public boolean containsKey(Object key) {
    return get(key) != null;
  }
Build a BaseCacheMap.

@param c the underlying client
@param expiration the expiration for objects set through this Map
@param prefix a prefix to ensure objects in this map are unique
@param t the transcoder to serialize and deserialize objects
src/main/java/net/spy/memcached/BaseCacheMap.java