翻转字符串

          synchronized (new Object()) {
            String word = "hello";
            String result = "";
            for (int k = word.length() - 1; k >= 0; k--) {
//          无效率，别这么做
//          每一轮都会创建新的String对象并舍弃旧的，可以使用StringBuffer或者StringBuilder
//          result = result + String.format("%s", word.charAt(k));
                result = result + word.substring(k, k+1);
//                System.out.println(word.substring(k, k + 1));
            }
            System.out.println(result);
        }