Why can't a Generic type containing a Generic type be assigned to a Generic typed class of wildcard type

Question

Sorry if the title seems confusing, but some examples are in order.

Let\'s say I have some Java class with a generic type parameter:

public class Gen         


        

Answer 1

The problem your facing is denominated Covariance.

List<GenericClass<String>> stringy = ...
List<GenericClass<?>> generic = stringy;
generic.add(new GenericClass<Integer>());

If this wasn't a compile error, then the last line of code would be possible.

You could get around the error by doing this:

 List<? extends GenericClass<?>> generic = stringy;

but you can not use add also because you don't really know what ? extends GenericClass<?> is (Covariance once again). In this case you can only enumerate through the List and expect GenericClass<?>.

Answer 2

Technically, that's because List<GenericClass<String>> is not a subtype of List<GenericClass<?>>. In order to make it work, you could do something like

List<? extends GenericClass<?>> generic = stringy

which should work as expected (though is pretty ugly...).

See, for example, this SO question for more details