UML: Class Diagrams

[2/24/2020] - I should update this page using examples from mexm-base, and rewrite the prose. A lot of this prose is taken from [1], but the examples aren't very clear. I should probably also implement this into python3 stub classes, so people who read this become familiar with it.

Declaration of Classes

The declaration of classes is pretty simple

@startuml

class class_1

class class_2

@enduml

Association and Multiplicity

f two classes in a model need to communicate with each other, there must be a link between them, and that can be represented by an association (connector).

Association can be represented by a line between these classes with an arrow indicating the navigation direction. In case an arrow is on both sides, the association is known as a bidirectional association.

We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association. The example indicates that a Student has one or more Instructors:

@startuml

class Student

class Teacher

'a single student can associate with multiple teachers

Student "1..N" -- Teacher: learns from >

@enduml

This example indicates that every Instructor has one or more Students:

@startuml

class Student

class Teacher

'a teacher has multiple students

Student "1..N" -- Teacher: learns from <

@enduml

@startuml

' this example doesn't work, need to look how to do this

class Student

class Teacher

Student "1..N" -- "1..N" Teacher: teacher <

Student "1..N" -- "1..N" Teacher: learns from >

@enduml

We will go over the three types of association connectors: association, aggregation, and composition. These examples are taken from [1]

Aggregation and Composition are subsets of association meaning they are specific cases of association. In both aggregation and composition object of one class "owns" object of another class. But there is a subtle difference.:

• Aggregation implies a relationship where the child can exist independently of the parent. Example: Class (parent) and Student (child). Delete the Class and the Students still exist. It's important to note that the aggregation link doesn't state in any way that Class A owns Class B nor that there's a parent-child relationship (when parent deleted all its child's are being deleted as a result) between the two. Actually, quite the opposite! The aggregation link is usually used to stress the point that Class A instance is not the exclusive container of Class B instance, as in fact the same Class B instance has another container/s.
• Composition implies a relationship where the child cannot exist independent of the parent. Example: House (parent) and Room (child). Rooms don't exist separate to a House. We should be more specific and use the composition link in cases where in addition to the part-of relationship between Class A and Class B - there's a strong lifecycle dependency between the two, meaning that when Class A is deleted then Class B is also deleted as a result

Inheritance: Generalization

Generalization is the term that we use to denote abstraction of common properties into a base class in UML. The UML diagram's Generalization association is also known as Inheritance. When we implement Generalization in a programming language, it is often called Inheritance instead. Generalization and inheritance are the same. The terminology just differs depending on the context where it is being used.

Composition Example:

We should be more specific and use the composition link in cases where in addition to the part-of relationship between Class A and Class B - there's a strong lifecycle dependency between the two, meaning that when Class A is deleted then Class B is also deleted as a result

References:

[1] Visual Paradigm. "UML Association vs Aggregation vs Composition"

• https://plantuml.com/class-diagram