public int compare(GroupDelta object1, GroupDelta object2) {
   final Long id1 = object1.getId();
   final Long id2 = object2.getId();
   if (id1 == null && id2 == null) {
     return 0;
   } else if (id1 == null) {
     return -1;
   } else if (id2 == null) {
     return 1;
   } else if (id1 < id2) {
     return -1;
   } else if (id1 > id2) {
     return 1;
   } else {
     return 0;
   }
 }
 @Override
 public long getChildId(int groupPosition, int childPosition) {
   final GroupDelta child = (GroupDelta) getChild(groupPosition, childPosition);
   if (child != null) {
     final Long childId = child.getId();
     return childId != null ? childId : Long.MIN_VALUE;
   } else {
     return Long.MIN_VALUE;
   }
 }