集合中取某一个字段求和

BigDecimal求和

BigDecimal totalPrice = list.stream().map(Apple::getPrice).reduce(BigDecimal.ZERO, BigDecimal::add);
 

interge求和

Integer sum = integers.stream().map(Apple::getPrice).reduce(0, Integer::sum);

stream 分组条件查询

    public static void main(String[] args) {
        Stu s1 = new Stu("机构1", 2,"商品1");
        Stu s11 = new Stu("机构1", 2,"商品2");
        Stu s2 = new Stu("机构1", 44,"商品44");
        Stu s3 = new Stu("机构2", 2,"商品1");
        Stu s4 = new Stu("机构2", 3,"商品1");
        Stu s5 = new Stu("机构2", 3,"商品1");
        List<Stu> list = new ArrayList();
        list.add(s1);list.add(s2);list.add(s3);list.add(s4);list.add(s11);list.add(s5);

        //当对象中某个属性相同时  把对象的另一个值存到List集合里面
//        Map<Integer, List<Stu>> map = list.stream().collect(Collectors.groupingBy(Stu::getAge));
//        list.stream().collect(Collectors.groupingBy(Stu::getAgency, TreeMap::new, Collectors.mapping(Stu::getSupplier, Collectors.toList())));
//

        //分组根据两个条件一致分组   把分组的后的数据放到map集合里  
        Map<String, Map<Integer, List<Stu>>> collect = list.stream()
                .collect(Collectors.groupingBy(Stu::getAgency, Collectors.groupingBy(Stu::getSupplier)));
        collect.forEach((key,val)->{
            System.out.println("机构id******："+key+" 供应商："+val);
        });
    }
}

@Data
class Stu{
    String agency;
    int supplier;
    String goods;
    public Stu(String agency, int supplier,String goods) {
        super();
        this.agency = agency;
        this.supplier = supplier;
        this.goods = goods;
    }
}

List<integer>  转 string

List<String> areaIdList = openAreaItemList.stream().map(x -> x.getGoodsOpenAreaId().toString()).collect(Collectors.toList());