p134 java

abstract class Geometry
{
public abstract double getArea();
}

class Pillar
{
Geometry bottom;
double height;
Pillar(Geometry bottom,double height)
{
this.bottom = bottom;
this.height=height;
}
public double getVolume()
{
if(bottom==null)
{
System.out.println("没有底，无法计算体积！");
return -1;
}
return bottom.getArea()*height; //求真正的体积;
}
}

class Circle extends Geometry
{
double r;
Circle(double rr)
{
this.r=rr;
}
public double getArea()
{
return 3.14*r*r;
}
}

class Rectangle extends Geometry
{
double a,b;
Rectangle(double aa,double bb)
{
this.a=aa;
this.b=bb;
}
public double getArea()
{
return a*b;
}

}

public class Appli {
public static void main(String []args)
{
Pillar pillar;
Geometry bottom=null;
pillar=new Pillar(bottom,100);
System.out.println("体积"+pillar.getVolume());
bottom=new Rectangle(12,22);
pillar=new Pillar(bottom,58);
System.out.println("体积"+pillar.getVolume());
bottom=new Circle(10);
pillar=new Pillar(bottom,58);
System.out.println("体积"+pillar.getVolume());
}

}