How to sum a list of integers with java streams?

Question

I want to sum a list of Integers. It works as follows, but the syntax does not feel right. Could the code be optimized?

Map<String, Integer> integers; integers.values().stream().mapToInt(i -> i).sum(); 

Answer 1

This will work, but the i -> i is doing some automatic unboxing which is why it "feels" strange. mapToInt converts the stream to an IntStream "of primitive int-valued elements". Either of the following will work and better explain what the compiler is doing under the hood with your original syntax:

integers.values().stream().mapToInt(i -> i.intValue()).sum(); integers.values().stream().mapToInt(Integer::intValue).sum(); 

Answer 2

I suggest 2 more options:

integers.values().stream().mapToInt(Integer::intValue).sum(); integers.values().stream().collect(Collectors.summingInt(Integer::intValue)); 

The second one uses Collectors.summingInt() collector, there is also a summingLong() collector which you would use with mapToLong.

---

And a third option: Java 8 introduces a very effective LongAdder accumulator designed to speed-up summarizing in parallel streams and multi-thread environments. Here, here's an example use:

LongAdder a = new LongAdder(); map.values().parallelStream().forEach(a::add); sum = a.intValue();