It is a kind of association where there is a part->whole relationship among the parts, but the object that represents the whole doesn't owns the objects that makeup the parts.

Ex: A university owns various departments (e.g., chemistry), and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will continue to exist. Therefore, a University can be seen as a composition of departments, whereas departments have an aggregation of professors. In addition, a Professor could work in more than one department, but a department could not be part of more than one university.

Figure 9.4. Aggregation X Composition

How do we implement those classes in Java?