Composition and Aggregation

Composition

Composition expresses relationship between objects. It is an association in which one class belongs to a collection. The containing object is responsible for the life time of the object it holds within it. The containing object cannot exist without the main object. It expresses the “has a” relationship among the objects

Example:  Office has a reception, Student has a name, Car has a tyre etc.

Aggregation

It is relationship which is just a reference between objects. The objects are independent of each other.

Example: Chair and person, Chair does not own the person.

Employee and address, address stays even after employee is destroyed.

The table below lists the key differences between composition and aggregation

Composition	Aggregation
Relationship where a part cannot exist without whole	Relationship where a part can exist without whole
Stronger relationship	Weaker relationship
Main object owns the containing objects	Objects are independent of each other
Main object instantiated the containing objects	Objects within the relationship are instantiated independently
Containing objects are destroyed when main object is destroyed	If one object is destroyed other object can survive.