@Override
        public int compare(ServerStats o1, ServerStats o2) {
            String zone1 = "";
            String zone2 = "";
            if (o1.server != null && o1.server.getZone() != null) {
                zone1 = o1.server.getZone();
            }
            if (o2.server != null && o2.server.getZone() != null) {
                zone2 = o2.server.getZone();
            }
            return zone1.compareTo(zone2);            
        }
returns map of Stats for all servers
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/LoadBalancerStats.java