@Override
    public Set<Host> getHostsByVlan(VlanId vlanId) {
        checkNotNull(vlanId, "VLAN identifier cannot be null");
        return filter(getHostsColl(), host -> Objects.equals(host.vlan(), vlanId));
    }
@Test(expected = NullPointerException.class)
    public void testGetHostsByNullVlan() {
        VirtualNetwork vnet = setupEmptyVnet();
        HostService hostService = manager.get(vnet.id(), HostService.class);

        hostService.getHostsByVlan(null);
    }