devslack's sum-files-challenge Java solution (Draft till when I can make it better to submit issue). Runs average of 6secs after warm up. https://github.com/devcenter-square/sum-files-challenge

sum-files-challenge

package com.topriddy.devslack;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static java.util.Arrays.asList;
import static java.util.function.Function.identity;

public class ProgramSolution {
    private final static String pathName = "/Users/topriddy/dev/sum-files-challenge-data/files";


    public static void main(String args[]) throws Exception {
        sumFiles(pathName);
        sumFiles(pathName);
        sumFiles(pathName);
    }

    static void sumFiles(String dataPath) throws Exception {
        System.out.println("Starting...");
        Long startTime = System.currentTimeMillis();

        List<Path> files = Files.walk(Paths.get(dataPath))
                .filter(p -> !Files.isDirectory(p))
                .collect(Collectors.toList());

        Long sum = files.parallelStream()
                .map(path -> {
                    try {
                        return Files.lines(path);
                    } catch (IOException ioex) {
                        ioex.printStackTrace();
                        return Stream.empty();
                    }
                })
                .map(lines -> lines.map(line -> asList(((String) line).split(",")).stream())
                        .flatMap(identity())
                        .map(value -> Long.valueOf(value))
                )
                .flatMap(identity())
                .mapToLong(v -> v).sum();

        Long endTime = System.currentTimeMillis();

        System.out.println("Sum of numbers in file is : " + sum);
        System.out.printf("\nDuration: %f seconds", (endTime - startTime) / 1000.0);
        System.out.println("\nEnd");
    }
}