java san jiaoxing

public class Geometry{
public abstact double area(){
return 0;
}

}

public class Circle extends Geometry{
private double r;
public Circle(double r){
this.r=r;
}
public double area(){
double t;
t=Math.PI*r*r;
return t;
}
}

 public class Triangle extends Geometry{
private double a;
private double b;
private double c;
public Triangle(double a,double b,double c){
this.a=a;
this.b=b;
this.c=c;
}
public double area(){
double p;
double t;
p=(a+b+c)/2;
t=Math.sqrt(p*(p-a)*(p-b)*(p-c));
return t;
}
}

public class Example{
public static void main(String ages[]){
Geometry g1;
g1=new Circle(2);
Geometry g2;
g2=new Triangle(3,4,5);
System.out.println(g1.area());
System.out.println(g2.area());
}
}