Generic classes and inheritance

Question

I am in the midst of converting some database object code over to split the data from the database functionality, and I am coming into issues with generics.
I have three bean classes:
abstract public class Payment {
  // common fields and their get/set methods
public final class AU_Payment
extends Payment {
  // extra fields and their get/set methods
public final class US_Payment
extends Payment {
  // extra fields and their get/set methods
}The code to write these classes to the database used to be in those classes, now I am moving them out to their own classes. I need to have a similar hierarchy so I can obtain an instance of the right class (through a factory) to write these objects to the database.
public interface PaymentDB<T extends Payment> {
  public Result load(Connection conn, T payment, ResultInfo resultInfo) throws SQLException;
  public Result loadAll(Connection conn, List<T> allPayments, ResultInfo resultInfo) throws SQLException;
}If I have the code for the AU_PaymentDB as the following, it won't compile:
public class AU_PaymentDB<AU_Payment> {
  public Result load(Connection conn, AU_Payment  payment, ResultInfo resultInfo) throws SQLException {
    return null;
  public Result loadAll(Connection conn, List<AU_Payment> payment, ResultInfo resultInfo) throws SQLException {
    return null;
}the compiler complains that the class doesn't override the interface, and I need to recode it to the following to get it to compile:
public class AU_PaymentDB<AU_Payment> {
  public Result load(Connection conn, Payment  payment, ResultInfo resultInfo) throws SQLException {
    return null;
  public Result loadAll(Connection conn, List payment, ResultInfo resultInfo) throws SQLException {
    return null;
load now takes just a Payment in it's signature list, and loadAll takes an untyped List.
This means I can actually call load() with a US_Payment and it works fine, which I don't want. Also, I cannot add an AU_Payment to the list in loadAll, because the list is of the wrong type.
How do I fix this? I want the AU_PaymentDB class to only act upon AU_Payments, and I need the loadAll to be able to add new AU_Payments to the list that is passed in.
I hope this all makes sense,
Chris

Answer 1

ok, I'm assuming that you just forgot to paste in that AU_PaymentDB implements PaymentDB :-)
parameterize AU_PaymentDB as follows:
public class AU_PaymentDB implements PaymentDB<AU_Payment> {
  public Result load(Connection conn, AU_Payment payment, ResultInfo resultInfo) throws SQLException {
    return null;
  public Result loadAll(Connection conn, List<AU_Payment> payment, ResultInfo resultInfo) throws SQLException {
    return null;
}so the interface defines a looser parameter ( Payment ) and your implementation drills it down to more specifically AU_Payment
Bob, I believe, is your uncle