java8-stream排序去重等操作

List<String> list =orderList.stream().
            collect(Collectors.groupingBy(OrderViewModel::getPcno,Collectors.counting()))
            .entrySet().stream()
            .filter(entry->entry.getValue()>1)
            .map(Map.Entry::getKey)
            .collect(Collectors.toList());

对List对象集合某个字段进行排重

private static List<OrderViewModel> removeDuplicateOrder(List<OrderViewModel> orderList) {
    Set<OrderViewModel> set = new TreeSet<>(Comparator.comparing(OrderViewModel::getPcno));
    set.addAll(orderList);
    return new ArrayList<>(set);
}

根据list对象中的两个字段进行排序，字段有空的就放最后

list.sort(Comparator.comparing(TmpOrderDto::getPrizeTime, Comparator.nullsLast(Date::compareTo))
.thenComparing(TmpOrderDto::getKoiTime, Comparator.nullsLast(Date::compareTo)));