4位或6位随机数生成（用于短信验证码）

import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Random;

/**
 * 生成四位和六位的随机数字
 */
public class RandomUtils {

   private static final Random random = new Random();

   private static final DecimalFormat fourdf = new DecimalFormat("0000");

   private static final DecimalFormat sixdf = new DecimalFormat("000000");

   public static String getFourBitRandom() {
      return fourdf.format(random.nextInt(10000));
   }

   public static String getSixBitRandom() {
      return sixdf.format(random.nextInt(1000000));
   }

   /**
    * 给定数组，抽取n个数据
    * @param list
    * @param n
    * @return
    */
   public static ArrayList getRandom(List list, int n) {

      Random random = new Random();

      HashMap<Object, Object> hashMap = new HashMap<Object, Object>();

      // 生成随机数字并存入HashMap
      for (int i = 0; i < list.size(); i++) {

         int number = random.nextInt(100) + 1;

         hashMap.put(number, i);
      }

      // 从HashMap导入数组
      Object[] robjs = hashMap.values().toArray();

      ArrayList r = new ArrayList();

      // 遍历数组并打印数据
      for (int i = 0; i < n; i++) {
         r.add(list.get((int) robjs[i]));
         System.out.print(list.get((int) robjs[i]) + "\t");
      }
      System.out.print("\n");
      return r;
   }
}