How to sum the values in List<int[]> using Java 8

Question

I want to find the sum of the List<int[]> using Java 8. Here is my attempt.

int sum = counts.stream().flatMap(i -> Stream.of(i).mapToInt(m)).sum(); 

However, I get the error cannot convert to Stream<Object> to <unknown>.

Answer 1

You want to flatMap to an IntStream. After that, taking the sum is easy.

int sum = counts.stream()         .flatMapToInt(IntStream::of)         .sum(); 

Answer 2

int sum = counts.stream().flatMapToInt(array -> IntStream.of(array)).sum();