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);
 }

  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);
 }

int sum = 0;

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

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

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);