HashMap统计词频的方法

             Map map = new HashMap();
             StringTokenizer st = new StringTokenizer(sb.toString(),",.! \n");
             while (st.hasMoreTokens()) {
                 String letter = st.nextToken();
                 int count;
                 if (map.get(letter) == null) {
                     count = 1;
                 } else {
                     count = map.get(letter).intValue() + 1;
                 }
                 map.put(letter,count);
             }