Task: CSV data manipulation
C#: using System.IO;
using System.Linq;

namespace CSV_data_manipulation
{
    class Program
    {
        static void Main()
        {
            var input = File.ReadAllLines("test_in.csv");
            var output = input.Select((line, i) =>
            {
                if (i == 0)
                    return line + ",SUM";
                var sum = line.Split(',').Select(int.Parse).Sum();
                return line + "," + sum;
            }).ToArray();
            File.WriteAllLines("test_out.csv", output);
        }
    }
}

Java: import java.io.*;
import java.awt.Point;
import java.util.HashMap;
import java.util.Scanner;

public class CSV {

    private HashMap<Point, String> _map = new HashMap<Point, String>();
    private int _cols;
    private int _rows;

    public void open(File file) throws FileNotFoundException, IOException {
        open(file, ',');
    }

    public void open(File file, char delimiter)
            throws FileNotFoundException, IOException {
        Scanner scanner = new Scanner(file);
        scanner.useDelimiter(Character.toString(delimiter));

        clear();

        while(scanner.hasNextLine()) {
            String[] values = scanner.nextLine().split(Character.toString(delimiter));

            int col = 0;
            for ( String value: values ) {
                _map.put(new Point(col, _rows), value);
                _cols = Math.max(_cols, ++col);
            }
            _rows++;
        }
        scanner.close();
    }

    public void save(File file) throws IOException {
        save(file, ',');
    }

    public void save(File file, char delimiter) throws IOException {
        FileWriter fw = new FileWriter(file);
        BufferedWriter bw = new BufferedWriter(fw);

        for (int row = 0; row < _rows; row++) {
            for (int col = 0; col < _cols; col++) {
                Point key = new Point(col, row);
                if (_map.containsKey(key)) {
                    bw.write(_map.get(key));
                }

                if ((col + 1) < _cols) {
                    bw.write(delimiter);
                }
            }
            bw.newLine();
        }
        bw.flush();
        bw.close();
    }

    public String get(int col, int row) {
        String val = "";
        Point key = new Point(col, row);
        if (_map.containsKey(key)) {
            val = _map.get(key);
        }
        return val;
    }

    public void put(int col, int row, String value) {
        _map.put(new Point(col, row), value);
        _cols = Math.max(_cols, col+1);
        _rows = Math.max(_rows, row+1);
    }

    public void clear() {
        _map.clear();
        _cols = 0;
        _rows = 0;
    }

    public int rows() {
        return _rows;
    }

    public int cols() {
        return _cols;
    }

    public static void main(String[] args) {
        try {
            CSV csv = new CSV();

            csv.open(new File("test_in.csv"));
            csv.put(0, 0, "Column0");
            csv.put(1, 1, "100");
            csv.put(2, 2, "200");
            csv.put(3, 3, "300");
            csv.put(4, 4, "400");
            csv.save(new File("test_out.csv"));
        } catch (Exception e) {
        }
    }
}