[Java知识库]list按照某个属性分组

import com.alibaba.fastjson.JSONObject;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import org.apache.commons.collections4.CollectionUtils;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListGroupByFieldTest {

    public static void main(String[] args) {
        
        List<LaneParamVO> laneParamVOList = new ArrayList<>();
        LaneParamVO laneParamVO = new LaneParamVO();
        laneParamVO.setParamKey("p1");
        laneParamVO.setParamValue("v1");
        laneParamVO.setGroupId(1);

        LaneParamVO laneParamVO1 = new LaneParamVO();
        laneParamVO1.setParamKey("p2");
        laneParamVO1.setParamValue("v2");
        laneParamVO1.setGroupId(1);

        LaneParamVO laneParamVO2 = new LaneParamVO();
        laneParamVO2.setParamKey("p3");
        laneParamVO2.setParamValue("v3");
        laneParamVO2.setGroupId(2);

        LaneParamVO laneParamVO3 = new LaneParamVO();
        laneParamVO3.setParamKey("p4");
        laneParamVO3.setParamValue("v4");
        laneParamVO3.setGroupId(2);

        laneParamVOList.add(laneParamVO);
        laneParamVOList.add(laneParamVO1);
        laneParamVOList.add(laneParamVO2);
        laneParamVOList.add(laneParamVO3);
        // java8流式写法
        if (CollectionUtils.isNotEmpty(laneParamVOList)) {
            Map<Integer, List<LaneParamVO>> collect = laneParamVOList.stream().collect(Collectors.groupingBy(LaneParamVO::getGroupId));
            System.out.println(JSONObject.toJSONString(collect));
        }

        Map<Integer, List<LaneParamVO>> integerListMap = listGroup2MapTwo(laneParamVOList);
        System.out.println(JSONObject.toJSONString(integerListMap));
    }


    /**
     * java8之前的写法
     * @param list
     * @return
     */
    private static Map<Integer, List<LaneParamVO>> listGroup2MapTwo(List<LaneParamVO> list) {
        Map<Integer, List<LaneParamVO>> userSignSerialMap = Maps.newConcurrentMap();
        if (CollectionUtils.isNotEmpty(list)) {
            // 分组
            for (LaneParamVO vo : list) {
                Integer groupId = vo.getGroupId();

                List<LaneParamVO> tempList = userSignSerialMap.get(groupId);
                /*如果取不到数据,那么直接new一个空的ArrayList**/
                if (tempList == null) {
                    tempList = Lists.newArrayList();
                    tempList.add(vo);
                    userSignSerialMap.put(groupId, tempList);
                }
                else {
                    /*某个vo之前已经存放过了,则直接追加数据到原来的List里**/
                    tempList.add(vo);
                }
            }
        }
        return userSignSerialMap;
    }
}

输出：

{1:[{"groupId":1,"paramKey":"p1","paramValue":"v1"},{"groupId":1,"paramKey":"p2","paramValue":"v2"}],2:[{"groupId":2,"paramKey":"p3","paramValue":"v3"},{"groupId":2,"paramKey":"p4","paramValue":"v4"}]}
{1:[{"groupId":1,"paramKey":"p1","paramValue":"v1"},{"groupId":1,"paramKey":"p2","paramValue":"v2"}],2:[{"groupId":2,"paramKey":"p3","paramValue":"v3"},{"groupId":2,"paramKey":"p4","paramValue":"v4"}]}