element cannot be mapped to a null key

Map<Object, Long> a1Map = groupStoreVisiList.stream().filter(map->map.getA1() != null)//这里需要加个判断，该字段不能为null
        .collect(Collectors.groupingBy(p -> p.getA1(),Collectors.counting()));

原文：https://www.cnblogs.com/magichu/p/14884331.html