Class diagram

Class Diagram¶

In software engineering, a diagram in the UML(Unified Modeling Language) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations(or methods), and the relationship among objects.

diagram

Visibility¶

To specify the visibility of a class member(i.e. attribute or method), these notations must be placed before the member's name:

+: Public
-: Private
#: Protected
~: Package

Relationships¶

A relationship is a general term covering the specific types of logical connections found on class and object diagrams.

diagram_relationship

Instance-level Relationships¶

Dependency¶

Dependency is the relationship of ... uses a ... that indicates that one class depends on another because it uses it at some point in time. One class depends on another if the independent class is a

parameter
local variable

of a method of the dependent class.

Association¶

Association is the relationship of ... has a ... which represents a family of links. An association can link any number of classes. It represents the static relationship shared among the objects of two classes. There are four types of association:

Bi-directional, which has two ends;
Uni-directional, which has one end;
Aggregation;
Composition.

Aggregation¶

Aggregation is the relationship of ... owns a ... which is more specific than association. It is an association that represents a part-whole or part-of relationship. Furthermore, there is hardly a difference between aggregations and associations during implementation, and the diagram may skip aggregation relations altogether.

Composition¶

Composition is the relationship of ... part of .... The main difference between aggregation and composition is:

composition: when the container is destroyed, the contents are also destroyed.
aggregation: when the container is destroyed, the contents are usually not destroyed.

Class-level Relationships¶

Realization/Implementation¶

A realization relationship is between two model elements, in which one model element(the client) realizes (implements or executes) the behavior that the other model element(the supplier) specifies.

Generalization/Inheritance¶

A Generalization relationship indicates that one of the two related classes(the subclass) is considered to be a specialized form of the other(the super type) and the superclass is considered a generalization of the subclass.