@Override
    public Host getHost(HostId hostId) {
        checkNotNull(hostId, HOST_NULL);
        Optional<VirtualHost> foundHost =
                manager.getVirtualHosts(this.networkId())
                .stream()
                .filter(host -> hostId.equals(host.id()))
                .findFirst();
        if (foundHost.isPresent()) {
            return foundHost.get();
        }
        return null;
    }
@Test(expected = NullPointerException.class)
    public void testGetHostByNullId() {
        VirtualNetwork vnet = setupEmptyVnet();
        HostService hostService = manager.get(vnet.id(), HostService.class);

        hostService.getHost(null);
    }