code: private static <E> SortedMap<PrefixKey, E> getPrefixSubMap(
        TreeMap<PrefixKey, E> map, PrefixKey lowerBound) {
      PrefixKey upperBound =
          new PrefixKey(lowerBound.getBucket(), lowerBound.getObjectName() + Character.MAX_VALUE);
      return map.subMap(lowerBound, upperBound);
    }
docs: Helper function that handles creating the lower and upper bounds for calling {@link
  SortedMap#subMap(Object, Object)}.
  
  @see SortedMap#subMap(Object, Object)