public class BookRental {
	String id;
	String customerName;

	...
}

public class BookRentals {
	private Vector rentals;
	public String getCustomerName(String rentalId) {
		for (int i = 0; i < rentals.size(); i++) {
			BookRental rental = (BookRental) rentals.elementAt(i);
			if (rental.getId().equals(rentalId)) {

				return rental.getCustomerName(); }
		}

		throw new RentalNotFoundException();
	}
}

public class RentalNotFoundException extends Exception {
	...
}