set1 <User>   //第一个user集合为set1

if(null!=set2&&set2.size()>0){
   if(set1.size()>0){
  set1.removeAll(set2);  //set1中去除set2已有的对象
  set1.addAll(set2);      //set1中添加set2
   }else{
       set1=set2;
   }
}

//重写equals、hashcode方法，只要userId一样，则认为两个对象相等
@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    User user= (User) o;
    return Objects.equals(appId, user.userId);
}

@Override
public int hashCode() {
    int result = 17;
    result = 31 * result + (userId== null ? 0 : userId.hashCode());
    return result;
}