example.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @see https://twitter.com/cobot_1/status/403501394528718848
 */
public class Example {
    static abstract class Dto {}
    static class DtoA extends Dto {}
    static class DtoB extends Dto {}

    static interface Dao<T extends Dto> {
        List<T> load();
    }

    static class DaoA implements Dao<DtoA> {
        @Override
        public List<DtoA> load() {
            return new ArrayList<DtoA>();
        }
    }

    static class DaoB implements Dao<DtoB> {
        @Override
        public List<DtoB> load() {
            return new ArrayList<DtoB>();
        }
    }

    public static void main(String[] args) {
        List<Dao<? extends Dto>> daoList = Arrays.asList(new DaoA(), new DaoB());
        for (Dao<? extends Dto> dao : daoList) {
            List<? extends Dto> values = dao.load();
        }
    }
}