25.07 - Class Diagrams

When making an application, you gotta know what you have to make. For Object Oriented Programming, this is knowing which classes interact with which and what methods these classes contain. One way of doing this is via modelling it in a subset of the Unified Modeling language(UML) called class diagrams. Another type of UML diagram is a use case diagram

UML is not a programming language, it is a visual modelling language made for Object Oriented software. UML is widely used all around the globe.

UML Class Diagrams

An UML diagram can also be called a class diagram. A class diagram can mean different things, but in UML it means the following:

“A class diagram is a type of static structure diagram that describes the structure of a system by showing the system’s classes, their attributes, operations, and the relationships among objects.”

So basically, it is a static(non-changing) representation of how classes, attributes(properties) and methods interact with each other and also their relations to one another.

Structural diagram

A structural diagram contains things like:

• classes
• objects
• components
• how the software will be deployed. Structural diagrams show the ‘static’ view of the program. It will always be like this and will not change during runtime.

Behavioural diagram

A behavioural diagram contains things like:

• use cases
• how things will play out (sequences)
• communication between things
• the state of things
• the activity of things Behavioural diagrams show the ‘active’ view of the program. It can change during runtime and shows the flow of the application.

The structure of a class diagram

Datatypes

Datatypes are written like this: name:type So for example: fruit:string is a string with the name fruit. If you want to assign a value to a variable, you use the = icon. For example: fruit="apple"

Classes

Classes are a collection of objects (or ‘instances’) with similar properties. You could compare it to a blueprint. A class consist of:

• The name of the class
• The description of the properties of the objects inside the class
  • Attributes: Information that’s contained within
  • Operations: The behaviour of the object (what can it do) A class can look 3 different ways, all with a different level of detail:

Attributes and operations(functions)

Attributes and operations are the properties inside a class. Attributes go on top, properties go below them. An attribute or operation can have an accessor modifier which shows how outside classes interact with them:

• Public (+): Can be seen an interacted with by outside classes.
• Private (-): Cannot be seen and interacted with by outside classes.
• Protected (#): Can only be seen and interacted with by classes that inherit the class. Operations are written like this: name(params):return. If the operation has parameters, you put those inside the brackets. If the operation has a return value, you put them behind the brackets (beginning with a :). If the operation has no return value, you type void

instances

Instances are objects that are derived from classes. They’re written like this: Notice how the second person has no name, it is still an instance from the class person.

Relations between classes (arrows and such)

There are a bunch of different associations, so i will just put a handy image in here.

Associations mean the classes can call one another’s methods. An arrow means one class knows the other, but not the other way around.

Below the line, you put the name of the relation.
Above the line, you put the roles of the two classes.
Assocations can also be a property of the class Some examples would look like this:

Multiplicity

Sometimes a class can have more than one relation to the same class. This is called multiplicity. The way of notation multiplicity is being done by the following list:

• None: The class contains exactly one relation
• Number: The class contains the number of relations given(1,2,3,etc..)
• Asterisk(*): The class can contain zero or more relations
• Range(2..5): The class can contain a number between the given range

Examples:
(All these examples have 1 car, since there’s none given)

The car moves 1 person

The car moves exactly 5 people

The car moves between 0 and an infinite amount of people

The car moves between 1 and an intfinite amount of people

The car moves between 1 and 5 people