Association, Aggregation and Composition

What is Association, Aggregation and Composition?

These are different terms which represents relationships among objects.

Association(i.e. a group of people organized for a joint purpose):

• It is a relationship among the objects. 
• It means a relationship between two or more objects which are linked to each other but each object has there own life cycle and there is no owner.

Example:

Let's take an example of the relationship between a Teacher and a Student.

• Multiple student can be associated with a single teacher.
• A single student can be associated with multiple teacher.

But there is no ownership between them. Each objects have there own life cycle. i.e. Both can be created and deleted independently.

Note: Association is represented by single straight line, as shown in the above image.

Aggregation(i.e. the collection of related items of content so that they can be displayed or linked to):

• It is a special type of Association, where all object have their own lifecycle but there is ownership.

Example:

Let's take an example of the relationship between Department and Teacher.

• A Teacher may belong to multiple Departments. i.e., teacher is a part of multiple departments.
• A department can have multiple teachers also.

But, if we delete Department object then Teacher object will not get destroyed. Teacher object will have it's own life cycle. In this scenario Department is the owner.

Note: Aggregation is represented by diamond straight line, as shown in the above image.

Composition(i.e. the nature of something's ingredients or constituents):

• It is specialized form of Aggregation. i.e. It is a strong type of Aggregation.
• In this relationship child object does not have a lifecycle without parent object.
• If parent object is deleted then child object is also deleted.

Example:

Let's take an example between Company and Company Location.

• Here Company Location is completely dependent on Company object.

If Company object is deleted then Company Location object is also deleted. So, child object is dependent on parent object.

Note: Composition is represented by solid diamond straight line, as shown in the above image.

Conclusion:

So if we closely observe the concepts of Association, Aggregation and Composition, we can say Aggregation and Composition are a special type of Association. Composition is again a special type of Aggregation. So in other words we can say that 

• Composition is a subset of Association and Aggregation.
• Aggregation is a subset of Association.