1．问题描述：
实现查询银行卡余额、取款、修改密码、退出系统等功能。
2．功能要求：
（1）卡号、密码输入最多三次，否则直接退出系统。
（2）取款功能：取款金额受卡余额、机单笔最大取款金额及机当前剩余金额的限制。
（3）查询功能：实现查询余额功能。
（4）更改密码：实现密码更改功能，对于密码要有2次验证。
（5）锁卡功能：实现卡被锁的功能。
（6）退卡功能：实现退出系统功能。(The 1. Problem Descr iption:

To realize the functions of bank card balance, withdrawal, password modification and withdrawal system.

2. functional requirements:

(1) the number of card number and password is up to three times, otherwise the system will be withdrawn directly.

(2) withdrawal function: the amount of withdrawals is limited by the balance of the card, the amount of the maximum amount of money taken by the machine, and the current residual amount of the machine.

(3) query function: to realize the function of the balance of the query.

(4) change the password: the password change function is implemented, and the password should be verified 2 times.

(5) lock card function: to realize the function of the card being locked.

(6) back card function: to implement the system function.)