Sample Solution:

Java Code:

$//$ Shape.java$-$ Base class Shape

public class Shape $\{$

public double calculateArea$()\{$

$\}$

return $0$ ; $//$ Default implementation returns $$

$\}$

$//$ Circle.java$-$ Subclass Circle

public class Circle extends Shape $\{$

private double radius;

public Circle$($double radius$)\{$

this.radius $=$ radius;

$\}$

@override

public double calculateArea$()\{$

return Math.PI $**$ radius $**$ radius; $//$ Calculate area of circle

$\}$

$//$ Rectangle.java$-$ Subclass Rectangle

public class Rectangle extends Shape $\{$

private double width;

private double height;

public Rectangle$($double width, double height$)\{$

this.width $=$ width;

$\}$

this. height $=$ height;

@override

public double calculateArea$()\{$

$\}$

return width $*$ height; $//$ Calculate area of rectangle

$\}$

$//$ Triangle.java$-$ Subclass Triangle

public class Triangle extends Shape $\{$

private double base;

private double height;

public Triangle$($double base, double height$)\{$

this. base $=$ base;

$\}$

this.height $=$ height;

@override

public double calculateArea$()\{$

$\}$

$\}$

$//$ Main.java$-$ Main class

public class Main $\{$

public static void main$($String$[]$ args$)\{$

Circle circle $=$ new Circle$(4)$;

System.out.println$("$Area of Circle: $"+$ circle.calculateArea$())$;

Rectangle rectangle $=$ new Rectangle$(12,34)$;

System.out.println$("$

Area of Rectangle: $"+$

rectangle.calculateArea$())$;

Triangle triangle $=$ new Triangle$(5,9)$;

System.out.println$("$

Area of Triangle: $"+$

triangle.calculateArea$())$;

$\}$

$\}$

Output: