Let's start off by creating a minimal ZooKeeper client: 

import java.io.*;
import org.apache.zookeeper.*;
import org.apache.zookeeper.data.*;

public class SimpleZoo {
    static class MyWatcher implements Watcher {
        public void process(WatchedEvent event) {
            System.err.println(event);
        }
        
    }
    public static void main(String[] args) throws Exception {
        MyWatcher myWatcher = new MyWatcher();
        ZooKeeper zk = new ZooKeeper(args[0], 10000, myWatcher);
        Thread.sleep(10000);
        zk.close();
    }
}

What events do you get when you start?

Lets create a program to create a znode (I'll use "/motd", but you can pick your own name). You'll need to add: 

        zk.create("/motd", "hi".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

What happens if you try to create /motd twice?

Create a new simple client to set "/motd": 

        zk.setData("/motd", "new data".getBytes(), -1);

Create a new simple client to read "/motd". 

        Stat stat = new Stat();
        byte data[] = zk.getData("/motd", true, stat);
        System.out.println(new String(data));

Try changing /data and watch the events you get. What if you change /data twice in a row? Could this be used for messaging?