Java lambda 循環累加求和代碼

java 8 stream 提供了下面幾種類型的求和

Stream::mapToInt

Stream::mapToDouble

Stream::mapToLong

public void test() { List<Person> people = new ArrayList<>(); people.add(new Person('zhangsan',20)); people.add(new Person('lisi', 26)); people.add(new Person('wangwu',35)); int sum = people.stream() .mapToInt(p -> p.getAge()) .sum(); System.out.println('Total of ages ' + sum); }

但是沒有BigDecimal類型，可以使用下面方法實現

public void test() { List<Person> list = new ArrayList<>(); list.add(new Person('zhangsan', 20, new BigDecimal(10.5))); list.add(new Person('lisi', 26, new BigDecimal(22.2))); list.add(new Person('wangwu', 35, new BigDecimal(15.54))); BigDecimal amounts = list.stream().map(item -> item.getAmount()) .reduce(BigDecimal.ZERO, BigDecimal::add); amounts = amounts.setScale(1, BigDecimal.ROUND_DOWN); System.out.println('Total of amounts:' + amounts); // 或者 BigDecimal sum = list .stream() .map(Person::getAmount) .reduce(BigDecimal::add) .get(); sum = sum.setScale(1, BigDecimal.ROUND_DOWN); System.out.println('Total of sums: ' + sum); }

補充知識：Java算法——求1到100累加的和，3種循環

1、for循環

int sum = 0;for (int i = 1; i <= 100; i++) {sum += i;}System.out.println('1到100累加的和為：' + sum);

2、while循環

int sum = 0;int i = 1;while (i <= 100) {sum += i;i++;}System.out.println('1到100累加的和為：' + sum);

3、do...while循環

int sum = 0;int i = 1;do {sum += i;i++;} while (i <= 100);System.out.println('1到100累加的和為：' + sum);

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