OOPS Concepts

Association , Aggregation and Composition is often confusing to developers . Although they have nearly same semantic but they have subtle difference.

     1)    Association – Relationship between two or more objects.  

  

     2)    Aggregation – Relationship between two or more objects where one object exists even if another object destroys.

Example – Organization exist even if an Employee leaves.

Here Organization is one object and Employee another object.

It is a special form of Association where:

§  It represents Has-A relationship.

§  It is a unidirectional association i.e. a one way relationship. For example, department can have students but vice versa is not possible and thus unidirectional in nature.

§  In Aggregation, both the entries can survive individually which means ending one entity will not affect the other entity

  

     3)    Composition- Relationship between two or more objects where one object destroys or become meaningless if associated object destroys.

Example -Car destroys if Engine destroys.

Here Car is one object and Engine is another object.

In Java, we make associated object as final if it is very much required.

Composition is a restricted form of Aggregation in which two entities are highly dependent on each other.

§  It represents part-of relationship.

§  In composition, both the entities are dependent on each other.

§ When there is a composition between two entities, the composed object cannot exist without the other entity.

References

http://javarevisited.blogspot.in/2014/02/ifference-between-association-vs-composition-vs-aggregation.html

http://www.geeksforgeeks.org/association-composition-aggregation-java/