public static String getCharAndNumr(int length) {
    String val = "";
    Random random = new Random();
    for (int i = 0; i < length; i++) {
        // 輸出字母還是數字
        String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num";
	// 字串
	if ("char".equalsIgnoreCase(charOrNum)) {
	    // 取得大寫字母還是小寫字母
	    int choice = random.nextInt(2) % 2 == 0 ? 65 : 97;
	    val += (char) (choice + random.nextInt(26));
	} else if ("num".equalsIgnoreCase(charOrNum)) { // 數字
	    val += String.valueOf(random.nextInt(10));
	}
    }
    return val;
}