Java黑皮书课后题第11章：11.3（Account类的子类）

题目描述：编写Account类的子类，名为CheckingAccount，该类继承Account类。该类必须包括一个名为overdraftProtection的double型数据域，该域存储与账户关联的可透支额。如果取款数超过账户余额和可透支额，则应打印一条消息，指出用户透支了多少美元。

解题思路：
1. CheckingAccount类继承Account类
2. CheckingAccount类包括一个名为overdraftProtection的double型数据域
3. 在取款方法中，如果取款数超过账户余额和可透支额，则打印透支消息，并显示透支金额

代码示例：

public class CheckingAccount extends Account {
    private double overdraftProtection;

    public CheckingAccount(double balance, double overdraftProtection) {
        super(balance);
        this.overdraftProtection = overdraftProtection;
    }

    public double getOverdraftProtection() {
        return overdraftProtection;
    }

    public void setOverdraftProtection(double overdraftProtection) {
        this.overdraftProtection = overdraftProtection;
    }

    @Override
    public void withdraw(double amount) {
        if (amount <= balance + overdraftProtection) {
            balance -= amount;
        } else {
            System.out.println("Sorry, you do not have enough money to withdraw $" + amount);
            System.out.println("Your balance is $" + balance + " and your overdraft protection is $" + overdraftProtection);
            System.out.println("Your maximum withdraw limit is $" + (balance + overdraftProtection));
        }
    }
}