Java随机生成手机短信验证码的方法

/**
* 创建指定数量的随机字符串
* @param numberFlag 是否是数字
* @param length
* @return
*/
public static String createRandom(boolean numberFlag,int length){
 String retStr = "";
 String strTable = numberFlag ? "1234567890" : "1234567890abcdefghijkmnpqrstuvwxyz";
 int len = strTable.length();
 boolean bDone = true;
 do {
  retStr = "";
  int count = 0;
  for (int i = 0; i < length; i++) {
  double dblR = Math.random() * len;
  int intR = (int) Math.floor(dblR);
  char c = strTable.charAt(intR);
  if (('0' <= c) && (c <= '9')) {
   count++;
  }
  retStr += strTable.charAt(intR);
  }
  if (count >= 2) {
  bDone = false;
  }
 } while (bDone);
 return retStr;
}