UML: Aggregation or Composition

In this Association there are two types mainly Aggregation Association and Composition Association.

Aggregation Association signifies that the whole object can exist without the Aggregated Object. For example in the below figure we have three classes university class, department class and the Professor Class. The university cannot exist without department which means that university will be closed as the department is closed. In other words lifetime of the university depend on the lifetime of department.

In the same figure we have defined second Association between the department and the Professor. In this case, if the professor leaves the department still the department continues in other words department is not dependent on the professor this is called as Composition Association.

Note: The filled diamond represents the aggregation and the empty diamond represents the composition. You can see the figure above for more details.

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