abstract class example in java

/**
 * @author lautturi.com 
 * Java example: abstract class
 */

import java.util.*;

abstract class Shape {
	double width;

	abstract double area();
}

class Square extends Shape {

	Square(double w) {
		width = w;
	}

	public double area() {
		return (double) width * width;
	}
}

class Circle extends Shape {

	Circle(double w) {
		width = w;
	}

	public double area() {
		return Math.PI * width * width;
	}
}

public class Lautturi {
	public static void main(String[] args) {

		int side = 5;
		int radius = 10;

		Square s = new Square(side);
		Circle c = new Circle(radius);
		System.out.println(s.area());
		System.out.println(c.area());
	}
}

output:

25.0
314.1592653589793