计算一个number类型的List对象总和

Integer[] integerArray = {1, 3, 5, 10, 18};

List<Integer> list = new ArrayList<>(Arrays.asList(integerArray));

IntSummaryStatistics summaryStatistics = list.stream().mapToInt((s) -> s).summaryStatistics();
System.out.println("总和：" + summaryStatistics.getSum());
System.out.println("平均数：" + summaryStatistics.getAverage());
System.out.println("总个数：" + summaryStatistics.getCount());
System.out.println("最大值：" + summaryStatistics.getMax());
System.out.println("最小值：" + summaryStatistics.getMin());

计算一个List对象中某个字段总和

List<User> userList = new ArrayList<>();
User user1 = new User();
user1.setAge(10);
userList.add(user1);
User user2 = new User();
user2.setAge(20);
userList.add(user2);
User user3 = new User();
user3.setAge(25);
userList.add(user3);

int ageSum = userList.stream().collect(Collectors.summingInt(User::getAge));
System.out.println("年龄总和 ：" + ageSum);
//下面一位网友提供了一种更加简洁的方式，非常感谢
int ageSumTwo = userList.stream().mapToInt(User::getAge).sum();
System.out.println("年龄总和2： " + ageSumTwo);