/**
	  * java统计List集合中每个元素出现的次数
      * 例如frequencyOfListElements(["111","111","222"])
	  *  ->
	  * 则返回Map {"111"=2,"222"=1}
	  * @param items
	  * @return  Map<String,Integer>
	  * @author wuqx
	  */
    public static Map<String,Integer> frequencyOfListElements( List<String> items ) {
        if (items == null || items.size() == 0) return null;
        Map<String, Integer> map = new HashMap<String, Integer>();
        for (String temp : items) {
            Integer count = map.get(temp);
            map.put(temp, (count == null) ? 1 : count + 1);
        }
        return map;
    }