Java 分組函數 groupBy

# Collectors.groupingBy()分組函數

4.1: 分組去重，多字段構成key, 返回 Map<String, List<Talk>> 類型數據
2 　　　　Map<String, List<Talk>> talkMap = talkList.stream().collect(Collectors.groupingBy(talk -> talk.getGradeId() + "" + talk.getSubjectId() + ""
3         + talk.getUnitId()));