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)

代码如下：

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();
	}
}

结果演示：