Java按值对映射进行分组,其中值为列表

只需循环浏览地图条目和其中的列表:

public static void main(String[] args) {
  Map<String, List<User>> map = new HashMap<>();
  map.put("projectA", Arrays.asList(new User(1,"Bob"),new User(2,"John"),new User(3,"Mo")));
  map.put("projectB",Arrays.asList(new User(2,"John"),new User(3,"Mo")));
  map.put("projectC",Arrays.asList(new User(3,"Mo")));

  Map<User, List<String>> result = new HashMap<>();
  for(Map.Entry<String, List<User>> e:map.entrySet()) {
    for(User u:e.getValue()) {
      result.putIfAbsent(u, new ArrayList<>());
      result.get(u).add(e.getKey());
    }
  }
  System.out.println(result);
}
public static record User(int id, String name) {}

打印

{User[id=1, name=Bob]=[projectA], User[id=2, name=John]=[projectB, projectA], User[id=3, name=Mo]=[projectB, projectA, projectC]}