Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Imperative programming (resit) / 6 / c

Class inheritance defines a software class by extending another software class, inheriting its available operations and optionally adding members or providing method overriding. In languages supporting it, traits and mixins can contribute reusable behavior as well.

Use inheritance for a sound specialization whose instances can satisfy the superclass's contract wherever its instances are expected. For example, every specific gate in part (d) is usable through the common gate object interface. Subtyping supports this substitution and dynamic dispatch selects the specialized calculation. Merely noticing shared code does not establish the required behavioral relationship; object composition is often better for a replaceable component or policy that an object contains. A subclass must preserve the superclass's promised behavior, rather than just sharing its field names.