产生一个随机数验证码

import java.util.Random;
public class random {
 public static void main(String[] agrs) {
  String base = "abcdefghijklmnopkrstuvwxyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  StringBuffer result = new StringBuffer();
  for (int i = 0; i < 4; i++) {
   int num =new Random().nextInt(base.length());
   result.append(base.charAt(num));
  }
  System.out.println(result);
 }
}