gistfile1.java

import java.time.Instant;
import java.time.LocalDate;
import java.time.ZoneId;
import java.util.*;
import java.util.stream.Collectors;

public class PlayingStream {

    static class A {
        public Date date;
        public String rfid;

        public A(Date date, String rfid) {
            this.date = date;
            this.rfid = rfid;
        }

        public Date getDate() {
            return date;
        }

        public String getRfid() {
            return rfid;
        }

    }

    public static void main(String[] args) {
        Date date1 = new Date();
        Date date2 = new Date(1420070400000L);
        Date date3 = new Date(1451606400000L);

        Map<LocalDate, Set<String>> phoneBook = (new ArrayList<A>(){
            {
                add(new A(date1, "test1"));
                add(new A(date1, "test2"));
                add(new A(date1, "test3"));
                add(new A(date2, "test4"));
                add(new A(date2, "test5"));
                add(new A(date3, "test6"));
            }
        }).stream()
                .collect(Collectors.groupingBy(A::getDate,
                        Collectors.mapping(A::getRfid, Collectors.toSet())))
                .entrySet()
                .stream()
                .collect(Collectors.toMap(e -> toLocalDate(e.getKey()), Map.Entry::getValue));
        System.out.println(phoneBook);
    }

    public static LocalDate toLocalDate(Date date) {
        ZoneId defaultZoneId = ZoneId.systemDefault();
        Instant instant = date.toInstant();
        return instant.atZone(defaultZoneId).toLocalDate();
    }

}