Aggregation can be said as a relation between two classes that is best described as a has-a and whole/part relationship. It is a more specialized version of the association relationship. Now, what is association? In Java, when you think of a class having that has a member of the different type, then there an association can be formed. In a Java class, where there lies an entity reference, it becomes an aggression. Let us take a situation, Student object contains many information such as roll, name, email_add etc. It contains one more object named rank, which contains its own information such as subj_Name, marks, board_name, exam-type etc. as given below.
class Student{
int roll;
String name;
Rank ranking;
}
In this case, Student has an entity reference ranking, so the relationship is Student HAS-A ranking Aggregation is also used for code reusability. An aggregation is a form of association where the relation of Association can be considered the containing class 'owning' the contained class. The lifetime of that relationship cannot be defined. 'Owning' can be determined as a single-direction Association.
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