@Override
    public String toString() {
        return MoreObjects.toStringHelper(getClass())
                .add("src", src)
                .add("dst", dst)
                .toString();
    }
@Test
    public void testToString() {
        LinkKey k1 = LinkKey.linkKey(SRC1, DST1);
        String k1String = k1.toString();
        assertThat(k1String, allOf(containsString("LinkKey{"),
                                   containsString("src=1/1"),
                                   containsString("dst=2/1}")));
    }