Converting a map into another map using the java 8 stream API

Question

Say I have the following map:

Map<Member, List<Message>> messages = ... //constructed somehow

I would like to use the java 8 stream api in order to obtain a:

SortedMap<Message, Member> latestMessages = ...

Where the comparator passed into the SortedMap/TreeMap would be based on the message sendDate field.

Furthermore, of the list of sent messages, I would select the latest message which would become the key to the sorted map.

How can I achieve that?

edit 1:

Comparator<Message> bySendDate = Comparator.comparing(Message::getSendDate);
SortedMap<Message, Member> latestMessages = third.entrySet().stream()
        .collect(Collectors.toMap(e -> e.getValue().stream().max(bySendDate).get(), Map.Entry::getKey, (x, y) -> {
            throw new AssertionError();
        }, () -> new TreeMap(bySendDate.thenComparing(Comparator.comparing(Message::getId)))));

I get the following compilation error:

The method collect(Collector<? super T,A,R>) in the type Stream<T> is not applicable for the arguments (Collector<Map.Entry<Member,List<Message>>,?,TreeMap>)

Answer 1

Let’s dissolve this into two parts.

First, transform Map<Member, List<Message>> messages into a Map<Message, Member> latestMessages by reducing the messages for a particular communication partner (Member) to the latest:

Map<Message, Member> latestMessages0 = messages.entrySet().stream()
    .collect(Collectors.toMap(
        e -> e.getValue().stream().max(Comparator.comparing(Message::getSendDate)).get(),
        Map.Entry::getKey));

Here, the resulting map isn’t sorted but each mapping will contain the latest message shared with that participant.

---

Second, if you want to have the resulting map sorted by sendDate, you have to add another secondary sort criteria to avoid losing Messages which happen to have the same date. Assuming that you have a Long ID that is unique, adding this ID as secondary sort criteria for messages with the same date would be sufficient:

Comparator<Message> bySendDate=Comparator.comparing(Message::getSendDate);
SortedMap<Message, Member> latestMessages = messages.entrySet().stream()
   .collect(Collectors.toMap(
       e -> e.getValue().stream().max(bySendDate).get(),
       Map.Entry::getKey, (x,y) -> {throw new AssertionError();},
       ()->new TreeMap<>(bySendDate.thenComparing(Comparator.comparing(Message::getId)))));

Since sorting by the unique IDs should solve any ambiguity, I provided a merge function which will unconditionally throw, as calling it should never be required.