#Collectors.groupingBy()分组函数

4.1: 分组去重，多字段构成key, 返回 Map<String, List<Talk>> 类型数据
2 　　　　Map<String, List<Talk>> talkMap = talkList.stream().collect(Collectors.groupingBy(talk -> talk.getGradeId() + "" + talk.getSubjectId() + ""
3         + talk.getUnitId()));

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