UML

UML

Class diagrams

Class

A class has a name, fields and operations

A class is NOT an object, objects are instances of classes

Each field or method has a visibility: public, protected, private or package internal

Association

Expresses: "Class A has a relationship with Class B"

An association might be directed and/or have a multiplicity/cardinality

Aggregation

Expresses: "The whole "comprises" the parts", "The parts "belong to" the whole"

Destroying the "whole" does not destroy the parts

The parts may be shared with other classes

Composition

Expresses: "The whole "has"/"contains" the parts", "The parts "are member of" the whole"

The parts are destroyed along with the "whole"

A composition is an aggregation and

Note about association/aggregation/composition

An aggregation and a composition are both more specific cases of an association

A composition is a strong relationship of the parts to the whole, if the parts can live without the whole, use an aggregation

Inheritance

Class inheritance

Expresses: "The derived class "is a" base class"

The base class generalizes/abstracts the derived classes

Derived classes specialize/extend the base class

Interface inheritance

Expresses: "The derived class "behaves like" the interface"

The interface is a specification/contract/behaviour for the class

The classe implements the behaviour of the interface

Use case diagrams

Actor

Actually a role taken on by some actor

Use case

Actions accessible to specific actors

Example

The rules for associations are similar to the ones of class diagrams