Java 实现将List中的实体类按照某个字段进行分组并存放至Map中操作

public static void main(String[] args) {
 List<User> list = new ArrayList<>();
 list.add(new User(1, 1));
 list.add(new User(1, 2));
 list.add(new User(2, 1));
 list.add(new User(2, 3));
 list.add(new User(2, 2));
 list.add(new User(3, 1));
 Map<Integer, List<User>> map = new HashMap<>();
 for(User user : list){
 if(map.containsKey(user.getId())){//map中存在此id，将数据存放当前key的map中
 map.get(user.getId()).add(user);
 }else{//map中不存在，新建key，用来存放数据
 List<User> tmpList = new ArrayList<>();
 tmpList.add(user);
 map.put(user.getId(), tmpList);
 }
 }
 System.out.println(map.toString());
 }

List<Map<String, Object>>>分组
List<Map<String, Object>>> list = new ArrayList<>();
Map<Long, List<Map<String, Object>>> contractIdMap =
 list.stream().collect(Collectors.groupingBy(m -> (Long.parseLong(m.get("contractId").toString()))));
Map<Long, List<Map<String, Object>>> nameMap =
 list.stream().collect(Collectors.groupingBy(m -> (m.get("name").toString())));

/**
* 按照List<Map<String,Object>>里面map的某个value重新封装成多个不同的list, 原始数据类型List<Map
* <String,Object>>, 转换后数据类型Map<String,List<Map<String,Object>>>
*
* @param list
* @param oneMapKey
* @return
*/
private static Map<String, Object> change(List<Map<String, Object>> list, String oneMapKey) {
Map<String, Object> resultMap = new HashMap<String, Object>();
Set setTmp = new HashSet();
for (Map<String, Object> tmp : list) {
setTmp.add(tmp.get(oneMapKey));
}
Iterator it = setTmp.iterator();
while (it.hasNext()) {
String oneSetTmpStr = (String) it.next();
List<Map<String, Object>> oneSetTmpList = new ArrayList<Map<String, Object>>();
for (Map<String, Object> tmp : list) {
String oneMapValueStr = (String) tmp.get(oneMapKey);
if (oneMapValueStr.equals(oneSetTmpStr)) {
oneSetTmpList.add(tmp);
}
}
resultMap.put(oneSetTmpStr, oneSetTmpList);
}
return resultMap;
}