public static ControllerNodeToNodeId toNodeId() {
        return INSTANCE;
    }
@Test
    public final void testToNodeId() {

        final Iterable<ControllerNode> nodes = Arrays.asList(CN1, CN2, CN3, null);
        final List<NodeId> nodeIds = Arrays.asList(NID1, NID2, NID3);

        assertEquals(nodeIds,
                FluentIterable.from(nodes)
                    .transform(toNodeId())
                    .filter(notNull())
                    .toList());
    }