WarehouseTest.java

public class WarehouseTest {

    @Test
    public void buyCd(){
        CD cd = new CD(10, 9.99);
        CreditCard card = mock(CreditCard.class);
        Warehouse warehouse = new Warehouse();
        warehouse.buy(cd, 1, card);
        assertEquals(9, cd.getStock());
        verify(card).charge(9.99);
    }
}