Java8 -- 分组

单字段分组

Map<String,List<String>> collect = list.stream().collect(Collectors.groupingBy(HistroySearch::getLx))

多字段分组

List<PrintValueVO> printValueVOListAllDeal = new ArrayList<>();                
Map<String, List<PrintValueVO>> groupMap = printValueVOListAll.stream().collect(Collectors.groupingBy(a -> a.getSupplierCommodityId() + "," + a.getLotNumber() + "," + a.getExpiryDate() + "," + a.getPrice()));
                for(Map.Entry<String, List<PrintValueVO>> entry : groupMap.entrySet()){
                    PrintValueVO printValueVO = new PrintValueVO();
                    BeanUtils.copyProperties(entry.getValue().get(0), printValueVO);
 
                    BigDecimal baseValue = BigDecimal.ZERO;
                    BigDecimal inputValue = BigDecimal.ZERO;
                    BigDecimal amount = BigDecimal.ZERO;
                    for(PrintValueVO subDTO: entry.getValue()){
                        baseValue = baseValue.add(subDTO.getBaseNumber() != null ? subDTO.getBaseNumber(): BigDecimal.ZERO);
                        inputValue = inputValue.add(subDTO.getInputNumber() != null ? subDTO.getInputNumber(): BigDecimal.ZERO);
                        amount = amount.add(subDTO.getEachPrice() != null ? subDTO.getEachPrice(): BigDecimal.ZERO);
                    }
                    printValueVO.setBaseNumber(baseValue);
                    printValueVO.setInputNumber(inputValue);
                    printValueVO.setEachPrice(amount);
                    printValueVO.setAmount(amount);
                    printValueVOListAllDeal.add(printValueVO);
                }

每组求最新

Java stream 处理分组后取最新时间的一组_Max-yang 的博客 - 优快云 博客

注意：

Stream的Collectors.groupingBy支持key为null_紫枫231的博客-优快云博客_collectors.groupingby key为空

public void groupBy(List<User> userList){
    //使用option包装一层
	Map<Option<String>, List<User>> userMap = userList.stream.collect(Collectors.groupingBy(x -> Optional.ofNullable(x.getSex())));
}