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MapQuest Senior Software Engineer Interview Question

I interviewed in Denver, CO and was asked:
"Explain the reasoning behind the OO practice of favoring composition over inheritance. When would you use one or the other?"
Tags: design, software, object oriented
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Part of a Senior Software Engineer Interview Review - one of 6 MapQuest Interview Reviews

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Composition means that an object's behaviors are specified by other objects it contains. For instance, a Dragon class might contain a Movement object that specifies flight. Inheritance, as is usually used in OO, means that behavior is specified by an object's position in a class heirarchy. A Dragon would inherit from a FlyingObject class, for instance, and flight behavior would be coded there.

Inheritance makes refactoring behavior tricky, because changing a behavior (e.g. a dragon that can fly in SPACE) implies changing that behavior for all objects that inherit from a superclass. With a design based on composition, the space dragon is as simple as creating a new Movement object that details how to move through space.
- Interview Candidate on Sep 14, 2012 Flag Response

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