Java8 stream 中利用 groupingBy 进行多字段分组

    public static void main(String[] args) {
        List items = Arrays.asList("apple", "apple", "banana", "apple", "orange", "banana", "papaya");
//        Map map = items.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
        Map map = items.stream().collect(Collectors.groupingBy(t->t,Collectors.counting()));
        System.out.println(JSON.toJSONString(map));
    }

@Data
public class GameMusicVO implements Serializable {

    /**
    * 游戏Id
    **/
    private Integer gameId;

    /**
    * 音乐Id
    **/
    private Integer musicId;
    /**
     * 音乐code
     **/
    private String musicCode;

    /**
    * 音乐配置名称
    **/
    private String codeName;

    /**
     * 音乐名称
     **/
    private String musicName;

    /**
     * 音乐大小
     **/
    private Float size;

    /**
     * 音乐长度
     **/
    private Long timeLength;

    /**
     * 存储的路径
     **/
    private String path;

    /**
    * 是否选中
    **/
    private String value;
}

@Data
public class GameNewMusicVO implements Serializable {
    /**
     * 音乐code
     **/
    private String code;

    /**
     * 音乐配置名称
     **/
    private String codeName;
    /**
     * 音乐列表
     **/
    private List children;
}

 List musicList = new ArrayList();
          //自己添加list

            Map> menuGroupMap = musicList.stream().collect(Collectors.groupingBy(GameMusicVO::getMusicCode));
            List musicNewVOList = menuGroupMap.keySet().stream().map(key -> {
                GameNewMusicVO temp = new GameNewMusicVO();
                temp.setCode(key);
                 //这里虽然code与codeName是一对一，但还需要再查询一次。
                //temp.setCodeName(GameStringUtils.matchCodeName(key,musicCodeVOList));
                temp.setChildren(menuGroupMap.get(key));
                return temp;
            }).collect(Collectors.toList());

Map> menuGroupMap = musicList.stream().collect(Collectors.groupingBy(v -> v.getMusicCode() + "_" + v.getMusicName()));
            List musicNewVOList = menuGroupMap.keySet().stream().map(key -> {
                String[] keyArr = key.split("_");
                String code = keyArr[0];
                String codeName = keyArr[1];
                GameNewMusicVO temp = new GameNewMusicVO();
                temp.setCode(code);
                temp.setCodeName(codeName);
                temp.setChildren(menuGroupMap.get(key));
                return temp;
            }).collect(Collectors.toList());