Classes and Class Diagrams

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Classes and Class Diagrams
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Learning Outcomes

• Students will be able to
• Describe a class
• Identify components of a class diagram
• Explain the terms
• Multiplicity
• Inheritance
• Aggregation
• Generalisation

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What is a Class ?

• A description of a set of objects that share the
  same attributes, relationships and semantics.
• An abstraction
• 3 Parts A Name, Attributes and Operations
• An object would be an instance of the class
• EG
• Class a libraryMember borrows a bookCopy
• Object Joe Bloggs borrows copy 1 of UML
  Distilled

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A Class
Eg
ClassName
Window
Attribute1 Attribute2, etc
origin size
Operation1 Operation2, etc
open() close() move() display()
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Attributes

• Named property of a class usually a noun
• Describes a range of values
• A class can have many attributes or none
• The attribute is shared by all objects of the
  class
• EG
• All students have name, address, date of birth
• All cats have ....
  
  
  
  

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Attributes ...
Can be further specified by stating their type
and even a default value
Eg
Wall
height Float width Float thickness
Float isLoadBearing Boolean false
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Operations

• The implementation of a service
• Something that can be done to an object, that is
  shared by all objects of that class
• Each class can have many operations or none
• Usually a verb

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Operations ...
WashingMachine
Can be further specified by stating parameter
parameter type and function return values
brandName String modelName String serialNumber
String Capacity Integer
acceptClothes (cString) acceptDetergent
(dInteger) turnOn( ) Boolean capacity( )
Boolean
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Attributes and Operations with Visibility
Television
Accessibility levels Public Protected -
Private
brandName String modelName String
changeVolume( ) changeChannel ( ) -
paintImageOnScreen ( )
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Class responsibilities
WashingMachine
In an area below The operations list you can
describe the classs responsibility (must be
done in an unambiguous way)
brandName String modelName String serialNumber
String Capacity Integer
acceptClothes (cString) acceptDetergent
(dInteger) turnOn( ) Boolean capacity( )
Boolean
Take dirty clothes as input and produce clean
clothes as output
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Class constraints
WashingMachine
brandName String modelName String serialNumber
String Capacity Integer
acceptClothes (cString) acceptDetergent
(dInteger) turnOn( ) Boolean capacity( )
Boolean
(capacity 16 or 18 or 20lb)
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What is a Class diagram?

• A description of types of objects in the system
• It has classes, relationships and multiplicities
• Constraints may be applied to the way objects are
  connected
• It may also have navigability or roles

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Example Class Diagram
borland.com
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Associations

• A relationship between classes
• Used if there is a real-world association
• A short sentence associations correspond to
  verbs
• E.g. A player plays on a team
• Links can be uni-directional or bi-directional
• In class associations each class usually plays a
  role e.g. in professional team, team is employer
  and player is employee

Player
Team
Plays on ?
Employee
Employer
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Multiplicity

• Records constraints on associations
• Caters for permitted or required number of links
  (a link is an instance of an association)
• Specify
• An exact number (e.g. use 1)
• A range of numbers (e.g. use 2..8 or 1..)
• Arbitrary, unspecified number (use )
• Discrete numbers (e.g. use 5,10)

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Terms of Multiplicity

• Optional
• lower bound of 0
• Mandatory
• lower bound of 1 or more
• Single-valued
• upper bound of 1
• Multivalued
• upper bound of gt 1 (usually )

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Multiplicity Examples
EggBox
Egg
1
holds
12,24
Car
Owner
Is owned by
1..
1
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Class Diagram revisited
borland.com
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Aggregation Composition

• Ways of showing more about an association
• An object of one class is part of an object of
  another class
• Denoted by a diamond at the whole end of the
  association
• Composition is a special kind of aggregation
  the whole owns its parts

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Aggregation Composition
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Class Diagram revisited
borland.com
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Generalisation

• Uses inheritance
• EG a business could have two types of customer
• Corporate customer
• Personal customer
• Both have similarities that can be placed in a
  general Customer class (supertype)
• Personal and Corporate customers are subtypes

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Generalisation Example
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Another Generalisation Example
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Classification

