The replace function in Java

I have this function in Java, that given a text, an array of words and a char, should check if the words in the "words" array exist in the text, and if they do, surround them with the given char "c".

Here's what i've come up with:

public void replaceString(BufferedReader in, BufferedWriter out, char c, String[] words){

    String line_in = in.readLine();
    String result = null;

        while (line_in != null) {

            for (int j = 0; j < words.length; j++) {

                if (line_in.toLowerCase().contains(words[j].toLowerCase())) {

                    result = line_in.replace(words[j], bold + words[j] + bold);

                }

                else
                    result = line_in;

            }

            out.write(result);
            out.newLine();
            line_in = in.readLine();

        }

}

But, for some reason, the replace method is not doing anything...

Any help would be greatly appreciated!