<figure class="highlight"><pre><code class="language-text" data-lang="text">name: uml class: middle, center template:inverse # UML [Unified Modelling Language] ## Class Diagrams by [João Rocha da Silva](https://silvae86.github.io), based on the [book](https://dl.acm.org/citation.cfm?id=1554749) by Ullman and Widom. --- name: agenda class: middle, center ## Agenda .index[ .indexpill[[Purpose](#purpose)] .indexpill[[Classes](#classes)] .indexpill[[Attributes](#attributes)] .indexpill[[Associations](#associations)] .indexpill[[Multiplicity](#multiplicity)] .indexpill[[Generalization](#generalization)] .indexpill[[Types of Generalization](#typesofgeneralization)] .indexpill[[Aggregation](#aggregation)] .indexpill[[Composition](#composition)] .indexpill[[Self-association](#selfassociation)] .indexpill[[Qualified association](#qualifiedassociation)] .indexpill[[N-ary associations](#nary_associations)] .indexpill[[N-ary association classes](#nary_association_classes)] .indexpill[[Derived elements](#derived_elements)] ] .index[ .indexpill[[Useful software](#software)] .indexpill[[References](#references)] ] --- name: purpose ## Purpose - To represent the data model of an information system - Programmers and system designers alike can understand - Often derived from the reality of a system as described by the client (usually in text) - Can later be converted into a relational schema .footnote[These slides will use the example of a furniture manufacturing company to illustrate the syntax of UML class diagrams.] --- name: classes ## Classes Classes are used to represent the main entities of the system. Their syntax consists of a box with two main sections. - The first section contains the **name** of the class - The bottom will contain all the Class's [Attributes](#attributes), **one per line**. .center[.imgscaledup[]] .footnote[Class names are always represented in the singular, as a class denotes a **type** of entity, instead of a **set** of all entities of a certain type.] --- name: attributes ## Attributes - Attributes represent characteristics of all objects of the Class. They have basic types like `integer`, `double`, `string`, etc*.* - Attributes cannot be multi-valued; if you have an attribute that can have multiple values for an object of a class, that attribute is likely a Class and should be promoted to that. --- name: associations ## Association - Associations are binary relationships between classes - Represented by a line drawn between the two classes that we want to associate .center[.imglg[]] At each end of the line we add the [multiplicity](#multiplicity) of the association*.* --- name: multiplicity ## Multiplicity - Serves to specify the cardinality (or possible number of elements) of some collections of elements. - It is specified through an interval with a lower and upper bound, which can be infinite. | Multiplicity | Equivalent | Cardinality | | ------------ | ---------- | --------------------------------------------------- | | 0..0 | 0 | Collection must be empty (rare, but can occur) | | 0..1 | | Must contain zero or one instance | | 1..1 | | Must contain exactly one instance | | 0...* | * | Must contain zero or more instances | | 1...* | | Must contain at least one or more instances | | 5...5 | 5 | Must contain exactly 5 instances | | m...n | | Must contain at least `m` and at most `n` instances | --- name: associationclass ## Association Class - Sometimes we need to represent attributes derived from an association of classes instead of classes themselves .center[.imgscaledup[]] - An attribute of the product? No! That would make the quantity of that product the same for every order in the system. That works if we wanted to save the **quantity in stock** of each product, for example, which only depends on the product and nothing else. --- name: associationclassexample1 ## Association Class (Example 1) - A Class that is derived from an association between several classes - Usually emerges to represent characteristics of the association. Groups of attributes that depend on the associated classes - Can be derived from many-to-many relationships, one-to-one and many-to-one associations .center[.imgscaledup[]] - Denoted by a box (usually without a title) connected by a dashed line to the solid line denoting the association between two classes. --- name: associationclassexample2 ## Association Class (Example 2) .center[.imgscaledup[]] --- name: generalization ## Generalization - Used to extract the common characteristics of a set of classes. A **superclass** is extracted, containing the common attributes of all **subclasses**. - It is represented by an equilateral triangle pointing to the superclass and attached to it, with lines coming from it towards all subclasses. .center[.imglg[]] .foonote[`Employee`s and `Customer`s are both `Person`s, so they have the attributes of a `Person`, as well as their own particular attributes] --- name: typesofgeneralization ## Types of Generalization - Generalizations can be - **Overlapping** (an object can be of more than one subclass) or **Disjoint** (can only belong to one subclass) - **Partial** (objects may not belong to any of the subclasses) or **Complete** (all objects of the superclass must be also objects of one or more subclasses) .center[.imgfull[]] --- name: aggregation ## Aggregation - A special kind of association, denoted by a line between two classes. - At one end we place a white diamond, which means `0...1`. - For the other end, if nothing is written, the syntax implies `*`. We can restrict the lower bound of that multiplicity by writing something different, e.g. `1..