Java - ¿Como modificar archivos txt en java?

public String [] cargarDatos() {/* Crear el registro y regresarlo*/

        String [] temp = {};

        try {

            if(fr != null){

                temp = br.readLine().split(",");

                total_c += Integer.parseInt(temp[3]);

            }

        } catch (IOException ex) {

            System.err.println(ex.getMessage());

        }

        return temp;

    }

}

total_c += Integer.parseInt(temp[3].isEmpty() ? "0" : temp[3]);

import java.io.BufferedReader;

import java.io.FileReader;

import java.io.IOException;

public class ReadCsv {

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

        int total = sum("src/main/test.csv");

        System.out.println("Result is: " + total);

    }

    private static int sum(String pathToCsv) throws IOException {

        BufferedReader csvReader = new BufferedReader(new FileReader(pathToCsv));

        String row;

        int total  = 0;

        boolean firstRow = true;

        while ((row = csvReader.readLine()) != null) {

            if(firstRow) {

                firstRow = false;

                continue;

            }

            String[] data = row.split(",");

            if(data.length == 4) {

                total += Integer.parseInt(data[3]);

            }

        }

        csvReader.close();

        return total;

    }

}

total_c += Integer.parseInt(temp[3].length == 4 ? temp[3] : "0");

String casos;

if(temp.length == 4){

    casos = temp[3];

} else {

    casos = "0";

}

total_c += Integer.parseInt(casos);

import java.io.BufferedReader;

import java.io.File;

import java.io.FileReader;

import java.io.IOException;

/**

 *

 * @author Billy Joel

 */

public class ArchivoCadena {

    public static void main(String[] args) {

        File archivo;

        FileReader fr = null;

        BufferedReader br;

        int total_c = 0;

        try {

            archivo = new File("C:\\casos_fran.txt");

            fr = new FileReader(archivo);

            br = new BufferedReader(fr);

            // Lectura del fichero

            String linea;

            while ((linea = br.readLine()) != null) {

                //System.out.println(linea);

                linea = linea.replace(",,", ", ,") + (linea.substring(linea.length() - 1).equals(",") ? " " : "");

                String[] r = linea.split(",");

                for (int i = 0; i < r.length; i++) {

                    System.out.print(i + ": " + r[i] + "\t");

                }

                System.out.println();

                total_c += Integer.parseInt(r[3]);

            }

        } catch (IOException e) {

            e.printStackTrace(System.out);

        } finally {

            // En el finally cerramos el fichero, para asegurarnos

            // que se cierra tanto si todo va bien como si salta

            // una excepcion.

            try {

                if (null != fr) {

                    fr.close();

                }

            } catch (IOException e2) {

                e2.printStackTrace(System.out);

            }

        }

        System.out.println("total: " + total_c);

    }

}

public String[] cargarDatos() {/* Crear el registro y regresarlo*/

    String[] temp = {};

    try {

        if (fr != null) {

            temp = br.readLine().split(",");

            String casos;

            if (temp.length >= 4) {

                casos = temp[3];

            } else {

                casos = "0";

            }

            total_c += Integer.parseInt(casos);

            if(temp.length==5){

                casos = temp[4];

            }else{

                casos = "0";

            }

            total_d +=Integer.parseInt(casos);

        }

    } catch (IOException ex) {

        System.err.println(ex.getMessage());

    }

    return temp;

if(temp.length>=5){

casos = temp[4];

}else{

casos = "0";

}

if(!Strings.isNullOrEmpty(casos)) {

            total_d +=Integer.parseInt(casos);

        }