public Map<Artist, Long> filter(){
        Map<Artist, List<Concert>> raport = concerts.findAll().stream()
                .collect(Collectors.groupingBy(Concert::getArtist));
        Map<Artist, Long> raportt = raport.entrySet().stream()
                .map(a -> new  AbstractMap.SimpleEntry<Artist, Long>(a.getKey(), a.getValue().stream().count()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        return raportt;
    }