Association , Aggregation and Composition

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with single teacher and single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently.

Aggregation is a specialize form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy.
UML:
An aggregation relationship is indicated by placing a white diamond at the end of the association next to the aggregate class. If B aggregates A, then A is a part of B, but their lifetimes are independent:


Composition is again specialize form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.
UML
Composition, on the other hand, is shown by a black diamond on the end of association next to the composite class. If B is composed of A, then B controls the lifetime of A.

Let’s take another example relationship between Questions and options. Single questions can have multiple options and option can not belong to multiple questions. If we delete questions options will automatically delete.
Composition is used for objects that have a HAS-A relationship to each other. A car has-a metal frame, has-an engine, and has-a transmission. A personal computer has-a CPU, a motherboard, and other components.

Compositions:
* Typically use normal member variables
* Can use pointer values if the composition class automatically handles allocation/deallocation
* Responsible for creation/destruction of subclasses

Aggregations:
* Typically use pointer variables that point to an object that lives outside the scope of the aggregate class
* Can use reference values that point to an object that lives outside the scope of the aggregate class
* Not responsible for creating/destroying subclasses

IS_A HAS-A difference
IS-A :Public inheritance Mango is a fruit , I am a man .
is-a (subsumption) is a relationship where one class D is a subclass of another class B (and so B is a superclass of D).
HAS -A : has-a relationship in an object is called a member field of an object. Multiple has-a relationships will combine to form a possessive hierarchy , Composition , Aggregation and association all comes under this But all have different Scope.
Private and Protected Inheritance alsocpmes under HAS-A relationship

REf
http://ootips.org/uml-hasa.html