private Person per = new Person("Per", new Address("A-gate 5", "0560", "Oslo"), 30);
private Person kari = new Person("Kari", new Address("B-gate 4", "0560", "Oslo"), 37);

@Test
public void knowsWhichRelativesLivingInTheSamePostalCode() {
    per.relateTo(SIBLING, kari);

    Collection<Relative> nearbyRelatives = per.getRelativesWithSamePostalCode();
    assertThat(nearbyRelatives, hasSize(1));

    Relative nearbyRelative = nearbyRelatives.iterator().next();
    assertThat(nearbyRelative.person, is(kari));
}