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Composition

When a field within a class has the type of a class or interface, objects of that class will be able to hold a reference to objects of another class. This is called an association.

There are multiple types of associations (for example, one-to-one, one-to-many, many-to-one, many-to-many) as well as some nuances. For example, when a class fully owns/controls the life cycle of one of its associations, then we talk about an aggregation.

As soon as you create such associations, you are composing OO elements together. Effectively, when you do so, you are reusing other elements to provide some or all of the functionality that you need.

Composition is, of course, central to OO code since it is natural to decompose complex systems into ...