定义一个矩形类Rectangle，再派生一个正方形类Square，自行扩充成员和方法。再定义接口EqualDiagonal，其中包含方法getDiagonal( ),在矩形类和正方形类中实现此接口，用来实现斜边的计算。-Define a rectangle class Rectangle, and then derive a square class Square, expand their own members and methods. And then define the interface EqualDiagonal, which contains methods getDiagonal (), in rectangles and squares class class implements this interface, used to achieve the hypotenuse calculations.