优化前：

Map<String,List<String>>precinctMap=newHashMap<>();

for(Map<String,Object>map:policeCardIdByJcwdm){
Stringkey=String.valueOf(map.get("jcwdm"));
Stringvalue=String.valueOf(map.get("policeCardId"));
if(precinctMap.containsKey(key)){
precinctMap.get(key).add(value);
}else{
List<String>newList=newArrayList<>();
newList.add(value);
precinctMap.put(key,newList);
}
}

优化后：

Map<Object,List<Object>>collect=policeCardIdByJcwdm.stream().collect(Collectors.groupingBy(
map->map.get("jcwdm"),
Collectors.mapping(
map->map.get("policeCardId"),
Collectors.toList()
)
));