发生定义字符中随机数

public class RandomFileName {
 /**
  * 产生一个随机的字符串
  *
  * @param 字符串长度
  * @return
  */
 public static String getRandomString(int length) {
  String base = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  Random random = new Random();
  StringBuffer sb = new StringBuffer();
  for (int i = 0; i < length; i++) {
   int number = random.nextInt(base.length());
   sb.append(base.charAt(number));
  }
  return sb.toString();
 }

 public static void main(String[] args) {
  System.out.println(RandomFileName.getRandomString(5));
 }
}