Die vervollständigte Book-Klasse

public class Book extends Goods implements Taxable
{
  protected String author;

  public Book(String des, double pr, String auth)
  {
    super(des, pr);
    author  = auth;
  }

  public String toString()
  {
    return super.toString() +
      "Autor: " + author ;
  }
  // die implementierte Schnittstelle
  public double calculateTax()
  {
    return price * TAXRATE;
  }
}