java定义一个接口Circlearea,声明一个方法用于计算园的面积,在定义Circle表示园,

接口Circlearea.java：
package test;
public interface Circlearea {
public static double PI = 3.14;
public double area(double r);
}
圆类Circle.java：
package test;
public class Circle implements Circlearea {
public