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151 lines
4.5 KiB
Markdown
151 lines
4.5 KiB
Markdown
---
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title: Lecture 11 Class diagrams
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sr-due: 2022-04-08
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sr-interval: 3
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sr-ease: 250
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---
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#review
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# Revision questions
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1. What is the purpose of stereotypes in UML?
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2. What is multiplicity and how is it represented on associations between classes? Provide a drawn example which uses an association between two classes.
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3. How are role names used for associations between classes and when should you use them?
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4. Discuss the issues that arise around the use of composition in the context of “cart-like” entities.
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5. Describe the relationship between role names and navigability in a class diagram.
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6. Explain the difference bewteen a domain class diagram and a system class diagram.
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---
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# Lecture 11 Class Diagrams
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e.g., 
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## 1 Stereotypes
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add further meaning to fields and methods
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- e.g., << unique >>, << abstrat >>, << interface >>,
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## 2 Packages
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group classes together
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break system to logical chunks
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package diagram, a class diagram with nothing but packages
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## 3 Associations
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UML anaglogue of ERD relationsips
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- multiplicity
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- realtionshpa nd role names
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PlUS
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- naviagability --> instances of one class can pass messages to instances of another
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- several differnt types, e.g., composition, aggregation, associateive classes
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### 3.1 multuplicity
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ERDsd effectively only do zero one many
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UML can to any non negative integer
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default is 1
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### 3.2 association names
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- usuallya verb phrase like "assings", "manages", "enrols in" ...
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- more useful is conpetual level diagrams
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- optional arrow head ()
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### 3.3 Role names
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At conceptual level, indicates role of class in association.
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At implementation level:
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- implies a field in class at opposite end
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- should include visibility
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- closely related to navigability
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### 3.4 Navigability
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specifies whether we can "navigate from one end of an association to another"
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affects how we code access paths between objects
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e.g.,
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- loan instance can see loanitem instances it contains via private field items
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- a loanitem instance can't see loan instance that contains it
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- must alwasy include relevant role names
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- no arrows = two arrows = bidirectional
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#### 3.4.1 why not always bidirectional
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- more complex code --> many references/collections to manage
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- navigation paths are not all equally important
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- e.g., "what items are in this loan" vs "what loans does this item appear in"
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- determined by requrements and typical usage
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- some classes are more "central"
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- usually at the "one" end of accociations
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- often represent transactional entities e.g., loan, sale, order
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- navigability readiates outwards from them
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there are exceptions as always e.g., patron <-> item
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### 3.5 Aggregation
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one class is made up of one or more other classes
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container and content instances _can_ exist separately
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usually implied by multiplicity and navigability
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e.g.,
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- computer is made u of several components
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- library catalogue is made up of many items
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### 3.6 Composition
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stonger form of aggregation
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container and content _cannot_ exist separately
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usually implied by multiplicity and navigability
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e.g.,
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- building contains many rooms
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- loan includes several items
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- coicident lifetime
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- multiplicity at least 1 at both ends
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- deleting an containter must also delete all associated contents
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- creating a container should also create some contents
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### 3.7 Associative classes
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- used for conceptual design
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- similar to associative entities
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- many to many relationship with additional independent fields
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- resolved into class at implementation level
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### 3.8 Specialisation generalisation
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class inheritance
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- e.g., book and disc are subclasses of (specialise) Item
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- inherit all public fields and methods of superclass
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- can add their own fields and methods
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- Compare with specialisation of actors and use cases
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## 4 Domain class model\
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only modles the associations among concepts from problem domain
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can be at conceptual level or implementation level
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## 5 System class model
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Models associations among domain objects and system components; implementation level only
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