Association :
Association is the relationships between classes. They can be one-to-one , one-to-many , many-to-many. Aggregation is a special case of Association.
For example : An object of a class Person can be related with only one object of person as a spouse. This is a one to one relation.
An object of class Person can be related to any number of Car object as a owner. This is a one-to-many relation.
Aggregation :
It is regarded as 'has a' relation between two objects. But it is less constrained in a way that one object can exist even if the other one perishes.
For example : A Desktop has a NIC(Network Interface Card). It means that they are related yet the desktop can exist without a NIC.
Composition:
It is a form of Aggregation , but it has more constraint applied over it. Here the object in the relationship will exist only if the other exists.
For example : A Desktop has a Mother Board. The desktop will exist only if the Mother Board Exists. Thus is a strong form of Aggregation .
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