Introduction to the Unified Modeling Language (UML)

Delos NoE , Preservation Cluster

Workshop: Persistency in Digital Libraries

13. February 2006, Oxford Internet Institute

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„Nobody, not even the creators of UML, understand or use all of it. Most people use a small subset of the UML and work with it.“

„UML is so complex that the standard is often open to multiple interpretations.“

„People do adopt conventions both in the industry widely and within a particular project.“

(Martin Fowler, „UML Destilled“)

Introductory Quotations

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UML – what is it good for?

• Visualisation: graphical representation of structures and processes

• Specification: Building models that are precise, unambigous and complete

• Construction: Converting UML-models into programming code

• Documentation: Illustration and mapping of systems architecture, requirements analysis, managing different versions of systems or parts of it

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Basic concepts: What is an object?

An object can be

A thing, e.g., car, office, book

A person, e.g., librarian, director

A term, e.g., programming language, disease

Synonymously, it is also called instance.

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An object can be featured

by

• State• Behaviour• Identity

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UML Representation of an object

elisabeth elisabeth:Cow :Cow

object:Class

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The state of an object includes its attributes and its relationships to other objects.

An attribute can have a value.

:Cow

Colour = brownAge

:Cow

Colour : stringAge : integer = 5

Object:Class

Attribute : type= value

Object: State

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Objects perhaps „know“ each other, that is there is some kind of relationship between the objects. Depending on the context, a relationship is called link or association or generalisation.

Examples:Cow ‚Elisabeth‘ is the daughter of cow ‚Victoria‘.Cow ‚Elisabeth‘ is suckling from ‚Victoria‘.A calf is a young cow.

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The behaviour of an object is described through one or more operations. They enable us to modify or request the object's state.

Object: Behaviour

getDigitalObject()

loadData()

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Every object has a distinct identity that is defined through its existence.

Object: Identity

Identity„Pseudo-Identity“

:Person :Person:Person :Person

:Person :Person :Person

Name=Michael Name=Michael

Name=John Name=John Name=John

Name=Mary Name=Linda

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The relations between objects can be visualised using an Object Diagram.

An Object Diagram describes objects, links and optionally attributes and their values at a specific point in time. An Object Diagram is a snaphot of a system or parts of it.

Object Diagram

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Example

Victoria:Cow

Elisabeth:Cow

Charles:Cow

Sex = mAge = 10

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A class represents a collection of objects. It defines the data structure (attributes), behaviour (operations) and relationships of the single objects.

Every object belongs to exactly one class.

Class

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UML representation of a class

Class

Class

Class

:DigitalObject

Attribute

Operation

Class

Attribute

Class

Operation

ID=1234DOType : IntegerREFNO=Q34R

ID=1234DOI=123.456

REFNO=Q34R

:DigitalObject

ID=5678DOType : IntegerREFNO=Q56S

DigitalObject

IDDOTypeREFNO

getDigitalObject()

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Attributes describe the properties of an object (class).

Attributes represent the data structure of an object (class).

All objects of a class share the same attributes. They differ in the values of the particular attributes.

Every attribute has a distinct type.

Attribute

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UML syntax for attributes

Attribute DigitalObject

ID ID: int ID: int {read only}

ID: int = 1234 ID: int [1] ID / ID

int ID

attribute attribute: type attribute: type {property string}attribute: type = Inital value attribute: type [x..y] class scope attribute /derived attribute type attribute

not normative!

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Operations are the actions that a class (the objects of a class) can carry out.

All objects of a class use the same operations.

All operations together represent the behaviour of the class (the objects).

Operation

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UML syntax for operations

Operation

name (parameter-list) : return-type {property-string}

assignID (int: ID) : void {read-only}

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Objects can be linked to each other. Classes can also be linked. This is called association.

Association

:DigitalObject

DigitalObject Date

:Date:DigitalObject

:Date

association

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Binary / Reflexive Association

CollectionDigitalObject11..*

DigitalObject

Reflexive A.

Binary A.

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Multiplicity

Multiplicities enable to model the number of possible links between objects of classes.

CollectionDigitalObject11..*

Multiplicities

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Examples for multiplicities

exactly one 1

one or none 0..1

uspecified number *

n to many 2..*

n to n 12..49

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An association can be labeled by using a verb phrase:

CollectionDigitalObject11..*

is part of

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An association can also be labeled by a property name

(role).

DigitalObject master

1

copy1..3

property names

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Associations can be completed by constraints.

DigitalObject master

1

copy1..3

constraint

{integrity checked}

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Aggregation and Composition

Aggregation and Composition are concepts for modelling a „whole-part“ relationship.

Example: „ A page is part of a book.“

They are both special kinds of association. They can also always be expressed as simple associations.

