public static String getRandomCharAndNumr(Integer length) {  
    String str = "";  
    Random random = new Random();  
    for (int i = 0; i < length; i++) {  
        boolean b = random.nextBoolean();  
        if (b) { // 字串  
             // int choice = random.nextBoolean() ? 65 : 97; 取得65大寫字母還是97小寫字母  
 

             str += (char) (65 + random.nextInt(26));// 取得大寫字母  
         } else { // 數字  
             str += String.valueOf(random.nextInt(10));  
         }
     }
    return str;  
}