Type mismatch: cannot convert from List

Newbie & Debugging Questions
#1 

public class EntityRef<T extends Entity> {
...
}



public interface Entity{
	public List<EntityRef<?>> getForeignEntities();
...
}

so why type mismatch?


public class GameStatistik implements Entity{
	private List<EntityRef<PlayerData>> players;
	
	@Override
	public List<EntityRef<?>> getForeignEntities() {
		return players;  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	}
}
#2

I don’t think wildcards work like that. Try changing it to


   @Override
   public List<EntityRef<PlayerData>> getForeignEntities() {
      return players;  //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
   }

[EDIT]:

If you want to store “anything” you can always store an Object. Though this means you need to check the returned objects with the instanceof operator for all the things.

#3

This looks like it has something to do with covariance and type erasure. Here is a link that talks about something that may be similar.

This stackoverflow question might help as well.

#4

[quote]I don’t think wildcards work like that. Try changing it to
[/quote]
dont work: The return type is incompatible with Entity.getForeignEntities()
Thats also confusing since that should be a case of covariant return type.

btw.: the folowing works:


public class GameStatistik implements Entity{
        private List<EntityRef<PlayerData>> players;
	@Override
	public List<EntityRef<?>> getForeignEntities() {
		return new ArrayList<EntityRef<?>>(players);
	}
}
#5

I think you’d define the method in this way:


   public <T extends Entity> List<EntityRef<T>> getForeignEntities() {
      return new ArrayList<EntityRef<T>>(players);
   }