java中各种类型用Stream流求和

一、BigDecimal类型

1. 对一个实体类是某一个字段求和

List<User> userList = new ArrayList<>(Arrays.asList(
                new User(1,new BigDecimal("10")),
                new User(2,new BigDecimal("100"))));

        BigDecimal sum = userList.stream().filter(Objects::nonNull).map(User::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);

2. 分组求和

数据准备

YearNum yearNum1 = new YearNum(2021, BigDecimal.valueOf(10));
        YearNum yearNum11 = new YearNum(2021, BigDecimal.valueOf(10));
        YearNum yearNum2 = new YearNum(2022, BigDecimal.valueOf(20));
        YearNum yearNum22 = new YearNum(2022, BigDecimal.valueOf(20));
        List<YearNum> years = ListUtil.of(yearNum1, yearNum11, yearNum2, yearNum22);

方式一

Map<Integer, BigDecimal> numByYear = years.stream()
                .collect(Collectors.groupingBy(YearNum::getYear,
                        Collectors.reducing(BigDecimal.ZERO, YearNum::getNum, BigDecimal::add)));

方式二

Map<Integer, BigDecimal> numByYear2 = years.stream().filter(Objects::nonNull).collect(
                Collectors.groupingBy(YearNum::getYear, Collectors.mapping(YearNum::getNum, Collectors.reducing(BigDecimal.ZERO, BigDecimal::add))));

二、int、double、long类型

1. Collectors.summarizingInt()实现

List<User> userList = new ArrayList<>(Arrays.asList(
                new User(1L,new BigDecimal("10")),
                new User(2L,new BigDecimal("100"))));
        
        DoubleSummaryStatistics collect = userList.stream().collect(Collectors.summarizingDouble(User::getId));
        double sum = collect.getSum();

2. stream().reduce()实现

对象的一个字段求和

List<User> userList = new ArrayList<>(Arrays.asList(
                new User(1L,new BigDecimal("10")),
                new User(2L,new BigDecimal("100"))));
        Long reduce = userList.stream().filter(Objects::nonNull).map(User::getId).reduce(0L, Long::sum);

Long类型的集合

List<Long> list = ListUtil.of(1L,2L);
        Long reduce = list.stream().reduce(0L, Long::sum);

3. stream().mapToLong()实现

Long类型的集合

List<Long> list = ListUtil.of(1L,2L);
        long sum = list.stream().mapToLong(s -> s).sum();

对象的一个字段求和

List<User> userList = new ArrayList<>(Arrays.asList(
                new User(1L,new BigDecimal("10")),
                new User(2L,new BigDecimal("100"))));
        long sum = userList.stream().mapToLong(User::getId).sum();