private final void renameUser(String oldNick, String newNick) {
        synchronized (_channels) {
            Enumeration enumeration = _channels.keys();
            while (enumeration.hasMoreElements()) {
                String channel = (String) enumeration.nextElement();
                User user = this.removeUser(channel, oldNick);
                if (user != null) {
                    user = new User(user.getPrefix(), newNick);
                    this.addUser(channel, user);
                }
            }
        }
    }