"There are several reasons to use mix-ins: they extend existing classes in new areas without having to edit, maintain or merge with their source code; they keep project components (such as domain frameworks and interface frameworks) separate; they ease the creation of new classes by providing a grab bag of functionalities that can be combined as needed; and they overcome a limitation of subclassing, whereby a new subclass has no effect if objects of the original class are still being created in other parts of the software."