public Group group(String groupId) throws GroupIdNotFoundException {
        Group group = groups.get(groupId, Long.MAX_VALUE);
        if (group == null) {
            throw new GroupIdNotFoundException(String.format("Group %s not found.", groupId));
        }
        return group;
    }
@return The group corresponding to the group id or throw GroupIdNotFoundException.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java