java lambda：如何计算列表中的项目总数？

Question

How can I return the following result by lambda?

int total = 0;
for (User user : userList) {
    total += user.getAge();
}

I know reduce's use. new LinkedList<Integer>().stream().reduce(0, (acc, x) -> acc + x)
I want to try(but failed). userList.stream().reduce(0, (acc, x) -> acc.getAge() + x.getAge());

Answer 1

You can use mapToInt

useList
    .stream()
    .mapToInt(User::getAge)
    .sum();

If you really wanted to use reduce , here it is (but I don't see a point in using it as the above is more readable)

useList.stream()
        .mapToInt(User::getAge)
        .reduce(0, (acc, current) -> acc + current);

Or as suggested by Holger@

user.stream()
    .reduce(0, (c, user) -> c + user.getAge(), (a, b) -> a + b);