Past exam of the year 1 of the computer science course of the University of Oxford / 2023 / Imperative programming / 7 / b / Solution

The design creates a separate concrete class in object-oriented programming for each selected platform combination. With $P$ independently selectable platforms, there are up to $2^P$ combinations. Adding a platform can therefore multiply the number of classes, even though each platform's send operation and per-message charge are simple independent behaviors.

Sending code and charges are duplicated across many classes; a changed charge must be updated consistently in every affected combination. A forgotten update can make billing and sending disagree. A customer's preferences are also tied to a concrete class, making recombination cumbersome. The repeated implementation has low modularity: it encodes a whole subset where a composition of independent capabilities would suffice. The Decorator pattern addresses exactly that structure.