Listener.java

import org.checkerframework.checker.nullness.qual.*;

public class Listener {

    @NonNull String f;

    public Listener() {
        Talker w = new Talker();
        //:: error: (argument.type.incompatible)
        w.register(this);

        f = "abc";
    }

    public void callback() {
        System.out.println(f.toLowerCase());
    }

    public static class Talker {
        public void register(Listener s) {
            s.callback();
        }
    }

    public static void main(String[] args) {
        new Listener();
    }
}