生成随机数（4位、6位）

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 * @param list * @param n * @return */
    public static ArrayList getRandom(List list, int n) {
    
        HashMap<Object, Object> hashMap = new HashMap<>();
        for (int i = 0; i < list.size(); i++) {
    
            int number = random.nextInt(100) + 1;
            hashMap.put(number, i);
        }
        Object[] objs = hashMap.values().toArray();
        ArrayList<Object> r = new ArrayList<>();
        for (int i = 0; i < n; i++) {
    
            r.add(list.get((int) objs[i]));
            System.out.println(list.get((int) objs[i]) + "\t");
        }
        System.out.println("\n");
        return r;
    }
}