编写一个矩形类，一个正方形类继承这个矩形类，实现一个接口中求对角线的方法。(Java)

import java.io.*;
import java.math.*;//导入math
interface Graph{
  abstract void catercorner();
}
class Rectangle implements Graph{
  double length, width;
  Rectangle(){
    length=0;width=0;
  }
  Rectangle(int i,int j){
    length=i;width=j;
  }
  public void catercorner(){
    double catercorner_long=length*length+width*width;
    System.out.println(Math.sqrt(catercorner_long));
  }
}
class ClassSquare extends Rectangle{
  double r=0;
  void fuzhi(double num){
    r=num;
  }
  public void catercorner(){
    double catercorner_long=r*r+r*r;
    System.out.println(Math.sqrt(catercorner_long));
  }
}
public class JieKou {
  public static void main(String args[]) throws IOException{
    double num=0;
    BufferedReader din=new BufferedReader(new InputStreamReader(System.in));
    System.out.print("请输入正方形边长：");
    num=Integer.parseInt(din.readLine());
    ClassSquare s=new ClassSquare();
    s.fuzhi(num);
    s.catercorner();
  }
}