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;
 }
}