public class Message {
private int id;
private User sender;
private User receiver;
private String text;
private Date senddate;
..
}
我有
List<Message> list= new ArrayList<>();
我需要将它们转换成
TreeMap<User,List<Message>> map
我知道如何使用以下方法将其转换为HashMap
list.stream().collect(Collectors.groupingBy(Message::getSender));
但我需要 TreeMap,其中: Key - 用户按最新消息发送日期排名,先发最新的消息 Value - 按发送日期最新排序的列表
用户类的一部分
public class User{
...
private List<Message> sendMessages;
...
public List<Message> getSendMessages() {
return sendMessages;
}
}
用户比较器:
public class Usercomparator implements Comparator<User> {
@Override
public int compare(User o1, User o2) {
return o2.getSendMessages().stream()
.map(message -> message.getSenddate())
.max(Date::compareTo).get()
.compareTo(o1.getSendMessages().stream()
.map(message1 -> message1.getSenddate())
.max(Date::compareTo).get());
}
}
Comparator<Message> byDate=Comparator.comparing(Message::getSenddate); Comparator<User> userComparator=Comparator.comparing(user -> Collections.max(user.getSendMessages(), byDate), byDate);
... - Holger