Money

Return type of method void; no return value

Figure 11.11: Adding Type Information.

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Adding Associations and Navigability

� Each end of an association is called a role, and in the DCDs the role may decorated with navigability arrow.

� Navigability is a property of role that indicates that it is possible to navigate uni-directionally across association from objects of source target class.

� Navigability implies visibility – usually attribute visibility as in Figure 11.12.

Captures

Register

currentSale : Sale

endSale()enterItem(...)makeNewSale()makePayment(...)

Sale

dateisCompletetime


Navigability arrow indicatesRegister objects are connecteduni-directionally to Saleobjects.

Absence of navigabilityarrow indicates noconnection from Sale toRegister.

Register class willhave an attribute pointing to aSale object.

1

1

the currentSaleattribute is oftenexcluded, as it isimplied by thenavigableassociation fromRegister to Sale.

Figure 11.12: Showing navigability

or attribute visibility

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MTI8305 - Software Modeling22

Associations and Navigability

� The required visibility and associations between classes are indicated by interaction diagrams, with common situations suggesting an association with a navigability adornment from A to B:

� A sends a message to B.

� A creates an instance of B.

� A needs to maintain a connection to B.

� Example is from interaction diagram in Figure 11.13 starting with the create message to a Store.

:Store :Register

pc:ProductCatalog

create() 2: create(pc)

1: create()

1.2: loadProdSpecs()

:ProductSpecification

1.1: create()

1.2.2*: add(ps)

1.2.1*: create(id, price, description)

ps:ProductSpecification

Figure 11.13: Navigability is identified

from interaction diagrams

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Navigability and Dependency Relationships

� Navigability

� Starting from create message to a Store and theinteraction diagram in Figure 11.13, Store has connectionto Register and ProductCatalog instances it created.

� Further the ProductCatalog needs an ongoing connectionto the ProductSpecifications it created.

� Based in above criterion for associations and navigability,analysis of POS application yields class diagram 11.14,shown by the arrows.

� Adding Dependency Relationships

� UML includes a general dependency relationship whichindicates that one element has knowledge of anotherelement.

� It is illustrated by the dashed arrow lines.

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SalesLineItem

quantity : Integer

getSubtotal()

ProductCatalog

...

getSpecification(...)


description : Textprice : MoneyitemID: ItemID

...

Store

address : Addressname : Text

addSale(...)

Payment

amount : Money

...

Contains

1..*

Contains1..*

Register

...

endSale()enterItem(...)makeNewSale()makePayment(...)

Sale

date : DateisComplete : Booleantime : Time


Captures

Houses

Uses

Looks-in

Paid-by

Describes

1 1

1

1 1

1

11

1

1

1

1

1

*

A dependency of Register knowing aboutProductSpecification.

Recommended when there is parameter,global or locally declared visibility.

Logs-completed4 *

1

Figure 11.14: Association with navigability adornment and dependency relationships indicating non-attribute

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Exercises

� 1. (Group) Information system for a Library

� Using the confirmMembership operation contract as a starting hint,complete the UML collaboration diagram. Annotate every messagewith the hint GRASP (Expert, Creator, and so on) and/or otherpattern that justifies it. If you add responsibilities not explicit in thecontract (because you think they are important to fulfill), pleasebriefly explain these additions.

confirmMembership(membershipID)

by Controller

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Exercises

• 2. (Individual) Information system for a Car Rental Centre

• Using the confirmMembership operation contract as a starting hint,complete the UML collaboration diagram. Annotate every message withthe hint GRASP (Expert, Creator, and so on) and/or other pattern thatjustifies it. If you add responsibilities not explicit in the contract(because you think they are important to fulfill), please briefly explainthese additions.

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8. Visibility and Class Diagrams

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