Bench.java

//this performs horribly. It's a programming error in the file :)
package com.company;

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.OpenOption;
import java.nio.file.Paths;

import static java.nio.file.StandardOpenOption.APPEND;

public class Bench {

    public static void main(String[] args) throws IOException {

        var start = System.currentTimeMillis();
        parse();
        var took = (System.currentTimeMillis() - start) / 1000f;
        System.out.println(took);
    }

    private static void parse() throws IOException {

        var path = Paths.get("C:\\Users\\cosmi\\Desktop\\random_numbers.txt");
        var charSet = StandardCharsets.UTF_8;

        try (var reader = Files.newBufferedReader(path, charSet)) {
            String line;
            while ((line = reader.readLine()) != null) {
                var x = Double.parseDouble(line.split("\t")[0]);
                var y = Double.parseDouble(line.split("\t")[1]);
                double res = x / y;

                writeResult(x, y, res);
            }
        }
    }

    private static void writeResult( double x, double y, double res) throws IOException {

        var resultFile = "C:\\Users\\cosmi\\Desktop\\results_contest_devforum.txt";
        var charSet = StandardCharsets.UTF_8;
        try (var writer = Files.newBufferedWriter(Paths.get(resultFile), charSet, APPEND)) {
            writer.write(x + "\t" + y + "\t" + res + "\n");
        }
    }
}

ReadAndWrite_10_mil_rows.java

package com.company;

import java.io.IOException;
import java.lang.management.ManagementFactory;
import java.nio.CharBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.EnumSet;

public class Bench {

    public static void main(String[] args) throws IOException {

        var start = System.currentTimeMillis();
        parse();
        var took = (System.currentTimeMillis() - start) / 1000f;
        System.out.println(took);
        System.out.println("Used memory: " + ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed() / 1024 / 1024);
    }

    private static void parse() throws IOException {

        var path = Paths.get("C:\\Users\\cosmi\\Desktop\\random_numbers.txt");
        var charSet = StandardCharsets.UTF_8;
        var sb = new StringBuilder();
        try (var reader = Files.newBufferedReader(path, charSet)) {
            String line;
            while ((line = reader.readLine()) != null) {
                var x = Double.parseDouble(line.split("\t")[0]);
                var y = Double.parseDouble(line.split("\t")[1]);
                double res = x / y;
                sb.append(x).append("\t").append(y).append("\t").append(res).append("\n"); //desi lumea zice sa nu folosesti sb
            }
        }
        writeResult(sb);
    }

    private static void writeResult(StringBuilder data) throws IOException {
        var data1 = CharBuffer.wrap(data);
        var pathToWrite = Paths.get("C:\\Users\\cosmi\\Desktop\\results_contest_devforum.txt");

        try (FileChannel fileChannel = (FileChannel) Files
                .newByteChannel(pathToWrite, EnumSet.of(
                        StandardOpenOption.READ,
                        StandardOpenOption.WRITE,
                        StandardOpenOption.TRUNCATE_EXISTING))) {

            MappedByteBuffer mappedByteBuffer = fileChannel
                    .map(FileChannel.MapMode.READ_WRITE, 0, data1.length());

            if (mappedByteBuffer != null) {
                mappedByteBuffer.put(
                        StandardCharsets.UTF_8.encode(data1));
            }
        }
    }
}