Java: Aggregation

"Aggregation is a special form of association. It is also a relationship between two classes like association, however its a directional association, which means it is strictly a one way association. It represents a Has-A relationship.

For example consider two classes Student class and Address class. each student must have an address so the relationship between student and address is a Has-A relationship. But if you consider its vice versa then it would not make sense as an Address doesn’t need to have a Student necessarily."

Why we need Aggregation?
To maintain code re-usability.

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