Association, aggregation and composition are three kind of relationships which classes can have in object oriented programming. Let’s understand the difference between them.
Table of Contents 1. Association 2. Aggregation 3. Composition 4. Summary
1. Association in Java
We call association those relationships whose objects have an independent lifecycle and where there is no ownership between the objects.
Let’s take an example of a teacher and student. Multiple students can associate with a single teacher, and a single student can associate with multiple teachers, but both have their own lifecycles (both can be create and delete independently); so when a teacher leaves the school, we don’t need to delete any students, and when a student leaves the school, we don’t need to delete any teachers.
2. Aggregation in Java
We call aggregation those relationships whose objects have an independent lifecycle, but there is ownership, and child objects cannot belong to another parent object.
Let’s take an example of a cell phone and a cell phone battery. A single battery can belong to a phone, but if the phone stops working, and we delete it from our database, the phone battery will not be deleted because it may still be functional. So in aggregation, while there is ownership, objects have their own lifecycle.
3. Composition in Java
We use the term composition to refer to relationships whose objects don’t have an independent lifecycle, and if the parent object is deleted, all child objects will also be deleted.
Let’s take an example of the relationship between questions and answers. Single questions can have multiple answers, and answers cannot belong to multiple questions. If we delete questions, answers will automatically be deleted.
Sometimes, it can be a complicated process to decide if we should use association, aggregation, or composition. This difficulty is caused in part because aggregation and composition are subsets of association, meaning they are specific cases of association.
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