• Relationship between an object and its type
• Single Classification
• Object belongs to single type
• May inherit from supertypes
• Multiple Classification
• Object may be described by several types
• Not necessarily connected by inheritance

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Multiple Classification Example
Agilemodeling.com
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Summary - Class Model

• Static structure of objects in a system
• Identity
• Relationships
• Attributes
• Operations

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Summary

• Class Group of similar objects
• Association group of similar connections
  between objects
• Generalisation structures description of
  objects by organising classes by
  similarities/differences
• Attributes and Operations elaborate structure

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Domain Class Model

• Describes real-world classes and their
  relationships to each other.
• Information gained from -
• Problem statement
• Artefacts
• Expert knowledge of application domain
• General knowledge of real world
• Does not rely on a single source

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Constructing a Domain Class Model

• Find classes
• Prepare data dictionary
• Find associations
• Find attributes of objects and links
• Organise and simplify using inheritance
• Iterate and refine

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Finding Classes

• Physical Entities
• Houses, People, Machines, etc
• Concepts
• seating assignments, payment schedules, etc
• Usually Nouns

Tentative Classes
Eliminate Spurious Classes
Extract Nouns
Requirements Source
Classes
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Eliminate Spurious Classes

• Is it beyond the scope of the system ?
• Does it refer to the system as a whole ?
• Does it duplicate another class ?
• Is it too vague ?
• Is it too specific ?
• Is it too tied up with physical I/Os ?
• Is it really an attribute ?
• Is it really an operation ?
• Is it really an association ?

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Exercise - scenario

• Littlesand, Pebblesea and Mudport are three
  charming resorts on the south coast which are
  very popular with tourists, since they score well
  on beach ratings and hours of sunshine for the
  sea.
• All three resorts have a large number of places
  to stay, ranging from one-room guest houses to
  the exclusive Palace Hotel at Pebblesea. The
  local tourist board wants to set up a central
  system to deal with room bookings in the area.

Britton Doake (2000)
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Exercise part a) b) - Tentative Classes

• Write out the scenario on previous slide and
  underline the tentative classes (possible
  classes)
• Some of the words/phrases that you underline will
  class attributes
• Then following the example on the next slide,
  categorise the classes into
• Vague too vague to use
• Redundant no use for it
• Implementation concerned with implementation
  issues
• Attribute the item has turned out to be an
  attribute of a class

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Exercise part b) - Bad Classes
Vague
Implementation
Attribute
Redundant
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Exercise part c) - Good Classes

• Write out a list of the classes that you have
  left
• These will be used to create you class diagrams

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Exercise part d) - Prepare Data Dictionary

• Isolated words can be misinterpreted
• Write a paragraph precisely describing each class
• Eg Room a room at a place to stay against
  which bookings can be made. Rooms may be of
  various types. A place to stay can contain more
  than one room.

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Exercise part e) - Initial Class Diagram
- construct a class diagram showing just class
names
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Exercise part f) - Find Associations and
multiplicity

• Show relationships between classes
• Often correspond verbs
• Physical Locations (NextTo, PartOf, ContainedIn,
  etc
• Directed actions (Drives)
• Communication (TalksTo)
• Ownership (Has, PartOf)
• Satisfaction of Condition (WorksFor, MarriedTo,
  Manages, etc)

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Exercise part g) - Find Attributes for the main
(resort) class

• Data Properties of individual objects
• Use
• Knowledge of application domain
• Artefacts
• Capture the most important you can add detail
  later

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Exercise part h) - Refine Using Inheritance

• Bottom-up generalisation
• Classes with similar attributes, associations,
  operations
• Define superclass to share common features
• Top-down specialisation
• Similar class names (eg fixed menu, pop-up menu,
  sliding menu)

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Further Reading

• Object-Oriented Modelling and Design with UML,
  Michael Blaha and James Rumbaugh,
• UML Distilled, Martin Fowler,Addison Wesley ISBN
  0-321-19368-7
• Using UML, Perdita Stevens
• www.agilemodeling.com
• Object-Oriented System Development (2000) Britton
  Doake