设计接口Method，该接口仅有一个方法：

double calArea( )，用于计算面积。

（2）设计矩形类Rectangle，并实现接口Method，要求如下：

成员变量length：int型，代表矩形的长度；

成员变量width：int型，代表矩形的宽度；

构造函数Rectangle(int l, int w)：将length和width初始化为参数给定的值；

接口方法calArea( )的实现：根据长length和宽width计算矩形的面积。

（3）设计正方形类Square，继承矩形类Rectangle ，要求如下：

成员变量edge：int型，代表正方形的边长；

构造函数Square(int e)：将边长edge初始化参数给定的值；

覆盖父类的方法calArea( )：计算正方形的面积；

成员方法int calPerimeter( )：根据边长edge计算正方形的周长。

（4）编写测试类Test，计算并输出边长为3的正方形的面积和边长

-Design interface method, the interface is just a method: double calArea, (), used to calculate the area. (2) the design of the rectangle class Rectangle, and interface Method requirements are as follows: member variable length: int, on behalf of the length of the rectangle　member variable width: int, width of the rectangle　constructor Rectangle (int l, int w) : the length and width initialize the value given for the parameter　implementation of the interface the method calArea (): according to the long length and wide width to calculate the area of ​ ​ a rectangle. (3) Design square Square, inheritance rectangle class Rectangle requirements are as follows: member variable edge: int, on behalf of the side of a square　constructor Square (int e): the side length of the edge initialization parameters given value　override the parent the class methods calArea (): calculate the area of ​ ​ a square　member method int calPerimeter (): calculated based on the long side e