Class Diagrams

Chapter 3

Classes and Objects

• Classes are the descriptions– definitions

• Objects are the things– instances

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Objects

• What’s an object?– It’s a “thing” that interacts with other

“things” to address the task at hand– It contains attributes (data items) which

represent its current state– It contains methods (functions) which are

used to manipulate it

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Objects

• What’s an object?– An object is a grouping of attributes and

behaviors into a single entity

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Primary and Support Objects• What’s a primary object and what’s a support

object?– Primary objects

• Those that are obvious• Those that are fundamental to the system• Those on which all team members agree must be

included in the system (without persuasion) – Support objects

• Those that you really have to stretch to come up with• Those which cause debates among team members• Those which you’re just not sure about

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Primary and Support Objects• Identify primary objects early in the design

process– This should be relatively easy

• Leave the support objects for later (maybe even for the implementation phase)

• Don’t hesitate to change the list of primary objects as the design progresses– Actually, it’s expected to change– Eventually, they’ll all be primary objects since they

compose the running system and without them the system will fail [to compile/link/bind at runtime]

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Tangible and Intangible Objects• What’s a tangible object and what’s an

intangible object?– Tangible objects generally represent things

(nouns) within the system– Intangible objects generally represent calculations

(verbs) within the system– It’s not important to specify whether an object is

tangible or intangible– It is important to consider both types of object

when identifying objects

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Object Identification• How do you identify objects?

– Nouns• People• Places• Things

– Things with multiple use– Things that store data or state– Things that are composites of other things– Things that are ancestors of other things– Things that perform a useful function

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Object to Object Relationships• How do objects relate to one another?

– Inheritance (“is-a” relationship)– Association/Composition (“has-a”

relationship)– Application specific (neither of the above)

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UML Class Symbol

• Class

• In the early stages of the design you should not be overly concerned (or concerned at all?) with the attribute and method fields

+Methods()-Attributes

Object-Name

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UML Relationship Symbols• Inheritance (Generalization) (“is-a”)

Base-Class (parent)

Derived (child)

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UML Relationship Symbols• Association/Composition (“has-a”)

• Containing contains 1 Contained Class object

• An instance of Contained Class may belong to either Containing Class 1 or 2 but not both

UML Relationship Symbols• Association/Composition (“has-a”)

– There is also something called aggregation which is nothing more than association

– Most use the association arrow and not the aggregation diamond

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UML Relationship Symbols• Application Specific (association)

– Use this when two objects are related but not by a “has a” or “is a” – you shouldn’t really need to do this but you might

Class1

Class2

Class1 role

Class2 role

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UML Relationship Symbols• Dependency

+Methods()-Attributes

Modify Me Too

+Methods()-Attributes

Modify Me

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Class Diagram• The result of identifying objects (classes) and

their relationships will be a UML Class Diagram• The Class Diagram represents the static

(unchanging) nature of the system design– It represents compile-time relationships, not run-

time usage• We will revisit class diagrams later, adding

more detail

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Object Identification Guidelines• Eliminate all preconceived notions.• Don’t strive for perfection…yet.• Don’t get into heated debates over the inclusion or exclusion of a class…

yet.• If you do debate, realize the difference between a debate and an argument

– Debate – good– Argument – bad

• Don’t get formal…yet.– Remember that UML can be used in “sketch”, “blueprint”, and “programming

language” modes – concentrate on “sketch” right now• Don’t toss out far fetched ideas too soon.• Remember the difference between interface and implementation

– Don’t think about the implementation right now.• Think about code reuse.• Keep in mind that this is an iterative process.

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Deliverables