Java 8 lambda Stream list to Map key 重复 value合并到Collection

List list = Lists.newArrayList("1", "2", "3", "1");
  Map> map = list.stream().collect(Collectors.toMap(key -> key,
          value -> Lists.newArrayList(value),
          (List newValueList, List oldValueList) -> {
              oldValueList.addAll(newValueList);
              return oldValueList;
          }));
  System.out.println(JSON.toJSONString(map));

  Map> partsMap = partList.stream().collect(Collectors.toMap(Parts::getMaintainCode, part ->
  Lists.newArrayList(part),(List newValueList,List oldValueList)->
       {oldValueList.addAll(newValueList);
          return oldValueList;
        }));