list stream：計算對象List中某個字段的總和

原文地址：https://blog.csdn.net/zhangzehai2234/article/details/88610738

目錄

• 計算一個number類型的List對象總和
• 計算一個List對象中某個字段總和

計算一個number類型的List對象總和

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("總和：" + summaryStatistics.getSum());
System.out.println("平均數：" + summaryStatistics.getAverage());
System.out.println("總個數：" + summaryStatistics.getCount());
System.out.println("最大值：" + summaryStatistics.getMax());
System.out.println("最小值：" + summaryStatistics.getMin());

計算一個List對象中某個字段總和

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("年齡總和：" + ageSum);