public class ShoppingCart {
    private double totalAmount;
    private PaymentStrategy strategy;

    public ShoppingCart(double amount) {
        this.totalAmount = amount;
    }

    // Επιτρέπει την αλλαγή στρατηγικής κατά το runtime (Dependency Injection)
    public void setPaymentStrategy(PaymentStrategy strategy) {
        this.strategy = strategy;
    }

    public void checkout() {
        if (strategy == null) {
            System.out.println("[ERROR]: No payment method specified!");
        } else {
            strategy.pay(totalAmount);
        }
    }
}