*`. .center[.imgscaledup[]] .footnote[Does **not** express parent-child relationships. If the diamond end object is deleted, the related objects still live on.] --- name: composition ## Composition - A special kind of Aggregation, where the diamond end denotes a `1..1` multiplicity - If the object of at the diamond side is destroyed, so must be the related ones in the opposite end of the Composition .center[.imglg[]] - If the campus is demolished, it makes no sense to keep track of its buildings and parks anymore. .footnote[.red[*] A mnemonic: black = "death". "If the whole is deleted, so are all the parts".] --- name: associationvsaggregationvscomposition ## Association vs. Aggregation vs. Composition - Compositions are special cases of Aggregations and Aggregations are special cases of Associations. .center[.largemargin[.imgscaledup[]]] --- name: selfassociation ## Self-Association - It is also possible to specify associations between a Class and itself. - Useful for representing hierarchies / subcomponents (one-to-many) or graphs (many-to-many). .center[.imgscaledup[]] --- name: qualifiedassociation ## Qualified association - One or more attributes of an association used to navigate from the class with the qualifier to the other - "Access key" from the qualifier to the qualified class .center[.imglg[]] --- name: nary_associations ## N-ary associations - n-ary associations express relationships between more than 2 classes. - Multiplicity is calculated one by one, by "fixating" all other classes to 1 and calculating the multiplicity of that "end" of the association according to the requirements. - Ternary = 3 classes involved; Quaternary = 4 classes; n-ary = n classes... .center[.imgscaledup[]] --- name: nary_association_classes ## N-ary association classes - Association classes can also be associated to n-ary associations. - Attributes need to depend on all of the associated classes! .center[.imgscaledup[]] --- name: constraints ## Constraints - Used to specify conditions that must be met by the system - Denoted by SQL-like text between brackets `{}`, either close to the relevant class (or classes) or connected by a dashed line to those elements .center[.imgscaledup[]] --- name: derived_elements ## Derived elements - There can be elements (classes, attributes or associations) that are computed from others but do not actually exist. - Notation: `/` before the name of the derived element. - Derived elements usually have an associated constraint that relates them with other elements. - They add complexity to the diagram, so should be used sparingly to highlight important pre-computed attributes, associations or classes --- name: derived_attributes ## Derived Attributes - Can be used to highlight a frequently used element that is expensive to compute - Indicate the need to pre-compute an attribute and save it in the system, as it will be typically read many times but written only a few - "Caching" of results .center[.imgscaledup[]] --- name: derived_associations ## Derived Associations - Similar to derived attributes, but express associations that result of computation - Expensive to compute: perform an expensive computation and save the result once on certain changes to the system (i.e. when a new `Order` is added for a `Client`, recompute the `mostPurchasedProduct` and save it). - Then just read the `mostPurchasedProduct` instantly many times, without having to re-compute. .center[.imgscaledup[]] --- name: derived_classes ## Derived Classes - Classes computed from other elements in the system - Same logic as the other derived elements .center[.imgscaledup[]] --- name: software ## Useful software - For diagramming - [draw.io](http://draw.io) - Free online collaborative diagramming, uses Google Drive - [Dia](http://dia-installer.de) - Free for all Operating Systems - [Visual Paradigm](https://www.visual-paradigm.com) - Paid for all Operating Systems - [OmniGraffle](https://www.omnigroup.com/omnigraffle) - Paid for Mac --- name: references ## References - *Jeffrey D. Ullman and Jennifer Widom. 2008. A First Course in Database Systems. 3rd Edition* - Section 4.7 Unified Modeling Language - *CS145 Lecture Notes (7) -- Higher-Level Design: UML. [Link](http://infolab.stanford.edu/~ullman/fcdb/jw-notes06/uml.html).*</code></pre></figure>