Composition

As the name suggests, a class is a composition of another one. This is somewhat similar to aggregation, with the difference being that the dependent class ceases to exist when the main class ceases to exist. For example, a House is made up of a Room, but the Room ceases to exist if the House is destroyed, as shown in the following diagram:

In practice, especially in languages such as Java that have garbage collectors, the boundary between composition and aggregation is not so well defined. Objects are not destroyed manually; when they are no longer referenced, they are automatically destroyed by the garbage collector. For this reason, from a coding point of view, we should not really be concerned if we deal with a composition or an aggregation relationship, but it's important if we want to have a well-defined model in UML.