What is the purpose of composition?

Composition allows software to be developed by assembling existing components rather than crafting new ones.

Composition (sometimes called aggregation) is the process of putting an object (a part) inside another object (the composite). It models the has-a relationship. For example, a FordTaurus can be composed of, among other things, an Engine, Transmission, InstrumentPanel, and so on. In other words, a FordTaurus has an Engine (equivalently, an Engine is part-of a FordTaurus):

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