public class AbstractAreas
{
    public static void main(String[] args)
    {
        //Figure f = new Figure(10 , 10);//illegal now
        Rectangle r = new Rectangle(9 , 5);
        Triangle t = new Triangle(10 , 8);
        Figure figref;//this is OK , no object is created
        figref = r;
        System.out.println("Area is " + figref.area());
        figref = t;
        System.out.println("Area is " + figref.area());
    }
}
//Using abstract methods and classes.
abstract class Figure
{
    double dim1;
    double dim2;
    Figure(double a , double b)
    {
        dim1 = a;
        dim2 = b;
    }
    //area is now an abstract method
    abstract double area();
}
class Rectangle extends Figure
{
    Rectangle(double a , double b)
    {
        super(a , b);
    }
    //override area for tectangle
    double area()
    {
        System.out.println("Inside Area for Rectangle.");
        return dim1 * dim2;
    }
}
class Triangle extends Figure
{
    Triangle(double a , double b)
    {
        super(a , b);
    }
    //override area for right triangle
    double area()
    {
        System.out.println("Inside Area for Triangle.");
        return dim1 * dim2 /2;
    }
}
/*
result:
Inside Area for Rectangle.
Area is 45.0
Inside Area for Triangle.
Area is 40.0
*/