MessagesQuery.java

import java.util.*;
import static java.util.stream.Collectors.*;
import java.util.stream.Stream;

/**
 * Created by alex on 23.02.15.
 */
public class MessagesQuery {
    public static void main(String... args) {
        Collection<Message> myCombinedCollection = getList();

        Map<String, Optional<Message>> map = myCombinedCollection.stream().collect(groupingBy(Message::mapping, maxBy(Comparator.comparing(Message::getSendDate))));
        List<Message> messages = map.entrySet().stream()
                                              .filter(e -> e.getValue().isPresent())
                                              .map(e -> e.getValue().get())
                                              .collect(toList());
        System.out.println(messages);
    }

    private static Collection<Message> getList() {
        Member member1 = new Member(1L);
        Member member2 = new Member(2L);
        Member member6 = new Member(6L);

        //Member 2 sending
        //2 -> 1
        Message m = new Message(3L, member2, member1, new Date(2015, 2, 28, 0, 0, 0), "Merci pour votre message");
        //2 -> 1
        Message m2 = new Message(8L, member2, member1, new Date(2015, 2, 28, 1, 0, 0), "Oui j'avais bien eu votre message");

        //Member 2 received messages
        //1 -> 2
        Message m3 = new Message(2L, member1, member2, new Date(2015, 2, 27, 0, 0, 0), "Bonjour je suis disponible");
        //1 -> 2
        Message m4 = new Message(4L, member1, member2, new Date(2015, 3, 02, 9, 45, 0), "Je suis disponible");
        //6 -> 2
        Message m5 = new Message(5L, member6, member2, new Date(2015, 3, 01, 0, 0, 0), "Hi there dude");
        //1 -> 2
        Message m6 = new Message(7L, member1, member2, new Date(2015, 2, 27, 1, 0, 0), "Avez vous eu mon message");

        return member2.combined();
    }
}
class Message {

    public Message(Long id, Member sender, Member recipient, Date sendDate, String text) {
        this.id = id;
        this.sender = sender;
        this.recipient = recipient;
        this.sendDate = sendDate;
        this.text = text;

        this.sender.sentMessages.add(this);
        this.recipient.receivedMessages.add(this);
    }

    public String toString(){
        return this.id +"-"+ this.sendDate+":"+text;
    }

    private Long id;

    public Member sender;

    private Member recipient;

    private Date sendDate;

    private boolean messageRead;

    private String text;

    private Integer version;

    public Date getSendDate() {
        return sendDate;
    }

    public Member getSender() {
        return sender;
    }
    public Member getRecipient() {
        return recipient;
    }

    public String mapping() {
        long minId = Math.min(sender.id, recipient.id);
        long maxId = Math.max(sender.id, recipient.id);
        return (maxId + "->" + minId);
    }

    public int getMapping() {
        long minId = Math.min(sender.id, recipient.id);
        long maxId = Math.max(sender.id, recipient.id);
        return (maxId + "->" + minId).hashCode();
    }
}

class Member implements Comparable<Member> {

    public Member(Long id) {
        this.id = id;
        this.sentMessages = new ArrayList<>();
        this.receivedMessages = new ArrayList<>();
    }

    private static final long serialVersionUID = 1L;

    public Long id;

    public Collection<Message> sentMessages;

    public Collection<Message> receivedMessages;

    public Collection<Message> combined() {
        return Stream.concat(sentMessages.stream(), receivedMessages.stream()).collect(toList());
    }

    private Integer version;

    public String toString(){
        return id+"";
    }

    @Override
    public int compareTo(Member o1) {
        return Long.compare(id, o1.id);
    }
}