Class diagram

The UML representation of a class is a rectangle containing three compartments stacked vertically.

The top compartment shows the class’s name. The middle compartment lists the class’s attributes. The bottom compartment lists the class’s operations.

Sometimes, attributes can have default value (Ex: flightNumber : Interger = 0).

To display visibility on the class diagram, you place the visibility mark in front of the attribute’s or operation’s name.

”-“ means private.

”+” means public.

”~” means package.

”#” means protected.

Inheritance is indicated by a solid line with a closed, unfilled arrowhead pointing at the super class.

An association is a linkage between two classes.

A bi-directional association is indicated by a solid line between the two classes. At either end of the line, you place a role name and a multiplicity value.

“0..*” means zero or more

“0..1” means zero or one

“0..5” means zero to five

In a uni-directional association, two classes are related, but only one class knows that the relationship exists.

A uni-directional association is drawn as a solid line with an open arrowhead

A class and an interface differ: A class can have an actual instance of its type, whereas an interface must have at least one class to implement it.

An interface is considered to be a specialization of a class modeling element. Therefore, an interface is drawn just like a class, but the top compartment of the rectangle also has the text “«interface»”.

A dotted line with a closed, unfilled arrow means realization (or implementation)

In the class diagram shown in this figure , the association between the Flight class and the FrequentFlyer class results in an association class called MileageCredit. This means that when an instance of a Flight class is associated with an instance of a FrequentFlyer class, there will also be an instance of a MileageCredit class.

Aggregation is a special type of association used to model a “whole to its parts” relationship.

In basic aggregation relationships, the lifecycle of a part class is independent from the whole class’s lifecycle.

To represent an aggregation relationship, you draw a solid line from the parent class to the part class, and draw an unfilled diamond shape on the parent class’s association end.

The composition aggregation relationship is just another form of the aggregation relationship, but the child class’s instance lifecycle is dependent on the parent class’s instance lifecycle.

That the composition relationship is drawn like the aggregation relationship, but this time the diamond shape is filled.