Association is a concept in OOP to represent relationship between two/more objects in the system.

Relationship can be categorized in many aspects: form, multiplicity, directional.

There are five forms of relationship:

• Association
  • is a superset of aggregation and composition
  • represents "using" relationship
• Aggregation
  • is a subset of association
  • represents "has-a" relationship
  • is a weak association
  • child can exist independently of the parent
• Composition
  • is a subset of association
  • represents "part-of" relationship
  • is a strong association
  • child can not exist independently of the parent
• Dependency
• Inheritance

For multiplicity (a.k.a. cardinality) we have:

• one-to-one
• one-to-many
• many-to-many

For directional we have:

• Unidirectional
• Bidirectional