public SkiPass(int id, String type, Date activation, Date expiration) {
        if (activation == null || expiration == null) {
            throw new IllegalArgumentException("Activation or expiration time is null");
        }
        if (activation.after(expiration)) {
            throw new IllegalArgumentException("Activation time is after expiration");
        }
        this.id = id;
        cardType = type;
        expirationDate = expiration;
        activationDate = activation;
    }