【java】将List中的实体按照某个字段进行分组的算法

直接看代码，就会明白。

//最后结果，发现集合大小由4变成3。集合根据特定条件被成功分组
    /**
     * 
Title:main  
     * 
Description: 
     * @author 鲁东顺
     * @date 2017-8-17下午6:23:31
     */
    public static void main(String[] args) throws Exception{
        List billingList = new ArrayList();
        SimpleDateFormat sdf=new SimpleDateFormat("yyyy/MM/dd");
        Article4temp a1 = new Article4temp();
        a1.setId(1);
        a1.setSign("GLOBAL_1_MOTHER");
        a1.setDate(sdf.parse("2015/01/01"));
        billingList.add(a1);
        Article4temp a2 = new Article4temp();
        a2.setId(2);
        a2.setSign("GLOBAL_1_MOTHER");
        a2.setDate(sdf.parse("2015/02/01"));
        billingList.add(a2);
        Article4temp a3 = new Article4temp();
        a3.setId(3);
        a3.setSign("GLOBAL_1_MOTHER");
        a3.setDate(sdf.parse("2015/01/01"));
        billingList.add(a3);
        Article4temp a4 = new Article4temp();
        a4.setId(4);
        a4.setSign("GLOBAL_1_MOTHER");
        a4.setDate(sdf.parse("2016/01/01"));
        billingList.add(a4);

        groupBillingDataByExcpBatchCode(billingList);
    }
    /**
     * 
Title:groupBillingDataByExcpBatchCode  
     * 
Description: 
     * @author 鲁东顺
     * @date 2017-8-17下午6:23:24
     */
    private static Map> groupBillingDataByExcpBatchCode(List billingList) throws Exception{
        Map> resultMap = new HashMap>();
        SimpleDateFormat sdf=new SimpleDateFormat("yyyy/MM/dd");
        try{
            System.out.println(billingList.size());
            for(Article4temp tmExcpNew : billingList){
                String key = sdf.format(tmExcpNew.getDate());
                if(resultMap.containsKey(key)){//map中已存在，将该数据存放到同一个key的map中
                    resultMap.get(key).add(tmExcpNew);
                }else{//map中不存在，新建key，用来存放数据
                    List tmpList = new ArrayList();
                    tmpList.add(tmExcpNew);
                    resultMap.put(key, tmpList);
                }
            }
            System.out.println(resultMap.size());
        }catch(Exception e){
            e.printStackTrace();
        }
        return resultMap;
    }