Payments.java

public interface Payments {
    Boolean process(double amount, CreditCard card);
}

public class BuyCdTest {

    private Payments payments;
    private CompactDisc cd;
    private CreditCard card;

    @Before
    public void setUp() {
        payments = mock(Payments.class);
        when(payments.process(any(), any())).thenReturn(true); // payment accepted
        cd = new CompactDisc(10, 9.99);
        card = new CreditCard(
                "MR P SQUIRE",
                "1234234534564567",
                "10/24",
                567);
    }

    @Test
    public void saleIsSuccessful(){
        cd.buy(1, card);
        assertEquals(9, cd.getStock());
    }

    @Test
    public void cardIsChargedCorrectAmount(){
        cd.buy(2, card);
        verify(payments).process(19.98, card);
    }
}