@Override
    public Iterable<Link> getLinks() {
        return manager.getVirtualLinks(this.networkId())
                .stream().collect(Collectors.toSet());
    }
@Test(expected = NullPointerException.class)
    public void testGetLinksByNullId() {
        manager.registerTenantId(TenantId.tenantId(tenantIdValue1));
        VirtualNetwork virtualNetwork = manager.createVirtualNetwork(TenantId.tenantId(tenantIdValue1));
        LinkService linkService = manager.get(virtualNetwork.id(), LinkService.class);

        // test the getLinks() method with a null connect point.
        linkService.getLinks(null);
    }