Software/Inheritance/Is-A

• Is-a relationship

  • Overview

    • An is-a relationship says that every instance of a more specific kind can be treated as an instance of a more general kind. MountainBike is a Bicycle only if code written for bicycles can use mountain bikes without losing the behavior it relies on.

  • In software

    • Software/Inheritance/Class-Based often encodes an is-a claim with a subclass declaration. The declaration alone does not prove the claim: an override can break a promise made by the parent.
    • Software/Subtyping gives the claim a type-level form; behavioral subtyping asks whether substituting the specialized value preserves client expectations.

  • Distinction

    • Software/Inheritance/Has-A means an object contains or uses another object. A Bicycle has a Wheel, but a bicycle is not a wheel. Software/Composition commonly models this relationship.
    • The informal English test is useful for spotting a mistaken hierarchy, but behavior and contracts decide whether substitution is sound.