public void addHost(String hostName) {
    synchronized(hostNames) {
      String lowerHostName = hostName.toLowerCase();
      if (!hostName.equals(lowerHostName)) {
        LOG.warn("Host name {} contains upper case letters, will be converted to lowercase!", hostName );
      }
      hostNames.add(lowerHostName);
    }
  }
Associate a single host name to the host group.

@param hostName  the host name to associate with the host group
ambari-server/src/main/java/org/apache/ambari/server/topology/HostGroupInfo.java