Stream List<br>>リストの転送
List collect = strLL.stream().flatMap(strs -> strs.stream()).collect(Collectors.toList());
または
List collect = strLL.stream().flatMap(Collection::stream).collect(Collectors.toList());
List collect = strLL.stream().flatMap(strs -> strs.stream()).collect(Collectors.toList());
List collect = strLL.stream().flatMap(Collection::stream).collect(Collectors.toList());