Java-Liste als Gruppe, Sortierung usw.

//Gruppe
Map<String, List<Object>> map = list.stream().collect(//
    Collectors.groupingBy(o -> o.getKey1()));
//Mehrere Schlüssel
Map<String, List<Object>> map = list.stream().collect(//
    Collectors.groupingBy(o -> o.getKey1() + o.getKey2()));
//Unterstützt mehrere Schlüsselnullen
Map<String, List<Object>> map = list.stream().collect(//
    Collectors.groupingBy(t -> t.getKey1() + (t.getKey2() == null ? "" : t.getKey2())));
 

//Sortieren
Comparator<Object> comparator = //
    Comparator.comparing(Object::getSort1, Comparator.nullsFirst(Comparator.naturalOrder()))//
    .thenComparing(Object::getSort2, Comparator.nullsFirst(Comparator.naturalOrder()))//
    .thenComparing(Object::getSort3, Comparator.nullsFirst(Comparator.naturalOrder()));
// limit
map.entrySet().forEach(e -> e.getValue().stream().sorted(comparator).limit(100).forEach(l -> System.out.println(e.getKey() + ":" + l.getValue())));

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