關於list.Stream的用法(去重，相加，分組)

1.通過list中的某個字段來對list分組，然后得到多組數據

   Map<String, List<WmsStockRefundItem>> refundMap = itemList.stream().collect(Collectors.groupingBy(WmsStockRefundItem::getSupplyNumber));

2.通過list中的某個屬性過濾去重，只留下不同的集合

List<WmsStockRefundItem> fiterList =list.stream().filter(distinctByKey(b -> b.getProdNumber())).collect(Collectors.toList());

//通過屬性去重
private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    Map<Object,Boolean> seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

3.相加list中的某個值 bigdecimal

 BigDecimal totalquantity = list.stream().map(WmsStockRefundItem::getReqQuantity).reduce(BigDecimal.ZERO, BigDecimal::add);

 

4.相加list中的某個值 Integer

Integer num=list.stream().collect(Collectors.summingInt(WmsStockRefundItem::getReqQuantity))