Aggregation

Composition

Aggregation :  An  association   in  which  one  class refers to collection or a another class. This is a part of  a  whole   relationship  where  the  phase   can  exist without the whole .

e.g.: A line item is whole and the products are the phases. If a line item is deleted then the products have not be deleted .

Composition :  An  association   in  which  one  class  refers  to another class or a collection. This is a part of a whole relationship where  the  phase   cannot  implement  without  the  whole .  If  the  whole  is deleted then the parts are deleted.  For example:  An Order is a whole and the line items are the phase. If an order is removed then all the line items should be deleted as well.

Aggregations are not used to be circular.

In  a  garbage-collected  language  like  Java,  The  whole   has  the responsibility of  operating   the  garbage  collector  to  prematurely collect the part by holding reference to it.