static interface Dao<I extends ResourceId<?>, R extends Resource<I>> {
        R findById(I id);

        void save(R u);
    }

static class UserDao implements Dao<UserId, User> {
        public User findById(UserId id) { return null; }

        public void save(User u) {}
    }

static interface Resource<I extends ResourceId<?>> {
        I id();
    }

public class NestedGenerics {

    static interface ResourceId<T> {
        T get();
    }

    static class UserId implements ResourceId<String> {
        public String get() { return null; }
    }

    static interface Resource<I extends ResourceId<?>> {
        I id();
    }

    static class User implements Resource<UserId> {
        public UserId id() { return null; }
    }

    static interface Dao<I extends ResourceId<?>, R extends Resource<I>> {
        R findById(I id);

        void save(R u);
    }

    static class UserDao implements Dao<UserId, User> {
        public User findById(UserId id) { return null; }

        public void save(User u) {}
    }
}