How do I create nested TreeMaps?

Question

This is somewhat related to my previous question but I've realised that I needed to deal with the issue of nesting earlier in the problem, so here's where I am. (I'm also not far off being a beginner in Java, so please bear with me).

I'm creating a simple booking system for 5 rooms which will take in names, times, room numbers and days. This has to be done using nested TreeMaps. Here's the layout of the data as I see it, where paretheses represent the boundaries of a TreeMap:

(Day, (Room #, (Time, Name)))

As far as I can see, I need one TreeMap for times and names, one for each room, then one for each day. That means one time/name treemap per room per day, which means 1 x 5 x 7 = 35 TreeMaps. Like this:

{Mon,  [Room 1,  (0600, NameA
                  0630, NameB
                  0700, NameC)
        Room 2,  (0600, NameD
                  0630, NameE)
        Room 3,  (0600, NameF
                  0630, NameG)]
Tues,  [Room 1,  (0600, Name1
                  0630, Name2)
        Room 2,  (0600, Name3
                  0630, Name4
                  0700, Name5)]}

(the different bracket types represent the boundaries of the nested TreeMaps)

Having come to that conclusion, my next problem is iterating through a loop to create all those TreeMaps. I can't seem to dynamically generate the TreeMaps using a for loop, because I can't stick a counter's variable number onto the newly-created TreeMap's name.

I did have this:

TreeMap keyDay = new TreeMap();
TreeMap keyRoom = new TreeMap();
TreeMap keyTime = new TreeMap();

but it is only three, which is clearly not enough to allow for duplication of keys - any new entries for e.g. '0900' (time key) or e.g. 'Room 1' (room key) will overwrite the old ones.

Does anyone have any suggestions? Would be much appreciated :)

Why in the world you love TreeMaps so much? You need to redesign your application until it is not too late. Triple-nested TreeMap is just Bad. It is really, Really Bad. I am sure that you can find the better solution. — xappymah, Mar 27 '11 at 18:25
There is nothing wrong with nested Maps, Sets, Lists, Arrays or whatever if this is appropriate for the problem. Yet what you want is kind of a relational database in memory and you just *assume* that your maps must be nested. — Ingo, Mar 27 '11 at 20:39

score 3 · Accepted Answer · edited Nov 22 '16 at 05:16

I agree it is really super mega bad. But if you want to implement this functionality, I recommend HashMaps instead. Here is a class that implements the functionality you want:

public class Reservations {

static final int  DAY_SUN = 0, DAY_MON = 1, DAY_TUE = 2, DAY_WED = 3, DAY_THU = 4, DAY_FRI = 5, DAY_SAT = 6;
static final int ROOM_1 = 0, ROOM_2 = 1, ROOM_3 = 2, ROOM_4 = 3, ROOM_5 = 4;
private static HashMap<Integer[], String[]> hMap = new HashMap<Integer[], String[]>();


static String [] getStringForValue(Integer[] i){
    return hMap.get(i);
}

static TreeSet<String> getOrderedOutputStrings(){
    TreeSet<String> set = new TreeSet<String>();
    for(Entry<Integer[],String[]> e : hMap.entrySet()){
        int day_int = Reservations.getDay(e.getKey());
        int room_int = Reservations.getRoom(e.getKey());
        int time = Reservations.getTime(e.getValue());
        String name = Reservations.getGuestName(e.getValue());
        String day = Reservations.dayToString(day_int);
        String room = Reservations.roomToString(room_int);
        if(time > 0)
        set.add("DAY: " + "(" + day_int + ")" + day + " (" + room_int + ")"+ "ROOM: " + room + " :: " + name + " @ " + time);
    }
    return set;
}



static void setupMap() {
    for (int day = 0; day < 7; day++) {
        for (int room = 0; room < 5; room++) {
            addGuest(day, room, (int)(Math.random()*1000), "Bob TestCase");
        }
    }
}

static void addGuest(int day, int room, int time, String name) {
    Integer[] ref = new Integer[2];
    ref[0] = day;
    ref[1] = room;
    String[] s = new String[2];
    s[0] = Integer.toString(time);
    s[1] = name;
    hMap.put(ref, s);
}

static String[] lookupRoom(int day, int room) {
    Integer[] i = new Integer[2];
    i[0] = day;
    i[1] = room;
    return hMap.get(i);
}

static int getDay(Integer[] i){
    return i[0];
}

static int getRoom(Integer[] i ){
    return i[1];
}

static int getTime(String[] s) {
    return Integer.parseInt(s[0]);
}

public static String getGuestName(String[] s) {
    return s[1];
}

public static String dayToString(int i){
    switch(i){
        case 0:
            return "SUNDAY";
        case 1:
            return "MONDAY";
        case 2:
            return "TUESDAY";
        case 3:
            return "WEDNESDAY";
        case 4:
            return "THURSDAY";
        case 5:
            return "FRIDAY";
        case 6:
            return "SATURDAY";
        default:
            return null;
    }
}

public static String roomToString(int i){
    switch(i){
        case 0:
            return "ROOM ONE";
        case 1:
            return "ROOM TWO";
        case 2:
            return "ROOM THREE";
        case 3:
            return "ROOM FOUR";
        case 4:
            return "ROOM FIVE";
        default:
            return null;
    }
}

