Item 33: Consider typesafe heterogeneous containers(考虑类型安全的异构容器)
Commonusesofgenericsincludecollections,suchasSetandMap,andsingle-elementcontainers,suchasThreadLocalandAtomicReference.Inalloftheseuses,itisthecontainerthatisparameterized.Thislimitsyoutoafixednumbero