Replacing random number of characters in a string on random places

I am trying to write a program which reads a txt file in which there are 4000 or (x) words. So far i manage to write a program which you see below. It kinda words it adds (random) number of stars (*) to a randon place in a word. However here is the problem. It always add a *. How can i change that so it would add random numbers of stars to a word.

For example: lets take word Mother. I want a program too add a random number of * to a random place in this word.That random number of stars would be either 0, 1, 2, 3, 4, 5 or 6 in this example. And if i would run program multiple times i would get different result like: (* are shown as ? below)

M??her , Mot??r, Mother, ?other, ??????, M????r ,ect.

Any ideas would be welcome.

public class random_menjava_z_zvezdico {

public static void main(String[] args) {
    String test;
    int dolzina = 0;
    String outputFile = "random_z_zvezdico.txt";

    ArrayList<String> list = new ArrayList();


    try {

        File file = new File("4000_random_besed.txt");
        FileReader fileReader = new FileReader(file);
        BufferedReader bufferedReader = new BufferedReader(fileReader);
        String vrstica = bufferedReader.readLine();
        while (vrstica != null) {

            list.add(vrstica);
            vrstica = bufferedReader.readLine();
            // dolzina=list.size();
            // System.out.println(dolzina);

        }

        FileWriter fileWriter = new FileWriter(outputFile);
        PrintWriter out = new PrintWriter(fileWriter);
        System.out.println("4000 besedam je bila dodana * random krat na random mestu!");
        for (int idx = 0; idx < list.size(); ++idx) {
            test = list.get(idx);
            dolzina = test.length();

            Random rGenerator = new Random();
            StringBuilder beseda = new StringBuilder(test);

            for (int i = 0; i < dolzina; ++i) {
                int randomInt = rGenerator.nextInt(dolzina);
                beseda.setCharAt(randomInt, '*');

            }
            out.println(beseda);

        }out.close();
    } catch (IOException e) {
        e.printStackTrace();

    }
}
}