重写有要求
1. 方法名：必须和父类被重写的方法名相同
2. 形参列表：必须和父类被重写的方法名相同
3. 返回值类型：
A. 基本数据类型和void:要求与父类被重写的返回值数据类型一致
B. 引用数据类型：要求子类重写的方法返回值类型 <= 父类被重写的方法的返回值类型
## 例如：子类方法的返回值类型是Student，父类被重写的方法的返回值类型Student
子类方法的返回值类型是Student，父类被重写的方法的返回值类型是Person
子类方法的返回值类型是Person，父类被重写的方法的返回值类型是Student（这个是错误的）
4. 修饰符
A. 权限修饰符：子类重写方法的权限修饰符的可见范围 >= 父类被重写的权限修饰符的可见范围
B. 其他修饰符：
When nothing seems to help, I go look at a stonecutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred and first blow it will split in two, and I know it was not that blow that did it, but all that had gone before. -- Jacob Riis