Java 8 Stream API toMap converting to TreeMap

Question

public class Message {
    private int id;
    private User sender;
    private User receiver;
    private String text;   
    private Date senddate;
..
}

Answer 1

If your list is sorted then just use this code for sorted map.

Map<String, List<WdHour>> pMonthlyDataMap = list
                .stream().collect(Collectors.groupingBy(WdHour::getName, TreeMap::new, Collectors.toList()));

Answer 2

You can use overloaded groupingBy method and pass TreeMap as Supplier:

TreeMap<User, List<Message>> map = list
            .stream()
            .collect(Collectors.groupingBy(Message::getSender,
                    () -> new TreeMap<>(new Usercomparator()), toList()));