java 生成不重复的6位数字 +年月日

        String bookNo="1";
        Date date=new Date();
        SimpleDateFormat sdf1=new SimpleDateFormat("yyyyMMdd");
        String times=sdf1.format(date);
        NumberFormat f= new DecimalFormat("000000");
        if(!bookNo.equals("") && !bookNo.isEmpty()){
            int count = Integer.parseInt(bookNo.substring(0));
            bookNo = f.format(count+1);
            }else{
                bookNo="000001";
            }