import java.util.Random;

 public String getCharAndNumr(int length)

 {

     String val = "";   

     Random random = new Random();

     for(int i = 0; i < length; i++)

     {

         String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; // 输出字母还是数字   

         if("char".equalsIgnoreCase(charOrNum)) // 字符串

         {

             int choice = random.nextInt(2) % 2 == 0 ? 65 : 97; //取得大写字母还是小写字母

             val += (char) (choice + random.nextInt(26));

         }

         else if("num".equalsIgnoreCase(charOrNum)) // 数字

         {

             val += String.valueOf(random.nextInt(10));

         }

     }   

     return val;

 }