Design a basic credit card system

Design an object-oriented credit card system for a retail bank. Focus on clean class design rather than distributed infrastructure. At a minimum, the system should support: - customers owning one or more credit cards - each card being linked to an account with a credit limit and available credit - recording purchases, payments, refunds, and declined transactions - tracking current balance and available credit - generating a monthly statement summary Explain the core classes, their relationships, and the main methods you would expose. Also discuss how you would handle validation, such as insufficient available credit, invalid card status, or overpayment.