Past exam of the year 1 of the computer science course of the University of Oxford / 2018 / Imperative programming / 6 / b

The Command pattern separates a request into a common command interface, concrete commands carrying arguments and execution behavior, a receiver doing the application work, an invoker requesting execution or retaining history, and a client assembling these software objects. A menu item, toolbar button or keyboard shortcut can invoke the same command without duplicating the playlist manipulation.

SongSuite is the receiver. The application/controller invokes its operations, and the returned Change software objects hold the concrete inverse/forward behavior. A command wrapper can expose an initial execute that calls the appropriate SongSuite operation and then delegates later undo/redo to its returned record. History is the history-managing part of the invoker. Separating these roles avoids making a graphical control know how the playlist representation is edited; requests can be retained and reversed through a common object interface.