Java Abstract Class And Generics

Question

I'm trying to modify a class AuthenticatedUser to store a list of AdminRole's. AuthenticatedUser is a class used by all my applications; it gets put into a session when the user logs in successfully. Now I want to add a list of authorization roles to the user session. However each application defines the an AdminRole class a little differently. For example, my DataCenter application stores in the database:

  employee_id
  role_name
  site_name
  receive_email

Not all of my applications will need the receive_email field or may want to extend their own methods. I figured this called for an abstract class. But Eclipse is complaining about the wrong Type on the Authorized_role_list setter here in this snippet.

  DataCenterAdminRoleDAO dcAdminDao = new DataCenterAdminRoleDAO();
  try {
      List<DataCenterAdminRole> authorized_roles = dcAdminDao.getAuthorizedRoleListByBadge(authenticatedUser.getBadge());
      authenticatedUser.setAuthorized_role_list(authorized_roles);

=== Classes

public class AuthenticatedUser extends Employee implements Serializable {   
  private static final long serialVersionUID = 1L;
  private List<AdminRole> authorized_role_list;
  ...
}

public abstract class AdminRole implements Serializable {
  private static final long serialVersionUID = 1L;
  private String role_name; //regular, admin, editor, etc..
  private String site_id;  //company branches
  ...
}

public class DataCenterAdminRole extends AdminRole implements Serializable {

Obviously a fix is to return a list of AdminRole in my Datacenter implemetation but I thought by extending the abstract class I could pass the subclass. What am I missing?

Please try to create a [sscce](Short, Self Contained, Correct Example). I realize you have compilation error but you can leave that line commented. Also paste the actual error reported by Eclipse without rephrasing. — Miserable Variable
– Miserable Variable, Commented Nov 1, 2012 at 23:02

cyon · Accepted Answer · 2012-11-01 23:04:04Z

2

If it's the setter that is giving you trouble you can use a lower bound with a wildcard on the authorized_roles_list

private List<? extends AdminRole> authorized_role_list;
....
public setAuthorized_role_list(List<? extends AdminRole> authorized_roles) {
...
}

answered Nov 1, 2012 at 23:04

cyon

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Comments

Alex Coleman · Accepted Answer · 2012-11-01 23:03:44Z

2

Generics do not behave like the rest of Java. Auto casting up does not occur, so if a method calls for List<List>, you can't hand it a List<ArrayList>. Similarly here, if your setter called for List, you cannot hand it a List<? extends AdminRole>, unless you define it as List<? extends AdminRole>. You can, however, override the setter method to take a List<DataCenterAdminRole> if you want, or convert your List<DataCenterAdminRole> to a List<AdminRole>, or just change your setter / the field to be a capture type <? extends AdminRole>

answered Nov 1, 2012 at 23:03

Alex Coleman

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JB Nizet · Accepted Answer · 2012-11-01 23:06:36Z

1

A List<Orange> is not a List<Fruit>, even if Orange is a Fruit. If it was a List<Fruit>, you could do the following:

List<Orange> orangeList = new ArrayList<Orange>();
List<Fruit> fruitList = orangeList;
fruitList.add(new Apple());

And it would thus completely break the type-safety of generic collections, since a list of oranges would contain an apple. The AuthenticatedUser should contain a List<? extends AdminRole> to be able to do what you're doing.

answered Nov 1, 2012 at 23:06

JB Nizet

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1 Comment

jeff Over a year ago

To better understand your analogy, I coded this in Eclipse. I get an error on line #2. Eclipse suggest changing type of fruitList to List<Orange>. I guess this is what you are pointing out, that Eclipse is preventing me from breaking type safety.

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