code: public <K2 extends K> BlockingListMaker<K2, E> useKeys(Collection<? extends K2> keys) {
          @SuppressWarnings("unchecked")
          BlockingListMaker<K2, E> stricterThis = (BlockingListMaker<K2, E>) this;
          stricterThis.keys = keys;
          return stricterThis;
      }
docs: Specify the keys associated with the eventual elements of this list