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Object-Oriented Software Engineering

Practical Software Development using UML and Java

Chapter 5: Modelling with Classes

5.1 What is UML?

The Unified Modelling Language is a standard graphical language for modelling object oriented software

• At the end of the 1980s and the beginning of 1990s, the first object- oriented development processes appeared
• The proliferation of methods and notations tended to cause considerable confusion
• Two important methodologists Rumbaugh and Booch decided to merge their approaches in 1994. —They worked together at the Rational Software Corporation
• In 1995, another methodologist, Jacobson, joined the team —His work focused on use cases
• In 1997 the Object Management Group (OMG) started the process of UML standardization

UML features

• It has detailed semantics
• It has extension mechanisms
• It has an associated textual language — Object Constraint Language (OCL)

The objective of UML is to assist in software development —It is not a methodology

What constitutes a good model?

A model should

• use a standard notation
• be understandable by clients and users
• lead software engineers to have insights about the system
• provide abstraction

Models are used:

• to help create designs
• to permit analysis and review of those designs.
• as the core documentation describing the system.

Classes

A class is simply represented as a box with the name of the class inside

• The diagram may also show the attributes and operations
• The complete signature of an operation is: operationName(parameterName: parameterType …): returnType

5.3 Associations and Multiplicity

An association is used to show how two classes are related to each other

• Symbols indicating multiplicity are shown at each end of the association

Analyzing and validating associations

• Many-to-one —A company has many employees, —An employee can only work for one company. - This company will not store data about the moonlighting activities of employees! —A company can have zero employees - E.g. a ‘shell’ company —It is not possible to be an employee unless you work for a company Employee (^) * worksFor^1 Company

Analyzing and validating associations

• Many-to-many —An assistant can work for many managers —A manager can have many assistants —Assistants can work in pools —Managers can have a group of assistants —Some managers might have zero assistants. —Is it possible for an assistant to have, perhaps temporarily, zero managers? * supervisor

Assistant 1..*^ ***** Manager

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Analyzing and validating associations

Avoid unnecessary one-to-one associations

Avoid this do this

A more complex example

• A booking is always for exactly one passenger —no booking with zero passengers —a booking could never involve more than one passenger.
• A Passenger can have any number of Bookings —a passenger could have no bookings at all —a passenger could have more than one booking
• The frame around this diagram is an optional feature that any UML 2.0 may possess.

Reflexive associations

• It is possible for an association to connect a class to itself

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Directionality in associations

• Associations are by default bi-directional
• It is possible to limit the direction of an association by adding an arrow at one end

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Avoiding unnecessary generalizations

Inappropriate hierarchy of classes, which should be instances

Avoiding unnecessary generalizations (cont)

Improved class diagram, with its corresponding instance diagram

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