@Override
    public List<Server> getServerList(ServerGroup serverGroup) {
        switch (serverGroup) {
        case ALL:
            return allServerList;
        case STATUS_UP:
            return upServerList;
        case STATUS_NOT_UP:
            ArrayList<Server> notAvailableServers = new ArrayList<Server>(
                    allServerList);
            ArrayList<Server> upServers = new ArrayList<Server>(upServerList);
            notAvailableServers.removeAll(upServers);
            return notAvailableServers;
        }
        return new ArrayList<Server>();
    }
return the server

@param index
@param availableOnly
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/BaseLoadBalancer.java