Important
Concepts of Aggregation, Association and Composition
- Inheritance
- Aggregation
- Association
- Composition
Scenario's
:-
- ProjectLead is an employee of Software Company.
- ProjectLead uses a swipe card to enter in Software Company.
- ProjectLead has Developers who work under him.
- ProjectLead has the responsibility of ensuring that the project is successful.
- ProjectLead's appraisal will be judged based on project success.
First
Scenario: - The IS A
Relationship
ProjectLead
is an employee of Software Company. It’s an inheritance
relationship or parent child relationship. Here Employee is the
parent class and ProjectLead is the derived class means ProjectLead
is a type of employee. ProjectLead is inherited from the Employee
class.
Second Scenario: - The USES A relationship: Association
ProjectLead uses a
swipe card to enter in Software Company. Here Both are using each other but
they are having their own object lifetime and they can exist without each
other. The point to think here in this relationship is there is no single
owner.
Association is a relationship where all object have their own lifecycle
and there is no owner. For example like 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.
Third Scenario: - The HAS A relationship with Parent:
Aggregation
ProjectLead has
Developers who work under him. It seems same type of relationship like
Association, but the difference here is that Project Lead is the owner of Developer object. In the meantime both
objects survives independently.
But… the Project Lead
object can have its own life time which is completely disconnected from the
Developer object. From a different perspective, it means that if the
ProjectLead object is deleted, the Developer object does not die.This
relationship is termed as an “Aggregation” relationship.
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. For example like Department and teacher. A single
teacher can not belongs to multiple departments, but if we delete the
department teacher object will not destroy. We can think about “has-a”
relationship.
Fourth Scenario: - The OWNS A relationship or The Death
relationship: Composition
Project Lead has the responsibility of ensuring that the
project is successful. Project Lead's appraisal will be judged based on project
success. Project Lead and the project objects are dependent on each other.
Both the objects is having the same lifetime. In other
words, if the Project Lead is not good
then the project will not be successful and if the project is having issues
then Project Lead will not get good appraisal.
This above
relationship is termed as the composition relationship. In this relationship,
both objects are heavily dependent on each other. Both the objects are having
same lifetime in other words, if one goes for garbage collection the other also
has to be garbage collected.
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 does not have their lifecycle and if parent object
deletes all child object will also be deleted. For example like 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.
