public int compareTo(Object o) {
        CollectionAndArtist other = (CollectionAndArtist) o;

        if (artist != null && collection != null)
            return artist.equalsIgnoreCase(other.artist) && collection.equalsIgnoreCase(other.collection) ? 0 : 1;
        else if (artist != null)
            return artist.equalsIgnoreCase(other.artist) ? 0 : 1;
        else
            assert false : "Es sollte keine Elemente ohne Namen geben!";
            return 0;
    }