将多个List对象合并一个List对象，且List对象保持不变
@Override
public List subAvg01(String subName) {
QueryWrapper wrapper = new QueryWrapper();
wrapper.eq("su.`subjectName`", subName);
wrapper.last("GROUP BY su.`subjectName` ");
//查询指定科目的平均分
List<ResultPo> list = resultMapper.subAvg01(wrapper);
//查询指定科目的最高分数
List<ResultPo> list1 = resultMapper.subMax(wrapper);
//查询指定科目的最小分数
List<ResultPo> list2 = resultMapper.subMin(wrapper);
List<List<ResultPo>> all = new ArrayList<>();
all.add(list);
all.add(list1);
all.add(list2);
List<ResultPo> collect = all.stream().collect(ArrayList::new, ArrayList::addAll, ArrayList::addAll);
return collect;
}