@Test(expected = IllegalArgumentException.class)
public void invalid_up_cast() {
CatCage catCage = new CatCage();
PetCage cage = (PetCage) catCage;
cage.enter(new Dog());
}
@Test
public void empty() {
PetCage cage = new PetCage();
PetCage catCage = new CatCage();
assertTrue(cage.isEmpty());
assertTrue(catCage.isEmpty());
Animal a = new Animal();
Cat c = new Cat();
cage.enter(a);
catCage.enter(c);
assertFalse(cage.isEmpty());
assertFalse(catCage.isEmpty());
cage.takeOut();
catCage.takeOut();
assertTrue(cage.isEmpty());
assertTrue(catCage.isEmpty());
}