code: public static <K, V> HashMap<K, V> newEmptyHashMap(Iterable<?> iterable) {
          if(iterable instanceof Collection<?>)
              return Maps.newHashMapWithExpectedSize(((Collection<?>) iterable).size());
          return Maps.newHashMap();
      }
docs: Returns an empty map with expected size matching the iterable size if
  it's of type Collection. Otherwise, an empty map with the default size is
  returned.