public class Message {

private int id;

private User sender;

private User receiver;

private String text;

private Date senddate;

..

}

我有

List list= new ArrayList<>();

我需要将它们转换为

TreeMap> map

我知道如何使用转换为HashMap

list.stream().collect(Collectors.groupingBy(Message::getSender));

但我需要TreeMap：

密钥 – 具有最新消息的用户首先发送日期

价值 – 列表按senddate排序第一

User类的一部分

public class User{

...

private List sendMessages;

...

public List getSendMessages() {

return sendMessages;

}

}

用户比较器：

public class Usercomparator implements Comparator {

@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());

}

}