Error handling

2. The program must emit an UnavailableException that is defined in Java by a class than inherits from Exception:

public class UnavailableException extends Exception { }

3. Next, the buy operation has to throws this exception when necessary:

public void buy(String name,int quantity) {
  for(Product book : this) {           // find product
    if (book.getName().equals(name)) { // String equality
      if (book.getQuantity()>=quantity) {
        book.buy(quantity);
        return;                        // exit operation
      }
      // Here: bad quantity !
      throw new UnavailableException();
    }
  }
  // Here: product not found ! SEE EXERCICES
}

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4. And the operation has to notify that an exception can occur:

public void buy(String name,int quantity) throws UnavailableException { ... }