package cn.exercise; //创建测试类 public class Demo4Test {     public static void main(String[] args) {         double len = 2;         double r = 3;         Square sq = new Square();//Square类实例化         Circle ci = new Circle();         System.out.println("边长为" + len + "的正方形面积为：" + sq.area(len));         System.out.println("半径为" + r + "的圆形面积为" + ci.area(r));     } } //创建接口 interface Shape {     public abstract double area(double len); } //创建正方形类并覆盖接口中的抽象方法 class Square implements Shape {     double len;     public double area(double len) {         return len * len;     } } //创建圆形类 class Circle implements Shape {     double r;     public double area(double r) {         return Math.PI * r * r;     } }