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Calculate Areas - Java Example Program


Calculate Areas
class CalculateAreas
{
    public static void main(String arg[])
    {
        Rectangle rect = new Rectangle(5.25, 4.0);
        System.out.println("Area of rectangle is " + rect.getArea());
        
        Circle circle = new Circle(7.5);
        System.out.println("Area of circle is " + circle.getArea());    
    }
}

abstract class Shape
{
    abstract double getArea(); // LINE A
}

class Rectangle extends Shape
{
    double length;
    double breadth;

    Rectangle(double length, double breadth)
    {
        this.length = length;
        this.breadth = breadth;
    }

    double getArea()
    {
        return length * breadth;
    }
}

class Circle extends Shape
{
    double radius;

    Circle(double radius)
    {
        this.radius = radius;
    }

    double getArea()
    {
        return 3.14 * radius * radius;
    }
}
OUTPUT

Area of rectangle is 21.0
Area of circle is 176.625

DESCRIPTION

Here we have created an abstract Shape class and two concrete classes namely Rectangle and Circle which extends from the Shape class. We have implemented the getArea() method in the sub-classes. Please note that every sub-class extending from an abstract class should implement all the abstract methods, otherwise it will cause compilation errors.

THINGS TO TRY
  • Define one more sub-class Triangle which extends from Shape class. Create a constructor, which takes the parameters base and height. Also implement the getArea() method. Area of triangle is (base * height) / 2.0
  • Add one more abstract method getPerimeter() in the Shape class and implement that method in Rectangle and Circle.
  • Define one more sub-class Polygon which extends from Shape. Do not define any constructors or methods in that class including the getArea() method. Observe the compilation error you get.

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