怎么在接口回调中的使用接口对象的实例化

interface Computerable  {  public double area();  }    class Rec implements Computerable  {  double a,b;  Rec(double a,double b)  {  this.a = a;  this.b = b;  }  public double area() {  return (a*b);  }  }    class Circle implements Computerable  {  double r;  Circle(double r)  {  this.r = r;  }  public double area() {  return (3.14*r*r);  }  }    class Volume  {  Computerable bottom;  double h;  Volume(Computerable bottom, double h)  {  this.bottom = bottom;  this.h = h;  }    public void changeBottome(Computerable bottom)  {  this.bottom = bottom;  }    public double volume()  {  return (this.bottom.area()*h/3.0);  }  }    public class InterfaceRecall {  public static void main(String[] args)  {  Volume v = null;  Computerable bottom = null;    //借口变量中存放着对对象中实现了该接口的方法的引用  bottom = new Rec(3,6);  System.out.println("矩形的面积是："+bottom.area());  v = new Volume(bottom, 10);  //体积类实例的volum方法实际上计算的是矩形的体积，下同  System.out.println("棱柱的体积是："+v.volume());    bottom = new Circle(5);  System.out.println("圆的面积是："+bottom.area());  v.changeBottome(bottom);  System.out.println("圆柱的体积是："+v.volume());    }  }