接口对象的实例化在接口回调中的使用方法

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()); 
 
 
 
} 
 
}