}

Here is a main that runs the Reservations class:

public class Main {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    Reservations.setupMap(); //Run to test TODO: remove setupMap() from Reservation class 
    Reservations.addGuest(Reservations.DAY_MON, Reservations.ROOM_2, 1230, "John Doe");

    TreeSet<String> set = new TreeSet<String>();
    for(String s: Reservations.getOrderedOutputStrings()){
        System.out.println(s + "\n");
    }


}
}

And finally, using SetupMap and the single entry, it produces:

DAY: (0)SUNDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 423

DAY: (0)SUNDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 637

DAY: (0)SUNDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 731

DAY: (0)SUNDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 424

DAY: (0)SUNDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 427

DAY: (1)MONDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 335

DAY: (1)MONDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 580

DAY: (1)MONDAY (1)ROOM: ROOM TWO :: John Doe @ 1230

DAY: (1)MONDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 700

DAY: (1)MONDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 556

DAY: (1)MONDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 92

DAY: (2)TUESDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 84

DAY: (2)TUESDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 964

DAY: (2)TUESDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 981

DAY: (2)TUESDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 294

DAY: (2)TUESDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 22

DAY: (3)WEDNESDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 216

DAY: (3)WEDNESDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 838

DAY: (3)WEDNESDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 198

DAY: (3)WEDNESDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 494

DAY: (3)WEDNESDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 796

DAY: (4)THURSDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 617

DAY: (4)THURSDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 866

DAY: (4)THURSDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 799

DAY: (4)THURSDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 242

DAY: (4)THURSDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 178

DAY: (5)FRIDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 491

DAY: (5)FRIDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 16

DAY: (5)FRIDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 838

DAY: (5)FRIDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 63

DAY: (5)FRIDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 860

DAY: (6)SATURDAY (0)ROOM: ROOM ONE :: Bob TestCase @ 73

DAY: (6)SATURDAY (1)ROOM: ROOM TWO :: Bob TestCase @ 759

DAY: (6)SATURDAY (2)ROOM: ROOM THREE :: Bob TestCase @ 15

DAY: (6)SATURDAY (3)ROOM: ROOM FOUR :: Bob TestCase @ 115

DAY: (6)SATURDAY (4)ROOM: ROOM FIVE :: Bob TestCase @ 21

That result was generated in less than a single second. I promise that is infinitely more efficient than nested TreeMaps. Good Luck!

For the analysis of the problem, it is entirely irrelevant whether a Tree- or a Hashmap is being used. The question is rather, whether Map is appropriate or not and if so, if Maps must be nested inside each other. — Ingo, Mar 27 '11 at 20:42
Although I agree that you never really answered the question, I'm giving you +1 for effort and going the extra mile. — bconneen, Mar 27 '11 at 21:49
I agree with bconneen - I can't use your code, but thank you for taking the time to write it! — melat0nin, Mar 28 '11 at 17:18

score 0 · Answer 2 · answered Mar 27 '11 at 18:45

0

One would assume that the day is actually an enum, so the top level map should be an EnumMap

Map<Day, Map<String, Map<Integer, String>>> bookings = new EnumMap<Day, Map<String, Map<Integer, String>>>(Day.class);

answered Mar 27 '11 at 18:45

MeBigFatGuy

28,272
7
61
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score 0 · Answer 3 · answered Mar 27 '11 at 18:50

Not really an answer, but rather a thought ;)

Indeed, as xappymah pointed out in comments, more natural approach would be to invent domain-specific types for your task. Like:

interface Room {
    String getName();
    public void bookRoom(Booking booking) throws AlreadyBookedException;
}

interface Person {
    String getName();
}

class Interval {
    Date startTime;
    long duration;
}

interface Booking {
    Interval getInterval();
    Room getBookedRoom() throws NotBookedYetException;
    Set<Person> getParticipants();
}

interface BookingUtils {
    Set<Booking> getBookingsForInterval(Interval interval);
    Set<Booking> getBookingsOfRoom(Room room);
}

Alas I have to use TreeMaps; this is for a university exercise :/ — melat0nin, Mar 27 '11 at 19:05

How do I create nested TreeMaps?

3 Answers3