随机生成ID属性值数字大小写字母

private  String randomString() {
  Random random = null;
  char[] numbersAndLetters =null;
        if (random == null) {
         random = new Random();
               numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                }
        char [] randBuffer = new char[8];
        for (int i=0; i<randBuffer.length; i++) {
            randBuffer[i] = numbersAndLetters[random.nextInt(71)];
         //randBuffer[i] = numbersAndLetters[randGen.nextInt(35)];
        }
        return new String(randBuffer);