private void tokenize(String s, int fileId) {
        StringTokenizer st = new StringTokenizer(s);
        String[] tokens = new String[st.countTokens()];
        for (int i = 0; i < tokens.length; i++) {
            tokens[i] = st.nextToken();
        }

        int lastIndex = 0;
        for (String t : tokens) {
            WordIndex wi = words.get(t);
            if (wi != null) { // word already met
                wi.met(fileId, lastIndex); // add new 'rendez vous'
                lastIndex += t.length() + 1; // word_length + spacing_character
            } else { // word has not met in any file before
                words.put(t, new WordIndex());
                words.get(t).met(fileId, lastIndex);
                lastIndex += t.length() + 1;
            }
        }
    }