Chapter 5 System Modeling Sommerville, "software engineering ", 9 th Ed., 2011

Chapter 5System Modeling

Sommerville, "software engineering " , 9th Ed., 2011

Structural models

Structural models of software display the organization of a system in terms of the components that make up that system and their relationships.

Structural models may be static models, which show the structure of the system design, or dynamic models, which show the organization of the system when it is executing.

You create structural models of a system when you are discussing and designing the system architecture.

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Class diagrams

Class diagrams are used when developing an object-oriented system model to show the classes in a system and the associations between these classes.

An object class can be thought of as a general definition of one kind of system object.

An association is a link between classes that indicates that there is some relationship between these classes.

When you are developing models during the early stages of the software engineering process, objects represent something in the real world, such as a patient, a prescription, doctor, etc.


Class diagram

In software engineering, a class diagram in UML is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among the classes.

Chapter 5 System modeling 4

The Consultation class


Object class aggregation models

An aggregation model shows how classes that are collections are composed of other classes.

Aggregation models are similar to the part-of relationship in semantic data models.


The aggregation association


Essential Elements of Class Diagram

ClassAttributesOperationsRelationshipsConstraint Rules and Notes

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Class diagram

Classes are composed of three things: a name, attributes, and operations. Below is an example of a class.

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Class attributes

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• attributes (fields, instance variables)– visibility name : type [count] = default_value

– visibility:+ public# protected- private~ package (default)/ derived

– underline static attributes

– derived attribute: not stored, but can be computed from other attribute values

– attribute example:- balance : double = 0.00

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Class operations / methods• operations / methods

– visibility name (parameters) : return_type

– visibility: + public# protected- private~ package (default)

– underline static methods– parameter types listed as (name: type)– omit return_type on constructors and

when return type is void

– method example:+ distance(p1: Point, p2: Point): double

Comments

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• represented as a folded note, attached to the appropriate class/method/etc by a dashed line

Classes and objects : summary

Objects are instances of classes.

Classes define the behaviour and the data types for an object

Data held in instances of the same class will typically vary.

Instances of the same class provide exactly the same services (subject to the data not controlling the flow).

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Relationships

In UML, object interconnections are modeled as relationships.

The kinds of relationships in UML: Generalizations Associations Aggregation Composition

Class Diagram Relationships

Class

Generalization

Object

Association

Aggregation

Composition

Class Diagram Relationships

o Generalization (Class-to-Class) (Superclass/Subclass)

• Inheritance

• Example: Person - FacultyPerson, StudentPerson, Staff...

• Example: ModesOfTravel - Airplane, Train, Auto, Cycle, Boat...

o [Object] Associations• FacultyInformation - CourseInformation

• StudentInformation - CourseInformation

o [Object] Aggregations & Composition (Whole-Part)

• Assembly - Parts

• Group - Members

• Container - Contents

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Multiplicities

Class

Class

Class

Class

1

0*..

0..1

m..n

exactly one

many(zero or more)

optional(zero or one)

numericallyspecified

Course CourseOffering1

0*..

Example:

Generalization Relationships

A generalization is used when two classes are similar, but have some differences. Look at the generalization below:

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Generalization is an everyday technique that we use to manage complexity.

Rather than learn the detailed characteristics of every entity that we experience, we place these entities in more general classes (animals, cars, houses, etc.) and learn the characteristics of these classes.

This allows us to infer that different members of these classes have some common characteristics e.g. squirrels and rats are rodents.



In modeling systems, it is often useful to examine the classes in a system to see if there is scope for generalization. If changes are proposed, then you do not have to look at all classes in the system to see if they are affected by the change.

In object-oriented languages, such as Java, generalization is implemented using the class inheritance mechanisms built into the language.

In a generalization, the attributes and operations associated with higher-level classes are also associated with the lower-level classes.

The lower-level classes are subclasses inherit the attributes and operations from their superclasses. These lower-level classes then add more specific attributes and operations.


A generalization hierarchy


A generalization hierarchy with added detail



A sub-class inherits from its super-class Attributes Operations Relationships

A sub-class may Add attributes and operations Add relationships Refine (override) inherited operations

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Association Relationships

If two classes in a model need to communicate with each other, there must be link between them.

An association denotes that link.

InstructorStudent

Associations

o A semantic relationship between two or more classes that specifies connections among their instances.

o A structural relationship, specifying that objects of one class are connected to objects of a second (possibly the same) class.

o Example: “An Employee works for a Company”

CompanyDepartmentEmployee

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Associations (cont.)

An association between two classes indicates that objects at one end of an association “recognize” objects at the other end and may send messages to them. This property will help us discover less trivial associations

using interaction diagrams.

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Association Relationships (Cont’d)

We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association.

The example indicates that a Student has one or more Instructors:

InstructorStudent1..*


The example indicates that every Instructor has one or more Students:

InstructorStudent1..*


We can also indicate the behavior of an object in an association (i.e., the role of an object) using rolenames.

InstructorStudent1..*1..*

learns fromteaches


We can also name the association.

TeamStudentmembership

1..* 1..*


We can specify dual associations.

