Code with generics won't compile

Question

I've failed to google this problem. Why would this line produce a compilation error.

wrapper.doSmth(wrapper.getCurrent());

I'm using java 7.

public class App {
 Wrapper<?> wrapper;

 class Generic<T>{

 }

 class Wrapper<T>{
  Generic<T> current;

  public void doSmth(Generic<T> generic){
  }

  public Generic<T> getCurrent(){
   return current;
  }
 }

 public void operation(){
  wrapper.doSmth(wrapper.getCurrent());
 }
}

The error is:

Error:(25, 24) java: method doSmth in class App.Wrapper<T> cannot be applied to given types;
  required: App.Generic<capture#1 of ?>
  found: App.Generic<capture#2 of ?>
  reason: actual argument App.Generic<capture#2 of ?> cannot be converted to conf.App.Generic<capture#1 of ?> by method invocation conversion

Answer 1

The compilation error should be something along the lines of "capture of ? #1 is not compatible with capture of ? #2". The cause of this error is that wrapper is a Wrapper<?>.

The compiler sees that the wrapper.getCurrent() returns a Generic<?>, and that wrapper.doSmth takes a Generic<?> as a parameter. But it won't equate the two ? wildcards, even if we can see that they come from the same instance and should be the same.

One solution here is to make the App class generic, so you can replace the wildcard.

public class App<T> {

Because of the fact that Generic and Wrapper are inner classes, T is still in scope, so you don't need to declare a generic type parameter for them any more.

    Wrapper wrapper;

    class Generic{

    }

    class Wrapper{
        Generic current;

        public void doSmth(Generic generic){
        }

        public Generic getCurrent(){
            return current;
        }
    }

    public void operation(){
        wrapper.doSmth(wrapper.getCurrent());
    }
}