//update return of item by user, remove the item from the aprropriate list
public void UpdateReturn(RentItem item)//OPTION: TO CREATE DICTIONARY BY KEY OF ISBN AND REMOVE BY ISBN?
{
if (_rentedItems.Contains(item))
{
_rentedItems.Remove(item);
}
}
//maybe add type if there are more class which inherited, only by condition that the type is of abstract item
//update rent by user
//return true if user can rent more items, else return false
public void UpdateRent(RentItem item)
{
_rentedItems.Add(item);
_NumOfRentedItems++;
}