code: public static String random(int len) {
          final char[] chars = {'0', '1', '2', '3', '4',
                  '5', '6', '7', '8', '9', '$', '#', '^', '&', '_',
                  'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
                  'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
                  'u', 'v', 'w', 'x', 'y', 'z',
                  'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
                  'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
                  'U', 'V', 'W', 'X', 'Y', 'Z',
                  '~', '!', '@'};
  
          final int max = chars.length;
          Random r = new Random();
          StringBuilder sb = new StringBuilder(len);
          while(len-- > 0) {
              int i = r.nextInt(max);
              sb.append(chars[i]);
          }
          return sb.toString();
      }
docs: Generate random string.
  
  The generated string is safe to be used as filename
  @param len
  @return a random string with specified length