Java generics, explanation needed

Question

There are several simple test-case classes:

public interface ListCriteria<T> {
  // some stuff here
}

public class UserListCriteria implements ListCriteria<User> {
 // implementation
}

public interface Editor<T> {
  // sample method 1
  List<T> listObjectsTest1(ListCriteria<T> criteria);
  // sample method 2
  <L extends ListCriteria<T>> List<T> listObjectsTest2(L criteria);
}

And there is an implementation of Editor which Java thinks it does not provide necessary implementation for both sample methods:

public class UserEditor implements Editor<User> {

  @Override
  public List<User> listObjectsTest1(UserListCriteria criteria) {
    //
  }

  @Override
  public List<User> listObjectsTest2(UserListCriteria criteria) {
    //
  }
}

Both method implementations are wrong. The question is why. Especially for the latter method. Sure I could do interface Editor<T, L extends ListCriteria<T>>, and that would solve the issue, but I don't want to, I want to understand why I can't use method-level generics here.

René Link · Accepted Answer · 2013-12-09 09:07:07Z

2

The errors you get have nothing to do with generics, because you implement the method with another type than the interface enforces.

The Editor interface defines

List<T> listObjectsTest1(ListCriteria<T> criteria);

Thus the UserEditor must implement in your case

public List<User> listObjectsTest1(ListCriteria<User> criteria) {

You should not mistake the parameter's type with the parameter's generic type. The Editor interface enforces a ListCriteria type. The ListCriteria's generic type is T and T can be bound by a subclass, e.g implements ListCriteria<User>. That is what I mean when I say "errors you get have nothing to do with generics".

I guess what you wanted was

public interface Editor<C, T extends ListCriteria<C>> {
    List<C> listObjectsTest1(T criteria);
}

and then the UserEditor can be implemented as

public class UserEditor implements Editor<User, UserListCriteria> {

   public List<User> listObjectsTest1(UserListCriteria criteria) {
      return null;
   }
}

why? Especially for the latter method

The second method in the Editor interface

  <L extends ListCriteria<T>> List<T> listObjectsTest2(L criteria);

does not mean that you can implement any type binding of L The UserEditor must still implement

public <L extends ListCriteria<User>> List<User> listObjectsTest2(L criteria) {
    return null;
}

This method defines the generic type L that will be bound when a client invokes the method. Thus you can invoke the method with any type that is a subtype of ListCriteria<User>.

edited Dec 9, 2013 at 9:07

answered Dec 9, 2013 at 7:53

René Link

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1 Comment

andbi Over a year ago

Thanks, do I get you right saying that I can override the return type, but not the method arguments, and that I must hold on to exactly the same arg signatures even if UserListCriteria perfectly fits at runtime?

Sage · Accepted Answer · 2013-12-09 08:09:14Z

Your listObjectsTest1(UserListCriteria criteria) function of UserEditor is not overriding the function listObjectsTest1(ListCriteria<T> criteria) of Editor<T> interface because they have two different signature, i.e., essentially the argument type.
The same goes for listObjectsTest2 which was declared in the interface: <L extends ListCriteria<T>> List<T> listObjectsTest2(L criteria); but you have declared it in the UserEditor class as: List<User> listObjectsTest2(UserListCriteria criteria)

So you will need to change the signature to:

class UserEditor implements Editor<User> {


    @Override
    public List<User> listObjectsTest1(ListCriteria<User> criteria) {

    }

    @Override
    public <L extends ListCriteria<User>> List<User> listObjectsTest2(L criteria) {

    }
}

Please read through jls 8.4.8. Inheritance, Overriding, and Hiding for more details.

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