Main_area.java

Use extends to calculate Rectangle,Triangle and Circle's areas.

class Point{
    public double area(){
    return 0;
    }
}

class Rectangle extends Point{
    private double length;
    private double width;
    public Rectangle(double s, double t){
        this.length = s;
        this.width = t;
    }
    public double area(){
        return length*width;
    }
}

class Triangle extends Point{
    private double bottom;
    private double height;
    public Triangle(double b, double h){
        this.bottom = b;
        this.height = h;
    }
    public double area(){
        return this.bottom*this.height/2;
    }
}

class Circle extends Point{
    final double pi = 3.14;
    private double radius;
    public Circle(double r){
        this.radius = r;
    }
    public double area(){
        return pi*radius*radius;
    }
}

public class Main_area{
    public static void main(String[] args) {
        Point w = new Point();
        System.out.println("point_area="+w.area());
        Rectangle x = new Rectangle(20, 30);
        System.out.println("rectangle_area="+x.area());
        Triangle y = new Triangle(20, 30);
        System.out.println("triangle_area="+y.area());
        Circle z = new Circle(20);
        System.out.println("circlele_area="+z.area());
    }
}

/*output*/

point_area=0.0
rectangle_area=600.0
triangle_area=300.0
circlele_area=1256.0