Association, Aggregation & Composition

Association : An association is  the relation between two objects. In other words its connectivity between two objects. Aggregation and compositions are form of association. 

Aggregation: This is a part of a whole relationship where a part can exist without a whole.

Aggregation is a weak association.  Also called “has a” relationship.

Composition : This is a part of a whole relationship where a part cannot exist without a whole. If a whole is deleted then all parts are deleted.

Composition is a Strong association.

Example:

A House contains Room and TV. Relationship between House and TV is aggregation and relation between House and Room is composition.

A TV can exist without a House-> Weak association (Aggregation)

A Room cannot exist without a House -> Strong association (Composition).

Java Example :

Aggregation

Class LPG{

}

Class Car{

Car(LPG objLPG) //Object is passed from outside   - weak relationship –LPG can exist without a car

{

}

}

Composition

Class Engine{}

Class Car{

                Car(){

                Engine objEng = new Engine();  //lives within car object   - Strong relationship: Engine cannot exist without car.

}

}