Relationships in OOAD

Relationships

• A relationship is a connection among things
• Graphically represented with different kinds of lines used to distinguish the kinds of relationships

There are three major kinds of relationships. They are:

Dependency Generalization Association

Dependency 

• A dependency indicates a semantic relationship between two or more classes in which a change in one may force changes in the other although there is no explicit association between them 
• Graphically, a dependency is rendered as a dashed directed line, directed to the independent thing 

Generalization 

• In generalization relationship one element (the parent) is generalization of another element (the child) 
• It is also called as "is-a-kind-of" relationship 
• Graphically represented as a solid directed line with a large open arrowhead, pointing to the parent

Association 

Association is a structural relationship where objects of one class are connected to objects of another class. 

• An association is

Used to show a 'knows-a' relationship  Either unidirectional or bidirectional  Represented by solid line with a label (optional) and with a direction indicator

• Associations may optionally have role names and multiplicity symbols on either end of the association line (next to the class icon)

Association properties 
Name 
Describes the nature of the relationship to convey the meaning clearly
Direction 
A triangle pointer in the direction that we want to read the name
Role 
A class may play specific role in a relationship A class can participate in many associations and thus have multiple (different) roles
Multiplicity

Multiplicity	Representation
Exactly one	1
Zero or one	0..1
Many	0..*
One or more	1..*

Association Types
Aggregation is

• Used to show a 'has-a' or a 'whole-part' relationship 
• It is represented as a solid line with an open diamond on the 'whole' end 

Composition: Stronger aggregation relationship

• The part cannot exist without the whole. For example, if the school closes down all the departments will go away with it