CD.java

public class CD {
    private int stock;
    private final double price;

    public CD(int stock, double price) {
        this.stock = stock;
        this.price = price;
    }

    public int getStock() {
        return stock;
    }

    public void buy(int quantity, CreditCard card) {
        card.charge(price * quantity);
        stock = stock - quantity;
    }
}