Use Case modelling

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Objectives

Document user requirements with a model

Describe the purpose of an actor and a use case

Construct a use case model Define use cases in detail

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Object-Orientation – why is it used?

Has been in existence since the late nineteen sixties

Covers all aspects of system development Throughout development of a system there

is a key concept of an object

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Why object-orientation ?

An object is something within the problem domain such as an Invoice or a Purchase Order

These real world objects become embodied in a model during analysis and design stages

They eventually end up as part of the final code that is produced

There is thus a continuity that exists throughout all stages of development

An approach based on objects rather than functions is more robust

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Object-orientation - Summary

Key concept follows through all stages of development - objects

Object - invoice, purchase order Real world objects are placed in models at

an early stage and continue throughout development to implementation

Approach based on objects (rather than what the system will do) is more stable

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Starting a project Imagine being asked to provide a new

system for a company. What would you do first?

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Use Case Models Use case models are a way of specifying

user requirements They specify what the system will do for

the people/systems who will interact with it They break a project down into the

functions that must be carried out by the system

The models produced will initially act as a means of communication between developers and users.

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Actors Actors are entities that interact with the

system Actors either provide or receive

information Actors are outside the system Names of actors are usually discovered

from dialogue with the users or may be specified in the problem statement

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Roles and Actors An actor plays a role It is the role that interacts with the system A person may play different roles An role may have a number of interactions

with the system

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UML representation of an Actor

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Actor

Role

Use Cases

A use case describes the interaction between users

and an application system describes what a user does with the

system to produce a business event, which forms part of a business process

models a dialogue between the actor and the system

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Use Cases

A use case represents the functions which are made

available by the system to the actors is a sequence of transactions performed by

a system that produce a measurable result for particular actors

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Representation of a use case in UML

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SFU example

Stafford Free University (SFU) needs a new information system for registration purposes. There are currently problems with registering students on modules and determining which modules their lecturers have elected to teach.

The registration section of SFU decides the modules to be offered in each semester of the coming year. This information is made available to the lecturers on-line. Each module has a number of credit points associated with it. It has a code, title, and it may have a module pre-requisite as well as a description.

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SFU example (cont) It may be delivered in any of three modes of

delivery (part-time, open learning, or full-time) in any semester. Every time a module is delivered, the start and end date is recorded and the total hours needed to study it.

The registration service adds, changes and removes the details held on each lecturer. Details stored on each lecturer include the highest qualification they have achieved and their subject specialism. If a lecturer has had a formal teacher training this is also recorded.

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SFU example (cont) The lecturers may elect to teach any of the

modules offered in a particular semester by accessing the registration system on-line. They may also change their minds and decide not to teach a module after they have previously elected to do so. At any time, they may wish to view the module offerings they have chosen. The students fill out a registration form on-line and indicate their choice of modules.

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SFU example (cont)

Both students and lecturers have a name and an id to use this registration service. The expiry date of their account is also recorded. Students may register to take up to any three modules in each semester. This information is then entered into the information system.

Students are then assigned to modules which, have a limit on student numbers. If a student has elected to take a module that is oversubscribed the Registration Service contacts the student and another module is usually substituted.

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SFU example (cont) After the registration period each lecturer receives

a module student list with the student names for that module and rooms and times for the module delivery they have elected to teach. A schedule of modules is sent to the student for confirmation of correct registration. This schedule shows the modules and the start and end dates that the student has registered for.

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SFU example (cont) Any module offering with fewer than the minimum

student numbers will be cancelled. The registration service makes the registration information available to a separate billing system that will send out invoices to the students. An invoice contains a date and amount together with a number.

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Use Case Diagrams

A use case diagram is a picture of some or all of the actors and their interactions with the system.

Each system will have a main use case diagram showing the system boundary the major functions provided by the system

Large systems may employ more than one use case diagram

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What are the use cases? Look for verbs in the problem description How much detail in a use case ? What about the

Registration Service? It must maintain the registration by adding deleting and modifying modules.

How many use cases are there here? This is matter of judgement

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Use cases for SFU Register for modules Select modules to teach Issue module student list Maintain module information Maintain student information Maintain lecturer information

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References• Bennett, S., McRobb, S. and Farmer,R. Object-

Oriented Systems Analysis and Design using UML, London: McGraw-Hill, (3rd Ed.) 2005

• Britton, C. and Doake, J. Object-Oriented Systems Development – a gentle introduction, London: McGraw-Hill, 2000

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