Error handling

1. Programs are subject to "errors", e.g. bad index in an array, read a file that does not exists, etc.
   As a example, let's consider a Catalog of Products below.
   What is the problem with the result ?

public static void main(String[] args) {
  Catalog bookstore = new Catalog();
  //                Product(name,price,quantity)
  bookstore.add(new Product("Core Java",  52.29, 13));
  bookstore.add(new Product("FP in Java", 41.63, 8));
  
  bookstore.buy("Core Java", 30);
  System.out.println(bookstore);
}

 Product	|	Qty	|	 Price
---------------------------------------------------
Core Java	|	-17	|	52.29 euros
FP in Java	|	8	|	41.63 euros