uml dynamic models

8/3/2019 UML Dynamic Models

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UML Dynamic Models

Introduction

This is one a part of a set of notes on the Unified Modelling language -- [ uml.html ]

Scenarios and UseCases

Scenarios and Use-Cases are an ideal place to start a project. They must be expressed in

terms that the user and client understands and can (and should) avoid all technical jargon.The set of usecases answer the questions:

1. Who wants to use this software?2.

What do they want to do with it?

A scenario describes a particular user getting something useful done. A usecase is a set of

similarscenarios all with the same type of user and all ending when the user finds out

that the process is complete. The user must begin a usecase and must know when it iscomplete. The user must be active entity - an actor- and communicates with the system

to achieve the goal of the usecase.

A usecase is written out in a natural language like English. UML provides a notation for

naming and classifying both actors and usecases. Here is a UML diagram that showsthree usecases: "add class", "drop class", "get roster", and "submit work" and two actors:"Student" and "Teacher".

Notice the use of a "stick figure" to indicate a class of actors. The usecase is indicated byan oval. Communication is modeled as a simple link between an actor and an usecase.

It is also possible to use the UMLgeneralization relationship between classes of actors:

For example a Graduate Student might be a special kind of Student and so could share the


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Add Class Scenario. The generalisation link is also used between Usecases to indicatethat one usecase is a special case of another one, or that one usecase makes use of another

one.

Interaction Diagrams

These allow you to work out how your classes can help the user solve problems.

Sequence diagrams make the timing of various operations clear. Collaborationdiagrams are better for modelling a complex interaction or algorithm. There is no

reason why you shouldn't draw both.

Sequence Diagrams

Historically, Charles Babbage was the first person to draw a diagram were eachpart of a machine was shown as a vertical line, time flows down the page, and

connections between the parts flow across the page. This idea has been reinvented

several times in the last 10 years.

The above diagram indicates the following sequence of events (inside our

program):

1. JoDoe asks to drop a class, the request is handled by Jo's Student object ---- --(jd)

2. jd asks the Section --(s) to dropjd.3. s (the section) passes the message to an Enrollment --(e) which self-

destructs.4. s reports success tojd.5. jd reports success to Joe.

Each vertical line indicates an existing object. When it is active it the linebecomes a rectangle. Connections can show flow and whether it is synchronous(--


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--->) or asynchronous(with only half an arrowhead!). Normally the steps aredescribed on the left hand side of the diagram.

Collaboration Diagrams

A collaboration diagram shows how a collection of objects and classes worktogether (Latin: co - labor = work together) to fulfill a use-case. Each step in theprocess is numbered and follows a link from one object to another. Here is an

example - a first sketch of how the system helps a student add a section:

The diagram above describes the following:

6. Jo Doe, a student asks to add a class by giving Jo's student id -- -- --(sid)and a section id--(sd).

7. The student class uses the sid to get Jo's object -- (jd) Thejd object ispassed to the section.

8. The Section class uses the find operation to find the asked for Sectionobject -- -- --(s)

9. s finds out which course it is and passes s andjd into the Enrollment classwhich

10.constructs a new added enrollment linkingjd to s.

Exercise. There are some obvious errors and ommissions in the above diagram --fix them!

. . . . . . . . . ( end of section Interaction Diagrams)

State Transition Diagrams


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State transition diagrams have been a part of computer science and computerprogramming since the 1940's. David Harel in the middle 1980's reinvented the state

transition diagram in a form that has become popular in many different notations andmethods. UML incorporates Harel's StateCharts as a way to work out, document and

verify the gross patterns of behavior of an object in a class.

The basic rules are easy to state.

o Draw only one object's chart at a time.o A state is drawn as a box with rounded corners.o Each state models a set of possible object values that have similar behavior - but

possibly different attribute values.

o A state is in a different box if objects in these states behave differently.o From each state draw an arrow to another state if the object can change from one

to the other in one step.o Label the arrow with the event that causes it.o

Show the initial state by drawing an arrow from a black blob to the initial state.o Show the destruction of the object (if there is one) as an arrow going from a state

to a blob inside a circle.

Here are a pair of examples showing the life of an idealized Student.

You will notice that in the above pair of diagrams the bottom one has the name of a state

in the upper diagram. This means that the the state has been refined. It is also possible toredraw the two diagrams as a single diagram with the refinement replacing the unrefined

state.


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uml dynamic models

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