Hi, This is a useful repo for zk study, while reading the code, I am curious why use ConcurrentHashMap in [Client.java](https://github.com/fpj/zookeeper-book-example/blob/135b79b472fd2c45b82d78a3653532eecd69b6fe/src/main/java/org/apache/zookeeper/book/Client.java#L137), is HashMap enough ?
Hi,
This is a useful repo for zk study, while reading the code, I am curious why use ConcurrentHashMap
in Client.java, is HashMap enough ?