int index = customersPersistentData.getCustomerIndexByCardNumber(cardNumber);
this.customers.get(index).loanBook(loanBook);
ArrayList<Customer> updatedCustomers = new ArrayList<Customer>(customers);
customersPersistentData.updateAll(updatedCustomers);
customersPersistentData.save();
}
public void setBookReturned(Book returningBook){
int bookIndex = this.getBooks().indexOf(returningBook);
Book bookTitle = this.getBooks().get(bookIndex);