Bigrams count in a file - Java

public class Bigrams {
    public static void main(String[] args) throws IOException {
        HashMap<String, Integer> hm = readFile();    }

    private static HashMap<String, Integer> readFile() throws IOException {
        ClassLoader classLoader = Bigrams.class.getClassLoader();        BufferedReader br = new BufferedReader(new FileReader(classLoader.getResource("file.txt").getFile()));        HashMap<String, Integer> hm;        try {
            StringBuilder sb = new StringBuilder();            String line = br.readLine();
            hm = new HashMap<String, Integer>();            while (line != null) {
                tokenizeAndPutIntoMap(hm, line);
                sb.append(line);                line = br.readLine();            }
            String everything = sb.toString();        } finally {
            br.close();        }
        return hm;    }

    private static void tokenizeAndPutIntoMap(HashMap<String, Integer> hm, String line) {
        StringTokenizer st = new StringTokenizer(line);
        int i = -1;        int j = 0;        ArrayList<String> al = new ArrayList<String>();        while (st.hasMoreElements()) {
            al.add((String) st.nextElement());
            if (i >= 0 && j >= 1) {
                String key = al.get(i) +"_"+ al.get(j);                if (hm.containsKey(key)) {
                    hm.put(key, hm.get(key) + 1);                } else                    hm.put(key, 1);            }
            i++;            j++;        }
    }

}