@Override
    public Set<Host> getHostsByMac(MacAddress mac) {
        checkNotNull(mac, "MAC address cannot be null");
        return filter(getHostsColl(), host -> Objects.equals(host.mac(), mac));
    }
@Test(expected = NullPointerException.class)
    public void testGetHostsByNullMac() {
        VirtualNetwork vnet = setupEmptyVnet();
        HostService hostService = manager.get(vnet.id(), HostService.class);

        hostService.getHostsByMac(null);
    }