java 随机生成字符串 （字母和数字）

public static String getRandomString(int length) { //length表示生成字符串的长度    String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";       Random random = new Random();       StringBuffer sb = new StringBuffer();       for (int i = 0; i < length; i++) {           int number = random.nextInt(str.length());           sb.append(str.charAt(number));       }       return sb.toString();    }