Shape接口： package Practise; public interface Shape {     /**      * @param value      * @return      */     abstract double area(double value); } -------------------------------------------------------------------------------------------------------- 正方形类： package Practise; class Square implements Shape{     @Override     public double area(double length) {         return length*length;     } } -------------------------------------------------------------------------------------------------------------  圆形类： package Practise; class Circle implements Shape{     @Override     public double area(double radius) {         return radius*radius*Math.PI;     }          } --------------------------------------------------------------------------------------------------------- 测试类： package Practise; public class ShapeArea {          public static void main(String[] args) {         Shape square = new Square();         Shape circle = new Circle();         System.out.println("Square area is " + square.area(3));         System.out.println("Circle area is " + circle.area(2));     }      } --------------------------------------------------------------------------------------------------------- 结果： Square area is 9.0 Circle area is 12.566370614359172