code: private void loadAllAndKeepExisting(Set<? extends K> keys) {
      List<K> keysToLoad = keys.stream()
          .filter(key -> !cache.asMap().containsKey(key))
          .collect(toList());
      Map<K, V> result = cacheLoader.get().loadAll(keysToLoad);
      for (Map.Entry<K, V> entry : result.entrySet()) {
        if ((entry.getKey() != null) && (entry.getValue() != null)) {
          putIfAbsentNoAwait(entry.getKey(), entry.getValue(), /* publishToWriter */ false);
        }
      }
    }
docs: Performs the bulk load where the existing entries are retained.