dispatch.java

it is a good tool to help you study java

abstract class Figure{
double a;
double b;
Figure(double a0,double b0){
a=a0;
b=b0;
}
abstract void Area();
}
class Rectangle extends Figure{
Rectangle(double a ,double b){
super(a,b);
}
void Area(){
System.out.println("area of rectangle is:"+a*b);
}
}
class Triangle extends Figure{
Triangle(double a,double b){
super(a,b);
}
void Area(){
System.out.println("Area of triangle is:"+a*b/2);
}
}
class Dispatch{
public static void main(String args[])
{
//Figure f=new Figure(10,10);
Rectangle r=new Rectangle(20,10);
Triangle t=new Triangle(10,30);
Figure ff; //声名Figure类型的引用变量
//ff=f;
//ff.Area();
ff=r;
ff.Area();
ff=t;
ff.Area();

}} 
/*由于被定义成为一个抽象类,Figure不能生成实例对象,但是可以创建一个Figure类型的引用变量,可以用来引用任何从Figure派生的对象.同时任何从Figure中派生的子类都必须重载Area()函数*/