@Test public void getCheckDigit() {

    CitizenId cid;
    cid = new CitizenId("0000000000003");
    assertEquals("check digit should be 3", 3, cid.getCheckDigit());
    cid = new CitizenId("0000000000005");
    assertEquals("check digit should be 5", 5, cid.getCheckDigit());
    cid = new CitizenId("000000000000x");
    assertEquals("check digit should be -1", -1, cid.getCheckDigit());
}