@Test
    public void num_should_be_add_1_when_out_a_car() {
        final int num = 20;
        Park park = new Park(num);
        Car car = new Car();
        Ticket ticket = park.in(car);
        Assert.assertEquals(park.getEmptyNum(), num - 1);
        park.out(ticket);
        Assert.assertEquals(park.getEmptyNum(), num);
    }