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Aggregation

UML syntax:

Collection DigitalObject

aggregated class

aggregation

partial class

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Collection DigitalObject

Collection DigitalObject1..*

1..*

1..* 1..*

is part of

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Composition

UML syntax:

Collection DigitalObject

aggregated class

composition

partial class

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Collection DigitalObject

Collection DigitalObject1..*

1 1..*is part of

Page273

Book

title=„UML“

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Navigability of associations

The navigabilty specifies the access to objects of given classes.

classX classY

classX

classX

classX

classY

classY

classY

unspecified

unidirectional

bidirectional

excluded navigability

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Properties modelled as attributes or associations

DigitalObject

ID: int [1]created: Date [1..*]

composed: File [1..*]

Digital Object

FileDate

1

1..*

1

1

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A generalisation describes a relationship between a super class and a sub class.

The sub class is a specialised class in regard of the super class. It is entirely consistent with the super class, but it adds some additional attributes or operations.

Every object of the sub class can be described with an „is a“ relationship.

Generalisation

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Abstract Class

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A package is used to merge elements of models, especially classes.

They can be grouped in practicable units.

Packages are a good concept for describing the structure of a system on an abstract level.

A complete software system is at its highest level one big package.

Package

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Visibility

Attributes and operations within a class can be labeled by a visibility indicator.

The same is valid for roles in associations.

Most common are „+“, that stands for public and

„-“ that stands for private.

Public elements can be used by any other class, private elements only by the class that owns them.

Additional features

38 Notes

There is a possibility to insert comments into any UML diagram using the following graphical elements:

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DigitalObject

- ID

- RefNo

- CatType

- checkIntegrity()

+ getDigitalObject()

DigitalObject

AccessUnit

+ copy

*

*

Visibility: Attribute, Operation and property name

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Dependency

A dependency indicates changes to one element caused by changes of the other.

UML allows the usage of dependencies for all sorts of elements.

AccessUnit DigitalObject

dependency

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Dependencies can be detailed using a keyword.

AccessUnit DigitalObject<<call>>

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Often, relationships imply dependencies.

AccessUnit DigitalObject**

Report Document

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„If someone were to come up to you in a dark alley and say, ‚Psst, wanna see a UML diagram?‘ that diagram would probably be a class diagram.“

(Martin Fowler, „UML Destilled“)

Class diagram

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The Class Diagram represents a static view of a system.

Therefore Class Diagrams are often used to model structural issues like the architecture of a system or parts of it.

The Class Diagram models single components of the system or parts of it and describes its relationships.

It can be a starting point for transforming the systems model into programming code.

Class Diagram

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Example 1 – Part of system architecture

Source: http://bdn.borland.com/

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Example 2 - Conceptual Model: Taxonomy

Source:http://www.agilemodeling.com

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Example 3 – Producing software

class Person { protected string name; public void generateAddress();} class Foreigner extends Person {}

class Citizen extends Person {}

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More static views

Package Diagram

Package diagrams are often used to show the static structure of large systems on a high level.

<<call>>

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So far we regarded the static and structural side of systems.

UML offers some more diagrams for modelling behavioural and dynamic aspects of systems.

Behavioural diagrams show systems in action.

Behavioural and Dynamic Aspects: More UML Diagrams

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A system in action can be described as one that has at any given time a set of defined states .

States change in a time span. This is called transition.

The UML diagram that describes states of a system or of elements of it (i.e. of particular objects) at a given time (span) is called State Machine Diagram.

State Machine Diagram

Example

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Core graphical elements

final state

inital statesub-state pointer

transition arrow

event

State box

decision node

Example (2)

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entry activityexit activity

fork

join

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A transition can be labelled in three parts (optional):

openkey turned [unblocked] /open door

trigger activity

guard condition

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Activity State: States with an ongoing activity

Query sentWorking

do/ search for entry

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Activity Diagrams provide a technique for describing work flows or procedures of a system.

An Activity Diagram shows the system in action in a more general view, not regarding just one particular object, but a sequence of actions where different objects may be involved.

Activity Diagram

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Initial node

action

exception

objectactivity final

condition

decision

Errorflow

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A Communication (Collaboration) Diagram is a technique

to show how messages flow between particular objects.

It visualises the interactions between the objects and shows how the objects are linked together.

The Sequence Diagram is an alternative kind of Interaction Diagram besides the Communication Diagram.

Communication Diagram

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Notation features

link

dataflow arrow

(flat) numbering object

self link

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Fowler, Martin: UML destilled (3d edition), 2004.

OMG: Unified Modeling Language (UML), Version 2.0 .

Link (01. Feb. 2006): http://www.omg.org/technology/documents/formal/uml.htm

(= Specification of the UML)

Balzert, Heide: Lehrbuch der Objektmodellierung, 2005.

References