@Test
public void testAddCity() {

    // when empty then
    assertEquals(0, model.getCityCount());
    // when
    model.addCity(null);
    assertEquals(0, model.getCityCount());
    // when
    model.addCity(new CityModel());
    // then
    assertEquals(1, model.getCityCount());
    // when
    for (CityModel city : cities) {
        model.addCity(city);
    }
    // then
    assertEquals(CITIES_NUM + 1, model.getCityCount());
}