Keeping array in sync between client and server.

Hey, I was wondering what's the best way to keep a custom 'tracker' in sync between the client and server. I have this class:

public class ObjectRegistry {

static HashMap<EntityPlayer, MyObject> map = new HashMap<EntityPlayer, MyObject>();

public static MyObject createNewObject(EntityPlayer entityPlayer) {
	MyObject t = new MyObject(entityPlayer);
	map.put(entityPlayer, t);
	return t;
}

public static MyObject getPlayerObject(EntityPlayer player) {
	return map.get(player);
}
}

And here is the MyObject class:

package mods.ages.civilization;

import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;

public class Town {

private static int lastObjectID;

public static int getNextObjectID() {
	return lastTownID + 1;
}

int id;
String name;
EntityPlayer owner;

public Town (EntityPlayer player) {
	owner = player;
	id = getNextObjectID();
	name = new String(player.username + "'s Object");
}

public EntityPlayer getOwner() {
	return owner;
}

public String getName() {
	return name;
}

public void setName(String s) {
	name = s;
}
}

This stores all MyObject classes in the hashmap. However, I need to access these classes from the client side and when I try (without packets) I get a NullPointerException. As would be expected. What would be the best way of making the HashMap and the static functions in the registry available for the client side to reference. I'm fine with creating packets to send the data, it's just I wouldn't know what to do with it after the packet arrives.