问答题

import static java.lang.System.out;
public class ShapeDemo{
  public static void main(String[] args){
    Shape shape=new Circle(5);
    out.println(shape.area());
  }
}
interface Shape{
                                ;
}
class Circle implements Shape{
  private double r;
  public Circle(double _r){
    this.r=_r;
  }
  @Override public double area(){
    return Math.PI*r*r;
  }
}