Why should you prefer EnumMap and EnumSet over HashMap and HashSet when keys are enums?

EnumSet is internally a bit vector and EnumMap is backed by a plain array indexed by the enum's ordinal, so both are dramatically faster and more memory-efficient than the hash-based versions and need no hashing or boxing.

They also iterate in natural (declaration) order, which is often what you want. The only caveat is that they're tied to a single enum type and are not thread-safe, so wrap them if shared across threads.