Java Data Type - 如何生成随机字母数字字符串

public class Main {
   private static final String ALPHA_NUM =
           "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
   public static void main(String[] args) {
      GenerateRandomString grs = new GenerateRandomString();
      System.out.println(grs.getAlphaNumeric(10));
      System.out.println(grs.getAlphaNumeric(20));
   }
   public String getAlphaNumeric(int len) {
      StringBuffer sb = new StringBuffer(len);
      for (int i=0;  i<len;  i++) {
         int ndx = (int)(Math.random()*ALPHA_NUM.length());
         sb.append(ALPHA_NUM.charAt(ndx));
      }
      return sb.toString();
   }
}