Java Stream grouping by multiple fields

Collection8Utils.java

  /**
   * Group on 2 levels
   */
  protected <K,V> Map<K, Map<K,List<V>>> groupByFields2 (List<V> modelList,Function<? super V, ? extends K> classifier1,Function<? super V, ? extends K> classifier2) {

    Collector<V, ?, Map<K, List<V>>> groupingBy2 = Collectors.groupingBy(classifier2);
    Collector<V, ?, Map<K, Map<K,List<V>>>> groupingBy = Collectors.groupingBy(
       classifier1
        ,groupingBy2
    );
    return modelList.stream().collect(groupingBy);
  }
  
  /* Simple Example */
  public static void main (String[] args) {
    Map map = groupByFields2(new ArrayList(Currency.getAvailableCurrencies()) , Currency::getDefaultFractionDigits, Currency::getDisplayName);
    System.out.println(map.toString().replace("{","\n{"));
    
  }