### Question:
GroupServiceImpl implements GroupService { @Override public Map<Long, Group> listByGroupIdIn(List<Long> groupIds) { Map<Long, Group> groupMaps = new HashMap<>(); List<Group> groupList = groupDao.listByGroupIdIn(groupIds); groupList.forEach(v -> { groupMaps.put(v.getGroupId(), v); }); return groupMaps; }  @Override Group findByGroupId(Long groupId); @Override Map<Long, Group> listByGroupIdIn(List<Long> groupIds); @Override boolean insertGroup(Group group); @Override boolean updateGroup(Group group); @Override boolean incMemberNumByGroupId(Long groupId, Integer memberNum); @Override boolean decMemberNumByGroupId(Long groupId); @Override boolean deleteByGroupId(Long groupId);  }### Answer:
@Test public void listByGroupIdIn() { }