quartz/content/notes/class-diagrams.md

title	aliases	tags
class-diagrams		info201

General example ERD vs class diagram

1 Stereotypes

add further meaning to fields and methods

• e.g., << unique >>, << abstrat >>, << interface >>,

2 Packages

group classes together break system to logical chunks package diagram, a class diagram with nothing but packages

Packages

3 Associations

UML anaglogue of ERD relationsips

• multiplicity
• realtionshpa nd role names

PlUS

• naviagability --> instances of one class can pass messages to instances of another
• several differnt types, e.g., composition, aggregation, associateive classes

3.1 multuplicity

multiplicity-1 multiplicity-2

ERDsd effectively only do zero one many UML can to any non negative integer default is 1

3.2 association names

named associations

• usuallya verb phrase like "assings", "manages", "enrols in" ...
• more useful is conpetual level diagrams
• optional arrow head ()

3.3 Role names

role names

At conceptual level, indicates role of class in association.

At implementation level:

• implies a field in class at opposite end
• should include visibility
• closely related to navigability

3.4 Navigability

navigability

specifies whether we can "navigate from one end of an association to another" affects how we code access paths between objects

e.g.,

• loan instance can see loanitem instances it contains via private field items
• a loanitem instance can't see loan instance that contains it
• must alwasy include relevant role names
• no arrows = two arrows = bidirectional

3.4.1 why not always bidirectional

• more complex code --> many references/collections to manage
• navigation paths are not all equally important
  • e.g., "what items are in this loan" vs "what loans does this item appear in"
  • determined by requrements and typical usage
• some classes are more "central"
  • usually at the "one" end of accociations
  • often represent transactional entities e.g., loan, sale, order
  • navigability readiates outwards from them

there are exceptions as always e.g., patron <-> item

3.5 Aggregation

aggregation

one class is made up of one or more other classes container and content instances can exist separately usually implied by multiplicity and navigability

e.g.,

• computer is made u of several components
• library catalogue is made up of many items

3.6 Composition

composition

stonger form of aggregation container and content cannot exist separately usually implied by multiplicity and navigability

e.g.,

• building contains many rooms

• loan includes several items

• coicident lifetime

• multiplicity at least 1 at both ends

• deleting an containter must also delete all associated contents

• creating a container should also create some contents

3.7 Associative classes

associative classes 1 associative classes 1

• used for conceptual design
• similar to associative entities
  • many to many relationship with additional independent fields
  • resolved into class at implementation level

3.8 Specialisation generalisation

specialisation and generalisation

class inheritance

• e.g., book and disc are subclasses of (specialise) Item
• inherit all public fields and methods of superclass
• can add their own fields and methods
• Compare with specialisation of actors and use cases

4 Domain class model

domain class model

only modles the associations among concepts from problem domain

can be at conceptual level or implementation level

5 System class model

system class model Models associations among domain objects and system components; implementation level only