program10_6.java

《A first book of java》by Gary J.Bronson 北大出版社

public class Circle
{
  // class variable declaration section
  public final static double PI = 2.0 * Math.asin(1.0);
  protected double radius;
  
  // class method definition section
  public Circle(double r)  // constructor
  {
    radius = r;
  }
  public double calcval()   // this calculates an area
  {
    return(PI * radius * radius);
  }
}
class Cylinder extends Circle  // Cylinder is derived from Circle
{
  
  // class variable declaration section
  protected double length;  // add an additional data member
  
  // class method definition section
  public Cylinder(double r, double l)
  {
    super(r);
    length = l;
  }
  public double calcval()   // this calculates a volume
  {
    return (length * super.calcval()); // note the base method call
  }
  public static void main(String[] args)
  {
    Circle circleOne = new Circle(1);  // create two Circle objects
    Circle circleTwo = new Circle(2);  
    Cylinder cylinderOne = new Cylinder(3,4);      // create one Cylinder object
    System.out.println("The area of circleOne is " + circleOne.calcval());
    System.out.println("The area of circletwo is " + circleTwo.calcval());
    System.out.println("The volume of cylinderOne is " + cylinderOne.calcval());
  }
}