UML Class Diagrams

Unified Modeling Language (UML) Set of MANY (visual) modeling techniques use case diagram class diagram object diagram component diagram deployment diagram sequence diagram collaboration diagram statechart diagram activity diagram Only 20% are necessary to represent 80% of most systems Static Views Dynamic Views

Unified Modeling Language (UML) Set of MANY (visual) modeling techniques use case diagram class diagram object diagram component diagram deployment diagram sequence diagram collaboration diagram statechart diagram activity diagram Only 20% are necessary to represent 80% of most systems Static Views Dynamic Views

Historical Note: Entity Relationship Diagrams name age student gender entities n student ID enrolled in attributes relationship between entities m subject class course ID max-enroll 4

Class Diagrams Entity Relationship SimpleWatch 1 1 1 1 2 1 2 1 PushButton state push() release() LCDDisplay Battery load() Time now() blinkIdx blinkSeconds() blinkMinutes() blinkHours() stopBlinking() referesh()

Class Diagrams - Entities Entities are abstractions of elements with common characteristics; at the finest-grained level they correspond to classes Entities have three basic components: Name Attributes Operations zone2price getZones() getPrice() TariffSchedule Name Attributes Operations Instance zone2price = { {‘1’, .20}, {‘2’, .40}, {‘3’, .60}} tariff_1974:TarifSchedule

Relationships Between Classes Classes are related to each other via: Generalization Association Aggregation

Generalization Generalization is implemented via “Inheritance” Children inherit attributes and operations of parent; the parent “generalizes” attributes and operations that apply to the children Generalization eliminates redundancy, and supports incrementality

Associations Associations are relations between entities, and can be named Endpoints can note “multiplicity constraints” Potential multiplicity values: 1 0..1 0..n 1..n * 1..*

Aggregation Aggregation is a special case of an association, and defines a “part of” relationship The aggregate is a parent class, the components are children classes Multiplicity can be used here too Exhaust System Part-Of 1 0..2 Muffler Tailpipe 34

Mapping to Ghezzi’s Terms Inheritance, Association, and Aggregation are all forms of USES Representation of IS_COMPONENT_OF is done via the package notation

Packages Organize classes into packages to simplify complex class diagrams A package is a collection of logically related UML elements.

Entities (Expanded) Person + name : String - ssn : String Visibility + public # protected - private Person + name : String - ssn : String # birthday : Date age : int +getName : String -calculateAge : int Visibility AttributeName:Type = Initial Value Visibility MethodName (Parameter List) : Return-List Protected = visible to classes that are inherited from this one

Visibilities Public (+): Any outside class can use the feature (method or data) Private (-): Only the class itself can use the feature Protected (#): Any descendant of the class can use the feature (pertains to the generalization relationship)

Aggregation (Expanded) Simple Aggregation Composite Aggregation With composite aggregation, the part objects may belong to only one whole, and are expected to live and die with the whole.

Self-relations manages employee

Navigabilities Bi-Directional Navigability: An Order has a responsibility to tell me which Customer it is for, and a Customer has a responsibility to tell me which Orders it has. Customer Order ID Date price Name Address Ordered by *  1  Uni-Directional Navigability: An Order has a responsibility to tell me which Customer it is for, but a Customer has no responsibility to tell me which Orders it has. Customer Order ID Date price Name Address Ordered by *  1 

Navigabilities Some people choose to interpret an association edge with no navigability arrows on it as being Bi-directional. Others choose to interpret such an edge as saying that navigability is not yet specified. The latter interpretation facilitates the specification of conceptual models; navigabilities can be added when we move to the specification/implementation models.

Association Classes Book Patron Checks Out Association Class are attributes of relationships between classes; there can be only one instance of the association class between any two participating objects Association Class

Levels of Class Diagrams Conceptual perspective – represents concepts in the domain under study Specification perspective – adds interfaces into the picture; what do classes export? How are associations kept track of? Implementation perspective – adds other elements An incremental refinement process moves you from the first to the last We start with initial analysis to create a high-level conceptual or domain model of the application. This typically contains just classes and maybe a few attributes. At the specification level, we complete all the major attributes, the public methods and understand the class relationships and their cardinalities. At the implementation level, we have translated all the relationships and cardinalities to data structures, and have identified additional classes necessary for implementation.

Constructing Class Diagrams Identify objects Construct conceptual perspective – concepts in the domain under study Refine to specification perspective – add interfaces into the picture and specify relationships more precisely Refine to implementation perspective – add other elements We start with initial analysis to create a high-level conceptual or domain model of the application. This typically contains just classes and maybe a few attributes. At the specification level, we complete all the major attributes, the public methods and understand the class relationships and their cardinalities. At the implementation level, we have translated all the relationships and cardinalities to data structures, and have identified additional classes necessary for implementation.

Object Identification No “magic formula” It relies on the skill, experience and domain knowledge of system designers Iterative process. Unlikely to get it right first time Approaches for identification Use a grammatical approach Identify tangible things Identify objects based on participation in behaviours Use a scenario-based analysis Use domain knowledge for refinement

Library Checkout System: Conceptual Level Patron Book Checks Out id name age isbn title author outDate returned fines

From Conceptual Level to Specification Level Book Patron Book Checks Out id name age isbn title author outDate returned fines Author isbn:string title:string name:string written by 1..5 * 1 Patron BookCopy id:int name:string age:int 1 0..5 copy_num : int 1..* represented by Checks Out outDate:date returned:date fines:currency

From Specification Level to Implementation Level Book Author #isbn:string #title:string #name:string written by 1..5 * Author Book #isbn:string #title:string #writtenby[5]: Author #name:string # writtenby :List<Book>

From Implementation Level to Code Author public class Author { protected: std::string name; std::list<Book> writtenby; } public class Book { std::string isbn; std::string title; Author writtenby[5]; #name:string # writtenby :List<Book> Book #isbn:string #title:string #writtenby[5]: Author

Class Diagram Hints Use initially to provide an abstraction of the problem domain, then refine further Keep a clear separation between specification and implementation Keep them understandable Beware of cyclical generalization Use associations where there are relationships

Example: Simple Online Bookstore Customers search for and select books, order them, provide credit card information to pay for them, and provide delivery information. Information on valid sales (sales for which the credit card is approved) is forwarded to a warehouse where ordered books are collected and shipped. Invalid sales are declined.

Examples of Class Diagram www.uml-diagrams.org http://www.uml-diagrams.org/class-diagrams-examples.html e.g., http://www.uml-diagrams.org/examples/online-shopping-domain-uml-diagram-example.html?context=cls-examples e.g., http://www.uml-diagrams.org/android-camera-uml-class-diagram-example.html