@Override
    public Set<Host> getHostsByIp(IpAddress ip) {
        checkNotNull(ip, "IP address cannot be null");
        return filter(getHostsColl(), host -> host.ipAddresses().contains(ip));
    }
@Test(expected = NullPointerException.class)
    public void testGetHostsByNullIp() {
        VirtualNetwork vnet = setupVnet();
        HostService hostService = manager.get(vnet.id(), HostService.class);

        hostService.getHostsByIp(null);
    }