1. Define abstract class Shape:
   - Abstract method numberOfSides()

2. Define class Rectangle that extends Shape:
   - Implement numberOfSides() to print "Rectangle has 4 sides."

3. Define class Triangle that extends Shape:
   - Implement numberOfSides() to print "Triangle has 3 sides."

4. Define class Hexagon that extends Shape:
   - Implement numberOfSides() to print "Hexagon has 6 sides."

5. Define class Demo:
   - In the main method:
     - Create a Rectangle instance as a Shape reference.
     - Create a Triangle instance as a Shape reference.
     - Create a Hexagon instance as a Shape reference.
     - Call numberOfSides() on each shape instance.
       - The appropriate numberOfSides() method for each shape will be invoked.

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abstract class Shape {
    abstract void numberOfSides();
}

class Rectangle extends Shape {
    void numberOfSides() {
        System.out.println("Rectangle has 4 sides.");
    }
}

class Triangle extends Shape {
    void numberOfSides() {
        System.out.println("Triangle has 3 sides.");
    }
}

class Hexagon extends Shape {
    void numberOfSides() {
        System.out.println("Hexagon has 6 sides.");
    }
}

public class Demo {
    public static void main(String[] args) {
        Shape rectangle = new Rectangle();
        Shape triangle = new Triangle();
        Shape hexagon = new Hexagon();

        rectangle.numberOfSides();
        triangle.numberOfSides();
        hexagon.numberOfSides();
    }
}