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

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

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

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

Your i is a primitive array (int[]), so Stream.of(i) will return a Stream<int[]>.

I suggest you first calculate the sum of each individual array and then sum all of them:

int sum=counts.stream()
              .mapToInt(ar->IntStream.of(ar).sum()) // convert each int[] to the sum 
                                                    // of that array and transform the
                                                    // Stream to an IntStream
              .sum(); // calculate the total sum