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Can composition be bidirectional in a way that both classes are aware of each other?
And if not, what is the default direction of composition?
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Bidirectional Association: Two objects might store each other in fields. For example, in addition to a Person object listing all the books that the person owns, a Book object might list all the people that own it. Aggregation: One object A has or owns another object B, and/or B is part of A.
A composition association relationship represents a whole–part relationship and is a form of aggregation. A composition association relationship specifies that the lifetime of the part classifier is dependent on the lifetime of the whole classifier.
Remember that aggregation and composition are both subsets of association. In both aggregation and composition, an object of one class can be the owner of an object of another class. And in both aggregation and composition, the child objects belong to a single parent object, i.e., they may have only one owner.
Composition is one of the fundamental concepts in object-oriented programming. It describes a class that references one or more objects of other classes in instance variables. This allows you to model a has-a association between objects. You can find such relationships quite regularly in the real world.
Yes, Composition does not add constraints with regards to the navigability of the association. More info on the difference between Accociation, Composition and Aggregations can be found here: UML Composition vs Aggregation vs Association
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