TeamStudent

member of

1..*

president of1 1..*

1..*


We can constrain the association relationship by defining the navigability of the association. Here, a Router object requests services from a DNS object by sending messages to (invoking the operations of) the server. The direction of

the association indicates that the server has no knowledge of the Router.

Router DomainNameServer


A class can have a self association.

LinkedListNode

next

previous

Course

0*..

0*..

has pre-requisite of

is pre-requisite for


•We can model objects that contain other objects by way of special associations called aggregations and compositions.

•An aggregation specifies a whole-part relationship between an aggregate (a whole) and a constituent part.

•The part can exist independently from the aggregate. Aggregations are denoted by a hollow-diamond adornment on the association.

Car

Engine

Transmission

Aggregation

A special form of association that models a whole-part relationship between an aggregate (the whole) and its parts. Models a “is a part-part of” relationship.

Whole Part

Car Door House1..*2..*

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Aggregation (cont.)

Aggregation tests:Is the phrase “part of” used to describe the

relationship?A door is “part of” a car

Are some operations on the whole automatically applied to its parts?

Move the car, move the door.Are some attribute values propagated from the

whole to all or some of its parts?The car is blue, therefore the door is blue.

Is there an intrinsic asymmetry to the relationship where one class is subordinate to the other?

A door is part of a car. A car is not part of a door.

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Composition

•A composition indicates a strong ownership and coincident lifetime of parts by the whole (i.e., they live and die as a whole).

•Compositions are denoted by a filled-diamond adornment on the association.

Window

Scrollbar

Titlebar

Menu

1

1

1

1

1

1 .. *

Composition

A strong form of aggregationThe whole is the sole owner of its part.

The part object may belong to only one whole

Multiplicity on the whole side must be zero or one.

The life time of the part is dependent upon the whole. The composite must manage the creation and destruction

of its parts.

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Aggregation and Composition

• Aggregation (shared aggregation):• is a specialized form of ASSOCIATION in which a whole is related to its

part(s).• is known as a “part of” or containment relationship and follows the “has a”

heuristic• three ways to think about aggregations:

• whole-parts• container-contents• group-members

• Composition (composite aggregation):• is a stronger version of AGGREGATION• the “part(s)” may belong to only ONE whole• the part(s) are usually expected to “live” and “die” with the whole

(“cascading delete”)

• Aggregation vs. Composition vs. Association???

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Aggregation Composition

0*..

1*..

Faculty

CourseTeaching

1*..

1

SalesOrder

SalesOrderLineItem

(another: hand --> finger)(another: assembly --> part)

(team-teaching is possible)

Static modeling: sum up

Steps during static modeling 1. Object/Class identification

• Start with the nouns in the use cases event flows 2. Find the attributes

• nouns 3. Find the methods 4. Find the associations between classes

• Association, generalization, aggregation, composition 5. Find the multiplicity relationship between classes

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Example

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1

0,m

UniversityCourse

StudentInCourse

StudentInformation

1

0,m

Example

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Class diagram example 2

DVD Movie VHS Movie Video Game

Rental Item

Rental Invoice

1..*1

Customer

Checkout Screen

0..1

1

Simple

Association

Class

Abstract

Class

Simple

Aggregation

GeneralizationComposition

Multiplicity

Dynamic Modeling:State Diagrams

State A condition during the life of an object during which it

satisfies some conditions, performs some actions or waits for some events

Shown as a rectangle with rounded corners State Transition

The changes in the attribute of an object or in the links an object has with other objects

Shown as a solid arrow Diagrammed with a guard condition and action

Event Something that takes place at a certain point in time

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Slide 46

Behavioral State Machines

• The behavioral state machine is a dynamic model that shows the different states of the object and what events cause the object to change from one state to another, along with its responses and actions.

Slide 47

Elements of a Behavioral State Machine

• States• Events• Transitions• Actions• Activities

Slide 48

Example Behavioral State Machine Diagram

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Behavioral State Machine Diagram Syntax

A STATE

AN INITIAL STATE

A FINAL STATE

AN EVENT

A TRANSITION

A Frame

Slide 50

Building Behavioral State Machine Diagrams

• Set the context• Identify the initial final, and stable states of the

object• Determine the order in which the object will pass

through stable states• Identify the events, actions, and guard conditions

associated with the transitions• Validate the statechart diagram

Analysis Versus Design

Start with existing set of analysis modelProgressively add technical detailsDesign model must be more detailed than

analysis modelComponent Diagram

A diagram that shows the software components or modules and their dependencies

Deployment Diagram A diagram that shows how the software

components, process and objects are deployed into the physical architecture of the system

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Summary

Object-Oriented modeling approach Benefits Unified Modeling Language

• Use cases

• Class diagrams

• State diagrams

• Sequence diagrams

• Activity Diagrams

Use-case modeling

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Summary

Object Modeling: Class Diagrams Associations Generalizations Inheritance and Overriding Aggregation

Dynamic Modeling: State Diagrams

Dynamic Modeling: Sequence Diagrams

Analysis Versus Design

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Questions???

Thanks for your Attention!

Slide 54

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Chapter 5 System Modeling Sommerville, "software engineering ", 9 th Ed., 2011