Class Diagram

A class diagram is used to describe:

• what the objects that should be stored in the system are.
• how these objects are structured.
• how these objects relate to each other.

Example

class diagram for a company's administration

Components:

• Relationships:
  • between 2 classes
  • aka association,
  • is shown with a solid line.
  • Associations may have a name.
• Multiplicities:
  • quantifies the relationships between classes.
  • for example: in the previous diagram an employee can have one address. and an address can have many employees living in it.
  • drawn at the end of the associations.

Notation

Class

• A class has zero or more attributes
• For every attribute a standard data type can be specified.
• Standard data types:
  • Integer
  • Date
  • Real
  • Time
  • Boolean
  • Money
  • String

Association

• n..m, k..i are the multiplicities.
• the association can also be directional represented with arrows.

There can be multiple associations between classes:

Multiplicities:

• many: *
• zero to many: 0..*
• one to many: 1..*
• only one: 1
• only two: 2
• 1 to 3: 1..3

Roles

A role name can be used rather than an association name, if a class has a specific role in the association.

Reflexive association

Used to model an entity having a relationship to itself. For example a manager .

Class Diagram as a model of the physical world.

• View: Focus is on data (not functions)
• Representation: the subject
• Simplified: only the data that should be in the system.