How can I pick a random word then replace it to a new word?

I need to replace a random word that I pick in the MovieList into underscore "___", I think about create a new method to handle it, the new method should be under the class guessMovies, and then I will call method under "object r" to replace every single word into underscore, but Java said it cannot find symbol ( it doesn't know what "r" is ). Please help.

Thank you

public class guessMovies {

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

        String MovieList;
        ArrayList<String> MovieNames = new ArrayList<>();

        try (BufferedReader br = new BufferedReader(new FileReader("MovieList.txt"))) {

            while ((MovieList = br.readLine()) != null) {
                MovieNames.add(MovieList);
            }

            for(String movieName: MovieNames){
                System.out.println(movieName);
            }


            Random r = new Random();
            System.out.println(MovieNames.get(r.nextInt(MovieNames.size())));

        }

        catch(FileNotFoundException exception) {
            System.out.println("I cannot find your file");

        }
        //pick random movie
    }
}