How to calculate the sum of the list using stream?

Robert Li :

I have a List<Integer> like { 1, 2, 3 ,4, 5 }, I'd like to get the result like 12345.

How to do this using Java8 streams or any smart way?

List consists of single digit non-negative integers.

I could definitely do like 1*10000+ 2 * 1000 + 3*100 + 4*10 + 5, but it is quite tedious.

Andy Turner :

Use IntStream.reduce:

int n = IntStream.of(array).reduce(0, (a,b) -> 10*a + b)

This is effectively the same as:

int n = 0;
for (int b : array) {
  n = 10 * n + b;
}

Personally, I would opt for the latter in the absence of other constraints, since it is much simpler code, doesn't involve the relatively heavyweight streams framework, easier to debug etc.