Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Imperative programming / 7 / a

Encapsulation exposes operations with contracts while hiding representation details and controlling changes to state. In a text editor, a document object can privately store text and cursor position and expose insert, delete and moveCursor. Clients cannot directly splice internal storage or set a cursor outside the document. The document's methods enforce its representation invariant, and the implementation can later replace a character array by a different text representation without changing the public editing interface. Merely declaring a field private is insufficient if an accessor returns the mutable representation itself.

Class inheritance defines a subclass by extending another class's behavior, often with method overriding and subtyping. An editor might have an abstract Command type with execute, and subclasses for insertion and deletion. The controller keeps commands through the common type and dynamic dispatch chooses each command's actual operation. Alternatively, a specialized document can extend a base document's documented behavior. An inheritance relationship used for substitution must preserve the superclass contract; borrowing its code alone does not justify exposing incompatible behavior. Commands exemplify the Command pattern without exposing document storage to the controller.