UML: Unified Modeling Language

1. Preface

2. The models

2.1. Use case diagrams

This first kind, as a scenario, describes what happens when an external observer, which is an actor, interacts with a system (use case). It represents a discrete unit of interaction between a user and the system. The connection between an actor and a use case is called communication (or communication association). In a set of single discrete units, many actors could have communication with a single use case, or a single use case could have interaction with many actors (actor could be human or machine entity). Booking a room in a hotel, is an example of the Use case model.

2.2. Class diagram

UML class notation is a rectangle with three parts: class name, attributes (properties or instances), and operations (methods). Names of abstract classes (interfaces), are written in italics.The association between classes are the connecting links. In class diagram, we can find the following relationships:
- association : relationship between instances of the two classes.
- aggregation : association in which one class is a collection. (It has a diamond end pointing to the collection class: here Hotel Reception has a collection of rooms).
- generalization: : inheritance link indicating one class extends of the other. It has a triangle which points to the superclass. Payment is a superclass for Credit and Internet Banking.
- navigability : arrow on an association that shows which direction the association will take. This arrow shows who own the other (here Hotel Reception owns payment class). An associations with no navigability arrows is bi-directional.
The multiplicity of an association is the number of possible instances of the class associated with a single instance of the other class. Multiplicities are single numbers or ranges of number.
Here are the most common used multiplicities.