Lambda表达式List分组合计

转：https://blog.csdn.net/gsls200808/article/details/86501905
//更新第2个List内容
List newListOrder = listOrder.stream().map(orderpart ->
listPartHave.stream().filter( one-> one.getPartno().equals(orderpart.getPartno())).findFirst().map(one -> {orderpart.setAmount(one.getAmount());return orderpart;}).orElse(null)
).collect(Collectors.toList());
//去掉空
newListOrder = newListOrder.stream().filter(orderpart -> orderpart != null
).collect(Collectors.toList());
合计
double amountsum = listSale.stream().mapToDouble(SaleListDetail::getAmount).sum();
分组
Map> groupBySaleNo = splitList.stream().collect(Collectors.groupingBy(SaleListDetail::getSaleno));

通过groupingBy可以分组指定字段
//分组
Map> groupBySex = userList.stream().collect(Collectors.groupingBy(User::getSex));
//遍历分组
for (Map.Entry> entryUser : groupBySex.entrySet()) {
String key = entryUser.getKey();
List entryUserList = entryUser.getValue();
}
多字段分组

    Function<WarehouseReceiptLineBatch, List<Object>> compositeKey = wlb -><br />                Arrays.<Object>asList(wlb.getWarehouseReceiptLineId(), wlb.getWarehouseAreaId(), wlb.getWarehouseLocationId());<br />        Map<Object, List<WarehouseReceiptLineBatch>> map =<br />                warehouseReceiptLineBatchList.stream().collect(Collectors.groupingBy(compositeKey, Collectors.toList()));<br />        //遍历分组<br />        for (Map.Entry<Object, List<WarehouseReceiptLineBatch>> entryUser : map.entrySet()) {<br />            List<Object> key = (List<Object>) entryUser.getKey();<br />            List<WarehouseReceiptLineBatch> entryUserList = entryUser.getValue();<br />            Long warehouseReceiptLineId = (Long) key.get(0);<br />            Long warehouseAreaId = (Long) key.get(0);<br />            Long warehouseLocationId = (Long) key.get(0);<br /> <br />        }<br />取某一列<br /> List<String> names = list.stream().map(p -> p.getName()).collect(Collectors.toList());<br />String offernos = StringUtils._join_(listOfferno , **","**);