@Test
public void testGetCity() {

    // given
    CityModel cityModel = new CityModel();
    // when empty then
    assertNull(model.getCity(0));
    // when
    model.addCity(cityModel);
    // then
    assertEquals(cityModel, model.getCity(0));
    assertNull(model.getCity(-1));
    assertNull(model.getCity(1));
    // when
    for (CityModel city : cities) {
        model.addCity(city);
    }
    // then
    assertEquals(cityModel, model.getCity(0));
    for (int i=0; i<cities.length; i++) {
        assertEquals(cities[i], model.getCity(i+1));
    }
    assertNull(model.getCity(-1));
    assertNull(model.getCity(CITIES_NUM + 1));
}